Effectiveness of oyster shell as alkali additive for two-stage anaerobic co-digestion: Carbon flow analysis

The objective of this study was to investigate the carbon flow of two-stage anaerobic co-digestion of kitchen waste, agricultural waste, and horse dung using oyster shells as pH control conditioners compared with using alkalis. The anaerobic system consisted of hydrolysis reactor (hydraulic retention time (HRT) 5 d, 35 °C) and methanogenesis reactor (HRT 9 d, 35 °C). The addition of oyster shells led to hydrolysis of 40.33% ± 3.1% of the carbon, whereas alkali reactor hydrolysed almost half (48.68% ± 1.4%). In methanogenesis stage, the highest methane yield under pH adjustment by oyster shells was 580 mL/gVS; 51.05% of the carbon in methanogenesis reactor was converted into methane, generating 1.49–2.00 kWh/kgVS of energy. In comparison, methane yield with NaOH as an alkali was higher (667 mL/gVS), and 59.71% of the carbon was converted into methane, generating 1.97–2.55 kWh/kgVS of energy. To facilitate the digestion of hydrolysate produced using NaOH, we had to remove excess sludge because of sludge accumulation, whereas this was not necessary in oyster-shell treatment. Although NaOH conditioner had higher methane production performance, the oyster-shell conditioner can function as a buffer that regulates itself to keep the pH of the reactor stable with a rapid increase in the hydrogen-ion concentration.