Microbiol. consortia contg. bacteria and yeasts were used to reduce the content of NH4+ ions in effluents from biogas plants based on sewage sludge or liq. manure, located in Poland and Slovakia, resp. The highest removal efficiency of NH4+ ions was 46% (after 7 days) for the effluent generated upon fermentation of sewage sludge, and 88.5% (after 14 days) for the effluent obtained upon biomethanization of pig manure.