Maintaining the long term stability of anaerobic digestion of maize straw in a continuous plug flow reactor by verifying the key role of trace elements

Abstract

Abstract A mesophilic anaerobic digestion bioreactor fed on maize straw was investigated in a plug flow reactor for 150 days. The process performance was evaluated through the stable, unstable, and recovery phases. Results showed that the system maintained stable performance within approximately two months but the following trace elements; Fe, Co, and Ni gradually declined and the volatile fatty acids accumulated to 8.07 g/L by the 120th day of operation. Pig manure containing higher trace elements, therefore, was mixed with maize straw at a ratio of 1:4 on a dry matter basis before feeding. The reactor recovery was observed with a significant downtrend of volatile fatty acids and an uptrend in biogas production. Upon recovery of the reactor and stable operation condition, a methane yield of 0.21 L/g-VSadded was obtained. Methane content stabilized at 54%. The quantitative utilization of the three elements on a unit mass of COD degraded was determined. Maintaining long-term stability was still a challenge without determining the minimum additional pig manure required. Conclusively, the co-digestion of trace elements-rich substrate or the addition of the trace elements into the substrate is required for the anaerobic digestion of straw.