SRT duality in densified activated sludge systems: Measurement methods, metrics and perspectives.

Densified activated sludge (DAS) systems improve settling and nutrient removal by encouraging biomass densification via physical and biological selection. However, a key operational challenge in such systems is characterising sludge age accurately, as flocs and granules coexist under distinct sludge retention times (SRTs). Herein, we developed a granular-weighted SRT (gwSRTDAS) metric to more accurately reflect the actual sludge age in DAS systems. A dynamic monitoring framework was implemented on a full-scale hydrocyclone-based DAS system at the Dijon Eauvitale water resource recovery facility, drawing on five years of operational data. The results revealed that granule SRT can exceed bulk SRT by up to 3.5 times, while floc SRT remains substantially lower. The gwSRTDAS metric captures these differences, offering a more reliable indicator of overall DAS mixture age. A strong linear correlation (R2 = 0.98) between gwSRT and biochemical methane potential highlights its relevance for optimising energy recovery. These findings indicate that gwSRT offers a more representative measure of sludge age than conventional bulk SRT, supporting clearer insights into DAS behaviour under full-scale conditions. Moreover, maintaining bulk SRT below nitrification washout threshold while keeping gwSRTDAS above it may improve sludge biodegradability without compromising effluent quality. Overall, gwSRTDAS provides a practical framework for optimising DAS operation under stringent nutrient limits and seasonal constraints.