Management of disturbances in agricultural digesters by post-digestion, total solid adjustment, and recirculation

• Remedies to manage process disturbances in agricultural digesters are evaluated • Post-digestion improved nitrogen mineralization and buffered disturbances • Total solid content adjustment within 2 to 7% of total weight proved necessary • Post-digester liquid for mixing with substrate reduced pH sensitivity of the process • Recirculation from post to main digester boosted process stability and efficiency Achieving process stability and efficiency are critical challenges in expanding the application of anaerobic digestion for biomethane production and nutrient recovery from agricultural residues. This study evaluated a series of methods for managing reoccurring disturbances during the operation of two serial laboratory continuous stirred tank reactors as a main digester and a post-digester, along with regular monitoring of enzymatic activities and microbial community dynamics. The study took place over 661 days, from February 2021 to December 2022, and six different operational phases with operational challenges including declining biogas production, loss of alkalinity, pH fluctuations, accumulation of intermediate degradation products, bulking and foaming. A total solid (TS) content within 2 to 7% in the main digester prevented disturbances and ensured efficient methane production. Using water to blend agricultural residues as the substrate can lead to a loss of alkalinity and increase process sensitivity to pH fluctuations. The liquid fraction after phase separation of digestate from the post-digester is a viable alternative to water, also mediating pH fluctuations. Recirculating from the post-digester to the main digester at rate of 0.3-0.8 g volatile solid (VS)/L/day created a homogenous microbial community across the serial digesters, improved alkalinity, and pH stability, and enhanced organic nitrogen mineralization. The serial reactors achieved a specific biomethane production of 270±30 mL/gVS/d, with a production of 211 ±13 mL/gVS/d in the main digester and 97 ± 36 mL/gVS/d in the post-digester. The study highlights the importance of a post-digester to improve substrate degradation and stabilize agricultural processes during operational instabilities.