The MBR Book: Principles and Applications of Membrane

160 The MBR Book
3.3.4.3 Tank sizing and redundancy
● DO levels are �0.5 mg/L in the anoxic zone, typically 1.0–2.5 mg/L in the
aerobic zone and relatively high in the membrane region (2–6 mg/L) where
aeration is intensive. If recycling for denitrification takes place from the membrane
aeration tank to the anoxic zone then the anoxic zone becomes slightly
aerobic at the sludge inlets, reducing denitrification efficiency. To compensate
for this the anoxic zone must be increased to extend the HRT in this region.
Alternatively, sludge can be recycled from the aerobic tank.
● Retrofitting places additional constraints on design of iMBRs, since the tank
size determines the HRT and the shape and placement of immersed membrane
modules. Retrofitting of sidestream modules, on the other hand, is not
constrained by the aeration tank dimensions.
● Spare capacity is required for membrane cleaning, which either involves
draining of the tank and cleaning in air (Section 2.3.9.2) or CIP. In the former
case the biomass in the membrane train being cleaned has to be transferred
either to a holding tank or to adjacent membrane tanks. In either case sufficient
installed capacity is needed to contend with the total volume entering
the works while the membrane train is being cleaned. For large plants with a
large number of trains (membrane tanks) the cleaning can be sequenced to
avoid large hydraulic shocks on the remaining in-service modules. For small
plants a buffer tank may be required.
3.4 Summary
The selection of appropriate design and operating parameter values for an iMBR
centres on:
1. choice of membrane module;
2. choice of membrane aerator, if the membrane module is not provided with an
integral aerator;
3. membrane aeration rate.
Many, if not all, of these facets are stipulated by the technology provider, and the
choice of technology itself (including that between iMBR and sMBR) will be strongly
influenced by the duty to which it is being put. However, broadly speaking the mean
permeability sustained in an iMBR is dependent on the aeration rate. Failure of
membrane surface fouling, which can be irrecoverable, and clogging of the membrane
channels. It is therefore essential that the maintenance schedule includes
cleaning of the aerators, normally achieved by flushing with a water or hypochlorite
solution.
Design of iMBRs relies on accurate information regarding oxygen transfer to the
biomass and maintenance of permeability by membrane aeration. The design proceeds
via calculation of the oxygen demand of the biomass, which relates primarily
to the feedwater composition, the solids retention time and the oxygen transfer efficiency.
This procedure applies to all biotreatment processes. However, for an MBR