In co-ordination with the EU-guidelines the large wastewater treatment plants in Switzerland have to be extended with enhanced nitrogen removal. Denitrification in tertiary filtration is a cost-effective alternative to extended denitrification in the activated sludge system, which needs additional reactor volume. At the wastewater treatment plant Zürich-Werdhölzli full-scale experiments of denitrification with methanol in tertiary filtration were performed during a summer and a winter campaign of 4 months each. For this purpose one of the original 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 attained. After main backwashing, denitrification is significantly reduced. Frequent backwashings (several times per day) led to methanol breakthroughs due to biofilm loss. The yield coefficient YCOD was 0.4 kg CODX kg−1 CODme. In spite of methanol dosage the quality of the filter effluent was very good during normal operation in the winter campaign. Accumulation of the nitrite intermediate product was observed in summer at temperatures of 20-22°C.