There is a need to implement high-efficiency solutions for groundwater treatment, especially in the context of manganese compounds removal. This study analyzed the impact of a water treatment plant modernization to a technology of fast contact filters with continuous bed rinsing. On the basis of basic indicators of water pollution, such as iron, manganese, turbidity, color, and nitrogen compounds, the raw water collected from the groundwater intake and the water treated after the filtration process were characterized. A comparison of the efficiency of water treatment installation before and after modernization indicated that there was a significant improvement in the quality of treated water, especially in terms of manganese removal.