Effect of Mechanical Stirring System on Biogas Production

A laboratory-scale stainless-steel horizontal biogas digester was designed, fabricated, and installed at the biogas laboratory of the Agricultural Engineering Department, Faculty of Agriculture, Mansoura University, to study the effects of stirring speed and time on biogas production from buffalo feces. Three stirring speeds (40, 80, and 120 rpm) and three mixing times (15 minutes every 1, 2, and 4 hours) were tested under mesophilic conditions (40°C), 12% total solids, and a 27-day hydraulic retention time. Daily and total biogas yield, and methane content were measured. Calorific value was calculated. Chemical analysis and biochemical oxygen demand (BOD) tests were also conducted on influent and effluent slurries. The results indicated that mixing improved digester performance, with the highest biogas yield and methane content at 80 rpm with a 15-minute mixing interval every 2 hours. According to chemical analysis, the effluent slurry from different experiments can be used as an effective organic fertilizer. It contains high levels of plant nutrients and organic matter, which can lead to higher crop yields and increased soil microbial activity. These findings highlight the importance of optimized agitation for enhancing anaerobic digestion efficiency and biogas quality.