Carbon and nutrient distributions between gas, solid, and liquid in anaerobic digestion facilities for various types of organic waste

Eighty-five methane fermentation facilities that treat various types of organic waste were studied. They were categorised into seven groups based on waste type, and a questionnaire was sent to collect data on the facilities and their operation. Digester residues from 42 facilities were obtained for analysis of elemental composition. The carbon distribution into solid, liquid, and gas was determined, and the proportion in the gas is an indicator of the biodegradation rate of organic material. Digester residue contained a significant amount of nitrogen, phosphorus, and potassium, whereas heavy metal levels were negligible. Although the phosphorus content was low compared with that of commercial liquid fertiliser, it can be higher when coagulants are not used. Separated solids from digester residue were comparable with compost produced from organic solid waste, in terms of their high nutrient content. The biogas generated was used to generate power or heat, but was consumed mostly within the facilities and outside supply was limited. Considering these findings, the digester residue seems to be the more promising output. Construction and operating costs were also examined.

► 85 Japanese anaerobic digestion facilities studied by survey and residue analysis. ► Mass and element balances in facilities estimated. ► C distribution into gas was a good index of biodegradation rate. ► Significant amounts of N, P and K in effluent and sludge. ► Phosphorus content can be increased by not using coagulants.