CALCULATING EXPECTED GPD/LPD FROM THE REVERSE OSMOSIS RO MEMBRANE
Membranes produce the rated gallons per day (GPD) at 60 psi (4.1 bars) operating pressure, 77°F (25°C) operating temperature and 500 ppm total dissolved solids. Membrane output gallons per day (GPD) depends on operating pressure, water temperature and the ppm TDS in the feed water.
Expected GPD = Rated GPD × PCF × TCF × OCF
where PCF is the pressure correction factor, TCF is the temperature correction factor and OCF is the osmotic correction factor.
1. Calculation of PCF:
The output GPD from the membrane are directly proportional to the applied pressure. Note: The membrane is rated to produce the rated GPD at 60 psi. For any pressure other than 60 psi the output GPD is multiplied by the PCF (Pressure correction factor).
PCF = Pressure available (in psi) ÷ 60
2. Calculation of TCF:
The output GPD also decreases with decrease in temperature. This is because the water viscosity increases with decrease in water temperature. The GPD increases by approximately 3% for every °C rise in temperature. (Refer to the following table for TCF values).
Table 1: Temperature Correction Factors (TCF)
| °F (°C) | TCF | °F (°C) | TCF | °F (°C) | TCF |
| 41.0 (5) | 0.521 | 59.0(15) | 0.730 | 77.0 (25) | 1.000 |
| 42.8 (6) | 0.540 | 60.8 (16) | 0.754 | 78.8 (26) | 1.031 |
| 44.6 (7) | 0.560 | 62.6 (17) | 0.779 | 80.6 (27) | 1.063 |
| 46.4 (8) | 0.578 | 64.4 (18) | 0.804 | 82.4 (28) | 1.094 |
| 48.2 (9) | 0.598 | 66.2 (19) | 0.830 | 84.2 (29) | 1.127 |
| 50.0 (10) | 0.620 | 68.0 (20) | 0.857 | 86.0 (30) | 1.161 |
| 51.8 (11) | 0.640 | 69.8 (21) | 0.884 | 87.8 (31) | 1.196 |
| 53.6 (12) | 0.661 | 71.6 (22) | 0.912 | 89.6 (32) | 1.232 |
| 55.4 (13) | 0.684 | 73.4 (23) | 0.941 | 91.4 (33) | 1.267 |
| 57.2 (14) | 0.707 | 75.2 (24) | 0.970 | 93.2 (34) | 1.304 |
3. Calculation of OCF:
The output GPD of the system decreases with an increase in ppm TDS of tap water. This is because the osmotic pressure increases with increase in ppm TDS, and the increased osmotic pressure reduces the net driving force for pure water passage through the membrane. The osmotic pressure is approximately 1 psi for every 100 ppm TDS.
Note: Generally speaking, the effect of osmotic pressure can be neglected for most tap water supplies and the OCF can be assumed to be 1.Example: What is the expected GPD from a 50 GPD CHP system at 40 psi pressure and 60°F water temperature?
PCF = 40 ÷ 60 = 0.666
TCF = 0.754 (from Table 1)
OCF = 1Expected GPD = 50 x 0.666 x 0.754 x 1 = 25.10 GPD
