Mixing intensity regulates kitchen waste anaerobic digestion through carbon flux and nitrogen metabolic pathways

Based on previous work on flow pattern dimensionality reduction, the control constant k was used to establish a quantitative bridge between the macro-hydraulic field and the micro-metabolic mechanism. Three impeller configurations with impeller diameter ratios of 0.4, 0.7, and 0.9, corresponding to control constant k of 33.3, 11.6, and 8.3, respectively, were selected for continuous anaerobic digestion of kitchen waste. At an organic loading rate of 2.5 g VS/L/d, the average methane production rates of the reactors with impeller diameter ratios of 0.7, 0.9, and 0.4 were 418.9, 363.7, and 356.5 mL/g VS, respectively. The reactor with an impeller diameter ratio of 0.4 achieved the highest volatile solids removal efficiency of 71.1%, compared with 65.7% and 65.9% for the reactors with impeller diameter ratios of 0.7 and 0.9, respectively. Weak mixing enhanced glucose phosphorylation in the early stage of glycolysis. The relative abundance of the key enzyme (EC: 2.7.1.147) under the mixing conditions (k = 11.6) catalyzing the conversion of d-Glucose to α-d-Glucose-6P was approximately 80.00% lower than that in the weak mixing condition. In contrast, increasing mixing markedly increased the abundance of key enzymes involved in pyruvate-to-acetate conversion, promoting carbon flux diversion from pyruvate to acetate and possibly be benefit improving methane production rate. Mixing intensity regulated carbon flux by controlling the relative abundance of key acid-metabolizing enzymes. High mixing intensity enhanced acetate production, whereas butyrate metabolism was affected by multiple regulatory factors. Weak mixing strengthened nitrogen metabolism and alleviated ammonia inhibition. However, nitrogen metabolism and methanogenesis may have competed for carbon resources, leading to carbon flux redistribution. This study provides new insights into the hydrodynamic regulation of anaerobic digestion.