The paper presents investigation results which were performed to evaluate the role of biological processes in elimination of Mn (II) and NH⁴⁺ ions from groundwater. The raw water containing up to 1.4 mg/dm³ manganese and up to 2.9 mg N/dm³ ammonia nitrogen after aeration was treated using the first and the second stage sand filters at the water treatment plant "Letniki" near Elbląg. Both ions were removed with the high efficiency and the concentrations of these components after two stages of filtration were lower than the admissible values for drinking water in Poland (0.1 mg Mn/dm³ and 0.5 mg N-NH4/dm³). It was found out that the physico-chemical and biological processes were responsible for ammonia nitrogen elimination, while catalytic processes (physico-chemical and biological) contributed in manganese removal.