A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering of the Asian Institute of Technology, Bangkok, Thailand

Thesis (M.Eng.) - Asian Institute of Technology, 1978

The purpose of this study was to investigate the optimum design parameters for biogas production in tropical rural villages using fixed cover type digesters which is similar to the Chinese system. Pig and poultry manures at different ratios of mixture were employed as raw materials fed to the digesters. For the batch biogas experiments (3 units) with varying pig : poultry ratios of 6.5 : 3.5, 5.0 : 5.0, 3.5 : 6.5, it was found that the highest gas production was obtained with the first ratio for a detention period of 20 days. With continuous feeding at different detention times of 20, 30 and 40 days, the following design parameters were observed to yield the most satisfactory results : Pig : Poultry ratio = 6.5 : 3.5, loading rate= 2 Kg VS/m3-day, detention time= 20 days, gas yield = 0.32 liter/g VS-day for a methane content of 71% and COD removal efficiency = 61%. Further experiments of these loading parameters with the pilot scale biogas units yielded similar results to that of the laboratory scale study. However, diurnal fluctuation in temperature was found to significantly affect the digester performance in term of gas production, but the mixed liquor pH in all digesters was within favorable range, i.e. 6.1-7.5. The red11ction of nitrogen and phosphorus after the study in batch digestion were 11% and 29%, respectively. This type of the biogas digesters was found to be relatively economic and easy in operation and maintenance.