Disinfection of drinking water is one of the major public health advances
in the twentieth century. In the early 1900s, typhoid and cholera
were common throughout American cities; disinfection was a large factor
in reducing these epidemics. Chlorine was the preferred disinfectant
back then and continues to be the most widely used substance for water
disinfection in the United States
Public health officials overwhelmingly agree: In a 1992 survey of
public health officials, 92% agreed that chlorine in drinking water
is safe. Safe drinking water has played a key role in: a 50% plus
increase in life expectancy, from about 45 years in the early 1900s
to about 76 years at present; a dramatic decline in infant mortality
rates and the virtual elimination of cholera, typhoid fever, dysentery
and gastroenteritis, as well as many other waterborne diseases which
once killed tens of thousands of Americans.
Disinfectants such as chlorine can react with naturally occurring
materials, such as plant and other organic matter, in the water to
form unintended by-products that may pose health risks. With more
and more chlorine added to drinking water supplies to meet increased
disinfection requirements, the risk of these unintended by-products
reaching unhealthful levels has increased. The challenge for water
suppliers is to meet new disinfection requirements, while minimizing
the levels of disinfection by-products (DBPs).
At the PWD, careful consideration has been given to this matter. In
1994, the PWD switched to an alternative disinfectant, ozone, to meet
the increased disinfection requirements necessary to protect our customers
from more resistant, microbial contaminants such as Giardia and Cryptosporidium.
An additional benefit of the conversion to ozone was a reduction in
the levels of chlorine needed to disinfect the water and the potential
for DBPs. One such group of DBPs, trihalomethane or THM as it is commonly
known, can be harmful if levels continually exceed 80 ppb in the drinking
water. As seen in the chart, THM levels have always been well below
levels that may prompt concern, and were basically eliminated after
the switch to ozone.