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Biodiesel is a renewable energy source that reduces dependence on fossil fuels and produces less pollution. However, its production creates waste, primarily glycerol and methanol. Glycerol is rendered unusable and toxic by the presence of methanol and other impurities. Several proposals have been made to use biodiesel waste, including converting it into chemicals, solid fuel, and polyurethane foam, or using it as a feedstock for the production of omega-3 fatty acids.
Biodiesel is one of the main sources of alternative energy made with “green” and sustainable raw materials. The advantages of biodiesel are that it reduces dependence on fossil fuels, produces less pollution and is renewable. One of the disadvantages of biodiesel fuel is the amount of waste it produces. Biodiesel waste consists primarily of glycerol, also known as glycerin, and methanol, with minor amounts of other impurities. Although glycerol is harmless and potentially useful, the waste from biodiesel production is of low purity and rendered unusable and toxic by the presence of methanol and other substances.
The production of biodiesel fuel involves a process known as transesterification in which vegetable oil is reacted with methanol, using sodium or potassium hydroxide as a catalyst, to produce the blend of fatty esters that make up biodiesel fuel. The other product of this reaction is glycerol. Typically, one part of glycerol is produced for every ten parts of biodiesel. In practice, most biodiesel manufacturers use up to twice as much methanol as is required in principle for the reaction to ensure that all of the oil is converted. The remaining glycerol therefore contains a large proportion of methanol and small amounts of soaps resulting from the reaction of the catalyst with the vegetable oil.
Methanol can be recovered by distillation and reused; however, the remaining glycerol will still contain traces of methanol and other impurities. Purifying glycerol to make it salable is expensive, and as of 2011 so much glycerol is produced this way that it is hard to find a market for it. This means biodiesel producers can be left with large quantities of low-grade glycerol to dispose of. Due to the presence of toxic impurities, it cannot be washed out, spread on the ground or buried in landfills, and manufacturers may have to pay to have it taken away.
The use of biodiesel waste presents a challenge, but since 2011 several proposals have been made. One idea is to use a strain of the common E. coli bacterium to convert waste glycerol, by fermentation, into succinates and formates, chemicals with a wide range of applications. Another suggestion is the combination of biodiesel waste with biomass waste from agriculture to create a solid fuel: this has the advantage of reusing two waste products. The use of waste glycerol, combined with other biomass products, as a feedstock for the production of polyurethane foam has been suggested as a cost-effective way to manufacture this widely used product. Experiments have confirmed that a type of seaweed called Schizochytrium limacinum can convert crude glycerol into docosahexanoic acid (DHA), a type of omega-3 polyunsaturated fatty acid with known health benefits for humans. to make good use of biodiesel waste.
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