The influence of pH on hydrolysis and acidogenesis of kitchen wastes in two-phase anaerobic digestion

Batch and semi-continuous experiments were carried out to determine the effect of pH on hydrolysis and acidogenesis of kitchen wastes in the two-phase anaerobic digestion. The results of the batch experiment showed pH adjustment could improve both hydrolysis and acidogenesis rates of kitchen wastes. Compared with pH at 5, 9 and 11, pH 7 provided an optimum working condition for anaerobic digestion of kitchen wastes. At pH 7, about 86% of the total organic carbon (TOC) and 82% of chemical oxygen demand (COD) were solubilized and the maximum volatile fatty acid (VFA) concentration of 36 g V was achieved on the fourth day. The total VFA yield was 0.27 g (g total solid (TS))(-1), which almost doubled the yield without pH adjustment (0.15 g (g TS)(-1)). The acidified products with lower concentration of lactic acid were more favorable to the subsequent methanogenesis. At pH 7, most of the protein was degraded into ammonia nitrogen (NH4+-N), resulting in additional buffer of acidified liquid. According to this, a novel method of pH adjustment applying NH4+-N buffer was established. The semi-continuous experiment indicated that adjusting pH to 7 in the acidogenic stage in the two-phase anaerobic digestion process would increase both TS loading rate (16g l(-1) d(-1)) and recovery of biological energy (520 ml methane (CH4) (g TS)(-1)).