Resource Magazine September/October 2013 : Page 17

to degradation of the color-causing compounds and recalcitrant organics. This treatment would help with reuse of the recov-ered water in the plant. If the amount of ener-gy required for distillation is calculated assuming that 15 lbs (approximately 16,000 Btu) of steam is required to distill 1 gal of ethanol from a mixture containing 10% ethanol, then this energy require-ment can be mostly (>90%) supplied by the biogas produced from the anaerobic digestion of Schematic of the biofuels pilot plant in Gainesville, Fla. stillage. Studies conduct-ed by the USDA on the • Continuous anaerobic digestion of the stillage was energy balance of a corn ethanol process showed that about successfully carried out in a fluidized bed reactor. Long-term, 13,679 Btu L -1 (51,779 Btu gal -1 ) of energy is used in the bench-scale digestion of the stillage was useful in determin-ethanol conversion process. If this figure is used as a basis for ing the feasibility of the process, the biochemical methane the cellulosic ethanol process, then the biogas produced by potential of the stillage, and the various parameters required anaerobically digesting the stillage can be used to compensate to design a large-scale digester. Methane yield from anaero-about 30% of the energy input in the conversion process. bic digestion of stillage was more than 12 v/v (volume of A mass balance was also carried out on the overall methane at 0 ° C and 1 atm pressure to volume of stillage). orthophosphate-phosphorous released in the process. About • Following energy recovery in the form of biogas, the 70% of the phosphate content of the stillage comes from the stillage effluent, which contains nitrogen and phosphorous, is acid pretreatment step, with the remaining 30% released from subjected to struvite precipitation. Struvite is a slow-release the feedstock itself. Other than the phosphate that is used for phosphate fertilizer, and its precipitation paves the way to biomass growth in the digester, there is no loss of phosphate recover and reuse plant nutrients from stillage. The phospho-throughout the process. Therefore, about 99% of the phos-rous concentrations can be reduced to less than 2 ppm using phate is recovered as struvite-containing sludge that can be this process. used as a fertilizer. • Next, the remaining effluent is passed through a Every ton of sugarcane produces 0.3 tons of bagasse. The TiO 2 /UV photoreactor for final cleaning. This photocatalytic amount of sugarcane required to meet the feedstock require-treatment allows recovery of water from the process, which ments of a 3.8 million L (1 million gal) per year ethanol plant can be recycled in the plant. would be 270 tons d -1 . The recommended phosphate dosage for Mass and energy balances for a biofuel plant producing P-limited soils is about 36 kg of P per hectare. Based on this 3.8 million L (1 million gal) of ethanol per year showed that information, the phosphate precipitated as struvite-containing the raw bagasse requirement would be about ten times the sludge can supply approximately 50% of the phosphate need-mass of ethanol produced, and a significant amount of water ed to cultivate the sugarcane required to produce an adequate would be required to make a pumpable slurry. Therefore, the supply of bagasse feedstock for bioethanol production. mass of stillage produced would be about 37% more than that This integrated treatment system allows successful of the raw bagasse feed. By anaerobic digestion, the organic recovery of resources while reducing the carbon and water content of stillage is converted to biogas (60% CH 4 , 40% footprints of the biofuel production process. Similar research CO 2 ) with a heating value of 46 MMBtu d -1 . Struvite precip-is now being conducted on stillages obtained from the fer-itation from the digested effluent yields 615 kg d -1 of struvite-mentation of other types of feedstock, including eucalyptus containing processed sludge. The remaining dark-colored, and wheat straw. nutrient-deprived stream, after exposure to advanced oxida-A S A B E m e m b e r P r at a p P u l l a m m a n ap p a l l i l , Associate Professor, tion via TiO 2 -mediated photocatalysis, would decolorize due Department of of Agricultural and Biological Engineering, University of Florida, Gainesville, USA, RESOURCE September/October 2013 17

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