In normal operation, the membrane in reverse osmosis elements can become fouled by mineral scale, biological matter, colloidal particles and insoluble organic constituents. Deposits build up on the membrane surfaces during operation until they cause loss in normalized permeate flow, loss of normalized salt rejection, or both.

Elements should be cleaned when one or more of the below mentioned parameters are applicable:
• The normalized permeate flow drops 10%
• The normalized salt passage increases 5 - 10%
• The normalized pressure drop (feed pressure minus concentrate pressure) increases 10 - 15%