The paper presents description of unit processes that proceed in onsite treatment systems (sedimentation, flotation, filtering, fermentation, sorption) with regard to nitrogen compounds removal from domestic sewage.Conditions necessary for optimal nitrification and denitrification have been described.Effectiveness of nitrogen compounds removal for typical onsite treatment systems has been shown (septic tanks, pits, trenches and drainage beds, constructed wetlands). Technical solutions with increased nitrogen removal have also been presented.