Using Online Ammonia and Nitrate Instruments to Control Modified Ludzack-Ettinger (MLE) Process

Modified Ludzack-Ettinger (MLE) is one of the most widely used processes to remove nitrogen from wastewater. It involves an anoxic zone and aerobic zone where nitrification occurs in the aerobic zone and nitrate is then recycled back to the anoxic zone for denitrificaiton reaction. At present, most plants using the MLE process are operated at constant dissolved oxygen (DO) in the aerobic zone and constant internal recycle flow (IRQ) ratio to the anoxic zone.A novel control system is presented in this paper. It uses on-line ammonia and nitrate measurement signals to dynamically control the MLE process at optimum DO set-points and IRQ ratios.The DO set-point control is realized by calculating loads to the aerobic zone and the DO levels required to complete nitrification. The IRQ control is realized by calculating residual nitrate concentration in the anoxic zone effluent, in order to maintain a reasonable denitrification reaction rate.The advantages of this new control system are calculated using on-line measurement results:Approximately 16% of energy consumed by aeration system can be saved. For a 10 MGD municipal WWTP, using fine bubble aeration system, the savings is about 755,000 kwh per year.Effluent nitrate concentration can decrease about 1 mg/l by controlling the IRQ in comparison with a constant IRQ ratio. Because nitrate removal is optimized, more denitrification oxygen credit can be realized.If post denitrification using external carbon is required to meet a stringent nitrate or total nitrogen permit, substantial external carbon source savings can be achieved. Approximately 100,000 pounds of methanol can be saved for a 10 MGD WWTP.Since the process is optimized by sophisticated control, it may be possible to stop one train or even re-rate the plant to a higher capacity, for significant operation and/or expansion costs savings.