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

This study evaluated high-rate activated sludge treatment across a broad range of short solids retention times (SRT)s (0.5-3d) 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 3d to 0.5d. This is higher than predicted by conventional models, likely due to additional mechanisms such as adsorption and storage, not included in these.