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 NO(3)-N removal through the sand filter was assessed for 22.6 mgNO(3)-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 NO(3)-N removal efficiency of 94%. Although increasing filtration rates decreased NO(3)-N removal, effluent NO(3)-N concentrations at the effluent port were lower than the limit value. The slow sand filter process was unable to provide NO(3)-N removal rate more than 27.1 gN/(m(2)day) (0.05 m/h flow rate). The NO(3)-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 NO(2)-N than the standard value.