Production of high-calorific biogas from food waste by integrating two approaches: Autogenerative high-pressure and hydrogen injection

Auto-generative high pressure digestion (AHPD) and hydrogen-injecting digestion (HID) have been introduced to directly produce high CH₄-content biogas from anaerobic digester. However, each approach has its own technical difficulties (pH changes), and practical issues (high cost of H₂) to obtain > 90% CH₄ containing biogas, particularly, from the high-strength waste like food waste (FW). To overcome this problem, in this study, AHPD and HID were integrated, which can offset each drawback but maximize its benefit. Substrate concentration of FW tested here was 200 g COD/L, the highest ever applied in AHPD and HID studies. At first, the reactor was operated by elevating the autogenerative pressure from 1 to 3, 5, and 7 bar without H₂ injection. With the pressure increase, the CH₄ content in the biogas gradually increased from 52.4% at 1 bar to 77.4% at 7 bar. However, a drop of CH₄ production yield (MPY) was observed at 7 bar, due to the pH drop down to 6.7 by excess CO₂ dissolution. At further operation, H₂ injection began at 5 bar, with increasing its amount. The injection was effective to increase the CH₄ content to 82.8%, 87.2%, and 90.6% at 0.09, 0.13, and 0.18 L H₂/g COD_FW.fed of H₂ injection amount, respectively. At 0.25 L H₂/g COD_FW.fed, there was a further increase of CH₄ content to 92.1%, but the MPY was dropped with pH increase to 8.7 with residual H₂ being detected (4% in the biogas). Microbial community analysis showed the increased abundance of piezo-tolerant microbe with pressure increase, and direct interspecies electron transfer contributors after H₂ injection. In conclusion, the integration of two approaches enabled to directly produce high calorific biogas (90% > CH₄, 180 MJ/m³ biogas) from high-strength FW at the lowest requirement of H₂ (0.18 L H₂/g COD_FW.fed) ever reported.

High-calorific biogas production from anaerobic digestion of food waste using a two-phase pressurized biofilm (TPPB) system

To obtain high calorific biogas via anaerobic digestion without additional upgrading equipment, a two-phase pressurized biofilm system was built up, including a conventional continuously stirred tank reactor and a pressurized biofilm anaerobic reactor (PBAR). Four different pressure levels (0.3, 0.6, 1.0 and 1.7MPa) were applied to the PBAR in sequence, with the organic loading rate maintained at 3.1g-COD/L/d. Biogas production, gas composition, process stability parameters were measured. Results showed that with the pressure increasing from 0.3MPa to 1.7MPa, the pH value decreased from 7.22±0.19 to 6.98±0.05, the COD removal decreased from 93.0±0.9% to 79.7±1.2% and the methane content increased from 80.5±1.5% to 90.8±0.8%. Biogas with higher calorific value of 36.2MJ/m³ was obtained at a pressure of 1.7MPa. Pressure showed a significant effect on biogas production and gas quality in methanogenesis reactor.