Two-step partial nitrification-anammox process for treating thermal-hydrolysis anaerobic digester effluent: Start-up and microbial characterisation

Abstract

Abstract Although the thermal hydrolysis has been proven to be an effective pre-treatment method for anaerobic digestion (AD), the ammonium-rich anaerobic digester effluents are unavoidably produced by thermal hydrolysis pre-treatment AD (THP-AD). In this study, a two-step partial nitrification-anammox (PN-A) process was conducted for the anaerobic digester effluents. By combining control of the solid retention time (SRT), dissolved oxygen (DO) and hydraulic retention time (HRT) (SRT 20 d, DO concentration 0.5–0.8 mg/L, HRT 11 h), start-up of the PN reactor was achieved with a total nitrogen loading rate of 2.34 kg N/(m3·d), generating an NH4+-N/NO2−-N molar ratio of around 1.0–1.4 in the effluent. The anammox reactor was then successfully started by feeding the effluent of the PN reactor, with a total nitrogen removal rate of 1.25 kg N/(m3·d). The system became unstable after 100 days of operation, and surface analyses of the anammox sludge particles suggested that the main reason was the adsorption of microsized suspended organic flocs in the influent onto the anammox sludge particles. Further study showed that the microsized suspended organic flocs contained large quantities of bacteria, which can compete with anammox bacteria. In addition, analyses of the microbial community reveal that Nitrosomonas and Candidatus brocadia were the main ammonium-oxidising bacteria and anammox bacteria in this study, respectively. The trends of their relative abundance during the PN-A system process confirmed the successful start-up of the PN-A reactors and the effectiveness of the start-up operations. These findings are expected to provide references for the treatment of anaerobic digester effluents from THP-AD via the biological removal of nitrogen.