In this paper the operation costs of the municipal water treatment plant in the biggest Polish town in Podlaskie province was analyzed. Capacity of this WTF is 600 m³/h. Water treatment processes are primarily focused on removal of iron, mangnese and turbidity and disinfection by UV rays. Water is taken by 19 wells and then it is oxygenated. From aerators water is addressed to ten filters, which filter water at a speed of 8.5 m/h. The analysis of the operation costs of the municipal water treatment plant was carried out by a method of testing and interpretation of the materials provided by the Water and Sewerage Company in a big town in the Podlaskie Province. It was established that, groundwater treatment plant operation costs, carried out in 2010–2012, showed the highest share of depreciation and remuneration costs.