U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2011–1096

In cooperation with The Ohio State University

Results of an Evaluation of the Effectiveness of 

Chlorine Dioxide as a Disinfectant for Onsite Household 

Sewage Treatment Systems

Results of an Evaluation of the 

Effectiveness of Chlorine Dioxide as a 

Disinfectant for Onsite Household Sewage 

Treatment Systems

By Christopher M. Kephart and Donald M. Stoeckel 

In cooperation with The Ohio State University

Open-File Report 2011–1096

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, , Director

U.S. Geological Survey, Reston, Virginia: 2011

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Suggested citation:

Kephart, C.M., and Stoeckel, D.M., 2011, Results of an evaluation of the effectiveness of chlorine dioxide as a disinfectant for onsite household sewage treatment systems: U.S. Geological Survey Open-File Report 2011–1096, 10 p.
iii

Contents

Abstract ...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Methods...........................................................................................................................................................2

Selection of Target Microorganisms .................................................................................................2

Bench-Scale Experiments ...................................................................................................................2

Field-Scale Trial Procedures ...............................................................................................................3

Results .............................................................................................................................................................3

Bench-Scale ClO2 Inactivation Capabilities .....................................................................................3

Field-Scale ClO2 Inactivation Capabilities .........................................................................................5

References Cited..........................................................................................................................................10

Figures

 1. Results of bench-scale dose-response of target microorganisms in experiments 

with chlorine dioxide as a wastewater disinfectant ...............................................................6

 2. Distribution and persistence of Escherichia coli in the holding tank at the Molly 

Caren Agricultural Center site, in the absence of chlorine dioxide disinfectant ...............7

 3. Results of field-scale trial of chlorine dioxide disinfection packets for target microorganisms at a single-family household sewage treatment system (HSTS) ......................9

Tables

 1. Test microorganism concentrations following three bench-scale trials of chlorine 

dioxide effectiveness for disinfection of wastewater. Effectiveness against 

Bacillus anthracis was evaluated in only one trial .................................................................4

 2. Test microorganism concentrations following a field-scale trial of chlorine dioxide 

effectiveness for disinfection of wastewater ..........................................................................8

Conversion Factors

Multiply By To obtain

gallon (gal) 3.785 liter (L) 

liter (L) 0.264 gallon (gal) 

Concentrations of chemical constituents in water are given in milligrams per liter (mg/L).

Results of an Evaluation of the Effectiveness of 

Chlorine Dioxide as a Disinfectant for Onsite Household 

Sewage Treatment Systems

By Christopher M. Kephart and Donald M. Stoeckel

Abstract

Scientists with the U. S. Geological Survey and The 

Ohio State University evaluated the effectiveness of chlorine 

dioxide (ClO2) as a wastewater disinfectant for onsite household sewage treatment systems. Effectiveness of ClO2

 was 

measured by evaluating the removal of six test microorganisms: Escherichia coli, enterococci, Clostridium perfringens, 

Bacillus anthracis Sterne strain spores, somatic coliphage, and 

F-specific coliphage. These evaluations were done in benchscale experiments and in a field-scale experiment where ClO2

was delivered to the sewage by use of a novel dry packet system. Results presented herein demonstrate effective inactivation of each test microorganism except for B. anthracis spores.

Introduction

As communities in Ohio continue to work toward 

eliminating pollutant discharge to streams to meet goals of the 

Clean Water Act, wastewater reuse through onsite wastewater 

treatment followed by irrigation is an important option. For 

example, it has been estimated that only 6.4 percent of Ohio 

soils are suitable for traditional leach-line or mound septic 

systems (Mancl and Slater, 2002). Consequently, on the basis 

of calculations using distribution of soil types, an estimated 

27 percent of all existing onsite household sewage treatment 

systems (HSTS) in Ohio are failing to adequately treat sewage effluent (Mancl, 1990). A solution to this problem may 

be to collect and disinfect HSTS effluent to make it safer for 

discharge into the environment and possible reuse as irrigation 

water.

Recycling or reuse of wastewater is a process that utilizes 

treated sewage in a manner that minimizes or eliminates the 

discharge of pollutants to resource waters. Reuse of wastewater, however, must be done with consideration of public health 

risk. Waterborne diseases, ranging from gastroenteritis to 

hepatitis, are caused by viruses, bacteria, and parasites that are 

spread through the fecal waste of infected persons. Wastewater from septic systems must be effectively disinfected to 

minimize the risk of pathogen breakthrough to nearby streams 

and ditches and subsequently minimize the incidence of waterborne disease in the reuse setting. 

Chlorine in multiple forms has been used as a disinfectant 

for wastewater. After its introduction in the 1950s, chlorine 

dioxide has recently reemerged as a disinfection agent. Chlorine dioxide has different activity than that of free chlorine or 

hypochlorite. Unlike free chlorine and hypochlorite, chlorine 

dioxide is active over a wide range of pH, and its use does not 

result in the creation of disinfection byproducts (Lykins and 

Griese, 1986). One disinfection byproduct that forms from 

the reaction of free chlorine with organics is trihalomethane, 

a halogenated hydrocarbon that has been linked to increased 

cancer mortality rates (Alavanja and others, 1980). Some 

studies have shown that microbial disinfection with chlorine 

dioxide is as effective or superior to disinfection with free 

chlorine—particularly in regard to inactivation of viruses 

(Narkis and Kott, 1992; Junli and others, 1997). Chlorine 

dioxide traditionally has been generated by combining sodium 

chlorite with free chlorine gas. Until now, the expense associated with the handling and injection of chlorine gas has made 

use of chlorine dioxide economical only for large water and 

wastewater facilities. A dry-packet delivery system (Avantec 

Technologies Inc., Columbus, Ohio) has recently made the 

chemistry behind chlorine dioxide generation safer and more 

economical, enhancing its usefulness for small treatment 

systems. 

Widespread use of a newly developed chlorine dioxide 

disinfection system would depend, in part, upon its ability 

to inactivate pathogenic microorganisms in sewage effluent 

to make the water safe for reuse. The inactivation capability 

of chlorine dioxide can be described by the product between 

the chlorine concentration (C) and contact time (t) required 

for a specific percentage of microorganism removal and is 

expressed as a chlorine contact time (Ct) (Gaudy and Gaudy, 

1980). In one study, the disinfection capabilities of free 

chlorine and chlorine dioxide in activated municipal sludge 

were compared. For approximately 7 mg/L chlorine dioxide, 

a 5-log decrease in fecal coliform concentration was achieved 

after 20 minutes of contact time (Ct = 140 mg*min/L). The 

same chlorine dioxide concentration caused a 6-log decrease 
2 Evaluation of Chlorine Dioxide as a Disinfectant for Onsite Household Sewage Treatment Systems 

in coliphage concentration after 20 minutes of contact time. In 

contrast, the same Ct value for free chlorine resulted in a 5-log 

decrease in fecal coliform concentration, but only a 1-log 

decrease in coliphage concentration (Narkis and Kott, 1992). 

The U.S. Geological Survey (USGS), in cooperation 

with The Ohio State University, evaluated the effectiveness of 

chlorine dioxide as a disinfection tool for household sewage 

treatment systems. The overall purpose of this study was to 

measure the ability of a novel chlorine dioxide delivery system 

to inactivate pathogens in onsite residential wastewater and 

thereby render the treated wastewater more suitable for discharge into the environment or reuse as irrigation water. 

One specific objective of this study was to characterize 

bench-scale Ct requirements to describe the inactivation kinetics of chlorine dioxide for different types of microorganisms 

in a wastewater matrix (fecal-indicator bacteria, spore-forming 

bacteria, and viruses). Another objective was to determine the 

effectiveness of the chlorine dioxide system against the same 

suite of microorganisms in a field-scale operational setting—

an onsite, single-family household sewage treatment system. 

Methods and results from these two experimental approaches 

are described herein.

Methods

The effectiveness of chlorine dioxide was tested in wastewater collected from the Molly Caren Agricultural Center 

wastewater stabilization pond, part of a three-person household septic system established and maintained as an educational tool by The Ohio State University. 

Selection of Target Microorganisms

Six different microorganisms were used to evaluate chlorine dioxide effectiveness: Escherichia coli, enterococci, Clostridium perfringens, Bacillus anthracis spores (Sterne), F-specific coliphage, and somatic coliphage. E. coli and enterococci 

were selected for study because they are the required indicators for monitoring and regulation of recreational waters for 

fecal contamination (U.S. Environmental Protection Agency, 

1986, 2004). C. perfringens is an alternate indicator of fecal 

contamination (Hill and others, 1996). C. perfringens is found 

both as vegetative cells and endospores. B. anthracis Sterne 

is a nontoxigenic strain of a spore-forming zoonotic pathogen. Evaluation of chlorine dioxide in reducing spore formers 

would provide a measure of effectiveness against stressresistant microorganisms. F-specific and somatic coliphages 

are viruses that infect E. coli by two different mechanisms. 

F-specific coliphages are RNA or DNA viruses that infect via 

the F-pilus of male strains of E. coli. Somatic coliphages are 

DNA viruses that infect via the outer cell membrane. These 

coliphage have been used as surrogate organisms to indicate 

capacity for transport and survival of enteric viruses from 

fecal contamination in water (Skraber and others, 2004; Helmi 

and others, 2008).

Wild type enrichments of each of the test microorganisms 

except for B. anthracis were cultivated from raw sewage from 

the Olentangy Environmental Control Center, a wastewater 

treatment plant in Delaware County, Ohio. Stocks of the test 

microbes were selected for and enriched immediately before 

each experimental trial. Each test microorganism was enumerated to determine concentrations after experimental treatment and, in some cases, to determine expected (theoretical) 

concentrations for seeding inocula. Specifically, E. coli was 

quantified by use of the modified mTEC membrane-filtration 

method 1603 of the U.S. Environmental Protection agency 

(USEPA) (2006b), enterococci was quantified by use of the 

mEI membrane-filtration method 1600 of the USEPA (2006a), 

C. perfringens was quantified by use of the mCP membranefiltration method (Bisson and Cabelli, 1979), B. anthracis was 

quantified by spread plating onto PLET agar (Knisely, 1966), 

and F-specific and somatic coliphage were quantified by the 

single agar layer method 1602 of the USEPA (2001). Theoretical concentrations of each seed inoculum were determined by 

factoring in the starting volume of the experimental matrix. 

General laboratory quality-assurance procedures followed 

those of Francy and others (2010).

Bench-Scale Experiments

For each of the three bench-scale trials, two 10-L samples 

of wastewater collected near the outlet of the Molly Caren 

wastewater stabilization pond were used as the test matrix. 

One 10-L sample was used as a recovery control, and ClO2

was not added to this sample carboy. The other 10-L volume 

was used as the test sample, and ClO2 was added to this carboy. The wastewater was collected either the day of the trial or 

the day before and stored at room temperature.

Evaluation by means of bench-scale experiments was 

done in three trials. Wastewater was seeded with inocula of 

each of the test microorganisms listed above. Initial testing did 

not include B. anthracis spores; this organism was included 

in the evaluation after determining that the C. perfringens

inocula consisted of predominantly vegetative cells and would 

not have represented a typical stress resistant spore-forming 

microorganism. As a result of the change, the bench-scale 

evaluation for the B. anthracis spores was only done once, 

and the field-scale evaluation did not include C. perfringens as a test microorganism. Each test microorganism was 

added to both carboys, and the seeded samples were mixed 

vigorously by shaking. Both carboys were then set up to stir 

slowly throughout the trial to keep an even distribution of the 

microorganisms but without creating significant aeration of the 

sample that might lower the effective ClO2 concentration. 

Sample collection began with a 1-L sample from the test 

carboy as the “time = 0” sample for microorganism analyses. 

For the bench-scale experiments, ClO2 was pipette-transferred 

to the test carboy from a recently prepared ClO2 solution and 
Results 3

allowed to mix for 30 seconds, then a 100-mL sample was 

collected as the initial sample for measuring ClO2 concentration. Samples were subsequently collected (100 mL for ClO2

concentration and 1 L for microorganism analyses) after 5, 10, 

30, 60, and 120 minutes. Samples were also collected from the 

recovery control vessel at these time points. For the third trial 

test carboy, a sample was also collected at 2.67 minutes, and 

the 120-minute sample was eliminated from the series. For 

the third trial recovery control carboy, samples were collected 

only at 0 and 120 minutes. Each sample for microbial analyses 

was collected in a bottle containing sodium thiosulfate to neutralize any residual ClO2

 and was stored on ice immediately 

after collection. Samples were brought back to the USGS for 

analysis on the day of the trial. 

Field-Scale Trial Procedures

Field-scale experiments were run in the final holding 

tank of the single-family household sewage treatment system 

at the Molly Caren site. Introduction of test microorganisms 

and ClO2 to the approximately 1,000-gallon holding tank was 

done by first mixing in a 100-gallon auxiliary polypropylene 

tank that received a constant flow of the wastewater moving 

from the underground holding tank to the auxiliary tank. The 

wastewater in the auxiliary tank was returned to the holding 

tank via an overflow outlet near the midpoint of the auxiliary 

tank. Distribution of wastewater in the approximately half-full 

holding tank was done by use of two pumps: a low-flow sump 

pump added to the tank near the inlet of the auxiliary tank and 

a supplemental, high-flow pump added to the tank near the 

outlet at the far end of the holding tank.

Before the field-scale trial was run, a distribution experiment was done to determine whether, through settling or 

inadequate mixing, concentrations of microbes sampled at 

different locations in the tank would vary significantly. Additionally, this experiment would demonstrate whether or not 

significant die-off would occur over time in the final holding 

tank. An inoculum of E. coli, prepared in the same manner 

as in the bench-scale experiments, was added to the auxiliary 

tank and was mixed by use of both pumps for 30 minutes. The 

initial samples were then collected at five different locations in 

the holding tank by using a peristaltic pump and sterile tubing. 

The high-flow, supplemental pump was then shut off. Single 

samples were then collected at the southwest corner of the 

holding tank after 60, 90, 150, and 165 minutes. A concurrent 

replicate sample was collected for the 60-minute sample. The 

location of these samples was the furthest from the mixing 

of the low-flow sump pump and was thought to represent a 

worst-case scenario for observing settling or inadequate mixing. Another step taken to determine whether mixing was not 

adequate was that after the 150-minute sample was collected, 

the high-flow, supplemental pump was turned back on for 15 

minutes before collection of the 165-minute sample. 

One field-scale paired trial was done on two separate 

days to evaluate ClO2 effectiveness; that is, recovery control 

samples (not treated with ClO2) were collected and analyzed 

one day and test samples (treated with ClO2) were collected 

and analyzed another day. For this paired trial, the experimental setup was the same as described above for the distribution 

experiment except that ClO2 was added to the wastewater in 

the auxiliary tank by use of the Avantec-prepared dry packets, 

and the sampling time intervals were not the same. Five test 

microbes (E. coli, enterococci, B. anthracis, F-specific coliphage, and somatic coliphage) were introduced to the auxiliary 

tank, and high-flow mixing was done with both pumps for 30 

minutes. All samples were collected in the same manner as in 

the distribution experiment. The time = 0 sample was collected 

and the high-flow pump was then shut off. For the recovery 

control experiment, subsequent samples were collected after 

5, 30, and 120 minutes. For the ClO2 test experiment, the 

disinfectant was added after the 30-minute high-flow mixing, 

and subsequent samples were collected after 5, 30, 90, 180 and 

190 minutes. After the 180-minute test sample, the high-flow, 

supplemental pump was turned back on for 10 minutes before 

collection of the 190-minute sample. Concurrent replicate 

samples were collected after the 30-minute sample for both 

the recovery control and test samples. All samples for the 

field-scale experiments were collected in sterile 1-L bottles 

amended with sodium thiosulfate and were preserved on ice 

immediately following collection for onsite processing within 

1 hour. 

Results

Bench-Scale ClO2 Inactivation Capabilities

Chlorine dioxide effectiveness was measured for six different microorganisms seeded into wastewater collected from 

the Molly Caren pond-based septic system. Concentrations of 

each test microorganism for each of three trials are shown in 

table 1. For B. anthracis, only one bench-scale trial was done. 

For the test carboys, concentrations at time = 0 indicate initial 

microorganism levels after initial mixing in the wastewater 

but before the introduction of ClO2. For the test microcosm in 

trial 3, a sample-collection time 2 minutes 40 seconds (2.67 

minutes) following the addition of ClO2 was added to help better describe microbe die-off characteristics in the early stages 

of ClO2 exposure. For the recovery control carboys, samplecollection time steps were reduced to two for trial 3 and three 

for the B. anthracis trial because steady concentrations were 

observed during trials 1 and 2. 

Rapid, high levels of die-off were observed in each of the 

bench-scale trial test carboys for each microorganism tested 

except for B. anthracis. After 5 minutes of ClO2 exposure, 

the log10-removal ranged from 4.22 to 5.97 CFU/100 mL for 

E. coli, 4.06 to 6.57 CFU/100 mL for enterococci, 

1.70 to 5.08 CFU/100 mL (initial concentration in trial 2 was 

only 2.70 log CFU/100 mL) for C. perfringens, 4.60 to 5.68 

CFU/100 mL for somatic coliphage, and 5.87 to 7.15 for 

F-specific coliphage. Factoring in the time of ClO2 exposure 

allows for the calculation of Ct values for these data. While 
4 Evaluation of Chlorine Dioxide as a Disinfectant for Onsite Household Sewage Treatment Systems Table 1. Test microorganism concentrations following three bench-scale trials of chlorine dioxide effectiveness for disinfection of wastewater. Effectiveness against B. anthracis was evaluated in only one trial. [All data are concentrations, in colony-forming units per 100 milliliters for bacteria and plaque-forming units per 100 milliliters for coliphage; — , sample not collected; <, no positive results for lowest volume evaluated] Test samples (treated with ClO2

)

Coliphage

E. coli Enterococcus C. perfringens Bacillus anthracis Somatic F-specific

Time

(minutes) Trial 1 Trial 2 Trial 3 Trial 1 Trial 2 Trial 3 Trial 1 Trial 2 Trial 3 Trial 1 Trial 2 Trial 3 Trial 1 Trial 2 Trial 3

0 2.8E+06 5.1E+07 3.5E+07 5.6E+06 6.7E+06 4.7E+07 1.2E+06 5.0E+02 4.2E+06 3.4E+06 2.4E+05 1.5E+06 7.5E+05 1.7E+08 7.3E+07 5.4E+08

2.67

—

— 400 — — 1200 — — 110 — — — 7 — — 280

5 3 310 300 <3 580 700 10 10 33 4.7E+06 <1 38 3 12 99 110

10 3 140 180 10 220 430 3 5 30 3.3E+06 <1 22 7 28 200 7830 3 150 210 <3 190 520 <3 <3 13 4.7E+06 <1 26 2 100 160 9460 <3 170 190 <3 140 590 <3 <3 10 2.9E+06 <1 57 1 3 110 73

120 <3 150 — <3 140 — <3 <3 — 2.6E+06 <1 35 — 4 50 — 

Recovery control samples (no ClO2

)

Coliphage

E. coli Enterococcus C. perfringens Bacillus anthracis Somatic F-specific

Time

(minutes) Trial 1 Trial 2 Trial 3 Trial 1 Trial 2 Trial 3 Trial 1 Trial 2 Trial 3 Trial 1 Trial 2 Trial 3 Trial 1 Trial 2 Trial 3

0 3.2E+06 3.2E+06 2.6E+07 6.5E+06 5.4E+06 3.5E+07 2.8E+05 9.0E+02 5.5E+06 4.2E+06 2.6E+05 2.0E+06 6.0E+05 1.2E+08 4.5E+07 6.1E+085 4.4E+06 4.8E+06 — 5.8E+06 5.9E+06 — 1.5E+06 5.0E+02 — — 2.5E+05 2.3E+06 — 1.1E+08 4.7E+07 — 

10 2.2E+06 2.8E+06 — 6.0E+06 3.8E+06 — <1.0E+05 3.3E+02 — — 2.8E+05 1.8E+06 — 8.8E+07 5.0E+07 — 30 3.1E+06 4.1E+06 — 5.6E+06 5.1E+06 — 8.6E+05 3.1E+02 — 3.6E+06 2.4E+05 2.0E+06 — 9.3E+07 4.2E+07 — 60 2.9E+06 5.5E+06 3.6E+07 5.5E+06 6.1E+06 3.5E+07 2.3E+05 1.5E+02 4.0E+05 — 1.9E+05 1.8E+06 4.8E+05 1.1E+08 4.3E+07 5.4E+08

120 3.3E+06 3.1E+06 — 5.2E+06 5.2E+06 — 8.4E+05 1.2E+03 — 3.0E+06 2.3E+05 1.8E+06 — 1.1E+08 2.7E+07 — 
Results 5

ClO2 concentrations are included as a function of the Ct values, the raw data for these concentrations are not presented. 

Plots of Ct values versus log10-removal in CFU/100 mL are 

presented in figure 1. 

Field-Scale ClO2 Inactivation Capabilities

Chlorine dioxide effectiveness was also measured in a 

1,000-gallon septic holding tank. Before challenging ClO2

against the test microorganisms, an initial experiment was run 

to determine how well distributed the microorganisms were 

in the large tank. Results from this experiment are presented 

in figure 2. E. coli concentrations for the initial five samples 

collected at various locations in the tank after 30 minutes of 

high-flow agitation ranged from 6.26 to 6.45 log10 CFU/100 

mL, thus demonstrating reasonably even distribution throughout the tank. These concentrations corroborated theoretical 

concentrations based on an estimate of the seed inoculum 

concentration of 6.52 log10 CFU/100 mL. The subsequent 

samples collected (under low-flow agitation) further supported 

that the E. coli were distributed evenly throughout the tank 

and also demonstrated that significant native die-off was not 

occurring during the allotted time. Additionally, the final sample collected after vigorous agitation reinforced that seeded 

E. coli did not settle out significantly in the tank throughout 

the 165-minute experiment. 

The paired trial results for the field-scale ClO2 evaluation 

are presented in table 2 and figure 3. Overall, removal caused 

by ClO2 exposure in the field-scale trial was not as rapid or 

severe as in the bench-scale trials. A delay in the initial effect 

of the disinfectant was expected, given that the ClO2 was 

delivered to the system in dry-packet form that required more 

time to activate and, once in solution, the ClO2 required time 

of travel from the auxiliary tank to the large holding tank 

where the samples were collected. For the test samples, log10-

removal peaked for all microorganisms except B. anthracis

after 180 minutes of disinfectant exposure. Somewhat contrary 

to the field-scale distribution experiment, some decreases were 

seen in microorganism concentrations after high-flow agitation was reinitiated (time = 190 minutes), indicating that some 

settling had occurred during the experiment. Corroborating the 

results from the bench-scale experiments, ClO2 exposure was 

ineffective against the B. anthracis spores in the conditions 

established for the field-scale experiment. 
6 Evaluation of Chlorine Dioxide as a Disinfectant for Onsite Household Sewage Treatment Systems 

F-specific coliphage

86420

10

0 20 40 60 80 100

Ct value (mg*min/L)

Log removal Trial 1 Trial 2

Trial 3

Bacillus anthracis (Sterne; spores)

-2

86420

0 20 40 60 80 100

Ct value (mg*min/L)

Log removal Trial 1

86420

10

0 20 40 60 80 100

Log removal

Ct value (mg*min/L)

Enterococci

Trial 1Trial 2Trial 3

Somatic coliphage

86420

10

0 20 40 60 80 100

Ct value (mg*min/L)

Log removal Trial 1 Trial 2

Trial 3

Clostridium perfringens (vegetative)

86420

10

0 20 40 60 80 100

Ct value (mg*min/L)

Log removal Trial 1 Trial 2

Trial 3

86420

10

0 20 40 60 80 100

Log removal

Ct value (mg*min/L)

Escherichia coli

Trial 1Trial 2Trial 3

Figure 1. Results of bench-scale dose-response of target microorganisms in experiments with chlorine dioxide as a wastewater disinfectant. Shaded data points indicate none detected; removal indicated is a minimum value.
Results 7

5.00

5.50

6.00

6.50

7.00

0 60 120 180

Time (minutes)

Escherichia coli density

Colony-forming units (CFU/100 mL)

Figure 2. Distribution and persistence of Escherichia coli in the holding tank at the Molly 

Caren Agricultural Center site, in the absence of chlorine dioxide disinfectant. Samples 

were collected at five locations in the tank at the first sample time (after 30 minutes of 

mixing). A concurrent replicate sample was collected for the 60-minute sample. Shaded 

data point indicates density measured following vigorous agitation (the difference 

between the density at 150 minutes and 165 minutes is attributable to settling).
8 Evaluation of Chlorine Dioxide as a Disinfectant for Onsite Household Sewage Treatment Systems Table 2. Test microorganism concentrations following a field-scale trial of chlorine dioxide effectiveness for disinfection of wastewater. [CFU/100 mL, colony-forming units per 100 milliliters; PFU/100 mL, plaque-forming units per 100 milliliters] Test samples (treated with ClO2

)

Coliphage

E. coli Enterococcus B. anthracis Somatic F-specific

Time 

(minutes)1 CFU/100 mL Log remaining CFU/100 mL Log remaining CFU/100 mL Log remaining PFU/100 mL Log remaining PFU/100 mL Log remaining

0 2.7E+06 6.4 1.5E+06 6.2 1.9E+06 6.3 1.9E+06 6.3 2.2E+07 7.35 2.6E+06 6.4 1.9E+06 6.3 1.5E+06 6.2 2.2E+05 5.3 2.1E+07 7.3

30 8.7E+05 5.9 9.3E+05 6.0 1.0E+06 6.0 4.5E+04 4.7 1.0E+06 6.0

312 8.9E+05 5.9 6.8E+05 5.8 1.3E+06 6.1 3.0E+04 4.5 9.0E+05 6.0

90 3.5E+03 3.5 1.4E+04 4.1 2.0E+06 6.3 1.8E+04 4.3 1.3E+04 4.1

180 73 1.9 1.2E+03 3.1 1.8E+06 6.3 1.3E+03 3.1 1.3E+03 3.1

1903 1.5E+03 3.2 4.6E+03 3.7 2.1E+06 6.3 4.2E+03 3.6 1.4E+03 3.1

Recovery control samples (no ClO2

)

Coliphage

E. coli Enterococcus B. anthracis Somatic F-specific

Time

(minutes) CFU/100 mL Log remaining CFU/100 mL Log remaining CFU/100 mL Log remaining PFU/100 mL Log remaining PFU/100 mL Log remaining

0 3.4E+06 6.5 1.9E+06 6.3 1.4E+06 6.1 1.2E+06 6.1 2.4E+07 7.45 3.2E+06 6.5 1.5E+06 6.2 1.7E+06 6.2 1.0E+06 6.0 2.1E+07 7.3

30 4.3E+06 6.6 1.9E+06 6.3 1.6E+06 6.2 1.4E+06 6.1 1.2E+07 7.1

312 4.5E+06 6.7 1.8E+06 6.3 1.6E+06 6.2 1.6E+06 6.2 1.9E+07 7.3

120 3.8E+06 6.6 2.3E+06 6.4 2.0E+06 6.3 1.3E+06 6.1 3.4E+07 7.5

1All microorganisms inoculated, mixed with high-flow agitation for 30 minutes, time = 0 collected, then supplemental pump deactivated.2Concurrent replicate sample collected immediately following 30-minute sample.3Supplemental pump reactivated following collection of 180-minute sample, run for 10 minutes, then final sample collected (time = 190).
Results 

9

Escherichia coli

-2

6420

0 60 120 180

Exposure time (minutes)

Log removal

TreatedUntreated

Enterococci

-2

6420

0 60 120 180

Exposure time (minutes)

Log removal

TreatedUntreated

Bacillus anthracis Sterne (spores)

-2

6420

0 60 120 180

Exposure time (minutes)

Log removal

TreatedUntreated

Somatic coliphage

-2

6420

0 60 120 180

Exposure time (minutes)

Log removal

TreatedUntreated

F-specific coliphage

-2

6420

0 60 120 180

Exposure time (minutes)

Log removal

TreatedUntreated

Figure 3. Results of field-scale trial of chlorine dioxide disinfection packets for target microorganisms at a single-family household sewage treatment system (HSTS). Shaded data points indicate densities measured following vigorous agitation (the difference between the density at 180 minutes and 190 minutes is attributable to settling).
10 Evaluation of Chlorine Dioxide as a Disinfectant for Onsite Household Sewage Treatment Systems 

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