Activated sludge from a wastewater treatment plant and pure culture of Hydrogenophaga pseudoflava were utilized for the development of a denitrifying biofilm in a submerged filter in order to remove nitrate from polluted groundwater. Nitrate removal efficiency, nitrite accumulation, turbidity, COD and faecal indicators persistence in the treated water were determined at different superficial hydraulic loading (10. 20 and 30 m(3)/m(2) d) and superficial nitrate loading rates (1, 2, 3, 6 and 9 Kg NO3-/m(2) d) in the submerged filter. The application of H. pseudoflava as inocula allowed better results in terms of system stability, higher superficial hydraulic loading and superficial nitrate loading rates (30m(3)/m(2) d and 9 kg NO3-/m(2) d, respectively). These values improve those obtained when the system was inoculated with activated sludge. In addition. the pure microbial inocula improved design parameters and running of the process due to its biofilm homogeneity obtaining treated water with better characteristics to its final use as drinking water than that obtained with an activated sludge inocula. (C) 2004 Elsevier B.V. All rights reserved.