This paper deals with the first part of an experimental programme in a pilot plant configured for advanced biological nutrient removal processes treating domestic wastewater of Bilbao. The IAWPRC Model No. 1 was calibrated in order to optimize the design of the full-scale plant. In this first phase the R-D-N process, incorporating an anoxic selector, was tested for eight months at three temperatures of approximately 20°C, 15°C and 11°C. Under quasi-steady state conditions the ammonia effluent varied from 1 to 5 mg N/l. A partial inhibition of nitrification in some periods was detected. This was clearly shown by regular maximum AUR batch tests and by the model calibration. The influent COD/TKN ratio was rather low (8-9) and the effluent nitrate concentrations varied from 10 to 12 mg N/l. The DSVI increased from 50 to 115 ml/g as the temperature decreased from 20°C to 11°C. An appropriate balance between floc-forming and filamentous microorganisms was maintained as a consequence of the combined configuration of the anoxic selector and the regeneration zone.

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