Production of polyhydroxyalkanoates through an integrated anaerobic-aerobic process fed with olive mill wastewaters as a renewable resource

Polyhydroxyalkanoates (PHAs) are biopolimers whose properties and applicability are quite similar to these of polypropylene. However, their production on the large scale is actually too expansive. In this research, an innovative biotechnological low-cost PHA producing process, consisting of three stages, is being studied. The aim of the first anaerobic stage is to obtain an effluent rich in volatile fatty acids (VFAs), which represents the substrate for microbial PHA production, through the acidogenic fermentation of olive mill wastewaters (OMWs), which are the liquid wastes of olive oil producing processes and are generally considered effluents of high environmental concern. In the second aerobic stage, the anaerobic effluent is fed to a Sequencing Batch Reactor (SBR) where microbial populations able to transform VFAs in PHAs are selected. In the third aerobic stage, the excess sludge from the second stage is fed with the effluent of the acidogenic fermentation in order to increase its PHA content up to the maximum value. Packed-bed biofilm reactor technology was chosen to develop the anaerobic first stage. Packing material and temperature parameters were studied by developing 4 identically-configured PBBRs: two of them were filled with granular activated carbon (GAC), the other two with ceramic cubes (VS); one reactor for each material was thermostated at 35°C, the other two at 55°C. 2 one-month batch experiments were previously carried out by feeding the reactors with a diluted and amended OMW, so that the initial waste COD was 6.33 g/l. The higher VFA production was observed within the second experiment in the SVS-PBBR operating at 35°C, where a conversion of the initial COD in VFAs of about the 60% was observed. Acetic acid was the main component of the VFA mixtures collected from all the PBBRs. Significant concentrations of both propionic and butirric acid were also observed.. Then, the PBBRs were forced to operate in continuous conditions and fed with an organic load of about 8 g/l/d. GAC-PBBR operating at 35°C gave rise to the highest VFA production, even if the COD conversion in VFAs was lower than the one reported above for batch experiments, due to a significant methanogenic activity. Higher organic loads will be fed to the 4 PBBRs in order to optimize the OMW acidogenic fermentation stage. The second and third stages of the process were studied in aerobic sequencing batch reactors, fed with a synthetic mixture of acetic (85% on a COD basis) and propionic (15 %) acids. In particular the effect of the pH in the second stage was studied. Three runs were carried out at three different pH values: 7.5, 8.5 and 9.5. The highest substrate removal rates and polymer production rates were obtained at pH 8.5. Under these conditions, 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.