A unidirectional submerged filter system was employed to purify groundwater contaminated with nitrate by biological denitrification. The influence of the concentration of dissolved oxygen (DO) in the process was tested using ethanol, methanol and sucrose as carbon sources. Inorganic-nitrogen removal, growth of the biofilm, platable denitrifying bacteria and nitrate reducing bacteria in biofilm were studied. With regard to the type of electron donor used, the presence of oxygen decreased the removal efficiency of inorganic nitrogen and caused an increase of nitrite concentration in the treated water. These negative effects depended on utilised carbon source. Biological denitrification with alcohols such as ethanol and methanol was less affected by DO than with sucrose. The development of the biofilm was also influenced by the DO concentration as excess O-2 caused reduced biofilm growth. These biofilms developed in oxygen presence had a smaller bacterial density and a lower denitrifying bacteria versus nitrate reducing bacteria ratio, which led to an unfavorable inorganic nitrogen removal and presence of nitrite in the treated water. All these effects are more pronounced when sucrose is used as carbon source. (C) 2002 Elsevier Science B.V. All rights reserved.