The U.S. Environmental Protection Agency’s (EPA) Water Infrastructure Resiliency and Finance Center, in collaboration with the ...
A disinfection choice for public and private water systems.
Chlorination and Its Alternatives
Purification of drinking water containing microbiological
contamination requires some form of disinfection treatment to kill or render
microbiological organisms harmless.
Of the available disinfection treatment methods for private
water systems, chlorination in the most commonly used.
Private Drinking Water
Nationwide, there are approximately 170,000 public water
systems (PWS) protected by focused teamwork at the federal, state and local
levels. A PWS is defined as a system for the provision to the public of water
for human consumption through pipes or other constructed conveyances, if such a
system has at least 15 service connections or regularly serves at least 25
individuals for at least 60 days a year. Under The Safe Drinking Water Act, a
clear partnership is established between the EPA and the states to protect
drinking water supplies and sources. Currently in the United States, 283.7
million people obtain their water supply from public water systems while 42
million have private drinking water supplies.
Typically, private drinking water supplies are drawn from
the ground water through wells, but some households also use water from streams
or cisterns. Private systems, while not federally regulated, are vulnerable to
biological contamination from sewage, improper well construction and
poor-quality water sources. The EPA encourages households to take special
precautions to ensure the protection and maintenance of their private drinking
Precautionary Measures and Testing
In households, water used for drinking and cooking should be
free of microorganisms that cause illnesses such as gastroenteritis, typhoid
fever, cholera and dysentery. To detect contamination problems early, the EPA
recommends that private water supplies be tested annually for nitrate and
coliform bacteria. Homeowners should test for other potential contaminants such
as radon and pesticides if a problem is suspected. When repairs or improvements
are made to a private well, testing also is recommended.
The biological quality of drinking water is determined by
tests for coliform group bacteria. The presence of coliform typically indicates
disease-causing microorganism contamination. The standard for coliform bacteria
in drinking water is less than 1
coliform colony per 100 milliliters of sample (<1:100 ml).
If a contaminant is detected, test results from laboratories
include the concentration of the contaminant and an indication if the
concentration exceeds a drinking water quality standard. The initial presence
of coliform requires retesting the water supply. The retested sample should be
analyzed for fecal coliform organisms. If positive, prompt action is required.
Conditions Causing Contamination
Coliform bacteria may contaminate well water from the
• Loose or worn seal on the drilled and driven well.
too short or inadequately sealed casings in the drilled and driven well.
or loose-fitting cover on a dug well.
wall lining or cracked concrete apron of a dug well.
to well structure of submerged pump.
of the well from hurricanes, floods, heavy rainfall or other natural disasters.
and repair of a septic tank system.
drilled into fractured rock formations.
located in areas where ground waters are subject to continuous contamination
from outside sources.
The addition of chlorine to a water supply readily combines
with chemicals dissolved in water, microorganisms, plant material, odors and
colors. Chlorine that is “used up” by these components comprises
the chlorine demand of a treatment system. Sufficient amounts of chlorine must
be added to a water supply to meet the chlorine demand and provide residual
disinfection. Free or residual chlorine is the amount of the disinfectant that does not
combine with components in the water, and the breakpoint is the point in which
free chlorine is available for continuous disinfection.
Prior to installing a permanent means of chlorine
disinfection onto a well, a homeowner must be sure that contamination
originates from the ground water itself and is not a temporary condition. When
the “one time only” addition of chlorine to a water
supply—known as shock chlorination—does not eliminate
bacteriological problems, a permanent means of continuous chlorination is
required (Table 1).
Permanent and continuous chlorination of a private water
supply can be done through a chlorine pump, solid feed unit, aspirator or
suction device. Unlike public water systems that utilize chlorine in the
gaseous form, a private water system uses liquid chlorine or dry chlorine
depending on the chosen method of chlorination.
In 1979, the EPA adopted a trihalomethane (THM) regulation
limiting the allowable level of this carcinogenic disinfection byproduct in
drinking water. The maximum contaminant level for total THMs in drinking water
is 0.10mg/L. THMs are chemicals that are formed, primarily in surface water,
when naturally occurring organic materials combine with free chlorine. Since
groundwater rarely has high levels of organic materials such as humic and
fulvic acids, chlorinated private wells contain much lower levels of THMs.
Despite the popularity of chlorination, the treatment method
has limitations when attempting to disinfect private wells that are heavily
contaminated and possess protozoan parasites such as Cryptosporidium parvum and
Giardia lamblia. Ultraviolet (UV) disinfection and reverse osmosis (RO)
filtration both have proved effective at inactivating specific protozoan. Both
methodologies purify water without the addition of harsh chemicals or the need to handle hazardous materials.
UV disinfection is the process where microorganisms are
exposed to UV light at a specified intensity for a specific period of time.
This process renders the microorganism to be considered
“microbiologically dead.” UV light penetrates the cell wall of the
microorganism affecting the DNA by fusing the Thyamine bond within the DNA
strand, which prevents the DNA strand from replicating during the reproduction
process. This fusing of the Thyamine bond is known as forming a dimerase of the
Thyamine bond. If the microorganism is unable to reproduce/replicate then it is
considered to be “microbiologically dead.” While providing a 99.99
percent inactivation of bacterium and viruses, UV will have no effect on water
RO filtration uses a semipermeable membrane that enables the water being purified to pass through while
contaminants remain behind. Traditionally, osmosis refers to the attempt to
reach equilibrium by dissimilar liquid systems trying to reach the same
concentration of materials on both sides of a semipermeable membrane. Reversing
the osmotic process is accomplished by applying pressure to stop the natural
osmosis process, creating RO. RO removes virtually all organic compounds and 90
to 99 percent of all ions from the processed water. In addition, RO can reject
99.9 percent of viruses, bacteria and pyrogens.
Alternative methods of treatment for private water supplies
such as UV and RO do not provide a residual effect like chlorination.
Without a residual, the regrowth of contaminants further down in the
distribution system becomes possible.
Chlorination generally is an inexpensive treatment method
and proven to be effective against a broad spectrum of pathogens. Although it
has shown itself to be effective
against waterborne bacteria and viruses, it provides only some degree of protection against protozoan
agents. Nevertheless, a private water supply should utilize a treatment system
that kills or neutralizes all pathogens in the water through an automatic, simply maintained and safe
process. Chlorination remains the most popular choice of treatment for private
water supplies by homeowners.