Membrane Treatment - Public Water Systems - page 494

Fig. 15-9. 4 MGD microfiltration plant, Erie, CO (Courtesy of Erie Public Works Department)
The Pall MF system utilizes a hollow-fiber membrane with outside-to-inside flow
during the service cycle. Backwashing is from inside-out with water and air. Figure
15-11 is a flow diagram of a Pall system, and Figure 15-9 is a photograph of Pall
membrane modules. Membranes in a Pall system for small plants are generally ar-
ranged in two parallel trains. The number of membranes in each train is adjusted to
provide the desired plant capacity. The systems are supplied for automatic operation,
including the service, backwashing, and cleaning cycles.
Immersed Membranes ( Zenon and Memcor CMF-S ). Membranes operate in the out-
side-to-inside flow mode. The membranes operate under a vacuum created within the
hollow membrane fibers by a permeate pump. Raw water is drawn through the mem-
branes and is pumped out to treated water storage or distribution. Air scour is contin-
uously introduced at the bottom of the membrane modules to create turbulence, which
helps keep the membranes clean and maintain flux.
Zenon supplies a proprietary MF system called ZeeWeed. Memcor has also devel-
oped a similar system, called CMF-S, in which the membranes are suspended vertically
in a basin containing the raw water. The immersed membrane systems are shown in
Figure 15-12.
The proprietary membrane material is resistant to chlorine up to concentrations of
1,000 mg / L. Membrane cleaning by backpulsing is achieved by reversing the permeate
flow and backwashing the hollow fibers with permeate at low pressure.
Pretreatment Requirements The objective of pretreatment for UF and MF is to
remove large solids to prevent plugging of the hollow fibers. Typically, a self-cleaning