Membrane and Desalination Technologies - TCE Moodle Website

Treatment of Industrial Effluents, Municipal Wastes, and Potable Water 209
Fig. 5.3. MBR process system with membrane module submerged in the bioreactor.
process. The high MLSS concentrations facilitate treatment within a smaller bioreactor
volume.
The hydraulic capacity of the MBR process is limited by the flow of water per unit area of
the membrane surface. The average flow rate per unit area, or flux, for membranes that are
used for wastewater treatment (WWT) is typically 10–15 gallons per square foot per day (gfd).
This type of MBR process can be built from the ground up, or retrofitted into an existing CAS
aeration basin, as shown in Fig. 5.3. In operation, air is supplied through coarse bubble diffusers
at the base of the membrane cassettes to agitate and scour the membranes for cleaning and to
provide oxygen for biological treatment. At regular intervals, automatic backwash (backpulse)
cycles clean and restore permeability to the membranes. The coarse bubble diffusers used for
membrane cleaning do not transfer oxygen efficiently, so fine bubble diffusers (or other means
of aeration) are added to supply more air for treatment.
2.2. Membrane Bioreactor with Membrane Module Situated Outside the Bioreactor
This type of MBR process system is schematically shown in Fig. 5.4. Screened influent
enters the bioreactor, where it is oxidized to remove organic pollution, as well as ammonia, if
any. The mixed liquor from the bioreactor at an MLSS concentration ranging from 10 to 20 g/L
is withdrawn and pumped through a crossflow membrane filtration module. The permeate
from the membranes constitutes the treated effluent. The retentate stream representing
concentrated biosolids is returned to the bioreactor. Excess biosolids are wasted from the
bioreactor or from the return line.
It may be noted that due to the membranes acting as absolute barrier for solids, it is possible
to maintain the desired sludge age or SRT accurately. Also, the micro- or ultrafiltration