Olive oil mill wastewater as renewable source in a multi-stage process from phenol recovery to polyhydroxyalkanoates production

Olive oil mill wastewater (OMW) valorization through polyphenols recovery and polyhydroxyalkanoate (PHA) production by mixed microbial cultures was investigated at lab-scale. The 1st step consisted of a solid phase extraction for recovering phenolic compounds, whose antioxidant features can be exploited for several industrial applications. The yield of phenol recovery was about 1.6 g/L (around 60.5% of the initial content). Subsequently, an acidogenic fermentation step was performed in packed bed biofilm reactor at high organic loading rate (OLR about 8 gCOD/L d), in order to convert the OMW organic matter into volatile fatty acids (up to around 16 gCOD/L), the most direct substrate for the aerobic bacteria responsible for PHA production. The PHA production steps involved a sequencing batch reactor (SBR) for selection and enrichment of PHA-storing microorganisms under feast and famine (F/F) regime and a batch reactor for PHA accumulation. The SBR was inoculated with an activated sludge and operated with fermented OMW (plus dilution water and additional N supply), at OLRs ranging from 2.37 to 8.42 gCOD/L d. The OLR variation strongly affected the ratio between the length of the feast and famine phase. The best performance was obtained at OLR 4.74 gCOD/L d with the lowest F/F ratio (0.10) and a storage rate of 448 mg CODPHA /gCOD h. At all investigated OLRs the intracellular polymer content in the biomass was increased in the accumulation reactor operated with undiluted OMW (with no further supply of N to that contained in the OMW) and pulse feeding strategy. Finally, the produced PHA was recovered through centrifugation followed by a chemical digestion with sodium hypochlorite and the recovered PHA was lyophilized to obtain a dry powder. Under the optimal conditions, around 66% of the produced polymer was recovered at a purity of around 72%.