Olive mill wastewaters anaerobid digestion in mesophilic and thermophilic batch packed-bed biofilm reactors for the accumulation of volatile fatty acids as monomers employed in the biological production of polyhydroxyalkanoates

Polyhydroxyalkanoates (PHAs) are bioplastic whose properties and applicability are quite similar to these of polypropylene. In order to make PHA industrial production feasible, new low-cost processes have to be developed. In this research, an innovative process for PHA production is being studied. This process consists of three stages. In the first stage (anaerobic conditions), the acidogenic fermentation transforms an organic substrate into a mixture of volatile fatty acids (VFAs), the most suitable substrate for PHA production. In the second stage (aerobic conditions), the effluent of the first stage is fed to a Sequencing Batch Reactor (SBR), where the periodic feeding (“feast and famine” conditions) enriches the mixed microbial culture of the PHA-producing microorganisms. In order to optimize PHA productivity, this stage is operated at the highest organic load allowed from the need of maintain strong selective pressure on the sludge. In the third (batch) stage (aerobic conditions), the excess sludge from the second stage is fed with the effluent of acidogenic fermentation in order to increase its PHA content up to the maximum value. Then this PHA-rich sludge flows to the downstream processing of PHA. Regarding the choice of the organic substrate to be used as feedstock for the process, olive mill wastewaters (OMWs) are particularly interesting. OMWs are the liquid effluent resulting from olive oil producing processes. Due to their high COD load, they are generally considered effluents of high environmental concern. The possibility of developing a biotechnological process fed with OMWs and addressed to the production of VFAs was explored in the present research. To this aim, Four packed-bed biofilm reactors (PBBRs) were employed in batch conditions in the anaerobic acidogenic digestion of an OMW. The effect on the process of the packing material and of the temperature were studied by filling two of the PBBRs with granular activated carbon (GAC) and the other two with ceramic cubes (VS), and by thermostating one GAC- and one VS-PBBR at 35°C and the other two at 55°C. The reactors were inoculated with an acclimated OMW-degrading consortium employed in a previous research (Bertin et al. 2004). Two one-month batch experiments were carried out by feeding the reactors with a diluted and amended OMW. For both experiments, the higher VFA production was observed in the PBBR filled with SV and operating at 35°C, where more then 3 g/l of VFA accumulated, this corresponding to a conversion of about the 60% of the initial COD. 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. Bertin L.; Colao M.C.; Ruzzi M.; Fava F. "Technological features and molecular microbial characterisation of a granular activated carbon packed-bed biofilm reactor capable of an effective anaerobic digestion of olive mill wastewaters." FEMS MICROBIOLOGY ECOLOGY, 48:413-423 (2004)