Effect of chemical treatment on biogas production from Bambara nut (Vigna Subterranea) chaff and its blend with some wastes

Abstract

A comparative study of the effect of chemical treatment on biogas production from Bambara nut (Vigna Subterranea) chaff and the chaff blended with; cow dung (BNC:CD), swine dung (BNC:SD), field grass (BNC:FG), and cassava peels (BNC:CP) in the ratio 1∶1, all treated with KOH (50% w/v) was investigated. The different variants from the treated wastes were charged into 50L metal prototype biodigesters in the ratio 3∶1 water to waste. They were all subjected to anaerobic digestion under a 65 day retention period and temperature range of 25°C–41°C. Results obtained showed that BNC had a cumulative gas yield of 186.9 ± 2.6 dm3/TMS (Total mass of slurry) with the flash point on the 5th day even though the flammability discontinued on the 9th day. Blending and chemical treatment increased the cumulative gas yields and sustained onset of gas flammability of the cow dung and field grass blends with the BNC: CD having cumulative gas yield of 223.80 ± 2.7dm3/TMS and flash point on the 26th day while BNC: FG had 207.70 ± 2.0dm3/TMS and flash point on the 56th day. However, there was no observed positive effect of the treatment on BNC: SD and BNC: CP with cumulative gas yields of 130.80 ± 2.3dm3/TMS and 132.30 ± 2.5 dm3/TMS respectively and with both systems not producing flammable biogas throughout the retention period. Results of mineral ions content showed that while most of the minerals were found in reasonable levels within the limits required, Ca2+ exceeded the limit and was actually at a level capable of inhibiting biogas production. Overall results showed that Bambara nut chaff is a highly unstable waste which blending and chemical treatment did not improve tremendously.