Low-temperature anaerobic digestion of chicken manure at high organic and nitrogen loads - strategies for controlling short chain fatty acids

• Tracking SCFA indicators prevented digester failure at early stage. • Acclimatised liquid inoculum and the mode of operation minimized SCFA accumulations. • Controlling SCFA/alkalinity ratio ensured digester stability with SMY > 0.6 LCH 4 /gVS. • Applied protocol can promote adoption of high-solids AD of chicken manure at 20 ± 1°C. Objective of this work was to investigate the technical feasibility of low-temperature, closed-loop two-stage (liquid–solid) anaerobic digesters to treat chicken-manure (TS:68%; NH 3 :8 g/L) as a sole-feedstock. Effect of pH, temperature, treatment-duration, organic loading rate (OLR) and inoculum-recirculation ratio on short chain fatty acids (SCFA) production was studied. Digesters were operated at 20 ± 1 °C for 282-d over 4 batch-runs (∼70-d/batch) at an OLR of 8.78–4.3 gVS/L/d. Results showed that specific methane yield above 0.6 LCH 4 /gVS was feasible with a methane concentration > 60%. SCFA speciation of the entire system was monitored through the liquid-digester. Among SCFA indicators, the ratios of propionic-to-acetic acids, (butyric + valeric)-to-acetic acids, and total SCFA-to-alkalinity were observed within the limit, i.e., below 1.4, 0.3 and 0.8, respectively, indicating high-digester stability. This strategy allowed early detection, diagnosis of process failures in high-solids digester in fed-batch mode, and re-evaluation of operating protocol to enrich performance with economic-benefits.