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.