BACKWASH OF MULTIMEDIA FILTER.
The multimedia filter should be backwashes:
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.
b. Backwash Timer and Multimedia Filter Control Valve (fig. 215). 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.
(1) Mechanical Timer (Model 600-1): The main components of the mechanical timer are the
reconditioning knob, timer mechanism motor, four controller cams, pilot valve motor, white cam, and the auxiliary
pilot switch. The reconditioning cycle, backwash, rinse, purge, and service operation begins when the backwash
switch, located on the control panel, is manually placed in the ON position. The recharge lever solenoid in the
timer is then energized and the level moves to the right, thus rotating the reconditioning knob. There is now a dead
time from 25 minutes (the time is still running) before backwash begins. The timer motor drives the controller
cams (4 cams). These cams, which are set at the factory, control the reconditioning cycle. Readjustment should
not be required. Backwash is initiated when the micro switch for the bottom cam (black cam with numbers) falls in
the cam slot. When the blade of the micro switch falls into the notch of the fourth cam, the white cam is in the
service (SR) position. The pilot valve motor receives signals from the white cam. The pilot valve sends water to
pressurize (close) control valve, or drains water (opens). The auxiliary pilot switch is used to start toe backwash
pump. The switch is operated by activation of its lever arm by the raised button on the white controller arm.
(a) Open timer cover.