Membrane and Desalination Technologies - TCE Moodle Website

Treatment of Industrial Effluents, Municipal Wastes, and Potable Water 211
Fig. 5.5. Working principle of crossflow membrane filtration.
In operation, the mixed liquor from the bioreactor (see Fig. 5.4) passes a security filter
and then enters the mixed liquor inlet of a membrane module, which consists of many
bundles of membrane filters. Through the mixed liquor inlet, the mixed liquor enters the
tube-type membrane filters where water–solid separation occurs under high pressure. The
liquid portion of the mixed liquor is forced by pressure to pass through the tube-type
membrane filter, becoming the treated effluent, while the suspended solids remain, becoming
highly concentrated retentate (or concentrate). The suspended solids are mainly microorganisms
from the bioreactor having particle sizes larger than that of the membrane filter’s
pores.
The treated effluent (or treated water), as shown in Figs. 5.4 and 5.5, is discharged to a
receiving water or reused.
The retentate from the membrane modules is partially returned to the bioreactor as return
activated sludge (RAS), and partially wasted as excess sludge (Fig. 5.4).
Since the direction of mixed liquor flow inside a tube-type membrane filter is perpendicular
to the direction of treated effluent passing through the membrane, this flow pattern is
called crossflow filtration – this is the first special membrane feature of the membrane
filtration operation.
The second special membrane feature, created by ONDEO Degremont, involves the use
of ceramic membranes, which have very high corrosion resistance, can be cleaned more
efficiently, and are less prone to biofouling.
Availability of various pore sizes in ultrafiltration and microfiltration range (Figs. 5.1 and
5.2) is the third special membrane feature.
The fourth special membrane feature is that the membranes can be cleaned by clean-inplace
(CIP) techniques, using crossflow filtration, and reverse backwash operation.