and the drain water rate is 40 gpd
Feed Rate = 10 gpd + 40 gpd = 50 gpd
% Recovery = 10 gpd x 100%
50 gpd
% Recovery = 0.20 x 100% or 20%
C. Net Pressure Differential
Most R.O. Membranes are rated at a standard-
ized condition of 77˚F (25˚C) and 65 psig
(450kPa) discharging to atmospheric pressure.
Product water quality and quantity greatly depend
upon the
Net Pressure Differential
(
∆
p)
across the R.O. Membrane. This pressure dif-
ferential 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
NOTES:
and the osmotic pressure, which also tries to
push the water backwards.
The
Osmotic Pressure
is in proportion to the
dissolved minerals in the water and can be ap-
proximated 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 pro-
duced. Some loss of production is normal
when using a pressurized Holding Tank.
