Trade-off between mesophilic and thermophilic denitrification: Rates vs. sludge production, settleability and stability Emilie N.P. Courtens a , Siegfried E. Vlaeminck a , Ramiro Vilchez-Vargas a , Arne Verliefde b , Ruy Jauregui c , Dietmar H. Pieper c , Nico Boon a,* a Laboratory of Microbial Ecology and Technology (LabMET), Ghent University, Coupure Links 653, 9000 Gent, Belgium b The Particle and Interfacial Technology Group (PaInT), Ghent University, Coupure Links 653, 9000 Gent, Belgium c Microbial Interactions and Processes Research Group, Helmholtz Centre for Infection Research, Braunschweig, Germany article info Article history: Received 1 April 2014 Received in revised form 5 June 2014 Accepted 16 June 2014 Available online 25 June 2014 Keywords: Biological nitrogen removal Industrial wastewater Nitrification Nitrous oxide Thermus abstract The development of thermophilic nitrogen removal strategies will facilitate sustainable biological treatment of warm nitrogenous wastewaters. Thermophilic denitrification was extensively compared to mesophilic denitrification for the first time in this study. Two sequential batch reactors (SBR) at 34  C and 55  C were inoculated with mesophilic acti- vated sludge (26  C), fed with synthetic influent in a first phase. Subsequently, the carbon source was switched from acetate to molasses, whereas in a third phase, the nitrate source was fertilizer industry wastewater. The denitrifying sludge maintained its activity at 55  C, resulting in an immediate process start-up, obtaining nitrogen removal rates higher than 500 mg N g 1 VSS d 1 in less than one week. Although the mesophilic SBR showed twice as high specific nitrogen removal rates, the maximum thermophilic denitrifying activity in this study was nearly 10 times higher than the activities reported thus far. The thermo- philic SBR moreover had a 73% lower sludge volume index, a 45% lower sludge production and a higher resilience towards a change in carbon source compared with the mesophilic SBR. The higher resilience was potentially related to a higher microbial diversity and evenness of the thermophilic community at the end of the synthetic feeding period. The thermophilic microbial community showed a higher similarity over the different feeding periods implying a more stable community. Overall, this study showed the capability of mesophilic denitrifiers to maintain their activity after a large temperature increase. Existing mesophilic process systems with cooling for the treatment of warm wastewaters could thus efficiently be converted to thermophilic systems with low sludge production and good settling properties. © 2014 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: þ32 9 2645976; fax: þ32 9 2646248. E-mail address: Nico.Boon@UGent.be (N. Boon). Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/watres water research 63 (2014) 234 e244 http://dx.doi.org/10.1016/j.watres.2014.06.026 0043-1354/© 2014 Elsevier Ltd. All rights reserved.