v>EPA
United States
Environmental Protection
Agency
Off ice 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,
Disinfect!on-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
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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.
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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 E-
Beam 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
• More powerful disinfectant than most chlorine
compounds.
• Inactivates most strains of bacteria and viruses
and is noted for destroying chlorine-resistant
strains of both. Highly effective for
Cryptospiridium eradication.
• Will oxidize phenols with no negative
residuals such as trihalomethane production.
• Does not produce a disinfection residual that
would prevent bacterial growth.
• Degenerates into oxygen, which can elevate
oxygen levels in treated water. It does not alter
pH of water.
• Increases coagulation.
• Helps remove iron and manganese.
• Has taste and odor control properties.
• Requires short contact time
Ozonation: Disadvantages
• More costly than traditional chlorinated
disinfection techniques.
• Forms nitric oxides and nitric acid which can
lead to corrosion.
• Ozone is chemically unstable as a gas, and
hazardous to transport. It must be generated on
site and used immediately.
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TABLE 1 SUMMARY OF GENERAL ATTRIBUTES OF
CHLORINE DIOXIDE, PAA AND UV RADIATION
CIO,
PAA
UV RADIATION
Stability
Persistent Residual
Potential Byproduct Formation
Reacts with Ammonia
pH Dependent
Ease of Operation
Temperature Dependent
Contact Time
Safety Concerns
Effectiveness as Bactericide
Effectiveness as Viricide
Likelihood of Regrowth
Moderate
Moderate
Yes
No
Moderate
Moderate
Moderate
Moderate
High
High
High
None
Low
None
No
No
No
Complex
Complex
Low
High
High
High
None
High
None
No
No
No
Simple to Complex
Simple to Complex
Low
Low
High
High
High
Source: Compiled from various sources.
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:
• No disinfectant chemicals required.
• No toxic byproducts are known to be produced.
• Short contact time required.
• Potential to deactivate a wide range of
pathogens.
• Potential to penetrate waste streams with high
solids concentrations.
E-Beam: Disadvantages
• Increased safety considerations due to use of
high- voltage technology and the generation of
X-ray radiation.
• No full scale application experience for CSOs.
• High capital costs.
• High O&M costs.
• Thin process flow stream.
• 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.
• Safety (transport and storage in inhabited areas,
potential for release)
• Effectiveness (ability to reduce indicator
organisms to target levels, reliability,
conditions of use)
• Cost (capital cost, operation and maintenance,
amortization cost)
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• Complexity of use (on-site generation,
application and control, flexibility)
• Environmental/adverse effects (aquatic
toxicity, persistent residuals, formation of toxic
or bio-accumulating byproducts)
• 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 C1O2 may be accomplished by
combining sodium chlorite with either aqueous or
gaseous chlorine (Reaction 1). C1O2 can also be
produced by combining sodium chlorite with
hydrochloric acid (Reaction 2).
Reaction 1:
2NaClO2 + C12 -> 2C1O2 + 2NaCl (chlorine)
Reaction 2:
5NaClO2 + 4HC1 -» 4C1O2 + SNaCl + 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 C1O2 (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 C1O2. Excess chlorine is typically
required in both of these reactions, potentially
resulting in chlorinated byproducts.
A new method of C1O2 generation that does not
involve chlorine is to radiate sodium chlorite with
UV radiation as follows:
Reaction 3:
NaClO2 + hv (254 nanometers)-
C1O,
Na+ +
Na+
H9O -» 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:
• A diffuser can be placed in the center of a pipe
or channel with flow running at full turbulence.
• A hydraulic structure may be placed in the flow
stream to induce turbulence (e.g., submerged
weir, hydraulic jump).
• A mechanical mixer (propeller, turbine) can be
used in conjunction with a small residence time
mixing chamber and the ejector.
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• 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:
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• Open channel (horizontal and vertical); and
• 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%
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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 1°C 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 C1O2 applied for a two minute
contact time achieved the following:
• Total coliform (TC) bacteria reduction to target
levels of 1,000 colonies/100 milliliters.
• 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
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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 (mW-
s/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:
• 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.
• 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+ bactedophage, and enterovirus.
• 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).
• 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).
• 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, andEscherichiacoli. 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.
• All tubing should be inspected every six
months and faulty or corroded tubing should be
replaced.
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• Any gas filters should be inspected every six
months.
• Chlorine dioxide pressure reducing valves
should be cleaned with isopropyl alcohol or
tri chl oroethy 1 ene.
• The valve spring should be replaced every two
to five years.
• Ejectors should be disassembled and cleaned
every six months.
• 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.
• Granular sodium chlorite should be stored in
its own building equipped with sloped floors
and equipment to hose down spills.
• Increases in temperature, exposure to light,
changes in pressure, and exposure to organic
contaminants should be avoided as they
increase the chance of C1O2 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:
• Prevent leaking connections or other leaks in
or around the ozonator because this presents an
electrical shock hazard.
• Schedule cleanings of ozonator and its parts.
• Lubricate compressor or blower as scheduled.
• Monitor ozone generator operating
temperature.
• 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:
• In-place recirculation;
• Mechanical wipers;
• Dip tanks; and
• Removing modules (rinse, clean with
chemical, rinse and return)
Several cleaning agents available for cleaning
lamps include:
• Citric acid;
Dilute HC1;
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• Phosphoric acid;
• Tile/bowl cleaner;
• Lime away;
• Detergent; and
• 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:
• Open-channel lamps should be arranged in
metal frames.
• 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.
• 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.
• Protective electrical devices should be included
in electrical designs for the facility.
• Precautions should be taken to avoid electrical
shocks. Facility plans should provide limited
access and traffic through electrical and
equipment areas.
• Access to those areas should be limited only to
necessary personnel.
• 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
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safety precautions are available from the manufacturer.
• 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.
• The storage room should be kept separate from
all other processes and should be separated
from acids, alkalies, organic materials, and
heavy metals.
• A backup power supply is necessary for
housing structures at all times.
• 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.
• PAA may only be stored in 30 gallon closed
drums and cannot be stacked.
• Personnel must wear special protective
clothing and positive pressure self-contained
breathing apparatus.
• 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.
• 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
Peak Design Flow (cfs)
Capital Costs
Annualized Capital Costs
Annual O&M Costs
Total Annualized Costs
Chlorine Dioxide
1,250
$651,000
$66,000
$275,000
$341,000
2,500
$1,085,000
$111,000
$275,000
$386,000
5,000
$1,808,000
$184,000
$275,000
$459,000
Ozone
1,250
$18,000,000
$1,833,000
$500,000
$2,333,000
2,500
$23,000,000
$2,343,000
$550,000
$2,893,000
5,000
$28,600,000
$2,913,000
$615,000
$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.
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Chlorine Dioxide
REFERENCES
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)
Avg. Flow (GPM)
Influent BOD mg/1
Influent SS mg/1
UV Transmission
7297.5 (10.5 mgd)
2919(4.2mgd)
30
30
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.
Arnett, C.J., and M. Boner, 1998. Wet
Weather Demonstration Programs.
Presented at WEFTEC '98 Workshop.
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.
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..
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.
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.
Kramer, S. and S. Leung, undated. "Dis-
infection with Ozone." Internet site at
http://www.ce.vt.edU/enviro2/wtprimer/o
zone/ozone.html, accessed July 1999.
Maher, M. B., 1974. Microstrainingand
Disinfection of Combined Sewer
Overflows-Phase III.
Metcalf and Eddy, 1995. Combined
Sewer Overflow Disinfection
Effectiveness.
Rein, D.A., G.A. Jamesson, and R.A.
Monteith, 1992. "Toxicity Effects of
Alternate Disinfection Processes." Water
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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
Number of Modules/Channel
Total Number of Modules
Number of Lamps/Module
Total Number of Lamps
Dose Level (megawatts per second
per square centimeter)
Retention Time (seconds)
Headless (inches)
Power Requirement
Capital Cost: $278,400
OPERATION AND MAINTENANCE
Electrical Consumption
Consumption/Lamp
Lamp Replacement Cost
Cleaning Frequency
Labor rate
Number of Lamps "On"
Electrical
Replacement
Cleaning
Replacement Labor
Total Replacement Costs
2; Water Depth will be a maximum of 62", Minimum of 57.5"
6
12
40
480
36
12-38
<3"
(2)208v, 3 ph, 60 Hz, 60 amp
$.055/kWh
$0.075/kW (Vertical)
$0.085/kW (Horizontal)
$55.00 every 8760 operating hours
Once/month (may vary depending on site-specific conditions)
$15.00/hour
Vertical Horizontal
160 176
$5,782 $7,208
$8,800 $9,680
$180 (30 minutes per channel per
month)
$2,400 (30 seconds per lamp per
year)
$17,162
$990 (15 minutes per module per
month)
$4,224 (8 minutes per lamp per year)
$22,102
Environment Federation, 65th Annual
Conference and Exposition. 11.
10. lift, E.G., 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 12.
Control Federation. July, 1977.
Ward, R.W., and G.M. DeGraeve, 1978.
"Residual Toxicity of Several
Disinfectants in Domestic Wastewater."
Journal of the Water Pollution Control
Federation.
U.S. EPA, 1975. Bench-Scale High Rate
Disinfection of Combined Sewer
-------
Overflows With Chlorine and
Chlorine Dioxide. EPA 670/2-75-
021.
13. U.S. EPA, 1984. Combined Sewer
Overflow Control Manual. EPA 625-R-
93-007.
14. U.S. EPA, 1986. Design Manual:
Municipal WastewaterDisinfection. EPA
625-1-86/021.
15. U.S. EPA, 1994. Stormwater Pollution
Abatement Technologies. EPA/600-R-
94/129.
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.
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.
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
For more information contact:
Municipal Technology Branch
U.S. EPA
Mail Code 4204
401 M St., S.W.
Washington, D.C., 20460
IMTB
Excellence fn compliance through optimal technical solutions
MUNICIPAL TECHNOLOGY
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