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 CH-content biogas from anaerobic digester. However, each approach has its own technical difficulties (pH changes), and practical issues (high cost of H) to obtain > 90% CH 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 H injection. With the pressure increase, the CH content in the biogas gradually increased from 52.4% at 1 bar to 77.4% at 7 bar. However, a drop of CH production yield (MPY) was observed at 7 bar, due to the pH drop down to 6.7 by excess CO dissolution. At further operation, H injection began at 5 bar, with increasing its amount. The injection was effective to increase the CH content to 82.8%, 87.2%, and 90.6% at 0.09, 0.13, and 0.18 L H/g COD of H injection amount, respectively. At 0.25 L H/g COD, there was a further increase of CH content to 92.1%, but the MPY was dropped with pH increase to 8.7 with residual H 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 H injection. In conclusion, the integration of two approaches enabled to directly produce high calorific biogas (90% > CH, 180 MJ/m biogas) from high-strength FW at the lowest requirement of H (0.18 L H/g COD) ever reported.