United States

Environmental Protection

Agency

Office of Water

Washington, D.C.

EPA 832-F-99-064

September 1999

Wastewater

Technology Fact Sheet

Ultraviolet Disinfection

DESCRIPTION

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

intensities.

TABLE 1 INFECTIOUS AGENTS

POTENTIALLY PRESENT IN UNTREATED

DOMESTIC WASTEWATER

Organism Disease Caused

Bacteria

Escherichia coli (enterotoxigenic) Gastroenteritis

Leptospira (spp.) Leptospirosis

Salmonella typhi Typhoid fever

Salmonella (=2,100 serotypes) Salmonellosis

Shigella (4 spp.) Shigellosis (bacillary

dysentery)

Vibrio cholerae Cholera

Protozoa

Balantidium coli Balantidiasis

Cryptosporidium parvum Cryptosporidiosis

Entamoeba histolytica Amebiasis (amoebic

dysentery)

Giardia lamblia Giardiasis

Helminths

Ascaris lumbricoides Ascariasis

T. solium Taeniasis

Trichuris trichiura Trichuriasis

Viruses

Enteroviruses (72 types, e.g.,

polio, echo, and coxsackie

viruses)

Gastroenteritis,

heart anomalies,

meningitis

Hepatitis A virus Infectious hepatitis

Norwalk agent Gastroenteritis

Rotavirus Gastroenteritis

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 122EF.

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

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.

ADVANTAGES AND DISADVANTAGES

Advantages

• 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

operators.

• 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.

Disadvantages

• 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." 

Source: Crites and Tchobanoglous, 1998.

(a) adapted from Trojan Technologies, Inc.

(b) adapted from Infilco Degremont, Inc.

FIGURE 1 ISOMETRIC CUT-AWAY VIEWS

OF TYPICAL UV DISINFECTION SYSTEMS

Flow

UV horizontal lamp

module with

supoort racks

Automatic

level control UV bank 1

(a)

UV bank 2

Note: A UV bank is

composed of a

number of UV

modules

Flow

Flap gate

level control

(b)

UV vertical lamp

module with

support rack
• 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.

APPLICABILITY

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

2.

UV disinfection can be used in plants of various

sizes that provide secondary or advanced levels of

treatment.

PERFORMANCE

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

TABLE 2 WASTEWATER

CHARACTERISTICS AFFECTING UV

DISINFECTION PERFORMANCE

Wastewater

Characteristic

Effects on UV

Disinfection

Ammonia Minor effect, if any

Nitrite Minor effect, if any

Nitrate Minor effect, if any

Biochemical oxygen

demand (BOD)

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

diminished.

Hardness Affects solubility of metals

that can absorb UV light. 

Can lead to the

precipitation of

carbonates on quartz

tubes.

Humic materials, Iron High absorbency of UV

radiation.

pH Affects solubility of metals

and carbonates.

TSS Absorbs UV radiation and

shields embedded

bacteria.
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.

OPERATION AND MAINTENANCE

The proper O&M of a UV disinfection system

ensures that sufficient UV 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

site.

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

cycles.

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.

COSTS

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'

reliability.

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 3A 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.

REFERENCES

1. Crites, R. and G. Tchobanoglous. 1998.

Small and Decentralized Wastewater

TABLE 3A LAMP COSTS FOR UV

DISINFECTION SYSTEMS

Item Range* Typical*

UV lamps ($/lamp) ($/lamp)

1-5 mgd 397-1,365 575

5-10 mgd 343-594 475

19-100 mgd 274-588 400

Construction cost

for physical

facilities

(% of UV

lamp cost)

75-200

(% of UV lamp

cost) 150

* Costs are based on a 1993 Engineering News Record

Construction Cost Index of 5,210.

Source: Adapted from: Darby et al. (1995) with permission

from the Water Environment Research Foundation.

TABLE 3B CAPITAL AND O&M COSTS

FOR UV DISINFECTION SYSTEMS

Cost Item UV System Cost ($)

Capital Costs

Equipment 120,000

Structural modifications 64,000

Electrical 20,000

Miscellaneous 40,000

Total: 244,000

Annual operating and maintenance costs

Energy 3300

Lamps and chemicals 2840

Cleaning 1180

Maintenance 1440

Process control 6240

Testing 4160

Total 19,190

Source: Hanzon and Vigilia, 1999.
Management Systems. The McGraw-Hill

Companies. New York, New York.

2. 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.

3. Eddington, G. June 1993. Plant Meets

Stringent Residual Chlorine Limit. Water

Environment & Technology. P. 11-12.

4. Fahey, R. J. Dec. 1990. The UV Effect on

Wastewater. Water Engineering &

Management. vol. 137. no. 12. pp. 15–18.

5. Hanzon, B.D. and Vigilia, R. 1999. UV

Disinfection. Wastewater Technology

Showcase. vol. 2. no. 3. pp. 24-28.

6. 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,

California.

7. 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.

8. Metcalf & Eddy, Inc. 1991. Wastewater

Engineering: Treatment, Disposal, and

Reuse. 3d ed. The McGraw-Hill Companies.

New York, New York.

9. 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.

ADDITIONAL INFORMATION

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 N5V 4T7

The mention of trade names or commercial products

does not constitute endorsement or recommendation

for use by the U.S. Environmental Protection

Agency.
For more information contact:

Municipal Technology Branch

U.S. EPA

Mail Code 4204

401 M St., S.W.

Washington, D.C., 20460

The technical content of this fact sheet was provided by the National Small Flows 

Clearinghouse and is gratefully acknowledged.