Monitoring Olive Oil Mill Wastewater Disposal Sites Using Sentinel-2 and PlanetScope Satellite Images: Case Studies in Tunisia and Greece

Wissal Issaoui,Evangelos Alevizos,Nikos Papadopoulos,Imen Hamdi Nasr,Mohamed Hédi Inoubli,Dimitrios D Alexakis,Athanasios V Argyriou

doi:10.3390/agronomy12010090

Abstract

Mediterranean countries are known worldwide for their significant contribution to olive oil production, which generates large amounts of olive mill wastewater (OMW) that degrades land and water environments near the disposal sites. OMW consists of organic substances with high concentrations of phenolic compounds along with inorganic particles. The aim of this study is to assess the effectiveness of satellite image analysis techniques using multispectral satellite data with high (PlanetScope, 3 × 3 m) and medium (Sentinel-2, 10 × 10 m) spatial resolution to detect Olive Mill Wastewater (OMW) disposal sites, both in the SidiBouzid region (Tunisia) and in the broader Rethymno region on the island of Crete, (Greece). Documentation of the sites was carried out by collecting spectral signatures of OMW at temporal periods. The study integrates the application of a variety of spectral vegetation indices (VIs), such as the Normalized Difference Vegetation Index (NDVI), in order to evaluate their efficiency in detecting OMW disposal areas. Furthermore, a set of image-processing methods was applied on satellite images to improve the monitoring of OMW ponds including the false-color composites (FCC), the Principal Component Analysis (PCA), and image fusion. Finally, different classification algorithms, such as the ISODATA, the maximum likelihood (ML), and the Support Vector Machine (SVM) were applied to both satellite images in order to assist in the overall approach to effectively detect the sites. The results obtained from different approaches were compared, evaluating the efficiency of Sentinel-2 and PlanetScope images to detect and monitor OMW disposal areas under different morphological environments.

Highlights

There are about 750 million productive olive trees worldwide, with 735 million of them being located in the Mediterranean region, contributing more than 97% of the olive oil world’s production
The results indicated that the Principal Component Analysis (PCA) approach (Figure 4) improved the spectral contrast of the olive mill wastewater (OMW) ponds compared to their neighborhood terrain by detecting OMW places for the majority of cases and increased the ability of Sentinel-2 and PlanetScope images to identify the OMW disposal sites
The enormous amount of OMWs generated annually from olive oil production suggest that optimum treatment is an inevitable necessity

Summary

Introduction

There are about 750 million productive olive trees worldwide, with 735 million of them being located in the Mediterranean region, contributing more than 97% of the olive oil world’s production. The olive oil production cycle generates large amounts of solid wastes and wastewater that pose a significant environmental risk due to their high phytotoxicity resulting from increased concentrations of inorganic constituents [2,3,4]. OMW disposal sites cause important environmental problems, such as the odd smell due to evaporation of ammonia and the infiltration of the organic and inorganic materials that flow in different levels of soil, eventually reaching the ground water [8]. For this reason, Bailey et al [9] indicated the vital necessity to have specific strategies in order to resolve and control OMW sites

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Discussion

Conclusion