Biological removal of nitrate from drinking water was studied in a slow sand filter. Optimum carbon to nitrogen ratio (C/N) was found to be 1.8 when using acetic acid in batch tests. The filtration rates impact on NO3-N removal through the sand filter was assessed for 22.6 mg NO3-N/l concentrations while keeping C/N ratio as 1.8 for acetic acid. The filtration rates varied from 0.015, 0.02, 0.03, 0.04,0.05, and 0.06 m/h, respectively, corresponding to an overall average NO3-N removal efficiency of 94%. Although increasing filtration rates decreased NO3-N removal, effluent NO3-N concentrations at the effluent port were lower than the limit value. The slow sand filter process was unable to provide NO3-N removal rate more than 27.1 N/(m(2) day) (0.05 m/h flow rate). The NO3-N removal efficiency slightly dropped from 99% to 94% when the loading rate increased from 27.1 to 32.5 g/(m(2) day), but the effluent water contained higher concentration of NO2-N than the standard value. (C) 2007 Elsevier B.V. All rights reserved.