Civil Engineering Reference
In-Depth Information
Measurements of heterotrophic bacteria can be substituted for measurements of chlo-
rine residual. A minimum of four-log removal / inactivation of virus and three-log
removal / inactivation of
Giardia
is required.
Disinfectants and Disinfection By-products
Table 1-8 summarizes the requirements for MCLs, health effects, and monitoring
requirements for disinfectants and disinfection by-products. For the first time, limits
have been placed on the maximum residual disinfectant concentrations that can be
present in the distribution system (MRDLs in Table 1-8). Chapter 3 provides further
discussion of DBP formation. Total THMs include chloroform, bromoform, bromo-
dichloromethane, and dibromochloromethane. HAA5 include chloroacetic acid, di-
chloracetic acid, trichloracetic acid, bromoacetic acid, and dibromacetic acid.
Special Requirements
With the Stage 1 D / DBP Rule, a treatment technique is
established for the removal of total organic carbon (TOC). By reducing the level of
TOC, the formation of the disinfection by-products will also be reduced. The treatment
technique is enhanced coagulation or softening and applies to all public water systems
using conventional filtration regardless of size. The treatment technique can be met in
one of two ways. The first way is meeting the removal percentage required (shown in
Table 1-9) of the influent TOC based on the raw water TOC and alkalinity. If a system
cannot meet the percentage removal requirements of the first step, then the other way
is to apply to the primacy agency to determine the point of diminishing returns
(PODR). PODR is the point on the TOC versus coagulant dose plot where the slope
changes from greater to less than 0.3 mg / L of TOC removal for 10 mg / L coagulant
dose. At this point of change, the alternative removal percentage for TOC can be set.
Exceptions
There are no exceptions to the rule, but there are alternative compliance
criteria. If the alternative compliance criteria are met, the system is deemed to have
met the TOC removal requirements. These criteria are:
The system's source water TOC is less than 2.0 mg / L.
•
The system's treated water TOC is less than 2.0 mg / L.
•
The system's source water TOC is less than 4.0 mg / L, the source water alkalinity
is greater than 60 mg / L (as CaCO
3
), and the system's DBP levels for TTHM and
HAA5 are less than 40
g / L and 30
g / L, respectively.
•
The system is using only chlorine as its disinfectant and the DBP levels for TTHM
and HAA5 are less than 40
g / L and 30
g / L, respectively.
•
The system's source water specific ultraviolet absorbance (SUVA) prior to any
treatment is less than 2.0 L / mg-m.
•
The system's treated SUVA is less than 2.0 L / mg-m.
•
Secondary Standards
Table 1-10 summarizes the current national secondary drinking water contaminant
standards.
