This paper reports the costs of municipal wastewater collection and treatment in Poland. Our study provides detailed cost estimates for the collection and treatment of municipal wastewater, based on an empirical sample of 1,400 operators, who jointly collect and treat over 80% of wastewater in Poland. The unit costs of collection and treatment, and the nitrogen and phosphorus treatment efficiency were investigated, and the effects of plant capacity on unit costs (scale effects) were explored. Primary, secondary and tertiary treatment cost functions were analysed econometrically using the Box-Cox regression model, indicating high non-linearity and significant scale effects. We found that wastewater treatment costs were increasing with technology efficiency (moving from the primary, through the secondary, to the tertiary treatment), and decreasing with higher wastewater treatment plant capacity. Our results provide a comprehensive picture of municipal wastewater treatment in Poland but potentially, as the technology is fairly generic, they can also be used for applications in other countries, after accounting for capital and labour cost differences. The paper thus provides a valuable input into cost-benefit analyses of nutrient loading reduction achieved by extending or intensifying municipal wastewater treatment.