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In Australia and New Zealand, gravity thickening and dissolved air flotation are commonly used to thicken primary sludge and waste activated sludge respectively. For plants with anaerobic digestion, the feed concentration to the digesters is often set by the limiting thickening capability of these upstream thickening processes. In some cases this does not fully utilise the capacity of this high capital value asset. Rather than build additional digester capacity (and possibly additional dewatering hydraulic capacity), consideration should be given to increasing the feed concentration to existing digesters to maintain or even extend the HRT. This paper describes how secondary thickening of primary and waste activated sludge has been successfully implemented at the Mangere Wastewater Treatment Plant (WwTP) in Auckland, NZ, to maximise the capacity of existing anaerobic digesters and improve digestion performance. It identifies the limiting performance of the first stage thickening units and describes how the second stage thickening can provide constant digester HRT control, increasing digester VS destruction to 55%VSR in conventional mesophilic digesters. This secondary thickening approach has also been adopted at two other major wastewater plants in the region.
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Document Type: Research Article

Publication date: 2007-01-01

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