Images of Nannochloropsis (TOP), Scenesdemes (MIDDLE), and Chlorella cells (BOTTOM).
Algae are diverse photosynthetic organisms that have an estimated 1-10 million different species. These organisms occur in microscopic (single cell) and macroscopic (multi cellular) form and can range from 0.2 microns (the diameter of a human hair is around 100 microns) to over 60 meters in length (more than half of a football field). For the purpose of biofuels, microscopic algae have been extensively due to their rapid growth rates, high lipid contents, and ability to grow in degraded water bodies. At the Illini Algae Biofuels project several species are being mass-cultured to study novel dewatering methods, lipid extraction technologies, and biodiesel conversion methods.
Illini Algae Species
Three primary species that are being investigated at the Illini Algae Project include Nannochloropsis, Chlorella Vulgaris, and Scenedesmus. Each of these species was selected for their distinct morphology characteristics, lipid profiles, and tolerable growth conditions. The first species, Nannochloropsis, consists of smaller cells (1-5 microns) that reproduce rapidly with a high membrane lipid to storage lipid ratio. The next species, Scenedesmus, consists of moderately sized cells (10-30 microns) that exhibit an elongated morphology arranged in rows. They have been shown to grow particular well in wastewater streams. Lastly, the Chlorella species consists of moderately sized cells (10-30 micron) that have been widely studied. These cells can display robust growth at high temperatures using flue gas and have also been converted to heterotrophic reproduction using organic molecules as a carbon source.
Other species that are also being investigated include wild-mixed species native to wastewater treatment facilities and genetic mutants that can operate in extremely harsh environments. Native wastewater algae are being studied to determine the feasibility of integrating algal cultivation with wastewater nutrient remediation. Likewise, genetic mutants that can operate in harsh environments, such as exposure to high irradiance, offer a self-selection process for maintaining mono-cultures while producing valuable co-products.
Algae can be grown in various culture systems including enclosed bioreactors (TOP), raceway ponds (MIDDLE), and agricultural troughs* (BOTTOM).*Photo courtesy of XL Renewables.
Algal strains are grown and maintained on several scales depending on the application. Initially, algal samples are obtained from laboratory culture libraries that house and preserve a wide variety of species. The initial culture sample is typically shipped in a test tube or culture dish and can be inoculated into a larger volume of media. The cells will rapidly multiply during the initial exponential and linear growth phase and can be reinoculated into larger volumes depending on the application. Inoculation cultures are then maintained under highly controlled conditions to ensure an adequate supply of fresh cells for the batch rotation.
Several methods exist to increase the scale of algae for larger applications. These methods can generally be divided into two categories, open-ponds systems and closed bioreactors tanks. Each method can be chosen based on the desired downstream processing and final application of the algal biomass.
Open-pond systems offer an agricultural-based approach to the growth and cultivation of algae. The ponds can be configured into a raceway paths which allows for the continuous circulation and harvesting of biomass. Ample circulation distributes nutrients and carbon dioxide to the culture to ensure optimum growth. The pond channels can be constructed out of low-cost materials and operated over large areas. Evaporation and contamination or potential concerns when the pond surface is left uncovered; however, culture conditions can be maintained to minimize the risk of invasive take over from competing species or bacteria.
Bioreactor tanks provide a controlled environment to optimize growth conditions and generate high yields of algal cells. Environmental conditions such as light, temperature, aeration, and culture density can be precisely maintained in an indoor environment and the sealed vessel minimizes the risk of contamination. The tanks can also be arranged in a variety of configurations to maximize vertical space and minimize the area footprint, making bioreactors a suitable option when space constraints are an issue. However, due to the high equipment costs bioreactor tanks are generally used to cultivate species that produce high value co-products, such as essential fatty acids or auxiliary pigments.
Samples from the bioreactor tanks are transferred to barrels that are pre-mixed with flocculant (TOP). The flocculant causes cells to conglomerate, allowing excess water to removed. The cells are then centrifuged to condense the biomass into pellets and further remove moisture (MIDDLE).
Once the algal cultures have reached sufficient density, the cells can then be harvested. Even at high densities, the majority of an algal culture consists of water. The amount of dry algae at peak culture densities can range from 1-10 grams per liter, which is only 0.1 to 1% mass. The remaining 99% of the culture is water which must be removed using a variety of methods including centrifugation, flocculation, and membrane filtration.
Cells can be removed from the culture water using the process of centrifugation. Centrifuges operate by spinning liquid samples at high revolutions per minute to generate a sufficient force for accelerating the cells radially outward to separate them from the liquid sample. While this method is effective at removing the majority of cells from suspension, a high energy inputs are required when processing large amounts of culture water.
Flocculating agents provide a chemical means of removing algal cells from the culture water. Flocculants consist of charged molecules that reduce the electrostatic repulsion forces between adjacent algal cells. This allows the cells to bridge together forming larger groups of cells, referred to as "flocs", that can then settle out of solution. The larger flocs can also be removed by air float ion, which uses tiny air bubbles fed in through the bottom of a tank to carry the flocs to the surface. Flocculants can be made out of several materials including multivalent ions and bio-polymers. The flocculant is incorporated into the harvested biomass so the type of material must be chosen according to the post-processing application.
Membrane filtration units can also be used to remove cells from the culture water without adding any additional materials to the harvested biomass. Similar to a spaghetti strainer, membrane units consist of polymer sheets or tubes that contain micro-pore to the allow the passage of water while retaining cells on the surface. As cells accumulate on the membrane, circulated air bubbles or pressurized water can be used to scour the surface and extend the membrane filtration capacity. Membrane units can also be configured into stacked modules to maximize the dewatering capabilities in a given area.
Traditional biomass processing includes dewatering algal cultures to a paste (TOP). The remaining moisture is then removed by drying the samples with heat (MIDDLE) or preserving the samples through lyophilization (freeze-drying). Our group is exploring novel methods to bypass these energy intensive processes while producing valuable co-products.
Preservation and Processing
Once algal cultures have been harvested, the resulting biomass must be processed to remove moisture depending on the desired downstream outcome. Traditional lipid extraction methods using hexane solvent require moisture contents ranging from 5-15%, significantly less then the 65-80% moisture content achieved with centrifugation or membrane filtration. This additional moisture removal requires significant energy consumption that must be further reduced in order for algal biofuels to become economically and environmentally competitive with other feedstock sources.
To overcome the energy consumption required for extensive dewatering, novel extraction and conversion methods are being explored that can operate in a highly aqueous environment. These include testing alcohol-based organic solvents in conjunction with cellular disruption methods and membrane separation processes to develop a high-value co-product refining operation. Additionally, the ability to convert rapid growing, low-lipid content algae that grow wastewater directly to a biocrude oil is also being explored. This process converts the carbon content present in the biomass into a crude oil using high temperature and pressure, similar to the original conditions on earth that produced our current fossil fuel reserves.