Several researchers have used crude glycerol as a source of substrate for methane production and power generation, which is a way of adding value to this residue that has a high chemical oxygen demand (COD) and is rich in impurities. This review article summarizes recent data and discussions on the use of crude glycerol as substrate and co-substrate for anaerobic digestion. In general, the dilution of glycerol has been used to avoid problems of inhibition due to the presence of inorganic salts of chloride and sulphates, and due to accumulation of metabolites. However, other methods have been proposed, such as the use of halo-tolerant biomass. It can be concluded that the anaerobic digestion of crude glycerol is technically viable, and an anaerobic reactor treating 25 m 3 per day of crude glycerol can produce 4.4 MW of thermal energy, which can be converted to 4.4 GW of heat or 1.2 GW of electricity.
The fermentative production of H2 from residual glycerol is an attractive alternative for clean energy production from a waste product. Selection of operational variables for microbial populations with an adequate diversity in order to improve H2 yields is an issue faced during optimization of biological production of H2. Operational and environmental factors affect both microbial diversity and the activity of specific enzymes. Therefore, these variables must be controlled to obtain the best H2 yields. This review covers the main variables involved in the fermentative production of H2 from crude glycerol and the biochemistry of the anaerobic digestion of glycerol, with a focus on the microbial diversity involved in this process.
Biological production of carboxylic acids during glycerol fermentation via the chain elongation process was investigated. Glycerol is a by-product of the biodiesel production process and a convenient feedstock alternative for the carboxylate platform. In batch experiments, glycerol was used as substrate, whereas sludge (granular and flocculent) and ruminal liquid served as inocula. To improve the production of carboxylates, bioaugmentation with Clostridium acetobutylicum ATCC 824 was performed. Maximal production of n-caproic and n-caprylic acids was achieved with the use of the granular sludge. Bioaugmentation with C. acetobutylicum enhanced production of n-caproic acid in the presence of 100 mM ethanol. However, increasing ethanol concentration to 200 mM had no further enhancing effect on n-caproic production with or without bioaugmentation. The enhancement of ncaproic acid production was probably due to the increased production of n-butyric acid by C. acetobutylicum, because n-butyric acid is an important intermediate in the chain elongation process when ethanol is used as an electron donor.
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