Achieving and maintaining good biomass settling characteristics is a critical process design objective for any activated sludge wastewater treatment plant (WWTP), whether intermittent or continuous technology. One way of ensuring good sludge settleability in intermittent WWTPs is the incorporation of bioselectors in the process. A bioselector is essentially a small discrete reactor volume designed primarily for carbon absorption, in which activated sludge organisms are exposed to a high substrate concentration for a relatively short time. It is normally very much smaller than an anoxic zone and the activated sludge recycle is only a fraction of that typically adopted in continuous plants. With proper conditioning, recycled biomass rapidly absorbs and stores soluble organic wastewater components before transfer to the main treatment basin. This absorption and storage mechanism, and careful management of aeration throughout the intermittent treatment cycle, plays a crucial role in many subsequent growth and treatment processes, including sludge floc formation, denitrification and biological phosphorus removal. This paper examines some design considerations, and reviews the benefits of bioselectors by reference to the commissioning and initial operation of the new 160ML/d Woodman Point Sequencing Batch Reactor in Perth, Western Australia. The applicability of bioselectors in continuous plants is discussed.

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