Olive oil mill wastewater to volatile fatty acids: statisticals study of the acidogenic process

Abstract

The aim of the present paper was to study the feasibility of using olive oil mill wastewater (OOW) to produce a mixture of volatile fatty acids (VFA) adequate for producing polyhydroxyalkanoates (PHA) with better mechanical properties in a later aerobic phase. Hence, thiswastewater can be valorized rather than only treated, and its organic pollutant content can result in valueadded products with a sustainable origin. The influences of alkalinity addition and initial substrate concentration on VFA formation were evaluated in anaerobic batch fermentation experiments of OOW. The highest acidification degree (DA) (60 %) was obtained with an intermediate alkalinity of 5 gCaCO3 L−1 and a high substrate concentration of 14 gCOD L−1. These operational conditions produced a mixture of VFA (7.4 gCOD L−1) composed predominantly by acetic, n-butyric, and ncaproic acids. Regarding VFA valorization into PHA, recovering an adequate VFA composition is crucial to produce biopolymers that are more attractive industrially. The most suitable VFA mixture for PHA production was obtained at the highest alkalinity addition (7 gCaCO3 L−1), with an odd-to-even VFA ratio ranging from 0.42 to 0.61 with increasing COD load, predominantly composed of odd-equivalent acids, mainly propionic acid, although resulting in a significant decrease of DA to values close to 20 %. These experimental results suggest that VFA produced in this process can be used as substrate in a subsequent process for PHA production, regulating its monomer composition and polymer properties, solely by a proper adjustment of the operational conditions of the acidogenic fermentation step.