How to determine whether to use a 4" or 8" membrane element for reverse osmosis?

According to the membrane company's design guidelines, the recommended average water flux is 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 (using permeate from the first-stage Reverse Osmosis system as feed water to the second-stage reverse osmosis system).

Based on these design guidelines, the minimum permeate flow rate of a single 8040 membrane element is 0.46 t/h (using surface water as the feed water source), 0.80 t/h (using groundwater as the feed water source), and 1.15 t/h (using permeate from the first-stage reverse osmosis system as feed water to the second-stage reverse osmosis system).

Theoretically, 8040 membrane elements can be used when the system permeate flow rate exceeds the above values. However, the decision to use 8040 membrane elements also depends on many factors, including the system's recovery rate requirements, whether concentrate recirculation is allowed, the system's footprint, the cost of the pressure vessel, and the overall economic viability of the system.

Based on a comprehensive balance of various factors, it is generally believed that 8040 membrane elements are more suitable for reverse osmosis systems with a capacity of 5t/h or above, 4040 elements are more suitable for reverse osmosis systems with a capacity of less than 3t/h, and either 8040 or 4040 membrane elements can be used for reverse osmosis systems with a capacity of 3-5t/h.