The purpose of the study is a scientific and theoretical substantiation of the energy characteristics of ultra and nano filtration, which directly depend on the quality of the source water, to ensure reliable and uninterrupted operation of a combined water treatment plant (CWTP), to obtain high-quality drinking water in water supply systems intended for settlements and industrial facilities. The developed method of combined operation of a water treatment plant is based on membrane technology, the efficiency of which directly depends on the preliminary improvement of the quality of purified low-mineralized water using an energy-efficient membrane, post-treatment and subsequent disinfection. indicators of the quality of treated water that meet regulatory requirements and indicators of improving the energy efficiency of the water treatment plant have been investigated and calculated on the basis of experimental data. The results of studies on low-mineralized water made it possible to obtain TDS (Total dissolved solids) with a total residual concentration of hardness and chlorides in the range of 0.77 mg/dm3 without any problems. The proposed combined water treatment plant method is a priority among fundamental and applied works in the field of water treatment, it is intended for the purification of natural waters under conditions of increased anthropogenic loads on natural water sources.