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

A mesophilic anaerobic digestion reactor fed with maize straw was investigated in a plug flow reactor for 150 days. The process performance was evaluated through the stable, unstable, and recovery stages. Results obtained in this study showed that the reactor maintained stable within approximately two months, but the gradual depletion of trace elements (Fe, Co and Ni) and significant accumulation of volatile fatty acids (8.07 g/L) occurred in the reactor by the 120th day. Pig manure containing higher trace elements, therefore, was mixed with maize straw at a ratio of 1:4 on a dry matter basis to feed the reactor. As a result of the consequent reactor recovery, volatile fatty acids decreased significantly, and biogas production increased. During the subsequent stable operation stage, a methane yield of 0.21 L/g-VSadded was obtained and methane content stabilized at 54%. Additionally, the uptake of the elements was determined quantitatively, highlighting the necessity of mixing maize straw with pig manure. Therefore, it is imperative to co-digest maize straw with trace element-rich substrates or supply trace elements to ensure the long-term stability of the anaerobic digestion process.