Sewage treatment

The sewage treatment process combines physico-chemical clarification using fine material or clay particles with anaerobic digestion of a concentrated sewage. The particles are preferably magnetite. The sewage (10) is mixed with magnetite (12) having a hydroxylated surface layer in tanks (14, 16, 18, 20) in which acid, inorganic flocculant and polyelectrolyte may be added if necessary. The organic material in the sewage is adsorbed on the particles during the mixing contact and a clarified liquid effluent (28) is separated from the loaded particles in a clarifier (22). The organic material may be separated from the particles before or after treatment in an anaerobic digestion assembly (36). The particles are preferably cleansed and regenerated in a two stage countercurrent flow washing system (38, 40, 58, 60, 62) into which a dilute solution of caustic soda or lime (32) is introduced.

This invention relates to a new process for sewage treatment. The process 
of the invention combines physico-chemical clarification using fine 
mineral or clay particles with anaerobic digestion of a concentrated 
sewage. 
Sewage can be regarded as the water-borne waste products of man. Ever since 
man has gathered himself into large communities, the effective treatment 
and disposal of sewage has been a matter for concern. Initially, the 
problem was solved by discharge into tidal waters or an inland 
watercourse. However, with an increase in population density, this option 
became unworkable and some form of treatment became necessary. 
The main aim of sewage treatment is to greatly reduce both the biological 
oxygen demand (BOD) of the sewage and the number of pathogenic organisms. 
Recently, the removal of inorganic nutrients (phosphorus and nitrogen) has 
also become important. Historically, both clarification in settling tanks 
and biological oxidation have been used. Biological oxidation can be 
achieved in lagoons, trickling filters or activated sludge plants, the 
particular process chosen depending on such factors as land availability, 
sewage strength and power costs. In an industrialized economy, the 
activated sludge process is usually selected because of smaller land 
requirements and the relative cheapness of power. 
Up to date capital and operating costs for an activated sludge plant are 
difficult to obtain, but some idea of costs can be gained by taking 
historical costs and updating to the present. Analysis of cost information 
for the years 1968 to 1977 shows that, when indexed to 1986, a direct 
operating cost of approx. 10.cent./m.sup.3 is obtained, while the capital 
cost of a 38 ML/day plant is estimated to be A$16.times.10.sup.6. 
Amortization of capital cost over 25 years at 15% p.a. results in a 
contribution to total treatment cost of approx. 20.cent./m.sup.3. Total 
treatment cost of the sewage is thus of the order of 30.cent./m.sup.3. 
With industrialization, sewage flows and, as a consequence, total costs 
have increased dramatically. While a large fraction of these costs can be 
attributed to the collection system (drains, sewers etc.), treatment costs 
are still a significant fraction of the total, especially when more 
stringent effluent standards are enforced. As a consequence, there is 
considerable incentive to reduce treatment costs, and recent efforts to 
apply anaerobic techniques to sewage treatment have had this aim. 
The invention seeks to provide a process for the treatment of domestic 
sewage, which will significantly reduce total treatment costs. While 
conventional processes generally rely on aerobic biological oxidation, 
this process proposes to use a combination of efficient physico-chemical 
clarification with rapid anaerobic biological digestion. The reduction in 
treatment costs can be achieved by greatly reducing capital costs, while 
limiting operating costs to below those for the conventional process. As 
capital costs represent about two thirds of the total treatment cost, a 
significant cost reduction should result. 
The first stage of the present process involves physico-chemical 
clarification of raw sewage or primary settled sewage. This part of the 
process derives from the so-called "Sirofloc" process for water 
clarification, various aspects of which are described in Australian 
Patents Nos. 512,553, 518,159 and 550,702, the full texts of which are 
hereby incorporated into the present specification. Basically, the 
Sirofloc process provides rapid and efficient removal of suspended 
impurities and coloured substances (turbidity and colour colloids) from 
water by treatment of the water with a finely divided particulate mineral 
or clay material (referred to as a "coagulant/adsorbent") the individual 
particles of which have a particle size of 10 microns or less and have a 
thin hydroxylated surface layer. A positive zeta potential on the surface 
of the coagulant/adsorbent particles is not considered necessary for the 
present process. 
Operation of the Sirofloc process is often enhanced by the addition to the 
water under treatment of a suitable coagulant, such as alum or ferric 
chloride, and/or a polyelectrolyte (cationic, anionic or non-ionic). The 
preferred coagulant-adsorbent in the process is magnetite. Apart from the 
ease with which the required hydroxylated surface layer can be formed (and 
regenerated) on magnetite, the magnetic properties of the mineral can be 
usefully employed to aid in separation of the coagulant/adsorbent from the 
treated water. 
We have found that there is a strong tendency for sewage organics to 
co-flocculate with coagulant/adsorbents such as magnetite. The rapid 
adsorption and flocculation with such materials contrasts strongly with 
the long contact times and slow clarification of the activated sludge 
process. 
We have also found surprisingly, that the coagulant/adsorbents can adsorb a 
significant fraction of the soluble chemical oxygen demand (COD) present 
in sewage as well as removing suspended and colloidal material. The 
ability of the coagulant/adsorbents, and particularly magnetite, to remove 
soluble organic material is contrary to expectations from the studies that 
have been made of the Sirofloc process, and it enables a high degree of 
clarification of the sewage to be achieved. 
Anaerobic digestion of concentrated sewage sludges has long been practised 
as a means of stabilizing the sludge prior to disposal, and in recent 
years there has been a strong resurgence of interest in the anaerobic 
treatment process. However, although raw sewage can be treated by 
anaerobic digestion the quality of the treated sewage is frequently 
unacceptable. The anaerobic digestion of raw sewage at ambient temperature 
is slow, especially in cold climates where the process may be inoperable. 
Moreover, the large volume of dilute material to be treated generally 
makes it impractical or uneconomic to carry out anaerobic digestion of raw 
sewage at elevated temperatures in order to accelerate the digestion and 
reduce the residence time required. For example, insufficient methane is 
produced in the anaerobic fermentation of raw sewage to serve as fuel for 
heating the large volume of dilute material, and the use of supplementary 
fuels is generally prohibitively expense. 
The present invention overcomes these limitations by employing 
physicochemical clarification to provide an effluent of high quality while 
simultaneously concentrating the organic material in the sewage into a 
sludge suitable for rapid anaerobic digestion. 
According to the present invention, there is provided a sewage treatment 
process which comprises the steps of: 
(a) mixing raw sewage or primary settled sewage with a coagulant/adsorbent 
which is a finely divided particulate mineral or clay material the 
individual particles of which have a thin hydroxylated surface layer, 
under conditions whereby at least a substantial proportion of the organic 
material in the sewage becomes attached to the coagulant/adsorbent; 
separating the coagulant/adsorbent with attached organic material from the 
mixture to leave a treated liquid effluent; and 
(c) subjecting the thus separated organic material to anaerobic digestion. 
As will appear from the following discussion, step (c) may be carried out 
while the organic material is still attached to the coagulant/adsorbent; 
or preferably, the organic material may be removed from the 
coagulant/adsorbent before anaerobic digestion. 
The anaerobic digestion may be performed in any known type of digester 
suitable for mesophilic or thermophilic digestion. 
The coagulant/adsorbents which may be used in accordance with the present 
invention may be of two notionally different types, i.e.: (I) those in 
which the hydroxylated layer is derived directly from the substance of the 
particles; and (II) those in which the layer is derived from another 
substance. 
The preferred coagulant/adsorbent materials are those of type I and these 
can be derived from a wide variety of minerals and clays provided the 
nature of the mineral is such as to permit the ready formation of the 
hydroxylated surface. In this respect oxides and silicates are 
particularly useful. Examples of such minerals include zinc oxide, silica 
and siliceous materials such as sand and glass and clay minerals such as 
mica, china clay and pyrophillite. This list is not exhaustive, however, 
and many other minerals are suitable for use in this invention. 
In the most preferred embodiment of this invention, the particulate 
material is a magnetic or magnetisable material. For this purpose iron 
oxides, such as gamma iron oxide or magnetite, which are eminently 
suitable, or ferrites, such as barium ferrite or spinel ferrite, can be 
used. 
The coagulent/adsorbent particles should have a particle size of 20 microns 
or less, preferably 1 to 10 microns. It is believed that the optimum size 
range for the preferred method of operating the process is 1 to 5 microns. 
The preparation of finely divided coagulant/adsorbent particles of type I 
to give each a thin hydroxylated surface layer is easily carried out, 
usually by suspending the particles in a basic, preferably an alkali, 
solution for a short period of time, preferably in the presence of air. 
Sodium hydroxide is suitable, but potassium hydroxide, lime or aqueous 
ammonia may also be used. Generally, alkali concentrations should be at 
least 0.01N, preferably about 0.05N to 0.1N, at which level the treatment 
is effective after about 10 minutes. Shorter treatment times can be 
achieved by the use of elevated temperatures and/or higher alkali 
concentrations. A suggested temperature range is 40.degree.-60.degree. C. 
For example, a satisfactory material is produced using either 0.1N sodium 
hydroxide at room temperature (i.e. about 20.degree. C.) for ten minutes, 
or 0.05N sodium hydroxide solution at about 60.degree. C. for five 
minutes. 
Because the hydroxylated layer of the type II coagulant/adsorbent is 
provided by a different substance, to the material of the mineral or clay 
particle the range of starting materials is broader. A wide variety of 
minerals and clays can be used provided the nature of the mineral or clay 
is such as to permit the ready deposition of a hydroxide gel on its 
surface. In this respect oxides, sulphates, silicates and carbonates are 
particularly useful. Examples of such minerals include calcium sulphate, 
calcium carbonate, zinc oxide, barium sulphate, silica and siliceous 
materials such as sand and glass and clay minerals such as mica, china 
clay and pyrophillite. This list is not exhaustive, however, and many 
other minerals are suitable for use in this invention. In some cases, 
pre-treatment of the surface of the mineral may be required to produce a 
satisfactory deposition of the hydroxide layer. Yet another alternative is 
to use hollow microspheres, e.g. of glass for the production of gel 
particles which can be separated from the liquid effluent, after the 
adsorbtion of the organic material in step (a), by flotation rather than 
sedimentation. 
The hydroxylated layer of the coagulant/adsorbent particles of type II can 
be provided by any of a number of metal hydroxides, the requirements being 
substantial insolubility in water and a valency preferably of three or 
more. 
Suitable metals with this characteristic are ferric iron, aluminium, 
zirconium and thorium. Ferric hydroxide is preferred because it is cheap, 
and exceptionally insoluble, over a wide pH range. For example, it does 
not readily dissolve at high pH, as does aluminium hydroxide. 
The preparation of the coated particle of type II is also easily carried 
out, usually by suspending the particles in water, adding a salt of a 
suitable metal followed by an alkaline material, preferably in aqueous 
solution which will precipitate the metal hydroxide which then forms a 
coating on the particle. Typically, chlorides, sulphates, nitrates or 
other mineral acid salts of the metals are suitable; ferric chloride or 
aluminium sulphate are examples. The alkaline material may be sodium 
hydroxide, calcium hydroxide, ammonia or similar soluble material. The 
concentration and temperature at which the preparation is carried out is 
generally not critical. 
In the case where magnetite or other iron oxide materials are used as the 
basis for type II particles, the metal salt which is employed to produce 
the hydroxide layer may be obtained by first adding acid to the suspension 
of the particles (to give ferric and/or ferrous salts in solution from the 
iron oxide) and then adding the alkaline material. 
It has been found advantageous, when forming the particles of type II to 
provide means for increasing the degree of polymerization of the hydroxide 
layer. Polymerization occurs due to elimination of water and the 
establishment of oxygen ("ol") linkages between the metal atoms: 
EQU 2MOH$MOM+H.sub.2 O 
This process occurs on standing, but can be accelerated by heating. 
After preparation, it is best if the coated particles are not permitted to 
dry out. This can be avoided by keeping them under water. The thickness of 
the hydroxylated layer on the particles is not important since the 
flocculation or coagulation is a surface effect. 
An important advantage of the process of the present invention is that the 
coagulant/adsorbent particles can be recycled many times. To achieve this, 
the adsorbed material is removed by raising the pH of a suspension of the 
adsorbent in water. In the case of type I coagulant/adsorbents, the 
coagulating properties may be regenerated by treatment with alkali 
solution; these two treatments may be combined. 
As in the Sirofloc process the process of the present invention may be 
enchanced by the addition to the liquid under treatment of a suitable 
coagulant, such as polyelectrolyte (cationic, anionic or non-ionic) and/or 
an inorganic coagulant which provides multi-valent cations such as 
Fe.sup.2+ (e.g. ferrous sulphate). More usually the multi-valent cations 
will have a valency of three or more, such as Fe.sup.3+ or Al.sup.3+, 
(e.g. from alum or ferric chloride). These coagulants are not essential 
but when both types (i.e. polyelectrolytes and the inorganic coagulants) 
are present they complement each other. The polyelectrolyte may be present 
in the range 0 to 10 mg/L, preferably from 2 to 5 mg/L. The inorganic 
coagulant may be present in the range 0 to 100 mg/L, preferably 20 to 50 
mg/L. 
The preferred coagulant/adsorbent is magnetite and the following detailed 
description will refer to that material. It will be appreciated however, 
that reference to magnetite includes mutatis mutandis reference to other 
coagulant/adsorbents.

The basic process of the invention is shown by the solid connecting lines 
in FIG. 1. 
Raw or primary settled sewage is mixed with finely-divided cleaned recycled 
magnetite particles which have been regenerated by suspension in a 
solution of caustic soda to produce a thin hydroxylated surface layer, and 
the mixture is stirred to provide good contact of sewage with the 
regenerated magnetite particles which are preferably in the size range 1 
to 10 microns. The pH level may be adjusted by acid addition to be in the 
range 5 to 9, preferably 5.5 to 6.5 and the addition of an inorganic 
coagulant and/or a coagulant aid (e.g. a polyelectrolyte) may also be 
necessary to achieve a satisfactory effluent quality depending on the 
strength and composition of the raw sewage. After 2 to 20 minutes, 
preferably 10 to 15 minutes, of contact the magnetite slurry is separated 
from the treated effluent which may go to polishing ponds for final 
treatment before discharge. The magnetite particles, which now have 
attached to them most of the organic material originally present in the 
sewage, can be passed to an anaerobic digester where, because of the high 
concentration of organic material, rapid digestion at an elevated 
temperature of 35.degree. to 40.degree. C. may be achieved. Digestion may 
be performed at a temperature in the range of about ambient to about 
70.degree. C. but temperatures below about 30.degree. C. result in 
insufficiently rapid digestion while temperatures between 40.degree. C. 
and about 70.degree. C. may speed up the process but would require 
additional heat. After exit from the digester, the magnetite may be 
cleaned by stripping of the digested sludge using a dilute solution of 
sodium hydroxide, ammonia, or potassium hydroxide or, for example, a lime 
slurry which also regenerates the magnetite. The magnetite can then be 
recycled while the stripped sludge is sent to drying beds for disposal. 
A successful sewage treatment process based on the above procedure offers 
great scope for a reduction in capital cost as the residence time of the 
main sewage flow in the plant would be no longer than 30 minutes compared 
with approximately 8 hours for an activated sludge plant. The flow through 
the anaerobic digestion stage would be only about 1-2% of the main sewage 
flow and, assuming a high rate of digestion can be achieved, the size of 
the digester should be smaller than the sludge digester associated with a 
conventional activated sludge plant. 
In the alternative, and more preferable process shown by the dashed lines 
in FIG. 1, the organic material is stripped from the magnetite before 
anaerobic digestion. The magnetite is recovered, regenerated and recycled 
as before. 
Various aspects of the process of the invention are further described and 
discussed hereinafter. This material should not be construed as limiting 
on the nature or scope of the invention. 
CLARIFICATION 
There is a variety of parameters which can affect the efficiency of the 
clarification process. However, once the mixing and settling conditions 
have been prescribed, it is the addition of chemicals (including the fine 
hydroxylated magnetite particles) which controls the effluent quality. The 
preferred dose of hydroxylated magnetite on a dry weight basis is in the 
range 5 to 40 g/L of sewage. Jar tests were used for a preliminary study 
of the clarification process, and a sample of results is given in Table 1. 
It was possible to obtain a high quality effluent, similar to that from an 
activated sludge plant, simply by increasing the dose of chemicals; e.g. 
at a chemical cost of 10.cent./m.sup.3 an effluent with a BOD of 25 mg/L 
and suspended solids of less than 1 mg/L was achieved, while at a more 
reasonable chemical cost of 5.cent./m.sup.3 effluent BOD and suspended 
solids levels were 38 and 3.4 mg/L respectively. The removal levels of COD 
in Table 1 vary from 70 to 90% and compare favourably with those reported 
for a full anaerobic treatment of raw sewage. 
TABLE 1 
__________________________________________________________________________ 
Clarification of Raw Sewage with Fine Magnetite Particles 
Raw sewage characteristics" 
SS 78 mg/L 
COD 500 mg/L 
BOD 220 mg/L 
Treated Sewage 
Suspended 
Quality 
Chemical 
Solids COD BOD 
Treatment Conditions 
Cost (mg/L) (mg/L) 
(mg/L) 
__________________________________________________________________________ 
FeCl.sub.3 - 20 mg/L 
4.cent./m.sup.3 
8 150 43 
Polyelectrolyte - 4 mg/L 
Acid - 0.5 mmol/L 
Magnetite - 10 g/L 
FeCl.sub.3 - 20 mg/L 
5.cent./m.sup.3 
3.4 120 38 
Polyelectrolyte - 4 mg/L 
Acid - 1 mmol/L 
Magnetite - 20 g/L 
FeCl.sub.3 - 50 mg/L 
10.cent./m.sup.3 
0.9 44 25 
Polyelectrolyte - 5 mg/L 
Acid - 1 mmol/L 
Magnetite - 10 g/L 
__________________________________________________________________________ 
ANAEROBIC DIGESTION 
The principal advantage of the present process, lies in the ability to 
achieve concentration of the organic material in raw sewage to a level 
that allows the organic material to be rapidly digested anaerobically. 
This concentrated material can be fed to the digester either attached to 
the magnetite particles used in clarification or as a concentrated slurry 
after stripping and separation from the magnetite particles. In either 
case the material is concentrated by a factor of about 60 over the raw 
sewage. Measured COD levels on a stripped slurry average around 30,000 
mg/L, an ideal level for feeding to a mesophilic or thermophilic anaerobic 
digester. Anaerobic digestion of concentrated wastes is now a commercially 
successful process with a variety of digester designs being available. 
DIGESTER OPERATION 
In initial investigations of anaerobic digestion the concentrated magnetite 
slurry as separated from the liquid effluent was used as the feed 
material. In this situation the sewage material remains attached to the 
fine magnetite particles having a particle size in the range 1 to 10 
microns, and it was hoped that in the digester these particles would be 
rapidly colonized by anaerobic bacteria, ensuring close proximity of food 
and microorganisms. The magnetite slurry had a water content of about 90% 
v/v (sp.gr. 1.4). 
However, the results from four weeks of digester operation at pH 7 and a 
temperature of 35.degree. C., with seed material from a conventional 
anaerobic sludge digester, were disappointing. At a feed rate of 5 kg 
COD/m.sup.3 d, gas analyses appeared satisfactory, the process taking 
about 2 weeks to reach a steady state with methane and carbon dioxide 
reaching 55 and 25% respectively on a w/w basis. Hydrogen concentrations 
reached undetectable levels after 2 weeks of operation. However, gas 
production rates were about 0.12 m.sup.3 /m.sup.3 d, an order of magnitude 
below what would be expected from a high rate digester, and showed no 
signs of increasing. A variety of reasons can be put forward to explain 
this poor result. The anaerobic bacteria may have been unable to attach 
themselves to the fine magnetite particles (1-10 mm), which are of a 
comparable size. As a consequence, access to their food supply may have 
been limited and this might have been alleviated by increasing the maximum 
particle size to 20 mm. Possible nutrient deficiencies, especially in 
phosphate which can be adsorbed by the magnetite particles, could also 
have caused or contributed to the poor result. The limited residence time 
of the magnetite particles in the digester may not have allowed sufficient 
bacterial colonization to occur. 
Whatever the reason for the failure of the initial approach, improved 
digestion can be achieved by the alternative procedure of stripping and 
separating the concentrated sewage material from the magnetite particles 
and then feeding the resultant slurry to an anaerobic digester. The 
regenerated magnetite particles can then be directly recycled for a 
further clarification step. 
In this approach, the anaerobic digester advantageously, but not 
essentially, contains larger magnetite particles, for example of size 
50-100 mm to serve as the site for bacterial attachment and growth and 
these particles are permanently retained within the digester by recycle 
e.g. from an attached settling cone. The anaerobic digester could be of 
any other type suitable for mesophilic or thermophilic digestion but the 
presence of the larger magnetite particles is advantageous because they 
remove H.sub.2 S from the reactor environment. 
The results from the first sixty days of operation of such a digester, with 
a concentrated regeneration effluent as feed, are given in FIGS. 2 and 3. 
For the first week the COD of the feed was around 14,000 mg/L, but this 
level was lowered to between 9 and 10,000 mg/L after the first week by the 
addition of a more dilute washing effluent. Nominal liquid residence time 
was 2.5 days. The digester was seeded from material supplied by Bunge Pty. 
Ltd. During the first ten days of operation between 80 and 100% removal of 
COD was obtained, with an initial gas evolution rate of 0.4 m.sup.3 
/m.sup.3.d and a methane content of around 50%. Unfortunately, during the 
first week an undetected blockage in the gas sampling line prevented 
further measurements of gas rates during this period. However, it is 
probable that the high COD removal rates observed during this period were 
the result of a combination of physical adsorption and microbiological 
digestion. After day 10, although gas evolution rates of up to 0.8 m.sup.3 
/m.sup.3.d were initially observed, COD removal fell to about 40% and 
remained at this level for the duration of the operation. Gas evolution 
rates slowly declined although methane content never fell below 50% and 
increased up to 70% after day 50. 
FIG. 4 is a process flow diagram of the presently preferred overall process 
which essentially follows the alternative process of FIG. 1. The raw 
sewage at 10 is mixed with cleaned, recycled and regenerated magnetite 
particles at 12 at a rate of 5 to 40 grams dry weight per litre of raw 
sewage (preferably 10 to 20 g/L) and the aqueous mixture is stirred to 
provide good conatct between the sewage and the hydroxylated magnetite 
particles. This stirred contact can be achieved over a period of 2 to 20 
minutes, preferably 10 to 15 minutes, in a series of four stirred tanks 
14, 16, 18 and 20 as shown with acid being added in tank 14 to adjust the 
pH to within the range 5 to 9, preferably 5.5 to 6.5, inorganic flocculant 
such as alum being added in tank 16 and polyelectrolyte being added in 
tank 18. The quantities of the latter two components added will depend 
upon the quality of the treated sewage required as well as the strength 
and composition of the raw sewage but these two components may not be 
required at all. After the final mixing tank 18 the sewage/magnetite mix 
passes to a clarifier 22 on the way to which the magnetite is magnetized 
by a flocculating magnet 24, which is placed around the entrance pipe 26 
to the clarifier 22. The clarifier may be of any suitable known type, but 
advantageously incorporates the improvement described in our Australian 
Patent specification No. 553,423 the contents of which are incorporated 
herein by reference. In the clarifier the loaded magnetite particles 
rapidly separate from the clarified sewage and this liquid effluent is 
exhausted from the process at 28. The loaded magnetite particles are 
extracted from the clarifier 22 at 30 and must then be regenerated, for 
example, by mixing them with a dilute solution of caustic soda which is 
introduced to the system at 32. Alternatively a lime slurry may be added. 
This regeneration process can produce a liquid stream 34 of sewage 
concentrate with a chemical oxygen demand (C.O.D.) in the range 
10,000-20,000 mg/L, which is in the ideal range for feeding to a 
mesophilic anaerobic digester assembly 36. 
As shown in FIG. 4 the preferred regeneration process involves a two stage 
countercurrent flow washing operation, which uses magnetic drum separators 
38 and 40, which may be of known type, to separate the magnetite particles 
from recycled wash water in the two stages. The liquid effluent 42 from 
the first magnetic drum separator 38 passes to the anaerobic digester 
assembly 36, where the sewage organics are broken down to form methane 
which is exhausted at 44. The effluent 46 from the digester assembly 36 
passes to a sludge settling pond 48 where particulate biomass settles out 
and may be disposed of as a dried sludge at 50. The overflow 52 from the 
pond 48 is relatively clear and colourless and has been found to have 
C.O.D. of less than 20% of that of the feed to the anaerobic digester 
assembly. Experiments have shown that this liquid overflow is suitable for 
recycling as the wash water 54. The caustic soda (NaOH) added to the waste 
water 54 at 32 raises the pH level to greater than 10 for regeneration of 
the magnetite and this also results in ammonia (which is generated in the 
anaerobic digester assembly 36) being stripped from the recycle stream by 
aeration at 56. This step alleviates the build up of ammonia in the wash 
water and thus allows the wash water to be recycled indefinitely. It will 
also prevent any ammonia, in excess of that coming in with the raw sewage, 
from finding its way into the clarified sewage effluent. Thus, the only 
liquid or solid effluents from the process will be the clarified sewage 28 
and the dried anaerobically stabilised sludge 50. 
The recycled wash water 54 with ammonia removed is mixed with partly 
cleansed and regenerated magnetite in a stirring tank 58 in the magnetite 
flow line between the magnetic drum separators 38 and 40. The overflow 
from the tank 58 flows to the separator 40 where the fully cleansed and 
regenerated magnetite with hydroxide layer is extracted for recycle. The 
separator wash water is directed along flow line 60 into a stirring tank 
62 in the magnetite flow line between the clarifier 22 and the magnetite 
drum separator 38. The recycle water and the loaded magnetite removed from 
the clarifier 22 are thoroughly mixed in the tank 62 and the overflow 
passes into the magnetic drum separator 38. As previously described the 
liquid effluent from the separator 38 passes to the anaerobic digester 
assembly 36 which may be as shown in FIG. 5. 
The presently favoured technique for operation of the anaerobic digester 
assembly is to strip the sewage material from the magnetite particles with 
a dilute caustic soda solution to produce a sewage concentrate, which is 
then fed to an anaerobic digester assembly 36 as described above. To date, 
the best digester results have been obtained with an upflow anaerobic 
sludge blanket digester 64, operating at a laboratory scale in a 10 cm 
diameter glass column 66. This digester is preceded by a 5.5 L stirred pot 
68, incorporating a magnetic stirrer 70, in which acidification of the 
sewage concentrate supplied from a feed tank 72 takes place at a pH of 
about 5.7. The anaerobic sludge blanket digester 64 was operated at a pH 
of about 7.2. The sewage concentrate had a pH level of about 10, but the 
acidification reactor 68 was still able to maintain its pH level with this 
feed. Some typical results of the digester operation are given in Table 2. 
TABLE 2 
______________________________________ 
Loading Rate 
g COD/g Hydraulic 
C.O.D. % removal dry wt Retention 
mg/L of C.O.D. biomass .multidot. d 
Time 
______________________________________ 
Feed 6,800 -- -- 
Acidifiction 
5,850 14% 0.11 6.1 days 
Reactor (at exit) 
Anaerobic 
1,200 82% 0.08 2.2 days 
Sludge (at exit) 
Blanket 
Digester 
______________________________________ 
Gas Production rate: 0.7 m.sup.3 /m.sup.3 .multidot. d 
Table 1: Digester Operating Data 
The results show a very good removal (82%) of C.O.D. through the digester 
system at quite reasonable loading rates. The effluent from the sludge 
blanket digester 64, after clarification in a settling pond (not shown), 
is sufficiently clear to allow the recycle and reuse of this material as 
wash water in the magnetite regeneration process as shown in FIG. 4. 75% 
of the gas given off at 24 from the digester was methane and 20% carbon 
dioxide. 
The results obtained clearly demonstrate that the sewage concentrate 
obtained by stripping the magnetic particles can be rapidly digested 
anaerobically.