Comparison of solid-state anaerobic digestion and composting of yard trimmings with effluent from liquid anaerobic digestion

Solid-state anaerobic digestion (SS-AD) and composting of yard trimmings with effluent from liquid AD were compared under thermophilic condition. Total solids (TS) contents of 22%, 25%, and 30% were studied for SS-AD, and 35%, 45%, and 55% for composting. Feedstock/effluent (F/E) ratios of 2, 3, 4, 5, and 6 were tested. In composting, the greatest carbon loss was obtained at 35% TS, which was 2–3 times of that at 55% TS and was up to 50% higher than that in SS-AD. In SS-AD, over half of the degraded carbon was converted to methane with the greatest methane yield of 121 L/kg VSfeedstock. Methane production from SS-AD was low at F/E ratios of 2 and 3, likely due to the inhibitory effect of high concentrations of ammonia nitrogen (up to 5.6 g/kg). The N–P–K values were similar for SS-AD digestate and compost with different dominant nitrogen forms.