Grease waste and sewage sludge co-digestion enhancement by thermal hydrolysis: batch and fed-batch assays

Grease waste (GW) is an adequate substrate for sewage sludge co-digestion since, coming from a waste water treatment plant, it has a high methane potential (489 NmLCH₄/gVSin); however, no synergistic effect takes place when co-digesting with 52%VS grease. Conversely, thermal hydrolysis (TH) improves the anaerobic digestion of GW (43% higher kinetics) and biological sludge (29% more methane potential). Therefore, the application of TH to a co-digestion process was further studied. First, biochemical methane potential tests showed that the best configuration to implement the TH to the co-digestion process is pretreating the biological sludge alone, providing a 7.5% higher methane production (398 NmLCH₄/gVSin), 20% faster kinetics and no lag-phase. Its implementation in a fed-batch operation resulted in considerable methane production (363 NmLCH₄/gVSin) and TH improved the rheology and dewaterability properties of the digestate. This leads to important economical savings when combined with co-digestion, reducing final waste management costs and showing interesting potential for full-scale application.