GIRE-097-2-2
15
In order to obtain enough suction pressure for the filtration considering the friction loss of
pipe and valves, the permeate water outlet should be located sufficiently below the liquid
level of the membrane submerged tank (normally 3 m below water level or lower).
It is recommended that the piping from the permeate water manifold to the permeate water
outlet should directly penetrate the tank wall, as shown in Fig.IV-2. Also if the permeate
water outlet is located in the open air, it is also recommended to make the pipe just before
the permeate water outlet U-shape to seal the piping with water.
The permeate water flow is controlled with the automatic control valve (the permeate control
valve). When the liquid level of the membrane submerged tank gets to the lower limit, this
control valve fully closes the permeate water line to stop the filtration. When the liquid level
gets to the higher limit, the automatic shutter valve on the raw water feed line closes its line
to stop raw water coming.
In this gravity filtration, the air accumulated in the permeate water line should be discharged once
a day at least; otherwise the effective water head is reduced seriously. The air purge nozzle
should be installed at the highest position of the permeate water line and the automatic shutter
valve (the air purge valve) is recommended to be installed on the line just before the nozzle. (As
shown in Fig.IV-2) Stopping the filtration (fully closing the permeate control valve) and opening
the air purge valve for a few minutes, the air is easily purged.
