Algae bioreactor

An algae bioreactor is used for cultivating micro or macro algae. Algae may be cultivated for the purposes of biomass production (as in a seaweed cultivator), wastewater treatment, CO₂ fixation, or aquarium/pond filtration in the form of an algae scrubber. Algae bioreactors vary widely in design, falling broadly into two categories: open reactors and enclosed reactors. Open reactors are exposed to the atmosphere while enclosed reactors, also commonly called photobioreactors, are isolated to varying extent from the atmosphere. Specifically, algae bioreactors can be used to produce fuels such as biodiesel and bioethanol, to generate animal feed, or to reduce pollutants such as NO_x and CO₂ in flue gases of power plants. Fundamentally, this kind of bioreactor is based on the photosynthetic reaction which is performed by the chlorophyll-containing algae itself using dissolved carbon dioxide and sunlight energy. The carbon dioxide is dispersed into the reactor fluid to make it accessible for the algae. The bioreactor has to be made out of transparent material.

The algae are photoautotroph organisms which perform oxygenic photosynthesis.

The equation for photosynthesis:

{\begin{matrix}\mathrm {6\;CO_{2}+6\;H_{2}O\quad \longrightarrow \;C_{6}H_{12}O_{6}+6\;O_{2}} \qquad \Delta H^{0}=+2870\ {\frac {\mathrm {kJ} }{\mathrm {mol} }}\end{matrix}}

Historical background

It has been documented that the first microalgae cultivation was unicellular Chlorella vulgaris by Dutch microbiologist Martinus Willem Beijerinck in 1890. Later during World War II, Germany used open ponds to increase algal cultivation for use as a protein supplement.[1] Some of the first experiments with the aim of cultivating algae were conducted in 1957 by the "Carnegie Institution" in Washington. In these experiments, monocellular Chlorella were cultivated by adding CO2 and some minerals. The goal of this research was the cultivation of algae to produce a cheap animal feed.[2]

Types of bioreactors

Bioreactors can be divided into two broad categories, open systems and photobioreactors (PRB). The difference between these two reactors are their exposure to the surrounding environment. Open systems are fully exposed to the atmosphere while PBRs have very limited exposure to the atmosphere.

Commonly used open systems

Raceway pond at the Bromley waste water treatment plant in Christchurch, New Zealand used for algae cultivation.

Simple Ponds

The simplest system yields a low production and operation cost. Ponds need a rotating mixer to avoid settling of algal biomass. However, these systems are prone to contamination due to the lack of environmental control.[3]

Raceway Ponds

A modified version of a simple pond, the raceway pond uses paddlewheels to drive the flow in a certain direction. The pond is continuously collecting biomass while providing carbon dioxide and other nutrients back into the pond. Typically, raceway ponds are very large in size due to their low water depth.[4]

Other Systems

Less common systems include an incline cascade system where flow is gravity driven to a retention tank then gets pumped back up to start again. This system can yield high biomass densities but requires higher operating costs.[5]

Commonly used photobioreactors (PBRs)

Nowadays 3 basic types of algae photobioreactors have to be differentiated, but the determining factor is the unifying parameter – the available intensity of sunlight energy.

plastic plate photobioreactor for the cultivation of microalgae and other photosynthetic organisms. It has an operational volume of 500 liters.

Flat plate PBR

A plate reactor simply consists of inclined or vertically arranged translucent rectangular boxes which are often divided in two parts to affect an agitation of the reactor fluid. Generally, these boxes are arranged into a system by linking them. Those connections are also used for making the process of filling/emptying, introduction of gas and transport of nutritive substances. The introduction of the flue gas mostly occurs at the bottom of the box to ensure that the carbon dioxide has enough time to interact with algae in the reactor fluid. Typically, these plates are illuminated from both sides and have a high light penetration. Disadvantages of the flat plate design is the limited pressure tolerance and high space requirements.[6]

tubular glass photobioreactor for the cultivation of microalgae and other photosynthetic organisms. It has an operational volume of 4000 liters.

Tubular PBR

A tubular reactor consists of vertical or horizontal arranged tubes, connected together to a pipe system. The algae-suspended fluid can circulate in this tubing. The tubes are generally made out of transparent plastics or borosilicate glass and the constant circulation is kept up by a pump at the end of the system. The introduction of gas takes place at the end/beginning of the tube system. This way of introducing gas causes the problem of carbon dioxide deficiency, high concentration of oxygen at the end of the unit during the circulation ultimately making the process inefficient. The growth of microalgae on the walls of the tubes can inhibit the penetration of the light as well.[6]

Bubble column PBR

Vertical bubble columns, a project at the Universidad EAFIT to utilize algae to reduce CO2 emissions.

A bubble column photo reactor consists of vertical arranged cylindrical columns made out of transparent material. The introduction of gas takes place at the bottom of the column and causes a turbulent stream to enable an optimum gas exchange. The bubbling also acts as a natural agitator. Light is typically sourced from outside the column, however recent designs introduce lights inside the column to increase light distribution and penetration.[6]

Industrial usage

The cultivation of algae in a photobioreactor creates a narrow range of industrial application possibilities. Some power companies [7] already established research facilities with algae photobioreactors to find out how efficient they could be in reducing CO₂ emissions, which are contained in flue gas, and how much biomass will be produced. Algae biomass has many uses and can be sold to generate additional income. The saved emission volume can bring an income too, by selling emission credits to other power companies.[8]

The utilization of algae as food is very common in East Asian regions.[9] Most of the species contain only a fraction of usable proteins and carbohydrates, and a lot of minerals and trace elements. Generally, the consumption of algae should be minimal because of the high iodine content, particularly problematic for those with hyperthyroidism. Likewise, many species of diatomaceous algae produce compounds unsafe for humans.[10] The algae, especially some species which contain over 50 percent oil and a lot of carbohydrates, can be used for producing biodiesel and bioethanol by extracting and refining the fractions. This point is very interesting, because the algae biomass is generated 30 times faster than some agricultural biomass,[11] which is commonly used for producing biodiesel.

References

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"Achmed Khammas - Das Buch der Synergie - Teil C - Die Geschichte der Solarenergie". www.buch-der-synergie.de. Retrieved 2022-05-03.
Costa, Jorge Alberto Vieira; Freitas, Bárbara Catarina Bastos; Santos, Thaisa Duarte; Mitchell, Bryan Gregory; Morais, Michele Greque (2019), "Open pond systems for microalgal culture", Biofuels from Algae, Elsevier, pp. 199–223, doi:10.1016/b978-0-444-64192-2.00009-3, ISBN 978-0-444-64192-2, retrieved 2022-05-03
Costa, Jorge Alberto Vieira; Freitas, Bárbara Catarina Bastos; Santos, Thaisa Duarte; Mitchell, Bryan Gregory; Morais, Michele Greque (2019), "Open pond systems for microalgal culture", Biofuels from Algae, Elsevier, pp. 199–223, doi:10.1016/b978-0-444-64192-2.00009-3, ISBN 978-0-444-64192-2, retrieved 2022-05-03
Richmond, Amos; Hu, Qiang, eds. (2013-05-07). "Handbook of Microalgal Culture". doi:10.1002/9781118567166. {{cite journal}}: Cite journal requires |journal= (help)
Yen, Hong-Wei; Hu, I-Chen; Chen, Chun-Yen; Nagarajan, Dillirani; Chang, Jo-Shu (2019), "Design of photobioreactors for algal cultivation", Biofuels from Algae, Elsevier, pp. 225–256, doi:10.1016/b978-0-444-64192-2.00010-x, ISBN 978-0-444-64192-2, retrieved 2022-05-03
Patel, Sonal (May 1, 2016). "A Breakthrough Carbon-Capturing Algae Project". Powermag. Texas, USA: powermag.com. Retrieved 16 November 2018.
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"Algae, The Food That Could Save Humanity". Le Monde. France: worldcruch.com. July 9, 2016. Retrieved 16 November 2018.
"Toxic diatoms". NOAA Northeast Fisheries Science Center. NOAA. September 1, 2014. Retrieved 16 November 2018. the family Pseudo-nitzschia; under certain conditions these diatoms can produce toxins harmful to humans
Ullah, Kifayat; Ahmad, Mushtaq; Sofia; Sharma, Vinod Kumar; Lu, Pengmei; Harvey, Adam; Zafar, Muhammad; Sultana, Shazia; Anyanwu, C.N. (2014). "Algal biomass as a global source of transport fuels: Overview and development perspectives". Progress in Natural Science: Materials International. 24 (4): 329–339. doi:10.1016/j.pnsc.2014.06.008.