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- 12 September 2007 -

The membrane bioreactor in sewage treatment

The membrane bioreactor has become an important processing tool in the treatment of waste liquids. Ken Sutherland looks at its technology and applications.

 

The secondary treatment of sewage is a large and energy intensive process, involving a biological digestion followed by a settlement of the solids created by the bioreaction. The membrane bioreactor takes the place of the whole secondary stage - and does it better, and in a much smaller space. It is a device for the biological oxidation of the organic material dissolved in sewage and the separation from the resultant slurry of a relatively clean liquid (it will also largely do away with tertiary liquid/solid separation). The excess solids created by the oxidation process can then be easily removed for subsequent treatment. It is a continuous process, and one that is quite easily controlled, and is rapidly becoming the best available technology (BAT) for waste water treatment.

The sludge settlement stage of the conventional secondary process is a fairly slow one, so the removal of the clear liquid from the slurry is a better option, and results in a cleaner liquid, because of the membrane filtration, at least to microfiltration standards, and quite commonly to those of ultrafiltration.

A major advantage of the MBR system is that it can operate at a much higher solids concentration in the bioreactor than that of a conventional activated sludge plant. The MBR plant can work effectively at MLSS (mixed liquor suspended solids) concentrations typically in the range 8000 to 12,000 mg/l (or 0.8 to 1.2%), and has been demonstrated successfully at up to 3%, whereas conventional activated sludge plants work at about 2000 to 3000 mg/l, because of the limitations on settling. This higher slurry concentration permits effective removal, not only of dissolved organic material but also of residual particulate solids.

This high sludge concentration capability enables an MBR system to deal effectively with strong industrial wastes, especially in places where water is short, and factories are seeking to close their water cycles.

The MBR is a comparatively recent development (it is not mentioned as such even in the second edition of Keith Scott's "Handbook of Industrial Membranes"), although it has been in use in wastewater treatment for a number of years. It was first developed to commercial use in the USA in the late 1970s and in Japan in the early 1980s. There are now well over 1000 MBRs in use (if not 2000), although a significant number of these are only of pilot scale.

The cost of an MBR plant for secondary processing is still higher than that for a conventional plant, but as the numbers of MBR plants increase, and as membrane costs fall, the life cycle cost differential will soon disappear, and the process advantages should lead to rapid uptake of the MBR system by the waste water treatment industry. The smaller footprint of an MBR plant will make it much more attractive for construction in developed urban areas.

The membrane bioreactor system

The membrane bioreactor in its present embodiments constitutes a complete plant for the treatment of sewage, once the large floating materials and the suspended grit have been removed. A normal sewage treatment works would have a primary settlement stage after screening and grit removal, but the ability of the MBR to deal with high solids loadings means that some of the primary settlement can be left to the MBR, and maybe all of it.

The basic MBR consists of two processing steps - a bioreactor, in which aerobic bacteria digest organic material in the presence of dissolved oxygen, and a membrane module, in which relatively pure water separates from the suspension of organic matter and bacteria. These two units may be set up to run in succession (i.e. the liquid flows first through the bioreactor, where it is held for as long as necessary for the reaction to be completed, and then through the membrane separation stage), with a recycle of some of the separated sludge to the bioreactor. This is often called sidestream operation.

Alternatively, the membranes are suspended in the slurry in the bioreactor, which is appropriately partitioned to achieve the correct air flow, with the surplus sludge withdrawn from the base of the bioreactor at a rate to give the required sludge retention time (and independent of the water offtake rate). This is then termed a submerged (or immersed) MBR.

Membrane format

Two main formats are used for the membrane material. One, developed by Kubota, uses flat rectangular sheets of membrane, welded in pairs around a panel for each pair, hanging vertically and parallel to other such cartridges on either side of it. Between the membrane sheets and the support panel are spacer sheets that allow the permeate to run from the inside of the membrane to a withdrawal nozzle at the top of the cartridge. Liquid flow is from the outside in to the centre of the cartridge.

The other, developed by Zenon, uses bundles of hollow fibres to form the membrane module. These also hang vertically, with liquid flow from the outside of the fibres in to their centres. The bottom end of the fibre is sealed shut, and the top end together with all of the other fibres is sealed through an end cap into a permeate offtake chamber. The hollow fibre, with this flow direction, is perfectly capable of resisting the relatively low transmembrane pressures entailed in MBR operation, but several manufacturers advertise their use of reinforced fibres.

Either of these formats can be used for submerged or sidestream operation. A third, employing capillary tubes of membrane, is mostly used for sidestream separation, with liquid flow from inside the tubes outwards into a permeate collection chamber. These membranes operate in through-flow mode, with separated sludge collecting inside the tubes, and having to be backwashed out at regular intervals, while the flat sheet and hollow fibre membranes operate in an approximation to cross-flow, with the solids flushed away from the membrane surface by an air-scour operation.

A fourth format has the membranes in the shape of circular discs mounted on a horizontal shaft. The whole array is submerged in the activated sludge suspension, and the discs are rotated. Permeate flow is from the outside of the double-sheet discs and in to the central shaft, which is hollow to provide permeate offtake flow.

Whichever the membrane format, air scouring is used to remove solids from the vicinity of the membrane surface. This may be the same as the air flow used to activate the solid suspension, or (and especially in sidestream arrangements) the two gas injection systems are quite separate.

The disassociation of bioreaction from sludge settlement means that the bioreactor can be much smaller than in the case of conventional activated sludge processes. This enables the skid mounting of MBR systems for easier installation.

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