Photobioreactor design: Mixing, carbon utilization, and oxygen accumulation

Photobioreactor design and operation are discussed in terms of mixing, carbon utilization, and the accumulation of photosynthetically produced oxygen. The open raceway pond is the primary type of reactor considered; however small diameter (1–5 cm) horizontal glass tubular reactors are compared to ponds in several respects. These are representative of the diversity in photobioreactor design: low capital cost, open systems and high capital cost, closed systems. Two 100‐m2 raceways were operated to provide input data and to validate analytical results. With a planktonic Chlorella sp., no significant difference in productivity was noted between one pond mixed at 30 cm/s and another mixed from 1 to 30 cm/s. Thus, power consumption or CO2 outgassing limits maximal mixing velocities. Mixing power inputs measured in 100‐m2 ponds agreed fairly well with those calculated by the use of Manning's equation. A typically configured tubular reactor flowing full (1 cm diameter, 30 cm/s) consumes 10 times as much energy as a typical pond (20 cm deep flowing at 20 cm/s). Tubular reactors that flow only partially full would be limited by large hydraulic head losses to very short sections (as little as 2 m length at 30 cm/s flow) or very low flow velocities. Open ponds have greater CO2 storage capacity than tubular reactors because of their greater culture volume per square meter (100–300 L/m2 vs. 8–40 L/m2 for 1–5‐cm tubes). However, after recarbonation, open ponds tend to desorb CO2 to the atmosphere. Thus ponds must be operated at higher pH and lower alkalinity than would be possible with tubular reactors if cost of carbon is a constraint. The mass transfer coefficient, KL, for CO2 release through the surface of a 100‐m2 pond was determined to be 0.10 m/h. Oxygen buildup would be a serious problem with any enclosed reactor, especially small‐diameter tubes. At maximal rates of photosynthesis, a 1‐cm tubular reactor would accumulate 8–10 mg O2/L/min. This may result in concentrations of oxygen reaching 100 mg/L, even with very frequent gas exchange. In an open pond, dissolved oxygen rises much more slowly as a consequence of the much greater volume per unit surface area and the outgassing of oxygen to the atmosphere. The maximum concentration of dissolved oxygen is typically 25–40 mg/L. The major advantage of enclosed reactors lies in the potential for aseptic operation, a product value which justifies the expense. For most products of algal mass cultivation, open ponds are the only feasible photobioreactor design capable of meeting the economic and operating requirements of such systems, provided desirable species can be maintained.