**3. Olive mill waste as renewable low-cost substrates**

According to the last report of Food and Agriculture Organisation of the United Nations (FAOSTAT 2009), 2.9 million tons of olive oil are produced annually worldwide, 75.2% of which are produced in Europe, with Spain (41.2%), Italy (20.1%) and Greece (11.4%) being the highest olive oil producers. Other olive oil producers are Asia (12.4%), Africa (11.2%), America (1.0%) and Oceania (0.2%). Olive oil production is a very important economic activity, particularly for Spain, Italy and Greece; worldwide, there has been an increase in production of about 30% in the last 10 years (FAOSTAT 2009).

Multiple methods are used in the production of olive oil, resulting in different waste products. The environmental impact of olive oil production is considerable, due to the large amounts of wastewater (OMWW) mainly from the three-phase systems and solid waste. The three-phase system, introduced in the 1970s to improve extraction yield, produces three streams: pure olive oil, OMWW and a solid cake-like by-product called olive cake or *orujo*. The olive cake, which is composed of a mixture of olive pulp and olive stones, is transferred to central seed oil extraction plants where the residual olive oil can be extracted. The twophase centrifugation system was introduced in the 1990s in Spain as an ecological approach for olive oil production since it drastically reduces the water consumption during the process. This system generates olive oil plus a semi-solid waste, known as the two-phase olive-mill waste (TPOMW) or *alpeorujo* (Alburquerque et al. 2004; McNamara et al. 2008; Morillo et al. 2009).

The olive oil industry is characterized by its great environmental impact due to the production of a highly polluted wastewater and/or a solid residue, olive skin and stone (olive husk), depending on the olive oil extraction process (Table 1) (Azbar et al. 2004).

Pressure and three-phase centrifugation systems produce substantially more OMWW than two-phase centrifugation, which significantly reduces liquid waste yet produces large amounts of semi-solid or slurry waste commonly referred to as TPOMW. The resulting solid waste is about 800 kg per ton of processed olives. This ''alpeorujo'' still contains 2.5–3.5% residual oil and about 60% water in the two-phase decanter system (Giannoutsou et al. 2004).

**INDUSTRY** 

**ENERGY** 

**AGRICULTURE**

 **Wastewater treatment Enzymes Organic acid Biopolymers Biosurfactant Food and Cosmetics Pharmaceutical** 

 **Biofuels - Bioethanol - Biodiesel - Biogas - Biohydrogen** 

 **Biofertilizers Biomass Compost Animal feed** 

Fig. 1. Potential uses of olive mill wastes in microbial biotechnology.

(Giannoutsou et al. 2004).

**OLIVE MILL WASTES** 

of plants (Roig et al. 2006; McNamara et al. 2008).

Moreover, olive oil production is no longer restricted to the Mediterranean basin, and new producers such as Australia, USA and South America will also have to face the environmental problems posed by OMWs. The management of wastes from olive oil extraction is an industrial activity submitted to three main problems: the generation of waste is seasonal, the amount of waste is enormous and there are various types of olive oil waste

OMWs have the following properties: dark brown to black colour, acidic smell, a high organic load and high C/N ratio (chemical oxygen demand or COD) values up to 200 g per litre, a chemical oxygen demand/biological oxygen demand (COD/BOD5) ratio ranging from 2.5 to 5.0, indicating low biodegrability, an acidic pH of between 4 and 6, high concentration of phenolic substances 0.5–25 g per litre with more than 30 different phenolic compounds and high content of solid matter. The organic fraction contains large amounts of proteins (6.7–7.2%), lipids (3.76–18%) and polysaccharides (9.6–19.3%), and also phytotoxic components that inhibit microbial growth as well as the germination and vegetative growth


Table 1. Inputs and outputs from olive oil industry (Adapted from Azbar et al. 2004)

The average amount of OMWs produced during the milling process is approximately 1000 kg per ton of olives (Azbar et al. 2004). 19.3 million tons of olive are produced annually worldwide, 15% of them used to produce olive oil (FAOSTAT 2009). As an example of the scale of the environmental impact of OMWW, it should be noted that 10 million m3 per year of liquid effluent from three-phase systems corresponds to an equivalent load of the wastewater generated from about 20 million people. Furthermore, the fact that most olive oil is produced in countries that are deficient in water and energy resources makes the need for effective treatment and reuse of OMWW (McNamara et al. 2008). Overall, about 30 million tons of OMWs per year are produced in the world that could be used as renewable negative or low-cost substrates.
