How to Initially Determine the Number of Membrane Elements Required for the System?

The designed permeate flow rate of a membrane element is the actual permeate flow rate assigned to each membrane element by the designer when designing the reverse osmosis system. As we know, the water source for preparing the standard test solution is reverse osmosis permeate, and therefore contains almost no impurities, eliminating the issue of membrane element fouling. In actual use, except for the secondary reverse osmosis system which uses permeate from the primary system as its feed water, the feed water for other reverse osmosis systems is almost always raw water that has undergone ordinary pretreatment. Although the pretreatment process removes some impurities, the feed water quality is still worse than that of the water source under standard test conditions. If the standard permeate flow rate is still used as the design flow rate in this case, the reverse osmosis membrane element will quickly become fouled, causing damage to the membrane element.

To avoid the above situation, membrane element manufacturers provide design guidelines to provide designers with a basis for their work. The design guidelines recommend selecting different design permeate volumes based on the different influent sources. Surface water sources are considered to contain more pollutants, with a Special Pollution Index (SDI) value between 3 and 5. Therefore, a lower design permeate volume per unit area should be selected. Groundwater sources contain fewer pollutants than surface water sources, with a lower SDI value, generally less than 3. Therefore, a higher design permeate volume per unit area should be selected. If permeate from a first-stage reverse osmosis system is used as feed water for a second-stage reverse osmosis system, it contains almost no impurities, with an even lower SDI value, generally less than 1. Therefore, a higher design permeate volume per unit area can be selected than for groundwater.

Therefore, it can be concluded that the designed permeate flow rate of each reverse osmosis membrane element is unrelated to its standard permeate flow rate, but only to its effective membrane area, feed water source, SDI value, etc. The designed permeate flow rate of CPA2 should not be 10000 gpd, but only as suggested in the company's design guidelines, i.e.: Design permeate flow rate = Average water flow rate × Effective area of membrane element.

Looking at the company's design guidelines, the suggested average water fluxes are 8-14 gfd (using surface water as the feed water source), 14-18 gfd (using groundwater as the feed water source), and 20-30 gfd (the permeate from the first-stage reverse osmosis system is used as the feed water for the second-stage reverse osmosis system). Therefore, when surface water is used as the feed water source, the designed permeate flow rate of the CPA2 membrane element = (8-14 gfd) × 365 ft² = 2920-5110 gfd, which is equivalent to 0.5-0.8 t/h. Similarly, when using groundwater as the feed water source, the designed permeate flow rate of the CPA2 membrane element is 0.8–1 t/h.

When using the permeate from the first-stage reverse osmosis system as the feed water for the second-stage reverse osmosis system, the designed permeate flow rate of the CPA2 membrane element is 1.2–1.7 t/h.

If a designer wants to design a reverse osmosis system with a permeate flow rate of 100 t/h, using CPA2 membrane elements and surface water as the feed water source, the required number of CPA2 membrane elements can be estimated as follows:

Number of membrane elements = System permeate flow rate / Design permeate flow rate of the CPA2 membrane element, i.e., Number of membrane elements = (100 t/h) / (0.5–0.8 t/h) = 200–125 elements.

Assuming each pressure vessel contains 6 membrane elements, then multiples of 6 can be used, and the designed number of membrane elements for this system should be 198-126. When groundwater is used as the feed water source, the designed number of membrane elements for this system should be 126-102. When the product water from the first-stage reverse osmosis system is used as the feed water for the second-stage reverse osmosis system, the designed number of membrane elements for this system should be 84-60.