ABSTRACT
This study was carried out to produce biogas from Cow dung, Chicken droppings and Cymbopogon citratus as well as the respective co-digestion of Cow dung and Chicken droppings with Cymbopogon citratus. 25 litre digesters and gas collection systems were designed and fabricated using locally available materials. The digesters were used to digest cow dung, chicken droppings and Cymbopogon citratus respectively as single substrates as well as to co-digest cow dung and chicken droppings respectively with Cymbopogon citratus. The respective feed materials were collected locally. They were pre-fermented, digested and analysed in accordance with standard methods. For the single substrates, the total volume of gas produced were 0.191m3 (0.032 m3/kg), 0.211m3 (0.035m3/kg) and 0.125 m3 (0.021 m3/kg) (before scrubbing) and 0.125m3 (0.021m3/kg), 0.130m3 (0.022 m3/kg), 0.090 m3 (0.015 m3/kg) (after scrubbing) for cow dung, chicken droppings and lemon grass respectively. For the co-digested substrates, Cow dung + Lemon grass produced 0.146m3 (0.024 m3/kg) before scrubbing and 0.100m3 (0.017m3/kg) after scrubbing while Chicken droppings + Lemon grass produced 0.193m3 (0.032 m3/kg) before scrubbing and 0.127m3 (0.0.021m3/kg) after scrubbing. The average ambient temperatures during the study were within the mesophilic range (20-40oC). The pH values were stable and always in the optimal range between 6.5-8.0. The reductions in total solid were 75.3 %, 60.1%, 98.2%, 61.9% and 35% for cow dung, chicken droppings, lemon grass, cow dung + lemon grass and chicken droppings + lemon grass respectively. The total coliform contents of the residue were 2.00 x107 CFU/100ml, 6.00 x 107 CFU/100ml, 1.00 x 107 CFU/100ml and 1.00 x 107 CFU/100ml for cow dung, chicken droppings, cow dung + Lemon grass, and chicken droppings + Lemon grass respectively with 95%, 70%, 95% and 99% reduction in each case. Pathogens such as Salmonella spp and Klebsiella sp. were still present in the residue although E.Coli and Shigella sp. were removed. The estimated methane contents of the gas from cow dung, chicken droppings, Lemon grass, Cow dung + Lemon grass and Chicken droppings + Lemon grass were 65.59%, 61.71%, 71.95%, 68.53% and 66% respectively. The cooking rates for water were 0.079L/min, 0.070L/min, 0.067L/min, 0.064L/min and 0.060L/min for unscrubbed lemon grass, cow dung + lemon grass, chicken droppings + lemon grass, cow dung and chicken droppings respectively while those for scrubbed lemon grass, cow dung + lemongrass, chicken droppings + lemon grass, cow dung and chicken droppings were 0.12L/min, 0.10L/min, 0.091L/min, 0.085L/min and 0.079L/min respectively. The rice cooking rates were 0.0038kg/min, 0.0034kg/min, 0.0033kg/min, 0.0031kg/min, and 0.0030kg/min for unscrubbed lemon grass, cow dung+ lemongrass, chicken droppings + lemon grass, cow dung and chicken droppings respectively while those for scrubbed lemon grass, cow dung+ lemongrass, chicken droppings + lemon grass, cow dung and chicken droppings were 0.0055kg/min, 0.0048kg/min, 0.0045kg/min, 0.0041kg/min and 0.0039kg/min respectively.