Solid-state anaerobic co-digestion of poultry litter and grass silage: Influence of nitrogen source and feedstock/inoculum ratio

Poultry litter (PL) has a high potential for energy recovery through anaerobic digestion, but excess nitrogen limits the process, making co-digestion with carbon-rich substrates like grass silage (GS), a promising alternative. This study evaluated solid-state anaerobic co-digestion of PL and GS, focusing on the effects of feedstock-to-inoculum ratio (F/I) and nitrogen effects on methane production and process stability. Biochemical methane potential assays were conducted under mesophilic conditions, 20% total solids, F/I ranging from 1.0 to 2.0, and GS:PL proportions of 100:0, 90:10, 70:30, and 50:50. The best performance occurred at F/I of 1.5 with C/N between 17/1 and 20/1, reaching methane yields of up to 137 Nm 3 CH4 t VS -1 and 34 m 3 CH4 t residue -1 . Up to 30% PL could be incorporated without compromising methane production, provided the F/I was at least 1.5 with no impact of the AD performance. Notably, methanogenic activity remained stable even at free ammonia nitrogen (FAN) concentrations exceeding 900 mg kg⁻¹, well above thresholds typically reported as inhibitory in liquid AD. Microbial analysis showed a predominance of hydrolytic and fermentative bacteria, mainly Firmicutes and Bacteroidota, while Methanothrix was the main methanogenic archaeal genus, suggesting dominance of the acetoclastic pathway. The results demonstrate that SS-AD of PL and GS not only achieves competitive methane yields but also withstands ammonia levels above conventional inhibition thresholds, reinforcing its potential for full-scale poultry litter management, when the organic load, C/N, and substrate proportions are properly balanced.