Increasing the tolerance to polyphenols of the anaerobic digestion of olive wastewater through microbial adaptation

The valorisation of olive oil mill wastewater (OMW) through anaerobic digestion requires identifying the concentration of polyphenols (PP) that causes failure of the process of digestion. In addition, the advantages of the possible microbial adaptation, in terms of increased methane production, to significant concentrations of PP as well as the kinetics of OMW anaerobic degradation requires evaluation. To fill these knowledge gaps, anaerobic digestion batch tests were carried out on three blends of OMW and inoculum (digestate from a biogas plant fed with agro-wastes) at a PP concentration of 0.5, 1.0 and 2.0 g l−1 in mesophilic conditions. Total inhibition of anaerobic digestion was found at a PP concentration of 2.0 g l−1 (non-adapted inoculum group). A positive effect of the adaptation to the substrate was, instead, observed for the blends with adapted inoculum at a PP concentration of 1.0 and 2.0 g l−1. Methane yields increased by 70% (PP = 1.0 g l−1) and 300% (2.0 g l−1) in the group with adapted inoculum compared to the group with non-adapted inoculum. The results suggest that OMW should not be subject to anaerobic digestion at high PP concentrations (i.e. higher than about 1 g l−1) due to the microbial inhibition detected. Moreover, given the benefits of the adaptation of the microbial population that was more evident at the highest PP concentration tested, it is advisable to allow the progressive adaptation of the digestion to OMW feeding. Thanks to the increased methane yield, because of the improved microbial tolerance to inhibiting compounds, the anaerobic digestion of OMW could be a viable and environmentally sound solution for the treatment of agro-industrial wastewater.