Process optimisation for simultaneous biological nitrification and chemical phosphorus removal

Abstract

Environmental regulations require sewage treatment plants originally designed for BOD and suspended solids removal to meet increasingly stringent effluent ammonia and total phosphorus standards. Operating a biological nitrification process in parallel with chemical phosphorus removal to achieve consistent effluent concentrations less than 1mg/L was successfully achieved through process optimisation. An optimisation program was developed and implemented which effectively allowed the combination of two essentially conflicting processes to produce high quality effluent. The process optimisation program utilised a structured approach to identify process deficiencies which reduced treatment capacity and reliability. Accurate identification of process deficiencies enabled appropriate solutions to be developed and implemented. Additional monitoring quantified improvements arising from process modifications or revised operating procedures. The average effluent ammonia concentration between June and September 1990 was 23.8mg/L. This was reduced to 0.6mg/L after optimisation in October 1992. The standard deviation of 3.4mg/L during the baseline monitoring program between February and April 1992 was reduced to 0.4mg/L after optimisation, at much lower mixed liquor temperatures. Similarly, the average effluent phosphorus concentration was reduced from 1.1mg/L with a standard deviation of 0.8mg/L to an average value of 0.6mg/L with a standard deviation of 0.3mg/L. Understanding process operations, combined with the ability to identify process deficiencies resulting from poor operating practices and environmental conditions, reduces the risk of generating poor effluent quality. The performance of any treatment process can derive significant benefits by implementing an effective process optimisation program. This work established that reliable process performance is possible, provided that process deficiencies are identified, and appropriate solutions are implemented and evaluated.