Biotechnology, in particular the fermentation sector, has become more and more attractive in recent years for the production of chemicals and biofuels from organic wastes (Willke and Vorlop, 2004). In fact, there are numerous possibilities for replacing chemical techniques with biotechnological methods based on renewable resources. The most important biogenic sources of raw materials for industrial chemicals are: oil plants (oil, fat, glycerol, celluloses); starch plants (starch, inulin, carbohydrates, celluloses); sugar beets and sugar cane (sucrose); wood (ligno-cellulose, cellulose); and waste and residues from agriculture and industry (biomass, fats, oils, whey, glycerol). The food industry is probably the main source for these materials.
Fermentative processes can be used for both production of biofuels (methane, hydrogen, ethanol, biodiesel) and building blocks, such as lactic acid, succinic acid, ascorbic acid, isomalt, cyclodextrines and polyamino-acids (Wilke, 1995; Gavrilescu and Chisti, 2005). Some important examples of chemicals and energy sources or vector production are reported below.
1 Building blocks. Costs for industrial chemicals are generally in the range
2-5 US$/kg, therefore the use of innovative bioprocesses using waste as substrates and (possibly) mixed cultures is very welcome. Typical examples of the bioproduction of building block chemicals from food waste(water) are lactic acid (anaerobic fermentation of starch, sugars or wheat) and succinic acid (fermentation of sugars by bacteria in anaerobic conditions). Lactic acid is produced at a cost of 0.5 US$/kg at 75 000 tonnes/year. Other compounds of importance are ascorbic acid (vitamine C), isomalt, cyclodextrines, 1-3 propandiol, polyaminoacids (PAA). These can be produced through the fermentation of sugar-rich waste(waters) (Wilke, 1995, 1999; Traverso et al., 2000; Willke and Vorlop, 2004).
2 Bulk chemicals. Bulk chemicals (production costs <1 US$/kg) produce a volume of sales of some 10 billion US$ (Wilke, 1999). These are alcohols (ethanol), organic acids, amino acids and other fermentation products. Fermentation of waste and wastewaters from the food processing industry can be the main route for the production of these compounds as the feedstocks are clearly cheaper than pure organic compounds (sugars). Traverso et al. (2000) and Bolzonella et al. (2005) have demonstrated the possibility of producing fatty acids at very high concentrations (up to 50 g/l) using vegetable and food wastes as substrates in anaerobic mesophilic (35 °C) reactors.
3 Bioenergy and biofuels. Biotechnology-based production of fuels continues to attract much attention (Kosaric and Velikonja, 1995). Bioethanol, biogas, biodiesel and biohydrogen are examples of biofuels. The use of these compounds directly reduces consumption of fossil fuels and environmental pollution (Gavrilescu and Chisti, 2005). Biodiesel, for example, can be produced through the transesterification of veget able oils and animal fats, all by-products of food processing wastewaters (Ma and Hanna, 1999).
The biotechnological production of acetone, butanol and ethanol (ABE process; Willke and Vorlop, 2004) works on non-purified substrates like hydrolysed starch or cellulose. Ethanol can be produced by yeasts and bacteria through the fermentation of sugars, generally glucose, coming as residues from the food industry (sugar cane, beets) while hydrogen can be directly produced from mixed cultures by anaerobic fermentation of food processing wastewaters (Ginkel et al., 2005) or waste (Valdez-Vazquez et al., 2005). Obviously, at the moment, production of methane is the only well-established technology for energy recovery from food processing waste(waters) (Kosaric and Velikonja, 1995; Carucci et al., 2005).
Another interesting option is the possibility of co-digesting the waste(waters) from the food industry in existing anaerobic digesters operating in wastewater treatment plants. In fact, at present, some 36 000 anaerobic digesters are operating in Europe (Mata-Alvarez et al., 2000); these are generally under-loaded and can be conveniently used to produce methane, and thus heat and power, in co-generation units. In Treviso WWTP (Cecchi et al. 1994), waste from markets and canteens/restaurants is co-digested together with waste activated sludge to produce important amounts of biogas (Bolzonella et al., 2006): typical productions are in the range 0.4-0.6 m3 of biogas (70% methane) per kilogram of solid fed into the reactor.
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