United States

Environmental Protection

Agency

Office of Water

Washington, D.C.

EPA 832-F-99-033

September 1999

Combined Sewer Overflow

Technology Fact Sheet

Alternative Disinfection Methods

DESCRIPTION

Combined sewer overflows (CSOs) occur when

flows exceed the hydraulic capacity of either the

wastewater treatment plant (WWTP) or the

collection system that transports the combined

flow of storm water and sanitary sewage to the

WWTP. When an overflow occurs, the excess

flows tend to be discharged into a receiving body

of water. CSOs typically discharge a variable

mixture of raw sewage, industrial/commercial

wastewater, polluted runoff, and scoured materials

that build up in the collection system during dry

weather periods. These discharges contain a

variety of pollutants that may adversely impact the

receiving water body, including pathogenic

microorganisms, viruses, cysts, and chemical and

floatable materials. Health risks associated with

bacteria-laden water may result through dermal

contact with the discharge, or through ingestion of

contaminated water or shellfish.

Preliminary reduction of microorganisms and

bacteria may be accomplished through physical

reduction of solids in the wastewater, primarily

through sedimentation, flotation, and filtration.

Following solids reduction, most systems further

reduce bacterial concentrations through

disinfection. Disinfection occurs as the

wastewater is brought into contact with oxidizing

chemicals (such as chlorine, bromine, ozone,

hydrogen peroxide, and related compounds).

Chlorine has long been the disinfectant of choice

for most disinfection systems. It offers reliable

reduction of pathogenic microorganisms at

reasonable operating costs. (See EPA's CSO

Technology Fact Sheet 832-F-99-021,

Disinfection-Chlorination, for more information).

While chlorine disinfection is the most common

method used to kill pathogenic microorganisms at

wastewater treatment plants, this methodology

may not be feasible at all CSOs for several

reasons, including:

• CSOs occur intermittently and their flow

rate is highly variable, thus making it

difficult to regulate the addition of

disinfectant.

• CSOs have high suspended solids

concentrations.

• CSOs vary widely in temperature and

bacterial composition.

• Residual disinfectants from chlorine

disinfection may be prohibited from

receiving waters.

• CSO outfalls are often located in remote

areas and thus may require automated

disinfection systems. 

In addition to these problems, the increased health

and safety concerns regarding the use of chlorine

to disinfect CSOs has prompted the development

of alternative disinfectants, which often pose

fewer problems and hazards. Alternatives to

chlorine have been developed and evaluated for

continuous disinfection of wastewater discharges

to small streams or sensitive water bodies, and are

now being considered for treatment of CSOs and

other episodic discharges. 

This fact sheet addresses the use of chlorine

dioxide, ozonation, ultraviolet radiation, peracetic
acid and Electron Beam Irradiation (E-Beam) to

treat CSOs.

Chlorine Dioxide

Studies have shown that chlorine dioxide is an

effective wastewater disinfectant, although its use

in the United States is limited. Chlorine dioxide

is applied to wastewater as a gas that is generated

on-site using excess chlorine. Although it is

relatively easy and economical to produce

chlorine dioxide is unstable and reactive and any

transport is hazardous.

Chlorine dioxide is effective at oxidizing phenols,

but does not react with aquatic humus to produce

trihalomethanes (THMs). However, any excess

chlorine remaining from the generation of

chlorine dioxide would react with THM

precursors and form THMs. Therefore, operators

must be careful to use the correct amounts of

chlorine when generating chlorine dioxide. And

while chlorine dioxide will not react with

wastewater to form chloramines, it can produce

potentially toxic byproducts such as chlorite and

chlorate.

Ozonation

Ozone is a strong oxidizer and is applied to

wastewater as a gas. Its use in CSO treatment

facilities for wastewater disinfection is relatively

new in the United States, and there are few

facilities currently using ozone for disinfection.

This can be potentially attributed to high initial

capital costs associated with ozone generation

equipment. Ozone is equal or superior to chlorine

in "killing" power, but it does not cause the

formation of halogentated organics as does

chlorination. 

Ultraviolet (UV) Radiation 

UV radiation is one example of electromagnetic

radiation used for disinfection. UV disinfection

incorporates the spectrum of light between 40

nanometers and 400 nanometers. Germicidal

properties range between 200 and 300

nanometers, with 260 nanometers being the most

lethal. The primary method for utilizing UV

disinfection is to expose wastewater to a UV

lamp. Historically, most UV disinfection facilities

have been designed to utilize Low Pressure Low

Intensity UV lamps for disinfection. For example,

low-pressure mercury arc lamps emit

approximately 90 percent of their light energy

around 254 nanometers. 

UV disinfection works by penetrating the cell

walls of pathogenic organisms and structurally

altering their DNA, thus preventing cell

replication and function. No hazardous chemicals

are produced or released while treating CSOs with

UV.

Because UV is not a chemical disinfection

method, it disinfects without altering the physical

or chemical properties of water. However, UV

efficiency is affected by suspended solids in the

wastewater, which can scatter and absorb light.

Thus, UV disinfection is not effective in

wastewaters with a high TSS level.

Peracetic Acid

Peracetic acid (CH3COOOH) (PAA), also known

as ethaneperoxoic acid, peroxyacetic acid, or actyl

hydroxide, is a very strong oxidant. Based on

limited demonstration data for disinfection of

secondary treatment plant effluent, peracetic acid

appears to be an effective disinfectant and should

be evaluated further for treating CSOs. The

equilibrium mixture of hydrogen peroxide and

acetic acid that produces PAA is too unstable and

explosive to transport, and so PAA must be

produced on site. The decomposition of PAA

results in acetic acid, hydrogen peroxide and

oxygen.

Electron Beam Irradiation 

Electron Beam Irradiation (E-Beam) uses a stream

of high energy electrons that are directed into a

thin film of water or sludge. The electrons break

apart water molecules and produce a large number

of highly reactive chemical species. There are a

few reactive species formed during this process

and include oxidizing hydroxyl radicals, reducing

aqueous electrons and hydrogen atoms.
APPLICABILITY

A brief summary illustrating the general

applicability of chlorine dioxide, peracetic acid

and UV radiation as alternative CSO disinfectants

is provided in Table 1. While ozonation and the

E-Beam process are discussed in this fact sheet as

potential alternative disinfectants for CSOs, they

are not currently considered practical for CSO

disinfection and thus they are not included in

Table 1. Because ozone must be generated on-site

and the amount generated is dependent on the

demand, ozone is not currently considered

practical for intermittent use in situations where

the system would be frequently turned on and off

or where there are wide fluctuations in flow rate

and disinfection demand, such as in CSO

treatment applications. The E-Beam system was

initially developed for the disinfection of

municipal wastewater treatment plant sludge and

the destruction of hazardous organic compounds,

and it has not been evaluated for CSO

disinfection. EPA will continue to evaluate the EBeam system as a promising innovative

technology for wastewater technology.

ADVANTAGES AND DISADVANTAGES

As discussed above, one of the primary reasons

for seeking alternatives to chlorine disinfection of

CSOs is the growing concern over safety in

handling gaseous chlorine and the possible toxic

side effects of treatment with chlorine. Studies on

alternative disinfectants such as peracetic acid,

ozone, E-Beam, and UV, have shown that these

alternatives serve as good substitutes for chlorine

because they produce no toxic byproducts.

Although chlorine dioxide does produce

byproducts and residuals, the limited use of

chlorine dioxide in this country has made it

difficult to assess these byproducts for toxicity.

The alternatives to chlorine for CSO disinfection

are not problem-free, however, and their

effectiveness depends on the physical and

chemical characteristics of the wastewater (e.g.,

the presence of large particles may hinder

disinfection). They require certain storage and use

precautions, and disinfection residuals and

byproducts may be a concern in receiving waters.

As discussed above, Table 1 provides comparative

data for chlorine dioxide, peracetic acid, and UV

radiation to help in determining which compounds

may be most advantageous for specific

applications. The following sections summarize

the advantages and disadvantages of using

ozonation and E-Beam as alternative CSO

disinfectants.

Ozonation: Advantages

C More powerful disinfectant than most chlorine

compounds.

C Inactivates most strains of bacteria and viruses

and is noted for destroying chlorine-resistant

strains of both. Highly effective for

Cryptospiridium eradication.

C Will oxidize phenols with no negative

residuals such as trihalomethane production.

C Does not produce a disinfection residual that

would prevent bacterial growth.

C Degenerates into oxygen, which can elevate

oxygen levels in treated water. It does not alter

pH of water.

C Increases coagulation.

C Helps remove iron and manganese.

C Has taste and odor control properties.

C Requires short contact time

Ozonation: Disadvantages

C More costly than traditional chlorinated

disinfection techniques.

C Forms nitric oxides and nitric acid which can

lead to corrosion.

C Ozone is chemically unstable as a gas, and

hazardous to transport. It must be generated on

site and used immediately. 
E-Beam: Advantages

Currently, there is insufficient information on the

E-Beam process to make a full determination of

its usefulness for CSO disinfection, but a pilot

study performed for the New York City

Department of Environmental Protection

(NYCDEP) determined several advantages of the

E-Beam system:

C No disinfectant chemicals required.

C No toxic byproducts are known to be produced.

C Short contact time required.

C Potential to deactivate a wide range of

pathogens.

C Potential to penetrate waste streams with high

solids concentrations.

E-Beam: Disadvantages

C Increased safety considerations due to use of

high- voltage technology and the generation of

X-ray radiation.

C No full scale application experience for CSOs.

C High capital costs.

C High O&M costs.

C Thin process flow stream.

C Abundant pretreatment straining of influent is

required for this delivery system.

DESIGN CRITERIA

Design criteria for different disinfection systems

will differ based on site-specific needs. However,

the following general factors should always be

considered when evaluating disinfection

alternatives.

C Safety (transport and storage in inhabited areas,

potential for release)

C Effectiveness (ability to reduce indicator

organisms to target levels, reliability,

conditions of use)

C Cost (capital cost, operation and maintenance,

amortization cost)

ClO2 PAA UV RADIATION

Stability Moderate Low High

Persistent Residual Moderate None None

Potential Byproduct Formation Yes No No

Reacts with Ammonia No No No

pH Dependent Moderate No No

Ease of Operation Moderate Complex Simple to Complex

Temperature Dependent Moderate Complex Simple to Complex

Contact Time Moderate Low Low

Safety Concerns High High Low

Effectiveness as Bactericide High High High

Effectiveness as Viricide High High High

Likelihood of Regrowth None None High

Source: Compiled from various sources.

TABLE 1 SUMMARY OF GENERAL ATTRIBUTES OF

CHLORINE DIOXIDE, PAA AND UV RADIATION
C Complexity of use (on-site generation,

application and control, flexibility)

C Environmental/adverse effects (aquatic

toxicity, persistent residuals, formation of toxic

or bio-accumulating byproducts)

C Flow and wastewater characteristics (range of

flows, duration of event, TSS concentration,

downstream channel conditions, monitoring

and control points)

Generally, the efficacy of a disinfectant depends

on factors such as the flow rate, volume, pH, TSS

concentration, and temperature of the wastewater.

For CSOs, these factors vary from location to

location, and even from discharge event to

discharge event, and thus a typical concentration,

contact time, and mixing intensity for a particular

disinfectant is difficult to characterize. CSO

facilities will need to conduct preliminary baseline

studies to characterize the range of conditions that

exist for a particular area and the design criteria to

be considered.

Chlorine Dioxide

Chlorine dioxide is an effective bactericide and

viricide that works over a wide range of pH values

but is unstable and explosive as a gas. On-site

generation of ClO2 may be accomplished by

combining sodium chlorite with either aqueous or

gaseous chlorine (Reaction 1). ClO2 can also be

produced by combining sodium chlorite with

hydrochloric acid (Reaction 2).

Reaction 1:

2NaClO2 + Cl2  ̃ 2ClO2 + 2NaCl (chlorine)

Reaction 2:

5NaClO2 + 4HCl  ̃ 4ClO2 + 5NaCl + 2H2O

(hydrochloric acid)

Reaction 1 requires 1.34 grams of sodium chlorite

to react with 0.526 grams of chlorine to produce

1 gram of ClO2 (pH of chlorine water 1.7-2.4).

Reaction 2 requires 1.67 grams of sodium

chlorite to react with hydrochloric acid to produce

1 gram of ClO2. Excess chlorine is typically

required in both of these reactions, potentially

resulting in chlorinated byproducts.

A new method of ClO2 generation that does not

involve chlorine is to radiate sodium chlorite with

UV radiation as follows:

Reaction 3:

• NaClO2 + hv (254 nanometers) ̃ Na+ +

ClO2

• Na+ + H2O  ̃ NaOH

Chlorine dioxide produced via UV radiation does

not use aqueous or gaseous chlorine, involves a

single chemical reactant, and is cheaper while not

producing any toxic by-products.

A chlorine dioxide disinfection system requires

chlorine dioxide generation on site by one of the

three generation methods above. An adequate

reactor vessel with metering pumps and ancillary

piping is required. Chlorine dioxide is directed

from the generator into an ejector, with the rate of

flow controlled by a chlorinator. The ejector is a

hydraulic chamber designed to carry a fraction of

wastewater flow through it, thereby creating a

vacuum or negative pressure. The chlorinator,

which uses a negative pressure diaphragm valve,

is triggered by the vacuum and releases gas into

the ejector as wastewater flows through. Once in

the ejector, chlorine dioxide enters into solution

and is sent to a diffuser where it is mixed with

wastewater. Mixing can be accomplished using

one of four available methods:

C A diffuser can be placed in the center of a pipe

or channel with flow running at full turbulence.

C A hydraulic structure may be placed in the flow

stream to induce turbulence (e.g., submerged

weir, hydraulic jump).

C A mechanical mixer (propeller, turbine) can be

used in conjunction with a small residence time

mixing chamber and the ejector.
C A jet mixer can discharge disinfectant solution

at high pressure into the wastewater. 

The degree of mixing at the point of disinfectant

application affects the initial rate of inactivation.

After mixing, wastewater enters into a contact

basin to be held for sufficient time until desired

microbial inactivation has been attained. The

discharge pipe or channel may be used for contact

if sufficiently long distance is available. If a

contact chamber is to be built, baffles should be

used to provide the longest possible pathway for

flow to minimize dispersion and approach plug

flow conditions. The design of the contact

chamber is dependent on the disinfectant dose, the

nature of the wastewater, and the required level of

disinfection.

Designs for an ejector should be based on the

maximum capacity of the chlorinator, the inlet

water pressure, the back pressure of the ejector

outlet, and the distance between the ejector and

the diffuser. The ejector should be located as

close to the mixing point as possible to minimize

both the lag in the solution line and the back

pressure at the ejector. The vacuum line carrying

gas between the chlorinator and ejector should

have a pressure drop of less than 0.7 pounds per

square inch. The temperature of the supply lines

and impurities in the gas are chief concerns in the

design of such a system.

Ozonation

Ozone disinfection is similar in most respects to

chlorine disinfection. The major difference is that

ozone is unstable, so it must be generated on site.

For water treatment, ozone is produced by an

electrical corona discharge or ultraviolet

irradiation of dry air or oxygen. Ozone can be

injected or diffused into the water supply stream.

Ozone can be generated from any gas containing

oxygen molecules. The most common sources for

ozone generation are oxygen gas or atmospheric

air. Although the use of pure oxygen gas will

result in a higher efficiency of ozone generation,

it will also increase the initial cost of the gas

source. On the other hand, using atmospheric air

requires preparation of the gas source. Most

ozone generators require clean, dry gas for

optimal conditions. Therefore, atmospheric air

must first be compressed, cleansed and

dehumidified. Compression of the air serves to

increase the concentration of ozone in the supply.

The removal of foreign particulates such as dirt

and dust is often accomplished through filtration.

Air humidity is usually decreased by lowering the

dew point through refrigeration. 

The solubility of ozone in water is a function of

temperature and pH. The amount of ozone in the

system can be regulated in the generator by

adjusting the voltage of the current or the flowrate

of the gas. The maximum ozone concentration

produced by a generator is 50 g/m3 and the

maximum solubility concentration of ozone in

water is 40 mg/L.

Ozone has an average half-life of 20 minutes if it

is not oxidized by particulates. Depending on the

water quality at a given time, a portion of the

dissolved ozone may pass through the system

unoxidized. Regulations require the resulting

concentration of ozone to be reduced to less than

0.002 mg/L before the water is released from the

plant. Ozone degradation can be accelerated by

the addition of hydrogen peroxide or by passing

the system through a UVc system. Contact time

can also be lengthened to allow for further ozone

destruction. 

Ultraviolet Radiation

The following factors must be monitored for the

successful operation of a UV system: flow rate,

suspended solids concentration, UV absorbency

coefficient, initial and final coliform density,

number of lamps in operation, average lamp

output, and average transmissibility of the

transmitting surface.

Two generic designs are available for disinfection

with UV. The non-contact design suspends lamps

away from contact with the wastewater. The

contact reactor-type design uses lamps encased in

a quartz sleeve submerged in wastewater at all

times. The two primary types of submerged UV

systems are:
C Open channel (horizontal and vertical); and

C Closed channel.

An open channel system submerges lamps in

either a horizontal or vertical arrangement in an

open channel from a self-supporting stainless steel

enclosure. Lamps in both positions are arranged

perpendicular to the horizontal flow. A closed

channel system is a sealed disinfection chamber

which can be used in pressurized systems. UV

lamps are installed in a stainless steel chamber

with removable ends for interior access. All

lamps are housed on racks (ballasts) that require

ventilation or cooling to reduce heat build up.

Excessive heat causes failure of ballasts.

The water level in the channels must be kept fairly

constant. Fluctuations in the water level may

result in several problems. In horizontal systems,

the top row of lamps can become exposed, or the

depth of water above this row can become so great

that adequate exposure of wastewater to UV does

not occur. Automated control valves downstream

from the UV lamps can control water level, but

these systems require electrically-operated valves

and electronic controllers. Self-adjusting level

control gates can control upstream water levels

over a wide range of flows. Although these gates

are inexpensive and require little maintenance,

they require proper installation and use more

hydraulic head than an appropriately sized control

valve or weir. Weirs may also be used to control

water level in UV tanks. Weirs are inexpensive,

reliable, predictable, and have no moving parts.

UV dosage is dependent upon the frequency and

the intensity of UV radiation, the number of lamps

and their configuration, the distance between the

wastewater and the lamp surface, the chamber

turbulence, the exposure time, and the absorption

coefficient of the wastewater. Design

considerations should account for the number of

lamps required for disinfection and the number of

channels required to minimize headloss. UV dose

is usually expressed as milliwatt-seconds per

square centimeter (mW-s/cm2). Lamps emitting

wavelengths in the range of 250 to 265

nanometers are usually adequate for disinfection.

Preliminary sizing and design for a system should

be based on 1.0 million gallons per day per UV

kilowatt.

UV disinfection of CSO wastewater requires that

proper UV intensity be applied to wastewater for

sufficient time to render pathogens inactive. A

spectrophotometer can be used in a UV system to

ensure that proper light intensity is delivered.

Unfortunately, many factors may limit the

intensity of the light delivered for treatment.

Suspended solids absorb and scatter UV light

while shielding microorganisms in the particles

from exposure to UV light. Studies have shown

that reduced TSS concentrations are necessary to

obtain adequate disinfection with UV systems;

therefore, sedimentation or filtration prior to UV

is usually required. Proper disinfection is also

dependent upon the light transmitting surface

remaining clear.

Peracetic Acid

Peracetic acid has been used as a disinfectant in

demonstration projects for the treatment of

primary effluent such as that found in CSOs.

PAA is a very strong oxidizer, and is produced by

combining glacial acetic acid, hydrogen peroxide,

and water. Sulfuric acid is typically used as a

catalyst for the reaction. A stabilizer chemical is

also added to solution to slow biodegradation.

Equilibrium concentrations are:

Peracetic Acid 15.0%

Glacial acetic acid 14.0%

Hydrogen peroxide 28.0%

Water 42.2%

Sulfuric Acid 0.8%

A 15 percent solution would be made with the

following proportions:

Glacial acetic acid 39.4%

Water 38.8%

Hydrogen peroxide 21.0%

Sulfuric Acid 0.8%
Because of the high reactivity of the final product,

liquid reactants must be delivered to the site in

separate tanks, and mixed on an as-needed basis.

PAA residuals appear to be non-toxic and are

readily biodegradable in receiving waters.

Electron Beam Irradiation

A limited amount of information exists on the

applicability, performance and optimum design

features for E-Beam systems. To limit the size of

particles to the E-Beam delivery system and

prevent clogging of the delivery unit, the pilot

study for NYCDEP allowed the wastewater to

flow through a basket strainer equipped with

either a 1/32-inch or a 1/64-inch screen basket

before reaching the E-Beam. Water was then

pumped through the E-Beam system using a

progressive-cavity, positive displacement pump.

The pump provided positive control of the

wastewater flowrate discharge. The pump

discharge was then fed to the E-Beam delivery

system where the wastewater film was scanned by

the electron beam. Thermocouples attached to the

delivery system influent and effluent monitored

the temperature change of the wastewater and

were used to calculate the absorbed dose.

According to the study, the absorbed E-Beam dose

was determined as a function of wastewater

temperature changes in the contact tank. A

temperature increase of 1EC equates to an

absorbed dose of 418.6 krads.

The E-Beam disinfection pilot was operated at a

flow rate of 20 gpm. The pilot flow rate was held

relatively constant during each of the test runs.

PERFORMANCE

Disinfectants for treating CSO events require

"high rate" disinfection practices. Ideally, high

bacterial kills should be accomplished with low

contact times, a high mixing intensity, and an

increased disinfectant dosage. Since most of the

alternative disinfectants evaluated in this fact

sheet are new and innovative, performance data is

limited. In cases where actual CSO data were not

available, primary and/or secondary effluent

treatment data were used to evaluate the

effectiveness of the disinfectant.

Comparisons of performance data from different

applications may be further complicated by the

fact that it is often difficult to determine the

appropriate disinfectant concentration to apply to

achieve the desired bacterial reduction.

Concentrations of fecal and total coliform bacteria

in the CSO are commonly used to assess the

performance of the disinfection process in

wastewater applications. However, the coliform

count for storm flow is often uncorrelated to the

pathogenic organism concentrations in the flow.

Therefore, other methods may be more

appropriate for determining the concentrations of

disinfectants to add to the flow to achieve the

desired results.

Chlorine Dioxide

The effectiveness of chlorine dioxide in treating

CSO discharges is measured in terms of reduction

in bacterial concentrations. According to a pilot

scale study in New York, a dosage of 12

milligrams/liter ClO2 applied for a two minute

contact time achieved the following:

C Total coliform (TC) bacteria reduction to target

levels of 1,000 colonies/100 milliliters.

C Fecal coliform (FC) and fecal streptococci (FS)

bacteria reduction to 200 colonies/100

milliliters.

Ozonation

Venosa (1983) compiled data on several studies

on ozonation as a CSO disinfectant. Venosa cites

a 1977 study by Scaccia and Rosen showing that

the disinfection efficiency of ozone is related to

the amount of ozone transferred into the process

water, regardless of the contactor type. Venosa

also cites his own 1978 study demonstrating that

the amount of ozone transferred to a municipal

effluent is directly related to the reduction in the

number of coliforms entering the disinfection

system. In this study, Venosa also developed a

model to predict the effluent coliform number

based on the amount of ozone being transferred

and the total chemical oxygen demand (TCOD).

This and other research demonstrates that ozone

disinfection is a function of the absorbed dose and
the effluent’s ozone-demanding properties. In

addition, monitoring off-gas ozone levels can help

to control disinfection efficiency.

Ultraviolet Radiation

A dose-response relationship exists between UV

dose and reduction of TC and FC. The quality of

wastewater will dictate what dose should be used.

A 30 megaWatt-second/square centimeter (mWs/cm2

) dose is recommended for a 4 log reduction

in fecal coliform for wastewater with a 65 percent

transmittance and a suspended solids

concentration of 30 milligrams/liter. Results of

several studies on UV disinfection are presented

below. In several of these studies, bacterial

concentrations are given in terms of the most

probable number, or MPN:

C A simulated CSO study carried out at the

Brampton WPCP in 1991 used a 60/40 blend

of raw wastewater and final effluent. A UV

dosage of 75 mW-s/cm2 was needed to achieve

a target bacterial concentration of 100

colonies/100 milliliters. Results from the

Central Contra Costa Sanitary District facility

(CCCSD) indicated that UV doses of 55

mW-second/square centimeter were required to

achieve 240 MPN/100 milliliters TC bacterial

densities in unfiltered secondary effluent

(WERF, 1995). Doses less than 50

mW-second/square centimeter were required to

achieve 2.2 MPN/100 milliliters total coliform

bacterial densities in filtered effluent.

C A study using a UV dose of 40 mW-m/cm2 on

tertiary effluent with flows ranging from 9 to

30 liters/second was able to achieve a 3 log kill

up to a 4.8 log kill (99.8 percent removal) of

fecal coliforms, total coliforms, Pseudomonas

aeruginosa, salmonellae, Staphylococcus

aureus, F+ bacteriophage, and enterovirus. 

C A study performed on wastewater being

discharged into shellfish waters around St.

Michael's, MD found that total coliform

densities could be reduced to 70 MPN/100 ml

or less with UV at 25,000 uW-second/square

centimeter and UV transmittance at an average

65 percent (as cited in U.S. EPA, 1986). 

C Under laboratory conditions, a study in

Syracuse, NY, suggested that a dose of 500

mW-s/cm2 would be required to achieve a

residual coliform level of 2500 MPN/100

milliliters (as cited in U.S. EPA, 1986).

C A similar pilot scale study at CCCSD designed

to examine UV and chlorine inactivation of the

MS2 bacteriophage indicated that UV doses of

20 to 100 mW-s/cm2 were sufficient to cause a

1 to 4.7-log reduction in these organisms. In

comparison, chlorination using a dose of 30 to

300 milligrams/liter per minute achieved little

or no inactivation of the MS2 bacteriophage.

Electron Beam Irradiation

According to the pilot study done for the

NYCDEP, E-Beam is not appropriate for the

disinfection of CSOs. The study showed no

increased disinfection with an increase in

disinfection dose for total coliform, fecal

coliform, and Escherichia coli. A weak trend was

observed for increased disinfection of the

Enterococcous group with increased disinfection

dose. 

OPERATION AND MAINTENANCE

In order to assure proper operation and

maintenance for CSO disinfection, operators and

facility managers should have access to, and be

aware of, all Material Safety Data Sheets on any

chemicals being used. These data sheets provide

information on several aspects of the chemical,

including handling and storage, regulations,

disposal considerations, and toxicological

information.

Chlorine Dioxide

Proper operation and maintenance of a chlorine

dioxide facility should include the following

procedures.

C All tubing should be inspected every six

months and faulty or corroded tubing should be

replaced.
C Any gas filters should be inspected every six

months.

C Chlorine dioxide pressure reducing valves

should be cleaned with isopropyl alcohol or

trichloroethylene. 

C The valve spring should be replaced every two

to five years.

C Ejectors should be disassembled and cleaned

every six months.

C Chlorine dioxide analyzers (if used) should be

inspected regularly. All lines should be

inspected daily. Results from the analyzer

should be compared with results from a manual

analysis.

C Granular sodium chlorite should be stored in

its own building equipped with sloped floors

and equipment to hose down spills.

C Increases in temperature, exposure to light,

changes in pressure, and exposure to organic

contaminants should be avoided as they

increase the chance of ClO2 explosions.

Ozonation

Maintenance of an ozone residual at a given

concentration for a specific period (detention

time) is necessary for proper disinfection.

Because ozone has a tendency to decompose

naturally and is consumed rapidly, it must be

contacted uniformly in a near plug flow

contactor. 

The ozone dosage must be calculated based on the

gas flow and the concentration applied to the

contactor versus the gas flow and the

concentration out of the contactor and the aqueous

ozone residual and flow. The calculation requires

consideration of gas volume and concentration

versus aqueous volume and concentration.

Maintenance activities that should be addressed

include:

C Prevent leaking connections or other leaks in

or around the ozonator because this presents an

electrical shock hazard.

C Schedule cleanings of ozonator and its parts.

C Lubricate compressor or blower as scheduled.

C Monitor ozone generator operating

temperature.

C Clean the ozone generation cells periodically to

maintain maximum efficiency.

Ultraviolet Radiation

Maintenance for a UV disinfection facility would

include area maintenance and component

cleaning/repair. As with most surfaces exposed to

wastewater, bacterial growth occurs in spite of the

disinfecting ability of the UV radiation (the

associated warmth facilitates growth). Bacterial

growth on the quartz tubes surrounding the UV

lamps gradually reduces the amount of UV

radiation reaching, and disinfecting, the

wastewater. UV systems have transmittance

meters that measure the amount of UV light

coming from the bulbs and passing through the

wastewater. When the transmittance reading

reaches a predetermined low level, the bulb covers

must be cleaned, chemically and/or physically.

Several methods are available for cleaning lamps

including:

C In-place recirculation;

C Mechanical wipers;

C Dip tanks; and

C Removing modules (rinse, clean with

chemical, rinse and return)

Several cleaning agents available for cleaning

lamps include:

C Citric acid;

C Dilute HCl;
C Phosphoric acid;

C Tile/bowl cleaner;

C Lime away;

C Detergent; and

C Sulfuric acid.

Chemical cleaning may not restore the lamp

covers to their original transmittance level. The

amount of restoration is likely to be less with each

subsequent cleaning. Ultimately, covers must be

manually cleaned or replaced.

The period of time between cleaning is

determined by the rate at which the quartz sleeves

become coated and reduce the ultraviolet output.

The coating composition and the rate at which it

accumulates are site-specific and depend on the

chemical properties of the water. Pilot studies are

recommended to produce data that correspond to

actual operating conditions. 

The lamps themselves will ultimately burn out or

lose their intensity. The normal life span of a UV

lamp is approximately 14,000 hours. Strict

inventory should be made of all lamp usage,

relative output, and estimated cumulative

operating life. Inventories should also be made on

quartz sheaths, Teflon tubes, and ballasts. The

ballast cages used to house the lamps will also

need to be replaced approximately every 10 years.

Reactors taken out of service should be rinsed

with clean water, drained, and held in a drained,

dry condition. A bypass should be constructed

around the entire UV disinfection system for use

during maintenance tasks.

Limited safety data is available on UV

disinfection systems. Moderate skin exposure to

UV can cause erythema, or reddening of the skin.

Excessive exposure may cause bleeding and

blistering. UV exposure to eyes can cause

kerato-conjunctivitis or inflammation of the eye,

retinal lesions, chronic yellowing of the lens, and

cataract formation. 

The following precautions are suggested to help

reduce UV exposure:

C Open-channel lamps should be arranged in

metal frames.

C Interlocks should be incorporated into the

channel lamp design to allow for the shutdown

of entire lamp blocks. Interlocks should also

prevent lights from illuminating when they are

not completely submerged.

C Protective goggles and a face shield should be

worn at all times around UV lamps even when

the lamps appear to be emitting little light.

C Protective electrical devices should be included

in electrical designs for the facility. 

C Precautions should be taken to avoid electrical

shocks. Facility plans should provide limited

access and traffic through electrical and

equipment areas. 

C Access to those areas should be limited only to

necessary personnel.

C Hazard placards should be placed in

appropriate areas of the facility describing the

risks associated with the UV equipment.

Peracetic Acid

Maintenance for a PAA facility should include

periodic inspections of feed lines, storage areas,

leakage detection equipment, and chemical

injectors. Blocked or silted sewer lines should be

flushed and cleaned on a regular basis. Spent

chemical containers should be discarded in

approved receptacles. Spilled chemicals should

be cleaned up immediately. The chemical housing

facility should be cleaned and inspected

periodically to ensure structural integrity. The

explosive nature of the chemical agents stored at

the facility requires that the sprinkler system be

inspected regularly.

The following basic safety precautions should be

observed while handling PAA. More detailed
safety precautions are available from the manufacturer.

C Chemicals for use in PAA disinfection

processes should be stored in a cool, dry,

well-ventilated area and should be kept in their

original, DOT-approved shipping containers

with hazard labels intact. PAA is thermally

unstable and decomposes explosively at high

temperatures. The storage building should be

constructed of materials with maximum

resistance to fire and explosive potential, and

should include deflagration venting. The

facilities must be designed with interior walls

capable of withstanding 125 pounds per square

foot internal overpressurization and ceilings

must be insulated. Storage buildings should

have an automatic sprinkler system designed

for 0.40 gallons per minute per square foot.

C The storage room should be kept separate from

all other processes and should be separated

from acids, alkalies, organic materials, and

heavy metals.

C A backup power supply is necessary for

housing structures at all times.

C No more than 4,800 gallons of the product

should be allowed inside the facility at any one

time; a greater quantity requires storage in

separate buildings.

C PAA may only be stored in 30 gallon closed

drums and cannot be stacked.

C Personnel must wear special protective

clothing and positive pressure self-contained

breathing apparatus.

C Release detecting equipment must be located

near chemicals, and near valves and equipment

that pose a potential threat. Releases in excess

of 1 pound should be reported.

C Chemical spills must be contained and diluted

with water.

COSTS

Costs for chlorine dioxide, ozonation, UV

radiation, peracetic acid, and E-Beam processes

cannot be easily compared due to the different

variables and the varying application methods for

the various disinfectants. Each alternative should

be evaluated based on the characteristics of a

particular site. Tradeoffs should be weighed to

determine any overriding factors to be ultimately

incorporated into the system. In a report prepared

for the NYCDEP, a cost projection was created

for chlorine dioxide and ozone. This data is

presented in Table 2. Due to limited information

on E-Beam processes there is no cost information

currently available.

TABLE 2 CSO DISINFECTION PILOT STUDY COST PROJECTIONS

Conceptual Level Disinfection Costs

Technology Chlorine Dioxide Ozone

Peak Design Flow (cfs) 1,250 2,500 5,000 1,250 2,500 5,000

Capital Costs $651,000 $1,085,000 $1,808,000 $18,000,000 $23,000,000 $28,600,000

Annualized Capital Costs $66,000 $111,000 $184,000 $1,833,000 $2,343,000 $2,913,000

Annual O&M Costs $275,000 $275,000 $275,000 $500,000 $550,000 $615,000

Total Annualized Costs $341,000 $386,000 $459,000 $2,333,000 $2,893,000 $3,528,000

Source: NYCDEP, 1997.

Notes:

1. Costs are present worth in 1997 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 at Spring Creek and are for process equipment only. Costs do not include additional contact

tankage (if required) or support facilities.
Chlorine Dioxide

Costs will vary for a chlorine dioxide generation

system based on design specifications. The

estimated cost for a chlorine dioxide system

(wired and installed) capable of producing a feed

rate of over 100 pounds ClO2/day is

approximately $21,000. 

The system would convert chlorine dioxide from

sodium chlorite or chlorine. Production of gas

would be dependent upon demand and would not

be affected by variations in flow rate.

Ultraviolet Radiation

A detailed cost analysis was obtained from a pilot

demonstration project performed at the

Downingtown WPCP in Downingtown, PA. This

system was not used for treatment of CSOs;

however, the equipment and O&M costs provide

baseline estimates for similar treatment flows.

Costs and equipment designs were based on the

following data:

Peak Flow (GPM) 7297.5 (10.5 mgd)

Avg. Flow (GPM) 2919 (4.2 mgd)

Influent BOD mg/l 30

Influent SS mg/l 30

UV Transmission

(%)

65

Based on this data, an open channel vertical

system with the specifications shown in Table 3

would be required to produce effluent which

meets the minimum fecal coliform count of less

than 200 colonies per 100 milliliters for any 30-

day geometric mean of daily samples at peak flow.

The present worth analysis based on a life

expectancy of 20 years and an 8 percent interest

rate for both capital cost and operation and

maintenance is approximately $446,897 for a

vertical system and $492,517 for a horizontal

system.

REFERENCES

1. Arnett, C.J., and M. Boner, 1998. Wet

Weather Demonstration Programs.

Presented at WEFTEC ‘98 Workshop.

2. Dunham, R., H. He, and K. Woodard, no

date. “Water Disinfection with Ozone.”

Internet site at

http://www.ce.vt.edu/enviro2/wtprimer/o

zone/ozone.html, accessed July 1999.

3. Goldstein, S., 1997. CSO Disinfection

Pilot Study-Spring Creek AWPCP

Upgrade - Final Report. Prepared for the

New York City Department of

Environmental Protection by Camp

Dresser & McKee..

4. Hall, Jr., L.W., D. T. Burton, and L. B.

Richardson, 1981. "Comparison of Ozone

and Chlorine Toxicity to the Development

Stages of Striped Bass, Morone Saxatilis."

Canadian Journal of Fish and Aquatic

Science, Volume 38.

5. Hass, C.N., K. Longley, and T. Selfridge,

1990. "High-Rate Coliform Disinfection

of Storm Water Overflow." Research

Journal of the Water Pollution Control

Federation, Volume 62, No. 3,

pp.282-287.

6. Kramer, S. and S. Leung, undated. “Disinfection with Ozone.” Internet site at

http://www.ce.vt.edu/enviro2/wtprimer/o

zone/ozone.html, accessed July 1999.

7. Maher, M. B., 1974. Microstraining and

Disinfection of Combined Sewer

Overflows-Phase III.

8. Metcalf and Eddy, 1995. Combined

Sewer Overflow Disinfectio n

Effectiveness.

9. Rein, D.A., G.A. Jamesson, and R.A.

Monteith, 1992. "Toxicity Effects of

Alternate Disinfection Processes." Water
Environment Federation, 65th Annual

Conference and Exposition.

10. Tift, E.C., P.E. Moffa, S.L. Richardson,

and R.I. Field, 1977. "Enhancement of

High-Rate Disinfection by Sequential

Addition of Chlorine and Chlorine

Dioxide." Journal of the Water Pollution

Control Federation. July, 1977.

11. Ward, R.W., and G.M. DeGraeve, 1978.

"Residual Toxicity of Several

Disinfectants in Domestic Wastewater."

Journal of the Water Pollution Control

Federation.

12. U.S. EPA, 1975. Bench-Scale High Rate

Disinfection of Combined Sewer

TABLE 3 ULTRAVIOLET RADIATION COST FACTORS FOR AN OPEN CHANNEL VERTICAL

SYSTEM CAPABLE OF PRODUCING AN EFFLUENT WITH A MAXIMUM FECAL COLIFORM

COUNT OF LESS THAN 200 COLONIES PER 100 MILLILITERS 

SPECIFICATION

Number of Channels 2; Water Depth will be a maximum of 62", Minimum of 57.5"

Number of Modules/Channel 6

Total Number of Modules 12

Number of Lamps/Module 40

Total Number of Lamps 480

Dose Level (megawatts per second

per square centimeter)

36

Retention Time (seconds) 12-38

Headloss (inches) <3"

Power Requirement (2) 208 v, 3 ph, 60 Hz, 60 amp

Capital Cost: $278,400

OPERATION AND MAINTENANCE

Electrical Consumption $.055/kWh

Consumption/Lamp $0.075/kW (Vertical)

$0.085/kW (Horizontal)

Lamp Replacement Cost $55.00 every 8760 operating hours

Cleaning Frequency Once/month (may vary depending on site-specific conditions)

Labor rate $15.00/hour

Vertical Horizontal

Number of Lamps “On” 160 176

Electrical $5,782 $7,208

Replacement $8,800 $9,680

Cleaning $180 (30 minutes per channel per

month)

$990 (15 minutes per module per

month)

Replacement Labor $2,400 (30 seconds per lamp per

year)

$4,224 (8 minutes per lamp per year)

Total Replacement Costs $17,162 $22,102
For more information contact:

Municipal Technology Branch

U.S. EPA

Mail Code 4204

401 M St., S.W.

Washington, D.C., 20460

Overflows With Chlorine and

Chlorine Dioxide. EPA 670/2-75-

021.

13. U.S. EPA, 1984. Combined Sewer

Overflow Control Manual. EPA 625-R93-007.

14. U.S. EPA, 1986. Design Manual:

Municipal Wastewater Disinfection. EPA

625-1-86/021.

15. U.S. EPA, 1994. Stormwater Pollution

Abatement Technologies. EPA/600-R94/129.

16. Venosa, A.D., 1983. “Current

State-of-the-Art of Wastewater

Disinfection.” Journal of the Water

Pollution Control Federation, Volume 55,

No. 5., pp 457-466.

17. Water Environment Federation, 1996.

Wastewater Disinfection. Manual of

Practice FD-10.

18. Water Pollution Control Federation, 1989.

Combined Sewer Overflow Pollution

Abatement.

ADDITIONAL INFORMATION

The City of Columbus, Georgia

Cliff Arnett

Columbus Water Works

1501 13th Avenue

Columbus, GA 31902

HydroQual, Inc.

Karl Scheible, Principle Engineer

1 Lethbridge Plaza

Mahwah, NJ 07430

Moffa and Associates

Peter Moffa

5710 Commons Park

P.O. Box 26

Syracuse, NY 13214

The City of New York Department of

Environmental Protection

Barry Zuckerman

96-05 Horace Harding Expressway, 5th Floor

Corona, NY 11368 

Wet Weather Engineering and Technology

Mark Boner

1825 2nd Avenue

Columbus, GA 31901

The mention of trade names or commercial

products does not constitute endorsement or

recommendation for the use by the U.S.

Environmental Protection Agency.