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

Abstract

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.