DEPARTMENT OF THE INTERIOR

UNITED STATES GEOLOGICAL SURVEY

GEOKGE OTIS SMITH, DIBECTOR

WATER-SUPPLY PAPER 229

THE DISINFECTION OF SEWAGE AND 

SEWAGE FILTER EFFLUENTS

WITH A CHAPTER ON THE

BY

EARLE BERNARD PHELPS

WASHINGTON

GOVERNMENT FEINTING OFFICE 

1909

Quality
DEPAKTMENT OF THE INTERIOR

UNITED STATES GEOLOGICAL SURVEY

GEORGE OTIS SMITH, DIRECTOR

WATER-SUPPLY PAPER 229

THE DISINFECTION OF SEWAGE AND 

SEWAGE FILTER EFFLUENTS

WITH A CHAPTER ON THE

PUTRESCIBILITY AND STABILITY OF 

SEWAGE EFFLUENTS

BY

EARLE BERNARD PHELPS

WASHINGTON

GOVERNMENT PRINTING OFFICE 

1909

CONTENTS.

Page.

Disinfection of sewage and sewage-filter effluents....J........................ 7

Introduction............................................................ 7

Sewage purification..................................................... 7

Purifying agents................................................... 7

Fate of bacteria in sewage filtration.................................. 8

Necessity for disinfection........................................... 12

Methods of disinfection.............................................. 15

Classification of methods....................................... 15

Heat.......................................................... 15

Lime........................................................... 16

Acids......................................................... '. 16

Ozone......................................................... 17

Chlorine and its compounds..................................... 17

Germicidal action......................................... 17

Chlorine gas............................................... 17

Hypochlorites............................................. 18

Use in disinfection..................................... 18

Hamburg experiments................................. 19

Berlin experiments..................................... 21

Experiments at Bengal, India........................... 24

Experiments in Ohio.................................. 25

Electrolytic chlorine processes.............................. 25

Electrolytic manufacture of chlorine..................... 25

Webster process........................................ 26

Woolf process Electrozone............................. 27

Hermite process........................................ 27

Oxychloride process.................................... 28

Copper and its compounds..................................... 29

Miscellaneous methods.......................................... 32

Permanganates............................................. 32

"Amines" process......................................... 32

Sodium benzoate and other organic compounds............... 33

Summary of disinfection methods............................... 33

Experimental investigations............................................ 34

History............................................................ 34

Bacteriologic methods and expression of results....................... 35

Investigations at Boston............................................ 37

Scope and character of experiments.............................. 37

Preliminary experiments with chloride of lime................... 38

Disinfection of trickling-filter effluent........................... 39

Disinfection of crude sewage.................................... 46

Disinfection of septic sewage at Red Bank, N. J...................... 51

Disinfection of trickling-filter effluent at Baltimore, Md.............. 56

Comparative germicidal efficiencies of chlorine and some of its compounds.. ....................................................... 60

Effect of calcium hypochlorite on colon and typhoid bacilli.......... 62

3
4 CONTENTS.

Page. 

Disinfection of sewage and sewage-filter effluents Continued.

Practical applications and costs of disinfection............................ 63

Conclusion............................................................ 70

Putrescibility and stability of sewage-filter effluents.......................... 74

Introduction.......................................................... 74

Putrescibility-........................................................ 75

Relative stability..................................................... 77

Definitions......................................................... 77

Estimation of reducing time........................................ 78

Theoretical relation between reducing time and relative stability.... 80

Determination..................................................... 83

Incubation periods............................................ 83

Effect of temperature on reducing time.......................... 86

Summary of method............................................ 87

Interpretation of results............................................. 88

DISINFECTION TABLES.

TABLE 1. Bacteria in sewage and in effluents of sewage filters at Boston in 1904. 11

2. Bacteria in sewage, septic effluent, and trickling-filter effluents at

Boston in 1906................................................. 11

3. Disinfection of three crude hospital sewages with chloride of lime... 20

4. Disinfection of the crude screened sewage of Eppendorfer, Germany,

by chloride of lime.............................................. 21

5. Disinfection of crude Berlin sewage with chloride of lime.......... 22

6. Destruction by chloride of lime of B. coli embedded in gelatine.... 23

7. Disinfection of septic sewage by chloride of lime at Bengal, India.. 25

8. Disinfection of effluents with "chloride of lime at Lancaster and

Marion, Ohio................................................. 25

9. Analyses of London sewage before and after treatment by the Webster

process........................................................ H 26

10. Summary of Rideal's experiments on the use of oxychloride at Guilford, England................................................. 28

11. Disinfection of trickling-filter effluents with copper sulphate at

Columbus, Ohio................................................ 29

12. Disinfection of sewage and effluents with copper sulphate at Columbus, Ohio.................................................... 30

13. Effect of organic matter and of alkalinity on the germicidal properties

of copper sulphate............................................. 31

14. Disinfection of trickling-filter effluent with copper sulphate at Boston .......................................................... 31

15. Disinfection of effluents with copper sulphate in Ohio................ 32

16. Chemical analysis of trickling-filter effluent at Boston before and after

disinfection with chloride of lime and sedimentation............. 40

17. Disinfection of trickling-filter effluent with chloride of lime at

Boston; weekly averages....................................... 42

18. Disinfection of trickling-filter effluent at Boston; summary of bacteriological results averaged by periods to show the effect of 

changes in temperature and in the amount of available chlorine.. 43

19. Relation between individual tests of bacterial removal and the

average result at Boston........................................ 44
DISINFECTION TABLES.

TABLE 20. Relation between individual bacterial counts and the average

 result at Boston............................................... 44

21. Relation between time of contact and efficiency of disinfection

with chloride of lime........................................... 45

22. Disinfection of crude sewage with chloride of lime at Boston; series

1 and 2...................................................... 48

23. Disinfection of crude sewage with chloride of lime at Boston; series 3. 49

24. Disinfection of crude sewage with chloride of lime at Boston; summary of results............................................... 50

25. Disinfection of septic sewage with chloride of lime at Red Bank,

N. J.; weekly averages ........................................ 54

26. Relation between individual tests of bacterial removal and the

average result at Red Bank, N. J................................ 55

27. Chemical analyses of trickling-filter effluent at Baltimore, before

and after disinfection with chloride of lime and sedimentation... 57

28. Disinfection of trickling-filter effluent with chloride of lime at Baltimore; weekly averages......................................... 58

29. Relation between individual tests of bacterial removal and the

average result at Baltimore.................................... 58

30. Relation between individual bacterial counts and the average result

at Baltimore................................................... 59

31. Average bacterial removal during the disinfection of trickling-filter

effluent at Baltimore........................................... 59

32. Relative germicidal properties of chlorine and some of its compounds 60

33. Comparative resistance to calcium hypochlorite of B. typhi and B.

coli in aqueous emulsion........................................ 63

34. Estimates of the cost of maintenance and operation of a plant for 

disinfecting sewage or effluent with chloride of lime; based on a 

capacity of 5,000,000 gallons a day. ............................ 66

RELATIVE STABILITY TABLES.

TABLE 1. Relation between reducing time and relative stability at 20° centigrade ......................................................... 82

2. Summary of stability tests ^of trickling-filter effluents, showing time

required to exhaust the available oxygen......................... 84

3. Comparison of relative stability results obtained from the twenty-day 

incubation period shown in Table 2 with those calculated from a 

four-day period................................................. 85

4. Comparison of relative stability results obtained from smaller numbers of samples by the use of four, six, and twenty day incubation 

periods........................................................ 85

5. Relation between relative stability and reducing time at 20° and at

37° centigrade.................................................. 87

ILLUSTRATION.

PLATE I. Plan of sewage-disposal works at Red Bank, N. J.................... 52

THE DISINFECTION OF SEWAGE AND SEWAGE FILTER

EFFLUENTS.

By EARLE BERNARD PHELPS.

INTRODUCTION.

The investigations on which this report is based were conducted 

by E. B. Phelps at the sanitary research laboratory and sewage 

experiment station of the Massachusetts Institute of Technology 

at Boston, Mass., and, in collaboration with Mr. Phelps, by Francis 

E. Daniels at the sewage-disposal works at Red Bank, N. J., and by 

Ezra B. Whitman at the Walbrook Testing Plant at Baltimore, Md., 

under cooperative agreements with the Massachusetts Institute of 

Technology, the State Sewerage Commission of New Jersey, and the 

city Sewerage Commission of Baltimore.

SEWAGE PURIFICATION.

PURIFYING AGENTS.

The essential agents of sewage purification are provided and employed by nature. The slow action of the soil bacteria, aided by 

atmospheric oxygen, eventually converts into harmless mineral 

ingredients all organic matter that comes within its sphere of activity, the process being analogous to that of combustion. Sewage 

purification as practiced to-day is but the intensive application of 

these natural processes under controllable conditions. The improvements that have been made in methods of treating sewage have not 

involved the discovery or application of new principles, but have 

merely increased the working efficiency of the natural bacterial 

agencies. The constant aim of the experimenters has been to increase the rate at which sewage can be treated on a given area of 

land. From the old-time sewage irrigation field, with its maximum 

capacity of possibly 10,000 gallons an acre in twenty-four hours, to 

the present-day trickling filter capable of dealing with two or three 

million gallons an acre a day, the march of improvement has been 

steady and continuous. The amount of sewage .that can be purified
8 DISINFECTION OP SEWAGE.

on an acre of filtering area has been increased two or three hundred 

fold, and investigators are working toward a still greater increase.

It must be admitted that the significance of the word "purify" has 

also undergone a radical change. The effluents are no longer pure 

ground water. The liquid flowing from a modern trickling filter 

looks to the untrained eye like the original sewage. The organic 

matter of the sewage is no longer "burned up" to harmless mineral 

matter; indeed, there is almost as much organic matter in the effluent 

as in the raw sewage, and sometimes more. What change then has 

taken place to justify the use of the term "purified?" The answer 

lies in the fact that the organic matter has been changed but not 

removed. To carry out the simile, the organic matter, though not 

burned, has been charred or partly oxidized, and this charring 

process has been sufficient to rob it of its putrescibility or foulness. 

In other words, its chemical composition has been so altered that it 

is no longer capable of undergoing rapid putrefactive decomposition.

On first consideration it appears inconceivable that the chief 

object of sewage disposal prevention of the fouling of streams 

could be attained by such subtle changes in the nature of the organic 

matter. Nevertheless, effluents containing comparatively large 

amounts of organic matter may be discharged into streams without fear of causing nuisances if the organic matter is nonputrescible 

and if conditions preclude immediate sedimentation. The work of 

purification proceeds in the stream as it does in the soil until the 

oxidation, or combustion, is complete, oxygen for that purpose being 

sufficiently abundant in a reasonably clean stream. On the other 

hand, too much crude sewage added to water first robs it of all its 

available oxygen, then, in the anaerobic condition thus established, 

kills the beneficent oxidizing bacteria and transforms the stream 

practically to an open sewer. It is apparent, therefore, that purification of sewage has come to mean primarily the removal of its 

tendency to putrefy and not the total oxidation and1 removal of all 

its organic matter.

FATE OF BACTERIA IN SEWAGE FILTRATION.

In the older and more perfect methods of sewage purification the 

bacteria of the sewage with the other organic matter were destroyed 

by the straining action of the soil and the oxidizing action of the 

normal soil bacteria; but a modern filter of coarse stones neither 

strains the material nor affords opportunity for vigorous oxidation. 

Much of the original organic matter passes through such a filter, 

having undergone changes so slight as almost to defy detection by 

ordinary chemical means. It is reasonable, therefore, to inquire as to 

the fate of the sewage bacteria and particularly as to the pathogens, or 

disease-producing microbes. The data on this point are somewhat
PATE OF BACTERIA IN SEWAGE FILTRATION. 9

conflicting. The question was raised in 1893 in connection with the 

first septic tank at Exeter, England. The fear was expressed that 

pathogenic germs might even multiply in the tank, as other forms 

of bacteria are known to do. Sims Woodhead conducted an investigation as a result of which he concluded that no organisms capable 

of setting up morbid changes in animals after inoculation came from 

the tank. Pickard introduced an emulsion of typhoid bacilli into 

the Exeter tank and observed a slow diminution in number. It is 

important to note, however,, that even after fourteen days 1 per 

cent of the original number was still alive. Pickard also reported 

a removal of over 90 per cent of the typhoid organisms introduced 

into a contact filter. Rideal, on examining the effluents of three 

Scott-Moncrieff filters at Caterham, England, found reductions in 

Bacillus coli ranging from 95 per cent to 98.5 per cent. He made 

the following statement before the Royal Sewage Commission: 0 

11 Satisfactory evidence in most of the systems is now available from 

which I think we are justified in concluding that even if towns on a 

river like the Thames adopted bacterial schemes the pathogenicity 

of the London water supply would not be adversely affected thereby."

On the other hand, there is some evidence that the pathogenic properties of sewage are not materially altered in its passage through a 

coarse-grained filter. Alfred MacConkey 6 made a series of tests 

upon the longevity of B. typM in various waters. Samples of the 

liquid under examination were inoculated with large numbers of 

typhoid bacilli and were kept under observation. In one experiment the organism was isolated from sewage thirteen days after 

inoculation. In a second set of two tests it was not found after fifteen and after seventeen days. In two contact-bed effluents it was 

found after fifteen and seventeen days, respectively, while in two 

other contact-bed effluents it did not survive beyond the sixth day. 

MacConkey concluded that the numbers of typhoid organisms reaching a filter are ordinarily so small that there is but slight possibility 

of their passing through, but "if from any cause they arrive to the 

tanks in such large numbers as the B. coli, then certainly they might 

appear in the effluent just as the B. coli does." In interpreting 

such results due allowance must be made for the fact that the isolation and identification of the typhoid bacillus under such conditions 

is extremely difficult and that failure to detect the organism carries 

much less experimental weight than a positive result.

Houston/ in a careful investigation of the subject for the Royal 

Sewage Commission, found that the effluents from septic tanks, contact beds, and trickling-filter beds contained enormous numbers of

a Interim Rept. Royal Sewage Commission, 1901, Question No. 4148, p. 251.

b Second Rept. Royal Sewage Commission, 1902, p. 62.

c Houston, A. C., Second Rept. Royal Sewage Commission, 1902, p. 26.
10 DISINFECTION OF SEWAGE.

bacteria. In some of the tests the per cent reduction ir> the effluents 

as compared with the raw sewage was striking; but as it was necessary to judge an effluent by its actual condition, and as the number 

of micro organisms still remaining was almost always very large, he 

concluded that per cent purification is of minor importance. In not 

a few of the tests the bacteria were practically as numerous in the 

effluent as in the raw sewage. The relative abundance of the different kinds of bacteria appeared to be much the same in the effluents as in the crude sewage. Of undesirable bacteria, such as B. coli, 

proteus-like germs, spores of B. enteritidis sporogenes, and streptococci, the effluents contained nearly as many as the crude sewage. 

The reduction in numbers of these objectionable bacteria was apparently not marked enough to be of consequence from the point of 

view of the epidemiologist. No definite proof was found that the 

effluents from bacterial beds were conspicuously safer than crude 

sewage in their possible relation to disease. Attention was especially 

called to the presence of streptococci in the effluent. Houston contends that if it be true that streptococci are more delicate germs than 

the typhoid bacillus, their presence in large numbers in the effluent 

indicates the possibility or probability that the typhoid bacillus also 

survives under similar conditions a view that leads to the inference 

that the biological processes at work are not strongly inimical, if 

hostile at all, to the vitality of pathogenic germs. Experiments 

with B. sporogenes, a spore-forming organism, have shown that it 

passes through filters in almost undiminished numbers.

Besides the facts already mentioned, little information is available which bears directly on the fate of the pathogens, and particularly of the typhoid organism, in sewage purification. It is therefore necessary to examine the available indirect evidence. By studying the removal of certain specific organisms that are easily detected 

and certain well-defined groups of organisms existing in sewage, the 

probability of the elimination of the typhoid organism can be determined. In the absence of further data it must be assumed for the 

sake of safety that the elimination of the typhoid bacillus is not 

materially greater than that of the other species and groups that 

maybe studied. During the summer of 1904 there were operated at 

the sewage experiment station four septic tanks running at storage 

periods of from twelve to forty-eight hours, nine contact beds differing in material, depth, and rate, three trickling filters, and three 

sand filters, one of each set being run with crude sewage and two 

kinds of septic effluents, respectively. The results of bacterial counts 

are shown in Table 1, in which the numbers are averages of all results 

obtained with one kind of filter.0

a Winslow, C.-E. A., The number of bacteria in sewage and sewage effluents: Jour. Infect. Dis., vol. 1, 

Suppl. 1,1905, p. 209.
FATE OF BACTERIA IN SEWAGE FILTRATION. 11

TABLE 1. Bacteria in sewage and in effluents of sewage filters at Boston in 1904-

[Wi-slow.]

Source of sample.

Trickling filters.....

Number 

of examinations.

56 

56 

140 

18 

15

Bacteria per cubic centimeter.

Lactose gelatin at 20° C.

Liquefiers.

365,000 

162, 000 

60,000 

134,000 

500

Acid 

formers.

1, 670, 000 

495,000 

270,000 

114,000 

1,360

Total.

5,430,000 

1, 750, 000 

1, 060, 000 

451,000 

9,160

Lactose agar at 37° C.

Acid 

formers.

1, 670, 000 

650, 000 

290,000 

284,000 

11, 400

Total.

3,760,000 

1,040,000 

570, 000 

1,170,000 

43,600

Anaerobic 

agar.

2,440,000 

930,000 

440,000 

200,000 

1,200

The septic tank and two trickling filters in operation at the same 

station during the summer of 1906 gave the average results recorded 

in Table 2, showing that less than one-half of the bacteria growing at 

37° C. were removed by filtration, and that the reduction in the number of colon bacilli was practically in the same proportion.

TABLE 2. Bacteria in sewage, septic effluent, and trickling-filter effluents at Boston in

1906.

Source of sample.

Bacteria per

cubic centimeter; lactose agar at

37° C.

1,300,000

750,000

1, 650, 000

750,000

B. coli; positive a tests in

one-millionth

of a cubic centimeter.

Per cent.

65

35

66

35

o The bile broth presumptive test recommended by D. D. Jackson was employed.

Johnson," in his experiments at Columbus, found a reduction in the 

total number of bacteria ranging from 33 per cent to 60 per cent in 

primary-contact filters, and a removal of about 39 per cent of the 

remainder in secondary-contact filters, and from 30 per cent to 80 per 

cent in trickling filters, depending largely on the depth of the filter. 

Subsequent sedimentation increased this removal to 87 per cent. 

Thumm and Pritzkow,6 at Berlin, report a reduction in the number of 

bacteria from 17,000,000 in the sewage to 6,000,000 in the effluent 

of a double-contact filter. At La Madelein, France, Calmette c found 

5,000,000 bacteria per cubic centimeter in crude sewage, 2,900,000 in a 

secondary-contact effluent, and 800,000 in the effluent from a trickling 

filter. The sewage contained 20,000, the contact effluent 4,000, and 

the trickling effluent 2,000 colon bacilli. At Plainfield, N. J., the

a Johnson, Geo. A., Report on sewage purification at Columbus, Ohio, 1905.

6 Thumm, K., and Pritzkow, A., Mitteilungen aus der Koniglichen Prufungsanstalt fiir Wasserversorgung und Abwasserbeseitigung zu Berlin, 1903, vol. 2, p. 127.

c Calmette, A., Recherches sur 1'epuvation des eaux d'6gout, Lille, 1907, vol. 2.
12 DISINFECTION OF SEWAGE.

double-contact filter was found to reduce the number of bacteria 

from an average of 1,000,000 per cubic centimeter to an average of 

322,000; B. coli ranged from 1,000,000 to 100,000 in the sewage and 

from 100,000 to 10,000 in the effluent. a . An experiment performed by 

Houston 6 is important in this connection: B. pyocyaneus, a pathogenic organism, was applied to the top of a trickling filter, and ten 

minutes later the bacillus appeared in the effluent, continuing to be 

discharged for ten clays. In a similar manner the same organism was 

found to pass through a septic tank and a contact filter successively, 

and to persist in both for nine days. At Baltimore, Md., the board of 

advisory engineers concluded that 95 per cent of the bacteria in the 

sewage could be removed by a system comprising a septic tank, 9-foot 

trickling filters, and supplementary sedimentation basins. 0 Such 

results are better than those usually obtained elsewhere, and if they 

can be maintained in practice they will go far toward solving the 

problem in that locality.

From a consideration of the available evidence it may be stated in 

a general way that coarse-grained, rapid sewage filters remove a considerable proportion of the sewage bacteria; that such removal has 

not been found to be sufficiently complete in practice to have great 

sanitary significance; that bacteria of various groups and certain 

specific organisms pass through such filters in practically the same 

proportions as the bacteria as a whole; and that, in the absence of any 

information to the contrary, it should be assumed that such filters 

have no greater effect on the typhoid and other pathogenic organisms 

than on B. coli, B. pyocyaneus, sewage streptococci, or the different 

groups of sewage bacteria.

THE NECESSITY FOR DISINFECTION.

It is probable that removal of bacteria will again be considered an 

essential factor in sewage purification. That it was so considered 

formerly is well known. The development of the modern rapid filter 

has made it possible to introduce sewage purification under conditions 

where it would have been impossible or prohibitively expensive in 

former days. In the acceptance of a partial solution of the problem 

much has been gained and but little lost. The process of purification 

that renders practically stable the offensive organic matter of sewage 

has accomplished the most important and in many cases the only 

essential requirement. If, however, sewage effluents find their way 

into the drinking waters of neighboring communities, the question of 

the relative responsibility of the settlements is debatable. It is generally conceded at present that efficient sewage purification should be

a Kept. Sewerage Commission of New Jersey for 1906.

6 Houston, A. C., Fourth Interim Kept. Royal Sewage Commission, 1904, vol. 3, p. 77.

c Report of the Board of Advisory Engineers to the Sewerage Commission of Baltimore, 1906.
NECESSITY FOR DISINFECTION. 13

undertaken by the communities in which the sewage originates, and 

that purification of the water supply is the urgent duty of the other 

communities. As streams flowing through populous districts are 

necessarily contaminated and unfit for domestic use without filtration, 

it is considered unjust to require a community to purify its sewage to a 

higher degree bacterially than that shown by the stream into which it 

is discharged. On the other hand, in the fight against infectious diseases, sound tactics demand an attack on the enemy as near as possible to the initial source of infection. , The best and easiest place to 

destroy typhoid germs is at the bedside of the typhoid patient; but 

this method can not be relied on to keep sewage free from infection, 

and the next strategic point is certainly the sewage. Once at large 

the germs may reach their victims in a score of well-known ways, 

and who will say by how many devious and unknown paths ? With 

increasing knowledge of these facts and with improved processes and 

reduced cost of disinfection it is not too much to expect that the 

disinfection of sewage will some day be regarded as an integral part 

of its purification and as a necessary measure of protection for the 

community.

One of the ways by which typhoid germs pass from the sewer to 

the victim is by means of shellfish fed or fattened in polluted water, 

and many of the oyster and clam beds of the eastern seaboard are 

subject to pollution by sewage. In England conditions are so 

serious that the demand for shellfish has perceptibly decreased. 

The situation presents obvious difficulties. The water of a polluted 

river may be rendered potable through filtration, but the purification 

of shellfish seems to be out of the question; it follows, therefore, that 

health must be safeguarded either by preventing the discharge of the 

bacteria of sewage on shellfish beds or by prohibiting the taking of 

the shellfish. In a large community where the shellfish industry 

is small it will probably be more satisfactory to adopt the latter 

method, but in small communities whose sewage pollutes large and 

important beds thorough bacterial purification is not an unreasonable requirement. Unfortunately the greatest shellfish areas of the 

country are situated near large cities, where they are subject directly 

or indirectly to possible pollution from sewage discharge.

In a carefully prepared paper on the pollution of shellfish beds, 

G. W. Fuller a states that the annual crop of oysters gathered along 

the Atlantic and Gulf coasts in 1902 amounted to more than 

25,000,000 bushels, exceeding in value $13,000,000, and that the 

crop of clams was more than 2,000,000 bushels, valued at $2,000,000. 

Over one-half of this total production came from New Jersey, 

Maryland, and Virginia, and the shellfish were grown mainly in the

a Concerning sewage disposal from the standpoint of the pollution of oysters and other shellfish, with 

especial reference to their transmission of typhoid fever: Jour. Franklin Inst., vol. 160,1905, p. 82,
14 DISINFECTION OF SEWAGE.

waters of Delaware and Chesapeake bays, which receive the sewage 

from many large cities. At present this sewage is so enormously 

diluted by the bay waters that the danger of pollution is in most 

places remote, but it is present and ever increasing.

In Baltimore a system of sewerage and sewage disposal is being 

planned to remedy the already serious pollution of Patapsco River. 

In calling on a commission of experts for advice the sewerage commission of Baltimore specified "that the effluent proposed to be 

discharged into Chesapeake Bay or its tributaries in the system to 

be recommended by the engineers shall be of the highest practicable 

degree of purity." a

By agreement between the States of New Jersey and Pennsylvania the condition of Delaware River has been thoroughly 

investigated, and the pollution of that stream by sewage from 

Trenton, Bordentown, and other smaller communities in New Jersey, 

and Philadelphia, Easton, and other Pennsylvania cities, is prohibited after certain specified dates. 6 The necessity for protecting 

these valuable shellfish beds makes bacterial removal an essential 

feature in any scheme of sewage disposal which may be considered 

for such places.

The board of advisory engineers at Baltimore recommended that 

the settled trickling-filter effluent be given a final treatment on sand 

filters. The cost of works for the complete treatment of 75,000,000 

gallons of sewage a day by septic tanks, trickling filters, sedimentation basins, and sand filters was estimated at $3,283,250, of 

which sum $1,040,750, or over 31 per cent, was for supplementary 

treatment. The annual cost of operation is-expected to be $115,500, 

of which $55,000, or 48 per cent, is for supplementary treatment. 

This gives some idea of the cost of complete bacterial removal by 

filtration processes over and above the cost of reasonable organic 

purification. In regard to disinfection by chemical means the 

advisory engineers state: "To remove all bacteria remaining in the 

settled effluent from the sprinkling filters by disinfectants, such as 

hypochlorite of lime or of sodium or sulphate of copper, would be 

prohibitively expensive" an authoritative opinion based on the 

best evidence then to be had. Almost no American data on chemical 

disinfection were available, and the results of experiments in 

Germany indicated that such disinfection could be accomplished only 

at high cost. The patented processes mainly used in England were 

also expensive. It was therefore desirable to learn just how effective 

disinfection processes could be made under conditions in America 

how much they would cost, and what after effects, objectionable or 

otherwise, might follow their introduction.

o Report of the Board of Advisory Engineers to the Sewerage Commission of Baltimore, 1906. 

6 Kept. New Jersey State Sewerage Commission, 1907.
METHODS OF DISINFECTION. 15

A review of the available data and a short experimental investigation made in 1906 at Boston® led the writer to believe that there is 

much value in the process and that it might afford the best possible 

solution of the whole problem under certain conditions common in 

this country, particularly in localities where the shellfish question is 

involved.

METHODS OF DISINFECTION. 

CLASSIFICATION OF METHODS.

In Great Britain the somewhat indefinite allusions of the royal 

sewage commission to sterilization as a finishing process in sewage 

treatment aroused a storm of discussion that resulted, at least, in 

clearing away many misconceptions. Sterilization processes of many 

kinds have been investigated, and the subject has been freely discussed. For the following classification of sterilizing agents and for 

many of the facts noted here the writer is indebted to Rideal, 6 whose 

carefully prepared paper on the subject discusses its possibilities in a 

thoroughly impartial manner. Other authorities are cited wherever 

possible. Except when otherwise stated, costs are based on current 

prices in eastern markets.

The different methods and substances proposed for the sterilization 

of effluents are considered in the following order:

1. Heat.

2. Lime.

3. Acids.

4. Ozone.

5. Chlorine and its compounds. 

(a) Chlorine gas.

(6) Hypochlorites, or oxychlorides. 

(c) Electrolytic chlorine processes.

6. Copper and its compounds.

7. Miscellaneous.

(a) Permanganates.

(6) "Amines" process.

(c) Sodium benzoate and other organic compounds.

HEAT.

The use of heat has been suggested for disinfecting sewage. In his 

testimony before the royal sewage commission E. E. Klein 0 referred 

to a patented process which he considered practicable, by the use of 

which sufficient ammonia could be recovered from the sewage nearly 

to pay for the treatment; but there is no record that this process

o Phelps, Earle B., and Carpenter, WilMam T., The sterilization of sewage-filter effluents: Tech. Quart., 

vol. 19,1906, p. 382; Contributions from the Sanitary Research Laboratory, vol. 4,1908. 

6 Rideal, S., On the sterilization of effluents: Jour. Royal Sanitary Institute, vol. 26, p. 378. 

c Interim Reptr. Royal Sewage Commission, 1901, question 9674, p. 519.

76474 IRE 229 09 2
16 DISINFECTION OF SEWAGE.

has ever been used on a working scale. A device for heat interchange 

similar to that employed in the Forbes sterilizer may possibly, as is 

claimed, raise water to the boiling point and cool it again within 5° F. 

of the initial temperature. With coal at $3 per ton having a calorific 

value of 10,000 British thermal units, the fuel cost alone of such an 

operation would be about $7 per million gallons of sewage. The 

ammonia in a million gallons of Boston sewage, if in the form of 

sulphate, would have a market value of $20. Whether the difference 

between the value of the ammonium sulphate and the cost of the 

fuel is sufficient to cover the cost of operation, including labor, 

evaporation of the dilute solution, and all fixed charges can be determined only by actual experiment, but the plan is not wholly without 

possibilities.

LIME.

Caustic lime acts only slightly as a germicide, and the considerable 

removal of bacteria that takes place when lime is used as a precipitant 

in sewage is undoubtedly due to the action of the precipitate itself 

in dragging down with it the bacteria which it has entangled. Such 

action occurs to some extent in the precipitation of any substance, 

and even the sedimentation of sewage is always accompanied by 

reduction in the numbers of bacteria. Lime alone, therefore, would 

be of little value for sterilizing effluents. Rideal states that 60 to 70 

grains per imperial gallon (860 to 1,000 parts per million) are inefficient 

in sterilization. Thresh, however, believes that lime would produce 

a satisfactory sterilization of effluents.a

ACIDS.

Most bacteria, and particularly typhoid and cholera germs, are 

more readily destroyed by acids than by alkalies. Rideal, therefore, 

considers it feasible to employ acids as germicides. He states that 

Stutzer found 0.05 per cent acid solutions fatal to bacteria in twentyfour hours ; that Ivanoff found that 0.04 per cent to 0.08 per cent of acid 

destroyed the cholera germs in the sewage of Berlin and of Potsdam; 

that Kitasato found 0.08 per cent of sulphuric acid fatal to typhoid 

bacilli in fifteen minutes, and that he had himself obtained similar 

results. To furnish 1,000,000 gallons of sewage with 0.08 per cent of 

sulphuric acid requires 6,650 pounds of acid, costing approximately 

$73. Smaller amounts of acid might be used, as it would be unnecessary to kill typhoid germs in so short a time as fifteen minutes; but, 

on the other hand, most sewage contains a considerable amount of 

free alkali which must be neutralized before any germicidal effect of

a Interim Rept. Royal Sewage Commission, 1901, question 8917, p. 502.
METHODS OF DISINFECTION. 17

the acid would be obtained. This process consequently would seem to 

be impracticable except in emergencies. It is interesting to note that 

the sewage of Worcester, Mass., contains normally an average of 0.01 

per cent of free sulphuric acid, or half enough to kill cholera germs in 

twenty-four hours.

OZONE.

Ozone has been used more or less successfully in Germany, particularly at Weisbaden, for sterilizing drinking water. Though the process has been most favorably commented on by those in immediate 

charge of the investigations, it has not been generally regarded as 

successful. The possibility of procuring a satisfactory effluent by 

this process, when the water is that of a highly-polluted river is beyond question, but whether the process can satisfactorily treat a sewage effluent of considerable turbidity has not. been determined. 

Rldeal calls attention to the fact that ozone is but sparingly soluble 

in water, and on that account it might fail to penetrate the solid 

masses in the effluent, since the rate at which a dissolved gas will 

penetrate solids in a liquid is a direct function of its solubility. The 

principal cause of failure of the ozone process, however, seems to be 

its expense. If this be true in waterworks it is hardly possible that 

such treatment could be applied to sewage effluents as an additional 

safeguard after purification. Data are not at hand for estimating 

the cost of applying ozone treatment to sewage effluents, but besides 

the cost of operation there is the very considerable cost of installing 

the necessary machinery and towers.

CHLORINE AND ITS COMPOUNDS.

GERMICIDAL ACTION.

Chlorine is well known as a powerful germicide. As a bleaching 

agent it acts on organic coloring matter indirectly by means of the 

free nascent oxygen which it liberates from the water in which it is 

dissolved, and it is probable that its germicidal action is similar. In 

other words, chlorine and ozone owe their germicidal power to the 

same thing nascent oxygen. Chlorine, however, has the advantages of being cheap, of being more readily soluble, and of being 

obtainable in compounds that are easily transported and handled.

CHLORINE GAS.-

Until within a few years chlorine has been manufactured commercially by the Weldon or some similar process. In the Weldon process 

hydrochloric acid is made to react with a complex mixture of man-
18' DISINFECTION OF SEWAGE.

ganese hydroxide and lime "Weldon mud" the reaction being 

essentially

MnO2 + 4HC1 = MnCl2 + 2H2O + C12 ,

though in reality it is much more complex. Kecently, however, 

electrolytic processes have been developed, by which the cost of 

manufacture has been materially reduced, particularly where cheap 

water power is available. Chlorine prepared by the mixing of common salt, an acid, and a suitable oxidizing agent was used in England 

as early as 1800 by Cruikshank, who recommended manganese 

dioxide and potassium bichromate as oxidizing agents. The objections to the use of gaseous chlorine are chiefly the cost of transportation, the difficulty and danger of handling the gas, and the difficulty 

of measuring accurately the amount of gas added to the effluent. 

Furthermore, in disinfection free chlorine is not so efficient as the 

hypochlorite.

Available chlorine, a term frequently used in the discussion of 

chlorine disinfection methods, is determined by titrating a solution 

with arsenious acid, or with some other reducing agent, and it 

represents in reality the oxidizing power of the substance expressed 

in terms of chlorine. For example, hypochldrous acid in the presence of a reducing agent is decomposed according to the following 

equation:

HC1O = HC1 + O.

The oxidizing power of this acid, or the available chlorine, is, therefore, two hydrogen equivalents per molecule, which is twice its total 

chlorine content, a fact that makes the term available chlorine a 

misnomer, but it has come into general use in the chlorine industries 

and it is a convenient expression. In this article it signifies oxidizing 

power, determined against arsenious acid, and expressed in terms of 

chlorine.

HYPOCHLORITE S.

Use in disinfection. Chlorine is handled commercially in the form 

of bleaching powder, or chloride of lime an impure product composed largely of calcium hypochlorite. Bleaching powder or " bleach " 

containing from 35 per cent to 40 per cent of available chlorine can 

be obtained in the market. The hypochlorite dissolves in water, 

leaving a residue composed chiefly of calcium hydrate and calcium 

carbonate. Hypochlorites in general are made by adding chlorine 

to caustic alkalies. Bleaching powder is made by passing dry chlorine gas over freshly-slaked lime. It is manufactured in this country 

in large amounts, the chlorine being obtained by the electrolysis of 

salt. Abroad the chlorine is made by the older methods, and much
METHODS OF DISINFECTION. 19

foreign bleach is sold in this country in competition with the electrolytic product. "Eau de Javelle" and "Labarraque's solution" are 

solutions of- sodium hypochlorite. "Chloros," a commercial preparation of sodium hypochlorite, contains 10 per cent by weight of available chlorine.

Hypochlorites have long been recognized as powerful and efficient 

disinfectants. The sodium and potassium compounds have not been 

generally used on a large scale because of their relatively high cost 

and the difficulty of preparing and keeping them in the dry state, 

but calcium hypochlorite has been extensively employed. The first 

Royal Sewage Commission of Great Britain used it in deodorizing 

London sewage in 1854. a The committee of 1885 of the American 

Public Health Association found it to be the best disinfectant available, cost and efficiency considered. It was used by Dibdin b in 1884 

to deodorize the sewage of London, but it was not successful for that 

purpose and was later abandoned in favor of sodium permanganate. 

Its action on specific bacteria was studied by Nissen ° in 1890. The 

use of bleaching powder as a sewage disinfectant has been more 

extensively studied in Germany than elsewhere. At the Hygienic 

Institute of Hamburg investigations have been made by Proskauer 

and Elsner, Dunbar and Zirn, Dunbar and Korn, Schumacher, and 

Schwarz. At the Royal Testing Station in Berlin, Kranepuhl and 

O. Kurpjuweit have each reported investigations.

Hamburg experiments.- Proskauer and Elsner d experimented at 

Hamburg with sewage which had been clarified by the Rothe-Degener 

system. They obtained satisfactory disinfection with chloride of 

lime, using concentrations of chlorine ranging from 2.7 to 4.0 parts 

per million, and ten minutes exposure sufficed practically to eliminate 

B. coli. Dunbar and Zirn d treated crude sewage and, in common 

with later workers, they imposed much more exacting standards of 

disinfection and used much greater concentrations of chlorine. 

After having employed cholera germs as test organisms, they concluded that the satisfactory disinfection of crude Hamburg sewage 

would require 25 parts per million of available chlorine and an exposure 

of two hours. Dunbar and Korn e studied the disinfection of crude 

sewage with special reference to its subsequent purification on biological filters. Schumacher^ investigated the problem of disinfecting 

hospital sewages that had not received any previous treatment. In a

a Second Rept. Royal Sewage Commission, London, 1861.

b Dibdin, W. J., Jour. Assoc. Eng. Soc., vol. 40, 1908, p. 310.

c Zeit. Hyg., vol. 8, 1890, p. 62.

d Vierteljahrsschr. ger. Med., vol. 16,1898, Suppl. Heft.

« Ges. Ing., vol. 27, 1904.

/ Idem, 1905.
20 DISINFECTION OF SEWAGE.

preliminary set of bottle experiments he obtained the following 

results with the sewages of three hospitals.

TABLE 3. Disinfection of .three crude hospital sewages with chloride of lime.

[Schumacher.]

Concentration of chloride of lime.

1:7,000............................... .... ..........

1:5,000........... .....

1 : 2,000. ...............................................

Hours of 

contact.

{ 2

4 

6

24

( 2

4

1 24

2

4

1 6

( 2

4

6

1

Bacteria per cubic centimeter.

Initial content: 

23,000,000.

After treatment.

540

140 

200

160

60

160

120 

30

80

20

20

60

120

40

Initial content: 

37,000,000.

After treatment.

200

260 

100

20

60

40

420

20

20

40

40

Initial content: 

21,000,000.

After treatment.

8,400

100 

20

60

100

800

DUU

40

60

20

80

180

The amount of available chlorine in the chloride of lime used in 

these experiments is not stated, but it was probably not far from 30 

per cent. On that assumption the concentration of available chlorine 

in the four sets of tests would be 43, 60, 150, and 300 parts per million, 

respectively. The high initial numbers indicate a very strong sewage. 

It is also worthy of note that the reduction in the number of bacteria 

with only 43 parts of chlorine is much greater than would be demanded 

in ordinary practice. Disinfection on a large scale was also conducted 

by the same investigator at two hospitals. After a storage period of 

two hours samples of one liter each were tested for B. coll. With a 

concentration of 1: 2,000, or about 150 parts per million of available 

chlorine, B. coli was isolated from a liter of water in only 6 samples 

out of 43, not being found in 88 per cent of the samples tested. With 

chloride of lime in the proportion of 1: 5,000, or about 60 parts per 

million of available chlorine, B. coli was destroyed in 62 per cent of the 

samples in two hours and in 64 per cent of the samples in four hours. 

Schwarz a called attention to the fact that excessive amounts of 

disinfectant are necessary on account of the large floating particles, a 

point previously commented on by Schumacher. Schwarz proposed, 

therefore, that all sewage should be carefully screened before disinfection in order to remove particles exceeding three millimeters in 

diameter. The screening of hospital sewage should be even more 

complete, removing particles exceeding one millimeter in diameter.

a Ges. Ing., vol. 29, 1906, p. 773.
METHODS OF DISINFECTION.

During experiments at the Eppendorfer purification works, a sewage 

flow of about 60,000 United States gallons a day was available, and a 

tank holding about four hours' flow was used. As information was 

especially desired regarding the effect of the treatment on the cholera 

vibrio, an emulsion of another vibrio (Leuchtvibrionen) was added to 

the sewage at a regular rate before treatment, after it had been determined that this test organism would not only pass through the tank 

but would persist for days in the filters. Table 4 summarizes the 

results.

TABLE 4. Disinfection of the crude screened sewage of Eppendorfer, Germany, with chloride, of lime.

[Schwarz.]

Concentration of chloride of lime. a

1:2,000....................................

1:5,000....................................

1 : 10,000. ...................................

1:20,000...................................

1 : 30,000. ..................................

1:40,000...................................

Tests for vibrio in 1 cubic centimeter amounts.

Total number of tests.

51 

28 

15 

10

8

Number of 

positive 

tests.

0 

0 

0 

0 

4

Test for B. coli in 1 cubic centimeter amounts.

Total number of tests.

17

51

7 

6

Number of 

positive 

tests.

0 

1 

0 

0

Final number of bacteria per 

cubic centimeter, b

15 

23 

36 

72 

3,620 

59,000 

950,000

a. 1:10,000 is about 30 parts per million of available chlorine, assuming that the chloride of lime contained 

30 per cent of available chlorine. 

>> Initial number of bacteria per cubic centimeter, 1,350,000.

It was noted that a concentration of chloride of lime of 1: 2,000 

materially affected the subsequent treatment of the disinfected 

sewage in trickling filters. Oxygen consumed and ammonia in the 

effluent were higher and nitrates lower than normal. An interesting 

fact noted was the production of chlorates in the filter, over 10 parts 

per million being recorded at one time. Schwarz concluded that 

sewage can be satisfactorily disinfected with chloride of lime after 

having been carefully passed through one millimeter mesh screens. 

One part in 5,000 (60 parts per million of available chlorine) was 

considered necessary for the destruction of typhoid germs and from 

one part in 7,000 to one part in 10,000 (30 to 40 parts of available 

chlorine) for cholera vibrio. The disinfected sewage can be subsequently purified without previous neutralization of the disinfectant.

Berlin experiments. At the royal testing station in Berlin the 

subject of sewage disinfection has been studied by Kranepuhl 0 and 

by Kurpjuweit. b Kranepuhl undertook to determine the concentration of chloride of lime and the time of contact necessary to destroy 

the colon bacilli in crude Berlin sewage. These bacilli numbered

« Mitteilungen aus der Konighlichen Prufungsanstalt fur Wasserversorgungund Abwasserbeseitigung zu 

Berlin, vol. 9, 1907, p. 149. 

b Idem, p. 1C2.
22 DISINFECTION OF SEWAGE.

about 100,000 per cubic centimeter. They were considered an 

index of the pathogenicity of the sewage, because they are more 

numerous and more resistant than the pathogenic forms. The 

bleaching powder employed contained available chlorine ranging 

from 25 to 35 per cent. One liter samples of sewage were treated 

with the desired amounts of chloride of lime, and at the expiration 

of the specified time the remaining available chlorine was determined and was then neutralized with sterile sodium thiosulphate. 

Nutrient broth was then added to the entire liter sample, after which 

the sample was incubated. B. coli was sought in the incubated 

sample by the usual means, and confirmatory tests for it were made. 

Kranepuhl's results are summarized in Table 5, in which positive 

tests mean that B. coli was found in 1 liter.

TABLE 5. Disinfection of crude Berlin sewage icith chloride of lime. 

[Kranepuhl]

Available

chlorine (in

parts per

million.)

50

50

60

60

150

150

300

300

Time of

exposure.

Hours.

2

4

2

4

2

4

2

4

B. coli in liter samples.

Number of

samples

tested.

20

9

17

6

19

10

16

7

Number of

positive

tests.

11

2

6

3

4

1

1

0

Per cent of

positive

tests.

55

22

35

50

21

10

6

0

Kurpjuweit a studied the. penetration of solid particles by the disinfectant. He made test cubes of gelatine having a volume of about 

10 cubic centimeters, which he immersed in solutions of chloride of 

lime for definite times, then removed and melted in warm water, 

after which he determined the available chlorine. He found the 

amount of available chlorine to be a regular function of the time of 

exposure and of the concentration of the solution. The most striking 

fact noted was the small quantity of chlorine actually absorbed. 

For instance, a cube that had been immersed for ninety hours gave 

a mean concentration of chlorine within its own volume equal to 

but 1 per cent of the concentration of the solution. There was also 

shown to be a chemical combination between the chlorine and the 

gelatine. In order to determine whether such chlorine was active 

in "disinfection before it became combined, similar gelatine cubes, 

inoculated with B. coli before setting, were immersed for two hours 

in solutions containing from 60 to 3,000 parts per million of available 

chlorine. The results in Table 6 were obtained.

a Loc. cit.
METHODS OF DISINFECTION. 23

TABLE 6. Destruction by chloride of lime of B. coli embedded in gelatine.

[Kurpjuweit.]

Concentration of solution (average available chlorine in parts per million).

60.... . --

350...

300..

300..

Number of

B. coli remaining in 10 

cubic centimeters after

2 hoars.

91,500

55,400

43,500

48,100

6,600

The value of these results impractical work is problematic for several 

reasons. The cubes employed are much larger than the particles that 

should be in any sewage to be disinfected, and the character of the 

material is still more significant, because cubes of solid gelatine do 

not represent in any way the porous, sernisoluble masses that occur 

in sewage. The question of penetration is an important one, and 

there can be no doubt that the practical efficiency of disinfection 

processes is limited by the ability of the disinfectant to penetrate 

small, solid particles that may be in the sewage. This point has 

been illustrated in a practical manner by Kurpjuweit's experiments. 

Four samples of crude sewage were screened through sieves having 

openings 2, 5, 7, and 10 millimeters in diameter, respectively. Four 

portions of each of the filtrates thus obtained were then treated with 

chloride of lime, so proportioned that the available chlorine was 150, 

300, 600, and 3,000 parts per million, respectively. In the sewage 

that was screened through a 2-millimeter mesh, 150 parts per million 

of available chlorine destroyed all the B. coli in four separate liter 

samples, while the same concentration of chlorine destroyed the B. 

coli in only 5 out of 8 liter samples screened through the 10-millimeter 

mesh. Indeed, 3,000 parts of available chloiine were required to 

remove completely the B. coli in the samples screened through the 

10-millimeter mesh.

It should be noted that the investigators at Hamburg and at Berlin 

dealt wholly with crude sewage. Even when purification plants are 

in operation the disinfection is invariably applied first. No good 

reason for this procedure is obvious, unless it is that preliminary sedimentation tanks are available and supplementary tanks are not. In 

Germany the method of operation is possibly justified by the fact 

that the processes are being studied in order that they may be adopted 

for temporary use during epidemics, and it is not proposed to practice 

disinfection regularly. The expense of treating crude sewage, however, is at least twice that of treating a well-purified filter effluent. 

The very high standards that have been established for this work are
24 DISINFECTION OF SEWAGE.

also of interest. It is proposed so to treat a sewage containing over 

100,000 B. coli per cubic centimeter that the number of that kind of 

bacillus will be reduced to less than one in a liter. The result can 

hardly be expressed in per cent purification, and it is proper to inquire 

why such severe standards are employed. If the number of colon 

bacilli were reduced even to one per cubic centimeter, it would mean 

a reduction of 99.999 per cent. It may safely be inferred that the 

number of typhoid and cholera germs would be reduced in about the 

same ratio, and furthermore that the number of typhoid and cholera 

cases due to the discharge of this sewage would be similarly decreased. 

In other words, if such disinfection were generally adopted, 99,999 

cases of disease out of every 100,000, which are due, directly or indirectly, to sewage pollution, would be eliminated. Such reduction 

would seem to be very satisfactory, and yet it is proposed to improve 

this a thousand fold by insisting on an elimination of the colon 

bacilli from 1-liter samples, thus making the cost so great that it 

practically prohibits the use of the process, except for short periods 

during serious epidemics. It is well worth considering whether a continuous removal of 99 per cent of the disease germs is not a better 

safeguard of the public health than an occasional complete removal 

during epidemics.

Experiments at Bengal, India. The government of Bengal,a in 1904, 

appointed a commission to report on the pollution of Hooghly River 

by the effluents of septic tanks. Though the commission decided 

that the physical and chemical pollution of the river by the effluents 

was improbable, as sufficient dilution took place at all seasons to prevent any nuisance, bacterial purification of the effluents was deemed 

advisable. Experimental sand filters and copper sulphate disinfection satisfactorily removed the germs, but substitution of chloride 

of lime for the copper salt accomplished the same result at much less 

expense.

A septic tank installed near Calcutta was connected with a 

public latrine serving about 2,000 persons. From 400 gallons to 

2,500 gallons of septic-tank effluent were daily treated with various 

amounts of chloride of lime, the available chlorine in which ranged 

from 20 to 60 parts per million. The numbers of bacteria initially 

present were not determined, but the final numbers are sufficiently 

low to indicate a satisfactory treatment. Furthermore, it was 

shown that increasing the concentration of the chlorine beyond a 

certain limit has very little effect on the residual bacteria. The 

results of this work are summarized in Table 7.

«Indian government resolution on the working of septic tanks. Calcutta, January 6,1906.
METHODS OF DISINFECTION. 25 

TABLE 7. Disinfection of septic seivage by chloride of lime at Bengal, India.

Available

chlorine (in 

parts per 

million).

20

30

40

60

Number of 

tests.

9

7

8

7

Number of 

samples.

31

24

28

25

Bacteria, remaining

(average 

number per 

cubic centimeter).

33

10

48

52

Experiments in OJiio. In 1907 the Ohio State Board of Health 

studied the disinfection of sewage effluents in cooperation with the 

Bureau of Plant Industry of the United States Department of Agriculture. The results, reported by Kellerman, Pratt, and Kimberly,0 

related in part to the use of chloride of lime. The average results of 

four series of tests made with this disinfectant are summarized in 

Table 8.

TABLE 8. Disinfection of effluents with chloride of lime at Lancaster and at Marion,

Ohio.

[Kellerman, Pratt, and Kimberly.]

Series.

A................

B. ...............

C..... ............

D. ...............

Available 

chlorine 

(in parts 

per million).

4.0 

2.8 

4.1 

6.0

Number of bacteria 

per cubic centimeter 

at 20° C.

Initial.

130,000 

60,000 

225,000 

2,000,000

Final.

140 

1,600 

- 1,600 

700,000

Number of bacteria 

per cubic centimeter 

at 37° C.

Initial.

14,000 

12, 000 

120, 000 

900,000

Final.

49 

120 

390 

230,000

Number of acid formers per cubic centimeter at 37° C.

Initial.

840 

3,000 

16,000 

70,000

Final.

0 

0 

1 

24,000

Each series represents a three-day test, during which 18 samples 

were examined in duplicate. Series A was made on the effluent of a 

sand filter at the Boys' Industrial School, Lancaster, and Series B, 

C, and D on the sand-filter effluent, contact-filter effluent, and septictank effluent, respectively, at Marion, Ohio. A subsequent study of 

the possibilities of treating the septic tank effluent at Marion led the 

authors to conclude that satisfactory disinfection could be accomplished by the use of sufficient bleaching powder to give 25 parts 

per million of available chlorine.

ELECTROLYTIC CHLORINE PROCESSES.

Electrolytic manufacture of chlorine. When an electric current under 

a tension of not less than 2.5 volts is passed through a solution of 

common salt or of calcium or magnesium chloride, chlorine gas appears

a Kellerman, K. F., Pratt, R. W., and Kimberly, A. E., The disinfection of sewage effluents for the protection of public water supplies: Bull. 115, Bur. Plant Industry, U. S. Dept. Agr., 1907.
26 DISINFECTION OF SEWAGE.

at one electrode and sodium hydroxide or the corresponding alkali 

at the other. In the electrolytic manufacture of chlorine the products of the dissociation are kept apart and are removed from the cell 

as quickly as possible, for if they are allowed to come together again 

they immediately unite and form a hypochlorite. This method of 

manufacturing hypochlorites has been employed in many of the 

so-called "electrolytic disinfection processes."

The Webster process. One of the earliest electrolytic treatments 

was devised by Webster over twenty years ago, when, in 1889, he 

installed an experimental electrolytic plant at Crossness, England, to 

treat London sewage.0 In his process crude sewage flowed between 

iron electrodes placed in long troughs, and an electric current was 

passed from one electrode to the other at a tension of only two volts 

and a current density of 0.9 ampere per square foot of electrode. It 

was estimated that the treatment of 1,000,000 gallons of crude sewage required the consumption of 240 pounds of iron and 450 kilowatt 

hours of electricity. Sedimentation followed the electrical treatment, 

and a large amount of material was removed. In fact the process 

was virtually one of chemical precipitation, the iron dissolved from 

the electrode being first converted into hypochlorite, or other salt, 

and then being decomposed by the alkali present and precipitated. 

The solution of iron at the positive pole allowed the electrolytic 

reaction to proceed at a lower voltage than that required to liberate 

free chlorine. The results shown in Table 9 are the averages of 20 

analyses, and they indicate the degree of purification obtained by 

the Webster process.

TABLE 9. Analyses of London sewage before, and after treatment by the Webster process.

Parts per million.

Initial.

333.5 

43.4 

5.0 

12.4

Final.

15.6 

32.2 

2.0 

5.2

Though the process was originally conducted as a chemical precipitation, credit is due Webster for first pointing out the disinfecting 

value of the hypochlorites that are formed. T. M. Drown,6 commenting on the process, observed that the American Public Health Association recognized the value of hypochlorites as early as 1888, and 

that their electrolytic manufacture was nothing new. Nevertheless 

its application to sewage was new and gave promise of success.

a The Engineer, London, vol. 67,1889, p. 261; also Eng. News, vol. 21, 1889, p.338; vol. 22, 1889, p. 388. 

6 Jour. New England Waterworks Assoc., vol. 8, 1894, p. 135; also Eng. News, vol. 31, 1894, p. 2*36.
METHODS OP DISINFECTION. 27

In a later paper before the British Medical Association a Webster 

called attention to the possibilities of electrolyzing sea water and thus 

laid the foundation for the many later processes that are based on that 

principle. A plant was later installed at Bradford, England, and in 

1890 Doctor McLintock stated that 70 per cent of the putrescible 

organic matter of the sewage had been removed.6

Fermi,c after having investigated the process at the hygienic 

institute at Munich, concluded that the process is one of chemical 

precipitation and that it is more expensive than the lime process, 

and similar conclusions were reached by Konig and Remele.d

The Woolf process Electrozone. Woolf's process differed from 

Webster's in that strong brine was electrolyzed, and the resultant 

chlorine and caustic soda were allowed to recombine to form sodium 

hypochlorite. The hypochlorite solution was then added to the 

sewage or water to be treated. In the spring of 1893 a plant of this 

kind was installed under the direction of the health department of 

New York City for treating the sewage of about 31 dwellings at 

Brewster, N. Y.,e a village situated on a small stream, the,waters 

of which discharge into Croton Lake. The experiment was considered so successful that the health department installed a similar 

plant at the same place to discharge hypochlorite solution into Tonnetta Creek/ Sixteen hundred pounds of salt per million gallons 

of sewage were used, and the plant required an electric current of 700 

amperes at 5 volts tension. This seems to have been the first plant 

established for the specific purpose of destroying bacteria; before 

that time the removal of organic matter had been the aim. An 

electrozone plant installed at Maidenhead, England, in 1897, was 

examined by Rideal, Robinson, and Kanthack in 1898. The bactericidal action was marked and the effluent was found to contain 

but few bacteria; this plant was not, however, continued^ in operation. At Havana, Cuba, the Woolf process was employed for preparing a disinfectant solution to be used for treating the streets and 

the harbor.

The Hermite process. The Hermite system differs from the Woolf 

system only in minor details. In reports on the process great stress 

is laid on the presence of magnesium hypochlorite hi the electrolyzed 

solution, and there is .much evidence that magnesium hypochlorite, 

owing probably to its lesser stability, is a more active agent than 

other hypochlorites. In later years this same fact has been observed,

a Eng. News, voL 22,1889, p. 388. 

6 Brit. Med. Jour., vol. 2.1890, p. 498. 

c Arch. f. Hyg., vol. 13,1891, p. 207. 

<i Arch. f. Hyg., vol. 28,1897, p. 185. 

e Eng. News, vol. 30,1893, p. 41.

f Eng. Record, vol. 29,1894, p. 110; Elec. Eng., vol. 18,1894, p. 101. 

g Rideal, S., Sewage and Its purification, 3d ed., New York, 1906, p. 185.
28 DISINFECTION OF SEWAGE.

and it has been the basis of fanciful claims for patented processes, 

whose owners have invoked hypothetical oxides of chlorine to explain 

the results. A commission appointed by the London Lancet to 

investigate the Hermite process found that the solution obtained was 

similar to ordinary hypochlorite ° in its chemical properties. A plant 

installed at Worthing, England, was investigated in 1894 by Kelly,6 

who reported that the hypochlorite solution contained from 0.22 to 

0.75 gram per liter of available chlorine. The claim made for this 

solution that mixed with equal parts of sewage it would instantly 

kill all germs was not substantiated. A plant employing the 

Hermite process, established at Havre, France, in 1893, was investigated by a commission appointed by the imperial board of health 

of Germany and by one sent from Paris by the council of hygiene, 

and both bodies reported adversely. Other Hermite plants were 

installed at various places, but were finally abandoned. At Poplar, 

England, the Hermite solution is prepared on a large scale, and it is 

used for general disinfecting purposes as well as for street watering.

The oxy chloride process. The oxy chloride process differs from the 

Hermite and Woolf processes only in matters of detail in the electrolytic cell. Greater efficiency than the older processes in the production of hypochlorites is claimed for it. At Guilford, England, 

Rideal made a test of the effect of oxychloride treatment on raw and 

septic sewages and on effluents of primary, secondary, and tertiary 

contact beds. The summary of his results given in Table 10 shows 

what was accomplished. The results, especially in respect to the 

removal of B. coli, are all that can be desired.

TABLE 10. Summary of Rideal's experiments on the use of oxychloride at Guilford,

England.

Source of sample.

Septic effluent . . . 

Effluent fromflrst 

contact bed. 

Effluent from seco n d contact 

bed

Effluent from 

third contact 

bed.

Available 

chlorine 

parts 

per 

million.

f 30 

50 

I 70 

25-44 

f 20 

1 20 

10.6

( 2.5 

1 2.5 

.5 

I -5

Time of 

contact.

4.3 hours... 

.....do....

.....do .... 

1 to 4 hours. 

40 minutes.

Number of organisms per cuhic centimeter.

Total.

Initial.

23, 000, 000 

23, 000, 000 

23. 000, 000 

2, 500, 000-4, 500, 000

1, 000, 000-2, 000, 000

Final.

50,000 

20 

10 

20-600

40

B. coli.

Initial.

1,000,000 

1,000,000 

1,000,000 

100,000-1,000,000 

100,000

1, 000, 000 

1,000-10,000

1,000-10.000

Final 

less 

than 

1.0 

.2 

.2 

1.0-.2

.2

.2 

.2

.2

B. enteritidis.

 

Initial.

1,000 

1,000 

1,000 

10-1,000

20-100 

10-1,000

10-100

10-100

Final 

less 

than 

10.0 

.2 

.2 

1.0-.2

.2 

.2

1.0

0.2

a Lancet, vol. 1, 1894, p. 1321. 6 Public Health, vol. 6,1894, p. 261.
METHODS OP DISINFECTION. 29

Examinations of the few organisms remaining in the sewages and 

effluents after treatment showed them to be largely organisms of 

the hay bacillus group aerobic spore-forming bacteria which are 

probably beneficial in the further oxidation of the organic matter. 

Absolute sterilization required very high concentration of chlorine.

COPPER AND ITS COMPOUNDSMoore and Kellerman, in 1904, suggested that copper sulphate be 

used in water sterilization.a Since that time a great deal of experimental work has been done, mainly in connection with water. The 

more important experiments, with sewage will be outlined.

Johnson and Copeland 6 at Columbus, Ohio, in 1904, in their work 

with trickling filter effluents, obtained the results given in Table 11.

TABLE 11. Disinfection of trickling filter effluents with copper sulphate at Columbus,

Ohio.

[Johnson and Copeland.]

Copper 

sulphate in 

parts per 

million.

( 5 i 10 

I 20

( 10 \ 20

( 40

Reduction in bacteria 

(percent).

In 3 hours.

90.0 

98.0 

98.5 

40.0 

60.0 

88.0

In 24 hours.

99.9 

99.95 

99.96 

99.7 

99.9 

99.95

They found the action of copper sulphate to be most rapid during 

the first hour. They estimate the cost for chemicals alone at $5 per 

million gallons of effluent treated with 10 parts per million of copper 

sulphate, and $10 if treated with 20 parts per million an expense 

which they consider prohibitive.

The use of copper sulphate as a disinfectant was more thoroughly 

investigated by Johnson at Columbus in 1905. c Experiments were 

made with crude sewage, and with effluents from trickling, contact, 

and sand filters. The effect of temperature, of organic matter, and 

of alkalinity on the efficiency of the process were determined and 

special studies were also made to determine the effect of the treatment 

on colon and typhoid organisms. The principal results with the 

various sewages treated are summarized in Table 12.

«U. S. Dept. Agr., Bureau of Plant Industry, Bull. 64,1904.

6 Jour. Infect. Diseases, Suppl. No. 1, 1905, p. 327; also Reports and papers, Am. Pub. Health Assoc. 

vol. 30, pt. 2, p. 327.

< Report on sewage purification at Columbus, Ohio, 1905, p. 471; also Jour. New England Waterworks 

Assoc., 1905, p. 474.
30 DISINFECTION OF SEWAGE.

TABLE 12. Results of disinfection of sewage and effluents with copper sulphate at Columbus, Ohio; total number of bacteria remaining after contact periods of one hour and of 

three hours. a

[Johnson.]

Copper 

sulphate 

(in parts 

per 

million. )

1,000 

200 

100 

40 

20 

10

Series A.

1 hour.

3,000

9,000

14,000

Series B.

1 hour.

1,000 

5,000 

6,000 

3,400 

11,000 

21,000

3 hours.

240 

600 

500 

700 

1,900 

4,500

Series C.

1 hour.

430 

1,100 

2,100 

1,200 

3,500 

7,500

3 hours.

60 

130 

230 

230 

600 

1.2CO

Series D.

1 hour.

2,100 

6,000 

5,500 

6,500 

13,000 ' 

20,000

3 hours.

280 

1,400 

1,800 

1,300 

2,600 

5,000

Bacteria per cc. 

a Series A. Crude sewage; initial number.................... .................................. 1,200,000

B. Effluent of sprinkling filter......................................................... 1,000,000

C. Effluent of contact filter...... ..................................................... 400,000

D. Effluent of sand filter............................................................... 280,000

Each series is the average of from three to five sets of tests.

Longer periods of contact up to twenty-four hours gf.ve results of 

no additional significance, except that when the lower concentrations 

of copper were used the number of bacteria showed a decided increase 

after about the third hour, an indication that the copper was removed 

from solution, probably by combination either with the organic matter or with carbonic acid. Subsequent increase in number of bacteria is a phenomenon of frequent occurrence in disinfection work. 

It is in fact neither possible nor desirable to prevent perpetually the 

feeding of bacteria on the organic matter of the disinfected effluents. 

The significant fact in Johnson's tests is that the bacteria originally 

present were practically eliminated, and it may safely be assumed 

that under conditions existing in a stream there would be no multiplication of the typhoid or other pathogenic bacteria.

Two important facts were brought out in this study. The disinfection obtained with 100 parts per million of copper sulphate is, for 

practical purposes, but little better than that obtained with 10 parts, 

and results obtained in a one-hour contact are practically as good as 

those obtained in a three-hour contact. Johnson's work was done at 

summer temperature, and in order to determine the effect of temperature on the germicidal action parallel experiments were conducted at 5° and at 20° C. It was found in general that a result 

which could be obtained at the higher temperature in thirty minutes 

would be attained at the lower temperature in about three hours. 

This is a point that has usually been overlooked, and it is of special 

significance in practical operation, as effluents in the northern latitudes would have a temperature of about 5° C. during much of the 

year. It is interesting to note that temperature has little effect on 

the germicidal efficiency of hypochlorites.
METHODS OF DISINFECTION. 31

The combined effect of organic matter and alkalinity was determined by treating the undiluted effluent and the same effluent after 

being diluted 1 to 1 and 1 to 2 with tap water. The effect was 

decidedly noticeable in the weaker concentrations of copper sulphate, 

and much less so where the concentrations of copper were excessive. 

The results with 10 parts of copper sulphate per million are summarized in Table 13.

TABLE 13. Effect of organic matter and of alkalinity on the germiddal properties of 

copper sulphate: per cent removal of bacteria at end of one hour's contact with 10 parts 

per million of copper sulphate.

[Johnson.]

Trickling filter effluent ...................................................

0

96.1

97.3

70

Dilution.

1:1

97.5

99.0

86

1:2

98.5

99.1

91

The efficiency of copper sulphate as a disinfectant was investigated 

in 1906 at the Sanitary Research, Laboratory of the Massachusetts 

Institute of Technology by treating the effluent from an 8-foot trickling filter. Table 14 gives the average results divided into two 

periods to show the effect of temperature of the effluent. At the 

conclusion of the tests a composite sample of the sediment drawn 

from the sedimentation tank during the experiment' was analyzed. 

The copper contained in the sediment accounted very closely for the 

total amount of copper added, a fact that makes it apparent that 

little copper left the tank in soluble form.

TABLE 14. Disinfection of trickling filter effluent with copper sulphate at Boston, Mass.;

average results.

Period.

1906.

October 13-31.. ..............

November 21-December 10. . .

Copper

sulphate 

(in parts

per

million).

4

4

8

Temper-

°F.

56

46

43

Number of bacteria per

cubic centimeter.

Initial.

230,000

250,000

240,000

Final.

14,000

51,000

5,000

Per

cent removed.

94.0

80.0

97.9

Number of B. eoli o per

cubic centimeter.

Initial.

44,000

48,000

32,000

Final.

640

770

390

Per

centremoved.

98.5

98.4

98.8

a Jackson's bile media used.

The work of Kellerman, Pratt, and Kimberly in Ohio during the 

winter of 1906-7 consisted mainly of experiments with copper sulphate as a disinfectant. Their investigations, probably the most 

comprehensive ever made with copper sulphate, were conducted under 

actual working conditions and with several kinds of effluent. The 

76474 IRR 229 09 3
32 DISINFECTION OF SEWAGE.

chemical composition of the water, particularly in regard to its hardening constituents, was found to exert an important influence on the 

results. Their original paper" on the subject contains complete 

chemical analyses of the effluents treated. Table 15, containing a 

brief summary of average results calculated from the original tables, 

shows in a general way what was accomplished. The authors concluded that copper sulphate is not so efficient as chlorine compounds, 

is more seriously affected by carbonates, and is much more expensive.

TABLE 15. Disinfection of effluents with copper sulphate in Ohio. 

[ Kellerman, Pratt, and Kimberly.]

Series.

A....................... ......

B....... ............ ........

C...... .........................

Copper 

sulphate 

(in parts 

per million )

5 

6.7 

10 

13 

20 

29 

40 

1. 67 

[ 7.3 

14 

I 22 

( 6.5 

15 

1 40 

1 116

Number of bacteria 

per cubic centimeter at 20° C.

Initial.

6, 000, 000 

250, 000 

60,000 

65, 000 

120,000 

60,000 

160,000 

200,000 

110,000 

75, 000 

55,000 

390, 000 

230, 000 

29,000 

84,000

Final.

1,200,000 

70,000 

16, COO 

23, 000 

20,000 

3,900 

8,500 

34,000 

23,000 

9,500 

7,000 

110, 000 

65, 000 

6,000 

7,000

Number of bacteria 

per cubic centimeter at 37° C.

Initial.

140,000 

19,000 

47,000 

37,000 

8,000 

24,000 

75,000 

81,000 

12,000 

16,000 

5,500 

170,000 

42,000 

30,000 

28,000

Final.

120, 000 

10,000 

10,000 

5, 500 

11,000 

6,000 

4,700 

14,000 

1,800 

600 

200 

41,000 

16,000 

8,500 

2,000

Number of acid 

formers per cubic 

centimeter.

Initial.

6,000 

6,000 

2,600 

4,600 

1,600 

1,200 

5,500 

7,000 

600 

75 

75 

8,000 

5,000 

5,500 

5,000

Final.

600 

900 

1,000 

600 

110 

10 

110 

700 

48 

0 

0 

3,000 

2,200 

750 

560

A. Effluent of contact filter at Westerville, Ohio. 

B. Effluent of sand filter at Lancaster, Ohio. 

C. Effluent of sand filter at Marion, Ohio.

MISCELLANEOUS METHODS.

PERMANGANATES.

Potassium permanganate and sodium permanganate have been 

used for the oxidation of organic matter in streams. At London 

when the Thames becomes extremely foul during low-water periods, 

sodium permanganate is added to its waters in order to destroy odors 

and putrescible material, but the treatment undoubtedly results in 

partly sterilizing the water. It is claimed that the germicidal action 

is not sufficiently great to interfere with the normal oxidizing changes 

in the stream. The use of permanganates has been proposed for rendering effluents of chemical precipitation plants noiiputrescible.

AMINES PROCESS.

The so-called "amines" process was developed by H. Wollheim in 

England. It is claimed that trimethylamine treated with lime or 

other alkali produces a very poisonous substance. Herring brine is

oKellerman, K. T., Pratt, E. W., and Kimberly, A. E.: The disinfection of sewage effluents for the 

protection of public water supplies, Bull. 115, Bur. Plant Industry, U. S. Dept. Agr. 1907.
METHODS OF DISINFECTION. 33

used to supply the amine. A large excess of lime is added and the 

mixture is used to precipitate crude sewage. Klein made a test of 

the process at West Horn in 1889, and found that a clear, nonputrescent, sterile effluent could be obtained. Similar results were 

obtained at Wimbledon," where 768,000 bacteria per cubic centimeter in the sewage were completely removed. The sludge is also 

nonputrescible. The process does not seem to have been further 

developed.

SODIUM BENZOATE AND OTHER ORGANIC COMPOUNDS.

Sodium benzoate is supposed to possess powerful germicidal properties, and its use as a disinfectant for sewage was suggested to the 

writer. It would apparently have the distinct advantage over such 

other disinfectants as chlorine and copper salts of not combining with 

organic matter, thus rendering all the disinfectant added available for 

a long time. Sodium benzoate was applied regularly to a trickling 

filter effluent at Boston during March and April, 1907. An addition 

of commercial benzoate at the rate of 0.8 part per million for twentysix days gave an average reduction of total bacteria from 140,000 to 

54,000 per cubic centimeter, or a 62 per cent removal. Doubling the 

amount of benzoate increased the efficiency somewhat, the average 

reduction then being from 370,000 to 84,000 per cubic centimeter, or 

78 per cent. The cost of benzoate for the treatment is $1 per million 

gallons for 0.8 part and $2 for twice that quantity. Obviously this 

substance is not an efficient disinfectant in sewage work.

Other organic substances, such as the phenols and the coal-tar 

products, were suggested, but in general their high cost eliminates 

them from consideration. The possibility of after effects must also 

be considered. There is a certain advantage in the use of a substance which is itself used up in the reaction or which is converted 

into harmless compounds. On the other hand, the organic disinfectants are extremely powerful and there is always a chance of the 

discovery of some new compound with the requisite germicidal properties and the low cost that will make it the ideal sewage disinfectant.

SUMMARY OF DISINFECTION METHODS.

Two of the methods of disinfection that have been mentioned 

appear not to have been sufficiently investigated, namely, disinfection by heat and by organic compounds. The heat method holds a 

reasonable possibility that sufficient ammonia may be recovered to 

pay for the necessary heating, but actual trial is essential for convincing proof. A systematic study of organic compounds as disinfectants has yet to be made.

a Interim Rept. Royal Sewage Commission, 1901, p. 304.
34 ' DISINFECTION OF SEWAGE.

Of the disinfectants that have been sufficiently investigated, 

chlorine compounds and copper salts alone appear to be applicable 

to the sewage problem. Moreover, a detailed study of results on a 

cost basis leaves no doubt that the efficiency of chlorine is much 

greater than that of copper. Even if the prices of these two materials were more nearly equal, many facts favor the use of chlorine. 

Both reagents unite with organic matter, but chlorine unites by oxidizing the organic matter, thus rendering it less putrescible, while copper precipitates it. The compounds formed by the copper tend to 

protect the solid particles from further action, and the diffusion of copper ions through such a precipitated copper envelope must necessarily 

be slow. As no such action occurs with chlorine compounds, the 

penetration of the chlorine into solid particles must be much more 

complete. Chlorine in the form of bleaching powder is to some 

extent a by-product, is very cheap, and will probably become cheaper 

as the methods of production are improved; on the other hand, 

copper is a staple, the price of which is likely to increase. Sewages 

may be found of such chemical composition that treatment with 

copper will be more effective than that with chlorine, but in general 

chlorine compounds are to-day by far the most economical and the 

most efficient disinfectants available in sewage work.

EXPERIMENTAL INVESTIGATIONS.

HISTORY.

In connection with general investigations in sewage disposal, 

which have been conducted since 1903 at the sanitary research 

laboratory and sewage experiment station of the Massachusetts 

Institute of Technology, the subject of the chemical disinfection of 

sewage and sewage effluents has been extensively studied during the 

past two years. In the spring of 1906 a cooperative arrangement 

between the United States Geological Survey and the institute made 

it possible materially to enlarge the scope of the work.

With a view also of gaining increased knowledge of the practical 

workings of the processes that had been developed at Boston, an 

arrangement was made with the state sewerage commission of New 

Jersey, through Boyd McLean, secretary, under which experiments 

were begun at Red Bank, N. J. The state sewerage commission 

later continued that work for its own information, retaining the 

writer in charge, and through the courtesy of the commission the 

results of the entire work are here presented. During the summer of 1907 the experiments were placed under the immediate 

charge of F. E. Daniels, to whose faithful efforts their successful 

completion is in large part due. Mr. Daniels was assisted at Red 

Bank by H. S. Crawford, and also did much of the work at Boston 

during 1908.
EXPERIMENTAL INVESTIGATIONS. 35

In December, 1907, the scope of the investigation was still further 

broadened by a cooperative agreement between the United States 

Geological Survey and the sewerage commission of Baltimore, of which 

Brig. Gen. Peter Leary, U. S. Army, retired, is chairman, and Calvin 

W. Hendrick is chief engineer. Experiments on a somewhat enlarged 

scale were immediately undertaken at the Walbrook testing station 

of the Baltimore sewerage commission, and they were continued up 

to July, 1908, at which date the agreement with the Geological Survey 

lapsed. The work at Baltimore was under the immediate charge 

of Ezra B. Whitman, division engineer of the disposal division, to 

whom the writer is under the greatest obligation for its successful 

outcome. Intimately associated with that work also and responsible in large measure for the results have been Charles A. Emerson, jr., 

assistant division engineer, Henry C. McRae, chemist, and Edward 

G. Birge, bacteriologist. Dr. R. P. Cowles, of Johns Hopkins University, and William T. Carpenter, Leyland Whipple, and Marvin H. 

Lillis, have also been associated with this work from time to time in 

connection with special lines of investigation.

Through the courtesy of A. G. Paine, jr., president, and J. R. 

Crocker, superintendent, of the McDonald Electrolytic Cell Company, of New York, an electrolytic chlorine cell having a capacity 

of 22 pounds a day of chlorine gas was placed at the disposal of the 

writer, and it proved to be of great assistance in the work, as a large 

supply of gaseous chlorine was thus made available for special experiments. The machine was used directly in disinfection and also in the 

preparation of a series of chlorine compounds that were studied.

BACTERIOLOGICAL METHODS AND EXPRESSION OF RESULTS.

The bacteriological methods employed in the different parts of this 

work have been kept as nearly uniform as possible in order that the 

results may be strictly comparable. In general, the methods have 

been those recommended by the committee on standard methods of 

water analysis of the American Public Health Association, laboratory section. Since the publication of those methods,*1 a new presumptive test for the colon bacillus has been described by Jackson 6 

and it has been used throughout this work. The medium employed 

in this test consists of ox bile to which lactose has been added. Its 

accuracy in sewage work has never been carefully studied, but 

recent studies by Prescott and Winslow c have confirmed the earlier 

statement of Jackson that in water work this presumptive test 

gives results which are similar to those obtained in complete B. coli

a. Jour. Infect. Dis. Suppl. 1, Chicago, 1905. Repts. and Papers American Pub. Health Assoc., vol. 30, 

pt. 2, 1905.

& Biological studies by the pupils of William Thompson Sedgwick, p. 292, Boston, 1906. 

c Elements of water bacteriology, 2d ed., New York, 1908, p. 149.
36 DISINFECTION OF SEWAGE.

determinations. It may fairly be assumed, therefore, that the B. 

coli results reported in the present paper represent a group of bacteria, including practically all the B. coli actually present, together 

with certain other bacteria which probably are not more than 10 

per cent of the whole number reported. The study of such a group 

gives valuable information concerning the probable elimination of 

typhoid and other pathogenic bacteria, while the additional information to be obtained from a-complete determination of the B. coli 

actually present is not deemed sufficient to warrant the additional 

labor involved. At Boston and at Baltimore counts of the bacteria 

on gelatin at 20° C. and on litmus lactose agar at 37° C. and 

counts of the acid-forming bacteria at 37° C. were recorded. The 

latter group includes the B. coli, and some bacteriologists believe 

that the number of acid-forming bacteria is a more accurate 

indication of the numbers of colon bacilli than the presumptive 

tests. It is indisputable, however, that the group of acidforming bacteria is large and that in sewage work at least it 

bears only a general relationship to B. coli. An additional group, 

the bacteria that liquefy gelatin, is also included in the Boston results, 

though the additional information afforded by including this group 

is slight. Inasmuch as adequate facilities for complete bacteriological 

work were riot available at Red Bank, the total count at 20° C. and 

the presumptive B. coli tests alone were made. Examinations of the 

trickling filter effluent before and after treatment have been made at 

Boston on. five or six days in each week. At Red Bank and at Baltimore two samples were tested each working day for B. coli and 

three samples were plated for the bacterial counts.

Considerable thought has been given to the manner of expressing 

results. It would obviously be inadvisable to attempt to tabulate 

the results of all examinations made, because such tables would be 

too unwieldy for general use. On the other hand, the use of averages, especially if they cover long periods, is open to the serious 

objection that a few bad results are easily covered up in general averages, and it is particularly necessary, in a process of this kind, to 

show that the results are not only satisfactory in general, but that 

they are fairly uniform. To cite a familiar illustration, a poor 

marksman's shots might be placed symmetrically about the center 

of the target and in that sense their average position might be as 

near the bull's-eye as the average position of better-placed shots. 

The actual deviations from the average position would indicate the 

character of the shooting. Similarly, in disinfection two series of tests 

might give the same average, even if one series were composed of 

fairly uniform results and the other series of very erratic ones. In 

other words, the deviations from the average are fully as significant 

as the average figure itself. Accordingly, the routine work is reported
INVESTIGATIONS AT BOSTON. 37

in the form of weekly averages. The total bacterial counts are 

accompanied by a statement showing the variations of the individual 

results and the variations of the individual bacterial removals from 

the average bacterial removal. In this way the record of what was 

actually accomplished is stated in the most compact form.

A new method of recording B. coli results is also employed. The 

procedures used in the determination of this organism do not permit 

a quantitative count of the number really present, but show merely 

the presence or the absence of the bacillus. Quantitative results are 

obtained in a rough way. by applying the test to various dilutions of 

the sample. For instance, in a disinfected effluent the test is applied 

on 0.1, 0.01, and 0.001 cubic centimeter, respectively, and the highest 

dilution giving a positive test is recorded. Hitherto a bare statement of the results is all that has been attempted in the way of giving 

quantitative significance to this test. But it has been shown a that 

where as many as fifty or one hundred results of that character from 

the same source are available, the probability law of distribution may 

be applied in order to estimate the average number 'of organisms 

present. The most probable value for the number of B. coli present 

in each sample is indicated by the reciprocal of the highest dilution 

giving a positive test, and while that figure may be far from the correct one for any one sample, the average of fifty estimates of this 

character closely approximates the actual value. The case is analogous to the well-known facts in relation to the expectancy of life at 

any age; an expectancy of life as given in the actuarial tables has 

but little weight when applied to any one person, but in the average 

of a large number of persons it approximates the truth. The number of B. coli present in a sample has therefore been recorded in this 

way. Individual results have but little weight and weekly averages 

must be properly interpreted, .but monthly averages are probably 

very near the average numbers.

INVESTIGATIONS AT BOSTON. 

SCOPE AND CHARACTER OF EXPERIMENTS.

Since the beginning of the work at Boston in February, 1906, 

various phases of the disinfection problem have been under investigation. The following summary of the studies shows their scope and 

character: (a) Preliminary experiments with chloride of lime; (6) studies 

of the comparative efficiencies of chloride of lime, sulphate of copper, and sodium benzoate; (c) small-scale experiments to determine 

the comparative efficiencies of free chlorine, commercial hypochlorites, electrolytic hypochlorites, chlorates and perchlorates, and the 

hypochlorites of several bases; (d) studies of the comparative effect

o Phelps, E. B., A method for calculating the numbers of B. coli from the results of dilution tests: Am. 

Jour. Pub. Hyg., vol. 18,1908, p. 141.
38 DISINFECTION OF SEWAGE.

of hypochlorites on the typhoid and the colon bacilli; (e) routine 

disinfection of 5,000 gallons a day of trickling-filter effluent with 

chloride of lime; (/) disinfection of crude sewage with chloride of 

lime in 700-gallon tests. The studies on copper sulphate and sodium 

benzoate have already been discussed (pp. 29-32). The remaining 

divisions of the work are reviewed in the succeeding pages.

PRELIMINARY EXPERIMENTS WITH CHLORIDE OF LIME.

The results of some preliminary experiments in the spring of 1906 

have already been published, a but as they have important bearing 

on the present work, a brief review of them is here presented. The 

experiments were made with chloride of lime and the effluent of a 

trickling filter in which Boston sewage was being treated. They 

consisted of 23 bottle tests in which the available chlorine that was 

added varied from 0.25 to 100 parts-per million, and the time of 

contact from thirty minutes to twenty-four hours. They were undertaken with the object of establishing practical working limits for 

available chlorine and contact period for future experiments, and 

particularly to determine whether the English and Continental practice of adding large, almost prohibitive, amounts of chlorine is justified by the results.

This preliminary study indicated that an effluent similar to the 

one used can be deprived of 95 per cent to 98 per cent of its total 

bacteria in two hours by the application of chloride of lime in concentrations having 2 to 5 parts per million of available chlorine. 

The impracticability of attempting to get much better results was 

well shown, for complete sterilization was never accomplished, though 

concentrations of chlorine up to 100 parts per million and storage 

periods of twenty-four hours were employed. It was found unnecessary, in brief, to use more than 5 parts of chlorine or to treat for 

over two hours.

It is a well-known fact that, in all processes involving the destruction of bacteria, it is comparatively easy to kill the first 95 per cent 

of the germs and very difficult to destroy the remaining 5 per cent. 

This phenomenon of the "resistant minority," as Whipple terms it, 

is common to all kinds of sterilization, whether it be by heat, cold, 

light, chemicals, or other means. It is therefore more practical to 

determine how far disinfection may be carried at a reasonable expenditure than to attempt the ideal complete sterilization. To state a 

concrete example, it might happen that the pathogenicity of an 

effluent could be reduced 96 per cent by the expenditure of a certain 

sum of money, 97 per cent by the expenditure of twice that sum, 

and 99 per cent by the expenditure of five times that sum. The 

first reduction might represent a feasible plan, and the last a proa Phelps, E. B., and Carpenter, Wm. T., The sterilization of sewage filter effluents: Tech. Quart., vol. 

19, 1906, pp. 382-403.
INVESTIGATIONS AT BOSTON. 39

hibitively expensive one. It is plainly more desirable at the outset 

to determine the relations between the various factors, such as concentration of disinfectant, time of contact, cost, and efficiency, and 

to establish certain working limits with reference to final costs, 

than to attempt the ultimate destruction of all germs regardless of 

the practical limits that necessarily exist. The preliminary experiments were undertaken with the idea of fixing these practical working limits. For the first time in the history of the subject the disinfection of sewage filter effluents and even of crude sewage itself was 

shown to be practicable and feasible, yielding results commensurate 

with the cost when compared with the results and costs of other 

purification processes.

DISINFECTION OF TRICKLING-FILTER EFFLUENT.

The most important work at Boston consisted in the routine 

disinfection of the combined effluents of two trickling filters. Each 

filter is 8 feet deep and has a surface area of 50 square feet. One is 

composed of crushed stone from 1 inch to 1^ inches in mean diameter; the other of stone from 1-J to 2 inches in diameter. The combined effluents, amounting to 5,000 gallons in twenty-four hours, 

were brought from the underdrains of the outdoor 'trickling filters 

to the filter house through a short length of iron pipe. Within the 

filter house the liquid was conducted by an open trough to the central 

channel of a sedimentation tank built on the Dortmund plan. It 

then passed downward nearly to the bottom of this conical tank, 

entered the main body of the tank, and, rising with constantly 

decreasing velocity, finally overflowed at the surface. The tank 

was designed to give a two-hour storage period. The disinfectant 

solution was made up in a 50-gallon barrel to a strength about one 

hundred times that required in the final mixture. A small orificebox working under constant head was connected with the barrel 

and was designed to deliver exactly 2 gallons an hour. As the flow 

through the sedimentation tank was a little over 200 gallons per 

hour, this arrangement gave a final mixture of the desired proportions. These rates and volumes were kept constant throughout, 

any change in the amount of disinfectant added being brought about 

by changing the strength of the solution. Readings of a float gauge 

set in the barrel and daily measurements of the flow of the tricklingfilter effluent into the tank served as checks on the accuracy of the 

dilutions, and together with daily analyses of the strong disinfecting solution, gave data from which the actual amount of disinfectant added during any required period of .time could be computed. Observations of the rate of flow of the disinfectant solution 

into the tank were made at hourly intervals for a period of three 

hours preceding the taking of samples, and the actual concentration 

of the disinfectant corresponding to the sample in question was
40 DISINFECTION OF SEWAGE.

calculated from these observations. The disinfectant solution flowed 

into the open trough previously mentioned, where it mingled with 

the effluent. Further opportunity for mixing occurred during the 

passage down the central channel of the sedimentation tank. Samples for bacterial examination were collected with the usual precautions, before the effluent had reached the wooden trough into 

which the disinfectant solution was run and also at the final outlet 

of the sedimentation tank after the combined effluent and disinfectant had been in contact for a period somewhat less than two hours. 

These samples are described as initial and final, respectively.

The chemical composition of an effluent has an important influence 

on the germicidal value of most disinfectants, particularly chloride 

of lime. The chlorine applied is eventually completely consumed by 

chemical reaction with the organic matter of the effluent, and the 

germicidal action takes place in the interval of time between the 

addition of the chlorine and its final exhaustion by chemical reaction. 

The amount of organic matter present, therefore, practically determines the amount of chlorine that it is necessary to use. For this 

reason the chemical analyses of the combined effluent are recorded 

by monthly averages in Table 16. The monthly averages of the 

analyses of the disinfected and settled effluent are also included to 

show the chemical character of the effluent after treatment.

TABLE 16. Chemical analyses of trickling-filter effluent at Boston before and after disinfection with chloride of lime and sedimentation; monthly averages.

[Parts per million.] 

INITIAL.

Month.

1907.

1908.

Average.....

Turbidity.

135 

155

140 

145 

130 

195 

275 

155

165

Suspended 

solids.

Total.

118

92 

177 

180 

313 

436 

174

213

Loss 

on 

ignition.

78

69 

156 

122 

192 

247 

96

137

Nitrogen as 

Organic.

Total.

3.5 

5.5

5.0 

6.0 

7.5 

10.5 

14.0 

11.5

8.0

Dissolved.

1.5 

3.0

3.0

2.5 

3.5 

5.5 

4.5 

6.0

4.0

Free am-" 

monia.

15.0 

15.5

15.0 

14.5 

15.5 

14.5 

14.5 

7.5

14.0

Nitrites.

0.2 

0.1

0.3 

0.1 

0.2 

0.4 

0.9 

0.8

0.4

Nitrates.

2.0 

1.0

3.5 

3.0 

4.0 

5.5 

6.0 

6.0

4.0

Oxygen 

consumed, o

Total.

40

38

45 

44 

48 

64 

70 

49

50

Dissolved.

32 

33

38 

32 

35 

46 

35 

35

36

Dissolved 

oxygen.

8.2

7.8

10.0 

12.5 

7.8 

9.4 

7.4 

7.1

8.8

FINAL.

1907.

1908.

May. ..............

Average .....

105

1 1 K

110

125

1 AK

135

95

115

71

124

259

125

147

92

51

124

CO

239

82

69

43

102

3 A

4.0

3 A

 4.5

9.5

5.5

5.0

7.0

5.0

1.5

1

C

3 A

2.5

3.5

3.5

3 A

4.0

3.0

14.5

14.0

13.0

13.5

15.0

H K

15.0

7.5

13.5

0.1

0.1

0.6

0.3

0.3

0.4

1.0

0.8

0.5

2 A

2.0

5.0

4.5

3.5

5 A

5.0

6.0

4.0

28

39

49

42

46

44

44

38

41

97

38

41

38

42

40

33

37

a Thirty-minute boiling method.
INVESTIGATIONS AT BOSTON. 41

This disinfection experiment was started November 11, 1907, and 

was continued practically without interruption till June 27, 1908, a 

period of thirty-three weeks. The value of such continuous experiment is obvious. A certain length of time is always required for 

for the establishment of uniform working conditions, not only in the 

personal element but in the tanks themselves. Furthermore, short 

special experiments receive an unusual and perhaps unfair amount 

of care and attention, which is not bestowed on work that has become 

part of the routine, and the natural defects of the processes under 

practical working conditions are, therefore, not always discovered. 

The most important value of long-continued tests, however, lies in 

their being carried out under various seasonal conditions. Fluctuating conditions of temperature and rainfall fundamentally affect the 

sewage and influence the work of the filter. It is consequently of 

prime importance to determine the efficiency of any purification 

process under an extreme range of seasonal conditions. It fortunately happened that the present experiments were continued 

through an unusually cold winter and through the hottest portions 

of an exceptionally hot, dry summer.

Table 17 contains the results of the experiments, given in weekly 

averages, which are the mean of four to six daily results. During 

the first five weeks the available chlorine added was about six parts 

per millipn, but during the remainder of the period this concentration was reduced to between two and four parts without materially 

affecting the results. The average results for the whole period are 

indicated in the last line of the table.
TABLE 17. Disinfection of trickling-filter effluent with chloride of lime at 

Week ending 

1907.

23................

qrt

14.................

21 .................

28.................

1908.

11...................

18...................

9<;

15. ................

22.................

29.................

14.....................

21.....................

28.....................

11......................

18......................

25. .....................

May 2.......................

9

16.......................

23.......................

30.......................

June 6 ......................

13......................

20......................

27......................

Temperature.

45 

46 

42

36 

42 

37 

44

40 

35 

34 

35 

34 

33 

35 

34 

34

35

38 

38 

42

40 

42 

42 

46

54 

48 

56 

58 

62

61 

68 

68

68

45

Available 

Cl (parts 

per million).

5.7 

5.9 

6.8

6.9 

5.0 

3.0 

3.1

2.8 

3.2

2.7 

2.7

3.3

3.6 

3.4 

3.4 

3.3 

3.8 

3.1 

3.0 

2.5 

3.2 

2.4 

1.7 

2.7

3.4 

3.1 

3.0

2.6 

2.7

2.8 

2.5 

2.8 

3.0

3.4

Number of bacteria per cubic centimeter at 20° C.

Total.

Initial.

1,100,000 

800,000 

1,000,000

600,000 

800,000 

550,000 

850,000

950,000 

550,000 

550,000 

800,000

460,000 

800,000 

660,000 

390,000 

550,000

430,000 

600,000 

600,000 

460,000

460,000 

2,500,000 

1,100,000 

600,000

450,000 

600,000 

600,000 

1,400,000 

700,000

850,000 

550,000 

630,000 

1,100,000

760,000

Final.

1,800 

7,500 

1,800

4,600 

15, 000 

85,000 

20,000

7,000 

17,000 

22,000 

35,000

75,000 

8,000 

43,000 

17,000 

13,000

3,400 

18,000 

7,500 

18,000

8,000 

37,000 

35,000 

100,000

2,000 

23,000 

9,000 

28,000 

18,000

34,000 

35,000 

32,000 

13,000

24,000

Liqueflers.

Initial.

160,000 

46,000 

48,000

60,000 

60,000 

27,000 

120,000

70,000 

55,000 

47,000 

26,000

30,000 

60,000 

30,000 

24,000 

40,000

30,000 

35,000 

65,000 

36,000

60,000 

500,000 

220,000 

70, 000

60,000 

180,000 

85,000 

220,000 

130,000

100,000 

160,000 

120,000 

120,000

95,000

Final.

100 

600 

43

90 

750 

4,500 

2,400

600 

460 

1,300 

2,800

4,000 

400 

260 

900 

100

190 

700 

320 

600

1,400 

6,000 

6,000 

2,400

200 

5,500 

1,200 

2,400 

3,800

3,800 

1,500 

3,600 

1,300

1,800

Number o

T

Initial.

330,000 

130, 00

65,000

140,000 

140,000 

95,000 

210,000

90,000 

86,000 

80,000 

110,000

70,000 

340,000 

60,000 

74, 000 

90,000

140,000 

170,000 

180,000 

140,000

140,000 

160,000 

160,000 

160,000

250,000 

210,000 

120,00

280,00

230,00

190,00

75,00

110,00

210,00

160,00

Jackson bile media used.
INVESTIGATIONS AT BOSTON. 43

The average efficiencies as measured by the per cent of bacteria 

removed for the whole period and for certain shorter periods, selected 

to illustrate the effect of different concentrations of chlorine and of 

fluctuating conditions of temperature, are shown in Table 18.

TABLE 18. Disinfection of trickling-filter effluent at Boston; summary of bacteriological 

results, averaged by periods, to show the effect of changes in temperature and in the amount 

of available chlorine.

Period.

November 12 to December 12. ..

Temperature.

o Ff

45 

42 

36 

60

Available 

chlorine, 

in parts 

per million.

3.4 

6.3 

3.2 

2.9

Per cent of bacteria removed.

Bacteria at 20° C.

Total.

96.8 

99.57 

95.8 

97.1

Liqueflers.

98.1 

99.73 

97.7 

98.0

Bacteria at 37° C.

Total.

97.4 

99.81 

96.6 

97.6

Acid 

formers.

97.3 

99. 91 ' 

96.4 

97.9

B. coli.

99.19 

99.99 

98.5 

99.07

Comparison of the results obtained in the periods November 12 to 

December 12 and January 27 to March 28 shows that the additional 

bacterial removal gained by increasing the available chlorine from 

3.2 to 6.3 parts per million, although it is considerable in amount, is 

hardly commensurate with the cost. The effect of temperature on 

the bacterial removal is shown in the periods from January 27 to 

March 28 and April 27 to June 27. The mean temperature is 36° F. 

in one period and 60° F. in the other, while the available chlorine is 

practically the same in both periods, though slightly higher during 

the period of lower temperature. An advantage in favor of the summer results is noticeable, and it would probably have been a little 

greater with exactly the same amount of chlorine. Yet the results 

show especially that the effect of temperature on efficiency is not 

great. This point is of special significance in comparing the results 

of chlorine disinfection with those obtained with copper. In experiments with copper, temperature has been shown to produce a marked 

effect; in fact, the concentration of copper must be doubled during 

the winter months to maintain the efficiency of the process. The 

bacterial removals in the several groups of bacteria recorded are 

nearly the same, though the removal of B. coli is higher in all experiments. It is probably safe to assume that the removal of the typhoid 

and other pathogenic organisms will be as nearly perfect as the 

removal noted in any of. these groups. The per cent removals of 

organisms shown by the counts at 37° C. and by the B. coli results 

probably represent most nearly the per cent removal of typhoid 

bacilli.

As has been stated, the reliability of a disinfection process depends 

quite as much on the general evenness of the results as on the aver-
44 DISINFECTION OF SEWAGE.

age efficiency. For that reason the per cent removal of total bacteria 

has been computed for each of the 158 individual tests, and the results 

of the calculations, given in Table 19, show that the per cent removal 

was between 98 and 100 per cent in over half the individual tests, 

and that it was less than 94 per cent in only 15 per cent of the tests.

TABLE 19. Relation between individual tests of bacterial removal and the average result

at Boston.

Per cent of total 

number of tests.a

54 

19 

12 

6 

2 

3 

2 

2

Total, 100

Per cent removal of total 

bacteria.

Not less 

than 

98 

96 

94 

92 

90 

85 

75 

65

Less than 

100 

98 

96 

94 

92 

90 

85 

75

Average, 96.3

a, Number of individual tests, 158.

The number of bacteria remaining in the disinfected effluent have 

been tabulated in a similar manner, in order that the bacterial quality 

of the final effluent may be properly understood. The results are 

given in Table 20.

TABLE 20. Relation between individual bacterial counts and the average result at Boston.

Per cent of total 

number of tests.a

47 

23 

20 

5 

3 

2

Total, 100

Total number of bacteria per 

cubic centimeter.

Not less than 

100 

10,000 

25,000 

50,000 

100,000 

200,000

Less than 

10,000 

25,000 

50,000 

100,000 

200,000 

250,000

Average, 24, 000

a Total number of tests, 158.

The average of the 158 individual tests of bacterial removal is 96.3 

per cent, as compared with 96.8 per cent removal (see Table 17, p. 42) 

obtained from the average numbers for the whole period. The "first 

figure is more nearly correct for estimating the efficiency of the process; but it is evident that the last figure, which is more commonly 

employed in such work, gives the average result with sufficient accuracy. The somewhat tedious calculation of the individual results
INVESTIGATIONS AT BOSTON. 45

and their variations has been made only for the total bacteria; but 

the results suffice to show that the average per cent removal of bacteria, computed from the average initial and final counts, is practically 

the same as the average of the per cents calculated from the individual 

tests, and that there is a certain variation in the results to the extent 

shown. As to .the other groups of bacteria studied, it is sufficient to 

state that similar computations with them gave practically identical 

results, subject to much the same deviations.

In order to study the effect of varying the time of contact with the 

disinfectant, samples of the trickling-filter effluent were collected from 

the mixing trough after the disinfecting solution had been added. 

After these samples had been well shaken, determinations were made 

of the number of bacteria remaining at the end of ten minutes, fifteen 

minutes, one hour, and two hours, the sample being shaken each time 

before withdrawing the test portion. The number of bacteria in the 

effluent before treatment, as determined on a separate sample, was 

between 250,000 and 1,000,000 per cubic centimeter; but the numbers in all tests have been converted to a uniform basis of 1,000,000 

initial bacteria per cubic centimeter for more ready comparison of the 

results in Table 21.

TABLE 21. Relation between time of contact and efficiency oj disinfection with chloride of

lime.®

Date.

9

10...............................................

11...............................................

14...............................................

15...............................................

16...............................................

17...............................................

20...............................................

21...............................................

23...............................................

Number of remaining bacteria per cubic centimeter b after contact for 

10 minutes.

2,500 

10,000 

3,500 

47,000 

4,200 

1,200 

9,800 

400, 000 

28,000 

1,300

50,000 

5.0

15 minutes.

1,100 

190 

270 

570 

1,100 

310 

240 

800 

12,000 

2,100 

230

1,700 

0.17

60 minutes.

160

58

154 

700 

120 

160 

260 

7,000 

- 1,300 

110

950 

0.10

120 minutes.

150 

7 

40 

100 

570 

120 

130 

150 

5,500 

1,000 

31

700 

0.07

a Available chlorine, 5 parts per million.

6 All numbers converted to a uniform basis of 1,000,000 initial bacteria per cubic centimeter.

The rapidity of the action of the disinfectant is somewhat surprising, and it indicates that long periods of contact are unnecessary. 

It is evident that the greater part of the disinfection is accomplished 

within the first fifteen minutes, and that a contact period of one hour 

is probably -ample for practical work. Another feature of interest in 

this experiment is its bearing on the character of the sediment in the 

disinfection tanks. It has been suggested that perhaps such sediment carries down with it large numbers of the bacteria that are
46 DISINFECTION OF SEWAGE.

recorded as being removed, though they may be still vigorous within 

the protecting body of solid matter. It is undoubtedly true that 

ordinary sedimentation tanks remove a considerable number of bacteria, and the actual removal is doubtless even greater than would 

appear from the data, owing to multiplication during sedimentation. 

But in their slow descent through the liquid, such bacteria are subject 

to the action of the chlorine, the supply of which is constantly renewed 

by the downward movement of the particles. The major portion of 

such sediment is suspended in the liquid-for periods varying from 

fifteen minutes to an hour, during which time the chlorine has 

evidently ample opportunity to act. In the series of tests just 

described this sediment was included in the bacterial determinations. 

It is interesting to note; therefore, that the average reduction of 

bacteria at the end of two hours was 99.93 per cent, while the routine 

tests on the same days in the sedimentation tank gave an average 

reduction of 99.96 per cent, results practically identical. The 

sediment is apparently as completely disinfected as the supernatant 

liquid, but naturally an accumulation of sediment at the bottom of 

the tank would soon rid itself of chlorine and permit the multiplication of the remaining bacteria. Such secondary growth of bacteria, 

however, has no sanitary significance, since it has been satisfactorily 

shown that the pathogens do not develop in such manner outside the 

body.

These results demonstrate the entire feasibility of satisfactorily 

disinfecting trickling-filter effluents with chloride of lime, and they 

indicate that about 3.5 parts per million of available chlorine and a 

contact period of about one hour are ample for an effluent like that on 

which the experiments were made. A general discussion of the 

results in their practical application to the disinfection problem with 

special reference to costs is given on pages 63-70.

DISINFECTION OF CRUDE SEWAGE.

Under certain circumstances it may be necessary and desirable to 

disinfect the crude sewage of a community, though such a procedure 

would not as a rule be considered. It can never take the place of 

purification except by the use of excessive amounts of disinfectant; 

and by destroying the bacteria always present it simply delays the 

natural processes of reduction, allowing them to proceed at a later 

time, possibly after the solid matter has had time to settle to the 

bottom of a stream where the greatest nuisance will be caused by its 

decomposition. It might happen, however, that a sufficiently great 

dilution to preclude anaerobic action and a sufficiently swift current 

or tidal flow to prevent sedimentation would obviate all danger of a 

nuisance except from a bacterial standpoint. Such conditions are,
INVESTIGATIONS AT BOSTON; 47

perhaps, most nearly realized in our seaboard cities, where there may 

be no question of a physical nuisance, but where bacterial pollution 

of the harbor waters may be undesirable for various reasons.

The disinfection of crude sewage was studied because it was 

believed that experiments with the material might help to solve the 

general problem. The relations between available chlorine, 

amount of organic matter, and germicidal effect suggest that an 

excessive amount of organic matter in raw sewage might affect the 

efficiency of the chloride of lime treatment by combining directly 

with the chlorine and thus preventing its activity. The quality of 

the organic matter might also influence the result; for, as the organic 

matter of crude sewage is more readily oxidizable than that of an 

effluent, it might be expected to consume a greater amount of chlorine 

than the latter in a short time. The experiments with crude sewage 

were undertaken with the object of studying such features and of 

ascertaining the concentrations of chlorine necessary to effect a 

satisfactory removal of the bacteria in crude sewage, having in view 

both the possible disinfection of crude sewage itself and the establishment of a maximum limit of available chlorine in the treatment of 

effluents.

The details of these experiments differ somewhat from those of the 

effluent tests that have been described. In treating an effluent, 

sedimentation is an advantageous process by itself and can be usefully employed as an adjunct in disinfection, but in treating raw 

sewage, the removal by sedimentation of large amounts of organic 

matter in an unoxidized condition is undesirable and is not permissible in practice. In the experimental work, therefore, it was 

necessary to prevent sedimentation during the treatment.

A rectangular tank having a capacity of about 700 gallons was 

employed for the experiments. A 2-inch pipe led from the bottom of 

the tank to a pump with sufficient capacity to draw out the entire 

contents of the tank in half an hour. The outlet pipe from the pump 

discharged into the top of the tank, and the rapid circulation maintained in the sewage in this manner during the experiment made 

sedimentation of solids practically impossible. Before the beginning 

of the test the sewage was pumped through the system for fifteen 

minutes in order to insure thorough mixing; then a sample was 

collected for chemical and bacterial analysis, and the calculated 

amount of disinfectant solution was slowly poured into the inflowing 

stream. The force of this stream entering the tank from a height of 

about six feet churned the sewage thoroughly and assured rapid and 

complete mixing. Samples were collected at the end of the second 

and fourth hours for determination of the total bacteria growing at 

20° C. and of the number of B. coll. The available chlorine employed

76474 ERR 229-
48 DISINFECTION OF SEWAGE.

was ten parts per million. In the first series of tests the total amount 

of chloride of lime was added at first, but in the second series it was 

added in four portions at hourly intervals. The results are tabulated 

in Table 22.

TABLE 22. Disinfection of crude sewage -with chloride of lime at Boston; series 1 and %. a

Series.**

1.......... ...................................................

Number of B. coll per cubic centimeter; average of both series.

Temperature.

'44 

46 

50 

60 

60 

60

54

56 

60 

60 

60 

60 

60

59

Number \>f bacteria per cubic 

centimeter.

Initial.

700,000 

260,000 

750, 000 

600, 000 

200,000 

270,000

460,000

600,000 

460,000 

360,000 

460,000 

280,000 

540,000

450,000 

35,000

At end of 

2 hours.

4,900 

220 

10 

620 

500 

500

1,100

700 

100 

260 

14,000 

22,000 

7,000

7,500 

24

At end of 

4 hours.

13,000 

5,700 

100 

80 

100 

100

3,400

190 

60 

40 

340 

200 

130

160 

15

" Available chlorine, 10 parts per million.

6 Series 1. Chlorine added at start. Series 2. Chlorine added in four equal hourly portions.

The average reduction of total bacteria in the first series was 99.76 

per cent at the end of two hours, and, owing to multiplication of 

bacteria, 99.26 per cent at the end of four hours. In the second 

series the reduction in two hours was only 98.3 per cent, five parts 

per million of available chlorine having been added up to that time, 

and the addition of another five parts of chlorine brought the total 

reduction up to 99.96 per cent at the end of four hours. This indicates that there is a distinct advantage in adding the disinfectant in 

successive portions, but the increased efficiency in this case was not 

great, and it is doubtful whether it would generally be sufficient to 

compensate for the more complicated method of operation.

The figures for B. coli have little individual significance, but the 

average values indicate a removal of B. coli of 99.93 per cent and 

99.97 per cent after two hours and four hours, respectively values 

that are considerably better than those derived for the total bacteria. 

The results of the disinfection of crude sewage therefore agree with 

those obtained with the filter effluent. It is also apparent from these 

experiments that ten parts per million of available chlorine are ample 

to disinfect the crude sewage of Boston, and it is probable that even a 

less concentration would give satisfactory results. It is evident also 

that two hours is a sufficient contact period, little advantage being 

gained by making it longer.
INVESTIGATIONS AT BOSTON. 49

In April, 1908, a third series of experiments with crude sewage 

was made to determine the minimum amount of chlorine and of contact time possible for satisfactory disinfection. These tests were 

essentially like those of the first series, except that the available 

chlorine was varied each day, so that on successive days it amounted 

to two, four, six, eight, and ten parts per million, respectively. On 

the sixth day it was started again at two parts. In this way the 

effect of chlorine concentration was studied with as little interference 

as possible from other variables, such as temperature and character 

of the sewage. A two-hour period of contact was given, and in addition to the initial sample, in each test, samples were collected after 

half an hour, one hour, and two hours. The results of this series are 

shown in Table 23.

TABLE 23. Disinfection of crude sewage with chloride of lime at Boston; series 3.

Available chlorine (in parts per 

million).

10......

Control.

mperture.

'o2

68

59

63

60

64

54

55

59

68

60

64

56

58

60

57

60

64

55

60

59

59

63

65

52

57

68

62

64

64

£6

Number of bacteria per cubic centimeter.

Initial.

1,100,000

1,600,000

1,200,000

900,000

1,900,000

1, 700, 000

2,100,000

1,100,000

1,400,000

950,000

1,900,000

3, 500, 000

1,700,000

1, 700, 000

2,800,000

1, 700, 000

2, 400, 000

3,300,000

1,600,000

1,500,000

2,000,000

1,100,000

2,600,000

2, 500, 000

1,600,000

610,000

1,900,000

1,100,000

2,500,000

3,100,000

1,200,000

One-half 

hour.

90,000

320,000

32,000

47,000

400,000

370, 000

50,000

5,000

30,000

3,400

12,000

33, 000

3,600

6,500

17,000

1,700

1,800

14,000

570

3,400

2,000

340

1,900

6,300

380

200

1,100

350

500

3,500

1,500,000

1 hour.

62,000

470,000

49,000

16,000

450,000

460,000

41,000

6,500

40,000

1,100

11,000

26,000

9,000

600

4,600

8,400

3,000

11,000

480

160

1,100

170

2,700

3,300

300

210

- 430

800

350

2,300

1,800,000

2 hours.

130,000

870,000

140,000

42,000

700,000

1,000,000

65, 000

4,400

85,000

4,200

23,000

80,000

1,700

6,000

4,400

3,900

1,700

18,000

600

730

3,700

750

3,300

9,500

 400

200

1,600

1,100

850

4,800

2, 100, 000

The results indicate that a large amount of chlorine is consumed 

by the sewage within two hours. Two parts per million are consumed 

within half an hour and ten parts per million are not sufficient to 

prevent the subsequent growth of bacteria. It has, however, already 

been pointed out that such subsequent growths have no significance 

and ^that the efficiency of the disinfection from the standpoint of the 

pathogenicity of the sewage is measured by the maximum reduction, 

independent of any subsequent increase.
50 DISINFECTION OF SEWAGE.

The average initial and minimum figures and the per cent of bacteria 

removed for each concentration of chlorine are brought together in 

Table 24.

TABLE 24. Disinfection of crude sewage with chloride of lime at Boston; summary of

results.

Available

chlorine (in

parts per

million).

2

4

6

8

10

Average number of bacteria per

cubic centimeter.

Average initial.

1, 400, 000

1,800,000

2, 200, 000

1, 900, 000

1, 800, 000

Average 

minimum.

210,000

23,000 

6,000

1,200

700

Bacteria

removed.

Per cent.

85

98.7

99.7

99.94

99.96

These experiments were not sufficiently prolonged to furnish complete data on the process, and it is not at all unlikely that at another 

season of the year somewhat different results might have been obtained, though it has been shown in connection with the work on 

effluents that the temperature effect on the disinfection itself, other 

conditions remaining constant, is but slight. The bacterial and 

chemical composition of crude sewage, however, is affected by temperature to a much greater extent than is that of filter effluents. 

The amount of dissolved oxygen present is an important index to the 

character of the sewage or the effluent in reference to its effect on 

chlorine; absence of dissolved oxygen in particular indicates ready 

oxidizability, which in turn means the rapid absorption of chlorine. 

It is probably true that a given amount of chlorine acting for a given 

time will bring about a definite reduction in the total number of 

bacteria, regardless of the character of the treated liquid. Unless 

excessive amounts of chlorine are added, however, the time of contact is determined by the character and the amount of the organic 

matter in the sewage; consequently the more rapidly the chlorine is 

exhausted by this organic matter the shorter is the time of contact 

and the less is the bacterial reduction. The sewage experimented 

with in these tests represents about an average condition in regard to 

reducibility, and it contained about one-half the amount of dissolved 

oxygen that would be found during the winter months. Results 

fully as satisfactory as these could doubtless be obtained during six 

months of the year with even less available chlorine. During the 

summer months, when dissolved oxygen is not present, more chlorine 

than is here indicated would be found necessary. The Red Bank 

experiments on septic sewage make it probable that a concentration 

of ten parts per million of available chlorine may be taken as a maximum amount during the summer. - *

Satisfactory disinfection of crude Boston sewage can be accomplished by adding chloride of lime in such amounts that the available
DISINFECTION OF SEPTIC SEWAGE. 51

chlorine will amount to about five parts per million during six months 

of the year, and to between five and ten parts during the other six 

months, or an average amount during the year of seven or eight parts 

per million. The addition of the disinfectant in portions at intervals 

during the treatment yields results that are somewhat better than 

those obtained by adding the entire amount at once, but it is not 

probable that this advantage is commensurate with the additional 

complications involved.

DISINFECTION OF SEPTIC SEWAGE AT RED BANK, N. J.

*

The successful results obtained in the disinfection of trickling-filter 

effluents at Boston in 1906 warranted the extension of the experiments 

to include a practical demonstration; and that step was made possible through the cooperation of the New Jersey State Sewerage Commission. Red Bank, a town of about 6,500 inhabitants, situated on 

Navesink River in Monmouth County, was selected for the tests for 

two reasons: The works at Red Bank were so arranged that the 

experiment could be undertaken at a minimum expense for new construction; and, like many other communities in that section of the 

State,.the town was discharging partly-purified sewage into waters that 

shortly reached important shellfish areas. A septic sewage of rather 

poor quality was to be treated. Work with an effluent of higher 

grade had been desired, but the novelty of treating a septic sewage 

and the immediate practical value of the results promised to make 

this experiment well worth undertaking. The conditions resembled 

those described on pages 12-13, where it was suggested that in certain 

localities thorough disinfection of crude or septic sewage will go far 

toward accomplishing all that is needful. At least the most necessary 

step is thus taken first, and it is a logical first step in the gradual 

improvement of sanitary and economic conditions along our water 

fronts.

It was realized that more chloride of lime would be needed than had 

been used in previous experiments, but how much more was not 

known. The Boston results with crude sewage were not available at 

that time, and it was to be expected that, owing to the highly unstable character of septic sewage, even more chloride of lime would 

be necessary than for crude sewage. The work was started in October, 1906. During the fall the disinfection apparatus was left to the 

care of a local attendant, and a representative of the commission 

visited the works twice a week to collect samples for analysis; but 

mechanical deficiencies in the dosing device led to intermittent and 

irregular dosing, so that the results obtained were far from satisfactory, and considerable time was consumed before it was learned that 

more than 10 parts per million of available chlorine would be required. 

The work was discontinued when cold weather began, but it was 

resumed under more favorable circumstances in the summer of 1907. 

A laboratory in charge of Mr. Daniels was installed at the works, and
52 DISINFECTION OF SEWAGE.

the entire experiment was then under careful observation, for samples 

could be taken and examined as frequently as might be desired. The 

results amply justified this course. The earlier work indicated the 

probable strength of chlorine necessary, and developed many defects 

and weaknesses in the dosing apparatus, all of which were remedied. 

The investigations that were conducted during the summer of 1907 

only are reported.

The sewerage system of Red Bank is constructed on the separate 

plan, with special drains to carry off rain water. On account of the 

sandy nature of the .soil and the poor construction of the sewers 

considerable storm water finds its way into the sewers. The average 

dry-weather flow of sewage is about 265,000 gallons a day. The 

following description of the disposal works will be more readily 

understood by referring to Plate I. After entering the works the 

sewage divides into two streams, passing through two grit chambers, 

each 5 by 8 feet in plan and 3 feet deep to the flow line. The flow 

may be entirely diverted through either tank to allow the other to 

be cleaned out. From these chambers the sewage flows to the septic 

tank, which is circular in plan, 43 feet in diameter, with a normal 

depth to the flow line of 8 feet 6 inches at the periphery and. 5 feet 

at the center, the bottom being conical. Its capacity at that depth 

is about 82,000 gallons, or approximately an 8-hour dry-weather 

flow. For the purpose of these experiments, however, it was necessary to raise the flow line throughout the system 1 foot, a change 

that increased the capacity of the tank to about 93,000 gallons. 

At the side of the tank opposite the entrance pipe the septic sewage 

passes out through a submerged 12-inch pipe i, which later divides 

into a Y. A branch then runs downward to the bottom of each of 

the filters, thence horizontally to a point beneath the center, thence 

upward into the filter, as shown in the sectional view. The so-called 

filters are two in number, and each is 12 feet in diameter. Tank a 

is 9 feet deep to the flow line, with a capacity of 7,600 gallons, and 

tank & is 8 feet 3 inches deep, with a capacity of 7,000 gallons. The 

tanks were originally filled with stone and brick over a false bottom 

and were practically strainers for the removal of suspended matter. 

Each filter is provided with a 12-inch overflow leading to a manhole 

outside the building. For these experiments the filtering material 

was entirely removed from both tanks, one of the outlets was closed, 

and a cross-connection was put in to conduct the sewage from the 

top of the first tank to the bottom of the second. By closing the 

inlet valve of the latter the entire flow was made to pass through 

the two tanks in series. Suitable baffles were provided, as indicated 

in the plan, so that the entire capacity of the tanks could be utilized 

to the best advantage. As thus arranged, each tank held about 

forty-five minutes' flow, and the efficiency of the process was tested 

at the end of forty-five minutes and ninety minutes.
U. S. GEOLOGICAL SI

 - - Superstru

 i. Course of
54 DISINFECTION OF SEWAGE.

day in hogshead d was dumped into tank a, where it settled to the 

bottom and was slowly leached out by the sewage. In this way all 

the available chlorine was utilized.

It was necessary to have some information about the chemical 

composition of the septic sewage under treatment and to make certain chemical tests of the effluent. For this purpose samples of the 

septic sewage and of the disinfected effluent were collected daily and 

determinations of the oxygen consumed were made on them. These 

estimates gave a comparative indication of the strength of the 

sewage from day to day. Daily determinations of dissolved oxygen 

in the final effluent were also made, and the effluent was tested for 

free chlorine. The septic sewage never contained any dissolved oxygen, and under normal conditions the effluent would be in the same 

condition, but the application of free chlorine to sewage gives rise to 

chemical reactions, whereby free oxygen is formed. This is largely 

consumed in the oxidation of the organic matter, but nevertheless 

some free dissolved oxygen was always found in the effluent. The 

presence of this oxygen is of great value in the further self-purification of the sewage after reaching the river. Free chlorine was never 

found in the final effluent. Its presence would be highly injurious 

to fish life, and disinfection processes of this nature must always be 

so controlled that no injurious chemical can escape into the stream. 

In addition to the daily determinations just mentioned, composite 

sterilized samples were preserved and sent to Boston regularly, 

where they were submitted to complete chemical analysis. Regular 

chemical analyses of the bleaching powder and daily assays of the 

disinfectant solution were made.

The results of the work at Red Bank are given in the form of 

weekly averages in Table 25, and' the relation between the_individual 

and the average results are shown in Table 26.

TABLE 25. Disinfection of septic sewage with chloride of lime at Red Bank, N. J.;

weekly averages.

Week ending.

1907. 

July 20..............

27 ..............

Aug. 3 ..............

10..............

17..............

24. .............

31. .............

Sept. 14 ..............

21..............

28..............

Average &......

Available chlorine in 

parts

million.

9.9 

10.6 

11.5 

11.4 

13.0 

7.3 

7.5 

11.8 

13.1 

10.5

11.5

Total number of bacteria per cubic 

centimeter at 20° C.

Initial.

800,000 

650,000 

1,80(,000 

85C.OOO 

760, -WO 

700,000 

1,200,000 

750,000 

750,000 

700,000

900,000

At end of 45 

minutes.

410 

800 

550 

240 

2,100 

45,000 

45,000 

13,000 

850 

120

2,300

At end of 90 

minutes.

460 

420 

130 

140 

1,500 

55,000 

26,000 

8,000 

800 

88

1,400

Number of B. cdli per cubic centimeter, a

Initial.

46,000 

80,000 

40,000 

55,000 

70,UOO 

70,000 

220,000 

300,000 

500,000 

550,000

205,000

At end of 45 

minutes.

4 

13 

21 

14 

30 

700 

16,000 

150 

270 

80

75

At end of 90 

minutes.

4 

11 

5 

2 

28 

600 

2,000 

140 

260 

28

60

a Jackson bile media used. & Exclusive of period August 19-31; temperature 56° to 58° throughout.
DISINFECTION OF SEPTIC SEWAGE. 55

TABLE 26. Relation between individual tests of bacterial removal and the average result

at Red Bank, N. J.

Per cent of total 

number of tests."

Total, 100

Total number of bacteria 

per cubic centimeter.

Not less 

than 

100

1,000

2,000

5,000

Less than 

1,000

2,000

5,000

Average, 1,900

a Total number of tests, 224.

During the period August 19 to August 31, the available chlorine 

was reduced to about 7.5 parts per million. A removal of the total 

bacteria amounting to about 95 per cent and of B. coli amounting to 

94.3 per cent after three-quarters of an hour and 99 per cent after 

one and a half hours was obtained, but the individual results were 

very erratic, and it was apparent that an insufficient amount of 

chlorine was being added. The chlorine was therefore increased 

again to about 12 parts per million and kept there during the remainder of the experiment. The averages in the last line of Table 25 do 

not include the results for the above-mentioned period. The average 

per cent removal of total bacteria was 99.7 at the end of 45 minutes, 

and the corresponding B. coli figure is 99.96. In 90 minutes the 

average per cent removals were 99.8 and 99.97, respectively. It is 

rather striking that such results can be obtained with 12 parts per 

million of available chlorine, while the results are poorer and very 

erratic with 7.5 parts. The explanation is probably found in the 

character of the sewage, which was very concentrated in its raw state 

and had passed through a septic tank that was working vigorously. 

A large amount of hydrogen sulphide was contained in the septic 

sewage. The characteristic odor of this substance was extremely 

noticeable in the vicinity of the works, and the addition of chlorine 

to the sewage produced a milkiness, due to the liberation of free sulphur. Titration of the septic sewage in the cold with potassium 

permanganate, a substance less easily reduced than chlorine, indicated that enough hydrogen sulphide and other easily oxidizable 

substances were present to reduce instantly about five or six parts 

per million of chlorine. Consequently the efficient results in this 

experiment were in reality obtained with about six parts of available 

chlorine and the poor ones with about three parts.

The disinfection of septic sewage, evidently requires so much 

chlorine that the expense will be considerable. It will probably be 

found to take twice as much chlorine for septic sewage as for the cor-
56 'DISINFECTION OF SEWAGE.

responding crude sewage. The combination of septic action and 

disinfection eliminates the suspended matter and the bacteria from 

the sewage and in many localities would constitute a very desirable 

system. In practice, however, the order of the processes should be 

reversed. The disinfection of the crude sewage can be done in a 

small tank of an hour's capacity. The disinfectant should be so 

regulated that little or no chlorine would flow into the septic tank. 

Unless a considerable amount of chlorine should thus escape at onetime there would be no objectionable effect on the action in the 

tank. There would be a great multiplication of bacteria in the 

tank, so that the number hi the final effluent would probably be as 

great as hi the raw sewage, and perhaps even greater. Nevertheless, 

the disinfection would be as effective on the pathogens as if it were 

applied as a final process. The subsequent development of saprophytes would have no sanitary significance and, would doubtless be 

of real value in the subsequent self-purification of the organic matter 

after it had been discharged into the stream.

DISINFECTION OF TRICKLING-FILTER EFFLUENT AT BALTIMORE.

Reference was made in the history of the experiments to the cooperative arrangement between the United States Geological Survey 

and the Baltimore sewerage commission. In December, 1907, the 

sewerage commission undertook a study of disinfection methods at 

its Walbrook testing plant. The writer assisted in planning this 

work, and was consulted from time to time during its progress. The 

disinfection experiments were made a part of the routine work of the 

testing station, and they were carried out by the station staff. A 

complete review of this investigation, the most important and most 

comprehensive one on the subject ever conducted, will, it is hoped, 

be made by the sewerage commission in the near future. The present report has been made as brief as possible and includes only the 

work done before June 30, 1908.

The Walbrook testing plant, in the extreme western section of 

Baltimore, was built for use in investigating certain problems in 

connection with the disposal works now being built at Back River, 

the main features of which screening, sedimentation, treatment on 

trickling filters, and final sedimentation had already been decided 

upon. Information was specially desired as to the probable bacterial 

efficiency of such a process and as to the necessity for further treatment of the effluent, because the statutes of the State of Maryland 

require rather high bacterial efficiency. As a detailed description of 

the testing plant has already been published,0 it is sufficient to state 

that a small section of the city was sewered and that sewage amounta Eng. News, vol. 57,1907, p. 235.
DISINFECTION OF TRICKLING FILTER EFFLUENT. 57

ing to about 50,000 gallons a day was brought to the plant and 

treated by a system comprising a grit chamber, a septic tank, trickling filters, and sedimentation tanks. Trickling filters of several 

depths and sizes of material are employed. The sewage is rather 

stronger than ordinary city sewage and is very fresh. The effluent 

used in the disinfection studies came from a filter 12 feet deep, built 

of crushed stone ranging in size from 0.5 inch to 1.5 inches. The rate 

of filtration fluctuated with the flow of sewage, but it averaged about 

3,000,000 gallons an acre a day. The effluent was conducted to.a 

sedimentation basin designed to give a two hours' flow. The disinfectant employed throughout this investigation was chloride of lime, 

dissolved and added to the effluent in practically the same manner 

as at Boston, as the effluent left the filter drain. The disinfecting 

solution and the effluent mixed during the short flow through a pipe 

to the sedimentation basin.

Monthly averages of the analyses of this effluent as it left the filter 

and after disinfection and sedimentation are given in Table 27.

TABLE 27. Chemical analyses of trickling-jilter effluent at Baltimore before and after 

disinfection with chloride of lime and sedimentation: monthly averages.

[Parts per million.] 

INITIAL.

Month.

1908.

Suspended 

solids.

Total.

56 

38 

53 

42 

33

45

Loss 

on 

ignition.

35 

25 

31 

28 

21

28

Organic matter.

Total.

8.3 

8.0 

8.9 

10.0 

7.0

8.4

Solids.

5.9 

6.8 

7.5 

9.0 

6.5

7.1

Nitrogen asFree 

ammonia.

7.4 

6.8 

5.5 

7.5 

4.0

6.2

Nitrites.

0.48 

.13 

.30 

.08 

.40

.28

Nitrates.

17 

17 

19 

21 

20

19

Oxygen 

consumed.o

16 

17 

16 

18 

13

16

FINAL.

1908.

April................................

38

, 25

26

34

21

29

26

17

17

20

16

19

8.5

6.8

7 9

9.6

7.0

8.0

6.2

5.8

7.1

7.6

6.5

6.6

7.2

6.8

5.5

7.9

4 0

6.3

0.49

.12

19

.08

9H

.23

16

17

20

22

20

19

17

15

18

13

16

a Thirty-minute boiling method.

Initial and final bacteriological samples were collected as usual. 

Bacterial counts were made three times a day for five days each week 

and twice on Saturday. B. coli determinations were made twice 

daily. The results are shown in Tables 28, 29, and 30. Table 28
58 DISINFECTION OF SEWAGE.

gives the average weekly results; Table 29 shows the relation between 

the removal of bacteria in individual samples and the average 

removal; Table 30 indicates the relation between the number of 

bacteria in the individual samples and the average number.

TABLE 28. Disinfection of trickling-filter effluent with chloride of lime at Baltimore;

weekly averages.

Week ending 

1908.

18.........

25.........

Feb. 1.........

15. ........

22.........

29.........

Mar. 7.........

14.........

21.........

28.........

Apr. 4.........

11.........

18.........

25.........

May 2.........

9.........

20 ........

27........

Average......

Temperature.

0 F. 

50 

45 

48

47

40 

43 

46 

45 

45

46 

47 

49 

52

50 

55 

54 

56

58 

56

66 

67

67

51

Available 

chlorine 

(parts per 

million).

Initial.

3.5 

2.0 

1.9 

2.0

2.6 

2.5 

0.94 

1.4 

1.5

2.0 

1.7 

2.0 

2.1

1.7 

2.6 

2.2 

2.6

3.1

2.8

2.3 

2.3 

1.9

2.2

Final.

1.3 

1.1 

0.1 

0.4 

0.4

0.6 

0.4 

0.4 

0.3 

0.4 

1.1 

0.6 

0.3

"i.'o"

0.4 

0.8 

0.1

0.6

Total number 

of bacteria per 

cubic centimeter 

at 20° C.

Initial.

170,000 

90,000 

120,000 

160,000

120,000 

70,000 

80,000 

65,000 

100,000

80,000 

80,000 

100,000 

150,000

120,000 

140,000 

120,000 

120,000

130,000 

160,000

120,000 

115,000 

190,000

120,000

Final.

1,600 

6,500 

3,300 

1,900

2,200 

4,100 

11,000 

20,000 

4,600

1,000 

1,700 

1,300 

1,700

1,800 

1,100 

2,500 

2,100

1,200 

2,300

17,000 

1,300 

3,700

4,300

Total number 

of bacteria per 

cubic centimeter at 37° C.

Initial.

70,000 

5,500 

3,100 

2,000

1,900 

1,000 

1,500 

4,000 

3,000

1,400 

1,000 

1,100 

2,400

4,100 

2,800 

13,000 

18,000

3,500 

2,200

21,000 

1,800 

2,900

7,500

Final.

600 

200 

250 

95

73 

70 

400 

440 

100

80 

60 

80 

160

120 

70 

110 

350

110 

70

3,600 

440 

1,100

390

Number of 

acid formers 

per cubic centimeter at 

37° C.

Initial.

8,000 

3,000 

2,200 

900

950 

400 

700 

3,000 

1,100

900 

500 

600 

1,500

2,100 

1,900 

3,600 

7,000

2,400 

1,600

9,500 

300 

430

2,400

Final.

107 

58 

165 

8

4 

4 

90 

180 

17

2 

3 

15 

3

10 

2 

25 

110

3 

9

6730 

18 

0

70

Number of 

B. coli per 

cubic centimeter."

Initial.

3,800 

1,700 

3,400 

1,300

2,000 

600 

440 

750 

2,400

1,600 

1,400 

2,600 

800

2,100 

3,700 

5,500 

2,200

1,400 

1,800

600 

3,000 

800

2,000

Final.

59 

380 

240 

45

490 

470 

330 

70 

210

150 

180 

180 

210

220 

39

55 

200

130 

43

100 

300 

400

200

o Jackson bile media used. & High, average due to one count of 8,000.

TABLE 29. Relation between individual tests of bacterial removal and the average result

at Baltimore. :«.

Per cent of total 

tests."

69 

14 

6 

0 

2 

2 

4 

1 

1 

1

Total, 100.

Per cent of bacterial 

removal.

Not less 

than 

98 

96 

94 

92 

» 90 

85 

75 

65 

40

Less than 

» 100 

98 

96 

94 

92 

90 

85 

75 

65 

40

Average, 95.5.

"Total number of tests, 255.
DISINFECTION OF TEICKLING FILTEE EFFLUENT. 59

TABLE 30. Relation between individual 'bacterial counts and the average result at

Baltimore.

Per cent of total 

tests."

36 

35 

8 

3 

4 

2 

2 

3 7-

Total, 100.

Number of bacteria per 

cubic centimeter.

Not less 

than 

100 

1,000 

2,000 

3,000 

4,000 

5,000 

6,000 

8,000 

10,000

Less than 

1,000 

2,000 

3,000 

4,000 

5,000 

6,000 

8,000 

10,000

: ""erage, 4,300.

« Total number of tests, 255.

The average of the individual bacterial removals is 95.5 per cent. 

The per cent removal calculated from the average numbers injtable 

28 is 96.6 per cent. The average efficiency, as in the Boston experiments, may be expressed in the latter form with sufficient accuracy, 

and this shorter method is employed in the other bacterial computations, by which the efficiencies in Table 31 were obtained.

TABLE 31. Average bacterial removal during the. disinfection of trickling-filter effluent

at Baltimore.

Per cent.

Total bacteria at 20° C..................................................... 96. 6

Total bacteria at 37° C..................................................... 94. 9

Acid formers at 37° C....................................................... 97. 0

B. coli a. ................................................................. 90. 0

The average results agree practically with those obtained at 

Boston. The trickling-filter effluent at Baltimore was of better 

quality than that at Boston and less chlorine was used. The average amount of available chlorine was 2.2 parts per million, as compared with 3.4 parts at Boston. Though-the average reduction of 

total bacteria at both places was practically the same, the variations 

were greater at Baltimore, and they indicate that a somewhat greater 

amount of chlorine would materially improve the results. It is 

probable that three parts per million of available chlorine are best 

for treating this effluent. The B. coli results are not in harmony 

with the rest of the figures. In all the work at other places the 

removal of B. coli has been without exception better than the removal of the total organisms. Soon after the beginning of the 

Baltimore studies it was noticed that the removal of B. coli, as indicated by the bile test, was far less perfect than the general results, 

but efforts to discover the cause of the discrepancy were only partly

o Jackson bile media used.
60 DISINFECTION OF SEWAGE.

successful. Complete identification of B. coli was carried out for a 

period, and the results seem to indicate that many of the positive 

tests after disinfection are due to the presence of an organism other 

than B. coli, which fermented the bile medium. Similar atypical 

results were obtained in the initial samples, but not in such large 

proportion. .The evidence of other work under the direction of the 

author indicates that in these experiments the removal of B. coli is 

better shown by the removal of acid-forming bacteria than by the 

bile test. The results as a whole demonstrate the entire feasibility 

of the process and the possibility of obtaining practical disinfection 

at a reasonable cost.

COMPARATIVE GERMICIDAL EFFICIENCIES OF CHLORINE AND 

SOME OF ITS COMPOUNDS.

Only a small amount of accurate data was at hand for the comparison of the germicidal efficiency of chlorine in its various forms. 

Accordingly, three series of comparative tests were made upon 

trickling-filter effluents for the purpose of obtaining such information. The results of these studies are collated in Table 32, each 

figure of which is the average of from three to twelve tests reduced 

to a uniform basis of one million initial bacteria per cubic centimeter.

TABLE 32. Relative germicidal properties of chlorine and some of its compounds. 

[All numbers converted to a uniform basis of 1,000,000 initial bacteria per cubic centimeter.]

Series.

I 

II 

III

Source of chlorine.

....do.&.. ........ .... .... . ...........

. . . .do.d. ..........................................

. . . .do./. ..........................................

Available 

chlorine 

(in parts 

per 

million).

3 

3 

3 

3 

3 

2 

2 

2 

2 

2 

2 

2 

2

Total number of remaining bacteria per cubic centimeter.

At end 

of 30 

minutes.

650 

500 

410 

800,000 

750,000 

17,000 

15,000 

4,600 

19,000 

23,000 

22,000 

23,000 

19,000

At end of 

1 hour.

390 

270 

260 

900,000 

1,400,000 

13,000 

6,000 

2,100 

18,000 

14,000 

12,000 

9,000 

7,500

At end of 

2 hours.

280 

230 

280 

1,000,000 

1,800,000 

17,000 

6,000 

3,400 

23,000 

19,000 

18,000 

10,000 

8,000

a Electrolytic.

& From bleaching powder.

c Chlorine added thirty minutes after the hydroxide.

d Chlorine added twenty minutes after the hydroxide.

« Chlorine added ten minutes after the hydroxide.

/ Chlorine added with hydroxide.

In series I comparison is made of free gaseous chlorine generated 

electrolytically, sodium hypochlorite, * and potassium hypochlorite. 

Potassium chlorate and potassium perchlorate are also included, 

because they are formed in many direct electrolytic processes. The 

available chlorine of the two latter compounds is taken in an elec-
COMPARATIVE GEEMICIDAL EFFICIENCIES. 61

trolytic equivalent sense, and would be better expressed by the term 

"oxidizing power." Three parts per million of available chlorine 

w^ere used in all the tests of series I.

In series II comparison is made of free gaseous chlorine, potassium 

hypochlorite prepared from bleaching powder, and the same compound electrolytically prepared by the recombination of the products 

of the electrolytic cell. In this and in the next series two parts per 

million of available chlorine were used, because it was found that 

comparisons are more readily made where the disinfection is not so 

nearly complete.

In series III free chlorine was again used and was compared with 

the electrolytic hypochlorite. In this set of experiments, however, 

the hypochlorite was made in the sewage by adding separatelyjthe 

chlorine water and the sodium hydroxide. In three sets of tests 

the addition of the hydroxide preceded that of the chlorine by ten, 

twenty, and thirty minutes, respectively. The ojbject of this procedure is to determine whether the caustic soda and the gaseous 

chlorine possess separately better penetrating powers than the hypochlorite does. If such were the case, formation of the hypochlorite 

within the solid particles of the sewage might result in more complete disinfection than could otherwise be obtained.

The results indicate plainly that hypochlorites are the most efficient germicides. Gaseous chlorine is almost as good, but in each 

series the free chlorine is somewhat inferior to the hypochlorite. 

Chlorates and perchlorates have almost no value in disinfection. 

The formation of these compounds in the electrolytic cell is, therefore, a total waste of energy, and should be prevented as far as possible. Production of these compounds explains in large measure the 

inefficiency of hypochlorite cells. Hypochlorites made electrolytically are slightly inferior to the market product, but this difference 

would probably be inappreciable in large-scale tests, where the conditions under which the hypochlorites are prepared are more nearly 

those of commercial practice. Hypochlorites of different bases evidently have the same value; the results obtained with the sodium 

and the potassium salts are practically identical and are similar to 

those obtained in practice with the calcium salt. The claim that the 

magnesium compound is more efficient than the calcium compound 

was not -investigated. Hypochlorites can be made advantageously 

by mixing dilute solutions of free chlorine and sodium hydroxide, the 

products of the electrolytic cell, but the mixing should take place 

either in the sewage or in cold dilute solution, as otherwise chlorates 

and perchlorates will t>e formed; if the Irydroxide is added a short 

time before the chlorine, it is removed from the solution, leaving free 

chlorine uncombined and leading to a more rapid exhaustion of the 

available chlorine.
62 . DISINFECTION OF SEWAGE.

These tests have an interesting bearing on the question of the exhaustion of the chlorine. It appears that the chlorine attacks the 

organic matter and the bacteria simultaneously, but that its effect on 

the former is a direct function of its concentration, while its germicidal effect does not bear such exact relation. If these are the true 

conditions, the successive addition of small portions of chlorine, or 

what amounts to the same thing, the addition of a substance that 

yields chlorine slowly, prevents the rapid reduction of the chlorine 

by the organic matter and prolongs the time of contact with the bacteria. In series III, Table 33, more or less of the hydrate was consumed in saponification before the chlorine was added, the amount 

so removed depending on the time that elapsed between the addition 

of £he hydroxide and the addition of the chlorine. The first set of 

series III, therefore, represents the action of free chlorine, and the 

last set the action of hypochlorite, with the intermediate tests representing different proportions of the two substances. The progressive 

nature of the final results indicates strongly the necessity of having 

the chlorine combined in such form that its action on organic matter 

is retarded. The same effect is shown in the work on crude sewage 

described on pages 37-51. In the experiments reported in Table 22 

decidedly better results were obtained by adding the hypochlorite 

in four equal portions at hourly intervals than by adding the entire 

amount at once. In practice the effluent to be treated should never 

be acid in reaction; it is probable that the addition of lime would 

still further decrease the rate of decomposition of the hypochlorite 

and increase its bactericidal efficiency.

EFFECT OF CALCIUM HYPOCHLORITE ON COLON AND TYPHOID

BACILLI.

The colon bacillus has been employed in this and in other work as 

a convenient test organism with which to measure the efficiency of 

the disinfection process. For obvious reasons experiments on a large 

scale with the typhoid bacillus are out of the question. Consequently, 

a comparison was made between the relative resistance of the typhoid 

and the colon bacilli under controllable conditions, with the idea that 

in practice the effect of the disinfection process on the former could 

be measured by its effect on the latter. Emulsions of the two organisms in tap water were treated with hypochlorite solution,- and the 

parallel tests with the two species were made at the same time and 

were kept as nearly alike as possible. The number of bacteria per 

cubic centimeter was determined at the end of twenty, forty, and 

sixty minutes, and two, four, and eighteen hours, and twelve tests 

of each kind were made in the same manner. The available chlorine 

ranged from 3.5 to 6 parts per million, averaging 5 parts. The 

results of the individual tests varied greatly from day to day, because,
PRACTICAL APPLICATIONS AND COSTS. 63

no doubt, of difference in the character of the growths and in the 

amounts of organic matter introduced with the organisms. Yet the 

average figures obtained from these twelve sets of tests probably give 

a fair basis for estimating the comparative resistance of the two 

organisms to the disinfectant. The per cent removal has been calculated and the average results are presented in Table 33.

TABLE 33. Comparative resistance to calcium hypochlorite of B. typhi and B. coli in

aqueous emulsion.®

Tests made at end of 

20 minutes ......................................................................

Removal of bacteria 

(per cent).

B. typhi.

90.5 

98.2 

99.45 

99.60 

99.92 

99.99+

B. coli.

92.0 

98.0 

99.53 

99.70 

99. 96 

99! 99+

« Average available chlorine, 5.0 parts per million.

The slight differences shown by the experiments on the two organisms may be attributed to experimental variations. The work is 

not conclusive because other, strains of organisms might have yielded 

different results, but it indicates in a general way that B. coli may 

reasonably be regarded as test organisms in disinfection work and 

that the process may be expected to destroy typhoid organisms 

present at least as thoroughly.

It is interesting also to note that the per cent removal of B. coli 

after four hours is about the same as that recorded in the experimental disinfection of crude sewage; namely, 99.96 per cent as compared with 99.93 per cent.

PRACTICAL APPLICATIONS AND COSTS OF DISINFECTION.

The experiments that have been described were sufficiently prolonged and varied in their scope to justify the application of the 

chloride of lime treatment to practical disinfection on a large scale. 

It is not possible, however, to draw general conclusions regarding the 

amount of chlorine necessary for the disinfection of effluents, much 

less of crude sewage, because the dose is determined largely by the 

character of the organic and reducing matters contained in the sewage 

or the effluent. But the investigations have shown what may be 

accomplished under several conditions and they have established 

certain probable maximum amounts of chlorine for particular classes 

of sewage and effluents. They have demonstrated, for example, that 

trickling-filter effluents similar to those tested may be satisfactorily 

76474 IRR 229 09 5
64 DISINFECTION OF SEWAGE.

disinfected with three or four parts per million of available chlorine, 

and that such quantity will usually be sufficient to effect the removal 

of 95 per cent or more of the total bacteria in the effluent. If the 

effluent should contain only 25 per cent of the original sewage bacteria, the whole purification process would result in a removal of 

98.8 per cent of the total number of bacteria. The per cent removal 

of B. coli and of typhoid organisms will be at least as high if not 

higher. Crude Boston sewage can be disinfected to about the same 

extent with from four to six parts per million of available chlorine. 

It may reasonably be inferred, therefore, that five parts per million 

of chlorine represent the maximum concentration that would ever 

be necessary for the disinfection of a fairly stable effluent, since no 

such effluent would contain as much oxidizable organic matter as 

the crude sewage that is treated. The septic sewage of Red Bank 

probably represents a maximum condition for crude sewage, because 

its high content of hydrogen sulphide and other oxidizable matters 

would probably never be exceeded in crude sewage, or at least in 

sewage from American cities. The disinfection of crude sewage, 

therefore, would require four to twelve parts per million, of available 

chlorine, depending on the character of the sewage and its content 

of oxidizable matters. It will probably be undesirable to treat septic 

sewage, but if such sewage should be treated, from ten to fifteen parts 

of chlorine, or perhaps more, would be necessary. The extreme 

variability of the composition of septic sewage makes it almost 

impossible to fix a maximum limit for the amount of hypochlorite. 

The amount of disinfectant required in any plant would necessarily 

be determined before final adoption of the plan, but these estimates 

will serve as useful guides, and the limits assigned include the majority 

of probable conditions.

No data have been obtained with effluents of higher degree of purity 

than those from trickling filters. The general conclusions from the 

work have been that the disinfectant action is a function of the chlorine concentration and of the time of contact, and that the time is 

determined largely by the rate at which the chlorine is consumed by 

the organic matter. Effluents of a better grade would probably 

require the same amount of chlorine, if the disinfection were to be 

accomplished within two hours. In the effluents with which experiments were made, the chlorine was practically consumed in two 

hours, but in a purer effluent chlorine would probably be left. This 

indicates that somewhat smaller quantities could be used with longer 

storage periods. A point is soon reached, however, beyond which 

the cost of storage is greater than the saving in disinfectant. Effluents 

of high degree of purity could probably be disinfected with one part 

of available chlorine, but a contact period of at least five hours would 

be required for satisfactory removal of the bacteria.- One part is
PRACTICAL APPLICATIONS AND COSTS. 65

probably the minimum amount of chlorine that can be used, as the 

necessary time of contact increases very rapidly with decreasing 

concentration of chlorine.

One of the most important practical points that has been developed is the relation between organic matter, time of contact, and 

amount of chlorine necessary. Obviously a definite concentration of 

chlorine acting for a given time will bring about a definite result 

independent of the nature of the solution. If, however, chlorine is 

being consumed by the solution itself a greater amount will have to 

added at first in order to maintain the same average concentration. 

On the other hand, longer periods of contact with lower chlorine concentration are possible with the better grades of effluent. The experiments with various kinds of sewage make it possible to formulate a 

crude rule, which may be stated as follows: The product of the 

initial concentration multiplied by the time in hours required for 

complete reduction of the available chlorine should be about five for 

satisfactory disinfection, except that the contact period must not be 

less than half an hour in any case.

It is possible to fix within narrow limits the cost of chlorine disinfection. The cost is determined chiefly by the concentration of 

chlorine necessary and the related factor of contact period, and 

secondarily by the size of the plant. The unit used in the following 

summary of costs is a plant with a daily flow of 5,000,000 gallons of 

sewage, such a plant being the smallest that would require the entire 

time of an attendant, a part of whose salary may legitimately be 

charged to disinfection. On larger works the labor costs would 

increase proportionately, and 011 smaller works arrangements would 

naturally be made by which the plant could receive proper attention 

in connection with the regular work of the sewer or street department. The price of bleaching powder has been estimated at $24 a 

ton delivered at the plant; it is quoted at from $22 to $25, and a 

price as low as $20 is obtained on large orders by certain paper mills. 

The price taken is, therefore, sufficiently high to cover the cost of the 

moderate-sized shipments that would be required for a 5,000,000- 

gallon plant. A 50,000,000-gallon plant could obtain its bleaching 

powder for at least 10 per cent less. This bleaching powder would 

contain over 35 per cent available chlorine, but in order to allow for 

waste 33 per cent has been taken as the average figure. Labor is 

computed at $2 for an eight-hour day. Two hours a day an ample 

allowance are reckoned for the care of a 5,000,000-gallon disinfecting plant using 5 parts or less of chlorine. For over 5 parts the time 

would increase proportionately. Interest, depreciation, and other 

fixed charges are computed as 6 per cent of the cost of the additional 

works made necessary by the disinfection treatment; as the con-.
66 DISINFECTION OF SEWAGE.

struction is chiefly masonry, but little depreciation need be allowed. 

In certain projects sedimentation tanks would already be available, 

so that the application of disinfection would not require the construction of storage tanks. For this reason the item storage tanks 

has been separated from the other fixed charges that include 6 per 

cent of the cost of mixing tanks and storage tanks for the solution, 

pumps, piping and connections, and suitable housing. In handling 

the chemicals for a plant requiring 3 parts per million or more of 

chlorine, some form of power mixer would be economical, and that 

item has been estimated. In small plants the mixture of bleaching 

powder and water would be settled and the solution decanted, but in 

the larger works the whole mixture would be used, and it would 

require constant stirring. The increased power item would be more 

than counterbalanced by the saving in tank construction. Table 34 

contains a summary of the cost estimates for chlorine concentrations 

from 1 to 15 parts per million.

TABLE 34. Estimates of the cost of maintenance and operation of a plant for disinfecting 

sewage or effluent with chloride of lime, based on a capacity of 5,000,000 gallons a day.

Available

chlorine

in parts 

per 

million).

1

9

3

4

5

10

15

Time of

contact

(in 

hours).

5.0

2.5

1.6

1.2

.8

.5

.5

Cost per million gallons.

Storage 

tanks.

an I n

05

Other 

fixed 

charges.

.04

.04 .05

.03

.03

.02

.07

.08

.16

.02 .24

Bleaching 

powder.

en qn

60

Labor.

am in

.10

.90 1 .10

1. 20 . 10

1.50

3.00

4.50

.10

.15

.20

Power.

$0.02

.02

.03

.06

.09

Total.

.79

1.11

1.42

1.74

3.39

5.05

The estimates made for a 5,000,000-million gallon unit can be 

safely applied to larger works, and they are applicable within reasonable limits to small works that are properly managed. The cost of 

the tanks and of covering them would become proportionately 

cheaper with increased size, but this saving would be offset by the 

advisability of better construction and architectural embellishment. 

The saving due to decreased price of bleaching powder for a 

50,000,000-gallon plant, using 3 parts per million of chlorine, would 

be about $4.50 a day, which would approximately pay for the 

additional chemical and bacteriological control required by a plant 

of such size. On very large works labor items would not increase 

proportionately, because most of the work would be done by 

machinery.

In an earlier paper a the opinion was expressed that the expense 

might be considerably reduced and efficiency increased by using

a Phelps, E. B., and Carpenter, W.' T., The sterilization of sewage filter effluents: Tech. Quart., vol. 19, 

1906, p. 382; Contr. from Sanitary Research Lab., vol. 4, 1908.
PRACTICAL APPLICATIONS AND COSTS. 67

electrolytic chlorine produced at the disposal works instead of 

bleaching powder. The relation of electrolytic processes to sewage 

disinfection is still unsettled, but a study of the present possibilities 

of such processes and experimental work with the McDonald cell 

and with small cells of special design have indicated that the margin 

of cost between the alternative methods is so slight that it hardly 

justifies the additional effort and the uncertainty involved in the 

establishment of an electrolytic plant. A brief discussion of recent 

developments in this field will show the reasons for this conclusion. 

Two general types of electrolytic process are available, in both of 

which the electric current is passed through a solution of sodium 

chloride, chlorine being liberated at one electrode and caustic soda 

at the other. In one type these products are allowed to recombine, 

forming sodium hypochlorite and certain other compounds. In the 

other type the products are removed from the cell as quickly as 

possible, the aim being to prevent their recombination. Numerous 

processes of the first kind have been developed, of which the Hermite 

and Woolfe processes have already been mentioned. The commercial preparation, called "Chloros," is made in this way. The 

most recent, and probably the most improved, cell of this type has 

recently been described by Digby and Shenton.a The reaction by 

which the hypochlorite is produced from chlorine and caustic soda 

in cold dilute solution is:

(1) 2 NaOH + 2Cl = NaOCl + NaCl+H2O. 

In the paper just cited Digby proposes the reaction, 

(2) NaOH + Cl = NaOCl+H,

He bases his view on the observation that the electro-chemical 

efficiency of the cell is over 50 per cent. Aside from the obvious 

impossibility that a reaction can produce at one and the same time 

nascent hydrogen and a strong oxidizing agent, it is apparent that 

reaction (1), if it were carried out completely, would yield a product 

containing not 50, but 100 per cent of the available chlorine initially 

present. The conception that this reaction represents a loss of half 

the available chlorine is due apparently to a mistaken idea of the 

term available chlorine, which, as has been explained on page 18, is 

really a misnomer. The fact is that the oxidizing power, or the 

available chlorine as ordinarily determined, of the products of reaction 

(1) is equivalent to twice the chlorine of the hypochlorite, or to the 

total chlorine present. There is, therefore, no apparent basis for the 

reaction proposed by Digby, which would yield twice as much available chlorine as the amount allowed by the law of electro-chemical 

equivalents. The reactions of equation (1) are complete only in cold

a Digby, W. P., and Shenton, H. C. H., Surveyor, vol. 30, 1906, p. 653.
68 DISINFECTION OF SEWAGE.

dilute solutions. If the solution is hot or if it is concentrated chlorates 

and perchlorates are produced simultaneously. It is for this reason 

that the disinfectant value of these two sets of compounds was determined in an earlier part of the present investigation. It was found 

that they possess practically no disinfecting power and that their 

production in the cell represents a loss of energy. Economy in electric current demands strong salt solutions and high current densities 

with consequent heating of the electrolyte. Electrical efficiency is, 

therefore, opposed to chemical efficiency, and the problem in designing cells of this type is to balance the two efficiencies in the most 

economical manner. The electro-chemical equivalent of a current of 

one ampere is 1.32 grams an hour of chlorine, and this equivalent is 

not modified by the voltage. However, as the total energy employed 

in a process determines its cost, it is necessary to consider voltage as 

well as current in discussing electrolysis. In any electrolytic reaction 

there is a definite minimum voltage required for carrying out the 

reaction, and this can be computed from thermal considerations as 

follows: The complete reaction in the cell before the recombination 

of the hydroxide and the chlorine may be written,

NaCl + H2O = NaOH + H + Cl.

Substituting the heats of formation of these compounds gives 

964 + 684 = 1118 + x, whence 

x = 530 calories (K).

Here x is the heat required by the reaction, expressed in calories (K) 

per gram-equivalent of the reacting substances, and its value is 530 

calories. One gram-equivalent of substance is transformed with the 

passage of 96,540 coulombs of electricity, and one coulomb transferred under a difference of potential of one volt has an energy 

equivalent to 0.00241 calories. Therefore, one electro-chemical 

equivalent of current at a difference of potential of one volt has a 

heat value of 96,540 multiplied by 0.00241, or 232.7 calories. Consequently the voltage required to effect the desired reaction, which 

calls for a heat absorption of 530 calories per equivalent, is 530 

divided by 232.7, or 2.28 volts. This is the minimum voltage with 

which the reaction can take place, and efficiency calculations to an 

energy basis must be referred to this voltage. At a difference of 

potential of 2.28 volts, one kilowatt gives 439 amperes, so that an 

output of 579 grams of chlorine per kilowatt-hour represents a process 

of 100 per cent efficiency on both a current and an energy basis. In 

practice a current at a tension of at least 4.5 volts is usually found 

necessary, even with strong solutions of salt and with electrodes 

placed as near together as possible. This factor alone reduces the 

energy efficiency to 55 per cent with perfect current efficiency. The
PKACTICAL APPLICATIONS AND COSTS. 69

current efficiency depends especially on the design of the cell. If 

there are no complicating secondary reactions during the recombination of the products it approaches 100 per cent very closely, and this 

is also the case in the most improved design of chlorine cell in which 

recombination does not take place. It is clear that the production of 

hypochlorite in one operation within the cell is not economical, and a 

review of the available information and a laboratory study of various 

hypochlorite cells have led to the conclusion that cells of that type 

can not be expected to yield much more than one-half the available 

chlorine that can be obtained from the same electric current by means 

of direct chlorine cells.

Direct chlorine cells have been developed to a high degree of efficiency, and this is the type of cell which has been considered in the 

present studies of sewage disinfection. The McDonald cell, which 

was used to some extent in this work, is giving in a regular installation in a large paper mill a current efficiency of over 80 per cent at 

4.5 volts, making the total energy efficiency over 43 per cent. The 

most recent development is the Townsend cell, for which current 

efficiencies exceeding 98 per cent are claimed at a tension of 5 volts 

or more. The special feature of the Townsend cell is an arrangement 

by means of which the caustic liquor drops into a bath of oil as soon 

as it is formed, a step that prevents recombination. In spite of the 

high efficiency of such cells, it is not practicable to employ them in 

sewage work on account of the small margin between the market cost 

of chlorine and the cost of its manufacture electrolytically. This condition is due to the fact that the demand for caustic soda is so much 

greater than that for chlorine that the chlorine is to a certain extent 

a by-product and can be made into bleaching powder and sold at low 

cost. On the other hand, the manufacture of small amounts of 

caustic liquor at the disposal works does not warrant the installation 

of the necessary machinery for the production of pure caustic soda; 

consequently, without a market for this by-product, the cost of the 

chlorine would be the entire cost of the operation. In addition, a 

skilled chemical engineer who would be required would increase the 

cost per million gallons much more in a small plant than in a large 

one, while the uncertainty of the process and the increased responsibility on the sewage works both offset any slight advantage in cost 

which might appear in favor of the electrolytic plant. It has also 

been made clear in the present studies that the use of free chlorine, 

as contemplated in the earlier plan, is not economical and that some 

base should be provided for the preparation of hypochlorite. This 

base might be the caustic soda yielded by the process, or if a market 

for that by-product were available, lime could be used.

For the reasons outlined, therefore, this investigation of the possibilities of the electrolytic processes indicates that, contrary to earlier
70 DISINFECTION OF SEWAGE.

views, such processes are not well adapted at the present time to 

sewage disinfection. Nor does it seem probable that hypochlorite 

from cells in which the products are allowed to recombine within the 

cell will ever be able to replace ordinary bleaching powder.

CONCLUSION.

The main reason for disinfecting sewage lies in the probable effect 

of discharging pathogenic bacteria into lakes, rivers, and harbors. 

Any such discharge is obnoxious to the sanitarian, and when the practice of computing the cost of typhoid-fever epidemics becomes more 

general the cost of disinfecting sewage will not appear excessive. 

At present, however, the demand for sewage disinfection is confined 

to two conditions, namely, the possible pollution of water supplies 

and of shellfish beds. It has not yet been decided upon whom the 

responsibility rests for protecting domestic water supplies. The 

sanitarian recommends that rivers be kept as clean as possible and 

that water be filtered, but in practice distinction is made between 

supplies which are filtered before use and those which are not, and 

complete disinfection of sewage that enters streams from which supplies of the first kind are derived is still regarded as an unreasonable 

demand. The concensus of competent opinion requires at least, 

however, that, if an effluent is discharged within the region of important shellfish beds or into a stream which is used as a source of 

domestic water supply without filtration, such effluent shall be free 

from pathogenic germs. Improved standards in sanitation and improved methods of disinfection will both operate to increase these 

minimum requirements, but in the meantime a thorough knowledge 

of disinfection methods and experimentation on the improvement and 

the cheapening of such methods will do much to hasten their general 

adoption. Slow sand filtration removes bacteria in a satisfactory 

manner and almost totally eliminates organic matter. Under certain 

conditions such a result is highly desirable, but the method is comparatively costly, especially in the larger communities, and it is 

practically out of the question in many sections of the country.

Chemical disinfection offers a means whereby a reasonable bacterial 

purification may be accomplished without complete purification of 

the organic matter. It is in no sense a substitute for sewage purification as ordinarily understood, for, though the application of chlorine 

compounds to an effluent oxidizes the organic matter in it to some 

extent and thereby increases its stability, such improvement is only 

incidental. It is not in anyway comparable with the cost of treatment, and it would be unwise to attempt to obtain stability in such 

manner. Incidentally, however, the advantages of this increased 

stability are obtained, and it is probable that rapid sewage filters
CONCLUSION. 71

may be worked at somewhat higher rates and with less margin for 

safe operation where chlorine treatment is employed as a finishing 

process. Under certain conditions it may be found desirable to 

effect bacterial removal without organic stability; along the seacoast 

and possibly along some great rivers of the Middle West the dilution 

factor is sufficienty high to preclude the danger of physical nuisance. 

Under other conditions the production of a stable effluent without 

regard to the amount of organic matter discharged may suffice; 

under others the removal of suspended matter may be of prune 

importance. Due consideration should be given in any particular 

process to the character of the organic matter in the effluent, and 

further treatment is advisable, not only where the discharge produces, 

or threatens to produce, an actual physical nuisance, but wherever 

the self-purifying powers of the stream will be appreciably drawn 

upon. In other words, stability is demanded in all effluents, unless 

the dilution is very great, not only in relation to the local discharge, 

but also in relation to all the sewage or effluent that the body of water 

in question may receive. This much is demanded from the standpoint of physical pollution alone. If, therefore, bacterial removal 

is also essential, disinfection is particularly satisfactory as a finishing 

process, because it can now be conducted at far less cost than the 

cheapest form of supplementary sand filtration.

. Comparison on a cost basis of the methods of chemical disinfection 

makes it apparent that chlorine in some form is the most efficient 

agent, though it must be admitted that the possibilities of heat and of 

certain organic compounds have not received adequate investigation. 

Calcium hypochlorite, or commercial bleaching powder, is by far the 

most satisfactory chlorine compound available. It has greater 

germicidal efficiency than equivalent amounts of free gaseous chlorine, 

chlorates, or perchlorates. It is equaled in efficiency by potassium 

and sodium hypochlorites, the products of certain electrolytic cells. 

The electrolytic production of hypochlorites or of free chlorine is not 

a satisfactory source of the disinfectant. The cost of such manufacture, on a scale necessarily small even at the larger sewage-disposal 

works, is at present so little below the cost of bleaching powder that 

no safe margin is left to cover the additional responsibility and uncertainty that are involved. Improved processes of manufacture may 

alter conditions somewhat, but the highest possible working efficiency 

and the cheapest power would not sufficiently reduce the costs to alter 

these general conclusions.

The application of 3 parts per million of available chlorine in the 

form of bleaching powder to a trickling-filter effluent similar to those 

on which experiments were made effects satisfactory disinfection. 

The removal of bacteria from the effluent averages over 95 per cent, 

making the removal for the whole purification process 98 to 99 per
72 DISINFECTION OF SEWAGE.

cent of the number in the crude sewage. The cost of disinfection 

ranges from $1 to $1.50 per million gallons of sewage, depending 

chiefly on the size of the plant. Effluents of higher degrees of purity 

can be disinfected at still lower cost. Five parts per million probably 

represents the maximum amount of chlorine required for the treatment of trickling-filter effluents of poorer quality. The results 

obtained with the amounts of disinfectant that are specified do not, 

of course, amount to complete sterilization, but they may reasonably 

be called "practical disinfection." Considerable additional cost is 

required to improve them but slightly.

The disinfection of crude sewage to the same final condition 

requires the removal of over 98 per cent of its total bacteria. This 

may be accomplished by the application of from 5 to 10 parts per 

million of available chlorine, the amount depending on the character 

of the sewage. Such disinfection costs from $1.50 to $3.50 per 

million gallons.

The disinfection of septic sewage requires the application of from 

10 to 15 parts per million of available chlorine. If no further purification is required than that given by septic action and by disinfection, it is advantageous to reverse the processes by disinfecting the 

crude sewage before it enters the tank. The resulting development 

of saprophytes within the tank has no sanitary significance, and it is 

doubtless of great advantage in the subsequent purification of the 

organic matter in the stream.

The removal of B. coli is usually somewhat more complete than 

that of the total organisms. Under the conditions of a laboratory 

experiment, the results of hypochlorite disinfection on typhoid and 

colon bacilli in tap water were identical. It seems reasonable to 

assume, therefore, that the viability of the typhoid organism under 

working conditions in practical sewage disinfection is at least no 

greater than that of the colon bacillus and no greater than that of the 

sewage bacteria as a whole. Consequently the disinfection results 

obtained with total bacteria may, in the case of chlorine disinfection 

at least, be referred directly to the typhoid bacillus with assurance of 

reasonable accuracy.

The results obtained in this investigation are so much more favorable than any results that have been reported for similar work that 

comment on their accuracy and general applicability seems justifiable. 

The more important portions of the work have been practically 

duplicated under as diverse conditions as possible and by different 

workers. There is no apparent reason for believing that the results 

are not of general applicability. The reactions involved are particularly free from interference on the part of the mineral constituents 

of normal sewage, a condition which has not been found where 

copper has been used as the disinfectant. The satisfactory results
CONCLUSION. 73

in the present work are largely due to the fact that many little 

difficulties inherent in new processes have been overcome by continuous work extending over considerable periods of time, and in 

particular to the fact that the experiments were made part of permanent laboratory routine and were free from temporary and special 

characteristics which usually involve discontinuity and interruption. 

This routine continued week after week without interruption and 

without undue attention in fact, just as it would naturally go on 

in practice. It is believed that the results represent what may be 

accomplished in practice and that they can be duplicated under 

working conditions on any scale which may be desirable.
PUTRESCIBILITY AND STABILITY OF SEWAGE 

EFFLUENTS.0

INTRODUCTION.

The development of modern rapid processes of sewage treatment, 

involving the use of coarse material, has resulted in a somewhat 

changed conception of the functions of sewage disposal, while the 

general introduction of contact and trickling filters in the newer and 

larger works has made it necessary to examine methods of sewage 

analysis from a new viewpoint. Certain hitherto important features 

of the analysis have assumed comparatively unimportant r61es, and 

new determinations have been developed on which the chief reliance 

is now placed. In the older methods of sewage purification, almost 

complete removal of organic matter and oxidation of the nitrogen 

were obtained, and the analytical methods employed in the control of 

such plants were designed to test their efficiency in accomplishing 

these ends. Consequently the determination of nitrogen in its 

different stages of oxidation and of organic matter in general were 

paramount. These determinations are only of minor importance in 

the practical control of modern rapid filters, where oxidation of 

nitrogen is incidental and removal of organic matter is but slight. 

Suspended solids, available oxygen, and the character of the effluent 

in reference to its stability now demand first consideration. As the 

production of a stable effluent is the primary function of such filters, 

the determination of stability becomes the most important analytical 

method in filter control. This point has been generally recognized, 

and incubation or putrescibility tests of one form or another are in 

general use, often at places where no further analyses are made. It 

unfortunately happens, however, that there are many different conceptions of what putrescibility really is and many different methods 

of determining it. Consequently statements of results lose much of 

their significance and comparisons are difficult or impossible. The 

present article is a review of the subject for the purpose of establishing the fundamental facts and of harmonizing current opinions. 

A method of determining stability, which has been in use by the 

writer for nearly three years, and a numerical method of expressing 

results, by which quantitative value is given to the test, are also 

presented.

a Investigation made at the sanitary research laboratory and sewage experiment station of the Massachusetts Institute of Technology. 

74
PTJTKESCIBILITY AND STABILITY OF EFFLUENTS. 75

PUTRESCIBILITY.

Putrescibility, as applied to organic matter in general, implies the 

ability of that matter to undergo offensive putrefactive decomposition. In a strict sense putrefaction is a term applied to nitrogenous 

matter only, though this is a popular rather than a logical conception. 

Exactly what constitutes offensive putrefactive decomposition in a 

sewage effluent is a matter on which opinions differ. Such decomposition is always anaerobic, and it is usually accompanied by the 

evolution of offensive odors. These two phenomena have, therefore, 

formed the basis of most putrescibility tests. Some criteria of 

putrefaction which have been employed are: (1) Development of 

offensive odors; (2) formation of black sediment; (3) reduction in 

the amount of dissolved oxygen; (4) loss of all dissolved oxygen; 

(5) loss of all available oxygen, including that of nitrates and nitrites; 

and (6) increase in the oxygen-consumed figure. Some of these 

tests are based on partial reduction of the available oxygen in the 

effluent; others depend on the complete reduction of the available 

oxygen and subsequent anaerobic fermentation. The tests most 

commonly employed belong to the latter group, depending on the 

production of odor or of hydrogen sulphide, blackening of the liquid, 

or reduction of organic dyes. The test which depends on an increase 

in the oxygen-consumed figure during incubation is also in that 

class, because anaerobic fermentation alone renders organic matter 

more readily oxidizable.

These two types of test illustrate two distinct points of view 

which should be clearly differentiated. An effluent may be regarded 

as being composed of a given mass of organic matter dissolved or 

suspended in a definite amount of water. The water contains also 

a definite amount of available oxygen in the form of free dissolved 

oxygen, nitrites, nitrates, and possibly of other compounds. All the 

organic matter is oxidizable to some extent, and to that extent it 

serves as bacterial food. The greater the amount of organic matter and the greater its oxidizability, the greater is the absorption 

of oxygen from the medium. Consequently a reduction of available 

oxygen in the effluent during incubation is a measure both of the 

amount of organic matter present and of its capability of oxidation. 

As a small amount of readily oxidizable matter has the same effect 

on the result as a larger amount of more stable matter, a test of 

this kind indicates whether or not the organic matter consumes 

oxygen; but it does not show whether or not the supply of available 

oxygen is sufficient to prevent the establishment of anaerobic conditions. This important question of the balance between the oxygen 

demanded by the organic matter and the oxygen available in the
76 PUTKESCIBILITY AND STABILITY OP EFFLUENTS.

liquid is taken into consideration by tests of the second kind mentioned, namely, those dependent on the establishment of anaerobic 

conditions. Such tests do not involve estimation of the amount 

and the kind of organic matter; indeed, organic matter which does 

not absorb any oxygen from the liquid under the conditions of an 

incubation test must be very highly oxidized; and, furthermore, 

most organic matter derived from sewage is putrescible in itself 

that is, if it is stored by itself in the absence of oxygen, it undergoes 

putrefactive changes. The question at issue is not, however, whether 

the organic matter itself will putrefy, but whether the effluent as a 

whole will become so reduced in oxygen that putrefaction will become 

possible. In other words, it is simply a question of a balance between 

the available oxygen of the effluent and the oxygen which the organic 

matter will require during the incubation period. It would seem 

that the problem might readily be solved by determining this balance, but, unfortunately, it is not a simple matter, because the 

action involved is bacterial. Many attempts have been made to 

determine the oxygen balance analytically, but such tests answer 

only with very good and very bad effluents, for which an inspection 

of the sample would serve just as well. When there is doubt about 

the character of the effluent the condition for which such information is of most value all such analytical procedures have heretofore 

failed. It is evidently impossible to imitate with any degree of 

precision the bacterial activities that are involved. There remains, 

then, but one satisfactory expedient: To let the reaction proceed 

by itself and to note the result. But here also there are difficulties, 

because bacterial reactions of this sort are necessarily slow in reaching equilibrium, and the time required by a nicely balanced effluent 

is greater than can be allowed in routine work. Some arbitrary 

period of time, therefore, is usually adopted, and it is in respect 

to this factor that the confusion rises. If stability is to be considered a definite qualitative characteristic of an effluent, that 

characteristic should be determined by a test sufficiently prolonged 

to insure equilibrium, but such procedure is not feasible for obvious 

practical reasons, and it is not desirable, because it is not enough 

simply to know that the available oxygen is sufficient or insufficient 

to satisfy the demands of the bacteria that are working on the 

organic matter. If the available oxygen is sufficient, there is perfect 

stability a definite condition; if it is insufficient, there is still stability in the quantitative sense a relative stability determined by 

the relation of the available oxygen to the total amount of oxygen 

required by the organic matter for perfect stability. In practice 

the latter condition is the one usually encountered.
PUTKESCIBILITY AND STABILITY OF EFFLUENTS. 77

RELATIVE STABILITY.

DEFINITIONS.

The term putrescibility has had so many and so varied meanings 

in dictionaries, in popular parlance, and particularly in the minds 

of water chemists, that it is proposed to employ the word stability 

for that desirable quality which is the usual object of sewage purification the transformation of the organic matter to such form that it 

is incapable of undergoing offensive putrefaction. This term has the 

added advantage of implying a positive characteristic that is acquired 

during purification, and it conveys a much more definite impression 

of the thing under discussion than the negative term putrescibility. 

A few more* definitions are necessary in order tro simplify the discussion. The time required to establish anaerobic conditions in an 

effluent which, on incubation in a closed bottle, is subject to bacterial 

activities producing such conditions may be called for brevity the 

reducing time; the total amount of oxygen initially present in the 

form of free dissolved oxygen, nitrites, nitrates, and possibly other 

combinations may be called available oxygen; the term oxygen 

required for equilibrium or simply required oxygen may be understood 

to express the total amount of oxygen which would be consumed by 

bacterial action in the effluent if the latter were supplied with an 

unlimited amount of oxygen and if the reaction were allowed to 

proceed to a condition of substantial equilibrium. An effluent of the 

character under discussion is not stable in the absolute sense, because 

its available oxygen is less than the oxygen required for equilibrium; 

but, of two such effluents, that one is obviously the better which 

contains the greater amount of available oxygen in proportion to its 

required oxygen. In other words, effluents of this class have a certain 

relative stability which is indicated by the ratio of the available 

oxygen to the required oxygen. This relative stability, as will be 

shown, can be measured by the time required to reach the anaerobic 

stage. The, term stability without qualification is employed in this 

paper to describe that condition in which the available oxygen exceeds 

the required oxygen, and the term relative stability is used to indicate the character of the effluent in the sense suggested. A perfectly 

stable effluent, therefore, has a relative stability of 100 per cent.

It is apparent that time is an important element in stability tests, 

and that it is not compatible with the idea of relative stability to 

select an arbitrary period of time for-establishing the line of demarcation between stability and putrescibility. It is obviously 

unfair to record one effluent as nonstable because it "holds up," or 

fails to putrefy, for only three days and to record another as stable
78 PUTEBSCIBILITY AND STABILITY OP EFFLUENTS.

because it "holds up" for four days. A filter might deliver during 

one week an effluent that would fail to pass a four-day incubation 

test by a narrow margin and might deliver during the next week 

almost crude sewage for four days and a passable effluent for three. 

Obviously, the first week's run would be the better and should be so 

recorded; but, under the present practice in many places, all the 

samples during the first week would be putrescible and 40 per cent 

of those during the second week would be nonputrescible. The first 

requisite, therefore, in logical study of the problem is that the time 

required for an effluent to reach a condition of anaerobic decomposition shall be taken as an index of its relative stability. This time 

element is absolutely indispensable, and any test that is adopted for 

the determination of relative stability should be of such a character 

that the length of time required for the sample to reach a given 

anaerobic condition may be recorded.

ESTIMATION OF THE REDUCING TIME.

Many tests have been devised to determine whether or not an 

effluent is putrescible, and a review of the subject with the details 

of the methods proposed has been given elsewhere.0 In any determination of the time required to exhaust the available oxygen in an 

effluent the following conditions must be fulfilled: (a) The sample 

must completely fill the bottle, the stopper of which must be tight 

and must not be removed during the test; (6) determinations must 

be made at a standard temperature; (c) observations must be made 

at least as frequently as once a day. If a test is employed which 

necessitates opening the bottle in order to observe the condition of 

the sample, one bottle must be incubated for each day that the effluent 

is under observation. This is true for any test which depends on a 

chemical determination of any constituent or which depends merely on 

the odor developed. Obviously such tests are not well adapted to the 

conditions heretofore stated. A simpler procedure is one in which 

the anaerobic condition can be detected by the appearance of the 

effluent without opening the bottle. The anaerobic fermentation that 

occurs immediately after the complete exhaustion of the oxygen 

is usually accompanied by a production of hydrogen sulphide. 

Consequently an indicator that is sensitive to hydrogen sulphide 

is advantageous in detecting the beginning of the anaerobic fermentation. Fortunately delicate indicators are available for this 

purpose, for certain organic dyes are readily reduced to corresponding leuco-compounds under anaerobic conditions. Methylene blue 

is an organic dye of this character, and it is reduced to its colorless 

leuco base by hydrogen sulphide, alkaline sulphides, and by the

- a Phelps, E. B., and Winslow, C.-E. A., On the use of methylene blue in testing sewage effluents: Jour. 

Infectious Diseases, Suppl. No. 3, 1907, p. 1.
BELATIVE STABILITY. 79

commercial reducers used in dyeing. Its employment for the study 

of stream pollution was first proposed by Spitta,0 and it was later 

more thoroughly investigated by Spitta and Weldert b as a test for 

sewage effluents.

Methylene blue, or tetra-methyl-thionin chloride, is a commercial 

dye of complex constitution, having the empirical composition 

C18H18N5SCL Merck's medicinal preparation is pure and it is preferable to the commercial article for sewage work. It is an extremely 

sensitive indicator for hydrogen sulphide and other reducing bodies, 

being decolorized at once in the presence of even small traces; its 

decolorization by bacterial action has been studied by many observers, 

the principal of whom are cited by Spitta and Weldert.c The 

technique of its use in testing a sewage effluent is extremely simple. 

One cubic centimeter of a one-tenth per cent aqueous solution of the 

dye is added to the effluent in a glass-stoppered bottle -of 250 cubic 

centimeters capacity, and the sample is then incubated either at 20° C. 

or at 37° C. The blue color of the solution remains practically 

unchanged till the available oxygen contained in it has been consumed 

and putrefactive conditions have been established. At this stage 

the dye is reduced and decolorized. The time required for such 

decolorization is, therefore, approximately the time required for the 

exhaustion of the available oxygen. The dye is an indicator for what 

may be called the oxygen neutral point, the point at which the available oxygen becomes exhausted and anaerobic conditions are established. Some studies d made at the sewage experiment station 

confirm the earlier conclusion of Spitta and Weldert, that the end 

point indicated by this dye is almost exactly the desired neutral 

point. The order in which different forms of oxygen are reduced 

appears to be: Dissolved oxygen, nitrates, nitrites, sulphates, and 

phosphates. Methylene blue was found to change color practically 

at the same time as the nitrites in this series. It is readily conceivable 

that an indicator might possess such properties that it would be 

reduced before the nitrates or even before the total exhaustion of the 

free oxygen. Another indicator might change only after the reduction of the sulphates. The fact of this varying end point has been 

well shown in a recent paper by Clark and Adams.e Comparative 

incubation tests were made with 17 dyes as indicators, only 6 of which 

were reduced during the incubation. Arranged in the order of their 

reducibility these are: Indigo carmine (sulphonated indigo), methyla Arehiv. fur Hyg., 1903, vol. 46, p. 113.

6 Spitta and Weldert, Mltteilungen aus der Kdniglichen Prufungsanstalt f iir Wasserversorgung und 

Abwasserbeseitigung zu Berlin,, vol. 6, 1906, p. 161.

« Loc. cit.

<J Phelps, E. B., and Winslow, C-E. A., On the use of methylene blue in testing sewage effluents: Jour. 

Infectious Diseases, Suppl. No. 3,1907, pp. 1-13.

« Clark, H. W., and Adams, G. 0., Studies in incubation tests: Jour. Am. Chem. Soc., vol. 30,1908, p. 1037.

76474 IRR 229 09 6
80 PTJTRESCIBILITY AND STABILITY OF EFFLUENTS.

ene green, and methylene blue; and then, order not stated, congo 

red, methyl orange, and tropseolin. The average time computed 

from 26 tests for the reduction of indigo carmine was 2 days; for 

methylene green the average time was 2.4 days; and for methylene 

blue, 3.9 days. As it is of course impossible to hasten or to retard 

the reactions that are taking place, these differences show differences 

in the end point recorded by the several indicators.

In the writer's experiments it was shown that the end point 

indicated by methylene blue is probably that point at which the 

free oxygen and the nitrates are practically exhausted and reduction 

of the sulphates is just beginning. This is understood to be the point 

at which anaerobic conditions are established. The work was not 

undertaken, however, for the purpose of determining the end point 

.accurately, and it is possible that the end point of methylene blue 

is a little too. far along and that either methylene green or indigotin 

indicates the desired point more closely. Just as it is essential in 

other branches of analysis to specify the indicator that shall be used 

in a given determination, in order to prevent confusion in comparative work, similarly it is important in this test to adopt a standard 

indicator as a basis of comparison. The results of all experiments 

thus far are in favor of methylene blue, and that dye is now widely 

and satisfactorily used in the laboratories of the country; consequently, its retention as a standard appears advisable, at least until 

further experimental evidence is available. The present series of 

comparative stability values is calculated on the assumption that 

methylene blue be used. A change in the final end point adopted 

would, of course, necessitate a remodeling of the computations.

THEORETICAL RELATION BETWEEN REDUCING TIME AND 

RELATIVE STABILITY.

The time required for complete exhaustion of the oxygen from 

an effluent the reducing time is obviously not a simple function 

of its relative stability; but there is a well-known theoretical relation 

between velocity of reaction and amount of reacting substance, 

from which it is possible to compute one from the other. It is a 

principle of physical chemistry that the velocity of a chemical reaction 

is a function of some power of the concentrations of the reacting 

substances. In the simplest cases the velocity varies directly as 

the concentration. The bacterial reactions that have been investigated also conform to this law,a and it has been, therefore, applied to

a Chick, H., An investigation of the laws of disinfection: Jour. Hyg., vol. 8,1908, p. 92.

Lubenau, C., Zur Saurebildung der Diphtheriebazillen: Arch. f. Hyg., vol. 66,1908, p. 305.

Famalener, W., and Madsen, T., Die Abschwiichung der Antigen durch Erwarmung: Bichem. Zeitung, 

vol 11, 1908, p. 186.

Nawiasky, P., Uber die Umsetzung von Aminosaureo durch B. proteus vulgaris: Arch, f. Hyg., vol. 66, 

1908, p. 209,
RELATIVE STABILITY. 81

the present study. The exact expression of the function need not 

be detailed, for it can easily be seen that this general law is approximately applicable; if one-half the work of oxidation is accomplished 

in one day, the availability of the organic matter as a food supply 

is reduced one-half, and the consequent bacterial activity on the 

organic matter is reduced accordingly; at the beginning of the 

second day food supply and bacterial activity are in the same relative proportions and the same relative amount of work is done again; 

that is, one-half of the remaining work of oxidation will be done 

during the second day. This law is expressed by the following 

equation:

log. x = log. a fc*

in which a is the amount of oxygen required for equilibrium at the 

commencement of the action, and x is the amount similarly required 

at the end of the time t, while Jc is a constant known as the velocity 

constant. If a grams of oxygen are required for initial equilibrium, 

and if x grams are required after the sample has been incubated in 

a tight bottle for a period of time which may be termed t days, 

(a x) grams of oxygen have been abstracted from the liquid by the 

organic matter. If the available oxygen of the liquid has just 

become completely exhausted at the end of t days, the value (a x) 

represents the amount of available oxygen originally present in the 

sample. It is not even necessary to know the actual amounts of 

oxygen, because the ratio of available oxygen to the oxygen initially 

required for equilibrium gives a relative stability factor that obviates 

the expression of the actual amounts. The ratio is

a x 

a

The value of this expression in terms of t and fc can be found by 

using the logarithmic equation given above:

 = 1-F 

a L ^

The second term of this equation, therefore, is equal to the ratio 

between the total available oxygen and the oxygen required for 

equilibrium, the ratio being expressed in terms of a constant and the 

reducing time. This ratio is the relative stability.

If a and x could be determined by analysis it would be possible to 

determine the value of fc by a few tests. It has already been shown, 

however, that there is no simple chemical method of determining 

the amount of oxygen which is consumed by the organic matter 

under natural conditions. A possible method involves actual 

measurement of the amount of oxygen absorbed by the liquid in a 

time sufficiently prolonged to insure virtual equilibrium. If, how-
82 PUTRESCIBILITY AND STABILITY OF EFFLUENTS.

ever, Jc is determined only approximately and if a series of values for 

(! &*) is obtained for all values of t, the terms of this series bear 

practically a constant relation to each other, even if Tc is varied considerably. The number expressing the relative stability is, in any 

case, a true index of the character of the effluent, independently of the 

further .requirement that it shall be the absolute ratio of the available 

oxygen to the required oxygen. If it is not this absolute ratio it 

stands in constant but unknown relation to it.

An indirect method has been devised for determining the value 

of & with a degree of accuracy which is ample for the present discussion. For this purpose the results of 2,649 separate stability 

tests have been analyzed. The nature of these samples and the 

manner of determining the reducing time t is described on pages 78 

to 80. It is sufficient to state that the reducing time was from one 

to twenty days in most of the tests, while many of the samples showed 

a relative stability of more than 100 per cent. As the samples which 

required a reducing time between one and twenty days had relative 

stabilities ranging from zero to 100 per cent, it was assumed for the 

purpose of approximating the value of Tc that one-half had values 

less than 50 per cent and one-half values greater than 50 per cent. 

Such assumption, of course, is justifiable only with a large number of 

.observations, but it is believed that it is sufficiently accurate in the 

present case. Inspection of the tabulated results showed that a 

stability of 50 per cent was attained, at a temperature of 20° C., in 

almost exactly three days, thus making

l-fc3 = 0.50; or fc = 0.794.

If this value of Jc is substituted in the equation before mentioned, 

the following values of the relative stability corresponding to the time 

t in days are obtained.

TABLE 1. Relation between reducing time and relative stability at "20° C.

Reducing 

time in days.

(«0

1

2

3

4

5

6

7

8

Relative 

stability.

(1-0.794*.)

21

37

50

60

68

75

80

84

Reducing 

time in days.

(«.)

9

10

11

12

14

16

18

20

Relative 

stability.

(1-0.794*.)

87

90

92

94

96

97

98

99

These values of relative stability are strict measures of the character of the effluent. An effluent which contains more than sufficient 

oxygen to establish stability should have a stability of 100; in other 

words, it would never reach the anaerobic stage. In practice it is 

necessary to set some time limit to the tests and give an average
RELATIVE STABILITY. 83

stability value to all tests exceeding this limit. This value is sufficiently high to indicate the character of the effluent. On the 

other hand, crude sewage containing a little dissolved oxygen is 

completely reduced in one or two hours, or, if it contains no dissolved 

oxygen, decolorizes methylene blue at once; in numerical expression its relative stability is practically zero. These figures are comparative, because they may be added and divided to obtain periodical 

averages and the averages thus obtained are properly weighted. 

This is not so if the reducing times themselves are averaged. The 

figures are also an approximate measure of the ratio between the 

total available oxygen in the effluent and the amount of oxygen 

required for the production of stable equilibrium in the organic 

matter, and it is believed that the approximation is sufficiently close 

for ordinary purposes of interpretation.

DETERMINATION OF RELATIVE STABILITY. 

INCUBATION PERIODS.

Relative stability as previously defined is a numerical measure of 

the relation between available oxygen and required oxygen and it is 

also a function of the reducing time. Some practical applications of 

the stability values in Table 1 remain to be outlined.

Incubation periods exceeding five days are inconvenient and 

probably unnecessary in practical work, but in experimental work 

where more detailed knowledge is desired, longer periods may be 

used to advantage. At the sewage experiment station, a fourteenday period has been adopted. During the past two years tests have 

been made of more than 2,600 samples of trickling-filter effluents of 

such a quality that most of them were near the border line between 

satisfactory and unsatisfactory effluents. Most of them were on the 

safe side, but some of them were unsatisfactory for considerable 

periods. The results of this large number of tests, therefore, constitute an admirable basis for studying the advisability of using shorter 

periods of incubation. In the summary of the results in Table 2 

the tests are divided first in ten groups corresponding to the different 

filters and the years during which the tests were made. Effluents 

A, B, C, and D are from trickling filters before sedimentation; E and 

F are the same effluents after two-hour sedimentation. The results 

in each group are subdivided in order to show the per cent of the 

number of samples that retained available oxygen at the end of 

stated periods. The results shown in this table have been platted 

and from the plats the number of samples which would have retained 

available oxygen after twenty days have been determined by exterpolation from a plat of the results for shorter periods of time.
84 PUTRESCIBILITY AND STABILITY OF EFFLUENTS.

TABLE 2. Summary of stability tests of trickling-filter effluents, showing time required 

to exhaust the available oxygen. 0

Effluent.

A..........

B. .........

C. .........

P.. ........

A..........

B. .........

C. .........

D...... ...

E. .........

F. .........

Period.

H906-7 

1907-8

Number of 

samples 

tested.

388 

388 

388 

388 

223 

225 

212 

189 

38 

210

Per cent of samples retaining oxygen after 

1 

day.

91 

93 

97 

97 

38 

39 

29 

17 

50 

20

2 

days.

83 

87 

91 

93 

32 

31 

19 

5 

37 

14

3 

days.

75 

81 

86 

91 

28 

25 

12 

1 

21 

10

4 

days.

67 

76 

83 

88 

25 

22 

6 

1 

11 

6

6 

days.

58 

67 

78 

84 

22 

14 

5 

0 

5 

3

8 

days.

54 

62 

74 

81 

20 

9 

4 

0 

5 

2

10 

days.

48 

58

72 

77 

17 

6 

3 

0 

2 

1

12 

days.

45 

56 

70 

74 

15 

5 

3 

0 

0 

1

14 

days.

42 

53 

70 

73 

15 

4 

2 

0 

0 

1

206 

days.

35 

45 

69 

71 

14 

3 

1 

0 

0 

1

a Indicator, methylene blue; temperature of incubation, 20° C. 6 Values exterpolated.

If it is assumed that in practice an incubation period of four days 

at 20° C. will be employed, it is possible to show from the data ir 

Table 2 what relative stability values should be assigned to samples 

that retain some available oxygen after four days. Such samples 

may be divided into two hypothetical groups, namely, those which 

would have lost their oxygen between four and twenty days and 

those which would have retained it more than twenty days. Obviously a relative stability of 100 may be assigned to the latter because 

the value at twenty days is 99. An average time of ten days ma}T 

be selected for the period between four and twenty days, thus making proper allowance for the decreasing number of samples, which 

reduce each day, and a relative stability of 90 may be assigned to that 

group. Between 43 and 63 per cent of the samples which passed the 

four-day period were reduced before twenty days. This per cent 

varied with the quality of the effluent, but it may be taken as 50 per 

cent without introducing an error of more than 5 per cent in the final 

result in any group. For practical purposes it may be assumed that 

one-half of all tests passing a four-day period of incubation will be 

reduced before twenty days, or in an average time of ten days, anc1 

that one-half will exceed twenty days. This gives 95 for an average relative stability value of the whole number of samples. If a 

four-day period of incubation at 20° C. is adopted, each sample 

reduced on the first, second, third, or fourth day may be recorded 

as having a relative stability of 20, 37, 50, and 60, respectively, and 

all samples retaining available oxygen after four days may be giver 

a relative stability of 95. An individual result obtained in tmV 

manner will have but little accuracy, and when only a few tests are 

made, an incubation period of at least 10 days should be employed. 

On the other hand, when daily tests are made, the method outlined 

will give for monthly periods average results that are very close tc 

the truth. Relative stability values calculated from the data ir
RELATIVE STABILITY. 85

Table 2 for the entire twenty-day period of incubation are compared in Table 3 with similar values obtained by the shorter, four-day, 

method. The agreement is satisfactory. If smaller numbers of 

tests are considered, the distribution naturally will not be so regular 

and greater errors will be introduced in the results calculated. In 

order lo determine the accuracy of the proposed method, the first 

776 samples of Table 2 have been subdivided into twelve quarterly 

groups, containing from 36 to 78 samples in ea.ch group, and the 

relative stability values have been calculated for these smaller 

groups, as shown in Table 4.

TABLE 3. Comparison of relative stability results obtained from the twenty-day incubation 

period shown in Table 2 with those calculated from a four-day period.

Effluent.

A.... ............................................................ --.-.-

B..................................... .................................

E............................................................. -.......-

F.. .....................................................................

A.... ..................................................................

B..................... -----.....------......-.---..-..-.---.....-----..

p

D... ......................-...........-.--..........----..---.-----.--.

E.................... ..................................................

P.......................................................................

Period.

ll906-7 

1907-8

Eelative stability.

20-day 

period.

( 76 

81 

] 88 

( 86 

43 

39 

31 

25 

38 

28

4-day 

period.

77 

82 

86 

84 

43 

41 

31 

25 

39 

28

TABLE 4. Comparison of relative stability results obtained from smaller numbers of 

samples by the use of four, six, and twenty day incubation periods.

Effluent.

A........ 

B........

Quarter.

1906. 

First ........................................

Fourth .....................................

1907. 

First .......................................

1906. 

First.......................................

Third......................................

Fourth. ....................................

1907. 

First.......................................

Number 

of 

samples 

averaged.

37 

73 

59

77

74 

68

36 

71 

59 

78

75 

69

Eelative stability based on a 

4-day 

period.

59 

57 

64 

89

92 

91

62 

75 

81 

86

91 

91

6-day 

period.

59 

53 

61 

88

92 

91

61 

69 

79 

84

90 

90

20-day 

period.

58 

52 

60 

88

92 

92

59 

69 

82 

90

95 

93

The maximum error with the smaller groups is about 10 per cent, 

and this error can be materially reduced by basing the calculation 

on a somewhat longer incubation period. The second column in 

Table 4 shows the relative stability values calculated from a six-day
86 PUTRESCIBILITY AND STABILITY OF EFFLUENTS.

period, and the maximum error in this computation is less than 5 per 

cent. If the greater number of the samples of a well-purified effluent 

retain oxygen after four days, the results with any short method of 

calculation will be low. Since a value of over 95 can not be obtained 

with this method, it is obviously impossible to provide a four or six 

day method which will give accurate results. If more accurate information is necessary, longer periods of incubation perhaps as long as 

ten days are advisable. It matters little, however, whether the relative stability is 95* or 99, as far as practical results are concerned, 

because either value represents a high degree of purification.

EFFECT OF TEMPERATURE ON THE REDUCING TIME.

It has been assumed in the foregoing discussion that the temperature of incubation is 20° C., and there are certain reasons why this 

temperature is better than higher ones. It probably represents 

more nearly than any other the average temperature of streams, 

and results depending on bacterial activity should be obtained near 

the normal temperature to which the bacteria are exposed in nature. 

Incubations at 37° C., for example, probably cause the development 

of a class of bacteria quite different from those normally at work. 

The most serious objection to higher temperatures, however, is the 

fact that certain effluents, particularly those from the rapid filters 

which are in such common use, contain a large amount of dissolved 

oxygen, frequently 8 or 10 parts per million, and this represents the 

greater part of the available oxygen. At 20° C. 9 parts per million 

of oxygen is the saturation point. The saturation point at 37° C. 

has not been determined, but it is not over 7 parts per million, so 

that there is a tendency for some of the dissolved oxygen to escape 

at high temperatures. Tight stoppers will not prevent the escape 

of this released gas unless great care is taken with them, and 

mercury seals are hardly adapted to routine work. It is therefore 

practically impossible properly to maintain the necessary saturation 

conditions at a temperature higher than 20° C. A variation of 

temperature with season would have certain advantages, but such 

adjustment is rather impracticable. Since results are frequently 

obtained at 37° C., the relation of such results to those obtained at 

20° C. has been investigated and has already been reported. a Variable relations were found, as was expected, but the average results 

may be used with some degree of accuracy. It was found that the 

time of reduction at 37° C. was from 37 per cent to 72 per cent of 

that required at 20° C., and the mean of 20 determinations was 

exactly 50 per cent. Carefully fitted glass stoppers were used in 

the bottles, and ordinary precaution was taken to prevent loss of

a Phelps, E. B., and Winslow, C. E.-A., loc. cit.
KELATIVE STABILITY. 87

oxygen. If haste is necessary, a temperature of 37° C. will give 

results in about one-half the time required at 20° C. But the use 

of a temperature of 20° is strongly recommended whenever it is 

possible to employ it.

SUMMARY OF METHOD.

Samples should be collected in glass-stoppered bottles of 150 or 

200 cubic centimeters capacity. No special precautions are necessary in collecting samples of ordinarily good effluents that are fairly 

high in dissolved oxygen. If the dissolved oxygen is low, precautions similar to those used in collecting dissolved oxygen samples 

should be observed. A one-tenth per cent solution of methylene 

blue, preferably Merck's medicinal quality, is used as indicator. One 

cubic centimeter of this solution is added to each of the samples, 

which are then incubated, preferably at 20° C., for four days, and 

observations are made at least once a day. The samples in which the 

methylene blue becomes decolorized are recorded as having a relative stability corresponding to the time required for reduction. 

Those that are blue at the end of four days are given a relative 

stability value of 95.

Table 5 gives the relation between time for reduction and the relative stability. Though the figures up to four days are the only 

ones required, the entire series up to twenty days is given for 

comparison. Relative figures for incubation at 37° C. also are given, 

but the use of that column is not recommended, except when it is 

absolutely necessary to use the higher temperature, or when it is 

desired to convert results to a standard basis of 20° C.

TABLE 5. Relation between relative stability and reducing time at 20° and at 37° C.

Relative stability numbers."

£20-

0.5 

1.0 

1.5 

2.0 

2.5 

3.0 

4.0 

5.0 

6.0 

7.0

<37.

0.5

1.0

1.5 

2.0 

2.5 

3.0 

3.5

«.

11 

21 

30 

37 

44 

50 

60 

68 

75 

80

Relative stabilty numbers."

te>.

8.0 

9.0 

10.0 

11.0 

12.0 

13.0 

14.0 

16.0 

18.0 

20

*37.

4.0 

4.5 

5.0 

5.5 

6.0 

6.5 

7.0 

8.0 

9.0 

10

S.

84 

87 

90 

92 

94 

95 

% 

97 

98 

99

a s=Relative stability or ratio of available oxygen to oxygen required for equilibrium. Expressed in 

per cent.

tso=Time in days to decolorize methylene blue at 20° C. 

<37=Time to decolorize at 37° C. 

Theoretical relation 

8=100 (1-0.794 « M) 

=100 (1-0.630 W)
88 PUTEESCIBILITY AND STABILITY OP EFFLUENTS. 

INTERPRETATION OF RESULTS.

The arbitrary tests for putrescibility now employed require no 

interpretation. An effluent is either putrescible or nonputrescible, 

according to whether it is on one side or the other of a certain line 

of demarcation. The fact that the dividing lines, for there are 

many of them, are perfectly arbitrary and have no real significance 

seems to have been overlooked. Fixed arbitrary standards in sewage 

analysis are undesirable in that they relieve the analyst of the important and difficult duty of interpreting his own results in the light of 

his own peculiar environment. Some general observations and rules 

may be stated for aid in interpretation, but it should always be 

borne in mind that such interpretations are dependent as much on outside conditions as on the analyses themselves. The mere statement, 

therefore, that 75 per cent of the samples of a tested effluent were nonputrescible has no bearing whatever on the broader phases of interpretation. These facts are well known and accepted in ordinary 

analytical features. The aim of the present investigation has been 

to. place the important determination of stability on the same basis. 

Under this proposed method of determining relative stability an 

interpretation of the information in hand is rendered possible. A 

relative stability of 75 per cent means that the effluent in question 

contains a supply of available oxygen equal to 75 per cent of the 

amount of oxygen which the effluent will eventually require before 

it will have become perfectly stable. The amount of this available 

oxygen is estimated fairly well by the chemical determination of 

dissolved oxygen and nitrates. The nitrites are usually so low that 

they are negligible, and it is unnecessary to decide whether or not 

the nitrates represent available oxygen, because they have been 

included in the test and must be considered in the interpretation. 

Undoubtedly the nitrates will not be used in the stream until the 

dissolved oxygen of the water has been reduced to a low point. 

Nevertheless, the fact remains that the available oxygen in the 

effluent, including the nitrates, is 75 per cent of that required for 

equilibrium, and that the remainder must come from the water of 

the stream, which must also supply enough additional oxygen to 

replace that which may be abstracted from the nitrates of the effluent, 

if aerobic conditions are to be maintained. Analyses of water from 

the stream and estimates of the relative volumes of the stream and 

the effluent complete the data necessary for a full interpretation.

In general, effluents having a relative stability greater than 90 per 

cent may be discharged into any stream without danger of their 

consuming any of the oxygen of the water, because effluents of such 

high stability will retain oxygen indefinitely on exposure to the air.
INDEX.

A. Page.

Acids, use of, for disinfection................ 16-17

Alkalinity, effect of, on copper sulphate

process........................... 31

Amines process, description of............... 32-33

B*.

Bacteria, fate of, in purification.............. 8-12

purification by.......................... 7

Bacillus coli, persistency of.................. 10,

12,22-23,36,62-63,72 

tests for, records of...................... 37

Bacillus pyocyaneus, persistency of......... 12

Bacillus sporagenes, persistency of........... 10

Bacillus typhus, persistency of............ 9,62-63

Bacteria, sedimentation of................... 45-46

Bacteriological methods, description of...... 35-36

Baltimore, Md., experiments at........ 12,14,56- 0

Baltimore sewerage commission, cooperation

of............................... 35,56

Bengal, India, experiments at............... 24-25

Berlin, experiments at................... 19,21-24

Birge, E. G., work of........................ 35

Bleaching powder. See Chloride of lime.

Boston, investigations at.................... 37-61

investigations at, character of............ 37-38

Brewster, N. Y., experiments at............. 27

C.

Calmette, A., experiments of................ 11

Carpenter, W. T., work of................... 35

Chemical disinfection, cost of................ 14

nature of................................ 70-71

Chesapeake Bay, pollution of................. 13-14

Chloride of lime, amount needed of......... 38-39,

40,43,46,48-51,59, C3-65 

amount needed of, relation of tempera-

. tureto.......................... 43

contact period needed for............... 38-39,

45^16,48-51,64-65 

disinfection by, cost of............. 65-70,71-72

effect of, on colon and typhoid bacilli.... 62-63

evenness of, results with........... 43^5,58-59

experiments with....................... 38-51

penetration of solid particles by......... 22-23

price of.................................. 65-66

use of, as disinfectant................. 18-24,71

Chlorine, electrolytic manufacture of......... 25-29

exhaustion of............................ 62

germicidal action of................... 17,60-62,

use of, as disinfectant.............. 17-29,34,71

Chlorine, available, definition of............. 18

Chlorine cells, description of................. 69

Chlorine compounds, germicidal action of.... 60-62

Chlorine gas, germicidal action of............ 60-61

production of............................ 17-18

Page. 

Chloros, nature of........................... 19,67

Clark, H. W., and Adams, G. O., on reducing

time............................ 79

Columbus, Ohio, experiments at......... 11,29-31

Contact-filter effluents, bacteria in........... 11

effect of chloride of lime on.............. 25

effect of copper sulphate on.............. 30,32

Contact period, length of, in chloride of lime

process.................... 38-39,44-45

length of, in copper sulphate process..... 30

Copeland, W. R., and Johnson, G. A., experiments by....................... 29-31

Copper, use of, for disinfection............ 29-32,34

Copper sulphate, disinfection by, relation of

temperature to.................. 30,43

Cowles, R. P., work of...................... 35

Crawford, H. S., work of.................... 34

Crude sewage. See Sewage.

D. 

Daniels, F.E., work of.................... 7,34,51

Delaware River, pollution of................ 14

Dibdin, W. J.,experiments of............... 19

Digby, W. P., experiments of............... 67

Disease, effect of filter purification on........ 10

Disinfection, degree of....................... 24

applications of.......................... 63-65

conclusions on........................... 70-73

costs of.................................. 65-70

methods of.............................. 15-34

classification of...................... 15

See also particular methods. 

necessity for............................. 12-15

See also Purification; Stability. 

Drinking water, purification of, responsibility

for........................... 11-12,70

Drown, T. M., on electrolytic process........ 26

D unbar and Korn, experiments on.......... 19

Dunbar and Zirn, experiments of............ 19

E.

Eau de Javelle, nature of.................... 19

Electrolytic chlorine processes, cost of. 66,69-70,71

description of..................... 25-29,67-69

. Elsner and Proskauer, experiments of....... 19

Emerson, C. A., work of..................... 35

Experimental Investigations, account of..... 34-65

expression of............................ 36-37

F. 

Fermi, experiments of....................... 27

Filter effluents, effect of chloride of lime on.... 25

effect of copper sulphate on.............. 30-32

effect of sodium benzoate on............. 33

Filtration, effect of, on bacteria.............. 9

Fuller, G. W., on oyster crop................ 13
90 INDEX.

G.

Gelatine, penetration of, by chloride of lime.. 22-23 

Germany, chemical disinfection in........... 14

H.

Hamburg, experiments at................... 19-21

Heat, use of, for disinfection.............. 15-16,33

Hermite process, description of.............. 27-28

Hospital sewage, experiments on............ 20

Houston, A. C., experiments of............ 9-10,12

Hypochlorites, use of, in disinfection........ 18-25

I.

Ivanofl*, investigations of. 16

J.

Jackson, D. D., bile broth test of............ 11,35

Johnson, G. A., experiments of.............. 11

Johnson, G. A., and Copeland, W. R., experiments by......... j.............. 29-31

Kellerman, K. F,, and Moore, G. T., experiments of........................ 29

Kellerman, K. F., Pratt, R. W., and Kimberly, A. E., experiments by. 25,31-32 

Kitasato, investigations of. ..*._.............. 16

Klein, E. E., experiments of............. 15-16,33

Korn and Dunbar, experiments of .......... 19

Kranepuhl and Kurpjuweit, experiments of. 21-22

L.

Labarraque's solution, nature of............. 19

Lillis, M. H., work of........................ 35

Lime, use of^ for disinfection................ 16

M.

MacConkey, Alfred, experiments of.......... 9

McDonald Electrolytic Cell Co., aid of...... 35

McLintock, experiments of.................. 27

McRae, H. C., work of...................... 35

Marion, Ohio, experiments at............... 25

Massachusetts Institute of Technology, cooperation of........................ 34

Moore, G. T-., and Kellerman, K. F., experiments of........................ 29

N.

New Jersey State Sewerage Commission, cooperation of..................... 34

O. 

Ohio, experiments in........................ 25

Organic matter, effect of, on copper sulphateprocess .......................... 31

Oxychloride process, description of.......... 28-29

Oxygen, available, definition of............. 77

Oxygen, required, definition of.............. 77

Ozone, use of, as disinfectant................ 17

P.

Pathogens, fate of, in purification............ 8-10

Permanganate of potash, use of, for disinfection. ............................ 32

Phelps, E. B., work of...................... 7

Pickard, experiments of..................... 9

Plainfleld, N. J., experiments at........:.... 11-12

Page. 

Potassium chlorate, germicidal action of..... 60-61

Potassium hypochlorite, germicidal action of. 60-61 

Potassium perchlorate, germicidal action of.. 60-61 

Pratt, R. W., Kimberly, A. E., and Kellerman, K. F., experiments by.. 25,31-32 

Pritzkow, A., and Thumm, K., experiments

of............................... 11

Proskauer and Elsner, experiments of....... 19

Purification, agents of....................... 7-8

essentials of............................. 12,74

significance of term...................... 8

speed of................................. 7-8

improvement in..................... 7-8

See also Stability; Disinfection. 

Putrefaction, prevention of ................. 8

Putrescibility, arbitrary standards of........ 88

control of................................ 74

tests of............................... 75-76,88

R. 

Red Bank, N. J., experiments at............ 51-56

sewage-disposal works at................ 52

plate showing....................... 52

Reducing time, definition of................. 77

estimation of...................... 78-80,83-86

stability and, relation of................ 80-83

temperature and, relation of............. 86-87

Relative stability. See Stability, relative. 

Rideal, Samuel, investigations of.......... 9,16,28

Sand-filter effluent, bacteria in.............. 11

effect of chloride of lime on.............. 25

eftect of copper sulphate on.............. 30-32

Sand nitration, effect of..................... 70

Schumacher, experiments of................. 19-20

Schwarz, investigations of..............:.... 20-21

Septic effluent, chemical composition of..... 54

disinfection of, cost of................... 55-56

effect of chloride of lime on.. 24-25,51-56,64,72 

effect of contact filter on................. 11

effect of oxychloride on.................. 28-29

effect of sand nitration on............... 11

effect of trickling filter on................ 11

treatment of, by chloride of lime......... 24-25

Septic tank, effect of........................ 9

Sewage, crude, disinfection of................ 46-47

disinfection of, cost of.................... 55-56

effect of chloride of lime on........'...... 19-

22,23,26,46-51,64,72 

effect of contact filter on................. 11

effect of copper sulphate on.............. 30-32

effect of oxychloride on.................. 28-29

effect of sand filter on................... 11

effect of trickling filter on................ 11

experiments on, in Germany............ 23-24

sedimentation of......................... 47

superabundance of, effect of, on streams. 8 

treatment of, by chloride of lime......... 23-24

Shellfish beds, protection of.............. 13-14,70

Sodium benzoate, use of, for disinfection..... 33

Sodium hydroxide, germicidal action of...... 60-61

Sodium hypochlorite, germicidal action of... 60-61 

Sodium permanganate, use of, in disinfection. 19,32 

Soil bacteria, purification by................ 7

Sprinkling filter effluent, effect of copper sulphate on........................ 30
Stability, definition of..........

necessity for................

relation of time and........

Stability, relative, definition of. 

determination of...........

Streams, purification of.........

Stutzer, investigations of.......

INDEX. 91

...... 77,88

...... 74

77-78,80-83 

... 77,85-86 

...... 83-86

...... 8,13

...... 16

T.

Temperature, effect of, on disinfection....... 43

effect of, on reducing time...... ........ 86-87

Thumm, K., and Pritzkow, A., experiments

of............................... 11

Tonnetta Creek, N. Y., experiments on...... 27

Trickling-filter effluents, bacteria in......... 11

chemical composition of................. 40

effect of chloride of lime on.............. 38-

46,56-60,63-65,71-72

Page.

Trickling-filter effluents, effect of copper sulphate on........................ 30-31

effect of sodium benzoate on............. 33

V. 

Vibrio, persistency of........................ 21

W. 

Walbrook, Md., testing plant at............. 56-57

Webster process, description of.............. 26-27

Weldon chlorine process, description of...... 17-18

Whipple, Leyland, work of.................. 35

Whitman, E. B., work of.................... 7,35

Wollheim, H., process of.................... 32

Woodhead, Sims, investigations of.......... 9

Woolf process, description of................ 27

Worcester, Mass., acid in sewage of.......... 17

Z.

Zirn and Dunbar, experiments of............ 19

o