Oct 24, 2002

Ultraviolet Disinfection of Drinking Water: A Growing Trend

Microbe Inactivation

The use of ultraviolet (UV) light for the treatment of
drinking water is becoming more acceptable by both the public and regulatory
agencies as an alternative disinfectant. Water suppliers that are developing
new water treatment facilities or modifying existing ones now commonly
investigate this technology to determine its applicability to their treatment

What is UV and How Does it Work?

UV light lies within the region of the electromagnetic
spectrum between visible light and x-rays (between 100 to 400 nanometers [nm]).
This region is further subdivided into four sub-regions; vacuum UV
(100–200 nm), UVC (200–280 nm), UVB (280–315 nm), and UVA
(315–400). Its disinfection effectiveness comes primarily from the UVC
and UVB portions of the light.

Whereas chemical disinfectants destroy or damage a
microbe's cellular structure, UV light inactivates microbes by damaging
their DNA, thereby preventing the microbe's ability to replicate (or
infect the host).

The dosage for a chemical disinfectant typically is
expressed in milligrams per liter (mg/L). However, a UV dose is expressed as
energy per area. It is calculated by multiplying irradiance by time (i.e., Dose
= I ¥ T). Irradiance is defined as the total radiant power incident from
all upward directions on a surface area divided by that area. The units for
irradiance are milliwatts per square centimeter (mW/cm2) and the units for time
are seconds. Therefore, the units for dosage are milliwatt-seconds per square
centimeter (mW-s/cm2) or millijoules per square centimeter (mJ/cm2).

What are the Pros and Cons of UV?

UV light disinfection offers many advantages.

-                It
does not impart any tastes or odors to the water as chlorine does.

-                It
does not form harmful disinfection byproducts that can result with chlorine and
other chemical disinfectants.

-                It
does not increase bacterial regrowth potential in distribution systems.

-                It
can effectively inactivate biological pathogens such as Giardia and

Some drawbacks also should be noted. For example, UV light
does not leave a disinfectant residual in the disinfected water as a
chlorinated disinfectant would, should one be desired or required.
Additionally, recent research suggests that UV light may not effectively
inactivate the Adenovirus at the same doses that are effective for other
microorganisms. Should either of these items be of concern, additional
disinfection with chlorine could be practiced to create a multiple disinfectant
barrier approach.

Types of UV Reactors

Commonly available UV reactors use low pressure (LP), low
pressure high-output (LPHO) and medium pressure (MP) UV lamps. The pressure
reference relates to the mercury vapor pressure within the lamp. The germicidal
UV light produced by the LP and LPHO lamps is monochromatic at approximately
254 nm and is polychromatic from approximately 200 to 320 nm for the MP lamp
types. In general, as you move from the LP to the MP lamp systems, fewer lamps
are needed to disinfect greater flows of water at the same dose.
Correspondingly, a higher power input also is required.

A basic rule of thumb for lamp type selection is that LP and
LPHO lamp systems would be suitable for flows up to 2 to 3 million gallons per
day (mgd) and that MP lamp systems would be suitable for flows above that
range. However, a detailed life cycle cost analysis should be performed for
each system to select the most economical lamp type.

Current UV Projects

Dufresne-Henry has been implementing UV technology in
drinking water supplies in the Northeast for several years. Among the states
that are demonstrating a strong interest in UV technology is the Commonwealth
of Massachusetts. Some of the most recent projects in Massachusetts to use UV
are listed in Table 1.

Of the five locations cited, a common reason for selecting
UV for disinfection has been its ability to not impart any tastes or odors to
the disinfected water. For these locations, the consumers and/or water
suppliers specifically asked that chlorine not be used. At the outset of the
project in Norfolk, Water Superintendent James Martin asked that an innovative
technology be selected to provide disinfection to an aerated water so that
chlorine would not have to be used. Similarly, West Bridgewater Superintendent
Rick Krugger specifically requested that UV be used so that his consumers would
not have to experience the tastes and odors typically imparted by chlorine. In
both of these communities, the municipal wells typically are not chlorinated.

A similarity among the water suppliers who have been
classified as GWUDI is the use of UV disinfection in a "multiple
barrier" approach for providing safe drinking water. Both the Plainville
and Westford projects are pressurized greensand filtration plants that will use
UV disinfection as a third barrier in their treatment processes.

Following are some important questions to ask in designing a
UV system for your treatment facility.

-                Is
pilot testing required?

-                What
is the maximum flow rate (present or future) to be treated?

-                What
dosage is needed?

-                What
type of lamp system will be most cost effective?

-                Will
a wiper system (to remove precipitate from lamp sleeves) be required?

-                How
will the UV reactor be validated to confirm it is applying the required dose?

-                What
type of monitoring will be required by the regulatory agency?

Looking Ahead

Although not yet accepted by the United States Environmental
Protection Agency as an approved technology for drinking water disinfection
under the Surface Water Treatment Rule (SWTR), studies conclude that UV light
can be an effective disinfectant for Giardia, Cryptosporidium and other
microorganisms at varying doses. This does not preclude water systems from
using UV; it just does not allow the granting of removal/inactivation credits
for microbial contaminants by the regulatory agencies. However, state
regulatory agencies have approved UV instead of other forms of chemical
disinfection on a case-by-case basis.

At this time, observers within the water treatment industry
anticipate that regulations pertaining to the use of UV disinfection for
compliance with treatment requirements will be contained in the Long Term 2
Enhanced Surface Water Treatment Rule (LT2ESWTR). This rule is expected to be
proposed in late 2002 or early 2003 and likely will include stipulations on UV
doses to achieve different levels of inactivation credits for various microbial
contaminants. Additionally, the EPA will publish a UV Guidance Manual
concurrently with the proposed LT2ESWTR.

About the author

Richard Protasowicki is a project engineer for Dufresne-Henry in the firm's Westford, Mass., office.