In terms of energy inputs, harvesting of algal biomass is the most energy-intensive process in biomass production. To date, there has been no specific commercial — scale algal harvesting technique that has been developed, and the approach has been to adapt separation technologies already in use in wastewater treatment and food processing industries. Therefore, the energy consumption and energy efficiency information available from those industries are discussed in this chapter to compare the energy efficiency of different algal harvesting techniques. The highest possible solids recovery (as %(w/v) total suspended solids (TSS)) and energy requirements for each of the harvesting processes are given in Table 6.1.

TABLE 6.1 Summary of Energy Usage and Highest Possible Solids (%w/v) Yields of Different Algae Harvesting Techniques Highest Yield Energy Usage Harvesting Process (% solids) (kW-hm-3) Ref. Centrifugation 22.0 8.00 Girma et al., 2003 Gravity sedimentation 1.5 0.1 Shelef et al., 1984 Filtration (natural) 6.0 0.4 Semerjian et al., 2003 Filtration (pressurized) 27.0 0.88 Semerjian et al., 2003 Tangential flow filtration 8.9 2.06 Danquah et al., 2009 Vacuum filtration 18 5.9 Girma et al., 2003 Polymer flocculation 15.0 14.81 Danquah et al., 2009 Electro-coagulation NA 1.5 Bekta§ et al., 2004 Electro-flotation 5.0 5.0 Shelef et al., 1984; Azarian et al., 2007 Electro-flocculation NA 0.331 Edzwald, 1995

Gravity sedimentation is relatively less energy intensive as fewer motors, pumps, and settling tanks are needed for its operation, thus resulting in low capital and operational cost and high expected life span. However, for a commercial-scale (>4 hectares) algae cultivation process and considering the slow sedimentation rates of algae, multiple tanks of large volumes (~100,000 L each) may be required.

Centrifugation is a very efficient technique, but the large energy requirement for the process clearly eliminates it as an option for harvesting a low-value energy crop. Therefore, this process can be ruled out for harvesting algae biomass for biofuel production, at least for first-stage dewatering (increasing solids content from algae culture on the order of 1%), on both cost and energy grounds.

Membrane filtration would be the next most efficient harvesting option; however, field experience on algae farms would be required to verify the lifetime and mainte­nance costs of the filter elements. Membrane filtration may be a competitive option if the back-flush function could be carried out with air knives in place of water jets, in order to achieve the desired consistency of 1% to 5% solids. Dissolved air flotation would be the expensive option in terms of cost and energy burden.

Of the flocculation-based processes, polymer flocculation is not only the most efficient, but also the most energy-intensive technique for dewatering. On the other hand, electro-flocculation techniques are cost-effective with low energy burden, but these techniques are still in their infancy and large-scale field testing is required to verify the overall process efficiency. Auto-flocculation is the lowest cost, low­est energy dewatering process by far, at one-tenth those of membrane filtration and polymer-based dewatering. Moreover, the chemicals required are pond nutrients, which can be recovered from the biomass for re-use either via anaerobic digestion, as would be the nitrogen, phosphorous, and potassium nutrients, or via an inexpensive carbonic acid extraction process if necessary, thereby avoiding the production-scale limitations imposed by synthetic polymer flocculants. As with the growth ponds and the anaerobic digesters, auto-flocculation employs managed natural processes to achieve its ends, at considerable savings in cost and energy.

Although the data presented in Table 6.1 appear straightforward and it would be easy for anyone to compare the harvesting and energy efficiencies of various processes, there are a variety of fundamental operational issues associated with each process. Therefore, it is important to carefully analyze several parameters, such as cell morphology, ionic strength of the media, pH, culture density, and final downstream processing of harvested biomass, when selecting a suitable harvesting technique. For example, very small sized algae could hinder the harvesting effi­ciency and would have a negative impact on the economics of biomass production if subjected to gravity sedimentation and filtration. However, if such algae could be made to float via the DAF (dissolved air flotation) flocculation process, this may facilitate harvesting. Furthermore, downstream processing of the harvested biomass to get final products will also be an important factor in selecting the har­vesting process. If the biomass will be subjected to anaerobic digestion (AD) for biogas production, a solids content up to 5% (w/v) would suffice; whereas, if lipid extraction followed by biodiesel production is the goal, the biomass needs to be dewatered to lower moisture contents. There is considerable interest in efficient but less energy-intensive harvesting technologies to make microalgae cultivation cost effective and competitive for renewable bioenergy production. Thus far, no single harvesting technique can be universally applied to algae cultivation systems, and a combination of different techniques could be applied in a specific sequence to achieve maximum biomass concentration with minimum energy usage. Moreover, there could be considerable costs and energy savings in custom-designed, multi­stage harvesting techniques for algal farms, in which a variety of harvesting techniques are arranged in a specific sequence based on culture chemistry, and the specific characteristics of each technique and its energy requirements. Such systems can achieve dewatering of pond water to either 5%, or 10% to 20% solids at the least energy input and cost. In an open pond system, dominant algal species could range from small unicellular to large colonial or filamentous species. In such cases, TFF and other filtration techniques could be used as the first stage to remove filamentous and auto-flocculated algae, followed by chemical flocculation, sedimentation, and/ or flotation to produce algal slurries with 1% to 5% solids, which could be either directly subject to the AD process if biogas is the final biofuel or subject to cen­trifugation to achieve >20% solids. Centrifuging 5% algal slurry would reduce the energy and cost requirements for this technique by 100 times, as opposed to direct centrifugation of otherwise very dilute algae culture (~0.05% solids). Similarly, the pond water temperature, alkalinity, and pH may also vary during different climatic conditions throughout the year and even during different times of the day, thus impacting the ionic strength, salts solubility, and eventually the biomass auto­flocculation properties. Auto-flocculation could be the lowest-cost, lowest-energy dewatering process by far, at one tenth those of membrane filtration and polymer — based dewatering.

6.2 CONCLUSION

There are several biomass harvesting techniques available for the recovery of algae from culture broth. However, no individual technique can be applied ubiquitously due to technical and economical limitations. Gravity sedimentation is relatively less energy intensive but the slow sedimentation rates of algae may negatively impact production economics. Centrifugation is very efficient, but the large energy require­ments for the centrifuge clearly eliminate it as an option for direct harvesting of a low-value energy crop. However, if used as a second-stage process for harvesting 5% algal slurries to higher solids concentrations, this could significantly reduce the energy and cost requirements. Membrane filtration is an efficient harvesting option; however, field experience on algae farms would be required to verify the lifetime and maintenance costs of filter elements. Dissolved air flotation would be an expensive option in terms of cost and energy. Polymer flocculation is also efficient but energy intensive for dewatering, while electro-flocculation is cost effective with low energy usage. Auto-flocculation is the lowest cost, lowest energy dewatering process by far. It is recommended to apply custom-designed multi-stage harvesting techniques for algal harvesting in which a variety of harvesting technologies are organized in some sequence to achieve the highest efficiency and lowest cost.

REFERENCES

Aragon, A. B., Padilla, R. B., and Ros de Ursinos, J. A.F. (1992). Experimental study of the recovery of algae cultured in effluents from the anaerobic biological treatment of urban wastewaters. Resources, Conservation and Recycling, 6: 293-302.

Azarian, G. H., Mesdaghinia, A. R., Vaezi, F., Nabizadeh, R., and Nematollahi, D. (2007). Algae removal by electro-coagulation process, application for treatment of the effluent from an industrial wastewater treatment plant. Iranian Journal of Public Health, 36: 57-64.

Bare, W. F., Jones, N. B., and Middlebrooks E. J. (1975). Algae removal using dissolved air flotation. Journal of the Water Pollution Control Federation, 47: 153-169.

Bekta§, N., Akbulut, H., Inan, H., and Dimoglo, A. (2004). Removal of phosphate from aque­ous solutions by electro-coagulation. Journal of Hazardous Materials, 106: 101-105.

Benemann, J., and Oswald, W. (1996). Systems and Economic Analysis of Microalgae Ponds for Conversion of CO2 to Biomass. Final Report to the U. S. Department of Energy. Pittsburgh Energy Technology Center, Grant No. DE-FG22-93PC93204.

Brennan, L., and Owende, P. (2010). Biofuels from microalgae—A review of technologies for production, processing, and extractions of biofuels and co-products. Renewable and Sustainable Energy Reviews, 14: 557-577.

Chisti, Y. (2007). Biodiesel from microalgae. Biotechnology Advances, 25: 294-306.

Danquah, M. K., Ang, L., Uduman, N., Moheimani, N., and Forde, G. M. (2009). Dewatering of microalgal culture for biodiesel production: Exploring polymer flocculation and tangen­tial flow filtration. Journal of Chemical Technology and Biotechnology, 84: 1078-1083.

Divakaran, R., and Pillai, V. N.S. (2002). Flocculation of algae using chitosan. Journal of Applied Phycology, 14: 419-422.

Edzwald, J. (1995). Principles and applications of dissolved air flotation. Water Science and Technology, 31: 1-23.

Gao, S., Yang, J., Tian, J., Ma, F., Tu, G., and Du, M. (2010). Electro-coagulation-flotation process for algae removal. Journal of Hazardous Materials, 177: 336-343.

Girma, E., Belarbi E. H., Fernandez, G. A., Medina A. R., and Chisti Y. (2003). Recovery of microalgal biomass and metabolites: process options and economics. Biotechnology Advances, 20: 491-515.

Harun, R., Singh, M., Forde, G. M., and Danquah, M. K. (2010). Bioprocess engineering of microalgae to produce a variety of consumer products. Renewable and Sustainable Energy Reviews, 14: 1037-1047.

Heasman, M., Diemar, J., Connor, O’., Shushames, T., Foulkes, L., and Nell, J. A. (2000). Development of extended shelf-life microalgae concentrate diets harvested by centrifu­gation for bivalve molluscs—A summary. Aquaculture Research, 31: 637-659.

Khan, S. A., Hussain, M. Z., Prasad, S., and Banerjee, U. C. (2009). Prospects of biodiesel production from microalgae in India. Renewable and Sustainable Energy Reviews, 13: 2361-2372.

Knuckey, R. M., Brown, M. R., Robert, R., and Frampton, D. M.F. (2006). Production of microalgal concentrates by flocculation and their assessment as aquaculture feeds. Aquacultural Engineering, 35: 300-313.

Lee, S. J., Kim, S. B., Kim, J. E., Kwon, G. S., Yoon, B. D., and Oh, H. M. (1998). Effects of harvesting method and growth stage on the flocculation of the green alga Botryococcus braunii. Letters in Applied Microbiology, 27: 14-18.

Matis, K. A., Gallios, G. P., and Kydros, K. A. (1993). Separation of fines by flotation tech­niques. Separations Technology, 3: 76-90.

Mohn, F. H. (1980). Experiences and strategies in the recovery of biomass in mass culture of microalgae. In Shelef, G., and Soeder, C. J., Editors, Algal Biomass: Production and Use, Elsevier Science Ltd., Kidlington, UK, pp. 547-571.

Oh, H. M., Lee, S. J., Park, M. H., Kim, H. S., Kim, H. C., Yoon, J. H., Kwon, G. S., and Yoon, B. D. (2001). Harvesting of Chlorella vulgaris using a bioflocculant from Paenibacillus sp. AM49. Biotechnology Letters, 23: 1229-1234.

Pittman, J. K., Dean, A. P., and Osundeko, O. (2011). The potential of sustainable algal biofuel production using wastewater resources. Bioresource Technology, 102: 17-25.

Schenk, P. M., Thomas-Hall, S. R., Stephens, E., Marx, U. C., Mussgnug, J. H., Posten, C., Kruse, O., and Hankamer, B. (2008). Second generation biofuels: High efficiency microalgae for biodiesel production. BioEnergy Research, 1: 20-43.

Semerjian, L., and Ayoub, G. M. (2003). High-pH-magnesium coagulation-flocculation in wastewater treatment. Advances in Environmental Research, 7: 389-403.

Shelef, G., Sukenik, A., and Green, M. (1984). Microalgae Harvesting and Processing: A Literature Review. Report, Solar Energy Research Institute, Golden, CO, SERI Report No. 231-2396.

Sim, T. S., Goh, A., and Becker, E. W. (1988). Comparison of centrifugation, dissolved air flotation and drum filtration techniques for harvesting sewage-grown algae. Biomass, 16: 51-62.

Spolaore, P., Joannis-Cassan, C., Duran, E., and Isambert, A. (2006). Commercial applications of microalgae. Journal of Bioscience and Bioengineering, 101: 87-96.

Sukenik, A., and Shelef, G. (1984). Algal autoflocculation—Verification and proposed mecha­nism. Biotechnology and Bioengineering, 26: 142-147.

Taylor, G., Baird, D. J., and Soares, A. M.V. M. (1998). Surface binding of contaminants by algae: Consequences for lethal toxicity and feeding to Daphnia magna Straus. Environmental Toxicology and Chemistry, 17: 412-419.

Tenney, M. W., Echelberger, Jr., W. F., Schuessler, R. G., and Pavoni, J. L. (1969). Algal floc­culation with synthetic organic polyelectrolytes. Applied Microbiology, 18: 965-971.

Uduman, N., Qi, Y., Danquah, M. K., Forde, G. M., and Hoadley, A. (2010). Dewatering of microalgal cultures: A major bottle to algal based fuels. Journal of Renewable and Sustainable Energy, 2: 012701-012715.

Van Vuuren, L. R.J., and Van Duuren, F. A. (1965). Removal of algae from wastewater matura­tion pond effluent. Journal of the Water Pollution Control Federation, 37: 1256-1262.