Discussion Paper Number 3
The Role of Treated Collection System Discharges in Sanitary Sewer Remediation Efforts
Draft - October 5, 1998

1. Introduction

There has been a historic bias against allowing treating infrequent, intermittent wet weather induced overflows from sanitary sewer collection systems. EPA is reevaluating the use of treated collection system discharges as either an interim measure until discharge frequencies are reduce or, under limited circumstances, where part of comprehensive sanitary sewer collection system management strategy, as a long-term measure. Several factors have lead to this reevaluation, including:

• the emergence of new technologies that provide high-efficiency sediment removal which can greatly increase the effectiveness of subsequent disinfection;
• greater attention given to improving the performance of sanitary sewer collection systems including the development of a comprehensive national effort which seeks to minimize human health and environmental risks in the most expedient way possible consistent with the CWA; and
• increased emphasis on watershed management approaches which seek to ensure appropriate resource allocation that is generally related to risk.

1.1 Comprehensive Remediation Programs

Remediating collection systems with complex wet weather SSO problems typically involves several steps including planning an investigation, assessing the systems structural conditions and hydraulic performance, developing and implementing a remediation plan*. Remediation plans typically involve a comprehensive set of measures, including:

• improved operation and maintenance (O&M) measures to increase the effective capacity of the system;
• reduction of inflow and rainfall-induced infiltration;
• capital improvements to increase conveyance capacity;
• capital improvements to increase WWTP capacity; and
• providing temporary storage.

*For a more detailed discussion of related approaches see "Existing Sewer Evaluation & Rehabilitation", WEF Manual of Practice FD-6, ASCE No 62, 1994, and "Sewer System Infrastructure Analysis and Rehabilitation", EPA, October 1991, EPA/625/6-91/030.

For many systems, developing and implementing remediation plans that fully eliminate wet weather discharges under all conditions at all locations within a sanitary sewer collection system can be difficult. In addition, the condition and capacity of sanitary sewer collection system are fairly dynamic and depend on system deterioration and population changes. Remediation programs are often uncertain in the degree to which they reduce peak flowrates (and therefore overflows), and altering facilities to fully convey and treat infrequent, intermittent wet weather flows can cause operational problems during periods of dry weather flow. Therefore, EPA is currently evaluating the appropriateness of treated discharges from a sanitary sewer collection system in two limited circumstances:

1. Interim measure - Treated discharges could be an interim measure to reduce health and environmental risks until the frequency of the discharge can be minimized. Given the significant capital costs of providing treatment, interim treatment may only be appropriate where a discharge presents a relatively high environmental or health risk and is expected to remain for a significant period of time until other capital improvements can be made to the collection system to reduce the discharge; and
2. Where conveyance is infeasible - In limited circumstances, treated discharges may be a long-term measure that was part of a comprehensive collection system management strategy where all feasible measures to reduce peak flows, store or convey flows are implemented but infrequent discharges (e.g. less than 6 per year) remain.

1.2 Concerns About Treated Collection System Discharges

Historically, key concerns against the treatment of wet weather induced SSOs include:

• Pose unacceptable risks - Treated collection system discharges may not provide adequate protection to public health, the environment, or wastewater treatment facilities. Flows delivered to the biological treatment plant are believed to receive more efficient treatment prior to discharge.
• Encourages insufficient operation and maintenance (O&M) and capital investment - Removing excess flows and ensuring conveyance of remaining flows to traditional wastewater treatment plants is a preferred long-term management strategy, because it ensures O&M and capital investment. If treated overflows are allowed, less O&M and capital investment may occur, leading to increasingly deteriorating systems.
• Impedes enforcement - Allowing treated discharges makes bringing enforcement actions more difficult because treated discharges could be allowed to meet different standards than those applied to POTWs. The secondary treatment regulations provide a clear framework for identifying non-compliance.
• Slows Remediation - Spending capital funds on interim treatment systems takes away or postpones funding for other capital improvements (e.g. increased conveyance capacity, storage or I/I reduction) that would reduce the frequency of the discharge. Large capital investment in interim measures are generally not favored because the interim measure may only have a relatively short operational life.

1.3 Why Consider Treated Discharges Under Limited Circumstances?

Supporters of using treated collection discharges in limited circumstances (e.g. on an interim basis or as part of a comprehensive remediation effort where infrequent discharges will remain) have raised the following considerations:

• Health risk reduction - For systems experiencing untreated collection system discharges, health risks can be reduced by installing facilities to treat discharges. For systems making comprehensive remediation efforts, the effectiveness of such efforts (e.g., infiltration and inflow (I/I) reduction) is often uncertain, bringing about a likelihood that some wet weather overflows will still occur and go untreated. Also, with some treatment technologies, treating collection system discharges may be as or more effective than traditional biological plants operating under peak flow conditions.
• Operational problems - Remediation efforts to fully eliminate wet weather induced collection system overflows can lead to operating problems during dry weather periods. Oversizing pipes to handle peak wet weather flow results in slower flow rates during dry weather conditions. These slower flow rates can lead to increased deposition and more corrosive conditions. Constructing offsite storage facilities to detain wet weather flows for future conveyance to the WWTP increases the need for collection system maintenance.
• POTW efficiency - The highest rate of wastewater flow to treatment plants typically occurs during large wet weather events. High rate flows that exceed the design capacity of a treatment plant can reduce treatment efficiency or make biological treatment facilities inoperable (e.g., wash out the biological mass necessary for treatment). Some treatment technologies that can be used in the collection system may be as or more efficient than biological units operating under peak flow conditions.
• Cost savings - Treatment facilities for wet weather collection system discharges can be more cost effective and space efficient than the treatment plant and collection system expansion efforts used to fully eliminate such discharges. Some of the technologies available for treating overflows are relatively compact and therefore, reduce site specific issues, such as land availability.
• Public acceptance - Treated collection system discharges may result in lower burdens on rate payers and have fewer construction-related hassles (e.g., from torn up roads) than major infrastructure projects that would likely be part of complete system remediation projects. In some cases, the public has supported treated collection system discharges over other management options such as providing in-system storage that might result in multiple storage facilities in residential or commercial areas.
• Watershed framework - Allowing treated collection system discharges under limited conditions as part of their comprehensive management strategy enables systems to better prioritize controls in a watershed framework. This may allow limited resources to be focussed on higher priority sources, such as discharges to more sensitive areas in the watershed or CSOs.

2. Treatment Objectives for Treated Discharges

If treated collection system discharges are allowed, the level of treatment needs to be based on health and environmental considerations, regulatory considerations and treatment capabilities and costs.

2.1 Health and Environmental Considerations

Many health and environmental considerations are site-specific and depend on the characteristics of the receiving water (e.g., presence of sensitive areas). For public health protection, the primary objective is generally pathogen removal since the bacterial pathogenic organisms found in wastewater are highly infectious and can cause gastrointestinal diseases. Effective removal of pathogens by disinfection processes depends on several factors including the concentration of suspended solids. A great deal of research has shown that pathogens can adsorb to suspended solids, aggregate to form clusters, and/or be sorbed within organic matter creating a shielding effect against disinfection (Hoff and Akin, 1986; White et al., 1983; LeChevallier et al., 1981). Microorganisms are protected from chlorination by suspended solids associated with fecal matter, wastewater effluent solids, and cell debris, particularly at high turbidity levels, greater than 13 NTU (Hoff and Akin, 1986). Similarly, since UV radiation must reach bacteria and other pathogens to destroy it, pathogen removal is more effective when wastewaters have low suspended solid concentrations. Thus, effective suspended solids removal needs to occur prior to disinfection in order to effectively remove pathogens.

For controlled, treated discharges to non-sensitive waters, the risk of water quality impacts is largely a function of the frequency of discharge, the condition and nature of receiving waters, and form, type and concentration of pollutants discharges. For relatively infrequent treated discharges, pathogens are typically the most immediate health and environmental concern.

2.2 Regulatory Considerations

The status of a specific discharge is related to the permit language and the circumstances under which the discharge occurs. Permits for POTW discharges generally prohibit discharges from a sanitary sewer collection system that are not in compliance with permit conditions based on secondary treatment standards or more stringent water quality-based requirements. The secondary treatment regulations establish 30-day average and 7-day average concentration limitations for suspended solids (SS) and biochemical oxygen demand (BOD) as well as a pH limitation. The regulations also establish a 30-day pollutant removal requirement for TSS and BOD. The 30-day averages (30 mg/l for SS and BOD) are based on the 95 percentile value of well operated POTWs. The 7-day averages (45 mg/l for SS and BOD) were set at 1.5 times the 30-day averages to account for the variability that occurs during a shorter operating time frame.

Where treated collection system discharges are to occur as part of a comprehensive collection system management program, they are typically expected to occur on an infrequent basis for relatively short time (e.g. most often under 24 hours). Given the short duration of the discharge, the variability is expected to be higher than would occur with a 30-day or 7-day continuous discharge.

The NPDES regulations (40 CFR 122.41(m)) prohibit bypasses from treatment facilities unless they do not cause effluent limitations to be exceeded and are for essential maintenance to assure efficient operation. The NPDES authority may take enforcement action against a permittee for a bypass unless the bypass meets the following three conditions: (A) the bypass was unavoidable to prevent loss of life, personal injury or severe property damage; (B) there was no feasible alternative to the bypass; and (C) the permittee complied with the notification requirements of the permit.

3. Performance Indicators

Suspended solids has been suggested as a key indicator for infrequent treated discharges for a number of reasons:

• the effectiveness of subsequent high-rate disinfection depends on suspended solids concentrations;
• removing suspended solids can also remove a high percentage of other pollutants that are associated with the solids, such as metals, phosphorus, and to a limited degree, BOD.
• physical/chemical processes which remove suspended solids typically do not remove soluble (dissolved) pollutants. Soluble pollutants would include a significant portion of total BOD (30% to 50%). However, if treated discharges occur infrequently and to turbulent waters experiencing wet weather flows, the environmental risks are lower because of less settling of pollutants in a concentrated area around the outfall.

3. Technology Options for Treated Discharges

The treatment options for intermittent, high-volume wet weather flows, including SSOs, are driven primarily by flow characteristics. Treatment systems must be able to handle high flowrates of wastewater at unplanned times, significant variation in flow and influent pollutant concentrations and extended periods of no flow (downtime).

Traditional biological treatment processes are generally not feasible for treating infrequent, intermittent, high volume wastewater discharges such as SSOs and other wet weather discharges. The impracticality of maintaining a viable biological mass to treat infrequent, intermittent flows of varying quantity and quality make biological treatment processes generally infeasible for treating wet weather discharges.

The physical and chemical treatment processes used for intermittent wet weather discharges generally focus on removing non-soluble pollutants which includes suspended solids and pollutants associated with solids. Where a treatment system provides a high rate of removal of suspended solids, it may, depending on the form of various pollutants in the wastewater, also provide a similar rate of removal for pollutants associated with solids, such as metals and phosphorus. However, physical/chemical treatment processes generally do not remove soluble or dissolved pollutants (unless the soluble pollutants can be turned into a soluble form by some means such as changing the pH of the wastewater).

High-rate disinfection treatment processes can be used after physical/chemical treatment processes to provide pathogen control. The effectiveness of most disinfection processes depends on the concentration of suspended solids (and hence the treatment efficiency of the physical/chemical treatment processes proceeding the disinfection) in the effluent.

Four classes of chemical/physical treatment processes are discussed below:

• Primary clarification
• Microscreening
• Chemical flocculation; and
• Ballasted flocculation

3.1 Primary Clarifiers

The removal of BOD₅ and TSS using screening and primary clarification are dependent on several design considerations including: surface loading rates (for sedimentation); particle size distribution (which affects settling velocities); frequency of screen cleaning and rotating speed of drum filter (for screening); and for BOD, and ratio of particulate to soluble BOD. The ratio of particulate to soluble BOD is an important factor in determining BOD removal since sedimentation generally removes little soluble BOD.

The sizing of a primary clarifier is based on minimum treatment level, and/or reducing the number of overflow events**. In general, as the size of a basin is increased, the surface loading rate decreases and the level of solids removal increases. At high surface loading rates (smaller volume clarifiers), the removal efficiency for solids decreases. To achieve a high level of solids removal, the surface loading rate must remain relatively low (larger clarifiers). Since the surface loading rate is approximated by the flow rate divided by the surface area of the basin, the desired surface loading rate is a key design parameter in determining the size of a sedimentation basin.

**Primary clarifiers can be dual purpose systems that act as storage facilities during smaller events (with flows bled back into the collection system when peak flows are reduced) and as a primary clarifier with a discharge during larger events.

Some existing primary clarifiers that are designed for treating wet weather induced SSOs have been effective in meeting the average 30-day concentration limits of 30 mg/L for BOD₅ and SS in the secondary treatment regulations. These facilities generally use large sedimentation basins, some with polymer addition to facilitate settling of solids. Data for four facilities in Illinois that have NPDES-permitted discharges from excess flow facilities show average effluent values ranging from 19.8-41.2 mg/L for SS and 19.6-54.2 mg/L for BOD (Metcalf & Eddy, 1997; SAIC, 1996). Fecal coliform effluent data are available for two of the Illinois facilities and range from 0-400 MPN/100 mL, with an average of approximately 160 MPN/100 mL, thereby meeting their permit limit of 400 MPN/100 mL (Metcalf & Eddy, 1997). Additional data on these facilities are provided in Table 3-1.

Table 3-1. Data Comparison from Four NPDES Permitted Dischargers in Illinois
Treatment Provided	Suspended Solids Effluent (mg/L)			Biological Oxygen Demand Effluent (mg/L)			Fecal Coliform Effluent (MPN/100 mL)
	No. of Data Pts.	Range	Median	No. of Data Pts.	Range	Median	No. of Data Pts.	Range	Median
Primary (settling via clarifier), chlorination	6	3-67	41.5	5	20-147	38	NA	NA	NA
Pre-sedimentation & retention basins, chlorination	13	22-52	34	12	12-25	20.5	NA	NA	NA
Settling, chlorination, dechlorination	12	21-107	32	13	0-78	20	8	0-400	27.5
Floatables & settleables removal (clarifier & polymer addition), chlorination	18	15-30	18	18	12-36	21.5	18	2-400	118
NA = not available Sources: Metcalf & Eddy, 1997; SAIC, 1996.

Data for two wet weather facilities in Oakland, California show effluent concentrations ranging from 6-110 mg/L for TSS and 20-140 mg/L for BOD (covering a period from 1992-1996). Influent concentrations ranged from 16-340 mg/L for TSS. It should be noted that the facilities were not designed for TSS/BOD removal, but for high-rate disinfection. The facilities provide primary sedimentation and disinfection. Sedimentation basins at each plant hold approximately 3 million gallons and capture the first flush of storm events, and the full volume of many smaller storms. (AMSA, 1996; Harvey, et al., 1996)

For many systems, developing wet weather facilities that treat excess flows to levels that consistently meet the secondary treatment limits of 30 mg/L for BOD₅ and SS may require very large basins, even when polymer addition is used to aid settling. Existing basins may not have either sufficient capacity to provide adequate detention time or the land necessary to build such basins. The land requirements and capital costs for enlarged primary treatment facilities make them impracticable for many WWTPs.

3.2 Microscreening

Microscreens remove solids from wastewater with fine screens. The fine screens can be placed on rotating drums which are lined with a filtering fabric. Wastewater flows into the side of the rotating drum and out through the filter. The pore size for microscreen’s filter material is typically between 25 and 35 μ. The solids are captured within the drum and collected in a trough for disposal. The suspended solids removal for microscreens ranges between 10 and 80 percent, with typical removals of 55 percent (Metcalf & Eddy, 1991).

Two excess flow facilities in Illinois (separate from the facilities listed in Table 3-1) have consistently met NPDES permit limits of 25-44 mg/L for CBOD and 30-49 mg/L for SS using rotating fine screens and chlorination for treatment. The limits are based on the number of discharges that occur in a month, ranging from 25 mg/L for CBOD and 30 mg/L for SS if discharges occur daily to 44 to 49 mg/L for CBOD and SS, respectively, if a single discharge occurs in a month. The two facilities do not meet their 400 MPN/100 mL fecal coliform limit as consistently as the CBOD and SS limits. Data from 1989 through 1994 indicate that one facility meets the limit 79 percent of the time and the other meets it 58 percent of the time. These two facilities use high capacity rotating screens that are compact in size (e.g., one facility has a 20-mgd capacity and fits on a 1/3-acre lot) (Gavle and Mitchell, 1996).

3.3 Rapid Sedimentation by Chemical Flocculation

This treatment system combines grit, grease and oil removal, lamellar settling, and sludge densification into one compact unit. The processing steps used in the physical-chemical treatment technology are as follows:

• A coagulant is injected into raw water while grit is removed.
• A flocculating agent and recirculated sludge is then added to the raw water.
• Grease and scum are separated and collected as flocculation occurs.
• Suspended flocculated matter is separated and thickened.
• Thickened sludge is drawn off while some gets recirculated.

The major benefits of this system include:

• Fast build-up of operational efficiency (pilot tests achieved approximately 80 percent TSS removal in about 30 minutes)
• Production of concentrated sludge, requiring no additional thickening equipment.
• Higher surface loading rates than conventional clarification systems (up to 30,000 gpd/square foot).
• Fairly stable operation over a significant range of operating conditions. This allows high-efficiency treatment of variable flows with limited equalization.

The chemical flocculation treatment technology is used in more than 130 international and European installations. In the United States, approximately 30 installations, including those in the pulp and paper, mining, petroleum, municipal, chemical and electric utility industries, use the technology to treat well, river, or waste water for softening, clarification, or metals removal. Operating costs for infrequent intermittent discharges would be lowered by reducing the frequency and volume of the discharge. Operating costs for this system can also be lowered through the automatic metering of reagent quantities on the basis of flow rate and turbidity data. EPA is currently in the process of gathering data from electric utilities and municipal users of the chemical flocculation process in the U.S.

3.4 Rapid Sedimentation by Ballasted Flocculation

This system is an advanced solid-liquid separation process. The major processing steps used in the ballasted flocculation system are as follows:

• After an initial screening, coagulant chemicals are added to raw influent.
• Sand, treated with polymers, is added to the main influent stream.
• Solids from the influent stream attach themselves to the sand forming larger, denser particles (floc).
• The floc thickens and matures and quickly settles to the bottom of a settling tank.
• While clean water is collected and exits the reaction vessel, sand particles are separated from solids and recirculated into the injection tank.

The ballasted flocculation system was developed around mid-1980. Initially it was primarily used in industrial applications. About seven years ago, some municipalities in Europe began using it for pretreating potable water supplies. More recently, municipalities in France, Great Britain, Germany, Mexico and Canada have used the technology in several different applications for treating municipal wastewater, including primary and tertiary clarification for the removal of TSS, turbidity, BOD, COD, and heavy metals. New installations are planned for operations in Turkey, Malaysia, Switzerland and the United States. Several pilot studies conducted by municipalities and/or vendors have recently been completed at sites in Galveston, Texas; Portland, Oregon; Cincinnati, Ohio; Papillion, Nebraska; and Newport, Kentucky. These pilot tests were conducted under a wide range of operating conditions to demonstrate effectiveness of the ballasted flocculation process in treating wastewater and potable water. Some of the pilot tests evaluated the impact of loading rate, operating pH, and coagulant dose on the overall performance of the process as well as the start-up efficiency of the plants. Pilot studies in Galveston, Portland and Cincinnati evaluated the removal performance of turbidity, TSS, BOD, COD, and phosphorus in combined sewer overflows. A summary of the data collected from these pilot studies is attached. Other studies conducted in Papillion and Newport demonstrated how the ballasted flocculation process can achieve the desired concentrations of turbidity and manganese in potable water, respectively. Two more pilot studies are scheduled for the Fall/Winter of 1998 in Fort Worth, Texas and Jefferson County, Alabama.

Major advantages of the ballasted flocculation system can be summarized as follows:

• Higher surface loading rates than conventional clarification systems (up to 58,000 gpm/square feet).
• Reduced capital cost because of smaller unit size and higher surface loading.
• Significant reduction in footprint (less than 10% of the space required for standard settling systems) mainly because of faster settling velocities (up to 10 times) than conventional clarification systems.
• Start-up efficiency is quickly achieved (in pilot tests, up to 90% TSS removal or 1 NTU or less could be achieved in less than 30 minutes).
• Fairly stable operation over a significant range of operating conditions. This allows high-efficiency treatment of variable flows with limited equalization.

A summary of the pilot test results, that was reported by vendors and/or municipalities is shown in Table 3-2. Since pilot studies are typically designed to get an idea of what the bounds of operating conditions for a technology, these removal rates represent a wide range of operating conditions. Detailed information from three of these pilot studies conducted in the U.S. this year is provided in Attachment 3-1. Data from additional pilot studies will be made available at the internet site listed below.

Table 3-2. Removal Efficiencies for Emerging Technologies

Technology Type	Loading Rate (gpm/sf)	TSS (%)	BOD (%)	Pb, Cu, Zn (%)	Phosphorus (%)
Chemical Flocculation Treatment1 (Densadeg)	6 to 66	80 to 99	60 to 75	87 to 92	56
Ballasted Flocculation3 (Microsep, Actiflo)	29 to 76	85 to 99	50 to 80	50 to 99	60 to 99
Notes: 1. The chemical flocculation treatment is being developed by Infilco Degremont, Inc. 2. Represents one pilot test result. 3. The ballasted flocculation treatment is being developed by Kruger, Inc. and U.S. Filter, Inc.

4. Key Issues

1. What are the major health and environmental concerns with treated discharges? What level of treatment is needed to adequately minimize health and environmental risks associated with infrequent discharges?
2. What are the best ways to characterize performance of treatment systems for intermittent wet weather flows?
3. What is the best way to apply 30-day and 7-day average concentration standards to intermittent wet weather flows?

5. Updates

This draft paper will updated periodically to reflect new data and comments. The most recent version of the paper will be posted at the following internet site:

www.epa.gov/OW-OWM.cfml/wet.cfm#sso.

Data and comments related to the discussion papers can be sent to:

Kevin Weiss
U.S. EPA (4201)
401 M Street SW
Washington DC 20460

or

e-mail address: Weiss.Kevin@EPA.gov

6. References

Association of Municipal Sewer Authorities (AMSA). 1996. "Technology-Based Standards for Wet Weather Facilities." Operator/Municipal Caucus Paper, Final Draft, 8/29/96. Developed for the SSO Federal Advisory Subcommittee.

Association of Municipal Sewer Authorities (AMSA). 1995. Separate sanitary sewer overflows. January.

Association of State and Interstate Water Pollution Control Administrators (ASIWPCA). 1996. Sanitary sewer overflow (SSOs) membership survey results.

Civil Engineering Research Foundation (CERF). 1994. State and Local Public Work Needs. American Society of Civil Engineers.

Gavle, D.R., and D.G. Mitchell. 1996. Innovative and economical SSO treatment utilizing fine screens and chlorination. In National Conference on Sanitary Sewer Overflows (SSOs), April 24-26, 1995, Washington, DC. U.S. Environmental Protection Agency, Office of Research and Development, Cincinnati, OH. EPA/625/R-96/007.

Harvey, T.E., J.D. Parker, and P.R. Giguere. 1996. Correcting sanitary sewer overflows: An evaluation of the East Bay infiltration/inflow correction program. In National Conference on Sanitary Sewer Overflows (SSOs), April 24-26, 1995, Washington, DC. U.S. Environmental Protection Agency, Office of Research and Development, Cincinnati, OH. EPA/625/R-96/007.

Hoff, J. and E. Akin. 1986. Microbial Resistance to Disinfectants: Mechanisms and Significance. Environmental Health Perspective, Vol. 69. Pp. 7-13.

LeChevallier, M., T. Evans, and R. Seidler. 1981. Effect of Turbidity on Chlorination Efficiency and Bacterial Persistence in Drinking Water, Applied and Environmental Microbiology, Vol. 42, pp. 159-167.

Mays, L.W. 1996. Water Resources Handbook. McGraw-Hill, New York, NY.

Metcalf & Eddy, Inc. 1997. "A Review of SSO Quality and Treatment Data - Draft Technical Memorandum." June.

Metcalf & Eddy, Inc. 1991. Wastewater Engineering: Treatment, Disposal and Reuse, 3rd Edition. McGraw-Hill, New York, NY.

Natural Resources Defense Council (NRDC). 1997. Testing the Waters, Volume VII: How Does Your Vacation Beach Rate? New York, NY.

Science Applications International Corporation (SAIC). 1996. "SS and BOD Data for Illinois SSO Treatment Facilities," data tables prepared for U.S. EPA. November 1996.

Stinson, M., Field. R, Moffa, P, et.al. "High-Rate Disinfection Technologies for Wet-Weather Flows", Advances in Urban Wet Weather Pollution Reduction, WEF Speciality Conference proceedings, 1998.

U.S. Environmental Protection Agency (U.S. EPA). 1990. Rainfall induced infiltration into sewer systems. Report to Congress. Washington, DC. EPA/430/09-90/005.

U.S. Environmental Protection Agency (U.S. EPA) Region VI. 1991. Study of 734 municipalities.

Water Pollution Control Federation (WPCF). 1989. Problem technologies and design deficiencies at publicly owned treatment works -- a survey.

White et al. 1983. Wastewater Treatment Plant Efficiency as a Function of Chlorine and Ammonia Content. In Water Chlorination: Environmental Impact and Health Effects, Vol. 4, pp. 1115-1125.

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