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HIGH-RATE GRANULAR MEDIA FILTRATION 8.11

......... SMITrER ~ --,~ F M:: FMLOoTwORLEMENT
TE
| , .-- ,, ..ira .~ . ,. ,. '~ ' CONTR ..... _]

--.-- .~,----- ! ..... w~ ...... , . . . . . .
INLET WEIR tu i

/ °1 z
.... t,_ ! ............. EVEL I ' ,,

FIGURE 8.4 Constant-rate filter with influent splitting and varying water level. (Source." Monk, 1987.)

Filter Media Washing
As the amount of solids retained in the filter media of a rapid sand filter increases, bed
porosity decreases. At the same time, head loss through the bed and shear on captured
floc increase. Before the head loss builds to an unacceptable level or turbidity break-
through occurs, washing is required to clean the bed.
Failure to clean filter media adequately can lead to a multitude of problems. Initially,
mudballs form and accumulate in the bed, causing clogging. Then the clogged areas con-
tract as head loss increases. This shrinkage opens cracks in the filter media surface and

FE: FLOW ELEMENT
LEVEL TRANSMIql"ER ~ PE: PRESSURE ELEMENT
l TE: TURBIDITY ELEMENT
M: MOTOR
r ~ ~ !
t . . . . • ~ ~ CONTROLLER RISE iN WATER LEVEL
~ FILTERBOX--,~ I I I[ ~-- -2' : L/t

i ) WAT:L .... 'i i ] ~l i.~
, 1I .......... I ~ , ,, ~ =*
t - : - ;;T---- , ! ;/
LARGE HEAD I r { j . / BACKWASH HEADER u~ |~
CHANNEL ~ I
,, tl~_~ o ~ j ,_ ~.~i J ~I ~
,2,% ,j~---------~ l I --,~.~,.._L I ; I ~.I
-- ! i ............ ~ t . z ,'r-'rL ~ ,~ ~ .....
, ~ , , ;.. ~.-
..... I UNDERD .... ~ i !~ ' # I~°i
BACKWASH I ' /" ,'
RiPE ,~ " ° I TM CONTROL WEIR
ORIFICE PLATE J
FIGURE 8.5 Declining-rate filter. (Source: Monk, 1987.)