Comparison of solid-state anaerobic digestion to composting of yard trimmings with effluent from liquid anaerobic digestion: Effect of total solids content and feedstock to effluent ratio

Solid-state anaerobic digestion (SS-AD) and composting of yard trimmings with effluent from liquid anaerobic digestion were compared at a range of total solids (TS) contents and feedstock to effluent (F/E) ratios under thermophilic condition. For SS-AD, TS contents of 22%, 25%, and 30% were studied, while for composting TS contents of 35%, 45%, and 55% were sued. Experiments were conducted at various feedstock to effluent (F/E) ratios including 2, 3, 4, 5, and 6. High TS content negatively affected degradation rates as indicated by low biogas and carbon dioxide yields for SS-AD and composting, respectively. 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 VS_feedstock obtained at 22% TS. Methane production from SS-AD was low at low F/E ratios (i.e. 2 and 3), likely due to inhibitory effects of high concentrations of ammonia nitrogen (up to 5.6 g/kg N). Nitrogen-rich (N, P, K) end products with similar N-P-K values but different dominant forms of nitrogen were generated from SS-AD and composting.