Loss of normalized permeate flow

Membrane fouling and scaling can both lead to a loss in normalized permeate flow. Additionally, membrane compaction will result in decreased permeate flow as well. 

Membrane fouling involves the deposition of suspended solids, including bacteria, on the membrane or components within the membrane module. These foulants form a layer on the surface of the membrane that becomes an additional barrier for water to flow through to the permeate side of the membrane. Hence, if the feed pressure is held constant, the permeate flow will decrease. 

In general, operators focus on observed permeate flow and adjust operating variables to deliver the required permeate flow. Thus, if the permeate flow is decreasing due to fouling, the operating pressure is usually increased to overcome the additional barrier to transport and to maintain a constant observed permeate flow. 

Membrane compaction involves the compressing of the membrane itself that, in essence, makes the membrane denser or thicker which reduces the flow and salt passage through it. Compaction can occur under higher feed pressure, high temperature, and water hammer. (Water hammer occurs when the RO high-pressure feed pump is started and there is air trapped in the membrane modules—see Chapter 6.2.) Most brackish water membranes, when operated properly, experience a minimal degree of compaction. However, seawater membranes and cellulose acetate membranes 

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Data, particularly normalized data, is evaluated to determine the nature of the loss in membrane performance (see Chapter 11.3 for a complete discussion on data normalization). Normalized permeate flow, salt rejection, and differential pressure should be evaluated to determine trends in performance. 

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