Improvement of coffee grounds high solid thermophilic methane fermentation by co-digestion with in-situ produced waste activated sludge: Performance and stability

Highlights: • A long-term operation was established by co-digested with sludge. • Nutrition elements supplying effect was evaluated with calculated microbial yield. • 20% and 30% of sludge addition could improve co-digestion stability. The feasibility of in-situ stabilization in the co-digestion of coffee grounds (CG) and waste activated sludge (WAS) was investigated. Two lab-scale thermophilic continuous stirred tank reactors (CSTR), R1 and R2 were operated with substrates that contained different WAS ratios, S1 (WAS% = 20%) and S2 (WAS% = 30%). During the whole process, there was no external supply of ammonia and trace elements. The volatile solid (VS) removal efficiency of R1 and R2 was comparable, and the biogas yield of R1 (0.467 ± 0.100 L/g-VS_in) was slightly higher than R2 (0.408 ± 0.020 L/g-VS_in). The total ammonia nitrogen (TAN) of R1 and R2 was 482 ± 32 and 884 ± 24 mg/L, respectively. The stoichiometry formulas of co-digestion were established to calculate the theoretical microbial yield coefficients and the requirements of microorganism reproduction. A comparison between the theoretical requirements and experimental values showed that co-digestion with WAS could avoid supply for an external supply of minerals. For the net energy production, R1 and R2 could generate 6342 and 5069 kWh of electricity daily, respectively.