Comparison of digestate from solid anaerobic digesters and effluent from liquid anaerobic digesters as inocula for solid state anaerobic digestion of yard trimming

Abstract. To select a proper inoculation method for the solid state anaerobic digestion of yard trimming with increased methane yield and stability, digestate from solid state anaerobic digesters and dewatered effluent from liquid anaerobic digesters were compared as inocula under different substrate-to-inoculum (S/I) ratios from 0.2 to 2, and total solids (TS) contents from 20% to 35%. The highest methane yield of around 244 L/kgVSfeed was obtained at an S/I of 0.2 and TS content of 20% for both types of inocula. The highest volumetric methane productivity was obtained with dewatered effluent at an S/I ratio of 0.6 and TS content of 24%. The two types of inocula were found to be comparable for methane yields and volumetric methane productivities at each S/I ratio, while using dewatered effluent as inoculum reduced the startup time. S/I ratio of 1 was a critical level and should be set as the upper limit of the inoculation ratio for mesophilic SS-AD of yard trimming.