Biofuel co-products as livestock feed - Opportunities and challenges

Micro-algae for fuel and use of spent biomass for feed and for other uses 431other contaminants like NO x and SO x . Cooling of the gasesis a costly option, but conversion of these gases into solublecarbonates such as Na 2 CO 3 and NaHCO 3 is an easy optionas some of the micro-algae can survive in extreme conditionsof high pH, thereby minimizing invasive species andother biological contaminants (Benemann 2003; Wang etal., 2008).Waste water utilization and cultivation of microalgaeOne of the economical means of cultivation of micro-algaefor biofuel production is utilizing municipal waste waterand industrial effluent. The utilization of effluents providesa way for removal of chemical contaminants, such as heavymetals. Scenedemus obliquus has been extensively studiedfor utilization of waste waters (Voltolina, Gomez-Villa andCorrea, 2005; Hodaifa, Martinez and Sanchez, 2008).Sawayama, Minowa and Yokayama (1999) developed acultivation strategy for uptake of nitrogen and phosphorousfrom sewage water and production of hydrocarbonrich biomass of B. braunii. Micro-algae are used in effluentsfrom aquaculture, dairy farms and the food processingindustry for removal of nutrients (nitrates, ammonia andphosphates) and odour, and to reduce acidity withoutchemicals. In oil drilling, micro-algae are used for reducingsludge and to remove metals and their precipitates.Effective utilization of algal biomass for waste treatmentwould ensure a net positive energy balance and economicallyviable algal cultivation technology.An integrated approach of growing micro-algae inwaste water and utilization of biomass for biofuel is shownschematically in Figure 3. The spent biomass obtained afterbiofuel extraction would be utilized further for bio-energyproduction through bio-methanation, unlike freshwatergrownmicro-algal spent biomass utilized for feed and otherbio-active molecules.Alternately the spent biomass can be converted to fuelproducts through pyrolysis, chemical catalysis or hydrocracking,and used in diesel, jet fuel or gasoline.DOWNSTREAM PROCESSING AND CONVERSIONTO BIOFUELSHarvesting and dewatering of micro-algaeMass production of micro-algae for metabolites requiresefficient harvesting and extraction methods. Harvesting ofbiomass requires one or more solid-liquid separation techniques,which depend on the nature of the alga, such assize, density and metabolite to be obtained. The commontechniques applied in the harvesting of biomass are centrifugation,flotation, flocculation and gravity sedimentation.Centrifugation recovery is the most preferred methodof harvesting for high value metabolites. Laboratory experi-FIGURE 3Scheme for integrated cultivation of micro-algae utilizing waste water and flue gasesWastewater afterprimary treatmentCO 2source(flue gas oralcoholicfermentation)Algal cultivationBiomass gasificationand anaerobic digestionEnergyAlgal biomassSpentbiomassBiofertilizerTransesterificationLipids orhydrocarbonsHydrocrackingValue added coproductrecoveryFeedapplicationGlycerolIndustrial chemicalsBiofuelrecovery