The acceptance criterion for cleaning efficiency is membrane and application specific and may vary between plants. This measurement becomes the baseline, which is compared to subsequent NWP measurements taken post-CIP. The first CIP removes membrane preservatives and conditions the membrane prior to the first exposure to process fluid. NWP is measured for a new set of membranes at installation (Section 8.6), after flush and after the first CIP cycle.

For lower permeability membranes, there is no need to run at high feed flows as this can accentuate feed channel pressure drop effects. This can be achieved through higher feed flows. It is critical that the retentate flow is at least 10% of the feed flow to avoid this. Air bubbles can build up in the retentate and blind off the membrane to alter the measured NWP. High permeate flows can also cause a pressure drop in the permeate stream so the TMP calculation in equation (8.4) will be thrown off. For high-permeability membranes, low feed flow and high TMP will cause dead-end flow where all the feed goes into the permeate stream. The calculated NWP value is subject to pressure gauge measurement error so it is useful to run at least at 10 psi TMP to minimize this effect. TCF = (viscosity of water at measured temperature)/(viscosity of water 25☌) as temperature correction factor (see Appendix).Īs described in Section 8.4, the hydraulics of modules are complex and the calculated NWP may be affected by membrane permeability and the selection of feed flow and TMP operating conditions. TMP = (( P feed + P retentate) / 2 – P permeate) as the module average transmembrane pressure. (8.4) NWP = ( litres per hour volumetric permeate flow ) ( TCF ) ( TMP psid ) ( membrane area m 2 )

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