Olive mill wastewater degradation in soil

Olive mill wastewater (OMW) is a by-product of olive processing for oil production. It consists of water initially present in the drupae (vegetation water), process water (added to improve oil separation during milling), and washing water of olives and milling plants. Olive milling causes a huge production of OMW: about 0.4-1.1 m3 of OMW are produced per 1000 kg of olives. This production takes place in a short time due to seasonal olive availability. Such large amounts cannot be disposed without any treatment in sewers or rivers because of the high COD (50000-200000 mg/L) and polyphenol content. Since the ’80s, different physico-chemical and biological treatments have been studied. Due to the high related costs, such treatments did not achieve wide diffusion except for demonstrative pilot plants. In Italy, the traditional solution of spreading OMW onto the soil continues to be applied according to the current regulation. In the case of OMW spreading onto agricultural soils, the high COD could be exploited to increase the organic fraction of poor soils. However, the phytotoxic and antimicrobial activity of OMW negatively affects such reuse. OMW toxicity has been ascribed mainly to the high polyphenol content (1.2-2.4%). Spontaneous remediation was observed with the spreading of moderate OMW amount onto the agricultural soil, probably due to reactions catalyzed by abiotic and biotic soil components. Indeed, the clay fraction of the soil generally contains iron and manganese oxides and adsorbs polyphenoloxidases contained in plants and fungi exudates. If too large amounts of OMW are spread on the soil, the natural remediation can be insufficient: the soil health is affected and OMW percolates down to the aquifer. In any case, the effects of prolonged OMW spreading on soil health are unknown, at the moment. In order to take full advantage of the OMW fertirrigating potential by preventing at the same time aquifer pollution, it would be interesting to develop a treatment that removes the polyphenols selectively, leaving unchanged the organic and salt content of the OMW. In this work, we propose to use the clay fraction of the agricultural soil as a catalyst for the reaction of humification of the polyphenols. The reaction would be carried out in a soil-OMW slurry in an aerated, stirred reactor to ensure optimal conditions for the oxidation reaction catalyzed by iron and manganese oxides and by polyphenoloxidases. The scope is a substantial reduction of free polyphenols concentration in the OMW without reducing the total organic load. Such a treatment, if effective, would combine simplicity and economy of realization by exploiting reagents and catalysts available in situ. The feasibility of polyphenol removal in soil has been assessed by experiments with model polyphenolic compounds and real OMW samples. Full conversion of low molecular polyphenols has been found in both cases. Experiments with model compounds have shown that the reaction is started by a preliminary oxidation by iron and manganese oxides and proceeds by further oxidation of intermediates by dissolved molecular oxygen. Final products are water-insoluble polymers. The treatment efficacy has been evaluated by the degree of polymerization of the polyphenols and by the reduction of phyto- and micro-toxicity. It has been found that phytotoxicity removal is marked with model polyphenols and only marginal with entire OMW. Therefore, in contrast with the common tenet, polyphenols do not seem to be the main cause of phytotoxicity in OMW.