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Olive oil is prized for its culi­nary uses and health ben­e­fits around the world. Yet the process of pro­duc­ing the oil results in copi­ous amounts of waste­water, which reduces soil fer­til­ity, con­t­a­m­i­nates water­ways and harms sur­round­ing ecosys­tems.
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Now, sci­en­tists have found a way to trans­form some of these neg­a­tives into pos­i­tives. They have devel­oped a pro­ce­dure that changes the pol­lu­tant into use­ful prod­ucts; namely, biofer­til­iz­ers, green fuels and safe water for crop irri­ga­tion.

In the olive oil man­u­fac­tur­ing process, olives are crushed and mixed with water. After­wards, the oil is extracted and the dirty waste­water is dis­carded. Most olive oil is pro­duced in Mediter­ranean coun­tries, where the milling pro­ce­dure gen­er­ates approx­i­mately 8 bil­lion gal­lons of waste­water per year. Such vast amounts present a chal­lenge in its dis­posal.

The removal of the waste­water can be prob­lem­atic. Dis­card­ing it in streams can pol­lute drink­ing water and pose a haz­ard to aquatic life. Using it in crops can be detri­men­tal to the soil and effect har­vest yields. Sci­en­tists have tried burn­ing the waste­water with other wastes, but the process has been either too expen­sive or has cre­ated unac­cept­able amounts of air pol­lu­tion.

In a new study, Mejdi Jeguirim and col­leagues won­dered if they could change olive mill waste­water (OMW) into prac­ti­cal sus­tain­able prod­ucts. They com­bined OMW with cypress saw­dust, another waste prod­uct com­mon in the Mediter­ranean area. After quickly dry­ing the mix­ture, they col­lected the evap­o­rated water, which could safely be used for crop irri­ga­tion.

The team then sub­jected the solid part of the mix­ture to pyrol­y­sis, which is the appli­ca­tion of high heat with­out oxy­gen to organic mate­r­ial. In the absence of oxy­gen, com­bus­tion doesn’t occur in the mate­r­ial; how­ever, it decom­poses into char­coal and com­bustible gases. The sci­en­tists col­lected the gas and con­densed it into bio-oil, a fuel that could serve as a heat source for dry­ing the OMW-saw­dust mix­ture and con­duct­ing pyrol­y­sis.

Lastly, they col­lected the char­coal, which was a rich source of potas­sium, nitro­gen, phos­pho­rus and other nutri­ents. After using this for five weeks as a biofer­til­izer for plants in flow­er­pots, they observed it sig­nif­i­cantly enhanced plant growth, result­ing in larger leaves and greater yields.

“This project offers the pos­si­bil­ity of man­ag­ing olive oil mill waste­water as a source of nutri­ents for plants,” said Jeguirim, of France’s Mul­house Insti­tute of Mate­ri­als Sci­ence. ​“The objec­tive is to use local resources in a sim­ple way in order to develop a cir­cu­lar econ­omy. The main ben­e­fits come from the envi­ron­men­tal impact which is low­ered, and also the pro­duc­tion of a biofer­til­izer.”

The study was pub­lished in the jour­nal ACS Sus­tain­able Chem­istry & Engi­neer­ing.

• ACS Sus­tain­able Chem­istry & Engi­neer­ing


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