Other membrane processes/membrane reactors

In a membrane bioreactor (MBR), a biological reactor (activated sludge) is coupled with membrane separation to retain the biomass. This not only produces a clarified effluent, but also allows operation at higher biomass concentrations. In doing so, the biotreatment process becomes more efficient, thereby reducing the required tank size. MBRs thus tend to generate treated water of a higher quality with respect to dissolved constituents. By removing the requirement for biomass sedimentation, biomass concentrations can be increased to 15 or 20 g.l-1 compared to 5 to 8 g.l-1 for conventional systems.

The two main MBR process configurations are submerged (or immersed) and sidestream. In the first, membranes are immersed directly in the aeration tank. Permeate is typically extracted by applying reduced pressure to the permeate side. In the latter case, membranes are installed outside the activated sludge tank. A crossflow pump is used to create the pressure and to prevent the build-up of solids on the membrane surface.

In MBRs, MF and UF membranes are used. In general, UF membranes are able to achieve higher levels of separation, in particular for bacteria and viruses. In sidestream MBRs, the external membrane module typically consists of tubular membranes with a fairly large diameter, and filtration occurs into- out. Submerged MBR membranes are largely configured as hollow fibres or flat sheets, and are operated out-to-in.

As is the case for all membrane filtration processes, fouling can manifest itself as flux reduction over 27 time when operating at a constant pressure or reversed. MBRs are usually operated under constant flux conditions, maintaining the convection of foulants to the membrane surface at a constant rate. MBR fouling is mostly affected by the interactions between the membrane and biological suspension rather than the feed.

Fouling related to concentration polarisation can be reduced by increasing the cross-flow velocity (for a sidestream MBR) or by increasing membrane aeration (for a submerged system). Further control of fouling relies on the following strategies:

• Appropriate physical cleaning: by relaxation under continued aeration or backwashing. This removes only reversible or temporary fouling.

• Chemical cleaning: with sodium hypochlorite, acid and/or base. A so-called maintenance cleaning (low concentrations of chemicals, short contact times) is done every 1 or 2 weeks using, an intensive cleaning is applied once or twice per year.

• Reducing the flux: sustainable operation implies that MBRs should be operated at moderate flux levels, preferably below the critical flux or sustainable flux.

• Modifying the mixed liquor or biological suspension: this can be achieved by adjusting the biological operating conditions such as sludge age, but is usually accomplished by adding chemicals such as coagulants/flocculants.

New developments

Capital costs for MBRs will presumably always remain higher than those of conventional treatment plants. MBRs, however, have the advantage of a smaller footprint and superior effluent quality. In addition, investment costs have decreased dramatically in the past 15 years thanks to continued technical improvements, the increased maturity of the technology, a more competitive market and growing demand. Although ongoing developments will lower membrane purchase costs, standardisation is probably needed to achieve a real breakthrough in price. Operating costs also have decreased dramatically. Mainly due to a reduction in energy requirements for membrane scouring. Ongoing developments to reduce capital or operating costs focus on four major domains:

• Filtration material/fouling control: e.g. cheaper membrane materials;

• Module-filtration system: e.g. optimisation of the packing density of hollow fi bres;

• System engineering: e.g. dual concepts including an MBR and a conventional system;

• MBR operation: e.g. evaluation of various chemical cleaning methods.

Nearly all commercial MBRs are aerobic units. Anaerobic MBRs exist, but they suffer from substantial fouling and are therefore operated at extremely low fluxes.