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. Regardless which water quality will be disinfected, there is a four step process (using the actual wastewater) that has to considered when judging the applicability of UV disinfection. Those four steps are effluent water-quality characterization, bench-scale evaluation (collimated beam dose-response curves), pilot-scale evaluation (pilot unit dose-response curves), and full-scale evaluation (dose-response curves and microbial monitoring). Other questions that must be answered include: • What type of lamp/s would be best? Low-pressure, low-intensity or low-pressure, high-output or low-pressure, high-intensity or medium-pressure, high-intensity. • Which automatic cleaning system (mechanical or mechanical/chemical) will meet the cleaning needs? • Will fats, oils, and greases (FOG) coat lamp sleeves? • What is the wastewater velocity through the reactor and what influence does it have on the reactor efficiency? • What lamp and/or lamp sleeve configuration changes can reduce the low UVT254 water quality impacts on the reactor? The results from the bench-scale and pilot-scale evaluations will be expanded on in the paper including the physical and microbial water quality.