Page 257 -

OXIDATION AND DISINFECTION 10.3

fect acceptance of water by the public, and for this reason, compliance with the limits es-
tablished by USEPA is strongly recommended. In addition, some states have enacted more
stringent regulations that require compliance with some of the maximum contaminant lev-
els (MCLs) in Secondary Standards.
The contaminants most commonly treated for removal by oxidation under the Sec-
ondary Standards are taste and odor, iron, and manganese. These oxidation processes are
discussed in greater detail later in this chapter.

The Surface Water Treatment Rule
The Surface Water Treatment Rule (SWTR) was promulgated by USEPA on June 29,
1989. This regulation applies to every public water system in the United States that uses
surface water as a source. It also applies to groundwater systems that the state determines
might become contaminated by surface water; these systems are labeled groundwater un-
der the direct influence of surface water, or GWUL
The purpose of the regulation is to protect the public from waterborne diseases that
are most commonly transmitted by contamination of surface water. Because it is difficult
to monitor for particular pathogens, such as Giardia lamblia and viruses, the SWTR em-
phasizes treatment techniques as the condition for compliance instead of having MCLs
for pathogens. Because of the wide variety of water qualities, local conditions, and meth-
ods of treatment, the rule does not prescribe a particular method of treatment but instead
offers several alternatives. Any of these methods may be used by a water system to meet
the overall goal, which is removal or inactivation of essentially all disease-causing
organisms.
To ensure that water quality goals are met, the SWTR contains many operation and
monitoring requirements. Studies indicate that viruses and the encysted protozoans (Giar-
dia and Cryptosporidium, which is addressed in the Enhanced Surface Water Treatment
Rule) are among the most resistant waterborne pathogens; therefore, water systems that
attain adequate removal or inactivation of these organisms will, to the best of current
knowledge, provide adequate protection from other waterborne disease organisms.

Disinfectant Contact Requirements. Most water systems using a surface water source
must use sedimentation and filtration to ensure adequate removal of pathogens. Under the
SWTR requirements, only water systems with extremely low-turbidity source water may
be allowed to operate without filtration, and then it is under very stringent operating and
monitoring conditions. All surface water and GWUI systems, whether they provide fil-
tration or not, must practice disinfection under highly specific conditions. Disinfection re-
quirements must be met prior to the water reaching the first customer.
The effectiveness of a chemical disinfectant in killing or inactivating pathogens de-
pends on
• The type of disinfectant used
• The disinfectant residual concentration (denoted by C)
• The time the water is in contact with the disinfectant (denoted by T)
• Water temperature
• The pH of the water, which has an effect on inactivation if chlorine is used

The residual concentration C of a disinfectant in milligrams per liter (mg/L), multi-
plied by the contact time T in minutes, is called the CT value. The CT values required by
the SWTR to guarantee the necessary reduction in pathogens by various disinfectants may
be obtained from tables in publications referenced at the end of this chapter. Each water