levels. Carbon adsorption is considered effective in removing high levels of
The air-water ratio required for aeration to effectively
remove volatile organic precursors is higher than the air-water ratios needed for taste
and odor control or iron and manganese removal. The high air-to-water ratio promotes
the growth of aerobic organisms (such as algae) and can be a significant problem.
Aeration is discussed in par. 5.4.12.
Some organic precursors are removed during coagulation.
These organics often adhere to the particulate matter that settles. Coagulation is
discussed in par. 5.4.2.
High doses of PAC removes only a portion of the
precursors. High costs and sludge problems limit the use of PAC for precursor control.
GAC can adsorb a wide spectrum of organics. Frequently,
GAC adsorbs enough precursor material so that chlorine disinfection can be practiced
following GAC treatment without forming excessive DBPs. Carbon adsorption is
discussed in par. 5.4.11.
One of the quickest and least expensive ways of maintaining
low DBP levels in chlorine-treated water is to chlorinate the highest quality of water
(water with the lowest possible organic content). If water is filtered, the highest quality
of water is filter effluent. However, unless additional contact tanks are constructed, the
contact time is not usually long enough for adequate disinfection. Chlorinating
coagulated and settled water reduces (but does not eliminate) DBP levels in finished
water. DBPs continue to form during distribution. Disinfection before filtration limits
bacterial growth in the filters. The absence of a disinfectant at the beginning of
treatment may cause problems because of the growth of algae, slime, and higher forms
in the early part of water treatment plants.
c) Technology available for DBP reduction includes PAC, ozonation,
GAC, and aeration.
(1) Very high doses of PAC and ozone are required to get
substantial (but not complete) DBP removal. These processes would be too expensive
in light of the removals obtained.
(2) GAC filters can effectively remove DBPs (as well as other
organics) below contaminant levels.