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8.38 CHAPTER EIGHT

Filter Ripening
"Clean ,i,r t ,i, i ,11
I I
Back- II Function of i
~,- Backwash i Function of I
"~ ~- Influent ~l
- Wash - i - Remnants i
I
Water ~
I I
Risinlg /~ Re in
Lim~--..x~ i, ~ Ledb g
Filter
¢p
B r e a k ~

Lag I / X
/2/ \
]Within Above
I I Media Media
Time
FIGURE 8.21 Characteristics of initial effluent quality. (Source: Amirtharajah and Wetstein, 1980.)

after filter ripening, but the mechanisms of particle attachment require attraction to the
filter media of opposite charges or coagulation with charge-neutralized particles previ-
ously attached to the filter media. Consequently, inadequate particle and turbidity removal
may occur for a period of time after a filter is put into service following backwash. This
time period typically lasts from 5 to 30 min. Amirtharajah (1980) thoroughly explores and
describes the filter ripening process (Figure 8.21). Moran et al. (1993) found that for
smaller particles, ripening continues well into the filter run, whereas for larger particles,
ripening occurs early in the filter run, but removal efficiency decreases afterward.
Methods successfully used to minimize the problem of filter ripening include a short
period of wasting the first portion of filtered water (i.e., filter-to-waste), a slow increas-
ing of the filtered water flow rate after a backwash, resting a filter, and adding a polymer
filter aid to the washwater supply.

Filter Efficiency. The computation of filter efficiency relates the effective filtration rate
to the operating filtration rate as in
R._.~e = UFRV - UBWV
Ro UFRV
where Re = effective filtration rate
Ro = operating filtratiori rate
UFRV = unit filter run volume
UBWV = unit backwash volume
As an example, a filter operating at 4.0 gpm/ft 2 (10 m/h) for 48 h and requiring 300
gal/ft 2 (12,200 L/m 2) for an adequate backwash would have an effective filtration rate of
3.9 gpm/ft 2 (9.5 m/h) and an efficiency of 97%.