At the waste water treatment plant Zurich-Werdhölzli full scale experiments of denitrification with methanol in tertiary filtration was performed during a summer and a winter campaign of 4 months each. For this purpose one of the 22 filter cells was equipped with a methanol dosage. At temperatures of 12-15°C rates of denitrification of about 1,0 kgN m-3 d-1 are reached. After main back-washing denitrification is significantly reduced. Frequently back-washings (several times per day) lead to methanol breakthroughs due to biofilm loss. Methanol consumption is between 4,0-4,5 kg methanol kgN-1 depending on oxygen concentration in influent. In spite of methanol dosage the quality of the filter effluent is very good during normal operation in the winter campaign. In summer at temperatures of 20-22 °C an accumulation of the intermediate product nitrite was obtained.