CO2 Biofixation by Microalgae

Modeling, Estimation and Control

Sihem Tebbani, Rayen Filali, Filipa Lopes, Didier Dumur, Dominique Pareau

• Publisher: Wiley-ISTE
• Year: 2014
• Print ISBN: 9781848215986
• eBook ISBN: 9781118984451
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Biotechnology
• Index: Biological Sciences

Due to the consequences of globa l warming and significant greenhouse gas emissions, several ideas have been studied to reduce these emissions or to suggest solut ions for pollutant remov al. The most promising ideas are reduced consumption, waste recovery and waste treatment by biological systems. In this latter category, studies have demonstrated that the use of microalgae is a very promising solution for the biofixation of carbon dioxide. In fact, these micro-organisms are able to offset high levels of CO2 thanks to photosynthesis. Microalgae are also used in various fields (food industry, fertilizers, biofuel, etc.). To obtain a n optimal C O2 sequestration us ing micr oal gae, their cul tivatio n has to be c arried ou t in a f avorable e nvironment, corresponding to optimal operating conditions (temperature, nutrients, pH, light, etc.). Therefore, microalgae are grown in an enclosure, i.e. photobioreactors, which notably operate in continuous mode. This type of closed reactor notably enables us to reduce culture contamination, to improve CO2 transfer and to better control the cultivation system. This last point involves the regulation of concentrations (biomass, substrate or by-product) in addition to conventional regulations (pH, temperature).
To do this, we have to establish a model of the system and to identify its parameters; to put in place estimators in order to rebuild variables that are not measured online (software sensor); and finally to implement a control law, in order to maintain the system in optimal conditions despite modeling errors and environmental disturbances that can have an influence on the system (pH variations, temperature, light, biofilm appearance, etc.).

1

Microalgae

1.1. Definition

Algae are photosynthetic organisms that develop in varied habitats, predominantly in aquatic environments, capable of converting light energy and carbon sources, such as carbon dioxide (CO₂), into “biomass”. Depending on their size, they can be classified into two broad categories: “macroalgae” and “microalgae”. Macroalgae are multicellular algae of around one centimeter in size which usually grow in ponds of natural fresh water or salt water. Microalgae have a size measured in micrometers and are considered to be single cell algae which grow in suspension, mainly in aqueous solutions [WEN 09].

These microorganisms are considered to be the first producers of oxygen (O₂). Their existence in the oceans dates back to more than three billion years ago. They are responsible for transforming the composition of the atmosphere (CO₂ fixation and O₂ emission) and have allowed the emergence of plant and animal life on Earth. Also referred to as Phytoplankton, microalgae represent a food source from the earliest stages of larval life right up to human beings, owing to their specific biochemical composition.

Their adaptation and survival capacities are such that they are able to colonize all types of environments. They are found in thermal waters as well as in ice, in acidic or even hyper saline waters, in caves, in symbiotic relationships with any other type of living organisms, and as parasites, even on humans. They are also able to develop on hard surfaces, such as walls or tree trunks, and even on immersed structures. Certain species can withstand very low or paradoxically extreme temperatures. This faculty of adaptation is the result of their morphological properties as well as their capacity to synthesize different varieties of secondary metabolites.

1.2. Characteristics

Through photosynthesis, these microorganisms synthesize O₂ and primary organic metabolites such as carbohydrates, lipids and proteins. From a cell structure perspective, a microalga has a nucleus, a plasma membrane and contains organelles, essential to its operation, such as chloroplasts, amyloplasts, elaioplasts and mitochondria. It contains three main types of pigments: chlorophylls, carotenoids and phycobiliproteins.

Microalgae take a variety of forms (Figure 1.1): spherical (Porphyridium), crescent-shaped (Closterium), spiral-shaped (Arthrospira), droplet-shaped (Chlamydomonas) and even star-shaped (Staurastrum).

From a nutritional point of view, microalgae are predominantly photoautotrophic¹ but they can also be heterotrophic or mixotrophic [CHE 11b]. An autotrophic metabolism uses inorganic carbon such as CO₂ or bicarbonate as a carbon source while a heterotrophic metabolism is characterized by a consumption of organic carbon as a carbon source for their development; mixotrophs use both types of carbon sources.

Figure 1.1. Morphological diversity of microalgae [SUM 09]. a) Gephyrocapsa; b) Haematococcus lacustris; c) Spirulina platensis; d) Chlorella vulgaris; e) Dunaliella tertiolecta; f) Chaetoceros calcitrans; g) Chaetoceros calcitrans; h) Dinophysis acuminate; i) Alexandrium; j) Bacillariophycea; k) Raphidophceae; l) Botryococcus. The length of the line in each figure is equal to 10 μm [SUM 09]

1.3. Uses of microalgae

Microalgae offer interesting perspectives for applications in diverse areas such as the pharmaceutical industry, agriculture, environment and renewable energy. The main uses are detailed below.

1.3.1. Nutrition

Microalgae represent an excellent source of nutrients. They are used for animal feed, as a human food source and in aquaculture. They are used in the manufacture of natural colorants in the food industry. Polysaccharides (hydrosoluble polymers) from microalgae are used in the food industry as gelling agents or thickeners. Glycerol (the molecule involved in the osmoregulatory systems of microalgae) is exploited in the food industry as a sweetener.

1.3.2. Pharmaceuticals

Microalgae are an interesting source of bioactive molecules and toxins that have notably been used in the development of new medicines for the treatment of cancerous diseases [PUL 04]. Polysaccharides extracted from microalgae allow the synthesis of antioxidant, antiviral, antitumor and anticoagulants agents. Microalgae are capable of synthesizing vitamins and natural antioxidants.

1.3.3. Cosmetics

Several species of microalgae are used industrially in the cosmetics industry [PUL 04, SPO 06], mainly the two species Arthrospira and Chlorella. Algae extracts with antioxidant properties are used in the manufacture of hair care products, anti-wrinkle products and sun creams. Pigments derived from microalgae are also used for cosmetics.

1.3.4. Energy

Algal biomass offers benefits in the production of energy in the form of electricity and/or heat by direct combustion, or in the form of biomethane or biofuels. However, these benefits are only competitive in cases with strong biomass productivity, using simple mechanical harvesting techniques and which present lower production costs than those involved in processes using other types of biomass [CAR 07].

1.3.4.1. Biomethane production

Several research projects have confirmed the technical and commercial feasibility of biomethane production from marine biomass, showing great potential [CHY 02]. However, technical obstacles such as the accessibility of the nutrients and high production costs limit the use of microalgae for this application. A way to reduce costs would be, for example, to link the production of methane with the production of secondary high value-added metabolites. Species such as Gracilaria sp. and Macrocystis are excellent methane-producing organisms.

1.3.4.2. Biofuel production

Considering the current global context (increase in the price of oil, depletion of fossil resources, production of greenhouse gases, etc.), it is interesting to consider microalgae as a source of production of different types of biofuel: bio-oil and biodiesel [PAN 11].

Bio-oil from microalgae represents an interesting alternative to liquid biofuels. It is produced by the thermochemical conversion of biomass at high temperatures in the absence of O₂. Two different processes are used: pyrolysis and thermochemical liquefaction. Several studies have been carried out based on the implication of microalgae in bio-oil synthesis [DOT 94, SAW 99, DEM 06]. Areas for improvement for this type of process include the reduction of production costs, the optimization of the culture system, and the improvement of separation and harvesting steps.

The most promising approach involves the production of second and third generation biofuels (ethanol production from lignocellulosic materials and biodiesel production from microalgae). The third generation biofuel addresses the major drawbacks observed in first and second generation biofuels (competing with food production, excessive water consumption and deterioration of soil). Due to certain valuable properties (important biomass productivity, high photosynthetic activity, large lipid storage potential up to 20–50% dry weight), microalgae are 500 to 1,000 times more effective than terrestrial species for biodiesel synthesis.

1.3.4.3. Biohydrogen production

Biohydrogen is an effective source of renewable energy and is currently the subject of extensive research and applications. The process of biohydrogen synthesis can take two forms: direct photolysis and indirect photolysis. Direct photolysis is based on the transfer of electrons from water molecules to protons, coupled with a reduction of ferredoxin (protein intervening at the level of the algae photosystem in the transport of electrons and protons) inducing hydrogen synthesis with hydrogenase enzymes [BEN 00]. The indirect method is based on the conversion of starch stored by algae to hydrogen under anaerobic conditions and sulfur limitation [CAR 07]. Several species of microalgae have shown interesting properties in relation to indirect processes, i.e. a large capacity for biohydrogen synthesis under sulfur deprivation conditions. Accordingly, the production of hydrogen from microalgae is a promising niche but requires a better understanding of microalgae metabolism and engineering [BEE 09].

1.3.5. Environmental field

The main environmental applications of microalgae are in wastewater treatment and consumption of CO₂ as a method for reducing greenhouse gas emissions.

1.3.5.1. Wastewater treatment

Their capacity to assimilate numerous nutrients necessary for their growth means that microalgae offer an interesting solution for the elimination of these elements; they are also able to fix heavy metals. They thus constitute the main biological element of certain municipal and industrial water treatment systems (mainly tertiary treatment). Due to the assimilation of nitrogen and phosphorus, they contribute in reducing the phenomenon of eutrophication (i.e. degradation) of certain aquatic environments.

In order to reduce the economic costs of these water treatment processes, generated microalgae biomass can be used to produce molecules with high added value, (such as biodiesel, methane, hydrogen etc.). These processes are typically coupled with the elimination of CO₂ in industrial gas emissions, leading to integrated processes.

1.3.5.2. Agriculture

Algal biomass constitutes a valuable asset as manure, fertilizer and soil stabilizer in agriculture, and also as a crop accelerator and protector by limiting the proliferation of epiphytes and parasites. Microalgae allow particle adhesion and storage of water in the soil as well as nitrogen fixation. The synthesis of bioactive molecules means that they are likely to influence the growth of terrestrial plants. Microalgae are used, for example, in the production of rice, ensuring nitrogen fixation in tropical and subtropical agriculture. They are also used for surface strengthening in arid regions in order to combat erosion.

1.3.5.3. Life-support systems

During space missions, the development of a life-support system for the crew is essential. These systems must fulfill four basic needs: regeneration of a breathable atmosphere (O₂ supply), water recycling, waste treatment and provision of food. In order to respond to these constraints, closed-loop regenerative life support systems use biological systems, such as algae and terrestrial plants. The Micro-Ecological Life Support System Alternative (MELISSA) project has been developed by the European Space Agency (ESA) for thesepurposes [GOD 0₂]. The microalga Spirulina is produced and then dried to be consumed or incorporated into food.

1.3.5.4. CO₂ sequestration

The current alarming situation concerning climate change has triggered worldwide awareness. The growing concentration of greenhouse gases (known as “GHG”) in the atmosphere has an increasingly important effect on climate change [MAT 95].

Natural absorption no longer compensates for the high production rate of these types of gas; CO₂ having the highest effect, representing more than 68% of total emissions [MAE 95, KON 07, ROM 07]. A dramatic increase in the release of CO₂ in the atmosphere has been observed due to anthropogenic sources. Indeed, released CO₂ was about 7.4 billion tons in 1997, and is estimated to be about 26 billion tons by 2100. There is need to remind the reader of the catastrophic results of climate warming on desertification, increase in the frequency of extreme weather events, disruption of ecosystems and melting of non-polar glaciers resulting in rising sea levels [MOR 97].

In order to reduce the levels of greenhouse gases in the atmosphere, intensive research has focused on the development of new CO₂ reduction techniques (Figure 1.2) [IPC 05]. There are three main types of processes: geological sequestration, chemical processes and bioprocesses [PIR 11].

Geological sequestration relies mainly on the storage of liquid or gaseous CO₂ in geological formations, in the soil [HER 01] or in deep ocean storages [ISR 09]. However, these technologies have many disadvantages such as the possibility of leakage, the contamination of drinking water aquifers, the increase in the acidity of water, the disruption of the marine ecosystem and significant financial costs.

Figure 1.2. Schematic view of the capture and storage of carbon dioxide [IPC 05]

Chemical processes include absorption by alkaline solutions [DIA 04], the use of multi-walled carbon nanotubes [SU 09], and adsorption–neutralization on amine enriched carbon [PLA 07]. These methods are expensive and energyintensive [WAN 08].

The use of biological systems is a very promising alternative solution, relatively efficient, economically feasible and sustainable. These methods are based essentially on photosynthesis, with the transformation of CO₂ into biomass [KON 07, DEM 07]. Two CO₂ sequestration biological pathways exist: one using terrestrial plants and the second photosynthetic microorganisms. In the first method, forests are used to convert CO₂ into cellulosic structures for plants (namely trees) and into humus for soils [ZAM 10]. However, due to limited conversion efficiency, low growth rate, economic and technical disadvantages (possibility of release of stored carbon following a forest fire or damage to trees), numerous studies have focused on the second pathway, which is the implementation of photosynthetic organisms such as microalgae. CO₂ sequestration by microalgae is the main subject of this book and will be discussed in Chapter 2.

1.4. Microalgae cultivation systems

Given the diversity of industrial applications and nutritional and environmental requirements of microalgae culture, the establishment of an efficient cultivation system is a specific and crucial step which depends on the application in question. There are two main categories of cultivation systems: open systems (natural and artificial ponds characterized by a low surface-to-volume ratio) and closed systems (photobioreactors with a high surface-to-volume ratio).

1.4.1. Open systems

This type of system is characterized by its technical simplicity, its ease of operation and a relatively long lifespan. These systems essentially involve shallow basins supplied, for example, with wastewater discharged from waste processing factories and fitted with a stirring system formed by rotating structures or impellers. Biomass itself is harvested at the end of the recirculation cycle (Figure 1.3). This type of configuration is known as a raceway basin [MOH 05].

Open systems have a wide variety of configurations depending on the size, depth, type of material, agitation system and inclination [TRE 04]. Three main types are used on an industrial scale: lakes and natural ponds (natural ecosystems), circular basins and raceway basins (Figure 1.3).

Figure 1.3. Variety of open system configurations: a) raceway basin used for the culture of Spirulina platensis in California; b) circular basin with central pivot for the culture of Chlorella in Taiwan; c) non-agitated broad basins used for the culture of Dunaliella salina in Australia

Natural ecosystems are the most economic mode of culture and the least technicall...

Table of contents

• Cover
• Contents
• Title Page
• Copyright Page
• Introduction
• 1 Microalgae
• 2 CO2 Biofixation
• 3 Bioprocess Modeling
• 4 Estimation of Biomass Concentration
• 5 Bioprocess Control
• Conclusion
• Bibliography
• Index