AbstractThe denitrification of biologically purified wastewater from the fibre‐board production is investigated in the batch experiment in a laboratory reactor with and without addition of methanol (0.5 ml/l) as an additional source of carbon. Without addition of methanol, in the main only a reduction of nitrate to nitrite occurs, which accumulates. The rate of reaction is low, so that the process is not yet finished after 10 h. The addition of methanol leads to an acceleration of the reactions and to a full denitrification of the wastewater within 4