Biorefineries: the case study of olive mill wastewaters (OMWs)

Agroindustrial wastes handling represents a serious economical and environmental concern. In some cases, large volumes of a toxic waste, such as olive mill wastewaters (OMWs), are produced within a short time in a limited area. In literature, a number of successful biotechnological approaches were attempted to reduce their toxic content while producing secondary metabolites of a certain industrial interest or biogas (CH4). However, to date the development of several biotechnological integrated processes aimed to fully recover or bioconvert the organic components naturally present in such wastes is a feasible option, and would allow their complete low-cost exploitation as renewable feedstocks in biorefineries. In the present communication, a case study related to the exploitation of OMWs is presented. Firstly, solid phase extraction of the polyphenolic fraction occurring in OMWs was conducted. Phenols (PHEs) are natural antioxidant compounds of high commercial value. Selective removal almost completely abated PHEs concentration in OMWs, their subsequent recovery being carried out with biocompatible solvents (i.e., ethanol). Thereafter, dephenolized OMWs were fed to a packed-bed-biofilm reactor (PBBR) for the acidogenic digestion of its organic content by means of microbial consortia. The removal of antimicrobial compounds such as PHEs was found to significantly enhance volatile fatty acids (VFAs) production respect to non-pretreated OMWs. In the following step, electrodyalisis of the VFA enriched effluent was performed in order to further increase VFAs concentration. This process also produces a secondary effluent with a reduced VFA content, which was yet sufficient to allow (1) CH4 production in PBBRs loaded with microbial consortia, (2) biohydrogen production with photosynthetic bacteria or algae, or (3) photoheterotrophic growth of algal biomass. The effluent further enriched in VFAs was fed to a second aerobic reactor loaded with microbial consortia for sustaining polyhydroxyalkanoates (PHAs) production and storage. PHAs are biodegradable and biocompatible microbial polymers which represent a renewable alternative to actual oil-derived plastics. Preliminary experiments showed that PHAs can be succesfully loaded with specific drugs, e.g., chlorexidine, which was then released leading to bacterial growth inhibition when in vitro tests with 3 strains of Streptococcus were carried out. This feature could be exploited in medical applications to deliver selected drugs in particular regions. Furthermore, it was observed that changes of the VFAs mixture fed for PHAs storage led to the synthesis of polymers with different chemico-physical properties. Therefore, culture conditions during acidogenic digestion could be adjusted in order to address the synthesis of desired biopolymers for following applications.