Page 93 - Water and wastewater engineering

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than 1 NTU in more than 5 percent of the samples taken each month. The 1 NTU limit may be
increased by the state up to 5 NTU if it determines that there is no significant interference with
disinfection. Other filtration technologies may be used if they meet the turbidity requirements set
for slow sand filtration, provided they achieve the disinfection requirements and are approved by
the state.
Turbidity measurements must be performed on representative samples of the system’s fil-
tered water every four hours or by continuous monitoring. For any system using slow sand filtra-
tion or a filtration treatment other than conventional treatment, direct filtration, or diatomaceous
earth filtration, the state may reduce the monitoring requirements to once per day.

Disinfection Requirements. Filtered water supplies must achieve the same disinfection as

required for unfiltered systems (that is, 99.9 or 99.99% removal, also known as 3-log and 4-log
removal or inactivation , for Giardia lamblia and viruses) through a combination of filtration and
application of a disinfectant.
Giardia and viruses are both fairly well inactivated by chlorine. Thus, with proper physical
treatment and chlorination, both can be controlled. Cryptosporidium, however, is resistant to
chlorination. Depending on the source water concentration, EPA establishes levels of treatment
that include physical barriers and disinfection techniques. Ozonation and disinfection with ultra-
violet light are effective in destroying Cryptosporidium.

Total Coliform. On June 19, 1989, the EPA promulgated the revised National Primary Drink-
ing Water Regulations for total coliforms, including fecal coliforms and E. coli. These regula-
tions apply to all public water systems.
The regulations establish a maximum contaminant level (MCL) for coliforms based on the
presence or absence of coliforms. Larger systems that are required to collect at least 40 samples
per month cannot obtain coliform-positive results in more than 5 percent of the samples col-
lected each month to stay in compliance with the MCL. Smaller systems that collect fewer
than 40 samples per month cannot have coliform-positive results in more than one sample per
month.
The EPA will accept any one of the five analytical methods noted below for the determina-
tion of total coliforms:

Multiple-tube fermentation technique (MTF)
Membrane filter technique (MF)
Minimal media ONPO-MUG test (colilert system) (MMO-MUG)
Presence-absence coliform test (P-A)

Colisure technique
Regardless of the method used, the standard sample volume required for total coliform test-
ing is 100 mL.
A public water system must report a violation of the total coliform regulations to the state
no later than the end of the next business day. In addition to this, the system must make public
notification according to the general public notification requirements of the Safe Drinking Water
Act, but with special wording prescribed by the total coliform regulations.