Photobioreactors have the ability to produce algae while performing beneficial tasks, such as scrubbing power plant flue gases or removing nutrients from wastewater (Carlsson et al. 2007). Photobioreactors are different types of tanks or closed sys­tems in which algae are cultivated (Richmond 2004).

Algae biomass can play an important role in solving the problem between the production of food and that of biofuels in the near future. Microalgae appear to be the only source of renewable biodiesel that is capable of meeting the global demand for transport fuels. Microalgae are photosynthetic microorganisms that convert sun­light, water, and carbon dioxide into algal biomass (Chisti 2007).

Most algal species are obligate phototrophs and thus require light for their growth. Phototropic microalgae are most commonly grown in open ponds and pho­tobioreactors (Patil et al. 2005). Open-pond cultures are economically more favor­able but raise the issues of land use cost, water availability, and appropriate climatic conditions. Photobioreactors offer a closed culture environment, which is protected from direct fallout and so is relatively safe from invading microorganisms. This technology is relatively expensive compared to the open ponds because of the infra­structure costs. An ideal biomass production system should use the freely available sunlight.

Tredici (1999) has reviewed mass production in photobioreactors. Many differ­ent designs of photobioreactor have been developed, but a tubular photobioreactor seems to be most satisfactory for producing algal biomass on the scale needed for biofuel production. Closed, controlled, indoor algal photobioreactors driven by ar­tificial light are already economical for special high-value products such as phar­maceuticals, which can be combined with the production of biodiesel to reduce the cost (Patil et al. 2008).

Photobioreactors have higher efficiency and biomass concentration (2 to 5 g/L), shorter harvest time (2 to 4 weeks), and higher surface-to-volume ratio (25 to 125/m) than open ponds (Lee 2001; Wang et al. 2008). Closed systems consist of numerous designs: tubular, flat-plated, rectangular, continued stirred reactors, etc. Photobiore­actors in general provide better control of cultivation conditions, yield higher pro­ductivity and reproducibility, reduce contamination risk, and allow greater selection of algal species used for cultivation. The bioreactor has a photolimited central dark zone and a better lit peripheral zone close to the surface (Chisti 2007). CO2-enriched air is sparged into the reactor creating a turbulent flow. Turbulent flow simultane­ously circulates cells between the light and dark zones and assists the mass transfer of carbon dioxide and oxygen gases. The frequency of light and dark zone cycling is
depended on the intensity of turbulence, cell concentration, optical properties of the culture, tube diameter, and the external irradiance level (Chisti 2007). Regulation of carbon dioxide and dissolved oxygen levels in the bioreactor is another key element to algal growth. The highest cost for closed systems is the energy cost associated with the mixing mechanism (Wijffels 2008).

Tubular photobioreactors consist of transparent tubes that are made of flexible plastic or glass. Tubes can be arranged vertically, horizontally, inclined, helically, or in a horizontal thin-panel design. Tubes are generally placed in parallel to each other or flat above the ground to maximize the illumination surface-to-volume ratio of the reactor. The diameter of tubes is usually small and limited (0.2-m diameter or less) to allow light penetration to the center of the tube where the light coefficient and linear growth rate of culture decrease with increasing unit diameter (Ogbonna and Tanaka 1997; Riesing 2006). The growth medium circulates from a reservoir to the reactor and back to the reservoir. A turbulent flow is maintained in the reactor to ensure distribution of nutrients, improve gas exchange, minimize cell sedimentation, and circulate biomass for equal illumination between the light and dark zones.

The most widely used photobioreactor is a tubular design, which has a number of clear transparent tubes, usually aligned with the sun’s rays. The tubes are generally less than 10 cm in diameter to maximize sunlight penetration. The medium broth is circulated through a pump to the tubes, where it is exposed to light for photosyn­thesis, and then back to a reservoir. A portion of the algae is usually harvested after it passes through the solar collection tubes, making continuous algal culture possi­ble. In some photobioreactors, the tubes are coiled spirals to form what is known as a helical-tubular photobioreactor. The microalgal broth is circulated from a reservoir to the solar collector and back to the reservoir (Chisti 2007).

Figure 4.1 depicts a tubular photobioreactor with parallel run horizontal tubes (Campbell 2008). A tubular photobioreactor consists of an array of straight trans­parent tubes that are usually made of plastic or glass. This tubular array, or the solar

collector, is where the sunlight is captured as seen in Figure 4.1. The solar collec­tor tubes are generally 0.1 m or less in diameter. Tube diameter is limited because light does not penetrate too deeply in the dense culture broth, which is necessary for ensuring a high biomass productivity of the photobioreactor. Figure 4.2 shows another type of tubular photobioreactor with parallel run horizontal tubes (Chisti 2007). Figure 4.3 shows a tubular (a) and a vertical (b) photobioreactor.

Flat-plated photobioreactors are usually made of transparent material. The large illumination surface area allows high photosynthetic efficiency, low accumulation of dissolved oxygen concentration, and immobilization of algae (Ugwu et al. 2008). The reactors are inexpensive and easy to construct and maintain. However, the large surface area presents scale-up problems, including difficulties in controlling culture temperature and carbon dioxide diffusion rate and the tendency for algae adhering to the walls.

An inclined triangular tubular photobioreactor was designed to install adjacent to a power plant utilizing flue gas as the feed gas. Flue gas entered the reactor from the bottom of the inclined tube. Gas bubbles traveled along the inner surface of the

Harvest

Degassing column

Fresh — medium

Cooling

water

Pump

(a) (b)

Figure 4.3 (a) Tubular and (b) vertical photobioreactors

tube generating eddies for mixing and preventing fouling. The upper surface of the inclined tube absorbed natural light. The mixing to the algal culture and the flow rate of flue gas influences the growth rate of algae. The system worked, and 15 to 30% of algae were harvested each day. The setup was able to remove 82% of the carbon dioxide on a sunny day and 50% of the carbon dioxide on a cloudy day. Nitrogen oxide was also lowered by 86% (Riesing 2006).

Rectangular tanks are another example of photobioreactors. Unlike the circular tank design, rectangular tanks do not require a stirring device when a sufficiently high gas velocity is used. Drain pipes and gas spargers are located at the bottom of the tank.

Continuous stirred tank reactors (CSTRs) consist of a wide, hollow, capped cylin­drical pipe that operates both indoors and outdoors with low contamination risk. A mechanical stirrer and a light source are inserted from the top of the reactor. Drain channels and gas injectors are positioned at the bottom (and midsection) of the reactor. The uniform turbulent flow established within the reactor promotes algal growth and prevents fouling.

Another photobioreactor uses helical coils made of plastic tubing placed across a columnlike structure. A group of helical coils make up one unit of photobiore­actor. Each helical coil runs independently with its own gas injector, pump, and gas removal system. The helical coils operate both indoors (fluorescent light) and outdoors (sunlight).

Similar to the helical coils, square tubular reactors consist of plastic tubing ar­ranged in a series of squares. One pump is used to provide algal flow through the series of squares and back. Compared with the helical coils, the square tubing is longer and holds more algal volume. The unit is also intended to be installed on the rooftop of a power plant and can operate both outdoor and indoor. However, light is a limitation where only one side of the square is exposed to the Sun at a time. To maximize light penetration, square tubular reactors cannot be packed as closely as the inclined triangular tubular reactor or the helical coils.