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SLOW SAND AND DIATOMACEOUS EARTH FILTRATION 9.19

Manganese may be removed in DE filtration with potassium permanganate (KMnO4)
added to the body feed, followed by flow detention, usually from 10 to 20 min. Deten-
tion time is important and should be determined in bench and pilot tests. The rate of
KMnO4 addition and body feed rate depend on the amount of manganese in the source
water and other water quality characteristics.
Where iron and manganese are both present in source water, supplementary condi-
tioning must usually be accomplished in separate steps, with iron treatment preceding the
KMnO4 addition. When there is a large amount of iron to be removed, it may be neces-
sary to have two filters in series, with iron conditioning preceding the first filter and the
addition of KMnO4 and detention between the first and second filters.
Practically the only carryover of solids in DE filter effluents would be very fine DE
particles used in precoat and body feed. Although the presence of these particles is in-
nocuous to health, effluent water turbidity must meet the established requirements. Use
of a finer DE for precoat can sometimes achieve lower turbidity levels. A slight reduc-
tion in the applied flow rate can also help to improve effluent turbidity. Where only slight
additional turbidity reduction is needed, the use of a simple cartridge filter following the
DE filter accomplishes the desired DE removal. Use of cartridge filters to polish the ef-
fluent will be more effective for the lower-capacity DE installations when solids carry-
over is minimal.
Note that without the addition of separate, additional treatment processes, DE filtra-
tion will not reduce the organic content of source water.

Filter Design Considerations
The principal elements of a flat-leaf DE filter are shown in Figure 9.9. Several options
are available in designing each of these elements, as well as in the integrated assembly
design.

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Baffled Inlet-
Containment
Vessel
Inlet
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Slurry Drain

FIGURE 9.9 Principal elements of a flat-leaf filter.