In this paper, a detailed presentation and analysis of treatment plant for drinking water at source of Ombla river (UPPV Komolac) was performed. The applied procedure includes an ultrafiltration separation technique that enables a stable and secure water supply of the city Dubrovnik, regardless of significant changes in the quality of "raw water". The paper analyzes all phases of the "raw water" treatment process: raw water pretreatment, ultrafiltration, backwash, disinfection and distribution of drinking water. The efficiency of the plant was assessed through the values for turbidity, the total number of coliforms and Escherichiae coli in raw and treated water in the period from October 2019 to May 2020. After the water treatment in UPPV Komolac, it can be concluded that the values of these parameters are significantly reduced by the ultrafiltration operation. The highest measured value of turbidity of treated water in the analyzed period was 0,6 NTU, which indicates the exceptional efficiency of the plant. The effectiveness of ultrafiltration related to the removal of coliform bacteria and Escherichiae coli cannot be defined, since in given period, the presence of these microbiological indicators in the raw water was not observed. The use of the analyzed membrane technology definitely ensures the high quality of drinking water in the Dubrovnik- Neretva County.