Denitrification with methanol in tertiary filtration

The large wastewater treatment plants in Switzerland have to be extended by enhanced nitrogen removal to comply with the relevant EU directives. Denitrification in tertiary filtration is a cost-effective alternative to extended denitrification in an activated sludge system which needs additional reactor volume. Full-scale experiments in denitrification with methanol in tertiary filtration were performed at the wastewater treatment plant in Zurich-Werdhölzli 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. Denitrification rates of about 1.0 kg-N m−3 d−1 are attained at temperatures of 12–15°C. The denitrification is reduced significantly after main back-washing. Frequent back-washings (several times per day) lead to methanol breakthroughs due to biofilm loss. The yield coefficient YCOD was 0.4 kg-CODxkg-COD−1me. In spite of the methanol dosage, the quality of the filter effluent is 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.