The large wastewater treatment plants in Switzerland have to be extend
ed by enhanced nitrogen removal to comply with the relevant EU directi
ves. Denitrification in tertiary filtration is a cost-effective altern
ative to extended denitrification in an activated sludge system which
needs additional reactor volume. Full-scale experiments in denitrifica
tion with methanol in tertiary filtration were performed at the wastew
ater treatment plant in Zurich-Werdholzli during a summer and a winter
campaign, each lasting 4 months. One of the original 22-filter cells
was equipped with a methanol dosage unit for this purpose. Denitrifica
tion rates of about 1.0 kg-N m(-3) d(-1) are attained at temperatures
of 12-15 degrees C. The denitrification is reduced significantly after
main back-washing. Frequent back-washings (several times per day) lea
d to methanol breakthroughs due to biofilm loss. The yield coefficient
Y-COD was 0.4 kg-CODX kg-CODme-1. In spite of the methanol dosage, th
e quality of the filter effluent is very good during normal operation
in the winter campaign. Accumulation of the nitrite intermediate produ
ct was observed in summer at temperatures of 20-22 degrees C. (C) 1997
Elsevier Science Ltd.