Performance of pilot scale two-stage anaerobic co-digestion of waste activated sludge and greasy sludge under uncontrolled mesophilic temperature

• Two-stage co-digest of WAS-GS has potential to biogas recovery and waste treatment. • Methane yield of 423 mL-CH 4 /g-VS, with 69% COD removal was obtained from two-CSTR. • Firmicutes bacteria and Methanosaeta archaea were dominantly found in this system. • The process' effectiveness and economic feasibility were successfully proved. Waste-activated sludge (WAS) and greasy sludge (GS) discharged from the canned tuna industry are considerably characterized as harsh organic wastes to be individually treated by using traditional anaerobic digestion. This study was attempted to anaerobically co-digest WAS and GS in continuous pilot scale two-stage process, comprising the first 50 L continuous stir tank reactor (CSTR 1 ) and the second 250 L continuous stir tank reactor (CSTR 2 ). The two-stage co-digesting operation of dewatered WAS:GS ratio of 0.4:1 (g-VS) at ambient temperature with the organic loading rate (OLR) of 12.6 ± 0.75 g-VS/L·d and 2.26 ± 0.13 g-VS/L·d, corresponding to 3-day and 17-day hydraulic retention time (HRT) for the first and second stage, respectively generated highest methane production rate of 957 ± 86 mL-CH 4 /L·d, corresponding to methane yield of 423.4 ± 36 mL-CH 4 /g-VS. Organic removal efficiency obtained was around 67.5% on COD basis. The microbial diversity was depended on the process's activity. Bacteria were mostly detected in the CSTR 1 , dominating with the phylum Firmicutes and Proteobacteria, whereas genus Methanosaeta archaea were found dominantly in the CSTR 2 . The economic analysis of process shows payback period (PBP), internal rate of return (IRR), and net present value (NPV) of 3 years, 30%, and 250,177 USD, respectively. This study demonstrated the potential approach to applying the two-stage anaerobic co-digestion process to stabilize both WAS and GS along with generating valuable bioenergy carriers.