Algal Cultivation
Algae grow in open, closed or semi-closed systems in round, long or tubular tanks that maximize access of the entire biomass to sunlight. Growth occurs only in the top layer, about two inches, of the growing medium unless mixing occurs. New cell growth blocks the sunlight for plants below. Semi-continuous mixing is necessary to give all the algae sufficient light. Some production systems put light sources near or in the water to augment sunlight.
Growth occurs based on a host of variables that not only constrain growth, but may change the algal composition. Primary variables include the following.
Light. Usually sunlight provides sufficient light but artificial light also works as well—especially for indoor growing systems. Some growing systems may be tilted to optimize orientation to the sun and reflected light. Several producers are experimenting with bent light using mirrors or glass cables and other are using LED lights that minimize energy consumption.
Mixing. Since most growth takes place in the top layer of the surface that faces the light source, mixing is imperative. Each cell needs to move in and out of the light for their light and dark growth periods as they take in CO2 and exhale O2. Algae are heavier than water and would sink away from their light source without mixing.
Algae grow so fast they become nutrient-limited quickly in still water. They cannot move and graze for food because they usually have no propulsion. Mixing brings nutrients and CO2 to each algae cell and provides intermittent light exposure. Mixing also helps release O2 from the water to the atmosphere. Too much or too little mixing impedes growth and rough mixing methods may create cell damage from shear stress.
Some algae have evolved two interesting differentiated features: flagella and eye spots. At a specific growth stage, some algae grow flagella, slender projections from the body like sperm tails that move in a whip-like motion to propel the algae. The eye spot recognizes light and the flagella propel the plant toward the light. Movement is very slow, possibly an inch an hour.
Water. Algae grow well in nearly any kind of water. They are especially good at using photosynthesis to convert dissolved nutrients and metals in wastewater to green biomass where the metals can be removed and recovered. Production systems can use wastewater, grey water and saline or ocean water, depending on the species grown. Growing systems can recycle the water so the only loss comes from evaporation.
CO2. Approximately half of the microalgal biomass dry weight is carbon, typically derived from CO2 or carbonates, and is fed continually during daylight. Each 100 tons of algal biomass fixes roughly 183 tons of CO2. Algae’s favorite food, CO2, needs to be added as a gas or in bicarbonate form because cultivated algae grow too fast to be able to take sufficient CO2 from the water. Most water is too dilute in CO2 for high production. Compressed air blended with CO2 up to 20%, typically provides carbon for algal photosynthesis. Industrial CO2 or waste flue gasses are typical sources but some coal-fired power plants overproduce sulfur, which may inhibit algal growth. Some producers such as Solazyme use an organic carbon source in the form of acetic acid or glucose.
Nutrients. Algae feed their growth with the same fertilizers used for land plants but the fertilizers can come from waste streams that are too salty for land plants. Algal growth consumes far less nitrogen and other fertilizers per pound of biomass than food grains such as corn and the nutrients are easier and less expensive to apply. Dissolved chemical fertilizer or waste stream nutrients are utilized by algae with far more efficiency than land plants because the tiny single celled algae consume the nutrients directly and do not have to transport the nutrients long distances. Unused fertilizer also may be reused with the recycled water.
pH. The acidity of water may be specific to the type of algae produced. Controlling the water’s pH represents a good strategy for retarding growth of competing algae. Water pH is likely to be highest at noon due to the high photosynthetic activity, which consumes maximum CO2.
Stability. Maintaining a stable growth environment presents difficulties with the high velocity of growth. The growing medium may retain too much of any nutrient or O2, which may create stress and or composition changes to the plants. Some producers capture released O2 and sell the pure gas as a value added product.
Algaculture Production
Algal biomass grows in ponds or containers called biofactories or cultivated algal production systems, (CAPS). Water, inorganic nutrients, CO2 and light is provided to the algal culture to promote biomass growth. Algae prefer diffused light that is not too bright so some systems use shading that limits light and diffuses it. Various species produce best at specific temperatures so some systems use recycled water on the outside of the biofactory to maintain optimum temperature.
Even though CO2 may be about 5% of production cost, that cost can be minimized by siting the biofactory near a power or manufacturing plant that produces CO2. Nutrients may be provided from wastewater, recovered from the algal tank or harvested fertilizer. After the algal oil is removed, the remaining biomass contains considerable nutrients.
Closed systems offer the advantage that high nutrient water may be recycled through the system. This practice significantly lowers the cost of added nutrients. It also minimizes water loss to evaporation. Algaculture systems that use high-saline water, such as agricultural waste streams or brine water, produce a biomass with considerable salt that needs to be removed during co-product extraction. Some business models indicate using algae to harvest heavy metals from industrial wastewater, which are then extracted and sold on the chemicals market.
Harvest may occur daily by filtering, centrifuge or flocculation. The cells suspended in the broth are separated from the water and residual nutrients are recycled to biomass production. Algal oil is extracted from the recovered biomass and converted to biodiesel. Some of the non-oil biomass may be used as animal feed, fertilizer and for other co-products.
Part of the spent biomass undergoes anaerobic digestion to produce biogas that generates electricity, which powers the biomass mixing and water transport. Effluents from anaerobic digestion may be used for more algal production or as nutrient-rich irrigation water. Most of the power generated from the biogas is consumed in biomass-production and any excess energy may be sold to the grid. Some systems use solar panels with photovoltaic cells to convert solar energy directly to electricity, which is typically used directly or stored in batteries.
In a continuous culture, fresh culture medium is fed at a constant rate and the same quantity of microalgal broth is withdrawn. Feeding stops during the night but mixing continues to prevent biomass settling. As much as 20% of the biomass, produced during daylight, may be consumed during the night to sustain the cells until sunrise. Nightly biomass loss depends on the growth light level, growth temperature and the temperature at night. Some production systems are experimenting with nightlights to boost productivity.
Microalgae contain high, but variable, percentages of the key macronutrients: typically 20-50% protein, 5-30% carbohydrates and 10-30% lipids, with about 10% ash or waste. The proportions of each nutrient may be modified by species selection, varying growth conditions or by harvesting the algae at different growth stages. Most species are rich in amino acids and offer a variety of pigments. The sugar composition of polysaccharides is highly variable, but most species have high proportions of glucose, 20-87%. Microalgae contain significant quantities of micronutrients and antioxidants such as vitamins, ascorbic acid, riboflavin, carotenoids and a variety of novel lipids.
After the oil component is used for biofuel, the remaining high protein biomass may be de-moistured and stored in a convenient form such as a cake, which does not require refrigeration and has about a two year shelf life. The algal cake may be separated into various food, food ingredients, fodder, fertilizer, fine medicines or other components.
Algal production for food, fuel, medicines or other co-products can be carbon neutral because the power needed for producing and processing the algae can come from the methane produced by anaerobic digestion of the biomass residue remaining after oil extraction. The modest energy requirement for mixing and harvest may also come from other non-carbon sources such as wind, geothermal or solar.
The harvested biomass is extremely malleable in the sense that it can be stored in the same form as corn, wheat, rice or soy products. These include protein-rich milk, soft mash of any size, shape or texture, tortilla, cracker or flour. The biomass may be made into texturized vegetable protein with added fiber or extruded to make additives for meats that improve moisture retention and increase protein while lowering fats.
Our future foods are likely to be enriched with algae and advanced compounds from algae.
Adapted from: Green Algae Strategy: End Oil Imports and Engineer Sustainable Food and Fuel, 2008.
