Polyphenol Removal from Olive Mill Wastewaters by Catalytic Soil Components.

Olive Mill Wastewaters (OMW) are one of the most common agricultural sources of pollution in the Mediterranean area. OMW are often disposed of by spreading onto soil for irrigation purposes. It should be noted that, in the countries of southern Mediterranean bank, water shortage is widespread. Furthermore OMW are also rich in organic and mineral compounds that could be usefully exploited as fertilizers. The high polyphenol content, however, can affect soil health because of the related microbial- and phyto-toxicity. Therefore, a detoxification treatment is mandatory before using OMW for agricultural purposes. The aim of this work is to test the feasibility of a simple treatment of OMW, with specific attention to the effects of spreading the treated wastewater onto the soil. The proposed technique exploits the natural catalytic activity of soil components towards polyphenol oxidation. The reactor used was an aerated stirred tank containing a slurry phase. Experiments were preliminarily performed on model phenolic compounds to check the soil capability to convert them, to elucidate the reaction mechanisms and, furthermore, to measure the residual toxicity of reaction products. Results obtained with model compounds were then compared with those achieved with the real OMW. Catechol and tyrosol were chosen as model compounds because they are widespread in OMW and they represent two of the main phenolic components detected in the OMW sample used. All phenolic content in OMW (hydroxytyrosol among the others) was monitored by reversed phase HPLC measurements. The experimental results indicated that soil is effective in converting phenols. As regards the model systems, complete conversion was achieved within 12 h, at room temperature, at the concentration detected in the OMW in either cases. These results were confirmed by experiments with OMW. Under the same experimental conditions, conversion of main phenolic compounds occurred in 24 h. In the short term, the observed reaction was found to be promoted by Fe and Mn-oxides contained in the soil. The experiments performed with model compounds suggested that reaction products are either polymers or polymeric aggregates. Indeed they were completely retained by filtration through 0,2 μm cut-off membranes. In the long term a reduction in overall TOC coupled with biomass growth was observed, thus suggesting that biotic mineralization occurred in the system, as well. Germination tests were performed with English cress seeds exposed either to the liquid or to the polymer containing solid phase from the slurry reactor. Tests indicated that the proposed treatment removes the phytotoxicity of the model compound solution and considerably reduces that of the real waste.