Nutrient removal and energy recovery from high-rate activated sludge processes - Impact of sludge age

•High-rate activated sludge enhances energy and nutrient recovery through short SRT.•Tested range of SRTs from 0.5 d to 3 d for nutrient removal and energy recovery.•Energy recovery maximised at low SRT while nutrient removal maximised at long SRT.•Sludge degradability significantly higher than predicted through standard model.

This study evaluated high-rate activated sludge treatment across a broad range of short solids retention times (SRT)s (0.5-3 d) and found a strong SRT-outcome dependence for performance and subsequent anaerobic degradability of the sludge. Up to 50% total nitrogen, and 35% ammonia removal was also achieved at the longer SRTs, via partitioning rather than reaction. The aerobic SRT significantly affected the anaerobic degradability of the sludge produced (p < 0.001), with degradability increasing from 66% to over 80% while reducing the SRT from 3 d to 0.5 d. This is higher than predicted by conventional models, likely due to additional mechanisms such as adsorption and storage, not included in these.

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