Effective mitigation of ammonia in sewage-sludge-derived fermentation liquid using flow-electrode capacitive deionization

Abstract

Due to the high toxicity of ammonia to organisms and its contribution to eutrophication in surface water, the risk of emission of ammonia and other nitrogenous ions to the environment and ecosystems has aroused wide concerns. Therefore, the discharge criterion on nitrogen in effluent from conventional wastewater treatment plants (WWTP) is very stringent. Furthermore, during the conventional denitrification processes, the relatively costly external carbon source is usually required. Nowadays production of volatile fatty acids (VFAs) from sewage sludge by alkaline anaerobic fermentation has regarded as an attractive carbon source. However, usually ammonia is quite abundant in the fermentation liquid and thus effective mitigation of ammonia in the fermentation liquid is also a significant step for its further utilization. In the present study, the flow electrode capacitive deionization (FCDI) was applied to remove ammonia in the fermentation liquid of sewage sludge. Firstly, response surface method (RSM) was employed to optimize parameters and then the performance of the FCDI in ammonia removal were examined. Results showed that optimal flow rates, carbon content and ammonia concentration were 8.0 mL min−1, 4.0 wt% and 110 mg N·L−1 and the ammonia removal efficiency (ARE) reached 42.7%, while treating the alkaline fermentation liquid. Seemingly the presence of Na+ and polypeptides in the liquid with their average RE of 53.3% and 11.1% substantially compromised ammonia removal probably due to the competition of adsorption sites. This present study serves as a proven concept for the feasibility of the application of the FCDI system in ammonia separation from the VFAs, which could realize economic and ecological benefits.