Chlorination: The Love/Hate Relationship
If left untreated, drinking water will cause waterborne diseases
such as typhoid, cholera and dysentery. History books are full of stories of
diseases being transmitted through drinking water.
However, it was not until 1854 that London physician John
Snow provided proof that public water supplies could spread diseases among
humans. Snow traced a cholera epidemic in London to a public well being
contaminated with human wastes from a broken sewer connected to the home of
someone stricken with the disease.
By the late 1800s, public health officials knew something
had to be done to protect drinking water supplies. While filtration was first used
in the United States around 1890, it was its combination with chlorine that
provided a practical, inexpensive way to control bacteria in water.
A one-time chlorination of a contaminated well was a success
in the 1850s, but it wasn't until the early 1900s that chlorination was applied
on a plant basis in the United States. George A. Johnson of Hering and Fuller,
a New York manufacturer of water treatment equipment, designed a full-scale
chlorine installation for the Bubbly Creek Filter Plant in Chicago in 1908.
This plant served the Chicago Stockyards. Johnson used chlorine through
chloride of lime, dramatically reducing the bacterial content of the water
In 1909, Jersey City, N.J., established the first facilities
for chlorinating an urban water supply. Curiously, this chlorination plant was
a successful attempt by a private water company to avoid a large expense. It
was cheaper to chlorinate the water than to build sand filters or prevent
contamination of the city's water source by sewage.
In 1913, Charles Wallace invented the chlorinator. It
provided the first practical and effective means for the controlled feeding of
chlorine gas. By 1914 most of the water supplied to U.S. cities was being
Disinfection with chlorine is very popular in water and
wastewater treatment (approximately 75 percent of municipal systems in the U.S.
use chlorine) because of its low cost, ability to form a residual and its
effectiveness at small doses.
Chlorine is a strong oxidizing agent, causing it to have a
tendency to withdraw electrons from other atoms and molecules. This allows it
to bond with and destroy the outer surfaces of bacteria and viruses.
Chlorine can be liquefied under pressure at room
temperature, making it easy to store and transport. Chlorine also is highly
soluble in water, making it easy to add to water supplies in controlled
amounts. Chlorine gas reacts rapidly with water to form hypochlorous acid and
hydrogen and chloride ions. In turn, hypochlorous acid reacts instantaneously
and reversibly with water to form hypochlorite and hydrogen ions (Equation 1).
Chlorination is not limited to drinking water supplies.
Industries use chlorine to prevent the fouling of cooling water. Food
processing plants use chlorinated water to preserve the freshness of foods by
killing bacteria that cause spoilage. Wastewater plants also use chlorine
before they release it into rivers or other bodies of water.
In recent years, there have been concerns about chlorine.
Although chlorine disinfects drinking water, it also reacts with traces of
other material or particles (e.g., organic matter such as decaying trees and
leaves as well as urban farm run-off) in the water and forms trace amounts of
substances known as disinfection byproducts (DBPs). The most common of these
are known as trihalomethanes (THMs). THMs (like chloroform) have been linked to
increasing cancer risks and birth defects. The U.S. Environmental Protection
Agency (EPA) has classified THMs as probable or possible carcinogens. Applying
drinking water treatment methods such as coagulation/flocculation and
sedimentation has reduced some of these risks.
In 1979, the EPA adopted a THM regulation limiting the
allowable ingestion level of this carcinogenic disinfection byproduct in
drinking water. The maximum contaminant level set for total THMs in drinking
water is 0.10 mg/L. In the 1990s, the Disinfection Byproducts Rule lowered this
level to 0.08 mg/L. Most municipal drinking water supplies maintain chlorine
levels such that the concentrations of chloroform in the systems range from
0.02 to 0.05 mg/L. However, THMs vary with seasons and water quality.
Since 1984, American drinking water utilities have spent
more than $23 million researching the production of DBPs, the risks posed by
them and the methods to treat them. In addition, $150 million has been spent by
the 300 largest drinking water utilities to conduct the information gathering
necessary for the Information Collection Rule (ICR). The ICR is the largest
study pertaining to the occurrence of DBPs and associated treatment practices.
Alternatives to the use of chlorine (e.g., chloramine,
chlorine dioxide, ozone and ultraviolet [UV] irradiation) have received
attention since concerns over the DBPs have emerged. Although these other
processes provide efficient disinfection capabilities, each has its own
disadvantages. Ozone and UV light do not provide residual disinfection or
lasting protection. Also, while all disinfection alternatives do not
necessarily produce THMs, they do produce some type of byproducts.
Since the attacks of September 11, there also have been
security concerns regarding chlorine. Vulnerability Assessments required by the
Bioterrorism Act are pointing out the risks of the use, storage and handling of
various chemicals. Because of these assessments, the Fairfax County Water
Authority as well as the District of Columbia Water and Sewer Authority have
switched from chlorine to sodium hypochlorite.
Chlorination has improved public health greatly by
eliminating or reducing the incidence of waterborne diseases. However, some
organisms that cause disease are resistant to chlorine treatment. For example,
chlorine disinfection is not effective for controlling protozoa such as
Cryptosporidium and Giardia. Other treatments and processes are used to remove
or inactivate these types of organisms. Ozone is the most commonly used
disinfectant in Europe. However, this process does produce ozonation
Since chlorination has been used for almost 100 years to
disinfect water supplies, many of the DBPs from chlorination have been
identified and researched. Much less is known about the kinds of DBPs produced
from alternative methods.
Many utilities now are using multi-barrier approaches to
disinfection. These methods are reducing DBPs while allowing utilities to
continue using disinfectants like chlorine.