Influence of recirculation of liquid fraction of the digestate (LFD) on maize stover anaerobic digestion

Influence of recirculation of the liquid fraction of the digestate (LFD) on anaerobic digestion of maize stover as sole substrate was investigated. Digestion without recirculation (R1) was compared with two LFD recirculation modes, direct recirculation (R2) and aerated recirculation (R3). There were no significant differences in biomethane production, biomethane yield, substance reduction rates, and pH. The biomethane yield in R1, R2, and R3 reached 73.8%, 68.2%, and 71.8% of the theoretical methane potential (TMP), respectively, and 55–60% of cellulose and hemicellulose were converted into biogas in three runs. Chemical oxygen demand (COD) and ammonia nitrogen (NH3–N) concentrations gradually increased in R2 over the whole period of digestion (360 days), but stabilised in R1 and R3 by the end of the digestion (225th−360th day). Aerated recirculation performed best in terms of COD reduction, and low NH3–N accumulation for long term operation, and therefore is recommended. The study indicated that it is technically acceptable to partially recirculate the liquid fraction for anaerobic digestion of maize stover, with the potential to reduce LFD discharge and avoid potential pollution associated with LFD.