Computers and Chemical Engineering 25 (2001) 585 – 593 Dynamic optimisation of small size wastewater treatment plants including nitrification and denitrification processes B. Chachuat, N. Roche, M.A. Latifi * Laboratoire des Sciences du Ge ´nie Chimique, CNRS -ENSIC, B.P. 451, 1 rue Grandille, 54001 Nancy Cedex, France Received 10 May 2000; accepted 5 January 2001 Abstract In this paper, dynamic optimisation of small size wastewater treatment plants is studied. The problem is stated as a hybrid dynamic optimisation problem which is solved using a gradient-based method. The aeration policy which minimises the energy consumption and satisfies discharge requirements under specified constraints (process and physical constraints) is then determined. The comparison between usual rule-based control policies and optimised aeration strategies showed that the optimised aeration profiles lead to reductions of energy consumption of at least 30%. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Wastewater treatment; Dynamic optimisation; Aeration energy minimisation www.elsevier.com/locate/compchemeng 1. Introduction The pollution of water is mainly due to rain storm, domestic and industrial activities. The polluted water contains among others large quantities of organic and nitrogenous compounds. For many reasons (regulation constraints, salubrity, environment, water reuse, …), the concentrations of these compounds must be re- duced. This is achieved by means of wastewater treat- ment plants which are mainly based on the activated sludge process. For small communities, i.e. 5000 population- equivalent (in this case, small size wastewater treatment plants are more appropriate), the activated sludge pro- cess is low organic loaded. The plants typically consist of: (1) a unique aeration tank (aerated and mixed using surface turbines) where a suspended microbial culture is used to treat the incoming wastewater; and (2) a settler in which the microbial culture is separated from the liquid being treated (Fig. 1). Most of the culture is recycled and mixed with incoming wastewater to main- tain convenient sludge age characteristics, thus guaran- teeing high degradation rates. Organic compounds are eliminated under oxic (aera- tion) as well as anoxic (non-aeration) conditions. Nitro- gen is removed in two steps: ammonium is first oxidised to nitrate under oxic conditions (nitrification step); the produced nitrate is then transformed into nitrogen gas under anoxic conditions (denitrification step). Both oxic and anoxic conditions are periodically carried out in the aeration tank by running the turbines sequentially. Hence, oxygen control is of great importance since the concentration of dissolved oxygen is directly related to nitrification and denitrification processes. In addition, the aeration process induces 60–80% of the global operation cost of a treatment plant (Vasel, 1988). Most of small size wastewater treatment plants: (1) do not deal with any type of control; (2) use very simple Fig. 1. Schematic diagram of the activated sludge process. * Corresponding author. Tel.: +33-383-175234; fax: +33-383- 175326. E-mail address: latifi@ensic.inpl-nancy.fr (M.A. Latifi). 0098-1354/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII:S0098-1354(01)00638-X