Recovery of Anaerobic Digestion Under Ammonium Inhibition: Impact of Substrate, Cod/Nh4+ Ratio, and Co2 Enrichment

Abstract

This study underscores the importance of maintaining an optimal chemical oxygen demand to ammonium (COD/NH4+) ratio to ensure stable anaerobic digestion (AD) performance. The optimal COD/NH4+ ratio ranges from 24.0 to 33.0, while a critical threshold is identified between 11.0 and 17.0, below which AD performance declines. However, the AD of protein-rich substrates typically operates under an "inhibited steady-state" condition, characterized by a significantly lower COD/NH4+ level. At lower COD/NH4+ ratios, the acetoclastic methanogenesis is significantly inhibited, while hydrogenotrophic methanogens remain active, as evidenced by an increase of over 96 mUr in δ2H-CH4 isotope values. The CO2 enrichment of AD under lower COD/NH4+ ratios demonstrate to be a promising approach for AD recovery and performance stabilization. The CO2 enrichment showed a rapid full recovery in methane production at a COD/NH4+ ratio as low as 7.0 (6.2 gNH4+-N/L) and a slower but still effective recovery up to a COD/NH4+ ratio of 5.0 (10 gNH4+-N/L). The δ2H-CH4 isotope values from CO2-enriched ADs indicated a well-balanced methanogenic community, in contrast to non-enriched ADs, which exhibited clearly lower acetoclastic activity. Therefore, CO2 enrichment is promising for enhancing AD recovery and stability at high ammonium stress, promoting methanogenic balance and boosting methane production.

Keywords: Anaerobic digestion, ammonium inhibition, inhibited steady state, carbon dioxide enrichment, process stability