Page 498 - Water and wastewater engineering

MEMBRANE FILTRATION 12-15

Process Design
Membrane Process Selection. The following factors should be considered in selecting the
membrane and membrane process (Bergman, 2005):
• Source water.

• Pretreatment requirements.
• Product water quality requirements.
• Product water quantity requirements.

• Residuals disposal.
• Capital and operating cost.
UF membranes are favored over MF membranes if virus or dissolved organic compound removal with-
out raw water pretreatment is a product water quality goal.
Because, at the time of this writing (2009), the use of MF and UF systems is relatively new
to the municipal water treatment industry, consultation with manufacturers and pilot testing is
highly recommended. This is particularly true because of the proprietary nature of the equipment.
Facility design is frequently preceded by “preselection” of the equipment supplier because the
facility is built around the special requirements of the proprietary design.

Operating Pressures. Transmembrane pressure is between 20 and 200 kPa for pressure systems
and from 7 to 80 kPa for vacuum systems. The applied pressure ranges from 70 to 200 kPa.
To minimize fouling the transmembrane pressure should be limited to 100 kPa (AWWA, 2005;
Bergman, 2005; MWH, 2005).
For the direct filtration mode, the transmembrane pressure may be calculated as (AWWA, 2005):
P P i (12-13)
P p
where P transmembrane pressure, kPa
P i pressure at inlet to module, kPa
P p permeate pressure, kPa
When the system is operated in the cross-flow mode, the average transmembrane pressure is
(AWWA, 2005):
P P
P i o P p (12-14)
2
where P o pressure at outlet of the module, kPa.

There is an accompanying pressure drop across the module (AWWA, 2005):
P module P P o (12-15)
i

where P module pressure drop across the module, kPa.