Methane fermentation of bean curd refuse

The methane fermentation of bean curd refuse was studied in a 1 l reactor with a draft tube which was operated in a fed-batch mode with a once-a-day feeding cycle using two kinds of methanogens. The effects of substrate loading rate on the methane yield and on the behaviors of the product species were examined. A unified approach was employed for the material balance between the substrate (reactant) and product species based on the elemental carbon content in the species being considered. The classified product species are methane and carbon dioxide in the gas phase, soluble TOCs excluding VFAs, ICs and VFAs in the liquid phase and cells, and the unconverted substrate in the solid phase. It is found that the methane yield increases with the increasing substrate loading rate, reaches a maximum and then decreases with a further increasing loading rate. The maximum methane yield is found to be as high as 53.7% which is very close to the theoretical yield of 55%. It is also found that there is a critical substrate loading rate beyond which the operation becomes impossible due to excessive accumulation of unconverted solids.