Substrate conditions for denitrifying bacteria were enhanced by adding carbon sources to a laboratory-scale sand filter system. Temperature, oxidation–reduction potential, and hydrogen ion concentration were measured through the recirculation of nitrogen-dosed wastewater and carbon sources that were mixed to encourage microbial growth, with denitrifying bacteria identified by standard plate counts. Two different external carbon sources (sucrose and ethanol) were added, with and without activated sludge amendments. Nitrate, nitrite, and chemical oxygen demand (COD) concentrations were monitored relative to an untreated control and a treatment with activated sludge under an initial hydraulic loading rate of 0.508 m3 /m2 d and a hydraulic retention time of 2.5 h. Nitrate decay rates were only significantly enhanced for the ethanol treatment without addition of activated sludge. Nitrite initially accumulated when carbon sources were added, but no accumulation was evident by the end of the experiment after 150 min. COD declined when carbon sources were added, but activated sludge had no effect on the rate at which the COD declined. The increased rate of nitrate removal with the addition of ethanol is of technical interest, as the volume of waste-water treated in a unit volume of filter medium for denitrification doubled with ethanol compared with sucrose at the same concentration.