Most separations and flux through membranes are controlled by the nature of the fluid. For salt rejecting membranes, such as RO and NF, the dominant variables are operating pressure and osmotic pressure (a solute concentration-dependent property which reduces net operating pressure with increased solute concentrate).

The pumping rate or fluid velocity across the membrane is another important operating parameter; an increased velocity results in improved mixing of the layer of feed solution directly above the membrane. The removal of fluid through the membrane results in accumulation of rejected solutes in this layer, often referred to as the boundary layer. The boundary layer can contribute a significant resistance to flux through the membrane as levels of solutes increase.

The accumulation of solutes in the boundary layer is often the most limiting factor in membrane flux, particularily for the larger pored membranes (NF, UF, and MF). There generally is a finite operating pressure, above which provides little or no flux benefit.

An increase in the feed solution velocity across the membrane, combined with turbulence promoting mesh spacers, can provide the optimal combination of operating conditions. Consideration of energy imput and mechanical load due to pressure drop across the membrane are practical limitations for operation of membrane systems.

To find maximum flux, we set the feed flow to a maximum practical rate, and increase the operating pressure incrementally while monitoring flux (filtrate) output. A given operating pressure will yield a certain maximum output for a specific feed solution.

If the feed solution becomes more concentrated, such as occurs for a dewatering objective, the optimal operating pressure will typically decrease as the solute concentration increases (the exception to this would be if the osmotic pressure increase due to concentration becomes significant). The input energy may be better applied to higher cross flow velocity if practical.

A practical method may include operation at a pressure setting slightly lower than the maximum initial rate determined. This is produce highest flow, best flux, and least amount of build up on membrane.