Activated carbon pre-treatment was applied to anaerobic digestion effluent in order to reduce the optical density. Up to 78% reduction of OD was achieved suggesting activated carbon pre-treatment a promising tool to increase the light transmission of agricultural digestate. The pre-treated digestate was applied for microalgae cultivation as a source of nutrients and compared to the untreated diluted digestate. Microalgae growth was similar at the first period of cultivation in pre-treated and diluted untreated digestate. However, at the other part of the cultivation higher growth rate and biomass accumulation in untreated digestate was observed. We discuss the possibility of too high nutrient load in pre-treated digestate that may have limited the microalgae growth. PAM measurements showed no stress from the substrate highlighting that activated carbon pre-treated digestate is a suitable medium for microalgae growth. Moreover, vitality of cultures in pre-treated PBRs dropped at the end of the cultivation suggesting some inhibitory effect may be present. Results of current study demonstrate that activated carbon holds a great potential for agricultural digestate pre-treatment to decrease the optical density.