Regulation of propionate degradation in anaerobic reactors: Roles of sludge retention time and organic carbon composition across different operational configurations

Abstract

Propionate accumulation is a challenge in anaerobic digestion due to the syntrophic lifestyle, slow growth rate, and environmental sensitivity of syntrophic propionate-oxidizing bacteria. The effects of sludge retention times (SRTs; i.e., 10 and 25 days, and without desludge) and carbon source compositions (a mixture of ethanol/acetate/propionate or sole propionate) on propionate degradation and microbial community structure were examined in sequencing batch reactors (SBRs) and continuous flow reactors (CFRs). SBRs outperformed CFRs in methanogenic propionate degradation, with nearly complete chemical oxygen demand (COD) removal achieved by day 13 (SBR with a mixture of ethanol/acetate/propionate) and day 41 (SBR with sole propionate), compared to day 54 in the CFR with sole propionate. The COD removal efficiency in the CFR with a mixture of ethanol/acetate/propionate stabilized at approximately 70 %. Reactors with a 10-day SRT were unable to efficiently remove propionate across different operational modes and carbon sources. Batch experiments showed that reactors utilizing mixed carbon sources exhibited shorter lag phases, increased acetate degradation activities, and higher maximum methane production rates compared to those using the sole propionate. SBRs enriched Syntrophobacter (16.0 %–22.8 %) and Desulfobulbus (4.1 %–5.0 %), whose relative abundances in CFRs were only 4.1 %–13.5 % and 1.0 %–1.2 %, respectively. Additionally, the relative abundance of genes involved in propionate oxidation increased by 9.7 %–47.0 % in SBRs compared to CFRs. A strategy involving the utilization of long SRTs, SBR operation mode, and mixed carbon sources was proposed to improve system startup and propionate removal in anaerobic reactors.