Effect of thermal pretreatment and nanobubble addition water on methane production from cowpea hull anaerobic digestion

Residual cowpea hull (CH) from akara (bean cake) processing constitutes a growing organic waste in countries with high production rates. CH has an unpleasant smell but rich in carbon, nitrogen, and micronutrients, which makes it a potential feedstock for energy recovery via anaerobic digestion (AD). However, the effect of the high antinutrient content, such as trypsin inhibitor (TI) previously reported in CH, and other parameters like moisture content on methane production need to be evaluated to promote its utilization as an AD feedstock. Anaerobic co-digestion of thermally pretreated cowpea hull and waste-activated sludge was investigated in this study, with a focus on the enhancing effect of thermal pretreatment (TP) at 83 and 100% moisture content reduction (MCR) and nanobubble water (air and N2-NBW) addition on methane production. TP of CH under 100% MCR increased methane production by 12.9% compared to fresh CH. Additionally, replacing deionized water with N2-NBW further enhanced methane production by 20.9%. However, the effect of Air-NBW on methanogenesis was not significant. Improved CH biodegradability and the suppression of TI by TP may have facilitated the F/M interaction rate. However, the mechanisms of TI on microbial activity in an anaerobic digester need to be clarified. The electric power generating potential from the methane obtained in this study was also estimated to promote sustainable disposal of CH as a waste management technique and green energy recovery.