Page 30 -

MASTER PLANNING AND TREATMENT PROCESS SELECTION 2.11

TABLE 2.4 Most Common Drinking Water Treatment Processes (Continued)

Water quality parameter Process components
By-product removal
* GAC adsorption
o Air stripping (partial)
Iron, manganese reduction/sequestering Filtration of precipitators formed by preoxidation
• Sand and/or anthracite media
• Green sand media
• Proprietary media
Polyphosphate sequestering agent
Hardness reduction Lime softening
• Ion exchange
• Nanofiltration
Inorganic, organic chemical reduction Ion exchange
Biologically activated carbon media
Adsorption
Reverse osmosis
Corrosion control Posttreatment
• pH adjustment
• Inhibitors

disinfection by-products. New D/DBP Rules require defined reduction in water TOC con-
tent which may require enhanced or increased coagulation. It is possible that the increased
solids generated could overload certain filters or filter types. While perfectly capable of
producing low-turbidity water in conformance with the microbiological rules, solids over-
loading may cause turbidity upset, especially in single-pass direct filtration, slow sand and
diatomaceous earth (DE) filtration, and ultrafiltration. Specific situations will vary. The
D/DBP Rule does permit alternative options to TOC reduction (Table 2.1). One option is
to maintain system running TTHM and HAA5 averages at half the Stage 1 D/DBPR re-
quirements. In addition to TOC reduction, another feature that will influence the magni-
tude of TTHM and HAA5 formation is the time of travel or the residence time in the dis-
tribution system. There are many systems with low retention where the TTHM and HAA5
levels tend to be low, but where TOC levels require removal. This would be especially
so in small to moderate-sized systems or in systems with an effective transmission net-
work feeding the smaller distribution mains. In these situations, where no reduction (or a
small reduction) in the TTHM and HAA5 levels may be required to meet the alternative
standard, it might be more advantageous to concentrate treatment modifications on TTHM
and HAA5 reduction than on TOC removal. This approach might result in reduced re-
sidual solids generation. In direct filtration and slow sand filtration plants, adding granu-
lated activated carbon (GAC) media and/or adding oxidants to increase TOC removals in
biologically activated media may be enough to meet the D/DBPR alternative to TOC
removal.
If simple process supplementation does not achieve the new rule requirements, treat-
ment addition should be considered. A possible addition to existing filtration processes to
improve effluent quality would be nanofiltration. First stage filtration would improve the
efficiency of nanofiltration, and the residual stream could be routed back to the initial fil-
ter (if appropriate) for recovery.
The purpose of the above discussion is simply to emphasize the need for the engineer
to evaluate the effect of each treatment change or addition in process. It is also essential