Enhancing low-temperature anaerobic digestion of nitrogen-rich feedstocks: Mitigating free ammonia and short-chain fatty acid inhibitions

Abstract

High ammonia levels (>5 g N/L) have been a critical barrier in the anaerobic digestion (AD) process, with limited studies achieving long-term stability—until now. This research investigates strategies to counteract inhibition from free ammonia nitrogen (FAN) and short-chain fatty acids (SCFAs) in low-temperature AD (LT-AD) of nitrogen-rich feedstocks. LT-sequencing batch reactors (LT-SBRs) were tested under total ammonia nitrogen (TAN) concentrations up to 12.5 ± 1.56 g N/L at 24.5 ± 0.5 °C and 20 ± 0.5 °C. Results demonstrated that LT-SBR systems maintained stability, with VFA/alkalinity ratio below 1 and the propionic/acetic acid ratio ≤ 1.4. FAN/TAN conversion ratio decreased from 2.61 % at 24.5 °C to 1.17 % at 20 °C, ensuring minimal inhibition. Despite high TAN, methane production was resilient, with specific methane yields of 0.44 L CH₄/gCOD at 24.5 °C and 0.22 L CH₄/gCOD at 20 °C. These findings demonstrate that LT-AD can handle high‑nitrogen feedstocks, achieving robust methane yields and stable performance.