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HIGH-RATE GRANULAR MEDIA FILTRATION 8.3

be maintained below 1.0 ntu at any time and below 0.5 ntu 4 h into a filter run to avoid
specific reporting and performance review action levels, based on continuous monitoring.
Reporting and filter evaluation action is required if two successive 15-rain individual fil-
ter turbidity measurements exceed 0.5 ntu after 4 h of filter operation and 1.0 ntu or 2 ntu
at any point during the filter run. Generally, achieving the required turbidity limits will
provide 2.5-1og Giardia removal credit, 2.0-log virus removal credit, and 2.0-log Cryp-
tosporidium removal credit for conventional filtration systems; and 2.0-log Giardia re-
moval credit, 1.0-log virus removal credit, and 2-log Cryptosporidium removal credit for
direct filtration systems. These credits vary from state to state, so consultation with your
local regulators is encouraged.
Pilot investigations and filter design must consider the filter's ability to achieve these
standards throughout the filter run. Filter-to-waste capabilities are not specifically required
by drinking water regulations; however, their use is recommended to achieve compliance
with individual filter turbidity limits. A short filter-to-waste period can "set" or ripen the
filter to reduce spikes of turbidity at the beginning of a filter run. For DBP standards, the
use of granular activated carbon filters may be considered to enhance removal of organic
DBP precursors. While the regulatory limit for combined filter effluent turbidity is 0.3
ntu, general industry practice is to maintain filtered water turbidity at less than 0.1 ntu.

Pretreatment
Effective operation of a high-rate granular media filtration system requires pretreating the
source water. The nature, as well as the quantity, of suspended material in the pretreated
water is critical to filter performance.
Unflocculated water can be difficult to filter regardless of the type of medium in use
(Cleasby, 1972; Hsiung, Conley, and Hansen, 1976). However, the work of Robeck,
Dostal, and Woodward (1964) with dual-media filters showed that if the applied water is
properly coagulated, filtration at rates of 4 or 6 gprn/ft 2 (10 or 15 m/h) produces essen-
tially the same filtered water quality as filtration at a rate of 2 gpm/ft 2 (5 m/h). Subse-
quent investigations have shown similar results for mixed-media filters (Laughlin and Du-
vail, 1968; Westerhoff, 1971; Conley, 1972). Recent research has pushed the filtration
rates for deep-bed filters to as high as 10 to 12 gpm/ft 2 (24 to 30 m/h); however, poly-
mer filter aids and deeper coarse bed filters were required to achieve these rates and the
raw water quality was excellent. Extended pilot studies and regulatory consultation are
recommended when filtration rates above 4 gpm/ft 2 (10 m/h) are proposed.
Chemicals used in conjunction with high-rate granular media filtration are limited pri-
marily to metal salts or cationic polymers as primary coagulants. Primary coagulants are
ideally fed into rapid mixing basins preceding flocculation. Whether clarification is also
required depends on the quantity of suspended solids, metals, and algae in the source wa-
ter. Primary coagulants are intended to produce agglomerations of natural and chemical
solids. Nonionic or anionic polymers are often added with the coagulant as a coagulant
aid to assist in strengthening and growth of these agglomerations during flocculation.
These same polymers can also be added as a filter aid to the filter influent water or to the
washwater to increase the strength of adhesion between media grains and floc in coarse-
to-fine filters. Proper pretreatment and mixing is essential to filter performance, especially
at higher filtration rates (AWWA Research Foundation, 1992). Pretreatment may also in-
clude aeration or introducing an oxidant if an objective of water treatment is to remove
iron or manganese.
A filter aid polymer can improve floc capture, provide better filtered water quality,
and increase filter runs with higher head loss before turbidity breakthrough. Filter aid
polymers are not generally used with fine-to-coarse filters because they promote rapid