Hydrothermal pretreatment of protein-rich substrate: Modified physicochemical properties and consequent responses in its anaerobic digestion

Wasted tofu rich in protein was subject to hydrothermal pretreatment (HTPT) under different conditions (at 120, 140, 160 and 180 ℃; for 0, 30, 60 and 90 min) followed by biochemical methane potential (BMP) tests, and 140 ℃ and 0 min were found to be respectively the best temperature and duration for HTPT of tofu in terms of its biogas production. Under the under the optimal conditions (140 ℃, 0 min) the accumulative methane yield reached up to 510.9 mL·(gVS)^-1, which was 26.98 % higher than that without HTPT (402.3 mL·(gVS)^-1). The start-up process of continuous anerobic digestion (AD) of the tofu before and after hydrothermal treated (HT) at the optimal HTPT conditions (140 ℃, 0 min) was examined, to investigate and compare how their consequent AD responded to HTPT. It was found that, for start-up of continuous AD, the HT tofu delivered more balanced nutrients and thus led to more stable AD and quicker biogas production. Unavoidably, HTPT generated products refractory to biodegradation, to slightly decrease the total biogas production. During AD of HT tofu some weak ammonia-tolerant microbes, such as methylotrophic methanogens, survived and played indispensable roles. Analyses of living microbial community structure indicated that, some hydrolytic acidification bacteria intolerant to ammonia nitrogen (such as Proteobacteria) were always active and appeared at high proportion. The viable methylotrophic methanogens, e. g. RumEn M2, took obvious responsibilities in start-up of the AD for HT tofu.