Nitrogen Removal to Minimize Energy Consumption of the Two WWTPs Choutrana II and Menzel Bourguiba in Tunisia

Abstract

Abstract The control of the aeration time and the dissolved oxygen in a reactor plays an important role in the management of the plant and the reduction of energy consumption. For this, two full-scale municipal wastewater treatment plants were chosen. The first plant (Choutrana II) designed for 600,000 population-equivalents (p.e), consists of a primary treatment step (screening, grit and grease removal), followed by four parallel oxidation ditches with a total volume of 60,000 m3. Aeration is ensured by air diffuser. The second plant (Menzel Bourguiba) designed for 140,000 p.e consists of a primary treatment stage (screening, grit and grease removal), followed by anoxic tanks and two aerobic reactors with a total volume of 14,000 m3. Aeration is ensured by six surface aerators which are operated episodically to create aerobic and anoxic conditions. Currently, the aeration management is controlled by a timer with a maintained oxygen concentration at a given level in the aerobic reactor. The nitrogen removal is done during air-on and air-off periods. The process can be seen as a succession of aeration periods followed by an anoxic period. This category of plant operation is far from the optimal and raises energy consumption and operative costs. The purpose of this study is to implement a new strategy to control aeration taking into account the variation of the daily organic load received by plants (feedforward control) and to create favorable conditions for nitrification and denitrification steps (feedback control). The Activated Sludge Model 1, which is calibrated and validated by respirometry tests, is used. Simulation results show the potentialities of the chosen strategy, which could save 28% of aeration energy and could adjust the aerobic and anoxic times. As first results, daily aeration time could be considered enough to produce treated water conform to standard terms. For Choutrana II plant, the results obtained have allowed to achieve a nitrogen removal rate of 85% and an output concentration between 6 and 12 mg/L. Rates of nitrification and denitrification were estimated at 4 mg NO3-N/L/h and 6 mg NH4-N/L/h, respectively. At the same time, aeration periods (on/off) are connected with daily organic load received by the plant and created favorable conditions for nitrification–denitrification. A good agreement between input biological loads, aeration profile and nitrogen removal is approved.