Water Research 39 (2005) 2676–2686 Effects of oxygen concentration on N-removal in an aerobic granular sludge reactor A. Mosquera-Corral a,Ã , M.K. de Kreuk b , J.J. Heijnen b , M.C.M. van Loosdrecht b a Department of Chemical Engineering, School of Engineering, Avda. Lope Go´mez de Marzoa s/n. E-15782 Santiago de Compostela, Galicia, Spain b Kluyver Laboratory for Biotechnology, Department of Biochemical Engineering, Delft University of Technology, Julianalaan 67, 2628 BC Delft, The Netherlands Received 11 November 2003; received in revised form 24 February 2005; accepted 21 April 2005 Available online 22 June 2005 Abstract In order to optimise nitrogen removal in an aerobic granular sludge system, short- and long-term effects of decreased oxygen concentrations on the reactor performance were studied. Operation at decreased oxygen concentration is required to obtain efficient N-removal and low aeration energy requirement. A short-term oxygen reduction (from 100% to 50%, 40%, 20% or 10% of the saturation concentration) did not influence the acetate uptake rate. A lower aerobic acetate uptake at lower oxygen concentrations was obviously compensated by anoxic acetate uptake. Nitrogen removal was favoured by decreased oxygen concentrations, reaching a value of 34% for the lowest oxygen concentration tested. Long-term effects were evaluated at two oxygen saturation levels (100% and 40%). Nitrogen removal increased from 8% to 45% when the oxygen saturation was reduced to 40%. However, the granules started to disintegrate and biomass washout occurred. It was impossible to obtain stable granular sludge at this decreased oxygen concentration under applied conditions. A solution to obtain stable aerobic granular sludge at low oxygen concentrations is needed in order to make aerobic granular sludge reactors feasible in practice. r 2005 Elsevier Ltd. All rights reserved. Keywords: Acetate; Oxygen; Granular sludge; N-removal; SBAR 1. Introduction Activated sludge systems generally require large surface areas for treatment of wastewater and settling of sludge flocs. In order to reduce space, research is focussed among others on more compact systems. These compact systems in aerobic wastewater treatment are generally based on realising high biomass concentra- tions, for example via enhancement of the settleability of the biomass. This can be achieved by growing biofilms adhered to a solid surface (airlifts, fluidised beds, etc.) or by cultivating biomass as anaerobic granules (UASB and EGSB reactors). Around ten years ago aerobic granular sludge was grown in an aerobic upflow sludge blanket where pure oxygen was supplied outside the reactor into a liquid recycle flow (Mishima and Nakamura, 1991; Shin et al., 1992). Only recently aerobic granular sludge has also been cultured in directly aerated and well-mixed reactors (Morgenroth et al., 1997; Heijnen and van Loosdrecht, 1998; Beun et al., 1999; Dangcong et al., 1999; Etterer and Wilderer, 2001; Tay et al., 2001). 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.2005.04.065 Ã Corresponding author. Tel.: +34981563100x16779; fax: +34981528050. E-mail address: eqanusmc@usc.es (A. Mosquera-Corral).