pH Adjustment Alleviates Ammonia Inhibition of Cell Proliferation During a Short Resting Period in Semi-continuous Anaerobic Digestion of Food Waste

Abstract

During the anaerobic digestion (AD) of food waste, the deliberate secession of substrate rapidly increases the microbial cell population, which can reach a maximum in 2–3 d. During short-term resting (STR), an increase in free NH₃ due to an increase in pH is a key inhibitor of cell proliferation; therefore, cell growth would be further promoted if free NH₃ was reduced. To explore adopting an STR technique to increase microbial cells in the AD of organic waste, we attempted to reduce free NH₃ by controlling the pH in the reactors. Two semi-continuously treated reactors were fed with food waste at a loading rate of 3.0 g-VS/L/d for 40 days and then the feeding was stopped in both reactors until day 47. One of the reactors was maintained at pH 7.37 ± 0.03, whereas pH was not controlled in the other. During STR, the cell density in the pH-controlled condition reached a maximum of 7.48 × 10¹⁰ cells/mL, which was twice as high as that before STR, and 1.7-times higher than that in the non-pH-controlled condition. These results demonstrated that mitigating NH₃ using pH can affect cell proliferation during STR.