Effect of fermentation type regulation using alkaline addition on two-phase anaerobic digestion of food waste at different organic load rates

In two-phase anaerobic digestion (TPAD) of food waste, alkaline addition is commonly used to alter fermentation types aiming to improve the subsequent methanogenesis. However, alkaline usage could also cause the accumulation of salt and inhibit methanogens. To discover the opposite effect, a series of continuous TPAD experiments with automatic pH control were conducted at different organic load rates (OLRs). The results indicated that pH regulation was not always effective for TPAD. At the OLR of 1.9 g/(L·d), mixed acid fermentation at pH 6.0 and lactic acid fermentation at pH 4.5 were achieved with the average NaOH dose of 1.69 and 2.45 g/(L·d), respectively, and the subsequent methane production increased to 460 and 482 ml/g in comparison to 380 ml/g in single-phase anaerobic digestion (SPAD). At the OLR of 2.4 g/(L·d), the Na+ concentration in methanogenic phase increased more than 3.5 g/L, resulting in the deterioration of methane production. At the OLRs higher than 3.2 g/(L·d), pH control cannot be applied to TPAD, and SPAD suffered from excessive acidification. Thus, TPAD without pH control became the only choice, but its methane yield was only 397 ml/g, implying new methods should be considered to improve TPAD.