University of Macau ABSTRACT Effect of Chloride on Biological Nutrient Removal from Wastewater by Hong, Chon Choi (Steve) Thesis Supervisors: Prof. Hojae Shim Prof. Shek-Kiu Chan Master of Science in Civil Engineering Water resources are crucial to socio-economical development of mankind. Macao's water comes from Xijiang of the Pearl River in Guangdong. During the drought period of 2003 - 2005, the salinity of drinking water in Macao showed an apparent increase, and salinity problems had continuously affected the quality of Macao's drinking water. Chloride ion is one of major components of salinity. This research was mainly to investigate the effect of chloride on biological nutrient removal from wastewater. An Anaerobic/Anoxic/Oxic (A2O) lab-scale reactor was used in order to achieve the nutrient removal from wastewater. The activated sludge was obtained from Taipa Wastewater Treatment Plant (WWTP). Internal recycled flow rate and solids retention time were varied respectively for determining the optimum operation parameter of A2O reactor using the synthetic wastewater. Experiments were performed in two phases and the results were compared. Phase 1: synthetic wastewater with chloride concentrations of 150 - 5,000 mg/L was injected into the A2O reactor in order to measure the effect of chloride on nutrient removal. Phase 2: the wastewater obtained from the Taipa WWTP was used with chloride concentrations of 500 - 2,500 mg/L. Results showed that 96.1 - 98.6% ammonia nitrogen were removed for phase one and 97.0 - 100% for phase two. The specific nitrification rate was 0.319 - 0.460 mg NH3-N/g VSS/hr for phase one and 0.503 - 0.537 mg NH3-N/g VSS/hr for phase two. The specific denitrification rate was 0.173 - 0.468 mg NO3 -N/g VSS/hr for phase one and 0.312 - 0.372 mg NO3 -N/g VSS/hr for phase two. The specific phosphorus uptake rate was decreased from 0.559 at 150 mg/L chloride concentration to 0.052 mg PO43- -P/g VSS/hr at 5,000 mg/L chloride concentration for phase one and from 3.613 at 500 mg/L chloride concentration to 0.451 mg PO43 -P/g VSS/hr at 2,500 mg/L chloride concentration for phase two. The soluble chemical oxygen demand (SCOD) removal efficiency was decreased from 94.8 at 150 mg/L chloride concentration to 63.0% at 5,000 mg/L chloride concentration for phase one and from 74.6 at 500 mg/L chloride concentration to 45.0% at 2,500 mg/L chloride concentration for phase two. The specific nitrification rate, specific denitrification rate, and ammonia removal were not significantly affected by the increased chloride concentration in both synthetic wastewater and real wastewater, while the specific phosphorus uptake rate and specific phosphorus release rate were negatively affected by increased chloride concentration. The removal efficiency for SCOD was poor when the chloride concentration of wastewater was raised. Overall, the quality of effluent generated from the A2O reactor meets the discharge limits for domestic wastewater of developed countries such as Germany and Canada. Furthermore, development of current technology may not only provide a better effluent quality but also contribute to the enhancement of sustainable development for wastewater treatment.