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A MBR consists of two parts: a bioreactor and a membrane installation. The biological processes that break down the pollution take place in the bioreactor (activated sludge). The membrane installation separates the water from the active sludge, after which it can be discharged or reused.

The biological processes in a membrane bioreactor (MBR) are almost identical to an active sludge system with the difference that the separation between active sludge and the purified water is achieved by means of membranes rather than settling and extraction with the conventional biological treatments. The advantages of a membrane bioreactor (MBR) over conventional techniques are:

  • No suspended matter in the effluent
  • The water quality after membrane passage (filtrate) is easy to reuse (water recycling/reuse)
  • Compact construction because higher sludge contents can be maintained in the reactor (aeration tank) (= higher load possible) and no major post-settling is required

Different types of membranes are used with MBR: Submerged membranes and crossflow membranes.

The submerged MBR membranes work on the basis of negative pressure. The flux (capacity per m² of membrane surface) is relatively low, which means that more membrane surface is required in relation to overpressure (crossflow) membranes. However, the advantage is that a submerged membrane consumes much less energy. So a shift from Opex to Capex. Another advantage is that the membranes are operated from the outside in. In this way, the dirt stays on the outside and cleaning is easier in the event of calamities or extreme pollution.

Besides the use of submerged membranes, there is also the option of applying overpressure MBR membranes, which is the case when using a crossflow membrane unit. The advantage of such a system is that much higher fluxes can be realised (50 – 80 l/m².h) so that the required membrane surface remains limited and a relatively compact installation can be built. On the other hand, one major disadvantage is the higher energy consumption compared to the use of immersion membranes.

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