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Biogas production from blends of bambara nut (Vigna subterranea) chaff with some animal and plant wastes

Ofoefule, Akuzuo U and Onukwuli, Okechukwu D

A study on the biogas production from blends of Bambara nut (Vigna Subterranea) chaff (BNC) with some animal and plant wastes was carried out. The wastes were blended as Bambara nut chaff and Cow dung (BNC: CD), Bambara nut chaff and Swine dung (BNC: SD, Bambara nut chaff and Cassava peels (BNC: CP) and Bambara nut chaff and field grass (BNC: FG) all in the ratio of 1:1, while the BNC alone served as control. The wastes were charged into 50L capacity metal prototype digesters in the ratio of approx. 3:1 of water to waste. The moisture content of the wastes determined the water to waste ratio. The anaerobic digestion was operated under a mesophilic temperature range of 23 - 37°C within a 19 day retention period. Results obtained showed that the BNC alone had a cumulative biogas yield of 3.16dm3/kg. slurry with mean volume of 0.16±0.12dm3/kg. slurry and onset of gas flammability on the 10th day. When blended with the animal and plant wastes, the cumulative biogas yield was increased to 3.30dm3/kg. slurry with mean volume of 0.17±0.15dm3/kg. slurry and 3.27dm3/kg. slurry with mean volume of 0.17± 0.13dm3/kg. slurry for the BNC: CD and BNC: SD respectively while the yield decreased to 2.87 dm3/kg. slurry with mean volume of 0.15±0.06dm3/kg. slurry and 2.23dm3/kg. slurry with mean volume of 0.12±0.13 dm3/kg. slurry for the BND: CP and BNC: FG respectively. All the blends commenced flammable gas production within 24 hr of charging the digesters, however, the gas flammability was not sustained and gas production stopped entirely after the 19th day. General results show that blending the Bambara nut with these animal and plant wastes did not improve its gas production parameters in terms of yield, sustained gas flammability / production and retention time. Overall results indicate that the waste requires chemical treatment to improve its gas production capability since it was found to be acidic throughout the period of digestion.