2/24/2009 US EPA

BASIC INFORMATION ABOUT DRINKING WATER DISINFECTION

4) What disinfectants are available for drinking water?

Most water utilities use chlorine as a primary disinfectant because of its
effectiveness in killing potentially harmful organisms.2

•	Chlorine is effective in killing bacteria, viruses, and other potentially harmful
organisms in water.

•	One disadvantage of chlorine is it can react with natural organic matter3
present in water to form potentially harmful disinfection byproducts.

•	Water utilities sometimes use chlorine several times during treatment
because the initial dose loses its effectiveness over time.

Monochloramine is commonly used as a secondary disinfectant to protect the
water as it travels from the treatment plant to consumers.

•	Monochloramine is effective in killing bacteria, viruses, and other potentially
harmful organisms but takes much longer to act than chlorine.

•	One disadvantage of monochloramine is it can react with natural organic
matter present in water to form potentially harmful disinfection byproducts.

•	Monochloramine is more chemically stable than chlorine, which makes it
longer lasting and an effective secondary disinfectant.

Water utilities may use ozone, UV light, or chlorine dioxide as primary
disinfectants in the treatment plant.

•	Ozone, UV light, and chlorine dioxide are effective in killing bacteria, viruses,
and other potentially harmful organisms in water at the treatment plant.

•	One disadvantage of ozone, UV light, and chlorine dioxide is they do not
provide protection as water travels through pipes.

•	Either chlorine or monochloramine should still be used in addition to any
primary treatment process to protect the quality of treated water as it travels
from the treatment plant to the customer.

Additional Supporting Information:

1.	See question 3 for a discussion of primary and secondary disinfectants. See questions 5
and 6 for a specific discussion of chlorine and monochloramine as a primary and secondary
disinfectant.

2.	Potentially harmful organisms include disease-causing bacteria, viruses, and protozoa.
Chlorination and chloramination are not effective at inactivating Cryptosporidium. EPA
requires that utilities that use surface water test and treat for Cryptosporidium where
necessary.

3.	Natural Organic Matter. Complex organic compounds that are formed from decomposing
plant, animal and microbial material in soil and water. They can react with disinfectants to
form disinfection by products. Total organic carbon (TOC) is often measured as an indicator
of natural organic matter.

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