Abstract
To introduce a methane production process in a small-scale sewer plant, thermophilic high solid codigestion of excess sludge and rice straw was proposed, and the effect of rice straw addition was examined by a pilot scale plant. Under the condition of input sludge concentration 9 % (mixture of excess sludge from an oxidation-ditch 6% and rice straw 3%) at SRT 15 days, stable methane production was observed, though the ammonium nitrogen (TAN) concentration increased over 2,500 mg/L. The methane conversion rate and methane yield were 0.35 and 0.186 Nm3/kgVS, respectively, which were agreed with the calculated vales from the biomethane potential of each biomass obtained by a batch experiment. Uncultured bacteria MBA08 belonging to Class Clostridia occupied 36.7-54.6 % in the digested sludge. High ammonium tolerance archaea, Methanosarcina thermophila was detected at high frequency (84-89%). Even though the viscosity increased by 10 times according to increase in TS concentration, the contents of the digester was well mixed. Although improvement effect of dewaterability of the residue was smaller than that observed in the methophiric digestion, water content of dewatered sludge was lower than those of input sludge and inoculum.
