Glycerol is an obligatory coproduct of biodiesel production, glycerol forming 10% by weight of the triglycerides that act as substrates for chemical or enzymic transesteri­fication (figure 6.1). The waste stream is, however, highly impure, with the glycerol mixed with alkali, unreacted methanol, free fatty acids, and chemical degradation products. The glycerol can, at the cost of considerable expenditure of energy in gas sparging and flash distillation, be recovered at more than 99.5% purity.74 Much sim­pler is to mix the crude glycerol with methanol and substitute waste petroleum oil and heavy fuel oil as a direct fuel.75

Glycerol represents a valuable chemical resource as a potential feedstock.15 Research into value-added utilization options for biodiesel-derived glycerol has, therefore, ranged widely in the search for commercial applications using both chem­ical and biotechnological methods (table 8.3).76 For the production of 1,3-PD for polymer manufacture, glycerol as source via microbial fermentation was (5-6 years ago) more expensive than either glucose or chemical routes from ethylene oxide or acolein.3 Those economics have significantly altered now, and glycerol represents the shortest, most direct route for bioproduction of 1,3-PD, a two-reaction sequence comprising an enzyme-catalyzed dehydration followed by a reduction:

glycerol ^ 3-hydroxypropionaldehyde ^ 1,3-PD

Microbial studies have focused on the fermentative production of 1,3-propandiol by clostridial species, but this is greatly complicated by the multiplicity of other products, including и-butanol, ethanol, and acids.77 Bioproduction of a nutraceutical fatty acid derivative (marketed as a dietary supplement) by microalgae is a route to a higher-value product than bulk chemicals.78 As examples of chemical engineer­ing, recent industrial patent documents have disclosed methods for converting the biodiesel waste glycerol into dichloropropanol and an alcohol-permeable membrane to generate a purified mixture of esters, glycerol, and unreacted alcohol.7980

For many years, glycerol was not considered fermentable by E. coli but only by a limited number of related bacterial species; a landmark publication in 2006, how­ever, reported that E. coli could efficiently ferment glycerol to ethanol (and a small amount of succinic acid) provided high pH in the culture was avoided — this is a cru­cial point because growth from glycerol requires an anaplerotic step (section 8.3.1), the CO2 being generated by the pyruvate formate lyase reaction whose activity may be much reduced by high pH in the growth medium (figure 8.6).81 The application of genetically engineered E. coli with superior ethanologenic potential (chapter 3,

Chemical and Biotechnological Transformations of Biodiesel-Derived Glycerol

Fermentation route

Clostridium butyricum, Klebsiella pneumoniae None

Gluconobacter oxydans Anaerobiospirillum succiniciproducens Enterobacter aerogenes None None

Various osmophilic microbial species

section 3.3.2.1), combined with further manipulations to eliminate competing path­ways of acid accumulation, could result in ethanol productivity approaching the theoretical molar production (mole per mole) from glycerol.