Alternative technologies for olive mill wastewater management with emphasis on soil application

The disposal of olive mill wastewater (OMW) produced during oil extraction in Mediterranean countries creates a significant environmental problem because of the great volumes of effluent produced (10-12 Mm3 year-1), heavy pollution load (40-80 g L‑1 biological oxygen demand (BOD), 50-150 g L‑1 chemical oxygen demand (COD)) and phytotoxic properties (because of phenolic compounds). During the last 30 years, many OMW treatment methods have been proposed and tested, but their application at the olive mill level is limited because of the high investment and/or running costs and technical expertise required. Existing technologies for OMW management have been identified and evaluated with the LIFE+ OLEICO+ project following certain criteria that are currently being used in Portugal, Spain, Italy and Greece. The results show that several viable technologies for OMW treatment do exist (composting, electro-coagulation, hydrolysis-oxidation, phyto-remediation, co-digestion, energy production); however, they require capital investment and maintenance costs that cannot be afforded by a small or medium olive mill plants. On the other hand, application of OMW to olive orchards can be a low-cost alternative method for OMW treatment in regions with small olive oil mill enterprises. Annual rates up to 1500 L tree-1 were applied in five equal doses, at 20-day intervals between November and February. Soil analyses were performed before the onset and after the end of the OMW application period. The response of olive trees to OMW application was monitored by measuring plant nutritional status, photosynthesis and yield. Furthermore, the possibility of groundwater pollution by the application of fresh OMW was investigated with lysimeters. OMW-treated soil had higher K content throughout the experiment. Phenols were decomposed rapidly, and therefore the phenolic content before the onset of a new OMW application period was negligible. The nutritional status, physiology and yield of olive trees were not affected by the application of OMW. The total olive orchard area required for the annual OMW production was 3.6 ha (280 trees ha-1), which is easily available around an olive mill. The cost of application is €0.007 L‑1 OMW, which seems reasonable compared with more sophisticated methods. Detailed study for each case is required in order to determine the application dose according to soil and climatic conditions of the area. The results show that several viable technologies do exist; however, the implementation cost is not negligible.