This paper presents the results of experiments to test biodiesel waste (glycerine-g-phase) as an organic carbon source for the removal of nitrate in a WWTP denitrification process. Investigation of g-phase was first centered on g-phase utilization as an external source for denitrification under laboratory conditions and consequently, after positive results from the laboratory investigation, g-phase was applied in the denitrification process in the WWTP Vrutky (35,000 PE). This WWTP had insufficient nitrogen removal via denitrification. Denitrification was insufficient due to an influent with a low BOD(5)/N ratio (1.7:1) entering into the activated sludge tank. Laboratory experiments and calculations showed that, to reach N(total) concentration under 10 mg l(-1) in effluent, a biodiesel waste dose of 500 kg(COD) d(-1) was necessary. Glycerol phase (g-phase) dosing into the denitrification tank increased denitrification efficiency by 2.0 - 5.0 mg(NO3-N)l(-1) per 100 l of g-phase dose into the denitrification tank. (c) 2008 Elsevier Ltd. All rights reserved.