<P> The disadvantage of the CASSBR process is that it requires a precise control of timing, mixing and aeration . This precision is typically achieved with computer controls linked to sensors . Such a complex, fragile system is unsuited to places where controls may be unreliable, poorly maintained, or where the power supply may be intermittent . Extended aeration package plants use separate basins for aeration and settling, and are somewhat larger than SBR plants with reduced timing sensitivity . </P> <P> Package plants may be referred to as high charged or low charged . This refers to the way the biological load is processed . In high charged systems, the biological stage is presented with a high organic load and the combined floc and organic material is then oxygenated for a few hours before being charged again with a new load . In the low charged system the biological stage contains a low organic load and is combined with flocculate for longer times . </P> <P> Membrane bioreactors (MBR) are activated sludge systems using a membrane liquid - solid phase separation process . The membrane component uses low pressure microfiltration or ultrafiltration membranes and eliminates the need for a secondary clarifier or filtration . The membranes are typically immersed in the aeration tank; however, some applications utilize a separate membrane tank . One of the key benefits of an MBR system is that it effectively overcomes the limitations associated with poor settling of sludge in conventional activated sludge (CAS) processes . The technology permits bioreactor operation with considerably higher mixed liquor suspended solids (MLSS) concentration than CAS systems, which are limited by sludge settling . The process is typically operated at MLSS in the range of 8,000--12,000 mg / L, while CAS are operated in the range of 2,000--3,000 mg / L. The elevated biomass concentration in the MBR process allows for very effective removal of both soluble and particulate biodegradable materials at higher loading rates . Thus increased sludge retention times, usually exceeding 15 days, ensure complete nitrification even in extremely cold weather . </P> <P> The cost of building and operating an MBR is often higher than conventional methods of sewage treatment . Membrane filters can be blinded with grease or abraded by suspended grit and lack a clarifier's flexibility to pass peak flows . The technology has become increasingly popular for reliably pretreated waste streams and has gained wider acceptance where infiltration and inflow have been controlled, however, and the life - cycle costs have been steadily decreasing . The small footprint of MBR systems, and the high quality effluent produced, make them particularly useful for water reuse applications . </P>

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