Nitrate is the most common form of nitrogen in water. The presence of nitrates in drinking water can harm human health, this is why the maximum concentration of nitrates in potable water is imposed by law. The main treatment technologies used for treating this kind of water are reverse osmosis, ion exchangers, electrodialysis and biological denitrification. The goal of this paper is to determine the efficiency of biofilters in reducing the ammonium concentrations in water, depending on the carrier/filtration media used in the biofilters, the C:N:P ratio used and contact time between water and the attached biomass. The paper is based on the result obtained during experimental tests in which the concentration of nitrates in the raw water was 150 mg/l.