Membrane Design

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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.

 

Design Guidelines

Feed Source	RO Permeate	UF Permeate	Well Water/ Softened Supply	Softened  Surface Supply	Surface Supply	Filtered Tertiary Effluent	Seawater
Feed Silt Density Index	DI<1	SDI <1	SDI<3	SDI 3-5	SDI 3-5		SDI<5
Max. Element Recovery % 40” (1m) Long Element	40	25	19	17	15*	10	10*
Max. Permeate Flow Rate Per Element gpd (m3/D)
8040	12000 (45)	10900 (41)	7500 928)	6500 (25)	5900 (22)	4000 (15)	6000 (22)
4040	3000 (11)	2700 (10)	1800 (6.8)	1600 (6.1)	1500 (5.6)	1000 (3.8)	1500 (5.6)
2540	1000 (3.8)	900 (3.4)	600 (2.3)	500 (2.0)	500 (1.9)	300 (1.3)	500 (1.9)
Max. Feed Flow Rate Per Element U.S. gpm (m3/h)
8040	70 (16)	70 (16)	62 (14)	60 (14)	55 (12)	50 (11)	60 (14)
4040	16 (3.6)	16 (3.6)	16 (3.6)	16 (3.6)	16 (3.6)	16 (3.6)	16 (3.6)
2540	6 (1.3)	6 (1.3)	6 (3.1)	6 (1.3)	6 (1.3)	6 (1.3)	6 (1.3)

* 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.