Membrane and Desalination Technologies - TCE Moodle Website

18 A.G. (Tony) Fane et al.
(VOCs) from water, silicone rubber composite membranes are the state-of-the-art material.
The membrane used depends on the nature of the organics for the case of organic/organic
separation (57).
2.8. Membrane Bioreactors
The integration of membrane technology and biotechnology has been applied in the food
and pharmaceutical industries. However, the major application is in wastewater treatment. In
this case, membranes are incorporated into the design of the wastewater bioreactors to
separate solids/liquid, and this combination has been termed the MBR. The first descriptions
of this technique dated from the late 1960s. A bench-scale membrane separation system
linked with an activated sludge process was reported, in which a UF membrane module was
placed externally to an aerated biological treatment basin in place of the gravity clarifier for
the separation of mixed liquor (64, 65) (Fig. 1.10a). At around the same time, the first MBRs
were developed commercially by Dorr–Oliver (66). Later in that period, similar external
MBRs were used in fermentation of ethanol or acetone/butanol from low-grade food processing
waste, such as cheese whey (67) and, in producing protein antibodies by genetically
engineered mammalian cells (68, 69). This “first generation” MBR system has benefits in its
continuous operation, the high cell densities achievable and easy control of the flux of
different molecular weight molecules. However, the power costs associated with the operation
of the external membrane MBR system limited its application to smaller wastewater
flows.
In the 1980s, the Japanese government invested in the development of MBR with a low
footprint that would be suitable for water recycling. In 1989, Yamamoto (70) demonstrated
the application of a vacuum-driven membrane immersed directly in the active sludge for
solid-liquid separation, which was later termed the submerged MBR (Fig. 1.10b). In the
submerged MBRs, the membranes are self-supported (no pressure vessel) and the modules
can be fabricated simply and economically. The submerged MBR operates at a much lower
TMP and at a lower liquid cross-flow velocity. This has paved the way towards a significant
reduction in capital and operating costs. The Kubota plate and frame submerged membrane
system, developed as a result of the Japanese Government initiative, has revolutionized
wastewater treatment in Japan. The other type of submerged membrane system uses hollow
fibers in bundles or curtains, either horizontally or vertically aligned. More details of submerged
membrane systems can be found elsewhere (71).
In MBRs, solids can accumulate at the membrane surface; thus, a back transport of solids is
necessary to sustain membrane permeability. In the external MBR, solids back transport is
provided by cross flow produced through recycling the mixed liquor at high flow rates. In
the submerged MBR, coarse bubble aeration is used to prevent solids accumulation. By using
the MBR, the conventional multi-stage activated sludge process, which includes primary
settlement, secondary biological treatment and possible tertiary disinfection/polishing, can be
simplified to a single filtration stage. The main advantages of MBRs lie in their high-quality
effluents (BOD/TSS