Post on 10-Oct-2014
Harvesting Unicellular Microalgae
Adam DasseyDr. Chandra Theegala
Louisiana State UniversityBiological and Agricultural Engineering
Products from Algae
• 5,000-10,000 tons worldwide[1]
– High value products ($8,000-15,000t-1)• food supplements
• nutraceuticals
• antioxidants
– Low value products ($810-1,080t-1)• Fuel
– 20% lipid content
– $3-4 per gallonhttp://www.ecoautoninja.com/eco-auto-technology/algae-fueled-cars-first-road-trip-96365/
http://ecx.images-amazon.com/images/I/41yv%2Bg6ATIL.jpg
Algae Concentration
• Raceway culturing
• Acre pond * 2 feet deep = 652,000 gallons
• Assume 200 mg/l concentration– 0.02% algal biomass
– 130 gallons of algal biomass
• Assume 20% lipids– 26 gallons of algal oil
http://trendsupdates.com/algal-biofuels-sustainable-energy-option-for-future/
Harvesting
• 20-30% of the total algal fuel cost
• 2 step process– Concentrate from 0.02% to ~2%
– Concentrate from 2% to 20%
http://www.our-energy.com/pictures/static_content/biofuel_production_from_algae/biodiesel_algae.jpg
http://4.bp.blogspot.com/_VyTCyizqrHs/ScvR9_jnuhI/AAAAAAAADQk/PV_-OJCriB0/s400/Bionavitaalgae.jpg
http://www.treehugger.com/20090424-algae-paste.jpg
Microalgae Characteristics [2]
• Size– 1-30 um in diameter
• Density– 1.07-1.2 kg/L
• Charge– -7 to -45 mV
• Concentration– 100-400 mg/L
http://www.devicedaily.com/wp-content/uploads/2008/12/microscopic-view-of-microalgae.jpg
Centrifugation
• Uses centrifugal forces as high as 13,000*g to separate algae from suspension[3]
– 95% algae recovery
– Concentration of 22%
• Very reliable
• Energy intensive
http://www.webeatdiabetes.com/algae_paste.jpg
Sedimentation
• Particles <100 um become difficult to settle• Algal cells <30 um
– settling velocities of 0.1-2.6 cm/h [4]
– Concentrations up to 500 mg/l can not be spontaneously settled [5]
• Induced sedimentation– Chemical flocculation– Autoflocculation– Bioflocculation
• Concentration of 1-2%
Filtration• Membrane
– Simple and reliable
– Aren’t effective with dilute concentration• Bind the filter
• Head loss
– Membrane replacement is a major costs
– May be more effective
when the culture is
preconditioned to 3-4%
solids
http://www.inc.com/magazine/20100501/innovation-a-lean-green-machine.html
Filtration
• Granular Media– Sand, gravel, diatomaceous earth, synthetic beads
– Small media is necessary to capture algal cells• Slow filtration rate
– Once captured, the algae must be separated from the media
– Backwashing, head loss, and biofouling are the major drawbacks
Dissolved Air Flotation
• Produces microbubbles (40 um diameter) to carry algal cells to the surface
• Moderate energy use
• Reliable
• Requires flocculent for effective separation
Recommend Process
• 1st step– Sedimentation, flotation, media filtration
• 2nd step– Centrifugation, membrane filtration
Benefits
• Centrifuge alone– 9.85 KW*h/m3 * (81.7 m3 culture/gal oil) *
($0.09/KW*h)• $72.43 per gallon
• DAF before centrifuge– 0.164 KW*h/m3 * (81.7 m3 culture/gal oil) *
($0.09/KW*h) + 9.85 KW*h/m3 * (0.33 m3
culture/gal oil) * ($0.09/KW*h)• $1.50 per gallon
Questions
References• [1] Pulz O and Gross W, Valuable products from biotechnology of microalgae. Appl Microbiol Biotechnol
65:625–648 (2004).
• [2]Henderson, R. et al. (2007) The impact of algal properties and pre-oxidation on solid-liquid separation of algae. Wat. Res
• [3] Heasman M, Diemar J, O’ Connor W, Sushames T, Foulkes L, Nell JA. Development of extended shelf-life microalgae concentrate diets harvested by centrifugation for bivalve molluscs—a summary. Aquacult Res 2000;31(8 – 9):637– 59.
• [4]Choi, S.K. et al. (2006) Settling characteristics of problem algae in the water treatment process. Wat. Sci. Tech. 53(7), 113-119
• [5]Salim, S. et al. (2010) Harvesting of microalgae by bio-flocculation. Journal of Applied Phycology, DOI 10.1007/s10811-010-9591-x