Investigating the influence of feedstock feeding frequency and carbon-to-nitrogen ratio on anaerobic digestion of food waste in lab-scale mesophilic reactors

There is no consensus regarding the influence of feedstock feeding frequencies (FF) on anaerobic digestion (AD) of food waste (FW) and lack of study on the long-term effects of extreme carbon-to-nitrogen (C:N) ratios. This study addresses these gaps by isolating FF or C:N ratio as the sole variable in separate reactors and employing long-term operation to capture acclimatisation dynamics. While methane yield were similar at both FFs, daily-fed reactors at organic loading rate (OLR) of 4 g VS/L $·$ day exhibited instability and inconsistency in methane production rate, pH levels, and concentrations of hydrogen and carbon monoxide. Hourly-feeding improved process stability by mitigating parameter fluctuations. FW with a C:N ratio of 20–30 achieved the highest methane conversion efficiencies (90.1–94.7 % of batch reactor yield in 21 days) at OLR 3–4, and the highest methane yield (567.4 mL/gVS) was recorded in C:N = 25 reactor at OLR 4. At OLR 5, methane yield dropped, and increased ammonia-nitrogen stress (C:N = 10 reactor) and microbial shifts due to nitrogen deficiency (C:N = 35 reactor) were observed. The findings highlight FF's significant role in data consistency and process stability and the importance of acclimatisation for extreme C:N ratios. They provide insights for optimising AD operations in both research and industrial applications.