The paper presents the methodology and results of cost-effectiveness analysis of selected methods of wastewater treatment: activated sludge and biofi lter. The analysis concerns small municipal wastewater treatment plants with capacity of 10 to 500 m3d-1 in Poland (~100 to 5000 PE). It is based on data on total investment outlays, annual operating costs and total average annual costs. It has been shown that, in the case of investment outlays, there are no statistically signifi cant differences between technologies. However, the annual operating costs and the total average annual cost of wastewater treatment are the lowest when applying the biofi lter technology. The models presented in the paper can be used for costs estimation at the initial stage of designing municipal wastewater treatment plants. The total average annual cost of wastewater treatment determines the charges for sewerage services. This charge, alongside technological and environmental factors, as well as local conditions, should be one of the criteria for choosing a method for wastewater treatment.