Autotrophic nitrogen removal appears as a prerequisite for the implementation of energy autarchic municipal wastewater treatment plants. Whilst the application of anammox-related technologies in the side-stream is at present state of the art, the feasibility of this energy-efficient process in main-stream conditions is still under investigation. Lower operating temperatures and ammonium concentrations, together with a demand for high and stable nitrogen removal efficiency, represent the main challenges to overcome for this appealing new frontier of the wastewater treatment field. In this study, we report the short-term effect of temperature on the maximum biomass specific activity of anaerobic ammonium oxidizing (anammox) bacteria as evaluated by means of batch tests. The experiments were performed on anammox biomass sampled from two full-scale reactors and two lab-scale reactors, all characterized by different reactor configurations and operating conditions. The results indicate that for the anammox conversion, the temperature dependency cannot be accurately modeled by one single Arrhenius coefficient (i.e., theta) as typically applied for other biological processes. The temperature effect is increasing at lower temperatures. Adaptation of anammox bacteria after long-term cultivation at 20 and 10 degrees C was observed. Implications for modeling and process design are finally discussed. (C) 2014 Wiley Periodicals, Inc.