Biodiesel is a major source of alternative energy made from “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 that is produced. Biodiesel waste consists mainly of glycerol, also known as glycerin, and methanol, with smaller amounts of other impurities. Although glycerol is harmless and potentially useful, waste from biodiesel production is of low purity and rendered unusable and toxic by the presence of methanol and other substances.
Making 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 mixture of fatty esters which comprise biodiesel fuel. The other product of this reaction is glycerol. Typically, one part glycerol is produced for every ten parts of biodiesel. In practice, most producers of biodiesel use up to twice as much methanol as is, in principle, required for the reaction, in order to ensure that all the oil is converted. The glycerol that is left therefore contains a large proportion of methanol, and small amounts of soaps resulting from the reaction of the catalyst with the vegetable oil.
The methanol can be recovered by distillation and reused; however, the glycerol that is left will still contain traces of methanol and other impurities. Purifying the glycerol to render it saleable is expensive, and as of 2011, so much glycerol is produced in this way that it is difficult to find a market for it. This means that biodiesel producers can be left with large amounts of low quality glycerol to dispose of. Due to the presence of toxic impurities, it cannot be flushed away, spread over the ground or buried in landfill sites, and producers may have to pay to have it taken away.
Using biodiesel waste presents a challenge, but as of 2011, a number of proposals have been put forward. 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 combining of biodiesel waste with biomass waste from agriculture to create a solid fuel — this has the advantage of reusing two waste products. Using waste glycerol, combined with other biomass products, as a raw material for the production of polyurethane foam has been suggested as an inexpensive way of manufacturing this widely used product. Experiments have confirmed that a type of alga 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 — this presents another opportunity to make good use of biodiesel waste.