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There are many alternatives to the direct line of treatment processes described in the preceding paragraphs. One such takes the whole sewage, before primary settlement, and treats it at above ambient temperatures with anaerobic bacteria, to produce methane and a benign liquid effluent and separated sludge. This process has many advantages, but is not yet accepted as a satisfactory answer to the sewage problem.

Most alternatives apply to the secondary process, including the use of rotating biological contactors (discs carrying the aerobic bacteria), and the sequencing batch reactor (which combines secondary treatment and sludge separation). The
most exciting development, of considerable interest to the filtration industry, is the membrane bioreactor (MBR), which uses a membrane to separate the clean effluent from the activated sludge zone.

The MBR uses microfiltration membranes, or loose ultrafiltration membranes, to separate clean water from the activated sludge broth. The membranes take the form of a module of either hollow fibres or flat sheets, and they operate at low transmembrane pressures, being driven either by suction, or hydrostatic head, or by a low system pressure. The module may either be submerged in the activated sludge suspension or be housed in a separate vessel. The MBR replaces the secondary activated tank of the conventional system, including its settlement zone, and takes up considerably less area in doing so. It can also cope with more suspended organics in its feed, so it reduces the size of the primary settlement system as well. Air streams through the MBR serve both to aerate the sludge and to scour the membrane surfaces.

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