4. Conclusion
Thermophilic anaerobic digestion of protein-rich stillage at 53 °C and 60 °C with a biogas recirculation ratio of 150 and an air supply facilitated stable digestion performance and biogas production at a higher OLR of 7 g VTS/L/d in the R1 system (recirculation of biogas into the headspace of the reactor). Alternatively, the NH3 removal efficiency increased 1.23-fold when the temperature increased from 53 °C to 60 °C. Under these conditions, it was found that the CH4 content in the biogas increased by 4.7% to 70.1%, and the removal efficiency of H2S reached up to 95.1%.
In order to improve the ease of operation of the system from an engineering standpoint, the possibility of reducing the biogas recirculation ratio was investigated. At 60 °C, the biogas recirculation ratio could be decreased in the R1 and R2 systems (recirculation of biogas into the liquid of the reactor) to 50 and 10, respectively, 4. Conclusion
Thermophilic anaerobic digestion of protein-rich stillage at 53 °C and 60 °C with a biogas recirculation ratio of 150 and an air supply facilitated stable digestion performance and biogas production at a higher OLR of 7 g VTS/L/d in the R1 system (recirculation of biogas into the headspace of the reactor). Alternatively, the NH3 removal efficiency increased 1.23-fold when the temperature increased from 53 °C to 60 °C. Under these conditions, it was found that the CH4 content in the biogas increased by 4.7% to 70.1%, and the removal efficiency of H2S reached up to 95.1%.
In order to improve the ease of operation of the system from an engineering standpoint, the possibility of reducing the biogas recirculation ratio was investigated. At 60 °C, the biogas recirculation ratio could be decreased in the R1 and R2 systems (recirculation of biogas into the liquid of the reactor) to 50 and 10, respectively, without decreasing anaerobic digestion performance. The NH4+ absorption rate of 8.3 mmol/L/d obtained at the biogas recirculation ratio of 10 in the R2 system was higher than the rate of 7.8 mmol/L/d, which was yielded at the ratio of 150 in the R1 system. The H2S content in the biogas was also reduced to less than 50 ppm by supplying air at 3% of the amount of biogas produced into the reactor. Consequently, the process for simultaneous decrease in NH3 and H2S inhibition could be achieved in thermophilic anaerobic digestion, and the protein-rich stillage was efficiently treated.