Valorization of olive mill wastewaters as a renewable resource for the biological production of polyhydroxyalkanoates

An innovative biotechnological anaerobic-aerobic integrated three-stage process for the production of polyhydroxyalkanoates (PHAs) is being studied. A low-cost renewable row material such as the liquid waste resulting from olive oil producing processes (olive mill wastewater, OMW) is employed to feed the first anaerobic step, by which obtaining an effluent rich in volatile fatty acids (VFAs). Four anaerobic packed-bed biofilm reactors filled with granular active carbon (GAC) or ceramic cubes (VS) were developed and employed in batch and continuous conditions in the OMW acidogenic fermentation. The effect on the process of temperature and organic loading rates were studied. Higher COD conversion in VFAs were obtained in the reactor filled with VS. In the second aerobic process stage, a sequencing batch reactor (SBR) is fed with the anaerobic effluent, with the aim of selecting microbial populations able to store PHAs through the metabolic conversion of VFAs. A SBR inoculated with an activated sludge was fed with a synthetic VFA mixture by different pH conditions. The highest substrate removal and polymer production rates were obtained at pH 8.5. The selected microflora is finally employed in a third aerobic batch stage fed with the VFA-rich anaerobic effluent. A batch experiment was carried out at pH 8.5. PHAs were produced at a rate of approximately 350 mgCOD/gCOD/h, with a storage yield of 45% (as COD). The produced polymer was a P(HB-HV) co-polymer, with 13% (mol/mol) HV content.