This study investigated the feasibility of using pre-treated plant liquors as organic carbon sources for the treatment of hydroponic wastewater containing high nitrate-N (> 300 mg N/L). The waste plant material was pre-treated to extract organic carbon-rich liquors. When this plant liquor was used as an organic carbon source in denitrification filters at the organic carbon:nitrogen dose rate of 3C:N, nitrate removal efficiencies were > 95% and final effluent nitrate concentrations were consistently < 20 mg N/L. However, at this dose rate, relatively high concentrations (> 140 mg/L) of organic carbon (fBOD(5)) remained in the final effluents. Therefore, a 'compromise' organic carbon:nitrogen dose rate (2C:N) was trialled, at which nitrate removal efficiencies were maintained at > 85%, final effluent nitrate concentrations were consistently below 45 mg N/L, and effluent fBOD5 concentrations were < 25 mg/L. This study has demonstrated that waste plant material is a suitable carbon source for the removal of nitrate from hydroponic wastewater in a denitrification filter. (c) 2007 Elsevier Ltd. All rights reserved.