An overview on olive mill wastes and their valorisation methods

Abstract

Olive mill wastes represent an important environmental problem in Mediterranean areas where they are generated in huge quantities in short periods of time. Their high phenol, lipid and organic acid concentrations turn them into phytotoxic materials, but these wastes also contain valuable resources such as a large proportion of organic matter and a wide range of nutrients that could be recycled. In this article, recent research studies for the valorisation of olive mill wastes performed by several authors were reviewed: second oil extraction, combustion, gasification, anaerobic digestion, composting and solid fermentation are some of the methods proposed. Special attention was paid to the new solid waste generated during the extraction of olive oil by the two-phase system. The peculiar physicochemical properties of the new solid waste, called two-phase olive mill waste, caused specific management problems in the olive mills that have led to the adaptation and transformation of the traditional valorisation strategies. The selection of the most suitable or appropriate valorisation strategy will depend on the social, agricultural or industrial environment of the olive mill. Although some methods are strongly consolidated in this sector, other options, more respectful with the environment, should also be considered.

Introduction

The olive oil industry is very important in Mediterranean countries, both in terms of wealth and tradition. Spain is the main world producer followed by Italy, Greece, Turkey, Syria and Tunisia. One-third of the total world olive oil production is concentrated in Andalusia, a region in the South of Spain. This corresponds to nearly 80% of the Spanish and to more than 40% of the European olive oil production, which represents more than 75% of the global oil production. Hence, this area is especially affected by olive mill waste pollution. However, many other countries such as Argentina, Australia and South Africa are becoming emergent producers since they are promoting intensive olive tree cultivation.

The extraction of olive oil generates huge quantities of wastes that may have a great impact on land and water environments because of their high phytotoxicity. Several studies have proven the negative effects of these wastes on soil microbial populations (Paredes et al., 1987), on aquatic ecosystems (DellaGreca et al., 2001) and even in air medium (Rana et al., 2003). Therefore, there is a need for guidelines to manage these wastes through technologies that minimise their environmental impact and lead to a sustainable use of resources.

For many years, olive mill wastewater (OMW) has been the most pollutant and troublesome waste produced by olive mills in all Mediterranean countries. Thus, the management of this liquid residue has been extensively investigated and some extensive and detailed reviews, which focus mainly on its management, have been recently published (Niaounakis and Halvadakis, 2004, Azbar et al., 2004). However, from the early 1990s, the implementation of a new extraction system (two-phase technology) in some countries has introduced a new solid waste, called two-phase olive mill waste (TPOMW), with specific management problems. This study provides a summary of updated information on research works that propose different valorisation methods based on scientific studies, laying special emphasis on TPOMW.