Volatile Fatty Acid Accumulation by Alkaline Control Strategy in Anaerobic Fermentation of Primary Sludge

Effects of pH (5–12) on primary sludge (PS) fermentation and volatile fatty acid (VFA) accumulation were studied in batch experiments. Results showed that under strong alkaline conditions (pH 10–12), hydrolysis could be accelerated and high soluble chemical oxygen demand (SCOD) produced, but acidification was inhibited, resulting in a decrease of VFA production; whereas in weak base (pH 8, 9) conditions, more VFAs accumulated, although less SCOD was produced. Optimal conditions for PS to accumulate VFAs in this study were pH 9 and a fermentation time of 10 days. The study also revealed that protein degradation played an important role in VFA production. More protein was hydrolyzed and acidified at pH 9 than at pH 10 in the system, although protein content at pH 10 was higher. Acetic and propionic acids were the dominant acid species. Furthermore, a comparison of VFAs, NH4+, and pH relationships in pH 9 and pH-uncontrolled systems showed that weak base conditions could significantly enhance the processes of sludge solubilization and protein and carbohydrate degradation, and thus reduce VFA consumption. This, eventually, resulted in high VFA accumulation in fermentation systems.