Recovering Carbon Source from the Thermophilic Aerobic Digested Sludge for Biological Nutrient Removal
Available carbon source is essential to enhance the biological nutrient removal (BNR) in wastewater treatment. Volatile fatty acids (VFAs) accumulated in the thermophilic aerobic digestion (TAD) supernatant under the microaerated operation is the potential carbon source for denitrification and phosphorus removal. This study investigated the use of VFAs recovered from the TAD supernatant in the BNR processes. Batch tests (2.8 L/batch, simulating the anaerobic, anoxic and aerobic conditions in sequences) and continuous feed simplified UCT processes (28.8 L/d) were conducted to evaluate the system performance with the TAD supernatant addition. Batch tests showed that in the anaerobic condition, the TAD supernatant performed comparable rate of P release as the pure sodium acetate addition cases. Denitrification rate was also enhanced by the TAD supernatant addition. P was then totally removed in the following aeration stage. Ammonia-N introduced from the TAD supernatant was entirely converted to NOx-N in the following aerated stage. In the continuous feed UCT experiments, the TAD addition performed comparable P and N load removal efficiencies at an 1.5 % of Q feeding rate. Higher rate (10% of Q) of TAD supernatant addition in the anaerobic zone resulted in higher P and N loading removals. However, high SCOD and colour deteriorated the effluent quality. Higher aeration and denitrification capacity demands are necessary to ensure the system performance in the TAD supernatant addition cases. The TAD supernatant must be used as fresh as possible after the preparation to prevent the VFAs diminishment during the storage and maximum the benefit of VFAs utilization.