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Water Science & Technology Vol 52 No 10-11 pp 79–87 © IWA Publishing 2005 Short circuiting in a denitrifying activated sludge tankR. Kjellstrand*, A. Mattsson**, C. Niklasson*** and M.J. Taherzadeh*****Chemical Engineering and Environmental Science, Chalmers University of Technology, SE-412 96 Göteborg, Sweden, (E-mail: robert.kjellstrand@hb.se; claes@cre.chalmers.se; mohammad@cre.chalmers.se); School of Engineering, University College of Borås, SE-501 90 Borås, Sweden ABSTRACT The presence of a short circuit flow in a denitrifying activated sludge tank was identified and modelled. Tracer tests with pulse addition of lithium salt were used to investigate the hydraulics of the tank. The lithium concentration in the effluent was detected and residence time distribution (RTD) curves were generated. Hydraulic models based on completely stirred tank reactors (CSTRs) in series were generated from the RTD curves and the models were compared. The short circuit problem was successfully described using the Martin model, where the inflow is divided into two strands. Each strand was modelled as a number of CSTRs in series. At a normal flow the results of the model show that the tank has 12.8% dead volume, 85.8% main volume and 1.3% short circuiting volume. The inflow was divided into 91.9% entering the main volume and 8.1% entering the short circuiting volume. The mean velocity of the short circuiting stream was estimated to 0.4 m/s. At maximum flow the short circuiting stream was even larger and handled 24.3% of the flow. The short circuiting stream was identified in the upper part of the tank due to the position of the inlet and the outlet. The configuration of a tank including the use of baffles, the geometry of the inlet and mixer configuration should be considered carefully if short circuiting is to be avoided. Keywords Activated sludge; dead volume; hydraulics; mixing; short circuiting eProduct: Buy this article for £24.00 (IWA MEMBER PRICE: £18.00) All prices include VAT. For customers where VAT should not be applied, the VAT amount will be removed upon payment |
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