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13.2 CHAPTER THIRTEEN

Pressure-driven and electrically driven membrane processes are the only commercially
available and commonly used membrane processes for water treatment.

Pressure-Driven Membranes
The pressure-driven membrane processes are
• Reverse osmosis (RO)
• Nanofiltration (NF)
• Ultrafiltration (UF)
• Microfiltration (MF)
Membranes are permeable to water but not to substances that are rejected and removed.
All membrane processes separate feedwater into two streams (see Figure 13.1). The per-
meate (for RO, NF, or UF) or filtrate (for MF) stream passes through the membrane bar-
tier. The concentrate (or retentate) stream contains the substances removed from the feed-
water after being rejected by the membrane barrier. The driving force for these membrane
processes may come from (1) a pressurized feedwater source with the membranes installed
in pressure vessels, called modules, or (2) a partial vacuum in the filtrate/permeate flow
stream caused by use of a filtrate/permeate pump or gravity siphon. The vacuum-driven
processes typically apply to MF and UF only and have membranes submerged or im-
mersed in nonpressurized feedwater tanks.
Pressure-driven membrane processes can be designed for cross-flow or dead-end op-
erating mode. In the cross-flow mode, the feed stream flows across the membrane sur-
face, and permeate (or filtrate) passes through the membrane tangential to the membrane
surface. Cross-flow operation results in a continuously flowing waste stream. Sometimes
a cross-flow system is designed with a concentrate recycle with a reject stream (feed-and-
bleed mode). Many MF and UF systems treating relatively low-turbidity waters are de-

Membrane Barrier
/

Product Stream
Feedwater
' (Permeate or Filtrate)

Waste Concentrate or Backwash
(Continuous or Intermittent)

:, =(2Y

Vacuum-Type with Permeate Pump
Pressure-Type with Feed Pump
(Membranes in Pressure Vessel) (Membranes in Open Tank)
FIGURE 13.1 Pressure-driven processes using feed or permeate pumps.