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Driving force

Most membrane filtration processes require quite a high transmembrane pressure in order to produce an acceptable permeate flow rate. By contrast, the membrane bioreactor operates with a low differential, of about 0.5 bar. This can be provided by a vacuum pump, sucking on the permeate discharge line, through a receiver, or by the hydrostatic head of a deep bioreactor tank, or by a low level of pressurisation of this tank.

The MBR mainly operates by ultrafiltration, but some microfiltration membranes are now used, as allowed by the degree of separation required.

An MBR is capable of removing suspended solids to levels of below 5 ppm and BOD to below 10 ppm (and much better in some cases), which figures are comfortably below the current 20/30 requirements for marine and watercourse discharges. By careful choice of membranes a membrane bioreactor system can retain chlorine resistant pathogens such as Cryptosporidium and Giardia.

The longer sludge retention times permit the reduction of molecules difficult to biodegrade, such as detergents. With proper system design nitrogen and phosphorus contents can also be significantly reduced,

The MBR business

The membrane bioreactor has lent itself very well to the "municipal" waste water treatment business. This is a huge, world wide activity, with very large companies (as well as municipalities) taking an increasing part. On the whole, the operators of treatment works tend not to be the suppliers of the equipment used in them. Rather is this done at two further levels: contractors engaged to build the works and specify the contained equipment, and then the specialist manufacturers of the equipment as appropriate.

The MBR business can thus be found among the waste water equipment suppliers, either the specialist builders of waste treatment equipment, and especially of secondary processes, who have acquired membrane technology (possibly by buying a membrane specialist company), or the membrane system manufacturers, who have developed an expertise in waste water treatment.

The earliest developers were Zenon in Canada and Kubota in Japan , closely followed by Wehrle Werk in Germany . There are now around 30 manufacturers of MBR systems world-wide, of which the largest include Zenon (now part of GE Water Technologies), Kubota (which has several licensees around the world), USFilter (now part of Siemens - which puts two of the largest manufacturers under the umbrellas of massive corporations), and Mitsubishi Rayon.

The numbers of installations are variously quoted, presumably from different times of writing, but it seems likely that Zenon has several hundred reference plants, Kubota has over 2,500, and Mitsubishi Rayon over 700 (the latter two mostly in Japan). The largest MBR plant (at the time this article is being written - which may well be exceeded very shortly) is the Brightwater plant, in King County, Washington State, which will treat a waste water flow of 495,000 m3/day (495 megalitres per day) when it starts up in 2010/2011, rising to 645,000 m3/day by 2040. The MBR plant for Brightwater will be supplied by GE Zenon.

Developments

In the long history of waste water treatment (over 110 years), the membrane bioreactor is quite a recent invention, and so, not surprisingly, is still in a period of intense development. Three key areas of system investigation can be identified: the nature of the membrane coupled with operating energy consumption, air/gas handling and the bioreaction itself.

By the standards of most membrane processes, the membranes in an MBR are very "loose", i.e. they have a low transmembrane pressure differential in operation (and, as a result, are relatively easily cleaned). Development will continue to find membranes with lower cut points (i.e. retaining finer solids) but without an increase in working pressure drop. One of the disadvantages of the MBR system is a relatively high energy consumption per unit of liquid throughput, and the energy required to drive the permeate through the membrane is a significant component of the total consumption, so higher pressure drops will be unacceptable.

The gas flow through an MBR has two purposes - to dissolve oxygen in the suspending liquid and so feed the aerobic bacteria in the bioreactor, and to scour the surfaces of the membranes to keep them free of solid deposits. Design developments will continue to maximise the efficiency of both of these gas flows - with a reduction in energy consumption, if possible.

Perhaps the most exciting development in the MBR system would be its conversion from an aerobic process to anaerobic operation. Conventional activated sludge plants are looking at anaerobic operation with a great deal of interest, because of its potential ability to treat whole sewage in one process, and at the same time generate (in the form of methane) sufficient fuel to drive the whole works, with some over to deliver to the electric power supply grid. (It is estimated that the energy content of a typical sewage inflow is about five times the energy required to run the treatment works, so the current processes are clearly some way from being energy efficient.)

Anaerobic operation of an MBR system would obviously need a change in the gas feed, as well as operation at higher temperatures, but the increased production of biogas would be a major advantage.

These developments have all concerned the MBR system itself, in its application as a sewage and industrial waste treatment process. There are, of course, a great number of other biological processes that might benefit from the continuous extraction of the liquid phase during the reaction, and developments of the system to suit other processes entirely can be expected.

Contact
Ken Sutherland
E-mail: ken.suth@ntlworld.com.

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