The MBR Book: Principles and Applications of Membrane

energy demand in kWh/m 3 permeate product, than sMBR technologies. The immersed
configuration employs no liquid pumping for permeation, instead relying on aeration
to promote mass transfer of liquid across the membrane (i.e. enhancing flux) by
generating significant transient shear at the membrane:solution interface (Section
2.1.4.4). Shear can also be promoted by directly moving the membrane, such as in the
Huber VRM (Vacuum Rotating Membrane) system (Section 4.2.4).
Whilst sMBRs cannot provide the same low energy demand as the immersed configuration,
they do offer a number of advantages:
1. Fouling has been shown to decrease linearly with increasing CFV. For example
a bench-scale study revealed that CFV values of 2 and 3 m/s were sufficient to
prevent the formation of reversible fouling in UF (30 kDa) and MF (0.3 �m)
systems, and that fouling was suppressed for CFV values up to 4.5 m/s test
(Choi et al., 2005b).
2. The membranes can also be chemically cleaned “in situ” (CIP) easily without any
chemical risk to the biomass.
3. Maintenance and plant downtime costs, particularly with reference to membrane
module replacement, are generally slightly lower because of the accessibility
of the modules which can be replaced in �5 min.
4. Precipitation of sparingly soluble inorganic solids (i.e. scalants) and organic
matter (gel-forming constituents) is more readily managed in sidestream MT
systems by control of the hydrodynamics both during the operation and the
CIP cycle.
5. It is generally possible to operate sMBRs at higher MLSS levels than HF iMBRs.
6. Aeration can be optimised for oxygen transfer and mixing, rather than demanding
a compromise between membrane aeration and oxygen dissolution, as
would be the case for single-tank iMBRs.
2.3.2 Extractive and diffusive MBRs
Fundamentals 57
Extractive and diffusive MBR processes (eMBRs and dMBRs) are still largely at the
developmental stage and are likely to be viable only for niche, high-added value
applications. They have been reviewed by Stephenson et al. (2000). In an extractive
system, specific problem contaminants are extracted from the bulk liquid across a
membrane of appropriate permselectivity. The contaminant then undergoes biotreatment
on the permeate side of the membrane, normally by a biofilm formed on the
membrane surface. In the case of the diffusive MBRs, a gas permeable membrane is
used to introduce into the bioreactor a gas in the molecular, or “bubbleless” (Ahmed
and Semmens, 1992a, b; Côté et al., 1988), form. This again normally feeds a biofilm at
the membrane surface. Hence, both extractive and diffusive systems essentially rely
on a membrane both for enhanced mass transport and as a substrate for a biofilm,
and also operate by diffusive transport: the pollutant or gas for the extractive or diffusive
MBR respectively travels through the membrane under a concentration gradient.
In principle, any gas can be transported across the membrane, though obviously
the choices are limited if it is to be used to feed a biofilm. Diffusive systems are generally
based on the transfer of oxygen across a microporous membrane and are thus