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