Environmental Modelling & Software 23 (2008) 1250–1261

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Risk assessment modelling of microbiology-related solids separation problems in activated sludge systems
´ J. Comas a, *, I. Rodrıguez-Roda a, K.V. Gernaey b, C. Rosen c, U. Jeppsson c, M. Poch a a Chemical and Environmental Engineering Laboratory (LEQUiA), University of Girona, Campus Montilivi s/n, E-17071 Girona, Catalonia, Spain Department of Chemical and Biochemical Engineering, Technical University of Denmark (DTU), Building 229, DK-2800 Kgs. Lyngby, Denmark c Department of Industrial Electrical Engineering and Automation (IEA), Lund University, P.O. Box 118, SE-22100 Lund, Sweden b a r t i c l e i n f o
Article history: Received 21 May 2007 Received in revised form 4 January 2008 Accepted 19 February 2008 Available online 19 May 2008 Keywords: Risk assessment Activated sludge Filamentous bulking Foaming Rising sludge Modelling Qualitative knowledge Benchmark simulation

a b s t r a c t
This paper proposes a risk assessment model for settling problems of microbiological origin in activated sludge systems (filamentous bulking, foaming and rising sludge). The aim of the model is not to diagnose microbiology-related solids separation problems with absolute certainty but to quantify in dynamic scenarios whether simulated operational procedures and control strategies lead to favourable conditions for them to arise or not. The rationale behind the model (which integrates the mechanisms of standard activated sludge models with empirical knowledge), its implementation in a fuzzy rule-based system and the details of its operation are illustrated in the different sections of the paper. The performance of the risk assessment model is illustrated by evaluating a number of control strategies facing different shortterm influent conditions as well as long-term variability