Optimization of Methane production from Cowpea Hull Anaerobic Co-Digestion: Moisture Content Reduction and Nanobubble Water Addition

Abstract

Cowpea hull (CH) is a by-product of cowpea food processing; its conventional use as animal feed hay declined due to flatulence characteristics. Therefore, sustainable utilization of CH for biogas production was investigated. Specifically, the effects of thermal pretreatment and nanobubble water (NBW) addition were examined during CH's co-digestion with waste-activated sludge for methane production. Oven-dried CH at 105 ℃ for 3 hours or 48 hours resulted in 83% moisture content reduction (MCR) or 100% MCR, respectively, and untreated CH at 81% moisture content was simultaneously set up as a control experiment. After 16 days of mesophilic anaerobic digestion (AD), The 100% MCR enhanced biogas production by 11% (219.14 mL/g−VS) compared to the untreated (197.93 mL/g−VS). In addition, 104.26, 114.41, and 116.15 mL/g−VS methane were produced from the untreated CH and two portions of thermally treated samples, correspondingly. Similarly, by adding N2-NBW, biogas production increased progressively, reaching the maximum daily value of 64.51 mL/d on day 3. As a result, the total accumulated biogas production from the N2-NBW reactor was 242.7 mL/g−VS, approximately 9.7 mL/g−VS more biogas and 10% increased methane production compared to 100% MCR when deionized water was applied. On the contrary, Air-NBW decreased methane production. Furthermore, the results reveal that 47.13 to 68.90 million m3 of biogas generated by co-fermentation of untreated and thermally treated CH with WAS can be converted to 75.36 GWh, which displaces 0.27% of the total fossil fuel-based electricity capacity generated in 2020. Besides, the green energy generated can mitigate methane emissions by recycling organic waste that would otherwise be landfilled.