Operational load enhancement for an anaerobic membrane bioreactor through ethanol fermentation pretreatment of food waste

Anaerobic digestion (AD) of food waste (FW) tends to result in the accumulation of volatile fatty acids (VFA), especially a large quantity of propionic acid that is inevitably produced. Propionic acid has a significant inhibitory effect on AD at a concentration far lower than acetic acid. Therefore, anaerobic digestion cannot be operated at an excessively high load. We proposed an AD system that used a considerable amount of starch-rich food waste for ethanol fermentation as a pretreatment and reported that it improved methane concentration in biogas with a low propionic acid accumulation even under high-load operation. This study examined whether ethanol fermentation pretreatment (EP) of food waste can contribute to high-load operation on AD using an anaerobic membrane bioreactor (AnMBR). This involved a sequential bath experiment using FW that was saccharized before fermentation to ethanol. The results are compared with those for FW without pretreatment. The organic loading rate (OLR) was stepwise increased to 43.5 g COD/L/d by increasing the feed OLR, the hydraulic retention time (HRT) was six days and methane content was 71%–74%, which is thrice the capability of the control series and the highest load in the reports of medium temperature AD of FW. Furthermore, the primary methanogens were tolerant to ammonia inhibition. Correspondingly, the ammonium produced improved the buffering capacity of AD at high load. Functional archaeal and bacterial communities in the two reactors were examined. The abundance of hydrogenotrophic methanogens in AnMBR are determined to highlight the impact of the EP for FW disposal through 16S rRNA sequence analysis. Because there were multiple methanogens that use hydrogen, the higher decomposition of the substrate was attributable to the maintenance of a lower hydrogen partial pressure. It contributed to create a virtuous cycle with the EP. This study demonstrates the performance of the EP during high loading and long-term AD operation using an AnMBR; moreover, ultra-high load is achieved by changing the metabolic pathway.