EXAMPLE: The product water rate is 14
gpd and the drain water rate is 56 gpd.
Feed Rate = 14 gpd + 56 gpd = 70 gpd
% Recovery = 14 gpd x 100%
70 gpd
% Recovery = 0.20 x 100% or 20%
C. Net Pressure Differential
Most R.O. Membranes are rated at a standardized
condition of 77˚F (25˚C) and 65 psig (450kPa) dis-
charging to atmospheric pressure.
Product water quality and quantity greatly depend
upon the
Net Pressure Differential
(
∆
p)
across
the R.O. Membrane. This pressure differential is a
summation of the feed water pressure at the
Membrane, which tries to push the water through,
the pressure in the Holding Tank, which tries to
push the water backwards and the osmotic pres-
sure, which also tries to push the water backwards.
The
Osmotic Pressure
is in proportion to the dis-
NOTES:
solved minerals in the water and can be approxi-
mated by 1 psig for each 100 ppm of TDS.
EXAMPLE: A feed water with 1500 ppm of TDS
would exert a backward pressure of about 15 psig
on the membrane.
Net Pressure Differential =
Feed Water Pressure – Holding Tank Pressure –
Osmotic Pressure
The higher the net pressure differential, the higher
the quantity and quality of water produced. Some
loss of production is normal when using a pressur-
ized Holding Tank.
