Alkaline treatment with KOH for enhanced biogas recovery from agricultural feedstock and digested residues

Recirculating the solid fraction of digestate could be an easy-to-implement strategy to increase methane production of biogas plants and further exploit expensive or scarce substrates. Solid-state alkaline treatment (at total solids content of 20 %) with 1–12 % KOH at 20 °C and a length of 1–8 days was explored as a method to enhance methane production from four solid digestate samples with the additional benefit of yielding a more valuable final digestate due to its enrichment in potassium. In comparison, the treatments were applied to a fresh commercial substrate mix. The ultimate methane yields of the solid digestate untreated samples varied between 122 and 162 ml/g of volatile solids, while that of the substrate mix was 294 ml/g of volatile solids. Post-treatments of the digestate samples resulted in ultimate methane yield changes between a 27 % decrease and a 41 % increase. These changes varied between a 4 % decrease and a 22 % increase when the substrate mix was pre-treated. In most cases, there was no evident correlation of KOH concentration and treatment duration on ultimate methane yields. However, increasing KOH concentration increased maximum methane production rates and shortened lag phase durations, while longer treatments reduced lag phase durations for solid digestate samples. Changes in maximum methane production rates and lag phase durations did not follow a clear trend for the substrate mix, and were, generally, less evident than for solid digestate. The economic viability of the process would be restricted by the high cost of KOH and low solid-liquid separation efficiency of digestate. • Four solid digestates and an undigested substrate were treated with KOH. • The treatments increased the ultimate methane yields by up to 41 %. • Reductions in the ultimate methane yields were observed in some cases. • Generally, higher KOH concentrations increased methane production rates. • Higher KOH concentrations and treatment durations decreased lag phase durations.