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

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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 200g 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 7bar without H2 injection. With the pressure increase, the CH4 content in the biogas gradually increased from 52.4% at 1bar to 77.4% at 7bar. However, a drop of CH4 production yield (MPY) was observed at 7bar, due to the pH drop down to 6.7 by excess CO2 dissolution. At further operation, H2 injection began at 5bar, 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 0.18L H2/g CODFW.fed of H2 injection amount, respectively. At 0.25L H2/g CODFW.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, 180MJ/m3 biogas) from high-strength FW at the lowest requirement of H2 (0.18L H2/g CODFW.fed) ever reported.