Pre-treatment of olive mill wastewaters at laboratory and mill scale and subsequent use in agriculture: Legislative framework and proposed soil quality indicators

Abstract

The present study investigates the potential of pre-treatment of olive mill wastewaters (OMW) at laboratory and mill scale and their subsequent use in agriculture to satisfy irrigation and fertilisation needs for widely cultivated crops. The ultimate objective of this approach is the development of a feasible decentralized treatment scheme in parts of the Mediterranean region where most olive mills are small family businesses, soils have low organic matter and the risk of desertification is high. Tests carried out both at lab and pilot scale involving mainly the use of low cost and readily available industrial/agricultural wastes/by-products such as saw dust, lime, iron fillings and goat manure. The aim of pre-treatment was two-fold (a) to produce OMW suitable for land application and (b) to allow its safe disposal in unprotected evaporation ponds, thus minimizing the risk for soil, surface- and groundwater contamination. Trials investigating the effect of pre-treated OMW on the growth of spinach and beetroot, using application rates 4, 10 and 20L/m2 (40, 100 and 200m3/ha), were also carried out in 0.5×0.5×0.2m containers. The parameters studied include emergence from soil, plant height, root length, aerial biomass, soil pH and conductivity as well as content of phenols, K, N and P in soils. Furthermore, in order to assess the long term impacts of OMW application on soil, soil quality data are provided from a field where occasional disposal of OMW from an operating evaporation pond takes place for a period of more than ten years. Furthermore, an overview of the existing EU legislation is provided to identify gaps and address important issues related to the management of OMW, its application/disposal on land and the potential risk for soil and water bodies. Finally, a set of indicators is proposed to define a code of good practice for OMW application on soils, safeguard soil quality in disposal areas and improve the overall sustainability of the olive oil sector.