Olive Mill Wastewater Agronomic Valorization by its Spreading in Olive Grove

Abstract

Olive mill wastewater (OMW) is one of the main waste streams of olive processing and its disposal can represent a relevant environmental issue in Mediterranean countries. OMW is characterised by high pollutant load, salinity and phytotoxic levels of polyphenols, but also by a high amount of organic compounds and plant mineral nutrients. This study aimed to investigate the OMW reuse as a whole effluent for its soil conditioner and fertilizer potentialities in agriculture, in the frame work of circular economy. For this purpose, OMW was applied at three doses (50, 100 and 200 m3 ha−1 year−1) over three successive years in olive field. Soil physico-chemical characteristics, enzymes activities and microbial properties, olive yield and olive oil quality were analysed. The findings revealed that the electrical conductivity, organic matter, total nitrogen, sodium, and potassium soil contents increased proportionally with OMW concentration and application frequency in the soil OMW-treated layers. Compared to their control soil counterparts, aerobic bacteria and fungi increased in proportion with the OMW spreading rates. Furthermore, all the soil enzyme activities tested (dehydrogenase, β-glucosidase and urease) were enhanced in the OMW-amended soils compared to the control. Vegetative activity and olive yield showed improvement according to the OMW level spread. With the only exception of the phenol content, which was significantly higher in the oils extracted from OMW treated olive trees than the control, the oil quality parameters did not show any significant difference. OMW agronomic application constitutes a suitable practice to better manage this effluent, with positive effects on olive production and oil quality. Consequently, OMW could be considered as a useful and low cost fertilizer in olive orchard requiring the use of suitable doses especially in the Mediterranean area where this practice has been extended.