What do we do with all the used oil we produce?

After cooking, throwing the already used oil in the kitchen sink is not a good decision, as it will bring some unfavorable consequences for the environment.

To win a dispute with Poseidon for the protection of a city, the goddess Athena made an olive tree sprout. Oil could be obtained from it to use as food, a remedy for wounds, and anointing oil. Thanks to the virtues of the tree and its product, Athena won. Since then she has been known as the goddess of wisdom and the city received the name of Athens.

Oil appeared as a natural step after the invention of agriculture. Humanity domesticated plants - such as olive trees, soybeans, sunflowers - and animals to extract their oils and fats, respectively, and use them both in food and in industry.

Since ancient Greece and the first agricultural civilizations, mankind has developed industrial production skills for these substances that would leave Zeus himself in awe.

In Spain, for example, we consume around 850,000 t of oil per year. This massive production of oil is associated with a proportional waste production. The US generates about 11 billion liters of used oil, while in Europe this figure is 1 billion. Spain generates about 150 million liters of used vegetable oil.

With respect to residual animal fats, 2.6 million tons (beef, pork, sheep fat, etc.) were discarded in the US alone.

Both of these substances are generated primarily from food processing industries, restaurants, and in our homes.

Management of a polluting waste

The waste oils and grease should be disposed of separately from the organic waste generated during cooking. They should never be dumped into the drainage network, since their contamination capacity is high. A landfill or mismanagement of this waste represents a real danger; It is estimated that a liter of oil can contaminate a thousand liters of water.

Due to their immiscibility, these substances create a superficial film preventing the exchange of oxygen in aquatic environments. They thus cause the degradation of rivers, lakes, aquifers and affect soils, ecosystems and, therefore, agriculture. Its persistence causes an increase in the costs of the infrastructures necessary for the remediation of contaminated soils and the purification of water.

Traditionally, a homemade form of recycling has been the production of soap. By combining the residual oil with caustic soda and applying heat, the saponification reaction takes place. The most daring can also produce their own fuel by producing homemade biodiesel.

However, modern life is hardly compatible with craftsmanship. Therefore, it is best to leave the recycling of this waste in specialized hands, which will carry out a controlled collection.

A second life for oils and fats

The scientific community is trying to give a value to these residues, so that, through a series of transformations, they can be used for other purposes.

The reuse or recycling, which comes to be the enhancement of this waste, is part of the circular economy model. This is done through a biorefinery model. In these facilities, a variety of products and bioenergy are obtained from biomass (or plant or animal waste).

The process is similar to traditional refineries, but more sustainable. Opting for this alternative allows us to mitigate our dependence on oil, its environmental impact and reduce the impact of dumping this food waste.

Manufacture of biofuels

Fats and oils are substances called esters. Generally, they are made up of three fatty acids (which can have a higher degree of saturation or unsaturation) and a glycerin molecule. We call this type of ester triglyceride and its synthesis in nature is related to the storage of energy for times of scarcity.

The triglycerides are synthesized by bacteria, fungi, plants, animals and humans. If at room temperature they are in a liquid state they are oils and if they are in a solid state they are fats.

The oil can be used as a fuel for diesel engines. In fact, the engine that Rudolf Diesel invented and presented at the 1900 World's Fair in Paris used peanut oil as fuel.

However, the viscosity of the oils is very high, which means that carbon can form when burned, among other problems. To reduce it, a transesterification reaction is used. This process basically consists of replacing the glycerin molecule with another lighter alcohol, transforming the ester into biodiesel. This reduces the viscosity and produces a fuel with properties similar to diesel.

Biodiesel has been losing weight as an alternative fuel, since its production usually starts from vegetable oils and could, therefore, compete with its use in food.

The use of residual oils can solve this conflict, since it has been shown that the biodiesel obtained has the same quality as that from vegetable oils.

In the BIOSAHE Research Group we have produced biodiesel from different oils and fats : recycled oils, inedible vegetable oils, oils extracted from garbage and kebab fats .

On the other hand, the emergence of electric cars and other alternative fuels such as hydrogen may further reduce the need for biodiesel, reducing its use to those situations in which no other emission-free source can be used.

Other uses: biolubricants and bioplastics

Oils and greases can also be used for lubrication  With a process similar to that of biodiesel production, new biolubricants have been obtained, which are biodegradable and less polluting than mineral oils.

In addition to applications in the automotive sector, these residues can be used in the pharmaceutical and nutraceutical sectors. The production of soaps via saponification has been a fact for a long time, but it has been observed that the composition of many of these residues is rich in antioxidants and that it can help in the treatment of many diseases.

Regarding the fish processing industry, we have discovered that the residues are rich in omega-3 fatty acids, which have a high added value to combat, for example, cholesterol.

A use of great value to develop in the coming years is the production of bioplastics from microorganisms fed with residual oils.

Many bacteria have the ability to break down triglycerides and include them in their metabolism (thanks to the secretion of an enzyme called lipase and a metabolic process called β-oxidation, which humans use to lose weight). Thus, they are capable of transforming them into other substances such as bioplastics.

Biopolymers have been shown to have similar properties to thermoplastics from petroleum. Therefore, they can be used in technology, such as 3D printing, opening an incredible range of possibilities.

As we can see, the oil that comes out of the fryer can have thousands of lives. What helped us today to cook some fabulous French fries, tomorrow may be the bioplastic of the pan handle.

In turn, by disposing of this utensil, other microorganisms can use this bioplastic to produce the hydrogen that takes us to work (in a new engine that takes advantage of the reaction of hydrogen and oxygen to produce energy) and water molecules, which will irrigate the olive tree that hundreds of years ago a goddess like Athena gave us. In this case, the miracle is the result of the circular economy.

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