Mesophilic and thermophilic fermentation of activated sludge for volatile fatty acids production: focusing on anaerobic degradation of carbohydrate and protein

The volatile fatty acids (VFAs) productions, as well as particulate organics decomposition, soluble chemical oxygen demand (SCOD) yield, and the VFAs production pathways from mesophilic and thermophilic anaerobic fermentation in waste activated sludge were investigated. Batch experiments showed that the decomposition rate of volatile suspended solids (VSS), particulate carbohydrate (P–C) and particulate protein (P–P) followed the first-order kinetic model at different temperatures. However, the intermediates, accumulated in the process of protein or carbohydrate digestion had a more significant inhibitory effect on the production of VFAs during the mesophilic anaerobic acidification process. The production of VFAs by thermophilic anaerobic fermentation is 2086.05 mg COD/L, which is about twice the production under mesophilic conditions. Among them, the concentration and proportion of high molecular weight organic acids such as isobutyric acid (320.29 mgCOD/L) and isovaleric acid (745.75 mgCOD/L) are relatively high. Then ¹³C stable isotope labelling experiment demonstrated that, the decomposition of carbohydrates yields 77% acetic acid and 86% butyric acid, while protein breakdown produces 85% propionic acid and 99% valeric acid. This confirms that carbohydrates are more favourable for the formation of even-carbon organic acids, while proteins tend to yield odd-carbon organic acids. Additionally, this helps refine the pathway for valeric acid formation during anaerobic acidogenesis.

本研究针对剩余活性污泥的中温与高温厌氧发酵过程，探究了挥发性脂肪酸（volatile fatty acids, VFAs）的生成水平、颗粒态有机物的降解特性、可溶性化学需氧量（soluble chemical oxygen demand, SCOD）的产率，以及VFAs的生成路径。批式实验结果表明，不同温度下挥发性悬浮固体（volatile suspended solids, VSS）、颗粒态碳水化合物（particulate carbohydrate, P–C）与颗粒态蛋白质（particulate protein, P–P）的降解速率均符合一级动力学模型。不过，在中温厌氧酸化过程中，蛋白质或碳水化合物消化阶段积累的中间产物对VFAs的生成具有更为显著的抑制作用。高温厌氧发酵的VFAs产量可达2086.05 mg COD/L，约为中温条件下产量的两倍；其中，异丁酸（isobutyric acid，320.29 mgCOD/L）与异戊酸（isovaleric acid，745.75 mgCOD/L）等高分子量有机酸的浓度与占比均相对较高。随后开展的¹³C稳定同位素标记实验证实，碳水化合物降解可生成77%的乙酸与86%的丁酸，而蛋白质降解则可生成85%的丙酸与99%的戊酸。这表明碳水化合物更利于偶碳数有机酸的生成，而蛋白质则倾向于产生奇碳数有机酸。此外，本研究结果有助于进一步完善厌氧产酸阶段戊酸的生成路径。