Treated wastewaters to be disinfected with ultraviolet (UV) radiation can be divided into three water quality groups: 1) high ultraviolet transmittance at 254 nm (UVT254 -90 percent and above), 2) medium UVT254 (40s to 80s percent) and 3) low UVT254 (below 40 percent). High UVT254 waters are secondary effluents that have received additional treatment with micro, ultra, or nano filters and in combination with reverse osmosis, and would be an easy application for UV disinfection. Medium UVT254 waters usually are secondary effluents and sand media filtered secondary effluent, which have well established UV applications. Low UVT254 waters are primary effluents or any other lower water quality effluents that are to be disinfected with typically unknown UV reactor performance.