EPA/600/R-02/077 

November 2003 

Research Summary
 

CSO Disinfection Pilot Study:
 

Spring Creek CSO Storage Facility Upgrade
 

by 

Izabela Wojtenko and Mary K. Stinson 

U.S. Environmental Protection Agency 

Urban Watershed Management Branch 

Edison, New Jersey, 08837 

Contract Number: 7C-R394-NTLX 

Project Officer 

Mary K. Stinson 

Water Supply and Water Resources Division 

National Risk Management Research Laboratory 

Cincinnati, Ohio 45268 

National Risk Management Research Laboratory 

Office of Research and Development 

U.S. Environmental Protection Agency 

Cincinnati, Ohio 45268 
Notice 

The U.S. Environmental Protection Agency (EPA) through the Office of Research and 

Development partially funded and collaborated in the research described here under contract 

No.7C-R394-NTLX to Camp Dresser & McKee of Woodbury, New York. It has been subjected 

to the Agency’s peer and administrative review and has been approved for publication as an EPA 

document. 

-ii­
Foreword 

The U.S. Environmental Protection Agency (EPA) is charged by Congress with 

protecting the Nation’s land, air, and water resources. Under a mandate of national 

environmental laws, the Agency strives to formulate and implement actions leading to a 

compatible balance between human activities and the ability of natural systems to support and 

nurture life. To meet this mandate, EPA’s research program is providing data and technical 

support for solving environmental problems today and building a science knowledge base 

necessary to manage our ecological resources wisely, understand how pollutants affect our 

health, and prevent or reduce environmental risks in the future. 

The National Risk Management Research Laboratory (NRMRL) is the Agency’s center 

for investigation of technological and management approaches for preventing and reducing risks 

from pollution that threaten human health and the environment. The focus of the Laboratory’s 

research program is on methods and their cost-effectiveness for prevention and control of 

pollution to air, land, water, and subsurface resources; protection of water quality in public water 

systems; remediation of contaminated sites, sediments and ground water; prevention and control 

of indoor air pollution; and restoration of ecosystems. NRMRL collaborates with both public 

and private sector partners to foster technologies that reduce the cost of compliance and to 

anticipate emerging problems. NRMRL’s research provides solutions to environmental problems 

by: developing and promoting technologies that protect and improve the environment; advancing 

scientific and engineering information to support regulatory and policy decisions; and providing 

the technical support and information transfer to ensure implementation of environmental 

regulations and strategies at the national, state, and community levels. 

This publication has been produced as part of the Laboratory’s strategic long-term 

research plan. It is published and made available by EPA’s Office of Research and Development 

to assist the user community and to link researchers with their clients. 

Lee A. Mulkey, Acting Director 

National Risk Management Research Laboratory 

-iii­
Abstract 

This Research Summary presents the results of a pilot-scale disinfection study performed 

for the New York City Department of Environmental Protection and the U.S. Environmental 

Protection Agency (US EPA) under a contract No. 7C-R394-NTLX to Camp Dresser & McKee 

of Woodbury, New York. The main objective of the pilot study was to demonstrate alternatives 

to hypochlorite disinfection for application to the Spring Creek facility and potentially to other 

combined sewer overflow (CSO) facilities. The pilot testing was divided into two phases. Phase 

I was performed from December 1996 through March 1997, and Phase II was performed from 

August through November 1999. US EPA provided technical assistance to the entire study. 

Phase I evaluated treatment performance of five technologies: ultraviolet (UV) irradiation; 

ozonation (O3); chlorine dioxide (ClO2) disinfection; chlorination/dechlorination (Cl2/deCl2); and 

electron beam irradiation (E-Beam). The fifth technology, E-Beam was evaluated during 

supplemental Phase I pilot testing sponsored by the New York Power Authority and the Electric 

Power Research Institute. Based on the results from Phase I, Phase II provided additional 

evaluation of technologies that had shown potential for CSO applications. These were UV, 

ClO2, and Cl2/deCl2. This Research Summary concentrates on these three technologies, but the 

overall results of both phases for each technology along with the cost are also discussed. 

Generally, all tested disinfection technologies, with the exception of E-beam, were able 

to achieve targeted bacterial reductions of 3 to 4 logs. Chlorination/dechlorination, ClO2, and O3 

at the doses tested were able to provide these levels of disinfection over the full range of 

wastewater quality tested. To date, ozonation was not found to be cost effective for CSO 

applications. While ClO2 was superior in effectiveness and similar in cost to Cl2/deCl2, the 

generation technology for ClO2 which avoids the need for gaseous Cl2 needs further 

development. Because an effective Cl2-gas-free process of ClO2 generation has not been proven 

to be reliable and, because Cl2 gas cannot be transported within New York City, disinfection 

with ClO2 cannot be recommended for use within New York City at this time. 

The Spring Creek facility upgrade construction was scheduled for the Fall of 2002. The 

upgraded Spring Creek facility will continue to use sodium hypochlorite for disinfection, with 

provisions to add dechlorination at a later date. Improvements will be made to increase 

disinfectant flash mixing and to automate hypochlorite feed and residual control. While the 

decision to continue the use of hypochlorination for the upgraded facility was based upon the 

Phase I study, it is recognized that the overall pilot results (Phases I and II) will be used to guide 

decisions at other CSO facilities. 

-iv­
CONTENTS


 Notice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii  


 Foreward . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii  


 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv  


 List of Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii  


 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii 
 

1. INTRODUCTION ..................................................... -1-
 

2. DESIGN AND OPERATION OF THE PILOT STUDY ....................... -1-
 

3. PILOT INVESTIGATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -4-
 

4.  PROCESSES INVESTIGATED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -4-
 

4.1 UV Light Disinfection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -4-
 

UV Pilot Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -7-
 

UV Pilot Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -8-
 

4.2 Chlorine Dioxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -8-
 

Chlorine Dioxide Pilot Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -9-
 

Chlorine Dioxide Pilot Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -13-
 

4.3 Chlorination/Dechlorination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -14-
 

Chlorination/Dechlorination Pilot Equipment . . . . . . . . . . . . . . . . . . . . . . . -14-
 

Chlorination/Dechlorination Pilot Operations . . . . . . . . . . . . . . . . . . . . . . . . . -14-
 

4.4 Ozone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -15-
 

Ozone Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -15-
 

Ozone Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -15-
 

5.  RESULTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -17-
 

5.1 Ultraviolet Irradiation ........................................... -17-
 

Dose-Response Relationships ..................................... -17-
 

Water Quality Relationships ...................................... -18-
 

Viral Reductions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -20-
 

Disinfection Byproducts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -20-
 

5.2 Chlorine Dioxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -20-
 

Dose-Response Relationships ..................................... -21-
 

Water Quality Relationships ...................................... -23-
 

Viral Reductions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -23-
 

Disinfection Byproducts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -23-
 

Chlorine Dioxide Residuals Vs Dose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -26-
 

5.3 Chlorination/Dechlorination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -27-
 

Dose-Response Relationships ..................................... -27-
 

Water Quality Relationships ...................................... -31-
 

5.4  Toxicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -33-
 

-v­
6. COST COMPARISON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -34-
 

7. CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -38-
 

7.1 Wastewater Quality ............................................. -38-
 

7.2 Treatment Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -40-
 

7.3 Disinfection Residuals and Toxicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -41-
 

7.4 Chlorine Dioxide Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -42-
 

8. SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -42-
 

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -44-
 

-vi­
List of Acronyms 

AOC - assimilable organic compounds 

APWA - American Public Works Association 

BMP - best management practice 

BOD - biochemical oxygen demand 

BOD5 - five-day biochemical oxygen demand 

Cl!

 - chloride 

Cl2 - chlorine 

ClO2 - chlorine dioxide 

ClO2 

!

 - chlorite 

ClO3 

!

 - chlorate 

COD - chemical oxygen demand 

deCl2 - dechlorination 

DNA - deoxyribonucleic acid 

DO - dissolved oxygen 

NaClO2 

-

 - sodium chlorite 

NPDES - National Pollutant Discharge Elimination System 

NRMRL - National Risk Management Research Laboratory 

NYCDEP - New York City Department of Environmental Protection 

O2 - oxygen 

O3 - ozone 

ORD - Office of Research and Development 

PAH - polycyclic aromatic hydrocarbon 

POTW - Publically Owned Treatment Works 

SS - suspended solids 

SSO - sanitary sewer overflow 

SW - stormwater 

TCOD - total COD 

THM - trihalomethane 

US EPA - United States Environmental Protection Agency 

UV - ultraviolet 

UWMB - Urban Watershed Management Branch 

WSWRD - Water Supply and Water Resources Division 

WWF - wet weather flow 

WWTP - wastewater treatment plant 

-vii­
Acknowledgments 

Camp Dresser & McKee (CDM) of Woodbury, NY, was contracted by the New York 

City Department of Environmental Protection (NYCDEP) to provide engineering design services 

for the upgrade to the Spring Creek CSO Storage Facility. This Research Summary is based on 

the original study reports prepared by CDM and CDM’s subcontractor, Moffa & Associates 

(currently Brown & Caldwell) of Syracuse, NY (CDM, 1997 and 1999). 

The authors are with the Urban Watershed Management Branch (UWMB) of the Water 

Supply and Water Resources Division (WSWRD) in Edison, NJ at the National Risk 

Management Research Laboratory (NRMRL) of the U.S. Environmental Protection Agency’s 

(US EPA’s) Office of Research and Development (ORD). 

Technical review was provided by Shirley E. Clark, Ph.D., P.E. Assistant Professor, 

School of Science, Engineering and Technology, Penn State, Harrisburg, PA. 

Editorial review was provided by Laura Panos of the Environmental Careers 

Organization. 

-viii­
1. INTRODUCTION 

Camp Dresser & McKee (CDM) of Woodbury, NY, was contracted by the New York 

City Department of Environmental Protection (NYCDEP) to provide engineering design services 

for the upgrade to the Spring Creek CSO Storage Facility. These services included a pilot study 

to evaluate the following disinfection technologies for treatment of combined sewer overflows 

(CSOs): ultraviolet (UV) irradiation; ozonation (O3); chlorine dioxide (ClO2) disinfection; 

chlorination/dechlorination (Cl2/deCl2); and electron beam irradiation (E-Beam). The pilot 

testing was divided into two phases. Phase I, from December 1996 through March 1997, 

evaluated performance of all five technologies listed above. The E-Beam was evaluated during 

supplemental Phase I pilot testing sponsored by the New York Power Authority (NYPA) and the 

Electric Power Research Institute. The E-Beam technology did not appear to be feasible for 

CSO disinfection, thus, the study results for this technology are not being discussed in this 

Summary. Phase II of the pilot testing, from August through November 1999, evaluated 

technologies which had been found promising for CSO disinfection during the first phase. Thus, 

Phase II further investigated UV, ClO2, and Cl2/deCl2 under additional wet-weather conditions. 

This Research Summary is based on the original study reports prepared by CDM and CDM’s 

subcontractor, Moffa & Associates (currently Brown & Caldwell) of Syracuse, NY (CDM, 1997 

and 1999). Because the original reports were prepared for internal use and were made not 

readily available to the Public, the authors believe that this summary will provide means to 

distribute the information to the Public. 

Chlorine has traditionally been used to provide disinfection due to its low cost. Since the 

1970s, growing awareness of the adverse environmental impacts associated with the byproducts 

of chlorination has led to increasingly restrictive residual Cl2 requirements. While the current 

State Pollution Discharge Elimination System permit for Spring Creek allows a maximum total 

residual Cl2 effluent limit of 2.0 mg/L, it is expected that more restrictive effluent limits, 

consistent with the water quality standard, will be required in the future. As a result of the 

impending restriction on the Total Residual Chlorine (TRC) in the effluent, dechlorination 

(deCl2) was considered for disinfection of CSO at the Spring Creek and at other CSO facilities. 

In addition, a need to minimize environmental risks, chemical demands, and contact times that 

are required using conventional Cl2/deCl2 has fostered a strong interest in alternative disinfection 

technologies for CSO. However, there is minimal data available on the effectiveness of 

alternative disinfectants for the treatment of CSO. This pilot study was conducted to provide the 

needed performance data on the alternative disinfection technologies for their possible future use 

at the Spring Creek or elsewhere. 

2. DESIGN AND OPERATION OF THE PILOT STUDY 

As mentioned previously, the objectives of the pilot study were to provide: (1) a basis for 

selection of a disinfection technology for use at the Spring Creek CSO Storage Facility, (2) fullscale design criteria for application at the Spring Creek CSO Storage Facility; and (3) treatment 

performance data to guide selection of disinfection technologies for other CSO facilities. The 

location for both phases of the pilot study was a designated area at the 26th Ward Wastewater 

-1-

Pollution Control Plant (WPCP) rather than the CSO Storage Facility due to space and logistical 

constraints. In addition, the 26th Ward site was able to provide a continuous supply of 

wastewater regardless of weather conditions and the 26th Ward primary effluent was 

representative of the range of wastewater quality found in the effluent from the Spring Creek 

CSO Storage Facility (CDM, 1997). However, it is important to indicate that the characteristics 

of wastewater in Phase I and Phase II were not the same. Figure 1 presents the schematic of the 

CSO Pilot location. 

Operation of the Phase I pilot units occurred over the period of December 17, 1996 to 

March 26, 1997. The 16 main test runs included a combination of 4 actual wet-weather and 12 

simulated wet-weather events. During Phase I, the UV, O3, ClO2, and Cl2/deCl2 pilots were 

operated in parallel for a total of 16 test runs. Testing of the units in parallel allowed comparison 

of disinfection efficiency of each technology on wastewater of identical quality. During testing, 

the operating conditions of each pilot unit were held constant over the duration of the test run. 

Flowrates, detention times, disinfectant dose, and mixing conditions were varied between test 

runs. The purpose of these test runs was to evaluate the performance of each technology over a 

range of operating conditions and over a range of wastewater quality typical of CSOs. 

Wastewater flow to the pilot facility was supplied from the primary settling tank effluent 

channel. Each pilot unit received flow from the common header. The flowrate to each pilot unit 

was manually controlled using in-line magnetic flowmeters and throttling valves. Treated pilot 

effluent was discharged directly into the 26th Ward plant recycle line through an existing 

manhole adjacent to the pilot facility. Figure 2 presents the pilot system flow schematic. Each 

pilot system was subjected to the same wastewater, in order to compare the performance of each 

pilot system directly against the other. 

Operation of the Phase II pilot units occurred over the period of August 25, 1999 to 

October 21, 1999. The same pilot location and source wastewater were used as in Phase I. 

During Phase II, the UV, ClO2, and Cl2/deCl2 pilots were operated in parallel over eight test 

events. Although these technologies had been investigated in Phases I and II, the tested units 

were from different manufacturers. The operating conditions of each pilot unit were held 

constant over the duration of the test run. Flowrates, detention times, disinfectant dose, and 

mixing conditions were varied between test runs. Phase II pilot study supplemented the Phase I 

results and performed additional research on the selected three technologies. The Phase II data 

was compiled with the Phase I data to evaluate the treatment performance over the full range of 

wastewater quality experienced. Ultimately, the Phase I and Phase II results were used to make 

recommendations and develop design criteria for full-scale CSO disinfection in New York City. 

This Research Summary concentrates on discussion of these results. 

In Phase II, the three pilot units were located side-by-side for concurrent operation 

alongside primary settling tank no. 4. This location provided a constant supply of wastewater 

and ready access to the primary settling tank effluent channel. The three pilot technologies were 

tested at various dosages, contact times and flowrates over a large range of wastewater 

-2-

Figure 1. CSO pilot location at the 26th Ward WPCP. 

-3-

conditions. Eight test runs were performed; four during wet-weather conditions and four during 

dry-weather conditions. The target dosages were designed to develop an appropriate doseresponse relationship for each technology and to supplement the Phase I results. As the 

analytical results from prior test runs were received, the dosages and operating conditions for 

subsequent test runs were adjusted and modified to achieve the desired range of bacteria kills. 

The actual chemical dosages and pilot unit operating conditions used during Phase II are 

provided in Table 1. This table illustrates the flowrate, disinfection dose and detention times 

which were provided for each event and for each individual pilot unit. 

The objectives of the entire study were limited to the relative comparison of alternative 

disinfection technologies and the evaluation of basic design parameters (e.g., dose, mixing 

configuration, contact time) for application to CSO. Optimization of other detailed design 

parameters and simulation of full-scale process configurations for use at the Spring Creek 

facility were beyond the scope of this effort. Additionally, the small size of the pilot units as 

compared to actual CSO flowrates, did not lend the pilot results to the determination of factors 

such as full-scale operational complexity and safety concerns, system constructability and size/ 

structural constraints, and technology cost effectiveness. This information can be determined 

from available full-scale facilities, but it was not within the scope of this study to do so. 

3. PILOT INVESTIGATIONS 

During the test runs, the pilot facilities were allowed to reach steady state conditions for a 

minimum of 30-min prior to initiating sampling. The UV unit was first started up on potable 

water in order to get an initial relative UV intensity reading on clean water. After the 

establishment of wastewater flow to the pilot facility, typical startup would proceed as follows: 

(1) balance wastewater flow to the individual pilot units, (2) restart the UV unit on wastewater, 

(3) startup of the Cl2/deCl2 and ClO2 mixers, (4) startup and set the hypochlorite and bisulfite 

feed pumps to the desired dose, and finally (5) startup and set either the ClO2 gas or liquid feed 

to the desired dose. Following this last step, at least 30 min were allowed to pass before 

beginning sampling. During each test run, samples of the common pilot influent and the treated 

pilot effluents were collected at various frequencies over the 2-h sampling period. In addition to 

offsite laboratory analyses, field monitoring of pH, temperature, dissolved oxygen (DO), 

oxidation/reduction potential probe (ORP), Cl2 and ClO2 residual were performed. Process 

monitoring and control parameters are summarized in Table 2. 

4. PROCESSES INVESTIGATED 

4.1 UV Light Disinfection 

UV disinfection is accomplished by electromagnetic radiation at specific wavelengths 

ranging from 100 to 400 nanometers (nm) with optimum disinfection at 253.7 nm. UV radiation 

-4-

Figure 2. The pilot system flow schematic. 

-5-

Table 1. Summary of pilot unit operating conditions during Phase II. 

Alternative 

DRY-WEATHER EVENTS WET-WEATHER EV

Run #1 Run #4 Run #7 Run #8 Run #2 Run #3 Run 

Viral #1 Viral #2 Viral #3 Viral #4 

UV Flowrate 140 113 140 58 113 77 58 

1 3 1 3 3 3 3 

UV Loading 3.6 7.1 3.6 13.8 7.1 10.4 13.Average UV 32.3 36.8 35.8 37.0 33.9 27.4 36.Absorbed 46.2 88.6 50.1 172.6 83.5 105.0 169

ClO2 Flowrate 32 32 32 32 32 32 32 

Concentration 8 6 8 6 8 6.5 

CDG UVD UVD, CDG UVD UVD UVD 

Mixing 1 stage 

Cl2/deCl2 Flowrate 32 32 32 32 32 32 32 

Chlorine 18 20 24 28 20 24 28 Mixing 1 stage 

Date 

Lamp Power 

10 

Generator 

2 stage 1 stage 1 stage 1 stage 2 stage 2 sta2 stage 1 stage 2 stage 1 stage 2 stage 1 sta

Note: 1. Power levels 1, 2 and 3 provide lamp power outputs of 125, 160, and 200 watts UV-C per lamp, respectively. 2. Absorbed UV dose calculated using UV manufacturer's empirical equation. 3. UVD Generator used during the first hour of sampling, while the CDG Generator was used during second hour. 

-6­
Table 2. Process Monitoring and Control Parameters. 

Process Monitoring Control Parameters 

Influent Wastewater Temperature 

pH 

Available Plant Wastewater Quality/Flow Data 

UV Pilot Disinfection System Wastewater Flowrate Through Pilot 

UV Dose 

Chlorine Dioxide Pilot Disinfection System Wastewater Flowrate Through Pilot 

Chlorine Dioxide Purity 

Chlorine Dioxide Dose 

Chlorination/deChlorination Pilot Wastewater Flowrate Through Pilot 

Sodium Hypochlorite Feed Concentration 

Sodium Hypochlorite Dose 

Sodium Bisulfite Dose 

Ozone Pilot Disinfection System Ozone Dose 

Wastewater Flowrate Through Pilot 

Beam Current 

E-Beam Pilot Disinfection System 	 Wastewater Flowrate Through Pilot 

Temperature Changes 

Absorbed Dose 

is the most common electromagnetic radiation technique used for disinfection. It is a physical 

process offering short detention times (5 to 7 sec). UV disinfection does not produce residuals 

or byproducts that are known to produce risk to humans or aquatic systems. Some concerns have 

been raised regarding the development of organism mutations, but no conclusive data exists. UV 

technology works on the principle that all microorganisms that contain nucleic acids are 

susceptible to damage through the absorption of radiation in the UV energy range. The extent of 

damage, mutation, or death will depend upon the organism’s resistance to radiation penetration. 

UV disinfection is well demonstrated for water and wastewater treatment. 

UV Pilot Equipment The UV pilot disinfection system was provided by Aquionics, Inc., of 

Erlanger, Kentucky, the same unit used during both phases. This skid mounted unit was 

manufactured by Berson Milieutechniek of Nuenen, Holland. The unit was a medium pressure, 

high intensity UV unit. The unit was housed inside a trailer to provide a barrier to climatic 

condition. A process flow diagram of the UV pilot unit is provided in Figure 3. 

The stainless steel reaction chamber was fitted with four high intensity mercury vapor 

lamps which were mounted for protection inside four quartz sleeves. Wastewater flowed 

through the unit in a horizontal, parallel-flow configuration. A quartz window was provided on 

the 

chamber wall through which UV intensity was monitored by a UV sensor. The bulbs were fitted 

-7-

with a mechanical wiper type automatic cleaning mechanism, mounted on a worm gear drive. 

The wiper removed deposits of materials from both the quartz sleeves and quartz window. The 

wiping frequency was set to once every 10 min throughout the pilot testing. A panel display 

indicated the wipe count. The effluent pipe from the reaction chamber was installed in such a 

way that it could be removed for inspection of the UV lamps. 

UV Pilot Operation The UV pilot unit was operated at a constant flowrate and lamp intensity 

throughout a sampling event. The flowrate and lamp power levels were varied between tests. 

Flowrate to the unit varied from 58 gal/min to 140 gal/min and was controlled manually using a 

flowmeter and throttling valve located at the influent side of the UV pilot. Lamp intensity was 

also controlled manually by adjusting the power level of the UV lamps to one of three preset 

power levels. Power levels 1, 2 and 3 provided lamp power outputs of 125, 160 and 200 w UVC per lamp, respectively. Treated UV effluent samples were collected from a sample port 

installed on the effluent discharge pipe and were analyzed for indicator bacteria (total coliform, 

fecal coliform, Escherichia coli, and enterococcus), UV transmissivity, particle size, volatile 

organic compounds (VOCs), semivolatile organic compounds (SVOCs), halogenated aromatic 

amines (HAAs,) Whole Effluent Toxicity (WET), and Microtox. All samples, except for the 

particle size samples, were discrete (grab) samples. The particle size sample was a composite 

sample made up of discrete samples taken at 15-min intervals over the duration of the test run. 

Absorbed dosage was also measured once per test event via a collimated beam test. The 

analytical results of the samples are summarized in Table 3. 

The UV pilot unit was found to be the simplest unit to operate, requiring minimal 

operator attention. As was the case with Phase I, the wiper mechanism periodically became 

jammed by rags or other bulk material in the wastewater. The jammed wipers were fixed easily 

by resetting the wiper fault condition and reversing wiper direction. At the end of the pilot 

operations, the quartz sleeves showed signs of slight reddish discoloration, possibly due to iron 

deposits, at the ends of the sleeves. In addition, a slight transparent film was present over the 

entire length of sleeve. In a full scale system this would likely necessitate periodic chemical 

cleaning of the quartz sleeves. 

4.2 Chlorine Dioxide 

Chlorine dioxide has proven its capabilities as an outstanding bactericide and viricide. It 

is ten times more soluble in water than Cl2. In contrast to Cl2, ClO2 does not react with ammonia 

and other nitrogenous compounds to form chlorinated organics, and its disinfection efficiency is 

high over a wide pH range. Due to its instability, ClO2 must be generated on-site on an as 

needed basis. It may be generated on-site by: acid/sodium chlorate generation; acid/sodium 

chlorite generation; chlorine/sodium chlorite generation (solution generators, gas-solid 

generators); and UV radiation/sodium chlorite generation. A recent advance involving UV 

radiation of sodium chlorite (NaClO2) has emerged as a new and innovative technology for ClO2 

generation. 

Chlorine dioxide is produced by this method through the disassociation of chlorite, a process that 

requires very little energy in the generation process. The primary benefit of this generation 

method compared to classical ClO2 generation methods is that Cl2 gas is not used in the 

-8- 
generation process. In Phase I, the Cl2 (gas) - NaClO2 (solid) generation process was used. 

During Phase II, the UV-chlorite generation process was intended to be used throughout the 

testing. However, due to operational problems with the UV-chlorite pilot generator, the backup 

gas-solid type generator was used. 

Typically for wastewater treatment, aqueous disinfectants such as sodium hypochlorite 

and ClO2 are injected into wastewater using a diffuser arrangement, without providing additional 

mixing. However, studies on CSO disinfection performed for the US EPA in Philadelphia, 

Syracuse, and Rochester demonstrated that disinfection performance could be significantly 

increased and required contact times reduced by providing mixing at high velocity gradients (G) 

(e.g., high-rate mixing) (US EPA 1978a and 1978b). These studies showed that values of GT (G 

x contact time [T]) between 10,000 and 100,000 provided effective disinfection at contact times 

of less than 5 min. Additionally, recent studies and operations experience with disinfection of 

secondary effluent have shown that high-energy mechanical mixing can improve chemical 

performance and minimize effluent residuals. Based upon this information, high-rate mixing 

was selected for use in the pilot testing. 

Chlorine Dioxide Pilot Equipment This pilot system consisted of two main components: the 

contact tank system and the ClO2 generator. The contact tank system included separate flash mix 

tank, contact tank and weir tank. The contact tank system was identical for the ClO2 and 

Cl2/deCl2 pilots. Two separate ClO2 generators were used as part of the pilot study. Ultra Violet 

Dioxide, Inc., (UVD) of Syracuse, New York, manufactured the first generator, which produced 

a ClO2 liquid using an innovative UV-chlorite process. The second generator, which produced a 

ClO2 gas using the Cl2 (gas)/chlorite (solid) process, was manufactured by CDG Technology, 

Inc., (CDG) of Bethlehem, Pennsylvania. Because the UVD system is a new technology, the 

system manufactured by CDG, used during Phase I was available as a backup system and was 

used during Test Runs No. 1, 7 and 8. 

UVD Pilot Equipment The UVD ClO2 generator generates ClO2 by passing an aqueous NaClO2 

solution through a UV reactor. The ClO2 gas is then stripped out of the NaClO2/ClO2 solution 

and sparked into water producing an aqueous ClO2 feedstock. Liquid ClO2 product, strength 

between 864 mg/L and 2,300 mg/L ClO2 was generated up to a week prior to testing. The ClO2 

product was then stored in a product tank, which was cooled to retard degradation. The product 

strength was then measured prior to testing. Based on the measured product strength and the 

desired dose, the ClO2 product was metered into the wastewater flash mix tank. 

Several difficulties were encountered in the operation of the UVD generator. Most of the 

difficulties can be attributed to the fact that the unit was still an early prototype. Chlorine 

dioxide could only be prepared on a batch basis and operator attention was required throughout 

the generation process. During test runs 2 through 6, volumes of gas in the product pump 

suction line would periodically air bind the product metering pump, potentially decreasing the 

-9-

Figure 3. A process flow diagram of the UV pilot unit. 

-10-

Table 3. UV pilot sample parameter table. 

SS

VSSSettleable SolidsBODSoluble BODTKN

CODTOCIron (total)

Escherichia coli 

Enterococcus Target Organic DBPs 

VOCs 

SVOCs HAAs Other 

Total 

76 

26 

26 

26 

76 

76 

26 

26 

26 

152 

656 

488 

152 

72 

68 

Analytical Reference 

Method 

EPA-160.2 

SM 20-9222 B (3) 

SM 20-9222 D (3) 

SM 20-9222 G (3) 

SM 20-9230 C 

EPA-624 / NYSDEC 

ASP 

ASP 

EPA-552.2 

(1) 

o C o C o C o C o C 

cool to 4o C, H2SO4 to 

pH<2 cool to 4o C, H2SO4 to pH<2 cool to 4o C, H2SO4 to pH<2 

cool to 4o

Na2S2O3 

cool to 4o

Na2S2O3 

o C 

cool to 4o C cool to 4o

chloride 

(2) 

7 d 7 d 

48 h 

48 h 48 h 

28 d 

28 d 

28 d 28 d 

6 h (4) 

(4) 

6 h (4) 

d analysis 

7 d 

2-L poly 2-L poly 

2-L poly 

1-L poly 1-L poly 

Conventional Parameters

Ammonia

Bacterial ParametersT. Coliform 

F. Coliform 

Number of 

Samples 

26 

80 

EPA-160.4 

SM 20-2540 F 

EPA-405.1 

EPA-405.1 

EPA-351.2 

EPA-350.1 

EPA-410.1/410.2 

EPA-415.1 

EPA-200.7 

EPA-625 / NYSDEC 

Sample Preservation 

cool to 4

cool to 4

cool to 4

cool to 4

cool to 4

 C, 10 mg  C, 10 mg 

cool to 4

 C, ammonium 

Maximum 

Holding Times 

6 months 

 6 h 

7 d 

5 d extraction, 40 

500-mL 

500-mL 

500-mL 500-mL 

250-mL 

250-mL 

250-mL 

250-mL 250-mL 

2 x 40-m

4 x 1-L a

250-mL 

-11­

Total Analytical Reference 

Method 

(1) 

(2) 

76 NA cool to 4o C NA 

(5) 76 NA cool to 4o C NA 

UV Collimated Beam 9 cool to 4o C 14 d 3 x 1-L p

cool to 4o C, 0.25 mL 10% 

Particle Size 9 NaOCl 14 d 500-mL 

Chlorate 93 EPA-300 D purge w/N2 gas (for 28 d 250-mL 

samples with ClO2), 50 

mg/L EDA, cool to 4o C, 

protect from light 

Chlorite 93 EPA-300 D 14 d 250-mL 

keep cool, protect from 

Whole Effluent Toxicity 34 EPA 600/4-90/027F light 48 h 5 x 1-gal 

cool to 4o C, 10 mg 

Viral 101 NA Na2S2O3 1 month 4 x 1-L p

Chlorine Residual (total and free) NA HACH DPD Method NA NA 

Chlorine Residual (amperometric NA SM 20-4500-CID NA NA 

titration) Chlorine Dioxide Residual NA NA 30 min 

HACH Chlorophenol Red Method 

Chlorine Dioxide Residual NA NA NA 

SM 20-8050, ASTM 

Microtox NA D5660-96 NA NA 

Number of 

Samples 

Sample Preservation Maximum Holding Times 

UV Transmissivity (unfiltered) 250-mL 

UV Transmissivity (filtered) 250-mL 

EPA-625.1/86.021 

SM 20-2560 

gas diffusion FIA and 

direct measurement 

Note: 1. Samples were preserved immediately upon sample collection; 2. Unless otherwise noted, all holding times are from the time o

For all 9222 methods, mastication of samples must be completed prior to analysis; 4. A maximum holding time of 6 hrs for all bacterial 

no time did holding times exceed 10 h; 5. Filtered UV transmissivity samples were filtered at the laboratory using a 0.45 un filter. A. Th

combined in the same 2-L polyethylene container; B. These parameters were combined in the same 1-L polyethylene container; C. Thescombined in the same 500-mL polyethylene container; D. These parameters were combined in the same sterilized 250-mL swirl containwere combined in the same 250-mL polyethylene container; F. These parameters were combined in the same 250-mL polyethylene con

-12-

disinfectant feed rate. Also, due to leaking fittings/valves, the NaClO2 solution leaked into the 

ClO2 product tank, resulting in a feedstock contaminated with up to 7,500 mg/L of chlorite. The 

maximum strength of ClO2 produced was 2,300 mg/L. 

CDG Pilot Equipment Because the UVD system was a new technology, the CDG ClO2 

generation system used during Phase I had been used as a backup system. In this system, ClO2 

gas was generated by passing a humidified 4% Cl2 gas in nitrogen blend through a packed 

column of solid NaClO2. A rapid reaction between the Cl2 gas and the NaClO2 results in a ClO2 

gas. The CDG unit was used to produce ClO2 during pilot test runs no. 1 and 8, and for half of 

pilot test run no. 7. In the contact tank system, wastewater flowed into the flash mix tank where 

ClO2 was mixed with the wastewater using a 1⁄2-hp, Series 32F, Gas Mastrrr, high-rate, chemical 

induction mixer. After the flash mix tank, the wastewater was directed into the contact tank. 

The contact tank was subdivided into 2 sections, one with corrugated longitudinal baffles and 

one with flat longitudinal baffles. Each section received equal flow, which was controlled by the 

adjustable v-notch weir plate located in the weir tank. The corrugated baffles were designed to 

provide a headloss of 1-in. H2O. The actual headloss was found to be approximately 0.75-in. 

H2O. This provided a gradient G for the corrugated baffled section of 303 s-1. In addition the 

contact tanks also had a second mixer located at a contact time of 0.5 min to evaluate the 

performance of single-stage versus two-stage mixing. For the mixing configuration used during 

each test event and the design conditions of the contact tank system please refer to the original 

CDM report (CDM, 1999). 

Chlorine Dioxide Pilot Operation The ClO2 pilot system was operated at a constant flowrate 

and chemical dose throughout each sampling event. Flash mixing and the addition of 

disinfectant into the wastewater stream was performed in all test runs using the high-rate 

chemical induction mixer in the flash mix tank. The corrugated and flat baffled tank sections 

were operated for all 8 test runs, and samples were collected at three different contact times (2.7, 

5 and 8 min) in each section to measure treatment performance. The majority of the sampling 

was performed at the 5-min sample port in the flat baffled section, to be consistent with the 

Phase I test conditions. Samples for bacterial parameters, target organic DBPs, chlorate, 

chlorite, WET and microtox were collected at the 5-min sampling location. Process monitoring 

for TRC and ORP was performed at a detention time of approximately nine minutes on the flat 

baffled section only. Monitoring for pH and ClO2 was performed at the effluent of the weir tank. 

Bacterial samples for fecal coliform and Escherichia coli were also collected at the 2.7-min and 

8-min contact times in both the baffled and unbaffled sections to correlate inactivation against 

contact time. The analytical results are summarized in the Results section. 

Because of questions on the accuracy of the UVD ClO2 product strength measurements, 

pilot test runs no. 2 through 7 may have been underdosed with ClO2. Without accurately 

knowing the ClO2 product strength, a reliable relationship between applied dose and toxicity 

could not be established. In order to make this correlation an additional test was performed. 

Using the ClO2 product from the CDG system, wastewater was dosed with five different known 

dosages of ClO2 (4, 6, 8, 10, and 12 mg/L ClO2). 

-13- 
4.3 Chlorination/Dechlorination 

Chlorine has been the most widely used disinfectant for wastewater and potable water in 

the United States due to its low cost, reliable disinfection capabilities, and adequate supply. 

Generally, bacteria are more susceptible to Cl2 than viruses. The disinfection effectiveness of 

Cl2 is largely a function of the chemical form of the disinfecting species. Chlorine is applied to 

the waste stream in molecular (Cl2) or hypochlorite (OCl-

) form. Chlorine is available in many 

forms including Cl2 gas and Cl2 products such as sodium and calcium hypochlorite. Liquid 

sodium hypochlorite has become widely used for wastewater disinfection due to its reliability 

and ease of handling. Sodium hypochlorite can be purchased in bulk forms of 10 to 15% of 

available Cl2 or can be manufactured on site. Sodium hypochlorite has limited shelf-life and is 

subject to loss of available Cl2 content by decay to Cl2 gas. Sufficient mixing, contact time, and 

dosages are necessary to maximize the use of Cl2 disinfection. 

Dechlorination may be accomplished through injection of a solution of sodium bisulfite 

(NaHSO3) or sulfur dioxide (SO2) gas into the process flow, following the chlorination process. 

Figure 4 presents Cl2/deCl2 pilot unit flow schematic. The deCl2 process is nearly an 

instantaneous process. A potential problem with deCl2 is the possible depletion of dissolved 

oxygen by excess sulfite ion. 

Chlorination/Dechlorination Pilot Equipment The Cl2/deCl2 pilot unit mixing configuration 

and sampling program was generally identical to that of the ClO2 system, with the exception of 

deCl2. In this pilot system an 8 to 15% solution of sodium hypochlorite was introduced, using a 

chemical metering pump, through the vacuum port on the 1⁄2-hp Gas Mastrrr chemical induction 

mixer. Dechlorination was performed in the weir tank downstream of the flow control weirs by 

injecting a 38% sodium bisulfite to the waste stream near the impeller of a small propeller mixer 

to provide for deCl2 of both free and combined Cl2 residual. Dosages of OCl- and NaHSO3 were 

controlled manually by adjusting the pump stroke/frequency. The pilot system included residual 

instrumentation for continuous monitoring of DO, total residual Cl2, deCl2 residual and ORP. 

The Cl2 and ClO2 contact tanks were both identical in design. 

Chlorination/Dechlorination Pilot Operations Operations of the Cl2/de Cl2 pilot were similar 

to the ClO2 pilot. The Cl2/de Cl2 pilot system was operated at a constant flowrate and chemical 

dose throughout each sampling event. Flash mixing and the addition of disinfectant into the 

wastewater stream were performed in all test runs using the high-rate chemical induction mixer 

in the flash mix tank. The corrugated and flat baffled tank section were operated for all 8 test 

runs, and samples were collected from three different contact times (2.7, 5, and 8 min) in each 

section to measure treatment performance. The majority of the sampling was performed at the 5­

min sample port in the flat baffled section, to be consistent with the Phase I test conditions. 

Samples for bacterial parameters, target organic disinfection byproducts (DBPs), chlorate, 

chlorite, WET, and Microtox were collected at the 5-min sampling location. Process monitoring 

for TRC and ORP was performed at approximately nine minutes on the flat baffled section only. 

Monitoring of the deCl2 effluent was performed by measuring the DO and the free residual Cl2 at 

the effluent of the deCl2 tank. Bacterial samples for fecal coliform and Escherichia coli were 

also collected at the 2.7-min and 8-min contact times in both the baffled and unbaffled sections 

-14-

to correlate inactivation against contact time. The analytical results can be found in the Results 

section. 

4.4 Ozone 

Ozone is a chemical oxidizing agent that has been widely used for disinfection of 

drinking water systems and bleaching in the pulp and paper industry. It is an extremely strong 

oxidant and is well established for its powerful antibacterial and antiviral properties (Wojtenko 

et al. 2001). Ozone is a rapid disinfectant, requiring substantially less contact time than 

conventional chlorination disinfection systems. Based upon research performed by the US EPA 

in the 1970s and early 1980s, O3 was considered to be one of the most feasible disinfection 

alternatives to Cl2; the other technology being UV radiation. However, there presently are few 

operating facilities using O3 for disinfection of municipal wastewater. This may be attributable 

to the relatively high initial capital costs associated with O3 generation equipment and the poor 

operating records of previous O3 generators. 

Ozone Equipment The ozone unit was a trailer mounted system manufactured by Aquifine 

Wedeco Environmental Systems, Inc., (AWES) of Valencia, California. Ozone was generated 

on-site and on-demand using 90% pure oxygen and a corona discharge type O3 generator. 

Oxygen was supplied using an air compressor and a pressure swing adsorption oxygen 

generation unit. Ozone was transferred to the wastewater using a pressure booster pump and a 

gas eductor. Following the eductor, contact time for disinfection was provided by a baffled 

contact tank. The tank was sized to provide a minimum detention time of 10 min at a flow of 10 

gal/min. Flow out of the tank was controlled by level sensors and an automatic valve. 

Ozone Operations The O3 pilot was operated at an approximately constant wastewater flowrate 

and O3 feed gas concentration throughout each sampling event. Some variation in wastewater 

flowrate occurred as a result of clogging of the strainer baskets. However, this variation was 

generally only significant when primary influent wastewater was used as the source water. This 

wastewater contained large particles of solids and waste which rapidly clogged the basket 

screens. Several difficulties were encountered in the operation of the O3 pilot. Some of these 

can be attributed to the use of the eductor mass transfer system rather than the technology itself. 

The O3 pilot could not be operated during test runs no. 1 and 2 due to a problem with a 

low differential pressure alarm. This may have been related to potential clogging of the booster 

pump impellers. The pressure transmitter alarm circuit was disconnected and the unit was 

successfully operated from runs no. 3 through 16. For runs no. 3 through 6, the booster pump 

discharge pressure was observed to be lower than required for normal operation. Lower 

pressures may have resulted in lower mass transfer efficiency in the eductor. During run no. 6, 

-15-

Figure 4. Chlorination/dechlorination pilot unit flow schematic. 

-16-

the unit was subject to numerous shutdowns during the test period due to clogging of the basket 

strainer and the pump impellers. Following cleaning of the booster pump, the pump discharge 

pressure in runs no. 7 through 16 was much improved. In these runs, the pump discharge 

pressure was generally in the range of 58 to 65 lb/in2

. In addition, this particular unit was subject 

to O3 gas leaks. Because of the difficulties observed with the system during Phase I, the 

technology was found not to be feasible for CSO applications at this time. As a result, this 

technology was not investigated during Phase II, and this Research Summary does not discuss 

the ozonation study results. 

5. RESULTS 

5.1 Ultraviolet Irradiation 

Dose-Response Relationships Scatter plots were developed for dose versus log reduction and 

effluent bacterial concentrations for each bacteria group. These relationships were developed to 

identify the dose required to achieve a range of bacterial log reductions and effluent 

concentrations. These dose-response relationships were generated for total coliform, fecal 

coliform, Escherichia coli, and enterococcus data. Blending was performed on the samples in 

order to release potentially entrained bacteria by shearing the solid particles without causing 

significant kills to the bacteria. Blending requirements for CSO wastewater samples were 

initially developed by US EPA (US EPA 1975). An example of the dose-response relationship 

generated for fecal coliform is presented in Figure 5. The figure shows the Phase I doseresponse curve along with the Phase II scatter plot. In general, these graphs demonstrate that as 

UV dose increases, log reduction of bacteria also increases while the effluent mean concentration 

decreases. 

The dose-response relationships provide two obvious trends: a tailing-off effect for UV 

effectiveness, and higher variability of wet weather data vs. dry weather data. Based on the 

dose-response relationship, it appears that as dose increases in the range of 10 to 75 mW/cm2

, log 

reduction of bacteria also increases. However, as dose increases above the 75 mW/cm2 UV, 

effectiveness tails-off. The tailing effect in the dose-response data is more clearly shown in the 

collimated beam data in Figure 6. Above approximately 100 mW/cm2

, the lab collimated beam 

dose curve becomes asymptotic to 100 cfu/100 mL of fecal coliform. The pilot unit data follows 

the same trend, although more variable and with a higher asymptote. 

The wet-weather runs captured during the Phase II pilot study were generally the result of 

large storms. Consequently, the dose-response data show much more variability for the wetweather data than for the dry-weather data. The bacteria log reduction data for dry-weather tend 

to be grouped together; whereas the wet-weather data are more widespread. The variability in 

the wet-weather dose-response data can be attributed to the variability in the pilot influent 

wastewater during wet-weather events. This is likely the result of variable solids concentration 

and particle size, both known to significantly affect UV disinfection performance. 

-17- 
Table 4 summarizes the estimated range of UV doses required to achieve corresponding 

bacterial concentrations of 1,000 cfu/100 mL, 3-log and 4-log bacterial reductions. These 

targeted doses were based on the Phase II and adjusted Phase I results. The dose required to 

achieve the fecal coliform effluent target compares favorably with the results of Phase I, after the 

UV doses were recalculated. Additionally, data from both Phase I and Phase II show that the 

lowest effluent fecal coliform concentration achieved by UV disinfection was approximately 100 

cfu/100mL. 

Table 4. Targeted UV dose. 

UV Dose (mWs/cm2) 

Parameter 1,000 cfu/100ml 3-log 4-log 

reduction reduction 

Total Coliform N/A 40 120-180 

Fecal Coliform 60-120 30 60-120 

Escherichia coli 40-80 30-40 75-160 

Enterococcus 75 40-80 130-160 

Water Quality Relationships Constituents such as suspended solids (SS) and iron absorb UV 

light, thus decreasing the available light intensity within the reactor, which in turn reduces the 

UV dose. Additionally, constituents such as SS limit the exposure of bacteria to UV radiation by 

shielding or harboring the bacteria from exposure to the UV light. 

Plots of UV transmittance versus SS and iron concentration are presented in Figure 7. 

Plots of SS concentration versus log reduction and effluent fecal coliform concentration by dose 

are shown in Figure 8. This relationship indicates a slight trend of reduced disinfection 

effectiveness with increasing SS concentrations. This trend is likely the result of harbored 

bacteria within the solids, a phenomenon that is not accounted for when measuring UV 

transmittance. The phenomenon of harbored bacteria is also likely the cause of the “tailing-off” 

effect. Because the bacteria are entrenched in the solids they are not exposed to the UV light, 

and therefore are not destroyed. The relatively constant bacteria concentrations at the higher UV 

doses could reflect a relatively constant concentration of harbored bacteria. Besides SS, no other 

water quality parameters showed a positive correction to UV disinfection effectiveness. These 

water quality results, as they relate to UV disinfection effectiveness, were similar to the trends 

observed during Phase I. 

-18-

Figure 5. UV dose response relationship for fecal coliform. 

-19-

Viral Reductions All four viral runs clearly demonstrated that UV disinfection produced 

nearly complete reductions of bacteriophage. As a result, no apparent relationship between dose 

and phage reductions could be drawn from this data set. However, the reductions provided by 

UV disinfection would likely inactivate most wastewater enteroviruses at concentrations in 

CSOs. Table 5 presents the results of the viral disinfection by UV dose and phage reductions. 

Because of the small number of positive observations of naturally occurring enteroviruses in the 

pilot influent, the UV treatment could not be evaluated satisfactorily on the basis of the tissue 

cultural infectivity assays. 

Disinfection Byproducts No disinfection byproducts were detected in the UV effluent with 

greater concentrations than in the pilot influent. UV has the distinct advantage of producing 

little or no byproducts that may cause a concern for toxicity. Haloacetic acids and semi-volatile 

and volatile organic compounds, as well as acute whole effluent toxicity were measured in the 

UV 

effluent. These samples were used as being representative of the pilot influent because there was 

essentially no difference between the UV effluent and the influent. 

Table 5. UV viral reductions. 

Parameter Sample location Run #1 Run #4 Run #7 Run #8 

UV Dose (mWs/cm2

) 43 75 43 145 

T4 & f2 

Seeded influent concentration (PFU/mL) 1x103 500 3.7x105 3.5x105 

Effluent concentration (PFU/mL) 15 0 0 60 

Log reduction 1.9 2.7 5.6 3.9 

S2 & X174 

0

Seeded influent concentration (PFU/mL) 1x104 2x104 7x105 * 1.1x106 * 

* 0 * Effluent concentration (PFU/mL) 150 0 

Log reduction 2.9 4.3 5.8 6.0 

* ·X174 not included in these samples 

5.2 Chlorine Dioxide 

The ClO2 disinfection pilot was operated for eight runs at a controlled flow of 32 gal/min. 

The flowrate was held relatively constant during each run. Difficulties were encountered with 

the ClO2 injection system, which are believed to have resulted in lower actual applied dosages 

than calculated. This became apparent when the results using the UVD system were compared 

against those for the CDG system at the same calculated dose. Due to these mechanical 

difficulties, the CDG ClO2 generator system was used for runs 1 and 8, and part of run number 7. 

The above mentioned mechanical difficulties excluded the use of data collected during runs 

number 2, 3, 4, and part of 7, while laboratory difficulties excluded the use of the data collected 

during the other part of run number 7. Only data from runs number 1, 5, 6, and 8 were 

considered valid and were used in the data analyses. This limited the amount of data and 

-20- 
restricted the ClO2 dose range tested to 6.5 mg/L to 10 mg/L. 

Dose-Response Relationships Scatter plots were developed for dose versus log reduction and 

effluent bacteria concentrations for each bacteria group. These relationships were developed (for 

total coliform, fecal coliform, Escherichia coli, and enterococcus data) to identify the dose 

required to achieve a range of bacterial reductions and effluent concentrations. The dose versus 

log reduction plot for fecal coliform is shown in Figure 7. The Phase II data exhibited a high 

degree of variability; more so than the Phase I data. This variability is similar to the UV and 

Cl2/deCl2 results and is at least partially attributable to more variable influent wastewater quality 

resulting from the greater number of and more intense storm events in Phase II. The Phase II 

data does show an increase in bacterial log reduction between 6.5 and 8 mg/L, as would be 

expected, and a slight decrease between 8 and 10 mg/L. It is possible that the actual dose was 

less than 10 mg/L due to the malfunctioning ClO2 injection system. Except for the total coliform 

data, Phase I data shows a higher degree of inactivation at a given dose than Phase II. This could 

be due to differences in wastewater quality between the two Phases. 

It was difficult to discern the difference between wet-weather versus dry-weather 

disinfection efficacies because of the way that the pilot units were operated. For instance, the 

CDG ClO2 product was used during runs number 1 and 8, both dry-weather runs, and the UVD 

ClO2 product was used during runs number 5 and 6, both wet-weather runs. This limited data set 

and its variability made it difficult to differentiate between performance during wet-weather and 

dry-weather events. Table 6 summarizes the estimated range of ClO2 doses required to achieve 

corresponding bacterial concentrations of 1,000 cfu/100 mL, 3-log and 4-log bacterial 

reductions. These target doses were based on both Phase I and Phase II results. In general, the 

performance of the ClO2 system was less effective during Phase II than during Phase I. For 

example, the Phase II results for fecal coliform reduction were approximately 0.5 log less than 

Phase I results at equivalent ClO2 dose (Figure 9). 

Comparisons of these pilot results with full-scale facilities cannot be done since there are 

presently no known facilities that use ClO2 to disinfect CSO or municipal wastewater. However, 

these results are consistent with bench-scale and field pilot work previously done under US EPA 

funded demonstrations (US EPA 1975; US EPA 1979). 

Table 6. Targeted ClO2 doses. 

Parameter ClO2 Dose (mg/L) 

1,000 cfu/100mL 

Total Coliform 9 

Fecal Coliform 7 5-7 

Escherichia coli 6-8 6-9 

Enterococcus 6-9 >10 

3-log reduction 4-log reduction 

N/A N/A 

8-10 

4-8 

6-9 

-21­
Figure 6. UV collimated beam results. 

-22-

Water Quality Relationships Disinfection performance is dependent upon wastewater 

characteristics such as SS and BOD. Many constituents found in wastewater limit disinfection 

by either exerting a disinfectant demand or shielding bacteria from contact with the disinfectant. 

Suspended solids limit the exposure of the disinfectant by shielding or harboring bacteria from 

contact with the disinfectant. 

Similar to Phase I, trends of reduced disinfection effectiveness as a result of increased 

concentrations of TKN, BOD, COD and TOC were not apparent. In general, the concentrations 

of these parameters measured during Phase I and Phase II were comparable, albeit Phase II was 

characterized by greater variability. Based on both Phase I and Phase II results, it does not 

appear that these parameters have a significant affect on ClO2 disinfection efficiency at the 

concentrations measured. 

In contrast to Phase I, SS concentrations appeared to affect disinfection effectiveness; as 

SS concentration increased disinfection effectiveness decreased. This relationship is depicted in 

Figure 10. During Phase II, a higher percentage of the samples had SS concentrations greater 

than 120 mg/L, thus offering more information regarding the effects of higher SS concentrations. 

These data indicate that as SS concentration increase, disinfection effectiveness decreases. 

Viral Reductions For test runs no. 1 and 7, ClO2 disinfection produced nearly complete 

reductions of bacteriophage. Data from the second viral run (run no. 4) was not included in this 

analysis because of the operational difficulties experienced with the ClO2 disinfection system. 

Data from the last viral run (run number 8) showed lower phage reductions than the other runs. 

It is not clear why the viral reduction is lower for this test run. By comparison the mean fecal 

reduction during test run no. 8 was 3.0 log. Because of the relatively limited data set, no 

apparent relationship between dose and phage reductions could be drawn from this data set. 

However the reductions provided by ClO2 disinfection would be expected to inactivate most 

wastewater enteroviruses at the concentrations in CSOs. Table 7 presents the results of the ClO2 

disinfection by dose and phage reductions. Because of the small number of positive observations 

of naturally occurring enteroviruses in the pilot influent, the ClO2 treatment could not be 

evaluated satisfactorily on the basis of the tissue cultural infectivity assays. 

Disinfection Byproducts The generation of toxic byproducts and disinfectant residuals has 

become a concern for chemical disinfectants. Byproducts from the reaction of ClO2 with 

wastewater, depending upon the generation process, include chlorate ion, chlorite ion, and Cl2. 

However, ClO2 produces far fewer byproducts than Cl2 and is a more effective disinfectant 

because of its superior penetration characteristics and bactericidal properties. The main 

byproducts of ClO2 disinfection are chlorite ion and chlorate ion. The presence of these ions can 

be the result of both the ClO2 generation process and reactions in the wastewater. 

There is little data available on the toxicity of chlorite and chlorate. Only two toxicity 

studies relating to lethal concentrations of chlorite and chlorate were found after a search of the 

-23-

Figure 7. UV transmittance versus SS and iron concentration. 

-24-

Figure 8. SS concentration versus log reduction and effluent fecal coliform concentration by

 dose. 

-25-

US EPA’s ECOTOX database. The first was a chlorate toxicity study predominately related to 

fresh and salt-water algae species. The second was a chlorite toxicity study by the US EPA’s 

Office of Pesticide Programs on many fresh water species including opossum shrimp. The 

opossum shrimp resulted in LC50 values of approximately 0.6 mg/L. It is important to note that 

the toxicity data provided in the US EPA’s ECOTOX database are from single chemical 

exposures, and therefore, do not reflect synergistic affects with other chemicals. Additionally, 

they may be the result of different experimental designs. 

Residual chlorate ion and chlorite ion concentrations were evaluated to determine if there 

was a correlation between these residuals and ClO2 dose, and if the concentrations of these 

byproducts exceed regulatory standards. The data does not show a trend of increasing chlorate 

or chlorite ion concentrations with increasing ClO2 doses as was observed in Phase I. However, 

the Phase II data set was very limited due to the operational problems experienced with the UVD 

Inc. generator, and therefore, the trends observed during Phase I should be considered more 

reliable. 

The chlorite and chlorate data from Phase I demonstrate that a relatively small amounts 

of chlorite and chlorate were found in the pilot effluent when using the CDG ClO2 product. 

During Phase I, effluent chlorite generally ranged from 0.5 to 4 mg/L for applied CLO2 doses of 

4 to 10 mg/L. Conversely, relatively high concentrations of chlorite and chlorate were found in 

the pilot effluent when using the UVD ClO2 product. This is the result of mechanical problems 

encountered with the UVD prototype ClO2 generator. These mechanical problems were 

discussed in greater detail in the previous section. It is important to note that UVD Inc. 

corrected these mechanical problems after the testing was complete, and produced a ClO2 

feedstock with chlorite and chlorate concentrations of 44 and 210 mg/L, respectively. At the 

highest ClO2 dose of 10 mg/L, these concentrations of feedstock would contribute approximately 

0.22 and 1.1 mg/L, to the effluent chlorite and chlorate concentrations, respectively. 

Total residual oxidant (as Cl2) was also evaluated to determine if there was a correlation 

between TRC and ClO2 dose, and if the concentration of TRC exceeds regulatory standards. 

Total residual oxidant concentrations were low; the highest observed concentration of TRC was 

1.5 mg/L. It should be noted that the DPD TRC method used did not differentiate between the 

various oxidizing forms of chlorine and included Cl2, ClO2, ClO2 and ClO3. 

Chlorine dioxide residual concentrations were measured to identify the lowest ClO2 dose 

at which a residual would be detected. However, this relationship could not be developed due to 

the limited data set. Measurable residual ClO2 concentrations were found at a dose of 8 mg/L, 

which corresponded to runs when the CDG ClO2 unit was used. However, no ClO2 residual was 

apparent when the UVD ClO2 unit was used, even at a dose of 10 mg/L. Based on water quality 

data such as organics, it is unlikely that the ClO2 demand explains the lack of residual when 

using the UVD ClO2 unit. It is more likely that the actual ClO2 dose using the UVD ClO2 unit 

was less than 10 mg/L due to the operational problems, discussed previously. 

Chlorine Dioxide Residuals Vs Dose Similar to Phase I, ORP sensors and data loggers were 

-26-

installed on the effluent side of the ClO2 contact tank. No relationship between ORP and ClO2 

residual concentrations was apparent. Conversely, a strong correlation between ORP and TRC 

was apparent, thus producing a n44early linear trend throughout the range of ORP and TRC 

values that were measured. The ORP values quickly rose and stabilized at a relatively high ORP 

reading (> 650 mV) and consequently the Phase I data was not suitable for developing a 

relationship as a process control technique. The Phase II ORP versus TRC data shows more 

potential for developing a relationship as a process control technique than the Phase I data. 

More testing is needed to determine the discrepancy between Phase I and Phase II data. 

Table 7. Chlorine dioxide viral reductions. 

Parameter Sample Location Run #1 Run #7 Run #8 

ClO2 Dose (mg/L) 8 10 8 

T4 & f2 

Seeded influent concentration (PFU/mL) 1x103 3.7x105 3.5x105 

Effluent concentration (PFU/mL) 0 0 5x103 

Log reduction 3.0 5.6 1.9 

MS2 & X174 

Seeded influent concentration (PFU/mL) 1x104 7x105 1.1x106 

Effluent concentration (PFU/mL) 0 0 3x103 

Log reduction 4.0 5.8 2.9 

5.3 Chlorination/Dechlorination 

The Cl2/deCl2 disinfection pilot was operated for 8 runs during the pilot study at a 

controlled flow of 32 gal/min. The pilot flowrate was held relatively constant during each run. 

Dose-Response Relationships Scatter plots were developed for dose versus log reduction and 

effluent bacteria concentrations. These were developed for each bacterial group to identify the 

dose required to achieve a range of bacterial log reductions and effluent concentrations. The 

dose-response relationships for total coliform, fecal coliform, Escherichia coli, and enterococcus 

data had been generated. The example of one for fecal coliform is presented in Figure 11. 

Similar to the UV and ClO2 results, the Phase II Cl2/deCl2 data exhibited a high degree of 

variability. This is at least partially attributable to the more variable influent wastewater quality 

resulting from the greater number and more intense storm events during Phase II. The Phase II 

effluent bacteria concentrations for doses of 18, 20, and 24 mg/L are generally consistent with 

the results from Phase I, although in most cases the Phase II data exhibits a trend of poorer 

disinfection performance than Phase I. It is interesting to note that between doses of 24 and 28 

mg/L, no further decrease in effluent bacteria concentrations is observed. In some cases an 

increase in effluent bacteria is apparent at the higher dose. 

-27- 
Figure 9. Chlorine dioxide dose versus log reduction plot for fecal coliform. 

-28-

Figure 10. Chlorine dioxide fecal coliform dose response relationship for SS. 

-29-

The factors which may contribute to these observations include variations in wastewater 

quality, pH, and temperature. Wastewater temperature, for instance, is known to significantly 

affect the rate of reaction of hypochlorous acid (free Cl2) with ammonia to form 

monochloramine. During Phase I, the mean wastewater temperature was 11.6 o

C, versus a 

temperature of 20.9 o

C for Phase II. The colder winter temperatures would impede the formation 

of monochloramine, which has approximately 25 time less germicidal efficiency as free Cl2. 

This effect is corroborated to some degree by the Cl2 residual measurements during Phases I and 

II. As seen in Table 8, the lower doses during Phase I, when the mean water temperature was 9.3 o

C cooler, produced significantly higher free residuals than were observed for the higher applied 

doses of Phase II. 

The decrease in treatment performance at a dose of 28 mg/L corresponds with a change 

in the chlorine-ammonia chemistry. The poor performance at this dose occurred during test run 

no. 5, a wet weather event when the mean ammonia concentration dropped to 5.6 mg/L versus an 

average dry weather concentration of 13.6 mg/L. The resulting Cl:N weight ratio of 5.0 

corresponds with the typical inflection in the chlorine-ammonia breakpoint curve. Beyond the 

break, further Cl2 addition will actually decrease the measured residual as dichloramine and 

organochloramines are formed. Although dichloramine is believed to be approximately twice as 

germicidal as monochloramine, organochloramines are generally nongermicidal. This and other 

factors may partially explain the tailing effect observed at these high doses. 

Table 8. Mean residual chlorine vs. applied dose for Phases I and II. 

Applied Cl2 Dose Mean Total Residual Chlorine mg/L (DPD Method) 

(mg/L) Phase I Phase II 

12 9.0 -

16 13.5 -

20-21 17.0 -

24-25 20.0 ­

18 - 5.5 

20 - 7.0 

24 - 10.0 

28 - 13.0 

The Phase II log reduction plots also depart from the Phase I data at the applied doses of 

24 and 28 mg/L. This can be attributed to the tailing effect observed in the effluent bacteria 

concentration plots and to the dilution of the untreated bacteria concentrations during wet 

weather events. The Phase II data for doses of 24 and 28 mg/L include data from wet weather 

events no. 3 and 5. In these events, the mean influent fecal coliform, for example, was only 

1.06E+05 and 8.32E+05 respectively versus an average dry weather concentration of 4.3E+06. 

-30-

So for similar treated effluent concentrations, the log reduction may be lower due to dilution of 

the influent. 

Table 9 summarizes the estimated Cl2 dose required to achieve corresponding bacterial 

concentrations of 1,000 cfu/100 mL and 3- and 4-log bacterial reductions. These doses were 

based on the results of Phase I and Phase II. The Phase II dose required to achieve the fecal 

coliform effluent target did not exactly match with the results of Phase I. The Phase II results for 

fecal coliform reduction were approximately 0.5 log less than the Phase I results. Similarly, the 

effluent fecal coliform concentration results were approximately 0.5 log greater than the Phase I 

results. These results are consistent with work previously done under US EPA supported efforts 

(US EPA 1975, US EPA 1979). Similar log reductions of bacteria resulted for the range of doses 

of Cl2 tested under this pilot study. 

Table 9. Targeted Cl2 doses. 

Parameter Cl2 Dose (mg/L) 

1,000 cfu/100mL 

25 N/A 

Fecal Coliform 20 12 20-28 

Escherichia coli 17-22 12 20-28 

Enterococcus >22-28 N/A 

3-log reduction 4-log reduction 

Total Coliform N/A 

>22-28 

Water Quality Relationships Generally, disinfection performance is dependent upon 

wastewater characteristics such as SS, ammonia, and BOD. Many constituents found in 

wastewater limit disinfection by either exerting a Cl2 demand or shielding bacteria from contact 

with Cl2. Suspended solids limit the exposure of embedded bacteria by shielding them from 

contact with the disinfectant. Plots of SS concentration versus log reduction and log 

concentration of fecal coliform for the different doses are shown in Figure 12. These 

relationships show that there is no apparent trend between disinfection effectiveness and SS. 

This is particularly evident at a dose of 24 mg/L for SS concentrations ranging from 200 to 500 

mg/L. 

Residual chlorate and chlorite concentrations were evaluated to determine if there was a 

trend between these residuals to sodium hypochlorite dose and the relative concentrations of 

these byproducts. Only a small percentage of the sodium hypochlorite remained in the form of 

chlorite, and essentially all of the sodium hypochlorite was converted into chlorate. Chlorate 

concentration increased with increasing dose hypochlorite concentrations. These chlorate 

concentrations are much higher than the chlorate concentrations measured during Phase I. In 

some cases the chlorate concentrations represent more than 80% of the sodium hypochlorite dose 

concentrations. 

-31-

Figure 11. Chlorine dose-response relationships for fecal coliform data. 

-32-

Oxidation-reduction potential versus Cl2 dose and TRC relationships were developed to 

determine the practicality of using ORP technology to control disinfection dosing processes. 

ORP is not very sensitive to changes in chlorine residual at the levels required for CSO 

disinfection. For the range and magnitude of the TRC values measured during Phase I and Phase 

II for Cl2/deCl2, ORP probes did not appear to work effectively because of the lack of sensitivity 

at higher TRC values. 

In general, the concentration of the haloacetic acid compounds increased after 

disinfection with Cl2; however, the magnitude of this increase was very small (on the order of 10 

to 50 ug/L). Bromochloroacetic acid and dichloroacetic acid were the haloacetic acid 

compounds with the greatest increase in concentration after disinfection. However, the absolute 

magnitude of these concentrations is low. The US EPA’s ECOTOX database did not contain 

information regarding bromochloroacetic acid but did include LC50 results for dichloroacetic 

acid. The LC50 results for dichloroacetic acid based on freshwater fish test species was on the 

order of 5 mg/L. It is important to note that the toxicity data provided in the US EPA’s 

ECOTOX database are from single chemical exposures, and therefore, do not reflect synergistic 

affects with other chemicals. Additionally, they may be the result of different experimental 

designs. A few semi-volatile and volatile organic compounds appeared to be produced during 

the Cl2 disinfection process. These compounds are listed in Table 10. Again, the absolute 

magnitude of these concentrations is low, and it is unlikely that these compounds will exceed 

discharge criteria or play a role in effluent toxicity because of the very low concentrations. Over 

78 haloacetic acids, and semi volatile and volatile organic compounds were measured during the 

Phase II pilot study. Table 12 presents only the compounds that increased after disinfection with 

Cl2 for one or more runs. 

5.4 Toxicity 

Due to operational problems with the UV-chlorite ClO2 generator, the ClO2 effluent 

toxicity data for test runs no. 2 through 6 are not qualified. During these test events, the ClO2 

feedstock from the UV-chlorite generator was high in chlorite and the ClO2 strength could not be 

determined accurately (Santos et al., 2000). This resulted in effluent wastewater chlorite 

concentrations as high as 55 mg/L and high chlorite likely was a significant contributor to 

effluent toxicity. As reported in the US EPA ECOTOX database, chlorite exhibits chronic 

toxicity to opossum shrimp at an LC50 of 0.6 mg/L. Since the high chlorite levels were due to 

equipment operational problems, the toxicity observed during these runs may not be indicative of 

ClO2 itself or of a properly operating ClO2 generator. To account for this, an additional ClO2 test 

event (test event no. 9) was performed using the CDG Inc., Cl2 (gas)/sodium chlorite (solid) 

generator. 

Toxicity to opossum shrimp or sheepshead minnow in the untreated influent wastewater 

was observed in runs no. 1, 4, and 5 with LC50 values ranging from 66 to 76% (Santos et al., 

2000). Somewhat higher toxicity effects were observed in the UV and Cl2/deCl2 effluent for 

runs no. 1 and 4 (LC50 of 39 to 52% effluent). However, when compared to the variability 

observed in the WET results, these values are generally within the observed influent toxicity. By 

comparison, results of WET analyses on field duplicate samples showed a mean relative percent 

-33-

difference 

(RPD) between duplicates of 24%. The WET data for runs no. 2 and 7 show a slight toxic effect 

for the UV and Cl2/deCl2 effluents compared to no toxicity on the influent wastewater. However 

again, the 95% confidence intervals for the data show that there may be no difference between 

the influent and effluent data for these samples. 

In contrast to the UV and Cl2/deCl2 effluents, the ClO2 effluent did show significant 

toxicity to opossum shrimp in runs no. 1, 2, 6, and 7 as compared to influent toxicity. LC50s in 

these test events ranged from less than 6% effluent up to 26% effluent. This could not be 

attributed to influent toxicity, which had an LC50 of 82% effluent for run no. 1 and greater than 

100% effluent for runs no. 2, 6, and 7. It is believed that the toxicity of the ClO2 effluent 

observed during this test runs was at least partially due to the high chlorite levels mentioned 

previously. This was assessed by plotting relative toxicity (i.e., influent LC50 minus effluent 

LC50) of the ClO2 effluent versus effluent chlorite concentration. A strong correlation between 

chlorite concentration and effluent toxicity was observed. One anomaly is apparent in this trend; 

no toxicity was observed in the effluent sample with the highest chlorite concentration (55 

mg/L). This data point is probably erroneous. The Cl2 (gas)/sodium chlorite (solid) generator 

was used in the toxicity analyses for test event no. 9. In this test event, the effluent chlorite 

concentration varied from less than 0.2 mg/L up to a maximum of 5.8 mg/L, and no toxicity was 

observed in the WET tests up to a ClO2 dose of 10 mg/L. Therefore, it appears the effluent 

toxicity observed in prior test runs was related to high chlorite concentrations resulting from an 

improperly operating generator. 

The field microtox data may still be used to identify relative trends in toxicity within a 

CSO event. One can see two trends in this data. First, the influent wastewater generally shows 

higher mortality than the UV effluent and similar, though in some cases higher, mortality as the 

Cl2/deCl2 effluent. This indicates that the toxicity observed in these effluents is most likely 

associated with that of the untreated wastewater. Second, the ClO2 effluent overall shows higher 

mortalities than the other two pilot units or the untreated influent wastewater. 

6. COST COMPARISON 

During the Phase I pilot study, conceptual level cost projections were prepared for each 

disinfection technology for comparison purposes, with the goal of recommending a technology 

for implementation at the Spring Creek CSO Storage Facility. The Phase II pilot study results 

served to verify the Phase I result; as such, the assumptions and approach used for the original 

cost comparison were applicable. Costs for each disinfection technology were prepared on a 

common flow basis and were prepared for a range of flow rates experienced at Spring Creek 

CSO Storage Facility. This approach shows the sensitivity of cost to flow rate, and allows 

independent comparison of technology costs at similar flow rates. 

Equipment capital costs were developed for peak design flow conditions of 1,250 cfs 

(800 mgd), 2,500 cfs (1,600 mgd), and 5,000 cfs (3,200 mgd) for a duration of 4 hours. The 

5,000 cfs flow rate represents approximately the maximum facility inflow for the 5-year storm. 

-34-

Table 10. Organic compounds concentration, chlorine effluent vs. pilot influent. 

Parameter Detection 

Limit (μg/l) 

Concentration (μg/l) 

Run #2 Run #3 Run #4 Run #5 Run #6 

Infl. Infl. Infl. Infl. Infl. Infl. In

VOCs: Bromodichloromethane 10 ND ND ND 3.5 ND ND ND 10 ND 22 ND ND NDibromochloromethane 10 ND ND ND 1.5 ND ND ND 14 ND 18 ND ND NBromoform 10 ND ND ND 0.9 ND ND ND 12 ND ND ND ND NChloroform 10 ND ND ND 10 ND ND ND ND ND 18 ND ND N4-Methylphenol 10 ND 22 ND ND ND ND ND 4 ND ND ND ND NBenzoic Acid 10 ND 69 ND ND ND ND 3 ND ND ND ND NHAAs: Bromochloroacetic acid 1 ND ND 1 6 1 1 7.4 1 20 1 

1 1 1 1 1 1 1 1 1 4 1 3 

Dichloroacetic acid 1 10 35 4 1 22 5 1 42 1 

Monobromoacetic acid 1 1 1 1 1 24 2 1 2.6 1 2 1 1 

Monochloroacetic acid 1 3 1 1 1 1 1 3 2 1 1 1 5 

 Trichloroacetic acid 1 17 19 7 2 6 9 10.4 4 13 4 8 

2 pilot effluent, respectively. 

Run #1 

Eff. Eff. Eff. Eff. Eff. Eff. 

SVOCs: 

23 

10 12 

Dibromoacetic acid 11 

27 64 38 

14 

Notes: 1. "Infl" and "Eff" denote pilot influent and ClO

2. ND denotes non-detected. 

-35-

Figure12. Plots of SS concentration versus log reduction and log concentration of fecal 

coliform for the different doses. 

-36-

 The original design report for Spring Creek CSO Storage Facility (Greeley and Hansen, 1962) 

calculated a peak 5-year storm inflow of 3,750 cfs. However, this flow rate was based upon a 

runoff coefficient of 0.65. As identified by CDM in the Spring Creek Stabilization Study (1990), 

a more realistic runoff coefficient would be 0.90 - 0.95. 

This would result in a peak inflow of approximately 5,000 cfs. This flow rate and 

selected duration are also consistent with inflows observed during the Stabilization Study. The 

lower flow conditions were selected at reasonable fractions of the 5-year condition. Operating 

costs were developed based on an estimate of approximately 40 events/year producing inflow to 

the Spring Creek CSO Storage Facility, at a volume of 15 million gallons (MG) treated per 

event. This condition was selected based upon a review of the design inflow volumes (Greeley 

and Hansen, 1964). 

Costs were developed for UV, ozone, chlorine dioxide, and chlorination/dechlorination. 

Due to the limited effectiveness and high power consumption of the E-Beam pilot unit, the Ebeam technology as tested was not considered feasible for CSO disinfection. By comparison, the 

power usage for E-beam based on the pilot unit was approximately 3.5 kW/gpm (or 2,430 

kW/mgd) while the power usage for UV at 4-log reduction was 0.0325 kW/gpm (or 22.6 

kW/mgd). Therefore, since the E-beam technology was not considered feasible for CSO, costs 

were not developed for the technology. The cost projections were developed for a 4-log 

reduction of fecal coliform and included the following process options: 

• UV - Medium-pressure, high-intensity lamps 

• Ozone - Oxygen feed ozonation with eductor or side stream venturi type mass transfer 

configuration 

• Chlorine Dioxide - High-rate mixing, generation of ClO2 using the chlorine (gas)/sodium 

chlorite (solid) process, with onsite generation of chlorine gas via the acidification of 

NaOCl with HCl, use of emergency gas scrubber for potential chlorine gas, and 5 minute 

contact time (provided by existing basins at approximately 5,000 cfs) 

• Chlorination/Dechlorination - High-rate mixing, use of 15% sodium hypochlorite and 

38% sodium bisulfite, and 5 minute contact time (provided by existing basins at 

approximately 5,000 cfs) 

The cost projections are shown in Table 11. The table presents estimated capital, annual 

O&M, and total annualized costs. Annualized costs were prepared on the basis of a 20-year 

period at an 8 percent discount rate. The capital costs only include the costs for the basic process 

equipment associated with each technology and do not include: 

• The construction of additional basin tankage or structures for contact/disinfection, 

• Modifications to the existing basins, 

• Building expansion to house disinfection equipment, 

• Support equipment or facilities, for example: additional power supply equipment, HVAC 

equipment, and plumbing equipment 

As shown in Table 11, chlorination/dechlorination and chlorine dioxide are significantly 

less costly than either UV or ozone. Due to the intermittent nature of CSOs, disinfection 

-37- 
technologies like chlorination and chlorine dioxide, which are less capital intensive with higher 

O&M costs are favored over high capital cost technologies with lower O&M costs. 

It is important to note that for other CSO facilities, the cost for construction of 

disinfection contact tanks for the chlorination/dechlorination and chlorine dioxide alternatives 

would need to be considered and may make UV somewhat more attractive. It is also important 

to note that the cost of contact tankage for chlorine dioxide could be almost 40% less than 

chlorination/ dechlorination. This difference is attributed to chlorine dioxide’s greater 

bactericidal properties and solids penetration characteristics than those of chlorination, as 

demonstrated during the contact time analysis performed for the Phase II contact time. 

Chlorine dioxide cost projections were developed using the demonstrated chlorine 

(gas)/sodium chlorite (solid) process. This system was chosen as more reliable for cost 

projections because it is in use at a number of full-scale installations. Because the UV-chlorite 

chlorine dioxide generator is currently in the prototype status, costs using this technology were 

not developed. 

7. CONCLUSIONS 

7.1 Wastewater Quality 

During the Phase I and Phase II pilot studies five disinfection technologies, UV, ClO2, 

Cl2, O3, and E-Beam were piloted to determine their effectiveness in reducing bacteria levels in 

water representative of the CSO at the Spring Creek CSO Storage Facility. These pilots were 

tested during wet and dry events. In general, the pilot influent water quality was variable but 

representative of CSO water quality from the Spring Creek CSO Storage Facility. The variation 

in wastewater temperature between Phase I (mean of 11.6 o

C) and Phase II (mean of 20.9 oC) is 

believed to have had a significant impact on the performance of Cl2 disinfection. While the 

majority of CSO discharges from Spring Creek are likely to occur during the late summer and 

early fall months, discharges have also occurred during the winter and early spring months. 

Therefore, it was an added benefit to characterize the difference in performance resulting from 

temperature effects. 

To achieve a four-log reduction of fecal coliform and fecal coliform effluent 

concentrations less than 1,000 colony forming units/100 mL (cfu/100 mL) required doses for 

UV, O3, ClO2, and Cl2 of 60-80 mWs/cm2

, 24 mg/L, 8-10 mg/L, and 20-28 mg/L, respectively. 

The spread in disinfectant doses for each technology reflects the variation in performance 

between Phase I and Phase II. 

-38-

Table 6-1. Cost projections. 

Flow (cfs) 

1,250 2,500 5,000 1,250 2,500 5,000 1,250 2,500 5,000 1,250 

912,000 1,045,000 1,219,000 695,000 1,159,000 1,932,000 30,539,000 

87,774,0

Annualized 93,000 107,000 124,000 70,000 119,000 196,000 1,957,000 2,502,000 3,111,000 

4,894,00

255,000 255,000 255,000 294,000 294,000 294,000 534,000 587,000 657,000 

Annual O&M 248,000 

Total 348,000 362,000 379,000 364,000 413,000 2,491,000 3,089,000 5,142,00

Annualized 

Conceptual Level Disinfection Costs ($) 

Technology 

Chlorination/Dechlorination Chlorine Dioxide Ozone

Peak Design 

Capital Costs 19,221,000 24,560,000 48,052,0

Capital Costs Cost 

490,000 3,768,000 

Costs 

Notes: 

1. Costs are present worth in 2000 dollars. 2. Capital costs are based upon sizing to meet peak design flow and a 4-log reduction in fecal coliform. 3. Capital costs are for installation of Spring Creek and are for process equipment only. Costs do not

 Include additional contact tankage (if required) or support facilities 

4. Annual operating costs are based upon an assumed typical 40 CSO events/year at a volume treated

 of 15 million gallons per event. 

5. Annualized costs are based upon a period of 20 years at an interest rate of 8%. 

-39­
7.2 Treatment Performance 

Four bacteria indicators were used as a measure of the effectiveness of each of the 

disinfection technologies; namely total coliform, fecal coliform, Escherichia coli, and 

enterococcus. Kills of each of the indicators, in terms of log reduction and concentration, were 

related to dose for each of the disinfection technologies. Currently, there are no effluent bacteria 

criteria established for the Spring Creek CSO Storage Facility or for other CSO facilities. 

However, these targeted bacterial reductions were selected as conservative estimates of levels 

that could be met by the technologies and that may represent permit criteria. Generally, all the 

tested disinfection technologies, with the exception of E-beam, were able to effectively provide 

bacterial reductions of 3 to 4 logs. Chlorination/dechlorination, ClO2, and O3 at the doses tested 

were able to provide these levels of disinfection over the full range of wastewater quality tested. 

UV disinfection effectiveness tended to drop off at higher SS concentrations (e.g., SS greater 

than approximately 150 mg/L). This was attributed to lower effective penetration of UV due to 

harboring of bacteria in solids. 

Fecal coliform and Escherichia coli exhibited similar dose-response relationships. 

However, total coliform and enterococcus generally required higher doses to achieve the same 

level of inactivation as that for fecal coliform and Escherichia coli. This was observed in all 

technologies except for the E-beam, where the inactivation results were inconclusive. The 

existing receiving water quality standards for Spring Creek and Jamaica Bay address only total 

and fecal coliform. However, in the future regulators may use indicator bacteria that are more 

specific to human waste such as Escherichia coli and enterococcus for water quality standards. 

In the absence of specific densities indicating health risk, this information can only be preserved 

for future reference at this time. 

The UV and ClO2 technologies provided nearly complete reductions of bacteriophage. 

However, the viral inactivation data for the ClO2 system was limited to only two out of the four 

runs due to operational problems. Of the valid data considered, the effluent concentrations of 

bacteriophage ranged from non-detect to 60 pfu/mL. Low influent concentrations of the seeded 

phage limited the maximum log reduction that could be observed. The log reduction of 

bacteriaphage ranged from 1.9 to 6.0. Because of the low concentrations of naturally occurring 

enteroviruses in the pilot influent, the UV could not be evaluated satisfactorily on the basis of the 

tissue culture infectivity assays. However, based upon the reductions of the marginal 

concentrations found and upon the bacteriophage results, it is likely that theses technologies 

would inactivate most natural enteroviruses found in wastewater at concentrations on the order 

6

of 10 pfu/mL. 

UV disinfection achieved 4-log bacteria reduction but at extremely high dosage levels 

owing to the impediments of poor water quality. UV effectiveness tended to be reduced by high 

SS concentrations (e.g., greater than 150 mg/L). Additionally, UV effectiveness tended not to 

increase at doses greater than 75 mWs/cm2

; a phenomena known as “tailing-off.” 

Ozone disinfection can be accomplished but at dosage levels more than one and one-half 

times that of Cl2. However, the O3 pilot unit did not include a contactor design appropriate for 

-40-

the wastewater conditions tested. Thus, the required O3 dosages may have been less if a more 

applicable O3 dissolution/contactor system were provided. An O3 disinfection system would 

require contact chambers other than the tankage that presently exists at Spring Creek. 

Chlorine disinfection included dechlorination to eliminate residual Cl2. Both Cl2/deCl2 

can be accomplished using the existing tanks at the Spring Creek CSO Storage Facility. Highrate mixing can be added to the head end of the tanks. Chlorine dioxide disinfection can be 

accomplished at doses on the order of 30% of the required Cl2 dose. 

Chlorination/dechlorination and ClO2 were determined to be the most cost effective 

technologies for application to Spring Creek. However, neither ClO2 generation method tested is 

currently feasible within New York City; the Cl2 gas solid sodium chlorite generation method 

because of its use of Cl2 gas, and the UV-sodium chlorite generation method because of its 

developmental status as a prototype. The capital costs for UV and O3 were significantly more 

expensive than Cl2/deCl2 or ClO2. For other CSO facilities that do not have existing tanks for 

contact time, UV could be somewhat more cost competitive. 

In the case of ClO2, there is no significant increase in disinfection performance beyond a 

contact time of 3 min. This is in contrast to the chlorination results, which show a greater 

dependence on contact time and required five minutes for comparable kills. The difference is 

attributed to ClO2’s greater bactericidal properties and solids penetration characteristics than 

those of chlorination. The results of this study confirm the optimum contact times for ClO2 and 

Cl2/deCl2 of 3 and 5 min respectively, originally determined in the Syracuse and Rochester 

studies (US EPA, 1979a and 1979b). Chlorination/dechlorination and ClO2 were determined to 

be the most cost effective technologies for application at this facility. Further development of 

the UV-chlorite ClO2 generator is required before reliable costs for this technology can be 

developed. 

Comparison of the dry weather performance data for single and two-stage mechanical 

mixing configurations for chlorine disinfection implied a slight increase in disinfection 

effectiveness for two-stage mixing. The wet weather data, with its higher variability, was 

excluded from this comparison as it appears to have obscured the effects of 2nd stage mixing. 

The evaluation of single versus 2-stage mixing could not be performed for the ClO2 system due 

to the limited data from the field operational problems. 

7.3 Disinfection Residuals and Toxicity 

Selected disinfection effluent residuals and byproducts, namely ClO2, chlorate, chlorite, 

TRC, volatile and semivolatile organics, haloacetic acids, were monitored to relate these 

residuals to disinfectant dose. UV disinfection had the distinct advantage of producing no 

byproducts. This is in contrast to Cl2 and ClO2, which produced increased levels of TRC, 

chlorate, chlorite and haloacetic acids in the effluent. The slightly increased haloacetic acid 

concentrations were considered insignificant. The increased TRC, chlorate and chlorite 

concentrations were directly related to increased Cl2 and ClO2 dose. 

-41- 
- -

No additional toxicity was observed in the UV effluent as compared to the UV pilot 

influent. In contrast, there were occurrences where the ClO2 effluent was considerably more 

toxic than the pilot influent. An attempt was made to correlate this toxicity with the specific 

disinfection byproducts, in particular TRC, chlorate and chlorite, but no correlation could be 

made. It is likely that the increased effluent toxicity is directly related to influent toxicity (i.e., 

influent water quality) or a synergistic effect of the disinfectant residuals, which could not be 

measured. Although the concentrations of TRC, chlorate and chlorite did not cause a concern 

for effluent toxicity, this relationship should be revisited when establishing ClO2 dose for 

specific sites. 

Effluent TRC was generally below 0.1 mg/L following deCl2, as compared to a receiving 

water quality standard of 0.0075 mg/L. This value of deCl2 effluent TRC reflects the practical 

quantitation limit of the process instrumentation used. Lower TRC values could not be 

quantified. Often, the deCl2 effluent TRC instrumentation displayed a negative value indicating 

the presence of excess bisulfite. Residual Cl2 was also monitored in the ClO2 effluent. However, 

these TRC values include all oxidizing species of Cl2 and the possible presence of free and 

combined Cl2 could not be differentiated from ClO2, ClO2 , and ClO3 . 

7.4 Chlorine Dioxide Generation 

The method of generating ClO2 must be considered when selecting the appropriate 

disinfection process. The chlorine gas/solid sodium chlorite generation method was tested 

during the Phase I and Phase II pilot studies. Although this pilot unit was reliable, the use of 

chlorine gas (either with chlorine cylinders or with on-site Cl2 gas generation) in this process 

may limit its application in residential and urban areas, including New York City. The UVsodium chlorite solution generation method was also tested during the Phase II pilot study. This 

method had the distinct advantage of not using or generating chlorine gas in the generation 

process. However, this technology is currently in the prototype stages of development and 

would need to be developed as a full-scale unit to be considered further. The UV-chlorite 

generator from the UVD Inc., was a prototype unit. 

8. SUMMARY 

Pilot testing of disinfection technologies on CSO wastewater, as part of the upgrade to 

the Spring Creek CSO Storage Facility, was performed in two phases. Phase I was performed 

over 16 pilot test events from December 1996 through March 1997. This testing evaluated the 

performance of five high-rate disinfection technologies: UV, O3, ClO2, Cl2/deCl2, and E-Beam. 

The results from Phase I were presented in a final report dated November 1997. The purpose of 

the Phase II pilot testing was to address data gaps identified in the Phase I study, provide 

additional wet-weather data, and perform additional research that was beyond the scope of the 

original study. The Phase II pilot program was performed from August through November 1999 

and evaluated the performance of disinfection with UV, ClO2, and Cl2/deCl2. 

Influent and effluent from each pilot unit were analyzed by a certified laboratory for 

-42- 
bacterial and conventional wastewater quality parameters: VOC’s, SVOCs, HAAs, and toxicity. 

Four indicator bacteria were used as a measure of the effectiveness of each of the four 

disinfectant technologies: total coliform, fecal coliform, Escherichia coli, and enterococcus. 

Bacteria kills for each of the indicator bacteria, in terms of 3 to 4 log reduction, were related to 

dose for each of the four technologies. 

Based upon the technologies evaluated in these pilot studies, only Cl2/deCl2 was 

recommended for CSO disinfection at the Spring Creek CSO Storage Facility and other New 

York City CSO facilities. While ClO2 was superior in effectiveness and similar in cost to 

Cl2/deCl2, the generation technology for ClO2 which avoids the need for gaseous Cl2 needs 

further development. Because an effective Cl2-gas-free process of ClO2 generation has not been 

proven to be reliable and, because Cl2 gas cannot be transported within New York City, 

disinfection with ClO2 cannot be recommended for use within New York City at this time. 

However, the City and its engineers should remain apprized of advances in alternative 

disinfection technologies, such as UV-chlorite ClO2 generation. These advances may make these 

technologies more effective, both in terms of cost and disinfection effectiveness. 

While UV and O3 treatment were technically viable, the study showed that, given the 

intermittent nature of CSO treatment and the high peak flows involved, the high capital cost of 

these technologies makes them cost prohibitive. Electron beam disinfection did not meet the 

treatment goals. 

The original, detailed reports from the two studies, Phase I and Phase II, were produced 

in 1997 and 1999 by CDM of Woodbury, New York, and CDM’s Subcontractor Moffa & 

Associates, a unit of Brown & Caldwell, of Syracuse, New York, for the NYCDEP and the US 

EPA (Urban Watershed Management Branch, Edison, New Jersey). Funding for the studies was 

provided by NYCDEP (NYCDEP Capital Project No. WP-225) and partial funding for Phase II 

only was provided by US EPA (US EPA Purchase Order No. 7C-R394-NTLX). Partial funding 

for Phase I only was provided by New York Power Authority (NYPA) and the Electric Power 

Research Institute, both of New York City, New York. 

-43-

REFERENCES 

Camp Dresser & McKee. 1997. “Spring Creek AWPCP Upgrade. CSO Disinfection Pilot StudyPart I” - Final Report. 

Camp Dresser & McKee. 1999. “Spring Creek AWPCP Upgrade. CSO Disinfection Pilot StudyPart II” - Final Report. 

Greeley and Hansen, Engineers. 1964. Report on Use of Hypochlorite for Treatment of Storm 

Overflow, Spring Creek Marginal Pollution Control Project. 

Santos, L. N., K. J. Smith, D. Davis, and E. Delva. 2000. Acute Toxicity Testing of Alternative 

High-Rate Disinfection Technologies for CSO Treatment in New York City. Presented at the 

WEF Disinfection 2000 Specialty Conference in New Orleans. 

Stinson, M. K., Field, R., Moffa, P.E., Goldstein, S. L., Smith,K. J., Delva, E. 1998. High-Rate 

Disinfection Technologies for Wet-Weather Flow (WWF), in Proceedings of Water Environment 

Federation’s Specialty Conference entitled "Advances in Urban Wet Weather Pollution 

Reduction," Cleveland, Ohio, June 28-July 1, 1998. 

U. S. Environmental Protection Agency. 1975. Bench Scale High-Rate Disinfection of 

Combined Sewer Overflows with Chlorine and Chlorine Dioxide. EPA/670/2-75-021, 

Cincinnati, OH. 

U. S. Environmental Protection Agency. 1979a. Combined Sewer Overflow Abatement Program 

Rochester, NY, Vol. II Pilot Plant Evaluations; EPA/600/2-79-031b, Cincinnati, OH. 

U. S. Environmental Protection Agency. 1979b. Disinfection/Treatment of Combined Sewer 

Overflows, Syracuse, New York; EPA/600/2-79-134, Cincinnati, OH. 

Wojtenko, I., M. K. Stinson, and R. Field. 2001a. Performance of Ozone as a Disinfectant for 

Combined Sewer Overflow. Manuscript to be published in the Critical Reviews in 

Environmental Science and Technology, 31(4): 295-309. 

Wojtenko, I., M. K. Stinson, and R. Field. 2001b. Challenges of Combined Sewer Overflow 

Disinfection with Ultraviolet Light Irradiation, Critical Reviews in Environmental Science and 

Technology, 31(3): 223-239. 

-44-