���Chapter 2�Design of Photobioreactors for Algal Cultivation

Tse-Min Lee

Photobioreactor: PBR

• A photobioreactor is a bioreactor that utilizes a light source to cultivate phototrophic microorganisms.[1] These organisms use photosynthesis to generate biomass from light and carbon dioxide and include plants, mosses, macroalgae, microalgae, cyanobacteria and purple bacteria. Within the artificial environment of a photobioreactor, specific conditions are carefully controlled for respective species. Thus, a photobioreactor allows much higher growth rates and purity levels than anywhere in nature or habitats similar to nature. Hypothetically, phototropic biomass could be derived from nutrient-rich wastewater and flue gas carbon dioxide in a photobioreactor.

PBR microalgae cultivation technology: �1. Operating conditions�2. Type and configuration of photobioreactor

• One of the key technologies that support the development of the microalgae industry is the cultivation of microalgae on a large scale and at low cost.
• This microalgae cultivation technology is associated with the design of the type and configuration of open or closed cultivation systems and photobioreactors, as well as the identification of the operating conditions leading to the optimal growth performance of the target microalgae.

operating conditions�FACTORS AFFECTING MICROALGAE GROWTH AND BIOFUELS PRODUCTION

• Carbon Sources
• Nitrogen Source
• Light Supply
• Temperature
• pH
• Salinity
• others

Carbon Sources�CO₂, HCO₃^-, Organic C

• the most critical factors for the growth of microalgae
• Photoautotrophic: carbon dioxide, bicarbonate
• Heterotrophic: methanol, acetate, glucose, or other organic compounds
• Mixotrophic: both resources
• organic carbon sources would be too expensive for producing low-price products such as biofuels

Photoautotrophic vs heterotrophic

• The perspectives of economic feasibility and environmental protection that microalgae-based biofuels should be produced via photoautotrophic growth of microalgae.
• Heterotrophic growth of microalgae is usually faster than autotrophic growth, but expensive!
• Condition dependence: choice

a net-zero CO₂ emission

• the aspect of CO₂ emissions reduction
• a net-zero CO₂ emission achieved when the biofuels are directly converted from using CO₂ as the substrate.
• Photoautotrophic growth of microalgae represents an ideal model of reutilization of CO₂ coming from flue gas of power plants and industrial activities (Packer, 2009), as microalgae biomass can be further utilized to produce biofuels or other value-added products (Hsueh et al., 2007; Raoof et al., 2006).

Nitrogen Source

• NO₃^-, NO₂^-
• NH₄⁺
• Urea ( )
• As N supply increases, microalgal growth increases.
• Linear growth followed by saturation
• Lipid accumulation-N limitation (Nile Red fluorescence)
• Nile Red“ A Selective Fluorescent Stain for Intracellular Lipid Droplets (THe JOURNAL OF CELL BIOLOGY - VOLUME 100 MARCH 1985: 965-973 )

Nile Red

Light Supply

• Photosynthesis is a light-dependent process.
• Movement and life cycle are affected by light periods and light spectrum.
• Light intensity: 0-2,500 μmol m^-2 s^-1

the light-limitation phase🡪 the light-saturation phase🡪 the light inhibition phase

• Light quality (wavelength): visible light from 400-700 nm

Blue and red light

• Length of light: daily variation, seasonal change, areal effect

Light and PBR for algal growth

• To maximize biomass productivity, the saturation light intensity needs to be distributed throughout the entire microalgae cultivation system.
• However, this is impossible in practical cultivation systems, since the light distribution inside the photobioreactor normally decreases significantly along with the distance due to the light shading effects (see Figure 2.1), especially when the cell concentration gets very high or when significant biofilm formation on the surface of the reactor vessel occurs.
• Improving the mixing of the cells can reduce the effects of light shading or photoinhibition at different zones of the photobioreactor.

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Temperature : growth

• Outdoor culture: variations in temperature greatly depend on the light exposure (i.e., day/night cycle) and seasonal changes.
• In Taiwan, for example, the temperature variation range is between 25 C and 45 C.
• Appropriate cultivation temperature could promote microalgae growth, whereas at a high temperature, microalgae biomass production would decrease, primarily due to denature of essential proteins/enzymes as well as inhibitory effects on cellular physiology.

Temperature effect on growth and components

• Thus, the operation temperature has a significant effect on biomass production.
• Hu et al. (2008) also indicated that the environmental temperature can affect the degree of saturation of the microalgae lipid, since an increase in saturated fatty acids has been observed when the culture temperature was increased.
• For some microalgae (e.g., Nannochloropsis salina and Ochromonas danica), increasing the cultivation temperature may also lead to an increase in the lipid content (Aaronson, 1973; Boussiba et al., 1987).

pH

• The optimal pH for most cultured microalgae species is between 7 and 9
• cyanobacterium Spirulina platensis exhibited optimal growth at pH 9.0 to 10.0 (Belkin and Boussiba, 1991).
• Apparently the suitable pH range for the growth of microalgae and cyanobacteria is greatly species-dependent.
• complete culture collapse may occur due to the disruption of cellular processes by extreme pH
• affects the biochemical reaction characteristics of microalgae
• the feeding of CO₂ obviously affects the culture pH as well as microalgae growth: biocarbonate formation
• The HCO₃^- is then utilized by microalgae via Ci-concentrating mechanisms (CCMs) (Miller et al., 1990).

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藍綠菌 (Synechocystis sp. strain PCC 6803) 鹼性環境 (pH 7.5 轉移到 pH 10) 誘導之基因表現。深綠色: 明顯表現; 淡綠色: 有部分表現。引用自Summerfield and Sherman (2008) Appl Environ Microbiol. 74: 5276–5284。

Ci-concentrating mechanisms (CCMs)

Salinity

• Salt injury: ionic effect, Cl^- and Na⁺, but usually not K⁺ or Mg²⁺
• Osmotic stresses: water deficit
• Alkaline condition: high pH with high cation concentration (Mg, Na)

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PHOTOBIOREACTOR DESIGN PRINCIPLES both lab-scale and pilot-scale microalgae cultivation systems

• highly efficient light sources: sun, light-emitting diode (LED)
• good circulation devices: keeping microalgae in suspension, decreasing heat generation within the microalgae cultivation system, uniform distribution of the cells and the liquid broth, improving CO2 mass-transfer efficiency, and degassing the O2 produced during photosynthesis
• Gas mixing

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(1) how to use appropriate light sources (intensity and wavelength)

(2) how to enhance light conversion efficiency

(3) how to maintain an appropriate microalgae biomass concentration during prolonged operation.

MICROALGAE CULTIVATION IN CLOSED AND OPEN PBRs FOR BIOFUEL PRODUCTION

• Closed system
• Open system
• Which one or your choice

Open Systems

• Simple Ponds
• Raceway Ponds
• major limitations in open ponds
• poor light utilization by the cells
• evaporative water losses
• diffusion of CO2 to the atmosphere
• the requirement of large areas of land
• Biotic effect: increase starting cell density

Closed Systems

• Not is a good system for the low-price biofuel production
• Due to the requirements of good manufacturing practice (GMP) guidelines, production of high-value products from microalgae for application in pharmaceuticals and cosmetics seems feasible only in well-controlled photobioreactors with closed system operations.
• The necessary gas exchange is performed through a sterilized gas filter, to avoid contamination inside the culture system. Therefore, closed systems are characterized by the minimization of contamination over open systems.

bubble column or airlift reactors

• Liquid flow patterns inside the photobioreactor
• Bubble column reactors are cylindrical vessels with height greater than twice their diameter.
• They are characterized by low capital cost, high surface-area-to-volume ratio, lack of moving parts, satisfactory heat and mass transfer, relatively homogenous culture environment, and efficient release of O2 and residual gas mixture (Loubie`re et al., 2009).
• The gas bubbling upward from the sparger噴頭provides the required mixing and gas transfer. Therefore, the sparger’s design is critical to the performance of a bubble column. In scale-up of the photobioreactor, perforated plates are adopted as the sparger used in tall bubble columns to break up and redistribute coalesced bubbles (Janssen et al., 2000).

Vertical Column Photobioreactors airlift reactors

Flat Plate Photobioreactors

• Light on both sides
• Problems: relatively high space requirements, high light energy requirements, difficulties in cleaning, and possible low efficiency in terms of mass production per unit of space

Horizontal Tubular Photobioreactors

• transparent polypropylene acrylic or polyvinylchloride pipes with small internal diameters to increase the penetration of light. Mixing and agitation of the culture are maintained by an air pump to provide circulation.
• More dissolved CO₂
• The cleaning problems of tubular systems are not easy to overcome due to the small internal tube size, which has no ready mechanical way to conduct the inside cleaning for a long tube.

COMMERCIAL MICROALGAE CULTIVATION SYSTEMS FOR BIOFUEL PRODUCTION

• Commercial company: Algenol Biofuels, Sapphire Energy, Seambiotic, Solazyme, and Solix BioSystems

Algenol

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The flexible plastic film photobioreactors used by Algenol; A) the structural diagram, B) the appearance (www.algenolbiofuels.com/media/media-gallery).

Solix Lumian: Algae Growth System(AGS™).

commercialized AGS system is the LumianAGS4000, which is a 4,000-liter cultivation system with 20 200-liter Lumian panels held in a 1260-foot waterfilled system

Sapphire

(a) Sapphire’s green crude farm with raceway open ponds (www.sapphireenergy.com/rendition.

• medium/images/multimedia/green%20crude%20farm%20ponds.jpg).

(b) Seambiotic’s pilot plant (www.seambiotic. com/uploads/Seambiotic%20Ltd.%20-%20Algae%20Pilot%20Plant%20white%20paper.pdf).

Solazyme’s heterotrophic algae cultivation platform (http://solazyme.com/technology).

• Solazyme uses large fermentation tanks to incubate algae in the dark and feed them plant sugars.
• This platform makes the feedstock more flexible, and it is able to use low-cost sugars, varying from sugarcane to corn stover, woody biomass, switchgrass, and other cellulosic materials. By this heterotrophic incubation, algae can accumulate more oil in cells.
• According to data shown on Solazyme’s Website, the oil content in the company’s algae cells is in excess of 80%