**4. Microalgae-based systems for CO2 sequestering and industrial biorefineries**

Microalgae have the capacity to adapt to changes in the environment, producing biomass that serves as a precursor for a variety of biomolecules; such as proteins, pigments, vitamins, lipids, and carbohydrates, in addition to finding applications in pharmaceutical, cosmetic, food and biofuel industries [32]. In **Figure 5**, a process flow diagram for micro-algal system in a combined biofuel production system is presented. Microalgae have a promising physiological plasticity in that they have a wide range of pH which allows for a range of species that can convert biomass to high value applications [33]. Pollutants in wastewaters present themselves as nutrients to microalgae, thus providing application of microalgae technology in the

**435**

*Valorization of Lignocellulosic and Microalgae Biomass DOI: http://dx.doi.org/10.5772/intechopen.93654*

hydrodynamic stress of the cultivation system [35].

**4.1 Thermal energy**

**Figure 5.**

*from wastewater [54].*

wastewater treatment sector. These photosynthetic organisms grow under diverse luminous intensities and electromagnetic radiations to produce biomass of desired compositions. In addition, they sequester CO2 from the environment and contribute to the global CO2 balance, thus addressing the global warming phenomenon induced by emissions from fossil fuel combustion processes. Microalgae cultivation combined with metabolic techniques range from autotrophy and heterotrophy to mixotrophy, allowing the biomass a wide latitude for varied specific growth rate, productivity, and composition, which in turn can be enhanced by the hydrodynamics that are governed by the reactor configurations. Lipid recovery continues to be a significant bottleneck in biodiesel production due to high costs of harvesting the biomass in the first instance and the available lipid extraction techniques [34]. Microalgae growth is induced and sustained by factors such as (i) thermal energy, (ii) inorganic carbon supply, (iii) nutrient availability, (iv) luminous exposure, (v) organic carbon and (vi) water. Under photosynthesis protocol, these factors usually work in combination through different metabolic scenarios, which include autotrophy, heterotrophy and mixotrophy. The response to these factors and the nutritional programs depends on the microalgae species and strains. However, the quality of biomass produced from these photosynthetic metabolic scenarios depends on the

*Process flow for a micro-algal system for combined biofuels production, CO2 biomitigation, and N/P removal* 

Most microalgae species are mesophilic in nature as they produce biomass in the temperature range of 15–35°C. However, some species are extremophiles, i.e. some strains are psychrophilic (*Chlamydomonas nivalis*, *Raphidonema* sp*.*, *Mesotaenium berggrenii,* and *Chloromonas* sp*.* [CCCryo 020–99]) as they produce biomass under snowy conditions, while few other strains are thermophilic (*Phormidium sp.* and *Thermosynechococcus elongatus* BP-1 which are cyanobacteria; and *Desmodesmus sp.* F51, *Chlorella sorokiniana* UTEX 2805, *Desmodesmus sp*. F2 and F18 are green algae and; *Galdieria sulphuraria* 074G and *Galdieria sulphuraria* CCMEE 5587.1 are the red algae); they produce biomass at temperatures as high as 55–74°C. Depending on the species, microalgae at an optimum temperature with suitable nutrients media (nitrogen, phosphorus, and sulfur) and luminous exposure, produce biomass of varying properties and composition [36]. Temperature is a key variable that

*Valorization of Lignocellulosic and Microalgae Biomass DOI: http://dx.doi.org/10.5772/intechopen.93654*

**Figure 5.**

*Biotechnological Applications of Biomass*

solvent recovery [20].

**3.4 Biological pretreatment**

**biorefineries**

**3.3 Acidic and alkaline pretreatment**

viscous in nature, requiring the use of co-solvents to enhance its fluidity and the recovery by a commonly employed aqueous biphasic system, or the use of acetone, sodium hydroxide or water [25]. Commonly used co-solvents are dimethyl sulfoxide (DMSO) and dimethylacetamide (DMAC). The application of ILs to LB in areas such as fractionation, cellulose composites preparation and its derivative and removal of pollutants is a new avenue for the efficient utilization of these solvents [26]. ILs have been found to be the most expensive research-grade solvents under investigation for the dissolution of biomass and provides further challenges with

Lignocellulosic pretreatment with acids at ambient temperatures are carried out to enhance hemicellulose solubilization, thereby, making cellulose accessible for enzyme degradation with a dilute or a strong acid [14]. In this process, solubilized hemicelluloses are exposed by hydrolytic reactions to produce monomers, furfural, and other volatile products under acidic conditions [27]. In this regard, solubilized lignin quickly condenses and precipitates into acidic conditions. Hemicellulose solubilization and lignin precipitation are therefore noticeable during strong acid pretreatment. A disadvantage of this method is the risk of the formation of inhibiting compounds [14]. However, the use of dilute acid pretreatment has gained numerous research interests over the use of concentrated acids [28]. This is due to the fact that concentrated acids are toxic, corrosive, hazardous, and require reactors

that need expensive construction materials which are resistant to corrosion.

**4. Microalgae-based systems for CO2 sequestering and industrial** 

Microalgae have the capacity to adapt to changes in the environment, producing biomass that serves as a precursor for a variety of biomolecules; such as proteins, pigments, vitamins, lipids, and carbohydrates, in addition to finding applications in pharmaceutical, cosmetic, food and biofuel industries [32]. In **Figure 5**, a process flow diagram for micro-algal system in a combined biofuel production system is presented. Microalgae have a promising physiological plasticity in that they have a wide range of pH which allows for a range of species that can convert biomass to high value applications [33]. Pollutants in wastewaters present themselves as nutrients to microalgae, thus providing application of microalgae technology in the

The delignification of LB could also involve application of biological methods using enzymes or microorganisms. Wood degrading microbes including white, brown, soft rot fungi, and bacteria are used in biological applications [28]. Biodegradation releases the chemical components and opens up the structure of the LB which promotes enzyme action leading to further breakdown. The brown and soft rots have been reported to attack cellulose leading to lignin modifications, whilst the lignin components are degraded by the white rot fungi [28]. The biological pretreatment of wood chips with four different white-rot fungi for a period of 30 days was studied [3]. The glucose yield of the pretreated wood by *Trametes versicolor MrP 1* reached 45% by enzymatic hydrolysis while 35% solid was converted to glucose during fungi incubation. Some microbes that have been employed in the past decades include *Ceriporia lacerate, Sterum hirsutum, Polyporus brumalis* and *Phanerochaete chrysosporium.*

**434**

*Process flow for a micro-algal system for combined biofuels production, CO2 biomitigation, and N/P removal from wastewater [54].*

wastewater treatment sector. These photosynthetic organisms grow under diverse luminous intensities and electromagnetic radiations to produce biomass of desired compositions. In addition, they sequester CO2 from the environment and contribute to the global CO2 balance, thus addressing the global warming phenomenon induced by emissions from fossil fuel combustion processes. Microalgae cultivation combined with metabolic techniques range from autotrophy and heterotrophy to mixotrophy, allowing the biomass a wide latitude for varied specific growth rate, productivity, and composition, which in turn can be enhanced by the hydrodynamics that are governed by the reactor configurations. Lipid recovery continues to be a significant bottleneck in biodiesel production due to high costs of harvesting the biomass in the first instance and the available lipid extraction techniques [34]. Microalgae growth is induced and sustained by factors such as (i) thermal energy, (ii) inorganic carbon supply, (iii) nutrient availability, (iv) luminous exposure, (v) organic carbon and (vi) water. Under photosynthesis protocol, these factors usually work in combination through different metabolic scenarios, which include autotrophy, heterotrophy and mixotrophy. The response to these factors and the nutritional programs depends on the microalgae species and strains. However, the quality of biomass produced from these photosynthetic metabolic scenarios depends on the hydrodynamic stress of the cultivation system [35].

#### **4.1 Thermal energy**

Most microalgae species are mesophilic in nature as they produce biomass in the temperature range of 15–35°C. However, some species are extremophiles, i.e. some strains are psychrophilic (*Chlamydomonas nivalis*, *Raphidonema* sp*.*, *Mesotaenium berggrenii,* and *Chloromonas* sp*.* [CCCryo 020–99]) as they produce biomass under snowy conditions, while few other strains are thermophilic (*Phormidium sp.* and *Thermosynechococcus elongatus* BP-1 which are cyanobacteria; and *Desmodesmus sp.* F51, *Chlorella sorokiniana* UTEX 2805, *Desmodesmus sp*. F2 and F18 are green algae and; *Galdieria sulphuraria* 074G and *Galdieria sulphuraria* CCMEE 5587.1 are the red algae); they produce biomass at temperatures as high as 55–74°C. Depending on the species, microalgae at an optimum temperature with suitable nutrients media (nitrogen, phosphorus, and sulfur) and luminous exposure, produce biomass of varying properties and composition [36]. Temperature is a key variable that

influences the composition of microalgae biomass. For instance, Varshney and coworkers [37] reported that an increase in temperature from 20 to 25°C doubled the lipid content of *Nannochloropsis oculata* (from 7.90 to 14.92%), whilst an increase from 25 to 30°C brought about a decrease of the lipid content of *Chlorella vulgaris* from 14.71 to 5.90%.

#### *4.1.1 Inorganic carbon*

Inorganic carbon that is accessible to microalgae is mostly CO2 gas. This gas is available in dilute concentrations in the atmosphere at 0.035 mole percent (dry basis) [38]. Microalgae absorb CO2 from the atmosphere to produce sugars by the physiochemical process of photosynthesis. The biological conversion of CO2 results in products of the photosynthetic metabolism such as cells, oxygen biopolymers which are soluble in the culture medium and volatile organic compounds (VOC's). Zhao and Su [39] described photosynthesis as a two-stage process. The first stage is the light-dependent reaction which captures the energy of light for oxidative phosphorylation in the metabolic cycle that produces the energy-storage molecules, adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH) as shown in Eq. (1).

$$2\text{H}\_2\text{O} + 2\text{NADP}^\* + 3\text{ADP} + 3\text{P}\_i \xrightarrow{hv} 2\text{NADPH} + 2\text{H}^+ + 3\text{ATP} + \text{O}\_2 \tag{1}$$

Eq. (2) shows the second-stage reaction, which is carbon dioxide fixation; and it is not directly light-dependent. This photosynthetic dark-reaction captures and reduces carbon dioxide to carbohydrates and releases molecular oxygen [40].

$$\text{C}\text{O}\_2 + 2\text{H}\_2\text{O} \overset{photons}{\rightarrow} \text{[CH}\_2\text{O}\text{]} + \text{O}\_2\tag{2}$$

With a solubility of 0.1449 g CO2/100 mL H2O at 25°C and 101.325 kPa vapor pressure, carbon dioxide gas dissolves in surface water and slowly reacts with water to alter its chemistry as shown in Eqs. (3) and (4).

$$\rm{CO}\_2 + \rm{H}\_2\rm{O} \rightleftharpoons \rm{H}\_2\rm{CO}\_3\tag{3}$$

$$\mathrm{H\_2CO\_3} \rightleftharpoons \mathrm{H^+} + \mathrm{HCO\_3^-} \tag{4}$$

The release of H<sup>+</sup> ion in Eq. (4) causes the reduction in pH of the culture medium. However, the ability to thrive in a wide pH range has given microalgae the privilege to access nutrients from municipal and industrial wastewaters. In the presence of hydroxide ions, carbonate ions are also released as in Eq. (5).

$$\mathrm{OH}^- + \mathrm{HCO}\_3^- \rightleftharpoons \mathrm{H}\_2\mathrm{O} + \mathrm{CO}\_3^{2-} \tag{5}$$

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*Valorization of Lignocellulosic and Microalgae Biomass DOI: http://dx.doi.org/10.5772/intechopen.93654*

*4.1.2 Nutrients availability*

products [42, 43].

*4.2.1 Organic carbon*

**4.2 Metabolic flexibility of microalgae**

to produce biomass [41]. The CO2 conversion into biomass is increased only under conditions where the CO2 mass loading rate is low. At a high CO2 mass loading rate,

Standard microalgae culture media have been developed and produced out of the need to produce desired products and are available in the market for freshwater microalgae growth management. Some of the media are the (i) Blue-Green medium, BG-11 (ii) Bold's Basal medium, BBM (iii) Bold's Basal medium modified, BBM-3 N (iv) CHU 13 and (v) Jaworski's Medium, JM. Both municipal and industrial wastewater have the basic nutrients common to all the artificial media designed for microalgae cultivation; and microalgae access these nutrients as nitrates and reactive phosphates from wastewaters to produce biomass and bio-

Microalgae have three different metabolic pathways, namely, autotrophy, heterotrophy and mixotrophy. While all algae species are autotrophic, some stains have the ability to exhibit heterotrophy and mixotrophy; and any of the chosen photosynthetic metabolic depends on the microalgae species, and the quality of the biomass desired. Autotrophic metabolism utilizes inorganic carbon in the form of CO2, gas and light energy. This mode of fixing CO2 produces low density microalgal biomass. Heterotrophic metabolism takes advantage of the presence of organic carbon and utilizes it both as a source of carbon and energy. This is the dominant pathway during the night or dark phases. Some microalgae do metabolize mixotrophically. Under mixotophic mode, light energy is not the absolute growth limiting factor as organic carbon sources are also accessed and utilized for microalgal biomass production. Photoinhibition, a phenomenon that describes excessive light intensity thereby arresting photosynthetic metabolism, is overcome under the mixotrophic metabolic mode. Consequently, the growth rate is not interrupted and high density biomass is produced with recorded higher productivities when com-

pared to autotrophic and heterotrophic metabolic scenarios [6].

Organic carbon present in municipal and industrial wastewater are carbohydrates, fats, volatile fatty acids (VFAs), soaps, synthetic detergents, lignin, proteins and their decomposition products; as well as various natural and synthetic organic chemicals. Wastewater treatment and concomitant algal biofuel production has received increasing attention in recent years owing to its diverse environmental and economic benefits [44]. Organic carbon is accessible through monosodium glutamate wastewater, cheese whey permeates, sodium acetate, fruit peel, glucose, fructose, glycerol, etc. via mixotrophic microalgal growth mode. Tan and coworkers [45] reported that productivities of *C. vulgaris* cultured in wastewaters containing glucose and sodium acetate were 63.5 and 55.2 mgL−1 day−1, respectively. This accounted for the leap of 2.61 and 2.27 times the productivities, respectively, achieved under autotrophic metabolic modes. Also, *Chlorella vulgaris* cultivated in sodium acetate and glucose wastewaters recorded productivities of lipid at 17.35mgL−1 day−1 and carbohydrate at 18.75mgL−1 day−1, respectively, indicating that sodium acetate and glucose wastewaters have the potential to boost microalgal lipid production, which in turn may serve as feedstock for the biorefinery [46].

the formation of VOCs is the main CO2 biotransformation strategy [33].

When the dynamic ionization equilibrium is attained, dissolved inorganic carbon (DIC) is available in the form of CO2, HCO3 − , CO3 2−, and H2CO3. However, only CO2 and HCO3 − are accessible to microalgae and both species are utilized simultaneously

to produce biomass [41]. The CO2 conversion into biomass is increased only under conditions where the CO2 mass loading rate is low. At a high CO2 mass loading rate, the formation of VOCs is the main CO2 biotransformation strategy [33].
