Environmental Modelling & Software 14 (1999) 409–419 Towards a model of input–output behaviour of wastewater treatment plants using soft computing techniques Lluı ´s A. Belanche a,* , Julio J. Valde ´s a , Joaquim Comas b , Ignasi R. Roda b , Manel Poch b a Seccio ´ d’Intel-lige `ncia Artificial, Dept. de Llenguatges i Sistemes Informa `tics, Universitat Polite `cnica de Catalunya, c/Jordi Girona 1-3, 08034 Barcelona, Spain b Laboratori d’Enginyeria Quı ´mica i Ambiental, Facultat de Cie `ncies, Universitat de Girona, Campus de Montilivi s/n 17071 Girona, Spain Abstract Wastewater Treatment Plants (WWTPs) control and prediction under a wide range of operating conditions is an important goal in order to avoid breaking of environmental balance, keeping the system in stable operating conditions and suitable decision-making. In this respect, the availability of models characterizing WWTP behaviour as a dynamic system is a necessary first step. However, due to the high complexity of the WWTP processes and the heterogeneity, incompleteness and imprecision of WWTP data, and finding suitable models poses substantial problems. In this work, an approach via soft computing techniques is sought, in particular, by experimenting with fuzzy heterogeneous time-delay neural networks to characterize the time variation of outgoing variables. Experimental results show that these networks are able to characterize WWTP behaviour in a statistically satisfactory sense and also that they perform better than other well-established neural network models.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Wastewater treatment plants; System identification; Heterogeneous neural networks; Environmental modeling Software availability Name of the software: HeterogNeu— Heterogeneous Neural Networks Developer: Lluı ´s A. Belanche, Julio J. Valde ´s and R. Garcı ´a Contact address: Seccio ´ d’Intel.lige `ncia Artificial, Dept. de Llenguatges i Sistemes Informa `tics, Universitat Polite `cnica de Catalunya., c/Jordi Girona 1-3. 08034 Barcelona, Spain. Telephone/Fax: + 34 93 401 56 44/ + 34 93 401 70 14 E-mail: {belanche, valdes}lsi.upc.es * Corresponding author. Tel.: + 34-93-401-5644; fax: + 34-93- 401-7014; e-mail: belanche@lsi.upc.es 1364-8152/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII:S1364-8152(98)00102-9 Year first available: 1999 Program language: C++ Availability: It is a prototype. Available only for personal use. 1. Introduction Nowadays, proper management of wastewaters in modern industrialized societies is not only an option, but a necessity. The main objective is to maintain natural water systems at as high a quality level as possible, and to ensure equilibrium between supply and demand through a rational use and management of water resources. Moreover, the wastewater treatment would help to reach the attainment of rivers as biological corri- dors, which ensures a good quality of life for animals and vegetables living in the water. Wastewater coming from different municipal uses contains a wide variety of contaminants. Among them, the most commonly found in municipal wastewater are total suspended solids