Biofuel co-products as livestock feed - Opportunities and challenges

432Biofuel co-products as livestock feed – Opportunities and challengesments conducted on harvesting of biomass indicated centrifugalrecovery is rapid and about 80–90 percent of algalbiomass can be recovered with 1000–5000 g for 2–3minutes (Chen et al., 2010). Centrifugation is a preferredmethod of harvesting algal biomass for producing extendedshelf-life concentrates for aquaculture hatcheries and nurseries(Knuckey et al., 2006). The only limiting factor is thehigh capital and operating costs for harvesting of largequantities of water and algae (Grima et al., 2003).Gravity sedimentation is a type of solid-liquid separationwhich is commonly applied for separating micro-algae inwater and waste water treatment. Particles with higherdensity can be separated easily by gravity sedimentation,whereas particles with a diameter of a few microns requireflocculants to form larger aggregates. Lamella separatorsand sedimentation tanks are used for enhanced micro-algalseparation through sedimentation (Uduman et al., 2010).Biomass-sediment is collected in a sump and recoveredby pumping. Gravity sedimentation in sedimentation tankis time consuming and very few reports are available onthis method; it relies on the autoflocculation principle.Addition of flocculants increases the efficiency of gravitysedimentation, which could be an inexpensive process.Filtration is used effectively for the harvesting of largermicro-algae like Coelastrum spp. or Spirulina spp., but failsto recover micro-algae with smaller cell dimensions (Mohn,1980). Filtration is theoretically simple, but very expensivein practice, and its understanding involves several issues,like filter pore size, filter material and also the design ofthe filters. Larger filter apertures allows smaller cells to passthrough, whereas decreased pore size results in binding orblocking of filter pores, and reduced filtration rates. Filterpore size is dependent on the size of the algal species andalgal aggregation rate. Filtration materials and filtrationdesign are of primary importance, which determine the costeconomics and efficiency of the process (Oswald, 1991).Flocculation can be a preliminary step in the bulkharvesting process, which helps to aggregate the microalgalcells in order to increase the effective particle size.Flocculation can be enhanced by addition of flocculantsthat can reduce or neutralize the surface negative chargeof the cells thereby increasing the effective particle size forgravity settling. An ideal flocculant should be inexpensive,non-toxic and effective at low concentrations, and alsoshould not affect further downstream processing (Grima etal., 2003; Murthy, 2005). Flocculants generally coagulatealgal cells by neutralizing the surface negative charge, asin the case of polycationic inorganic or organic compoundssuch as polyvalent metal salts, which are iron- or aluminumbasedcoagulants, like ferric chloride, aluminum sulphateand ferric sulphate. Coagulation efficacy of metal ionsincreases with increasing ionic charge (Brennan and Owende,2010). Multivalent salts like alum are used effectively toharvest Chlorella and Scenedesmus in waste water treatmentprocesses (Grima et al., 2003). Organic flocculants, apartfrom reducing or neutralizing the surface charge, can bringparticles together by physical linkage through a processcalled bridging (Grima et al., 2003). Chitosan is being usedas a biodegradable organic flocculant that can be synthesizedfrom natural sources (Divakaran and Pillai, 2002). Polymericorganic flocculants such as polyacrylamide, cationic starch,poly-ferric sulphate, etc., are commonly used for harvesting.Marine organisms cannot be effectively harvested throughflocculation due to high ionic concentrations within cells(Bilanovic, Shelef and Sukenik, 1998). The main problemsassociated with flocculation are accumulation of the flocculantin the media and its effect on possible further recycling ofmedia (Chen et al., 2010).Flotation relies on the attachment of air or gas bubblesto solid particles, which are then carried to the liquid surfaceand accumulate as float, which can be easily separated.Flotation is a more effective and beneficial harvestingmethod than flocculation since it obviates the use ofchemicals. Solid particles can be separated by colliding airbubbles with the particles; for capturing particles smallerthan 500 µm, flotation efficiency increases with decreasingparticle size (Yoon and Luttrell, 1989; Uduman et al., 2010).Although flotation could be a potential harvesting system,its efficacy has yet to be clearly evaluated (Brennan andOwende, 2010).According to Uduman et al. (2010), any dewateringtechnology can be quantitatively evaluated by the followingparameters: the rate of water removal of the dewateringtechnique; the solid content of the recovered micro-algaewaterslurry (percent total suspended solids - TSS); andthe yield of the processed micro-algae by the dewateringtechnique. They proposed a single-step simultaneousharvesting and dewatering process for micro-algae, froman initial 0.02 to 0.06 percent TSS to a primary harvest of2 to 7 percent TSS, followed by dewatering to give 15 to25 percent TSS.The final step in harvesting is the complete dewateringand drying of the micro-algal slurry. This step is one ofmajor economic importance. Selection of the dryingmethod depends on the use for which the dried productis intended, and also the scale of operation. Some of thecommonly available drying methods include sun drying,drum drying, freeze drying, air drying, oven drying andspray drying. Freeze drying and spray drying methods arefound to be rapid and the most suitable drying methodfor algae, but is comparatively energy intensive. Spraydrying is often used for algae, finding food applicationssuch as with Dunaliella spp. and Spirulina spp. (Leach,Oleveira and Morais, 1998). Oven-type driers are alsofound to be effective and less energy intensive, butare not suitable for heat-sensitive metabolites (Mohn,