Anaerobic co-digestion of food waste, poultry litter and sewage sludge: seasonal performance under ambient condition and model evaluation

With the increasing population, food waste, sewage sludge, and poultry litter management problem are scaling up even in low-income countries. The management of these wastes has therefore been challenging. Anaerobic digestion of food waste alone is not very stable due to its acidic nature and high degradability whereas sewage sludge and poultry litter have low biochemical methane potential and a high nitrogen concentration. Co-digestion of suitably selected substrate leads to enhanced biogas production potential, system stability due to synergetic effects, and resolving the problem of waste management in the vicinity in a holistic approach. However, these wastes have varying characteristics and composition, in terms of carbon-to-nitrogen (C/N) ratio, pH, and alkalinity. In addition, millions of the rural household bio-digesters operating in low-income countries are working under ambient conditions and are primarily unheated. Therefore, there is a need of research to assess the viability of biogas production of co-digestion of all above substrates in an optimal mixing ratio operating in an ambient temperature condition. In this study, food waste (FW), sewage sludge (SS), and poultry litter (PL) were co-digested at ratios (SS: PL: FW: 3:2:1, 2:1:1, 1:1:1) with 8% total solid (TS) content at ambient temperature in summer and winter seasons. Biogas yield was highest with mixing ratio of 2:1:1 with the values of 640 L/kgVS in summer while it gave extremely low biogas yield of 106 L/kgVS in winter. The 2:1:1 mixture also had the highest methane composition of nearly 65% as well as the highest VS removal efficiency of 60%, making it the most viable option for increasing biogas production. Mathematical modeling results using Gompertz model and first order model predicted well with R-2 value ranging from 0.91 to 0.98, which upheld the experimentally obtained values. Findings from this study suggest that co-digestion substrates (SS:PL:FW) mixing ratio of 2:1:1 is an optimized ratio among the studied co-substrate ratio for enhanced biogas production. Furthermore, winter biogas yield is almost one sixth of summer biogas yield, which means use of temperature enhancement techniques to anaerobic digesters operating at ambient conditions would be essential to increase the biogas yield during winter.