pH and oxidation-reduction potential control strategy for optimization of nitrogen removal in an alternating aerobic-anoxic system

An alternating aerobic and anoxic (AAA) system is a continuous-flow, activated-sludge process in which the environmental conditions necessary to meet the specific requirements for aerobic nitrification and anoxic denitrification are created. Because of stricter regulations on wastewater effluent and concerns on energy consumption, the process control of the system is now becoming more important. A bench-scale AAA system with different fixed aeration ratios was operated for more than 8 months to evaluate effects of the total cycle time and aeration ratio on the system performance and to develop a feasible control scheme. With supplemental organic addition, the system shows removal efficiencies of 85 to 90% and 78 to 82% for chemical oxygen demand and total nitrogen, respectively. The oxidation-reduction potential (ORP) and pH profiles indicate that the aerobic cycle can be controlled by a control point (ammonia valley) on the pH profile indicating the end of nitrification and the anoxic cycle controlled by another point (nitrate knee) on the ORP profile signifying the end of denitrification. Thus, a dual control strategy with the pH and ORP control points was used for terminating aerobic-anoxic and initiating anoxic-aerobic cycles in an AAA system. The performance of the online control system is excellent, with a significant energy saving (average aeration ratio,f_a = 0.23) as compared to the fixed time systems (f_a = 0.33 to approximately 0.5).