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8.8 CHAPTER EIGHT

filter was computed as a weighted average. Data in the figure indicate that as the media
used become coarser, the required depth is increased, and as the media become finer, the
depth required is reduced. Kawamura (1999) uses a length-to-diameter L/d ratio to select
the proper depth and size of filter beds (with both L and d measured in millimeters). A
value of the L/d ratio should be > 1,000 in rapid sand filters, > 1,250 in trimedia filters,
and > 1,300 in most coarse deep beds where d is 1.2 to 1.4 mm and > 1,500 is most coarse
deep beds where d > 1.5 mm.

Filtration Rates
Slow sand filters, designed for filtration rates of 3 to 6 mgd/acre at a rate of 0.05 to 0.10
gpm/ft 2 (0.1 to 0.2 m/h), were initially replaced by rapid sand filters that operated at rates
of 1 to 2 gpm/ft 2 (2.4 to 5.0 m/h). The 2 gprn/ft 2 (4.9 m/h) rate became widely accepted
as an upper limit in U.S. water supply practice for many years. For the past 30 years, it
has been demonstrated that dual-media and mixed-media, as well as single-medium (sand
or anthracite), filters can be successfully operated at much higher rates.
A number of investigators found dual- and mixed-media filters to operate successfully
at rates from 3 to 8 gpm/ft 2 (8 to 20 m/h) in a variety of locations (Conley, 1961, 1965;
Robeck, Dostal, and Woodward, 1964; Dostal and Robeck, 1966; Laughlin and Duvall,
1968; Tuepker and Buescher, 1968; Rimer, 1968; Westerhoff, 1971; Kirchman and Jones,
1972). The quantity of evidence of the practicality of high-rate filtration was such that in
1972 the AWWA Committee on Filtration Problems concluded that it had been amply
demonstrated that filters could be designed and operated to produce water of acceptable
quality at flows substantially higher than the rate of 2 gpm/ft 2 (5 m/h), once considered
the maximum. Over the last 30 years, a number of pilot-scale and full-scale deep-bed uni-
formly graded anthracite filters have been operated reliably at rates of 10 to 15 gpm/ft 2
(24 to 37 m/h).
Average filtration rates of roughly 2 to 7 gpm/ft 2 (5 to 17 m/h) are reported for the
upflow, biflow, and deep-bed filters discussed previously (Hamann and McKinney, 1968;
Jung and Savage, 1974).
Logsdon et al. (1993) demonstrated that deep-bed monomedia direct filters could be
operated at filtration rates up to 9 gpm/ft 2 on higher-turbidity waters (up to 60 ntu) with
proper chemical pretreatment and polymer filter aid selection. Filtration rates are impacted
by water temperature. Generally, when water temperatures drop below 45 ° F (8 ° C), wa-
ter quality and filter run length deteriorate in high-rate filters (Kawamura, 1999). Many
regulatory agencies will not approve rates in excess of 4 to 5 gpm/ft 2 (10 m/h) without
successful pilot-scale testing. The designer should make every effort to obtain approvals
for operation at higher rates. The quality of the raw water and extent of pretreatment will
play a large role in the acceptable filtration rate.

Filter Operational Control
Filtration process control is critical to successful operation. Decisions regarding control
methods must be made early during the design because they affect the physical layout of
the filtering facilities and its performance. Filter control may be predicated either on head
loss through the filter bed or on the rate of filtration. In either case, smooth transition dur-
ing changes in filtration rate is highly desirable. The adverse effects of sudden flow surges
on filtered water quality have been well documented (Cleasby, Williamson, and Baumann,
1963; Tuepker and Buescher, 1968).