While the ability of slow sand filters to remove total suspended solids (SS), turbidity, and organics from wastewaters is well known, this study has demonstrated that they can also achieve simultaneous nitrification-denitrification, producing effluent total Kjedahl nitrogen (TKN) and total nitrogen (TN) concentrations as low as 0.6 and 1.5 mg/l, respectively, utilizing particulate and slowly biodegradable COD in the process. The impact of filtration rates in the range of 0.15-0.38 m/h, filter depth of 0.5-1.5 m, and sand size 0.3-0.5 mm on nitrogen removal processes at temperatures of 10-39degreesC was assessed. Nitrification efficiency, denitrification efficiency, and total nitrogen removal efficiency correlated well with filtration rate and sand size only, with all three parameters inversely proportional to the square root of the aforementioned two process variables. Nitrification exhibited the most sensitivity to filtration rate and sand size. The filters produced effluent with turbidities of 0.1-0.5 NTU, SS concentrations of 3-6 mg/l in the fine sand and 6-9 mg/l in the coarse sand. Effluent BOD5 and COD concentrations were mostly in the 0.8-2.6 and 15-34 mg/l range, respectively. (C) 2002 Elsevier Science B.V. All rights reserved.