Biotechnology Advances 29 (2011) 686–702 Contents lists available at ScienceDirect Biotechnology Advances journal homepage: www.elsevier.com/locate/biotechadv Research review paper Production and harvesting of microalgae for wastewater treatment, biofuels, and bioproducts Logan Christenson, Ronald Sims ⁎ Department of Biological Engineering, Utah State University, 4105 Old Main Hill, Logan, UT 84322-4105, United States article info abstract Article history: The integration of microalgae-based biofuel and bioproducts production with wastewater treatment has Received 18 March 2011 major advantages for both industries. However, major challenges to the implementation of an integrated Received in revised form 21 May 2011 system include the large-scale production of algae and the harvesting of microalgae in a way that allows for Accepted 27 May 2011 downstream processing to produce biofuels and other bioproducts of value. Although the majority of algal Available online 2 June 2011 production systems use suspended cultures in either open ponds or closed reactors, the use of attached cultures may offer several advantages. With regard to harvesting methods, better understanding and control Keywords: fl fl Algae production of auto occulation and bio occulation could improve performance and reduce chemical addition Algae harvesting requirements for conventional mechanical methods that include centrifugation, tangential filtration, gravity Biofuel sedimentation, and dissolved air flotation. There are many approaches currently used by companies and Wastewater treatment industries using clean water at laboratory, bench, and pilot scale; however, large-scale systems for controlled Photobioreactor algae production and/or harvesting for wastewater treatment and subsequent processing for bioproducts are Raceway lacking. Further investigation and development of large-scale production and harvesting methods for biofuels fi Algae bio lm and bioproducts are necessary, particularly with less studied but promising approaches such as those involving attached algal biofilm cultures. © 2011 Elsevier Inc. All rights reserved. Contents 1. Introduction .............................................................. 687 2. Major challenges ............................................................ 687 2.1. Nutrient supply and recycling .................................................. 688 2.2. Gas transfer and exchange .................................................... 689 2.3. PAR delivery .......................................................... 689 2.4. Culture integrity ........................................................ 689 2.5. Environment control ...................................................... 689 2.6. Land and water availability ................................................... 689 2.7. Harvesting ........................................................... 690 2.8. Genetic and metabolic engineering ................................................ 690 2.9. Summary of major challenges .................................................. 690 3. Algae production methods ....................................................... 690 3.1. Suspended cultures ....................................................... 690 3.1.1. Open ponds ...................................................... 690 3.1.2. Closed reactors ..................................................... 691 3.2. Immobilized cultures ...................................................... 691 3.2.1. Matrix-immobilized microalgae ............................................. 691 3.2.2. Algal biofilms...................................................... 691 3.3. Algae production cost ...................................................... 692 4. Algae harvesting methods ....................................................... 692 4.1. Chemical based ......................................................... 692 4.2. Mechanical based ........................................................ 693 ⁎ Corresponding author. Tel.: +1 435 797 2785; fax: +1 435 797 1248. E-mail address: [email protected] (R. Sims). 0734-9750/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.biotechadv.2011.05.015 L. Christenson, R. Sims / Biotechnology Advances 29 (2011) 686–702 687 4.3. Electrical based ......................................................... 693 4.4. Biological based ........................................................ 693 5. Approaches to algae production and harvesting in industry ....................................... 694 5.1. Reactor designs for algae production ............................................... 694 5.1.1. Open ponds ...................................................... 694 5.1.2. Closed reactors ..................................................... 694 5.1.3. Hybrid designs ..................................................... 697 5.1.4. Biofilm reactors .................................................... 697 5.2. Harvester designs and harvesting processes ........................................... 697 5.2.1. Mechanical harvesters ................................................. 697 5.2.2. Biological based harvesting ............................................... 697 5.2.3. Bypassing the harvesting step .............................................. 698 5.3. Process design ......................................................... 698 5.3.1. Culture control ..................................................... 698 5.3.2. Lipid accumulation ................................................... 698 5.3.3. Nutrient and gas recycle ................................................ 698 5.4. Genetic manipulation ...................................................... 698 5.5. Summary of approaches in industry ............................................... 699 6. Conclusions .............................................................. 699 References ................................................................. 700 1. Introduction without sufficient production and harvesting of the algae crop. Unfortunately, no current approach has been demonstrated to be simple With growing concerns surrounding the continued use of fossil fuels, and inexpensive enough for economical large-scale use with algae. renewable biofuels have received a large amount of recent attention. The U.S. Department of Energy has recognized the potential While biofuels produced using oil crops and waste oils cannot alone meet synergy of wastewater treatment and biofuel production from algae, the existing demand for fuel, microalgae appear to be a more promising stating that “Inevitably, wastewater treatment and recycling must be feedstock option (Chisti, 2007, 2008). Microalgae include microscopic incorporated with algae biofuel production (U.S. DOE, 2010).” Because eukaryoticalgaeaswellascyanobacteria(Acreman, 1994). Such algae much of the infrastructure is already in place, algae based wastewater could provide substantially more biodiesel than existing oilseed crops treatment can be deployed relatively soon. The use of wastewater can while using less water and land (Sheehan et al., 1998). Algae biomass offset the cost of commercial fertilizers otherwise needed for the may also be fed to an anaerobic digester for methane production production of algae, and wastewater treatment revenues can offset (Golueke et al., 1957; Gunaseelan, 1997; Yen and Brune, 2007), or used to algae production costs. It is apparent that overcoming the current produce bioplastic materials (Chiellini et al., 2008). Residual biomass challenges to the production and harvesting of algae will be beneficial from these processes can potentially be used as a fertilizer, soil for both wastewater treatment and for the production of biofuels and amendment, or feed for fish or livestock (Mulbry and Wilkie, 2001; bioproducts. Mulbry et al., 2005; Roeselers et al., 2008). However, the production of Considering the benefits of cost-effective algae production and biofuels and bioproducts using algal biomass has been handicapped by an harvesting to both wastewater treatment and the production of inability to find a reliable and cost effective method of producing and biofuels and other bioproducts, this review has the following harvesting large quantities of algae feedstock. objectives: In addition to biofuel and other bioproduct applications, large- scale methods of producing and harvesting algae have uses in 1. Identify the major challenges to cost-effective production and wastewater treatment (Hoffmann, 1998; Oswald, 2003). Without harvesting of algae. proper treatment, excess nitrogen and phosphorus in discharged 2. Compare the benefits and limitations of the different approaches to wastewaters can lead to downstream eutrophication and ecosystem algae production, including open ponds, closed reactors, and damage (Correll, 1998). The negative effects of such nutrient over- immobilized systems. loading of receiver systems include nuisance algae, low dissolved 3. Compare the benefits and limitations of algae harvesting ap- oxygen concentrations and fish kills, undesirable pH shifts, and proaches, including chemical, mechanical, biological, and electrical cyanotoxin production. While chemical and physical based technol- based harvesting. ogies are available to remove these nutrients, they consume 4. Examine algae production and harvesting approaches in industry. significant amounts of energy and chemicals, making them costly 5. Identify research needs and potential solutions to the major processes (Tchobanoglous and Burton, 1991).