NIRE-170-1-6
14
It is recommended that the piping from the filtrate water manifold to the filtrate water
outlet should directly penetrate the tank wall,
as shown in Fig.IV-2. In addition, if the
filtrate water outlet is an open air discharge, it is recommended that a U-shaped
“trap” be
constructed to seal the filtrate piping with water.
The filtrate water flow rate is controlled with the automatic control valve (filtrate control valve).
When the liquid level of the membrane tank gets to the designed low level limit, the filtrate
control valve fully closes to effectively stop the filtration process. When the liquid level of the
membrane tank reaches the designed high level the automatic shut off valve on the raw
water influent line closes to stop raw water flow to the membrane tank.
In the gravity filtration mode air may accumulate in the filtrate water lines. To prevent
reduction or loss of suction the air in the filtrate line should be purged to atmosphere at least
once per day. The air purge nozzle should be installed at the highest position of the filtrate
water line. The line should also be fitted with an automatic shut off valve (the air purge valve)
just upstream of the air purge nozzle (As shown in Fig.IV-2). The filtration process should be
suspended for a few minutes while purging any entrained air from the filtrate line. Close the
filtrate control valve and open the air purge valve. After all air has been purged return the air
purge valve to the closed position and open the filtrate control valve.