Olive mill wastewater: a source of microbial- and phyto-toxicity for soil.

OMW are high phenolic content wastewaters produced during olive milling. In Mediterranean countries, where olive oil production is widespread, disposal of OMW is a serious problem because of their anti-microbial activity and phytotoxicity. Usually, OMWW are spread onto the soil and this practice is responsible of a continuous decreasing of soil quality and fertility. OMW are often stored in ponds for aging before spreading. In this case, however, soluble toxic compounds can permeate through soil and reach the groundwater table. Furthermore, long-term storage causes production of malodorous compounds due to anaerobic fermentation processes. Several treatments have been proposed for OMW but usually they are too expensive and not easy enough to be applied in low technology plants such as those where olive oil is produced. Generally, no tests are made to assess if the treated OMW can be used as water source for irrigation purposes. Target of this work is to assess microbial- and phyto-toxicity of OMW when spread onto the soil and to focus attention on reactions involving polyphenols and promoted by soil compounds. Phytotoxicity (measured by germination of Lepidium sativum) and microbial toxicity (tested on the growth of Bacillus cereus) of OMW are quite strong and a high dilution of the wastewater is needed to observe germination of the seeds or growing of bacteria. Oxidation of catechol and tyrosol, as models of phenolic compounds present in OMW, has been studied thoroughly in soil slurries. Final reaction products are high weight polymeric compounds whose structure is similar to that of humic and fulvic acids. The same reaction occurs when OMW are added to soil. Polyphenols contained in OMW are converted in 24 h, if oxygen is provided to the system. The measured reduction of phenolic compounds is bound to a slight decrease in phyto- and microbial toxicity. This indicates that reaction products are still toxic or other compounds in OMW are responsible of toxicity in the soil environment. Further studies will be devoted to assess the nature of these compounds and to find a possible treatment to reduce the damaging of soil quality when OMW are spread onto soil.