A Combined Activated Sludge Anaerobic Digestion Model (CASADM) to understand the role of anaerobic sludge recycling in wastewater treatment plant performance

► The Combined Activated Sludge-Anaerobic Digestion Model (CASADM) is introduced. ► CASADM is applied to WWTPs with and without anaerobic digester sludge (AD) recycle. ► Negative AD SRTs with sludge recycle results in more CH4 and less sludge wasting. ► Longer system SRTs with sludge recycle provide greater EPS and biomass hydrolysis. ► Fermenters and methanogens survive in activated sludge, making CH4 in the settlers.