Controlling nitrogen removal using redox and ammonium sensors

At the Ejby Mølle WWTP (300,000 PE) operated by the Odense Water Ltd. a system of aeration control which combines ammonium concentration and redox potential has proven itself as a method of reducing nitrogen levels in the effluent. The nitrogen removal process at this plant proceeds in parallel aeration tanks that alternate between periods of aeration and denitrification. At the same time influent is redirected from one tank to the other such that influent and effluent is primarily to and from the tank where denitrification is going on. The aeration stops and the tanks open and close when the ammonium concentration reaches its set point. Aeration is restarted again when redox potential drops to its low set point or when ammonium reaches its upper set point. The average total nitrogen concentration in the effluent from the Ejby Mølle WWTP went from 4.7 down to 2.1 mgN/l after this control system was implemented. Not all of this reduction can be explained by better control. However we believe that at least 1 mgN/L of the reduction and probably more was the result of the control system based on the combination of ammonium and redox sensors.