Enhancing a multi-stage process for olive oil mill wastewater valorization towards polyhydroxyalkanoates and biogas production

Olive oil mill wastewaters (OMW) cause serious environmental pollution problems due to their high level of production and specific chemical characteristics. An innovative disposal treatment and simultaneous valorization technology consists of using OMW as a no-cost substrate for mixed microbial culture (MMC)-based polyhydroxyalkanoates (PHAs) production process. Here, an MMC multi-stage process was developed in which after acidogenic fermentation, OMW was subjected to a solid liquid separation step by centrifugation. The resulting solid fraction was converted into methane through anaerobic digestion in order to recover energy for the process, whereas the liquid fraction was used as feedstock for both the biomass selection and the PHA accumulation step. Most of OMW soluble chemical oxygen demand (COD) content in the liquid fraction (approximately 70%) was removed in a sequencing batch reactor (SBR) obtaining a high PHA production (about 532 mgCOD, in the feast phase). In particular, the strategy to uncouple COD feed from nitrogen supply within the SBR cycle, along with the operation of a settling phase before the famine phase, allowed to enhance the selective pressure towards PHA-storing microorganisms. Polymer production significantly increased in the accumulation reactor up to approximately 2470 mgCOD/L. Finally, anaerobic digestion tests on the OMW solid fraction showed a relevant COD conversion into methane (up to 100%).