Figure 2-72. Distribution Pump Switch
BACKWASH OF MULTIMEDIA FILTER
The multimedia filter should be backwashed:
After 20 hours of ROWPU operation, or
Multimedia filter gage reading is over 10 psid, or
Multimedia filter gage reading exceeds 5 psid of log reading at
ROWPU is to be shut down.
General. The backwash cycle is an up-flow operation. Water flows from the bottom to the top of the tank
and then out to drain through the backwash flow control, washing out turbidity and settling the filter bed. The filter has three
separate backwash stages. The first stage of backwash is at 70 gpm to fluidize the bed. A second backwash stage
begins at approximately 120 gpm, tumbles at the bed, and scours the particles against one another when the backwash
diaphragm valve opens. The backwash diaphragm valve then closes, returning the backwash rate to 70 gpm to restratify
the media bed. The rinse cycle is a down flow operation. Water flows from the top of the tank through the filter bed and
out to drain. This displaces the backwash water in the tank, and preconditions the filter bed before the filter reaches the
purge cycle. The purge cycle is rapid downflow rinse to remove turbidity from the bottom of the filter and recondition the
filter bed prior to the return to service.
Backwash Timer and Multimedia Filter Control Valve (fig. 2-15). The direction and rate of water flow through
the multimedia filter are controlled by the control valve and backwash diaphragm valve. The control valve consists of six
diaphragms which open and close during the timer cycles of backwash, conditioned rinse, and service (service means
normal ROWPU operation). As the backwash timer automatically changes cycles, its pilot valve changes position. When
the backwash timer pilot valve changes position, it hydraulically closes and opens the diaphragms in the control valve.
During the 120 gpm portion of the backwash cycle (referred to as "secondary backwash") the backwash timer solenoid
valve sends water to the backwash diaphragm valve (located just under the control valve). This opens the backwash
diaphragm valve, resulting in the higher rate of backwash water flow.