The Olive Mill Wastewater (OMW) is the principal 
industry residue extraction of olive oil. Generally is 
characterized by very high organic load, due to high levels 
of phenolic compounds and sugars, and have minimum 
levels of nitrogen compounds and low pH. A variety of 
biological processes and microorganisms have been tested to 
treat OMW, including bacteria, yeasts and fungi, where the 
effectiveness in reducing the toxicity varies greatly. 
Taking this into account, the aim of this work is to assess the 
potential of microalgae in the decrease of phenolic 
compounds in OMW. For this purpose, the effect of 
immobilized Chlorella vulgaris in OMW biotreatments has 
been tested in batch cultures. The effectiveness of the 
process has also been assessed by phytotoxicity, in 
germination trials of Lactuca sativa. Biotreatments have 
been performed with OMW dilutions of 35%, 50% and 
60%. Results showed that C. vulgaris has the ability to 
remove phenolic compounds, achieving a final Phenolic 
Loss Index (PLI) of 73%. Regarding the phytotoxicity after 
biotreatment we can conclude that microalgae can reduce 
the toxicity of olive mill wastewaters also having the ability 
to degrade the phenolic compounds that inhibit the 
germination and growth of Lactuca sativa. Furthermore, the 
algae biomass obtained in this process could be valorized in 
other applications.