DENITRIFICATION WITH METHANOL IN TERTIARY FILTRATION

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.