From pollutant to sustainable product — transforming olive mill wastewater

The more than 30 billion litres of wastewater generated by olive oil processors each year represents a huge challenge for the conventional wastewater treatment techniques.

The dark liquid effluents are characterised by high concentrations of organic compounds, including organic acids, sugars, tannins, pectins and phenolic substances that make them phytotoxic and inhibit bacterial activity. Each litre of olive mill wastewater is estimated to be the equivalent of 100 to 200 litres of domestic sewage.

During processing, olives are crushed and mixed with water in mills. The oil is separated out of this mixture, and the dirty water and solid residue are discarded. This water can foul waterways, reduce soil fertility and trigger extensive damage to nearby ecosystems. Discarding into rivers there is the risk of contaminating drinking water and harming aquatic life; pumping it onto farm land, there is the risk of damaging the soil and reducing crop yields; burning it — air pollution.

Now a team of Tunisian and French researchers have developed an environmentally friendly process that could transform olive mill wastewater into ‘green’ biofuel, biofertiliser and safe water for use in agricultural irrigation; converting the pollutant into sustainable products for practical use.

The researchers first embedded the wastewater into cypress sawdust — another common Mediterranean waste product. Then they rapidly dried this mixture and collected the evaporated water, which they say could be safely used to irrigate crops. Next, the researchers subjected the OMW-sawdust mixture to pyrolysis, a process in which organic material is exposed to high temperatures in the absence of oxygen. Without oxygen, the material doesn’t combust, but it does thermally decompose into combustible gases and charcoal. The researchers collected and condensed the gas into bio-oil, which could eventually be used as a heat source for OMW-sawdust drying and the pyrolysis process. Finally, they collected the charcoal pellets, which were loaded with potassium, phosphorus, nitrogen and other nutrients extracted from the breakdown of OMW-sawdust mixture during pyrolysis. Used as biofertilisers, the researchers found that after five weeks these pellets significantly improved plant growth, including larger leaves, compared to vegetation grown without them.

The research has been published in ACS Sustainable Chemistry & Engineering.

Image: Making olive oil produces lots of wastewater; a new environmentally friendly process could help transform this pollution into useful products. Credit: Marco Ossino/Shutterstock.com