🏭 Olive oil mill

An olive mill is a facility where oil is made from olives. The fruit is first cleaned of leaves and washed, then crushed into a paste, the paste is slowly mixed, and finally the oil is separated from the water and solid remains. There are about 12 thousand olive mills in the world, and more than 80 per cent of them use centrifuges.

An old olive mill had stone millstones and a press. The paste was spread on mats, the mats were stacked and squeezed in a hydraulic press, and the oil was left to settle. The hydraulic press was first used for olive oil in 1833 in Montilla, near Córdoba.

In a modern olive mill the paste is separated by a horizontal centrifuge called a decanter. A three-phase decanter needs 40–60 litres of warm water per 100 kilograms of olives and gives three streams: oil, fairly dry pomace and bitter wastewater. A two-phase decanter works without added water and gives oil and a single wet mass, pomace together with the fruit's juice. Oil from three-phase systems has fewer phenolic compounds, because the water washes some of them out.

An olive mill's waste is valuable too: pomace olive oil is extracted from the pomace, which is also made into fertiliser, animal feed and fuel. The wastewater, however, is acidic and rich in phenols, so it cannot simply be poured into rivers.