Olive oil processing wastes: a pressing issue

While olive oil is enjoyed the world over for its culinary and medicinal benefits, its production is integral to both the rural heritage and economy of North African and the Mediterranean. An estimated 2.9m tons of olive oil was produced worldwide in 2012, with some 2.5 million producers in the European Union’s olive sector alone — roughly one third of all EU farmers. This industry offers valuable opportunities, generating both seasonal work on farms and in the off-farm milling and processing industry; but while olive oil production has significant economic benefits, its downside is a surprisingly severe level of environmental harm and degradation.

The dark side of the olive

Two different processes can used for the extraction of olive oil: the ‘three-phase’ and the ‘two-phase’ systems, both of which generate large quantities of byproducts. The three-phase system has two main byproducts, a solid residue known as olive press cake (OPC) and a large amount of aqueous liquid known as olive-mill wastewater (OMW). Typically, it yields 20% olive oil, 30% OPC waste, and 50% OMW, which means that four times more waste is produced than actual product.

More contemporary technology has led to the introduction of the two-phase system. This reduces the volume of OMW, both by using less water and by causing more of the water used—and associated toxic substances— to be held within the olive cake in the form of a semi-solid residue (SOR). However, while the two-phase system produces less OMW, it brings its own pollution risks owing to its high organic matter concentration. Furthermore, traditional treatment technology is designed to cope with waste produced by the three-phase process, and cannot easily handle SOR.

Regardless of these differences, both processes produce  effluents exhibiting highly phytotoxic and antimicrobial properties, mainly due to the presence of poisonous, caustic crystalline compounds known as phenols. If not properly disposed of these effluents can result in serious environmental damage. Troublingly, there is no general policy governing the disposal of this waste across olive oil producing nations, and as a consequence there is inconsistent monitoring and non-uniform application of guidelines across these regions.

Snake oil solutions

Each year the Mediterranean alone produces around 30m m³ of OMW which cannot be sent to ordinary wastewater treatment systems, making safe disposal of this waste of serious environmental concern. Moreover, due to the presence of complex compounds, olive processing waste (OPW) is not easily biodegradable and needs to be detoxified before it can properly be used in agricultural or industrial processes.

This poses a serious problem for developing countries such as Morocco, Algeria and Tunisia, where the sophisticated treatment and detoxification solutions needed are simply too expensive. Subsequently, it is common for OMW to be dumped in rivers and lakes or used for farming irrigation, resulting in the contamination of ground water and the eutrophication of lakes, rivers and canals. Eutrophication in turn promotes excessive growth of algae, which deplete the water of oxygen as they die and decompose, causing aquatic populations of plants, fish, and other animals to plummet.

Developments in infrastructure are needed for dealing with olive oil processing wastes. Image by Giovanni Stradano, via Wikimedia Commons

Another common disposal method is to collect and retain OMW in large evaporation basins or ponds, where it is dried to a semi-solid fraction. In less developed countries, this waste—as well as OPC and SOR waste—is often unloaded and spread across the surrounding lands, where it builds up throughout the olive oil production season. Over time these toxic compounds accumulate in the soil, saturating it, and are often transported by rain water to nearby areas resulting in serious hazardous runoff. Because these effluents are generally untreated, this casual method of disposal leads to land degradation and contamination of soil, groundwater and the water table itself.

Even a small quantity of olive wastewater in contact with groundwater has the potential to cause significant pollution to drinking water sources. The problem is more serious where chlorine is used to disinfect drinking water as this reacts with the phenol contaminants to form chlorophenol, which is even more dangerous to human health than phenol alone.

Oil on troubled water

The problems associated with OPW have been extensively studied for the past 50 years. Unfortunately, research has continued to fall short of discovering a technologically feasible, economically viable, and socially acceptable solution. The most common solutions to date have been strategies of detoxification, production system modification, and recycling and recovery of valuable components. Because the latter results in reductions in the pollution and transformation of OPW into valuable products, it has gained popularity over the past decade.

For example, due to its plant inhibiting characteristics OPW, once properly treated, is increasingly being used as an alternative to chemical weed control. Research has also been conducted into using SOR as an absorbent for the management of hazardous oil spills; and in the field of health, studies are suggesting that OPW may be an affordable source of natural antioxidants due to the high rates found in phenolic compounds.

Still, none of these techniques alone offers a complete and exhaustive solution to the problem of OPW disposal. Industry has shown little interest in supporting any current technology or traditional process (physical, chemical, thermal or biological) on a wide scale, due to the high investment and operational costs, the short duration of the production period (three to five months) and the small size of olive mills.

The problems associated with OPW are compounded by a lack of common policy among the olive oil producing regions, a lack of funding and infrastructure for proper treatment and disposal, and a general lack of education regarding the environmental and health disbenefits of improper disposal. While some progress has been made with methods of treatment and detoxification of OPW, there is still significant scope for further research. Given the severity of the environmental impacts, it is imperative that policy-makers and industry leaders undertake more concrete initiatives to develop a sustainable framework for tackling the problem.

We are grateful to EcoMENA for the opportunity to reproduce this article, a version of which first appeared here. EcoMENA is a website focused on raising awareness of renewable energy, sustainability, waste management, environment protection, energy efficiency and resource conservation in the Middle East and North Africa (MENA) region.