United States
                        Environmental Protection
                        Office of Water
                        Washington, D.C.
EPA 832-F-99-064
September 1999
Waste water
Technology  Fact Sheet
Ultraviolet Disinfection
Disinfection  is considered  to  be the  primary
mechanism  for  the  inactivation/destruction of
pathogenic organisms to prevent the spread of
waterborne diseases to downstream users and the
environment.  It is important that wastewater be
adequately treated prior to disinfection in order for
any disinfectant to be effective.  Some common
microorganisms found in  domestic wastewater and
the diseases associated with them are presented in
Table 1.
An Ultraviolet (UV) disinfection system transfers
electromagnetic energy from a mercury arc lamp to
an organism's genetic material (DNA and RNA).
When UV radiation penetrates the cell wall of an
organism, it destroys the cell's ability to reproduce.
UV radiation, generated by an electrical discharge
through mercury  vapor,  penetrates the genetic
material of microorganisms and retards their ability
to reproduce.
The  effectiveness  of a  UV  disinfection system
depends on the characteristics of the wastewater,
the intensity of UV radiation, the amount of time the
microorganisms are exposed to the radiation, and
the reactor configuration. For any one treatment
plant, disinfection success is directly related to the
concentration  of  colloidal  and  particulate
constituents in the wastewater.
The main components of a UV disinfection system
are mercury arc lamps, a reactor, and ballasts. The
source of UV radiation is either the low-pressure or
medium-pressure mercury arc lamp with low or high
                           TABLE 1 INFECTIOUS AGENTS
                              DOMESTIC WASTEWATER
 Disease Caused
                       Escherichia coli (enterotoxigenic)
                       Leptospira (spp.)
                       Salmonella typhi
                       Salmonella (=2,100 serotypes)
                       Shigella (4 spp.)

                       Vibrio cholerae
                       Balantidium coli
                       Cryptosporidium parvum
                       Entamoeba histolytica

                       Giardia lamblia
                       Ascaris lumbricoides
                       T. solium
                       Trichuris trichiura
                       Enteroviruses (72 types, e.g.,
                       polio, echo, and coxsackie
                       Hepatitis A virus
                       Norwalk agent
Typhoid fever
Shigellosis (bacillary

Amebiasis (amoebic


heart anomalies,
Infectious hepatitis
                      Source:  Adapted from Crites and Tchobanoglous, 1998.

The optimum wavelength to effectively inactivate
microorganisms is in the range of 250 to 270 nm.
The intensity of the radiation emitted by the lamp
dissipates as the distance from the lamp increases.
Low-pressure lamps emit essentially monochromatic
light at a wavelength of 253.7 nm. Standard lengths
of the low-pressure lamps are 0.75 and 1.5 meters
with diameters of 1.5 - 2.0 cm. The ideal lamp wall
temperature is between 95 and 122°F.

Medium-pressure lamps are generally used for large
facilities. They have approximately 15 to 20 times
the germicidal UV intensity of low-pressure lamps.
The medium-pressure lamp disinfects faster and has
greater penetration capability because of its  higher
intensity. However, these lamps operate at higher
temperatures with a higher energy consumption.

There  are two  types  of UV disinfection reactor
configurations  that   exist:  contact  types  and
noncontact types.  In both the contact  and the
noncontact types, wastewater   can  flow  either
perpendicular or  parallel to the lamps.   In the
contact reactor, a series of mercury lamps are
enclosed in quartz sleeves to minimize the cooling
effects of the wastewater. Figure 1 shows two UV
                                  Note: A UV bank is
                                     composed of a
                                     number of UV
  Flap gate
  level control ~
                         UV vertical lamp
                         module with
                         support rack
Source: Crites and Tchobanoglous, 1998.
       (a) adapted from Trojan Technologies, Inc.
       (b) adapted from Infilco Degremont, Inc.

contact reactors  with submerged lamps placed
parallel and perpendicular to the direction of the
wastewater flow. Flap gates or weirs are used to
control the level of the  wastewater.   In the
noncontact reactor,  the UV lamps are suspended
outside a transparent conduit, which carries the
wastewater to be disinfected.  This configuration is
not as common as the contact reactor. In both types
of reactors, a ballast—or control box—provides a
starting voltage for the  lamps  and maintains  a
continuous current.



•      UV disinfection is effective at inactivating
       most viruses, spores, and  cysts.

•      UV disinfection is a physical process rather
       than  a   chemical   disinfectant,  which
       eliminates the need  to  generate, handle,
       transport, or store  toxic/hazardous  or
       corrosive chemicals.

•      There  is  no residual effect  that can  be
       harmful to humans or aquatic life.
                                                         UV   disinfection
                          is  user-friendly  for
       UV disinfection has a shorter contact time
       when compared with  other  disinfectants
       (approximately 20  to 30 seconds  with
       low-pressure lamps).

       UV  disinfection equipment requires  less
       space than other methods.
       Low dosage may not effectively inactivate
       some viruses, spores, and cysts.

       Organisms can sometimes repair and reverse
       the destructive effects of UV through a
       "repair  mechanism,"  known  as  photo
       reactivation,  or  in  the absence  of light
       known as "dark repair."

•      A  preventive  maintenance  program  is
       necessary to control fouling of tubes.

       Turbidity and total suspended solids (TSS)
       in   the  wastewater   can   render   UV
       disinfection  ineffective.   UV disinfection
       with low-pressure lamps is not as effective
       for secondary  effluent  with  TSS  levels
       above 30 mg/L.

•      UV disinfection is not as cost-effective as
       chlorination, but costs are competitive when
       chlorination  dechlorination is used and fire
       codes are met.


When choosing a UV disinfection system, there are
three critical areas to be  considered.  The first  is
primarily  determined  by the manufacturer; the
second, by design and Operation and Maintenance
(O&M); and the third  has to be controlled at the
treatment facility.

Choosing  a UV disinfection system  depends on
three critical factors listed below.

       Hydraulic properties of the reactor: Ideally,
       a UV  disinfection system  should  have a
       uniform flow  with enough axial  motion
       (radial mixing) to maximize exposure to UV
       radiation.  The path that an organism takes
       in the reactor determines the amount of UV
       radiation it  will  be  exposed to  before
       inactivation.  A reactor must be designed to
       eliminate short-circuiting and/or dead zones,
       which can result in inefficient use of power
       and reduced contact time.

       Intensity of the  UV radiation:  Factors
       affecting the intensity are the age of the
       lamps, lamp fouling, and the configuration
       and placement of lamps in the reactor.

•      Wastewater  characteristics: These include
       the flow  rate, suspended and  colloidal
       solids, initial bacterial density, and other
       physical and  chemical parameters. Both the
       concentration of TSS and the concentration
       of  particle-associated   microorganisms
       determine  how   much  UV   radiation
       ultimately reaches the target organism.  The
       higher these concentrations,  the lower the
       UV radiation absorbed by the organisms.
       Various wastewater characteristics and their
       effects on UV disinfection are given in Table

Effects on UV
Minor effect, if any
Minor effect, if any
Minor effect, if any
 Biochemical oxygen
 demand (BOD)
 Humic materials, Iron


Minor effect, if any.
Although, if a large
portion of the BOD is
humic and/or unsaturated
(or conjugated)
compounds, then UV
transmittance may be

Affects solubility of metals
that can absorb UV light.
Can lead to the
precipitation of
carbonates on quartz

High absorbency of UV

Affects solubility of metals
and carbonates.

Absorbs UV radiation and
shields embedded
UV disinfection can be used in plants of various
sizes that provide secondary or advanced levels of


Gold Bar Wastewater Treatment Plant in
Edmonton, Alberta, Canada

The  Gold  Bar  Wastewater  Treatment  Plant
(GBWTP) in Edmonton, Alberta, was required to
use disinfection to meet water quality standards for

contact recreation in Alberta. During that period,
the average and peak  design flow rates for this
treatment facility were  82 and 110 million gallons
per day (mgd), respectively. A  pilot  study  was
conducted  to review  current  UV disinfection
systems, effectiveness of lamp intensities, and costs.
UV disinfection was determined  to be the  most
efficient disinfection system to achieve the required
treatment levels.

Lamp fouling is a potential problem among UV
systems,  but with  proper cleaning  and O&M,  it
should  not interrupt  the  system's disinfection
capability.   Lamp  cleaning at the  GBWTP  was
achieved by  a  mechanical  wiping  mechanism
accompanying each cluster of lamps. Lamps were
cleaned on  a regular  basis using  an  in-channel
cleaning  system.   The safety  concerns for both
low-pressure  and  high-intensity  UV  systems
regarding exposure to UV radiation and electrical
hazards are low under normal operating conditions.
However, precautionary measures should be taken
when  operating high-intensity  lamps  to  avoid
overexposure. The risk was not considered major
by  the  GBWTP  and  was outweighed by the
potential  savings  of  using high-intensity  UV
systems.   At  the  GBWTP, a medium-pressure,
high-intensity  system  was found to   be more
economical  than  the  conventional  low-pressure
systems in both capital  and life-cycle costs.

Northwest Bergen County Utility Authority
Wastewater Treatment Plant in  Waldwick,
New Jersey

The  use  of  UV  disinfection   for wastewater
treatment has increased dramatically in the last few
years due to the impact of chlorinated organics from
sewage effluent on receiving waters. Such was the
case with the Northwest Bergen County Utility
Authority (NBCUA) Wastewater  Treatment Plant
located in Waldwick, New Jersey.   In  1989, the
treatment plant had to convert from chlorination to
an alternative disinfection  technology  with zero
residual after treatment. This change was brought
about when  the "zero residual"  regulation  was
imposed  by  the  New  Jersey   Department of
Environmental Protection with the passage of the
Toxic Catastrophic Prevention Act.
Several factors, such  as public safety and recent
findings  and  concerns  over  the  environmental
impact of chemical releases and spills, have led to
more stringent permit requirements for chlorine.
Also, there were other conditions that the treatment
plant had to meet if chlorine use was to continue.
To avoid the escalated costs that could be incurred
and to be in compliance with the new regulations,
the wastewater treatment plant switched  to  UV
disinfection. The UV system was installed within
the existing chlorine contact tanks, along with an
extension  to  the  existing  building  for  easy
maintenance during bad weather. The UV system at
NBCUA was able to meet fecal coliform levels (200
count per 100 ml) better than chlorination since its
installation in August  1989.


The proper O&M of a UV disinfection  system
ensures that sufficientUV radiation is transmitted to
the organisms  to render them sterile.  All surfaces
between the UV radiation and the target organisms
must be clean, and the ballasts, lamps, and reactor
must be functioning at peak efficiency.  Inadequate
cleaning is one of the most common causes of a UV
system's ineffectiveness.   The  quartz  sleeves or
Teflon  tubes  need to be  cleaned  regularly  by
mechanical wipers, ultrasonics, or chemicals.  The
cleaning frequency  is  very site-specific,  some
systems need to be cleaned more often than others.

Chemical cleaning is  most commonly  done with
citric acid.  Other cleaning agents include  mild
vinegar solutions and sodium hydrosulfite.   A
combination of cleaning agents should be tested to
find the agent most suitable for the  wastewater
characteristics without producing harmful or toxic
by-products. Noncontact reactor systems are most
effectively cleaned by using sodium hydrosulfite.

Any UV disinfection system should be pilot tested
prior to full-scale operation to ensure that it  will
meet discharge permit requirements for a particular

The average lamp life  ranges from 8,760 to 14,000
working hours, and the lamps are usually replaced
after 12,000 hours of use.  Operating procedures
should be set  to reduce the on/off cycles of the

lamps, since their efficacy is reduced with repeated

The ballast must be compatible with the lamps and
should be ventilated to protect it from excessive
heating, which may shorten its life or even result in
fires.    Although the  life cycle of ballasts  is
approximately 10 to 15  years, they are usually
replaced every 10 years.  Quartz sleeves will last
about 5 to 8 years but are generally replaced every
5 years.


The cost of UV disinfection systems depends on the
manufacturer, the site, the capacity of the plant, and
the  characteristics   of  the  wastewater  to be
disinfected. Total costs of UV disinfection can be
competitive   with  chlorination   when  the
dechlorination step is included.

The  annual operating costs for UV disinfection
include power consumption; cleaning chemicals and
supplies; miscellaneous equipment repairs (2.5% of
total equipment cost); replacement of lamps, ballasts
and sleeves; and  staffing requirements.

Costs  have decreased in recent  years  due  to
improvements in lamp and system designs, increased
competition,  and improvements in  the systems'

Medium-pressure lamps cost four to five times as
much as low-pressure lamps. However, the reduced
number of lamps necessary for adequate disinfection
could make medium-pressure lamps cost-effective.
Table 3 A summarizes the costs of some of the lamps
used in UV disinfection.   This information was
collected in  a  study conducted by the  Water
Environment  Research Federation  in  1995 for
secondary effluents  from disinfection facilities at
average dry weather flow rates of 1,  10, and 100
mgd (2.25, 20, and  175  mgd peak wet weather
flow, respectively). Table 3B describes the typical
capital and O&M costs that are associated with a
UV disinfection.
UV lamps
1-5 mgd
5-10 mgd
19-100 mgd
Construction cost
for physical
(% of UV
lamp cost)
(% of UV lamp
cost) 150
* Costs are based on a 1993 Engineering News Record
Construction Cost Index of 5,210.
Source: Adapted from: Darby etal. (1995) with permission
from the Water Environment Research Foundation.
       Cost Item
UV System Cost ($)
 Capital Costs


 Structural modifications





 Total:                         244,000

       Annual operating and maintenance costs

 Energy                          3300

 Lamps and chemicals              2840

 Cleaning                        1180

 Maintenance                     1440

 Process control                   6240

 Testing                          4160
Source: Hanzon and Vigilia, 1999.

                                                  1.      Crites,  R.  and G.  Tchobanoglous. 1998.
                                                         Small   and   Decentralized   Wastewater

Management  Systems. The McGraw-Hill
Companies. New York, New York.

Darby, J.; M. Heath; J. Jacangelo; F. Loge;
P. Swaim; and G.  Tchobanoglous. 1995.
Comparison   of  UV   Irradiation   to
Chlorination:   Guidance  for   Achieving
Optimal  UV  Performance.   Water
Environment   Research  Foundation.
Alexandria, Virginia.

Eddington,  G. June  1993.  Plant Meets
Stringent Residual Chlorine Limit. Water
Environment & Technology. P. 11-12.

Fahey, R. J. Dec. 1990. The UV Effect on
Wastewater.   Water  Engineering   &
Management, vol. 137. no. 12. pp. 15-18.

Hanzon, B.D.  and Vigilia, R.  1999.   UV
Disinfection.    Wastewater Technology
Showcase, vol. 2. no. 3. pp. 24-28.

Hrentstein, B, Dean, T., Anderson, D.,  and
Ellgas, W. October 1993.  Dechlorination
at EBMUD: Innovative and Efficient and
Reliable.    Proceeding  of the  Water
Environment Federation Sixty-sixth Annual
Conference  and  Exposition.   Anaheim,

Kwan, A.;  J.  Archer; F.  Soroushian; A.
Mohammed; and G. Tchobanoglous. March
17-20, 1996. "Factors for Selection of a
High-Intensity UV Disinfection System for
a Large-Scale Application." Proceedings
from  the Water Environment Federation
(WEF) Speciality Conference: Disinfecting
Wastewater for Discharge and Reuse. WEF.
Portland, Oregon.

Metcalf & Eddy, Inc. 1991. Wastewater
Engineering:  Treatment,   Disposal,   and
Reuse. 3ded. The McGraw-Hill Companies.
New York, New York.

Task  Force  on Wastewater Disinfection.
1986. Wastewater Disinfection. Manual of
Practice  No.  FD-10.  Water  Pollution
Control Federation. Alexandria, Virginia.
10.    U.S.  Environmental  Protection  Agency
       (EPA).  1986a. Design Manual: Municipal
       Wastewater Disinfection. EPA Office of
       Research  and  Development. Cincinnati,
       Ohio. EPA/625/1-86/021.

11.    U.S.  EPA.   1986b.  Disinfection  with
       Ultraviolet Light—Design, Construct, and
       Operate for Success. Cincinnati, Ohio.

12.    U.S. EPA. 1988. Ultra Violet Disinfection:
       Special Evaluation Project. EPA Region 5.
       Chicago, Illinois.


Brown and Caldwell
Raymond Matasci
P.O. Box 8045
Walnut Creek,  CA 94596

Roy F. Weston Inc.
Peter J. Lau
1515 Market Street, Suite 1515
Philadelphia, PA 19102-1956

Salcor Engineering
Dr. James E. Cruver
P.O. Box 1090
Fallbrook, CA 92088-1090

Tacoma-Pierce County, WA
Steve Marek
Water Resources Section
3629 South D.  Street
Tacoma, WA 98408-6897

Trojan Technologies, Inc.
David Tomowich
3020 Gore Road
London, Ontario N5 V 4T7

The mention of trade names or commercial products
does not constitute endorsement or recommendation
for use by  the U.S.  Environmental Protection

The technical content of this fact sheet was provided by the National Small Flows
Clearinghouse and is gratefully acknowledged.
                                                        For more information contact:

                                                        Municipal Technology Branch
                                                        U.S. EPA
                                                        Mail Code 4204
                                                        401 M St., S.W.
                                                        Washington, D.C., 20460
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                                                         MUNICIPAL TECHNOLOGY BRANCH