Investigation of the effect of C/N, material loading rate and pH on biogas production from animal wastes and rice straw

One of the useful and innovative solutions to solve the problems of agricultural and animal wastes is biogas production from these materials that is valuable and environmentally friendly fuel. In this study, the factors affecting biogas production including C/N ratio, loading rate, retention time and pH were investigated in combination of poultry manure, cattle dung and rice straw. The experiments were carried out in three stages and in each stage three digesters were used in each experiment. The first digester contained the combination of cow manure and rice straw, the second digester contained the combination of cow manure, poultry manure and rice straw, and the third digester was devoted to the cow manure. The ratio of materials to water was approximately 1 to 2 in all digesters. The results showed that the maximum amount of biogas (11200 ml) obtained at the first digester with C/N=27.1, and the minimum biogas (2717.5 ml) was produced in the second digester with C/N=9.81. It was determined that the large reduction in C/N caused a sharp drop in the amount of biogas production so that in the second test at digesters No.2 with C/N=9.81, the rate of biogas production dropped to 97.83 liters. At the third digester with 3 liter loading rate per three days, the maximum biogas production was 472.6 liter during 36 days. Also, the closer pH to neutral resulted in higher performance. Cow manure was resistant against pH changes so it can be used as a stabilizer of the digestion process. In general, adding rice straw to cattle dung increased the amount of produced biogas. Flame test showed that the highest amount of methane in the biogas is in the middle of digestion process.