Olive oil is obtained from olive fruit by mechanical procedures, whereas its production involves one of the following extraction processes: i) discontinuous (press) extraction, ii) 3-phase centrifugal extraction or iii) 2-phase centrifugal extraction. Each of these processes generates in different forms and compositions.
The traditional olive pressing and the three phases continuous systems produce three streams: olive oil, olive cake (or kernel) and olive mill wastewater (OMWW). The annual world OMWW production is estimated between 10 and 30 million m3. The discontinuous process (not used often anymore) produces less but more concentrated wastewater (0.5–1m3 per 1000 kg) than the centrifugation process (1–1.5m3 per 1000 kg). The 2-phase centrifugal system was introduced during the 1990s in which the olive paste is separated into phases of olive oil and wet pomace (sludge by-product) that enables reduction of the volume of OMWW. Wet olive pomace is usually further extracted with n-hexane yielding olive cake oil, although it has no significant value because of the required energy for the drying process.
OMWW is a dark-colored, acidic (3< pH value <5.9) suspension of three phases: water, oil and solids (smashed particles of olive paste and kernel). It has a characteristic unpleasant odour and high organic content, whereas is claimed to be one of the most polluting waste produced by the agro-food industries. Typically OMWW consists of: 83-94% water, 0.4-2.5% mineral salts, 0.03–1.1% lipids and 4-16% organic compounds such as carbohydrates (2-8 g/100 g), pectin, mucilage, lignin and tannins.
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