Effect of calcium saponification on anaerobic co-digestion of sewage sludge with fat, oil, and grease at low addition rates: Performance, kinetics and microbial responses.

Most studies on the anaerobic co-digestion of fats, oils and greases (FOG) with sewage sludge have focused on high FOG addition rates, which are unlikely to be achievable in full-scale practice. This study investigated whether calcium saponification could improve the co-digestion of FOG with sewage sludge under controlled low addition rates (2-10 % volatile solids, VS). Three representative model FOGs-olive oil (OO), sunflower oil (SO) and pork fat (PF)-and their calcium-saponified products were used to isolate the effects of FOG composition and calcium saponification under controlled batch conditions. The raw FOGs showed strong substrate-dependent behaviour: OO increased cumulative specific methane yield (CSMY) by up to 40 %, SO produced only a limited increase in CSMY (<10 %), whereas PF increased CSMY at 2-5 % VS but showed no CSMY improvement at 10 % VS. Calcium saponification generally improved process performance, particularly for SO and PF, increasing CSMY by 14-28 % relative to the corresponding controls and improving the high-loading response of PF. Microbial community analysis showed that a shared core community was retained across treatments, while adaptive shifts occurred in syntrophic long chain fatty acid (LCFA) degraders and methanogens in response to substrate type and saponification. Overall, these findings provide a controlled mechanistic assessment of substrate-dependent calcium-saponification effects and a basis for subsequent validation using heterogeneous sewer-derived FOG deposits under continuous operating conditions.