Ultraviolet disinfection is the most efficient mode in combination with other disinfection methods within multi-barrier approach for the water treatment. UV disinfection being effective against a wide range of pathogens including the chlorine-resistant (viruses and protozoa) significantly reduces chlorine byproducts. This paper presents a review of results of the implementation of multi-barrier lay-out with the application of UV disinfection at water and wastewater treatment plants of large cities: St. Petersburg and Nizhniy Novgorod.