**1. Introduction**

274 Olive Oil – Constituents, Quality, Health Properties and Bioconversions

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#### **2. Olive mill extraction wastes**

#### **2.1 Olive oil extraction technology**

The olive mill elaboration system has evolved over time for economic and environmental reasons. The traditional system or "pressing system" was replaced in the 70s by the threephase continuous centrifugation system. The centrifugation of the milled and beaten olives to obtain olive oil by the three-phase system produces 20% oil, 50% three-phase olive mill wastewaters (3POMWW) and 30% three-phase olive mill solid wastes (3POMSW). This three-phase system led to an increase in the processing capacity and consequently to an increase in the yield of the mills and the growth of the average mill size. However, large quantities of water needed for carrying out the three-phase process generate a high volume of olive mill wastewaters. The uncontrolled discharge of 3POMWW brought environmental problems. In some countries, technology manufacturers developed the "ecological" two-phase process. This system enables

Olive Oil Mill Waste Treatment: Improving the Sustainability

**3. Olive oil mill wastes - Current treatments** 

**3.1.1 Three-phase olive mill solid waste (3POMSW)** 

energy for their own drying process before extraction.

**3.1.2 Three-phase olive mill wastewater (3POMWW)** 

2006) and for composting with olive leaves (Michailides et al., 2011).

the decomposition processes (Khoufi et al., 2006).

**3.2.1 Two-phase olive mill solid wastes (2POMSW)** 

**3.2 Two-phase olive mill wastes** 

**3.1 Three-phase olive mill wastes** 

1992).

of the Olive Oil Industry with Anaerobic Digestion Technology 277

The traditional use of 3POMSW is to extract the residual olive oil. 3POMSW have around 4% residual oil on wet basis, which can be extracted by mechanical and/or chemical treatments. There are several extraction methods, the most usual being a first centrifugation where 40%-50% of the residual oil is extracted (Sánchez & Ruiz, 2006), followed by a drying process from 60-70% to 8% moisture (400ºC-800ºC) and extraction with solvents (hexane). Finally, the oil extracted 3POMSW is used for co-generation of heat and electricity in combustion-turbine cycles or a gas-turbine cycle. Oil extraction factories usually use this

Due to their high organic load and problematic disposal, the depuration of 3POMWW has been the subject of a great number of studies over the years. Initial treatments in the 60s focused on the use of 3POMWW as a soil conditioner if previously neutralized with lime (Albi Romero & Fiestas Ros de Ursinos, 1960). The addition of 3POMWW to the soil seems beneficial, as it produces an increase in nitrogen-fixing organisms (Garcia-Barrionuevo et al.,

3POMWW are a potential source of biophenols, some being studied for potential industrial exploitation (Cardoso et al., 2011). The extraction of polyphenols provides a double opportunity to obtain high added value biomolecules and to reduce the phytotoxicity of the effluent (Bertín et al., 2011). López and Ramos-Cormenzana (1996) showed the possibility of obtaining 4.4 g L-1 of Xanthan with 3POMWW diluted to 30%-40%. The 3POMWW have also been studied as a source of natural pigments (anthocyanins) and different exopolysaccharides, and as a growth medium for algae (Ramos-Cormenzana et al., 1995). 3POMWW have been used as a growth media for the microbial production of extra-cellular lipase (D'Annibale et al.,

Most of these studies, although very interesting, do not solve the problem because the quantities required for these studies are very small in contrast to the high quantity generated annually. The final destination of these wastewaters is mainly evaporation ponds. In the Mediterranean countries the summers are very hot, the evaporation ponds are large pools built with waterproof materials where the wastewaters can be stored for their evaporation in the summer period. After solar drying, the remaining solids can be used as fertilizer (Rozzi & Malpei, 1996). Although the evaporation ponds are very simple constructions, failure in the insulation of the basin can contaminate the ground water. Another disadvantage of these ponds is the production of putrid odors and insects during

2POMSW have around 3.5% residual oil in wet basis. Like 3POMSW, this waste is also used for residual olive oil extraction. However, the humidity of 2POMSW is higher than

reduced fresh water consumption in the centrifugation phase. The two-phase process has attracted special interest in countries where water supplies are restricted. The quantity of water required to carry out the two-phase process is much lower than in the three-phase process and a considerable reduction in generated two-phase olive mill wastewaters (2POMWW) is achieved. However, the two-phase process led to a slight increase in solid wastes. The quantities of two-phase olive mill solid wastes (2POMSW) are 60% higher than those generated in the three-phase system (3POMSW).

Over 2.9 million tonnes of virgin olive oil are produced annually worldwide, of which 2.4- 2.6 million tonnes are produced in the European Union (IOOC, 2009). Currently, both elaboration systems, three- and two-phase, coexist in the Mediterranean area (Niaounakis & Halvadakis, 2004). Spain, the largest producer of olive oil in the world, currently uses the two-phase system in 98% of its olive mills. Over the past few years, Spain has produced between 1,412,000 tonnes (2003/2004 season) and 1,028,000 tonnes (2008/2009 season) of olive oil, which meant 57.7% and 53% of European production (IOOC, 2009). Croatia uses the two-phase system in 55% of its mills and produces 4,500 tonnes of olive oil (2008/2009 season). In olive oil producing countries such as Cyprus, Portugal and Italy, only around 5% of the mills use the two-phase system (Roig et al., 2006). Other large producers such as Greece or Malta have continued using mainly the three-phase system although the twophase system is being introduced slowly. The high quantities of wastes produced in both systems makes sustainable treatments necessary.

#### **2.2 Waste quantities and characteristics**

3POMSW are produced in a proportion of 500 kg per ton of olives and are basically made up of dry pulp and stones.

3POMWW are the main wastes generated in the three-phase olive mill system (1,200 L ton-1 milled olives). The annual 3POMWW production of Mediterranean olive-growing countries is estimated between 7-30 million m3. The chemical composition of 3POMWW is complex due to the water from the milled olives (vegetation water) and the soft tissues from the olive fruit. Typical composition of three-phase olive mill wastewaters is: pH 5.04, COD 43.0 g L-1 (COD: Chemical Oxigen Demand), total sugars 17.4 g L-1, total phenols 2.5 g L-1 and lipids 0.75 g L-1 (D'Annibale et al., 2006).

The change from the three-phase to the two-phase elaboration system reduces the high generation of wastewaters produced in the three-phase process. The two-phase elaboration process generates 800 kg of 2POMSW per ton of olives processed. 2POMSW have a 60%-70% humidity content, 13%-15% lignin, 18%-20% cellulose and hemicellulose and 2.5%-3% oil (Borja et al., 2002). In a similar way to 3POMWW, the composition of 2POMSW is complex due to the vegetation water. Consequently, 2POMSW and 3POMWW are the main problematic streams.

2POMWW are a mixture between the water used for olive washing before the milling process and the water coming from washing the oil in a vertical centrifuge. Initial studies gave volumes of 2POMWW of around 250 L ton-1 of olives in total. However, current studies have suggested a significant reduction in the amount of water to be added to the vertical centrifuge.
