Exogenous pH Buffer System with K2HPO4/KH2PO4 Addition Improving Thermophilic High-Solid Anaerobic Digestion of Waste-Activated Sludge

Abstract

Volatile fatty acids (VFAs) and ammonia accumulation showed high inhibition for the high-solid anaerobic digestion (HSAD) of waste-activated sludge. This study investigated the effects of an exogenous pH buffer system with the addition of K2HPO4/KH2PO4 at different initial pH values in order to mitigate the inhibition of VFAs and ammonia accumulation. Results showed that the addition of K2HPO4/KH2PO4 with an optimal initial pH of 7.0 could prevent the pH from decreasing at the initial stage or rising at the final stage. In this study, 30.6% higher cumulative methane production was obtained compared to the control. A volatile solid (VS) removal rate as high as 42.3% was also achieved. Three key enzymes’ activities in the HSAD process were investigated, including alkaline protease, acetate kinase, and Coenzyme F420. Moreover, a model simulation presented that a modified Gompertz model fit the actual methane production best, as supported by the highest R2 value. The addition of phosphorus buffer substantially increased the hydrolysis rate (khyd) and maximum methane potential (fd) by parameters analysis, which explained the increased VS removal and methane production. High-throughput sequencing analysis clearly showed differences in the microbial community between samples from different treatments. Tepidimicrobium and Rhodobacter were the dominant bacterial genera involved in the HSAD system, which presented a high degradation capacity of organic material. The archaea community was dominated by Methanosaeta and Methanoculleus, and these are the typical aceticlastic methanogens and hydrogenotrophic methanogens, respectively. Herein, K2HPO4/KH2PO4 addition proved to be an effective method to overcome the problems related to the free ammonia and VFA accumulation during the sludge HSAD process under thermophilic conditions on a lab-scale setting (using a 7-L bioreactor).