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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.
B. Water Quantity
Water quantity is termed Flux or Product Water
Rate and is measured as the amount of water
produced in one day. It is reported as Gallons per
Day (gpd) or Liters per Day (lpd).
The flow of water to drain is the Reject Water
Rate and is measured as Gallons per Day (gpd) or
as Milliliters per Minute (ml/min).
Milliliters per minute x 0.38 = gallons per day
EXAMPLE: The drain flow will fill a graduated
cylinder to the 150 ml mark in one minute.
150 ml/min. x 0.38 = 57 gpd
If the container available measures ounces, use
the following conversion:
Ounces per minute x 11.2 = gallons per day
EXAMPLE: The product flow will fill 2½ ounces
in two minutes.
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%
