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HIGH-RATE GRANULAR MEDIA FILTRATION 8.39

Net Filter Production. A filter operating at 4.0 gpm/ft 2 (10 m/h) for 48 h and requiring
300 gal/ft 2 (105 L/m 2) for an adequate backwash would have a net filter production of 4
gpm/ft 2 × 60 rain × 48 h - 300 gal/ft 2 = 11,220 gal/ft 2 (3,946 L/m2). Properly designed
and operated filters should exhibit net filter production volumes ranging from 7,500 to
12,500 gal/ft 2 (306,000 to 509,000 L/m2). A net filter production volume of 5,000 gal/ft 2
(204,000 L/m 2) or less could indicate inadequate pretreatment, filter-clogging algae in the
influent water, excessive fines or mudballs in the filter media, mineral precipitates in the
underdrains, air binding, or hydraulic restrictions causing inadequate head between filters
and clearwell.

Common Filter Problems
Common filter problems include inadequate pretreatment or filter washing, gravel bed up-
set, air binding, restart after shutdown, and filter media replacement.
Inadequate Pretreatment. The pretreatment process (i.e., coagulation, flocculation, and
clarification) in a conventional plant generally should produce pretreated waters with tur-
bidities no greater than about 4 ntu with levels less than 2 ntu preferred. Pretreated wa-
ters with turbidities much greater than 4 ntu are indicative of floc carryover that tends to
cause short filter run lengths. Conversely, pretreated waters with turbidities of 1 ntu or
less in a conventional plant may result in inefficient filter operation or inadequate partic-
ulate removal.
In a direct filtration plant, pretreated water turbidity typically exceeds source water
turbidity. The flocculation process should be operated to minimize floc size and allow
penetration of the floc deep within the filter media, effectively using its entire depth.
Jar tests or on-line pilot filters, zeta potential, or streaming current instrumentation can
greatly assist the operator in optimizing the coagulation process and ensuring adequate
pretreatment for effective filtration.

Inadequate Filter Washing. Inadequate filter washing can result in poor filtered water
quality and mudball formation. Cracks can occur in filter media when compressible solids
remaining from previous filter runs pull filter media together and away from the filter box
wall. Pretreated water can then travel through the cracks and bypass much of the filter
media. Mudballs result from residuals remaining from previous filter runs sticking to fil-
ter media and forming agglomerations that grow too large to reach the washwater col-
lection troughs during washing. As they grow heavier, mudballs can sink to create im-
passable regions within the filter media, typically at the anthracite-sand or sand-gravel
interface. The impassable regions result in higher effective filtration rates, poorer filtered
water quality, and shorter filter runs.
Air scour and surface wash systems can prevent the formation of cracks and mudballs,
but previously formed mudballs may have to be removed manually or by soaking filter
media with acidified water. Some plants include waste washwater turbidimeters as an op-
erational tool to monitor the waste washwater quality and to minimize the volume of wash-
water usage.

Support Gravel Upset. Filter media support gravel upset may occur from operational
errors such as washing a dry or drained filter. The initial rate for filling the filter should
not exceed 5 gpm/ft 2 (12.2 m/h). Otherwise, rising water can compress air within the fil-
ter media pores to short-circuit the washwater, channel through the filter media, and dis-
turb the support gravel,
Opening the washwater rate control valve too fast can cause gravel upset. Also, trapped
air in the washwater header piping or in the underdrain system may be released in an un-