© 2022, The Author(s), under exclusive licence to Springer Nature Switzerland AG.Any type of wastewater should be treated before being released into the environment to protect marine life and water ecology. The wastewater harms its environment regarding the concentration of the pollutants, which are monitored via organic matter, suspended solids, nutrients, and color. Slaughterhouses are a part of a huge industry and massive amounts of water are required during and after slaughtering. Even though generated wastewater has similar charasteristics to domestic wastewater, it has an unusually dark color due to the high concentration of pollutants. Recent environmental legislations and regulations require the decolorization of wastewater before discharge. Besides, colored water absorbs part of the sunlight and decreases underwater illumination, which in turn can be harmful to the aquatic ecosystem. The dark color is the result of the dense nutrient composition of the slaughterhouse wastewater which makes it a great habitat for microalgae. Considering the circumstances, this study aims to decolorize the real slaughterhouse wastewater with a freshwater microalga Chlorella ESP-6 by cultivating them in 0%, 20%, and 30% diluted media. In the study, the bioreactors were set at a fixed temperature, continuously stirred, and aerated. The first color change was observed on the second day of the experimental period. At the end of the experimental period, 96.1% of the color of each media was removed successfully and the nutrient removal efficiency was calculated as 96.7% for COD and more than 98.5% for TN, and finally more than 98.1% for TP.