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2.5 oz. ÷ 2 min. = 1.25 oz./min.
1.25 oz./min. x 11.2 = 14 gpd
The Reject Ratio is the amount of water produced compared to the amount of water flowing to drain.
Reject Ratio = Reject Rate
Product Rate
EXAMPLE: The product rate is 14 gpd and the reject rate is 56 gpd.
Reject Ratio = 56 ÷ 14
Reject Ratio = 4 or 4 - to - 1
The Percent Recovery is another way to measure the amount of water produced as compared to the amount actually
used.
% Recovery = Product Rate x 100%
Feed Rate
NOTE: The total flow or feed water rate into the system is the sum of the product flow and the drain flow.
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) discharging 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 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 approximated 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 pressurized Holding Tank.
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