Journal of Methods Microbiological Journal of Microbiological Methods 32 (1998) 193–204 Sensors to monitor biological nitrogen removal and activated sludge settling a a a b Krist Gernaey , Alexis Vanderhasselt , Herwig Bogaert , Peter Vanrolleghem , a, * Willy Verstraete a Laboratory for Microbial Ecology, University Gent, Coupure Links 653, B-9000 Gent, Belgium b BIOMATH Department, University Gent, Coupure Links 653, B-9000 Gent, Belgium Abstract A brief overview of methods and sensors to characterize activated sludge is presented, summarizing techniques related to two important activated sludge processes: biological nitrogen removal (nitrification and denitrification) and sludge settling. Traditional off-line methods, typically applied in a laboratory environment to determine nitrifying / denitrifying sludge activities and sludge settling properties are briefly described. The main part of the paper covers a more detailed description of on-line sensors which were recently developed to continuously provide information about important activated sludge properties in a full-scale plant. The most important future work in this research field is the development of control strategies based on the data provided by these sensors.  1998 Elsevier Science B.V. Keywords: Activated sludge; Denitrification; Nitrification; Respirometry; Sludge settling; Titration experiment 1. Introduction due to bacterial growth during degradation processes is normally discarded as a fraction of the concen- The activated sludge process is one of the most trated sludge flow withdrawn at the bottom of the widespread wastewater purification technologies. In decanter, and treated separately in the sludge treat- this process, wastewater is mixed with a concen- ment facilities of the activated sludge plant. trated biomass suspension (the activated sludge) Originally, activated sludge system design was responsible for the degradation of the pollutants. only concerned with the removal of organic carbon When microbial degradation processes have come to substances from the wastewater. During the last two an end, the sludge flocs are separated again from the decades however, more stringent effluent standards purified wastewater through sedimentation in a de- for nutrients (nitrogen (N) and phosphorus (P)) canter. The purified water is discharged into surface imposed by legislation have in many countries led to waters while the concentrated sludge suspension is the development of more complicated activated continuously withdrawn at the bottom of the decan- sludge process configurations specifically designed to ter. The majority of the concentrated sludge suspen- achieve biological nutrient removal. A general sion is recycled and mixed again with wastewater characteristic of all systems is that mixed liquor is entering the treatment plant. Excess sludge produced cycling through aerobic, anoxic and anaerobic con- ditions to obtain nitrification, denitrification and * biological P removal (Randall, 1992). Corresponding author: Tel. 132 9 264 59 76; fax: 132 9 264 62 48; e-mail: Willy.Verstraete@rug.ac.be In view of the variety of biological processes 0167-7012 / 98 / $19.00  1998 Elsevier Science B.V. All rights reserved. PII: S0167-7012(98)00023-2