Technical Bulletin on Membranes
Membrane System Design Guidelines
The factor which has the greatest influence on the membrane system design is the fouling tendency of the feed water. Membrane fouling is caused by particles and colloidal material which are present in the feed water and are concentrated at the membrane surface. The concentration of the fouling materials at the membrane surface increases with increasing permeate flux and increasing element recovery. A system designed with high permeate flux rates is therefore likely to experience higher fouling rates and more frequent chemical cleaning.
The Silt Density Index (SDI) value of the pretreated feed water correlates fairly well with the amount of fouling material present.
Only experience can set the limits on permeate flux and element recovery for different types of waters. When designing a membrane system for a specific feed water, it is advantageous to know the performance of other membrane systems operating on the same water. However, quite often there are no other membrane systems to compare with and then the guideline below should be followed.
Permeate flux: permeate flow rate per unit membrane area. Element recovery; ratio of permeate flow rate to feed flow rate for a single element.
* At permeate flow rates lower than shown in the column for Filtered Tertiary Effluent, maximum element recovery is 20% for surface water and 15% for seawater.
1We recommend that minimum concentrate flow rates be increased at the discretion of the designer by 20% for feed waters known to be high fouling tendency (SDI>3).
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