Dairy wastewaters can be remediated through assimilation of nutrients into algal biomass, providing upcycling into valuable products. Unfortunately, these wastewaters are non-ideal for phototrophic organisms; high levels of dissolved organic matter (DOM) and heterotrophic microbial organisms result in limited light and nutrient availability. Ultraviolet (UV) radiation can modify aquatic systems, changing DOM quantity and quality and lowering microbial densities. We hypothesized that dairy wastewater would be similarly modified by UV radiation, allowing for improved algal growth. Two distinct dairy wastewaters, anaerobic digester effluent (ADE) and polyhydroxyalkanoate reactor effluent (PHAE) were investigated. UV pre-treatment of ADE resulted in modified growth kinetics of Chlorella vulgaris, increasing exponential growth rates up to 4 fold and decreasing time to stationary phase by 8 days. UV pre-treatment of PHAE resulted in no growth effects for C. vulgaris. A UV fluence of 1892.7 mW·s·cm− 2 reduced bacteria colony forming units in ADE and PHAE by 99% and 100% respectively, modified fluorescent DOM, and reduced light attenuation in both effluents. These findings suggest that UV successfully modifies and enhances ADE's utility as a growth medium for C. vulgaris, improving the economics for a waste-to-algae process, and providing potential for recycling of nutrients from agricultural waste.