Integration of microbial hydrolysis by the hyper-thermophilic cellulose- degrading Caldicellulosiruptor bescii with anaerobic digestion

Integration of 15-d HRT mesophilic primary sludge (PS) digesters with hyperthermophilic hydrolysis (HTH) was evaluated in continuously stirred tank reactors (CSTRs) operated with and without enrichment of Caldicellulosiruptor bescii (CBC). Two CSTRs (R1control and R2 CBC) were operated over four phases, including periods without HTH, HTH operation at a 2-day hydraulic retention time (HRT), CBC inoculation, and reinoculation with an increased HRT of 4 days, with the HTH effluent recirculated to the mesophilic digesters. Across all phases, PS biodegradation efficiency in the R1 control was not different from the R2 CBC i.e. the recirculation of the HTH with and without CBC did not improve the methane yield, with average values ranging from 53%–64%. Offline testing of the control and test HTH reactors 2, and 14 days after CBC inoculation with cellobiose and cellulose confirmed that the CBC did not offer any improvement in the acidification rates. To further assess CBC viability and competitive dynamics, confirmatory offline batch tests conducted using PS slurry and dried PS with HTH inoculum, in the presence and absence of CBC demonstrated that CBC fermentation occurred with a more pronounced effect observed for dried PS where competition from native fermenters was reduced. A modified ADM1-based kinetic model calibrated for cellulose removal showed that under the applied operating conditions (HTH HRT 2–4 days; mesophilic HRT 15 days), more than 80% of the CBC inoculum was washed out 7 days after inoculation. Microbial analysis showed that the inoculated species (CBC) failed to achieve long-term dominance. Instead, reactor performance was dictated by native communities selected by the temperature and the substrate used