Optimization of Biogas Production from Cow Dung using Response Surface Methodology

Biogas is produced through anaerobic digestion of organic wastes. Hydrolysis, the first step of anaerobic digestion, is rate limiting due to the presence of complex polymers in biomass which results in long retention period and low efficiency. Pretreatment is a process in which the organic waste is made ready for microbial attack before entering the anaerobic digester in order to encourage faster digestion and improvement of yield. This research was conducted on the ultrasonic pretreatment of cow dung for production of biogas, where Response Surface Methodology (RSM) was used to achieve the optimum sonication time, slurry ratio and retention time. 10 experimental runs were developed according to Central Composite Design with different set up conditions to observe the Responses, i.e., volumes of biogas produced across 21 days. 30 runs were further generated from the responses in order to optimize the retention time (days) for biogas production. This was achieved with the aid of a software package (Design Expert 12.0.1.0). Quadratic models were developed for the responses and a 3D response surface plot was obtained to analyze the effect of the variables and their interactions to determine their optimum levels. The numerical optimization results gave a sonication time of 25 minutes, slurry ratio of 1.85 and retention time of 18 days. Under these conditions, the predicted maximum volume of biogas production was 18.97 ml. The validation of this prediction was tested, and the volume of biogas produced under same conditions was 17.772 ml, which confirmed the adequacy of the predicted model as only a 6.75% error was recorded. Hence, the optimum sonication time, slurry ratio and retention time were achieved for maximum biogas production from cow dung.

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