Open ponds are the oldest and simplest systems for mass cultivation of microalgae. The pond is designed in a raceway configuration, in which a paddlewheel circu­lates and mixes the algal cells and nutrients. The raceways are typically made from poured concrete, or they are simply dug into the earth and lined with a plastic liner to prevent the ground from soaking up the liquid. Baffles in the channel guide the flow around the bends in order to minimize space. The system is often operated in a continuous mode, i. e., the fresh feed is added in front of the paddlewheel, and the algal broth is harvested behind the paddlewheel after it has circulated through the loop.

Open-pond systems are shallow ponds in which algae are cultivated. Nutrients can be provided through runoff water from nearby land areas or by channeling the
water from sewage/water treatment plants (Carlsson et al. 2007). The water is typi­cally kept in motion by paddlewheels or rotating structures, and some mixing can be accomplished by appropriately designed guides. Algal cultures can be defined (one or more selected strains), or are made up of an undefined mixture of strains. For an overview of systems used see Borowitzka (1999) and Chaumont (1993).

The only practicable methods of large-scale production of microalgae are race­way ponds (Terry and Raymond 1985; Molina Grima 1999) and tubular photobiore­actors (Molina Grima et al. 1999; Tredici 1999; Sanchez Miron et al. 1999).

Open architecture approaches (e. g., ponds or traditional racetracks), while pos­sibly the cheapest of all current techniques, suffer challenges with contamination, evaporation, temperature control, CO2 utilization, and maintainability.

Figure 4.4 shows open-pond “algae farm” systems. The ponds are “raceway” designs in which the algae, water, and nutrients circulate around a racetrack. Pad­dlewheels provide the flow. The algae are thus kept suspended in water. Algae are circulated back up to the surface on a regular frequency. The ponds are kept shallow because of the need to keep the algae exposed to sunlight and the limited depth to which sunlight can penetrate the pond water. The ponds are operated continuously; that is, water and nutrients are constantly fed to the pond while algae-containing water is removed at the other end. Some kind of harvesting system is required to recover the algae, which contain substantial amounts of natural oil.

Large-scale outdoor culture of microalgae and cyanobacteria in open ponds, race­ways, and lagoons is well established (Becker 1994). Open culture is used commer­cially in the USA, Japan, Australia, India, Thailand, China, Israel, and elsewhere to produce algae for food, feed, and extraction of metabolites. Open-culture systems allow relatively inexpensive production but are subject to contamination. Conse­quently, only a few algal species can be cultured in open outdoor systems. Species

that grow successfully in the open include rapid growers such as Chlorella and species that require a highly selective extremophilic environment that does not favor the growth of most potential contaminants. For example, species such as Spirulina and Dunaliella thrive in highly alkaline and saline selective environments, respec­tively. Algae produced in quantities in open systems include Spirulina, Chlorella, Dunaliella, Haematococcus, Anabaena, and Nostoc (Chisti 2006).

Algae farms are large ponds. The size of these ponds is measured in terms of sur­face area, since surface area is so critical to capturing sunlight. Their productivity is measured in terms of biomass produced per day per unit of available surface area. Even at levels of productivity that would stretch the limits of an aggressive research and development program, such systems require acres of land. At such large sizes, it is more appropriate to think of these operations on the scale of a farm. Such al­gae farms would be based on the use of open, shallow ponds in which some source of waste CO2 could be efficiently bubbled into the ponds and captured by the al­gae. Careful control of pH and other physical conditions for introducing CO2 into the ponds allows for more than 90% utilization of injected CO2. Raceway ponds, usually lined with plastic or cement, are about 15 to 35 cm deep to ensure ade­quate exposure to sunlight. They are typically mixed with paddlewheels, are usually lined with plastic or cement, and are between 0.2 to 0.5 ha in size. Paddlewheels provide motive force and keep the algae suspended in the water. The ponds are sup­plied with water and nutrients, and mature algae are continuously removed at one end.

Figure 4.5 shows a raceway pond (Chisti 2007). A raceway pond is made up of a closed-loop recirculation channel that is typically about 0.3 m deep. As shown in the figure, mixing and circulation are produced by a paddlewheel. Flow is guided

Figure 4.5 Arial view of a raceway pond

around bends by baffles placed in the flow channel, and raceway channels are built in concrete or compacted earth and may be lined with white plastic. During daylight, the culture is fed continuously in front of the paddlewheel where the flow begins. Broth is harvested behind the paddlewheel on completion of the circulation loop. The paddlewheel operates all the time to prevent sedimentation.

Photosynthesis is the most important biochemical process in which plants, al­gae, and some bacteria harness the energy of sunlight to produce food. Organisms that produce energy through photosynthesis are called photoautotrophs. Photosyn­thesis is a process in which green plants utilize the energy of sunlight to manufac­ture carbohydrates from carbon dioxide and water in the presence of chlorophyll (Viswanathan 2006).

Raceway ponds for mass culture of microalgae have been used since the 1950s. Extensive experience exists on the operation and engineering of raceways. The largest raceway-based biomass production facility occupies an area of 440,000 m2 (Spolaore et al. 2006). Productivity is affected by contamination with unwanted algae and microorganisms that feed on algae. Raceway ponds and other open cul­ture systems for producing microalgae are further discussed by Terry and Raymond (1985).

There are quite a number of sources of waste CO2. Every operation that involves the combustion of fuel for energy is a potential source. One program targeted coal and other fossil-fuel-flred power plants as the main sources of CO2. Typical coal — flred power plants emit flue gas from their stacks containing up to 13% CO2. This high concentration of CO2 enhances transfer and uptake of CO2 in ponds. Figure 4.6 shows the production of algae in open ponds.

For large-scale biofuel production, which would require systems of hundreds of hectares in scale, this would mean deploying tens of thousands of such repeating units, at great capital and operating cost. Open ponds, specifically mixed raceway ponds, are much cheaper to build and operate, can be scaled up to several hectares for individual ponds, and are the method of choice for commercial microalgae pro­duction. However, such open ponds also suffer from various limitations, including

Figure 4.6 Production of algae in open ponds

more rapid (than closed systems) biological invasions by other algae, algae grazers, fungi and amoeba, etc., and temperature limitations in cold or hot humid climates.

Microalgae can be cultivated in coastal areas. The raceway pond system of biomass culture must be approved to achieve high and sustained growth rates and oil yields that are essential to developing an algal-based biofuel industry.