Threshold and recovery of anaerobic digestion under high ammonia stress: Influence of COD/NH4+ ratio and CO2 enrichment
Mohammad Javad Bardi, Felix Müller, Daniela Polag, Nigus Gabbiye Habtu, Konrad Koch
Abstract
This study underscores the importance of maintaining an optimal chemical oxygen demand to ammonia (COD/NH 4 + ) ratio to ensure stable anaerobic digestion (AD) performance. The optimal COD/NH 4 + 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 lower COD/NH 4 + level. At lower COD/NH 4 + ratios, the acetoclastic methanogenesis is inhibited, while hydrogenotrophic methanogens remain active, as evidenced by an increase of over 96 mUr in δ 2 H-CH 4 isotope values. The CO 2 enrichment of AD under lower COD/NH 4 + ratios demonstrate to be a promising approach for AD recovery and performance stabilization. The CO 2 enrichment showed a rapid full recovery in methane production at a COD/NH 4 + ratio as low as 7.0 (6.2 g NH4+-N /L) and a slower but still effective recovery up to a COD/NH 4 + ratio of 5.0 (10 g NH4+-N /L). The δ 2 H-CH 4 isotope values from CO 2 -enriched ADs indicated a well-balanced methanogenic community, in contrast to non-enriched ADs, which exhibited clearly lower acetoclastic activity. Therefore, CO 2 enrichment is promising for enhancing AD recovery and stability at high ammonia stress, promoting methanogenic balance and boosting methane production.