Solid and Soluble Organic Compounds Changes During Thermophilic Aerobic Digestion of Sewage Sludge

Over the past decade, thermophilic aerobic digestion (TAD) has been considered a practical approach to treating high-strength wastewater because of its several advantages. In this study, batch-mode TAD of sewage sludge was conducted using a lab-scale bioreactor to evaluate the changes in solid and soluble organic compounds. The TAD process achieved rapid degradation of the solid compound at 55^oC. The removal efficiency of volatile suspended solids (VSS) increased rapidly at the early operation stage and achieved 25% at 48 h. Then, the VSS removal efficiency increased gradually up to approximately 40% at the end of the digestion (168 h). As a result of VSS degradation, the soluble chemical oxygen demand (SCOD) increased significantly up to 12 g/L at 36 h and then showed a slow decline pattern. Furthermore, the pattern of protease activity was similar to that of the SCOD. This result indicates that the rapid degradation of VSS is highly linked to the increase of protease activity and thermophilic conditions. This study used acetic acid and propionic acid. The variation patterns were similar under both acetic acid and propionic acid. The concentrations of acetic acid and propionic acid rapidly increased up to 3.05 g COD/L at 48 h and 2.51 g COD/L at 36 h, respectively. Then, major volatile fatty acids concentrations declined sharply and became undetectable after 144 h. Similar to the SCOD variation pattern, the soluble total nitrogen and NH4+-N also decreased sharply after 72 h. This might be because ammonia stripping is enhanced by pH rising and continuous aeration during the TAD.