Water Research 39 (2005) 2125–2135 Detailed spatio-temporal solids concentration proﬁling during batch settling of activated sludge using a radiotracer Jeriffa De Clercq a,Ã , Filip Jacobs b , David J. Kinnear c , Ingmar Nopens c , Rudi A. Dierckx b , Jacques Defrancq a , Peter A. Vanrolleghem c a Department of Chemical Engineering and Technical Chemistry, Ghent University, Technologiepark 914, 9052 Zwijnaarde, Belgium b Department of Nuclear Medicine, Ghent University Hospital, De Pintelaan 185, 9000 Gent, Belgium c BIOMATH: Department of Applied Mathematics, Biometrics and Process Control, Ghent University, Coupure Links 653, 9000 Gent, Belgium Received 5 November 2004; received in revised form 4 March 2005; accepted 8 March 2005 Abstract In building and tuning good settling models for secondary clariﬁers of wastewater treatment plants, there is a need for measured continuous solids concentration proﬁles during batch settling. Conventional measuring techniques have difﬁculties in recording this kind of data, either because they are invasive, or because of the low solids concentration and/or solids density of activated sludge. This paper investigates a novel non-invasive measurement technique borrowed from nuclear medicine, using a solids radiotracer and gamma cameras, to obtain solids concentration proﬁles during the batch settling of activated sludge, in a pilot-scale column with a height of 1 m. The technique does not disturb the settling process, does not alter the settling characteristics, gives proﬁles every minute and every few millimeters, and is capable of measuring in a range of 0–25 g/l with high accuracy. Dynamic solids concentration proﬁle measurements were performed for sludges of different wastewater treatment plants, and at different initial concentrations. The results show a quantitative representation of the settling process, and reveal hindered and compression settling. r 2005 Elsevier Ltd. All rights reserved. Keywords: Activated sludge; Batch settling; Pilot-scale; Concentration proﬁles; Radiotracer; Gamma camera 1. Introduction Batch settling experiments are commonly used for hindered settling experiments and process monitoring intended to measure how well sludge can be thickened in a continuous thickening process, as well as to validate thickening models. In activated sludge batch settling usually only the batch settling curve is recorded. As far as known to the authors, the only exceptions are Kinnear (2002), who measured equilibrium solids concentration proﬁles, and Chu et al. (2003), who measured dynamic solids concentration proﬁles during the settling of waste activated sludge. In soil mechanics on the other hand, such proﬁles (Bergstro¨ m, 1992; Scott, 1968; Dreher, 1997; Shih et al., 1986; Been and Sills, 1981; Wells, 1990; Tiller et al., 1991) are more frequently ARTICLE IN PRESS www.elsevier.com/locate/watres 0043-1354/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2005.03.023 Ã Corresponding author. Tel.: +32 264 56 15; fax: +32 264 58 35. E-mail address: jeriffa.declercq@ugent.be (J. De Clercq).