BACKGROUND Mixotrophic and heterotrophic growth of microalgae through biodegradation of olive mill wastewaters (OMW) is a promising strategy to improve the economic viability of microalgae production processes. However, OMW are characterized by an elevated phenols content, which makes their treatment by biological processes difficult. In this work, the influence of OMW phenols on microalgae growth was investigated by using the phenols extracted with a resin from OMW. RESULTS CONCLUSIONS OMW phenols, at 0.5 g L-1, were found to inhibit the growth of Scenedesmus sp. This inhibition was reduced by replacing an inoculum in the stationary phase with an inoculum in the exponential phase. Even with this strategy, a relevant negative effect (- 30%) on maximum cell concentration was found, which could be prevented by decreasing the phenol concentration to 0.1 g L-1. Under mixotrophic conditions, phenols reduced light penetration, limiting photosynthetic activity without significant improvement in phenols biodegradation. Under both mixotrophic and heterotrophic condition, percentage phenols removal up to 57% was achieved. The most promising strategy to achieve integrated microalgal cell growth and OMW phenols biodegradation is the heterotrophic cultivation with phenols concentration not greater than 0.1 g L-1 and an inoculum in exponential phase. (c) 2018 Society of Chemical Industry