Nowadays in water supply engineering systems, water preparation is very topical, especially paying attention to water biological condition. Among numerous methods used for water disinfection, electrochemical water treatment must be singled out. During the electrolysis process generation of specific disinfectants is possible, which are able to eliminate the microorganisms present in water, as well as decrease their ability to reproduce. In the research, the influence of water hardness on water electrolysis has been investigated using titanium oxide ceramic electrodes. High water mineralization can facilitate by-reactions on the surface of electrodes, as well as the formation of sediments that reduces the current of electricity in the electrolytic cell. If the solution contains calcium and hydrocarbonate ions, then during the electrolysis process CaCO3 sediments precipitate on the electrodes and in the solution. The turbulence of water flow and the increase of concentration of calcium hydrocarbonate facilitate the precipitation of CaCO3, which can be explained with the intensification of mass exchange process at the surface of electrodes. The increase of sediments on cathode depending on the changes of calcium hydrocarbonate concentration at different flow conditions can be described with the help of a linear equation. It has been detected that water hardness can affect the total amount of produced chlorine during electrolysis, which can be explained by the by-reactions on the electrode surface and the produced chlorine by-reactions. In case of hard water, the production of chlorine can be detected only after the first ten minutes.