Keywords

9.1 Introduction

Energy is one of the most indispensable parts of human civilization. With the progress of science and technologies, the requirement of energy has been increased exponentially. Bitumen as an energy source has been used for centuries, whereas the refinery-based operation was started in the seventeenth century (Craig et al. 2018). Even after the thriving of electrical energy (considered as low entropy, more efficient), the requirement of petrol and diesel keeps on increasing, mainly to meet the need of transportation fuel. The world was shocked, when suddenly, in 1973, OPEC (Organization of Petroleum Exporting Countries) announced the unexpected rise in crude oil price. The same scenario happened in 1979–1983 several times. This coerced scientists and technologists to find alternative energy sources.

Biodiesel is considered as a potential solution to the current threat. However, despite several advantages, the overall production process has faced different technical difficulties, such as high cost, lack of availability for raw materials, etc.

Algal biodiesel, the production of biodiesel through algal roots, is a new, suitable, and advanced method. During the production of biodiesel, alkyl group of oils is replaced by different alkyl groups to form mixture of esters whose properties (viz., octane and cetane number, kinematic viscosity, calorific value, etc.) are similar to petro-diesel. This conversion method is known as transesterification. In the case of algal biodiesel, the oil is extracted from high lipid content microalgae. Compared to animal oil and plant oils, algal oil can be produced in a faster way, but the growth of algae and oil extraction from algae comes under different strategies. Algal biodiesel comes under second and third generation of biofuels because they can absorb CO2 from the atmosphere as well as from flue gases with 30–40% of CO2.

9.2 Algae Biology

Algae are recognized as the fastest growing and one of the oldest life forms. They are primitive plants (thallophytes), i.e., lacking roots, stems, and leaves, have no sterile covering of cells around the reproductive cells, and have chlorophyll-a as their primary photosynthetic pigment. Algae have a diverse form of morphologies and are generally classified into two, microalgae and macroalgae. Macroalgae are a multicellular organism which can grow more than 100 feet in length. Macroalgae can be classified based on their pigmentation such as red (Rhodophyta), brown (Phaeophyta), and green (Chlorophyta). The composition present in macroalgae is mainly carbohydrates, starch, mannitol, and cellulose, which makes them suitable to be utilized for production of food product and bio-ethanol (Rajkumar et al. 2014). Microalgae are tiny unicellular organisms normally found in both marine and freshwater habitats. Microalgae have higher surface to quantity ratio which makes them grow faster by absorbing high amount of nutrients. The lipids present in the range of 50–60% in many microalgae make them promising source for biodiesel production in comparison to land crops (Griffiths and Harrison 2009).

Algae can either be autotrophic or heterotrophic; the former require only inorganic compounds such as CO2, salts, and a light energy source for growth. The most common types are green algae, red algae (Rhodophyta), diatoms (Bacillariophyta), and multicellular form (kelp). Heterotrophic algae are non-photosynthetic, and their growth depends upon nutrients absorbed from external source of organic compounds. For autotrophic-type algae, photosynthesis is an essential component for their growth, where they absorb solar radiation to fix atmospheric carbon dioxide and reduce their dependency on sugar for fermentation. Algae have higher photosynthesis capability than plants which make them efficient to grow rapidly under desired conditions (Fabris et al. 2020). Microalgae are mixotrophic, and they have the ability to acquire nutrients from both photosynthesis and external organic nutrients for their growth.

9.3 Advantages of Using Microalgae for Biodiesel Production

Microalgae are one of the most growing feedstock for biodiesel production because they are easy to cultivate and require little or even no maintenance for growth. Microalgae use sunlight for their growth. The growth rate can be accelerated by addition of nutrients and sufficient aeration. The yield of oil obtained is 3–17 times more than other terrestrial oil-producing plants (Avagyan and Singh 2019). Microalgae-based biofuels have higher calorific value and lower viscosity and density in comparison to plant-based biodiesels (Costa and De Morais 2011). They have much higher growth rates and require much less land area in comparison to other energy crops. The total cost of producing biodiesel from microalgae is cheaper than other sources as no lands are required and algae can be cultivated and produced with wastewater which significantly reduces operational cost. Apart from biodiesel, microalgae can be utilized as feedstock for several different types of renewable fuels such as methane, hydrogen, ethanol, etc. Some possibilities currently being considered are listed below:

  • Capturing CO2 from flue gases by algae-based bio-fixation process. The study shows that CO2 fixation via algae is 10–50 times greater than terrestrial plants (Chisti 2007).

  • Algae used in wastewater treatment for a range of purpose such as removal of heavy metals, bacteria, NH4+, PO43−,NO4−, and reduction of COD and BOD.

  • Algal biomass after oil extraction can be utilized for the production of methane, ethanol, and organic fertilizer or directly used as electricity or heat generation by burning.

  • Algal biomass has the ability to grow under harsher conditions; they can be grown in areas unsuitable for conventional agricultural products.

  • Microalgal species can also be utilized for large range of fine chemicals and industrial products such as fats, oil, dyes, pigments, and antioxidants.

  • Microalgae have the potential to be raw materials for cosmetics, nutrition, pharmaceuticals, and food and aquaculture industry (Mata et al. 2010).

9.4 Technologies for Microalgal Biomass Production

As in natural habitat, phototrophic algae absorb CO2 and sunlight in the presence of suitable temperature and some nutrients and grow themselves; thus, during artificial production, the environment should be replicated as much as possible (Brennan and Owende 2010; Muller-Feuga et al. 1998). But, natural sunlight has limitations due to seasonal variations, day-night cycle, etc. which is a bottleneck problem for the commercial production of algal biodiesel in different areas of the world (Pulz and Scheibenbogen 1998). Thus, use of artificial light source such as fluorescent lamp is almost essential for pilot plant projects. Although artificial light helps in continuous production, it increases the energy demand, thus, questioning low-cost production. At the same time, it also increases the carbon footprint. Another complicacy arises due to the presence of different photosynthetic pigments in different algae (diatoms have chlorophylls a and c and fucoxanthin; green algae have chlorophylls a and chlorophylls b and zeaxanthin). This leads to the requirement of a varity artificial light wavelength (Muller-Feuga et al. 1998; Razzak et al. 2013).

Microalgae can collect CO2 from atmospheric air, from industrial flue gases, and from soluble carbonates (Wang et al. 2008). Most of the species are able to capture CO2 from normal level of CO2 concentration in air (around 360 ppm), but some species are able to assimilate very high concentration of CO2 (up to 1,50,000 ppm) (Bilanovic et al. 2009; Chiu et al. 2009). Thus, several works have been conducted where high CO2 which contains flue gases (such as exhaust from thermal power plant) is used as a carbon source for microalgal growth (Rosenberg et al. 2011; Zhu et al. 2014). Another interesting method is to use soluble carbonates (such as Na2CO3 and NaHCO3) as carbon source for the growth of algae (Lohman et al. 2015).

Except carbon, other required micronutrients are nitrogen, phosphorous, and silicon (Suh and Lee 2003a, b). Although some species can absorb nitrogen from atmospheric NOx, most species are only able to absorb from soluble nitrogen slats (Hsieh and Wu 2009). Although phosphorous is required in a very little amount, due to high reactivity with metallic ions, it must be added in high amounts (Çelekli et al. 2009; Chisti 2007). Silicon is specifically important for the growth of diatoms (Martin-Jézéquel et al. 2000).

Growth of algae toward biodiesel productions is mainly done in three different ways, viz., phototrophic, heterotrophic, and mixotrophic. In the next section, we are going to discuss them in detail.

9.4.1 Phototrophic Production

Phototrophic production is considered as the most economically and technically feasible method (Abomohra et al. 2016). Depending on the local weather, locally available strains, scale of production, cost, and availability of land/water, two main methods are used, viz., open/raceway ponds and closed photobioreactor (Moreno et al. 2014).

9.4.1.1 Open Pond Production System

Culture of algae could be done in natural water bodies like ponds and lakes, or it could be raceway ponds. Raceway ponds are of oval shape with free land space in the middle. Water is circulated through the water body around the periphery of the middle land (Jiménez et al. 2003; Pugazhendhi et al. 2020). It has the advantages of efficient water circulation, optimum water usage, facility of CO2 sparing, and better exposure of algae to natural light. Figure 9.1 shows the circulation of water with the help of a paddlewheel. Broth medium is introduced continuously just after the paddlewheel, and grown algae is collected just before the paddlewheel. This arrangement helps in the efficient use of the broth; at the same time, the flow generated by the paddlewheel doesn’t allow the settlement of algae at the bottom (Rogers et al. 2014).

Fig. 9.1
A schematic diagram depicts two rounded rectangles, one inside the another. The inner rectangle is labeled paddlewheel. The outer rectangle has one upward vertical arrow labeled harvest, a downward vertical arrow labeled algal broth, nutrient medium. C O 2 is supplied at the top and the bottom. A left arrow labeled direction of flow is present.

Schematic diagram of an open raceway pond (Brennan and Owende 2010)

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Raceway ponds are optimally 0.2–0.5 m deep, built in concrete, and sometimes covered with white colored plastic which are found to be most suitable for the growth of algae (Chisti 2008; Kumar et al. 2015). Compared to the closed photobioreactor system, raceway ponds are cheaper, require less energy, and are easy to clean. But, on the other hand, they require more operating area and higher installation cost compared to photobioreactors (Rodolfi et al. 2009; Ugwu et al. 2008; Baldev et al. 2018). Due to exposure from the atmosphere, monoclonal growth is possible only under high maintenance, and due to the absence of sterile atmosphere, contamination by other algae and protozoa is also highly vulnerable (Abomohra et al. 2016; White and Ryan 2015).

Overall effectiveness of raceway ponds compared to closed photobioreactors is less (Pugazhendhi et al. 2020). This is due to several factors such as uncontrolled temperature, loss of water due to evaporation thus causing ionic concentration to change, uneven mixing of CO2, uncontrollable light intensity, etc. (Chisti 2007; Ugwu et al. 2008; Pulz 2001).

9.4.1.2 Closed Photobioreactor

As discussed in the last section, raceway ponds have some major disadvantages, and closed photobioreactors are designed to address those technical drawbacks. The main advantage of closed photobioreactors is that they can deal with single-strain culture without contamination, which is very beneficial for industries like cosmetics or pharmaceutical (Chisti 2007; Ugwu et al. 2008). Except this, closed photobioreactors have the advantages of efficient mixing, better utilization of broth medium and CO2, lower harvesting cost, etc. But, due to the requirement of artificial light, precise circulation system, higher maintenance, and cleaning cost, the overall expenditure is quite high compared to raceways (Carvalho et al. 2006; Kunjapur and Eldridge 2010).

These photobioreactors consist of transparent tubular arrays (made of plastic or glass) with diameter less than 0.1 m to allow microorganisms to be exposed in the light (Chisti 2007). The arrangements of these arrays could be horizontal (Molina et al. 2001), vertical (Mirón et al. 1999), or even in the form of a helix (Watanabe and Saiki 1997). Algal broth is circulated with the help of mechanical pump or airlift systems (Eriksen 2008a, b). Airlift system also helps in the mass transfer of O2 and CO2 between the broth and air (Singh and Sharma 2012).

Closed photobioreactors can be classified as tubular, flat plate, and column photobioreactors. A diagram of a flat plate photobioreactor is shown in Fig. 9.2. These are the initially developed photobioreactors, which consist of rectangular cuboids with large transparent surfaces (Samson and Leduy 1985). These surfaces are exposed to artificial light or to sunlight. Very dense cultures are processed in these photobioreactors; thus, the thickness of the plates must be very less for the penetration of light inside the broth (Hu et al. 1998; Richmond et al. 2003). The main advantages of a photobioreactor is that a high concentration mass culture can be easily processed with a higher level of photosynthetic efficiency (Richmond 2000; Razeghifard 2013).

Fig. 9.2
An illustration depicts the front and the side view of a flat plate photobioreactor that consists of rectangular cuboids with large transparent surfaces. The algal broth is depicted as small bubbles. The labels present are, air outflow, culture outflow, liquid level, temperature sensor, medium inflow, D O sensor, p H sensor, transparent polycarbonate sheets, reactor compartment and water jackets.

Front and side view of a flat plate photobioreactor (Singh and Sharma 2012)

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Tubular photobioreactors are designed as transparent helical pipes through which algal broth is circulated. This horizontal part is known as a solar receiver as this portion is utilized for photosynthetic activity (Singh and Sharma 2012). At the two ends of the pipe, airlift section is placed which is responsible for O2 accumulation, CO2 depletion, and the circulation of the broth (Fig. 9.3). Another research shows the growth of two different strains Nannochloropsis salina and Scenedesmus obliquus in marine and freshwater environment in a specially designed flat plate photobioreactor with biomass recycle facility (Sforza et al. 2014). Growth of microorganisms was modelled, and the performance of the lab scale model was simulated with the method of Pruvost et al. (Pruvost et al. 2011).

Fig. 9.3
A diagram depicts the design of the horizontal tubular photobioreactor. It has helical zig zag pipes through which the algal broth is circulated. The primary components are exhaust gas, culture medium, harvest, air, airlift system, C O 2, samples, and solar receiver. The horizontal part is solar receiver. At the two ends of the pipe, an airlift section is placed.

Design of the horizontal tubular photobioreactor (Brennan and Owende 2010)

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Tubular photobioreactors are known for their high surface area to volume ratio and for precise maintenance of pH (Eriksen 2008a, b; Ashok kumar et al. 2015). But the scale-up is difficult for this type of design (Olaizola 2000).

Column photobioreactors are one of the most advanced designs which contain vertical columns aerated from the bottom. Artificial light sources are used instead of direct sunlight which have the advantage of changing the intensity, wavelength, and light cycles (Fig. 9.4) (Suh and Lee 2003a, b). These bioreactors are considered as compact, high performance, low cost, and easy to operate. But the operating cost is on the higher side due to the use of external light sources (Mirón et al. 2002; Naira et al. 2020). Bubble column photobioreactor was used for the growth of marine microalgal biomass by Itoiz et al. The energy balance is also considered to prove the efficiency, as the creation of the artificial environment requires some extra energy (Itoiz et al. 2012).

Fig. 9.4
The diagram illustrates a column photobioreactor with external light sources. The flow starts from the carbon dioxide entry point outside the pipeline into the cylinder labeled column photobioreactor, and then to the degasser, and the flow is cyclic thereafter entering the reactor again. The photobioreactor is exposed to the L E D or cold light source.

Column photobioreator with external light sources

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Overall, closed photobioreactors got more attention from researchers, as they help in monoclonal cell culture, have comparatively higher biomass production rate, and are friendlier toward the process control operations (Singh and Sharma 2012).

9.4.1.3 Hybrid Production System

This process is the combination of both closed photobioreactors and open raceway ponds (Adesanya et al. 2014; de Jesus and Maciel Filho 2017). In the first stage, initial growth of monoclonal culture was done in sterile condition in closed systems. Afterward, in the second stage, algae are able to handle the harsh conditions, and then, the broth is transferred to an open raceway (Singh and Sharma 2012). This not only reduces the cost of mass culture, but also nutrient stress (at low concentration of nutrients, algae start storing lipid within their cell structure as future food material, which is an example of microbial intelligence) can be applied which increases the lipid content inside the cell body (Huntley and Redalje 2007; Rodolfi et al. 2009). It is reported that nitrogen and glucose deprivation condition helped to increase the lipid content from 16% to 57% for the Auxenochlorella protothecoides (a green microalga) in mixotrophic growth condition (Bohutskyi et al. 2014).

9.4.2 Heterotrophic Production

Glucose or other carbon sources are used by algae for growth rather than using CO2 during photosynthesis (Liu et al. 2011). That kind of growth is known as heterotrophic production which can be used for biomass production as well as for the production of other useful products (Miao and Wu 2006). This type of productions are very easy to scale up, as surface-to-volume ratio is not an important factor here (Eriksen 2008a, b). These systems don’t depend on light sources and provide faster growth, and biomass harvesting is also easier due to high cell densities (Chen and Chen 2006).

As per some reports, biodiesel production by heterotrophic production is more viable than phototrophic production due to the high degree of lipid content (viz., lipid content in C. protothecoides could be up to 55%) (Miao and Wu 2006).

9.4.3 Mixotrophic Production

Some algal species are able to grow in both phototrophic and heterotrophic conditions. Some examples of such organism are Spirulina platensis (cyanobacteria) and Chlamydomonas reinhardtii (green alga) (Chen 1996). Due to the capability of both phototrophic and mixotrophic production, availability of light is not only the growing factor; and at the same time, biomass loss during the dark phase is much less (Andrade and Costa 2007). The growth rate in mixotrophic production is observed to be somewhat middle between phototrophic production and heterotrophic production (Wang et al. 2014). Overall, the success of mixotrophic production process depends on the efficient utilization of both light and dark cycles (Zhang et al. 1999).

9.4.4 Factors Affecting the Microalgae Production Process

In the previous section of this chapter, we have seen how the solar energy is trapped inside the biomass (phototrophic production) or biomass is grown with the help of carbon sources such as glucose (heterotrophic production). Regarding the discussion of factors affecting growth rate, we can easily say that photosynthetic efficiency and utilization of sugar are the main factors for phototrophic and heterotrophic production, respectively. Except this, impact of strain selection and lipid productivity are two more important factors for biodiesel production.

9.4.4.1 Effect of Photosynthetic Efficiency (PE)

PE is defined as the fraction of solar energy absorbed during the photosynthesis (Alberts et al. 2015). Solar radiation contains spectrum ranging from 250 nm to 2500 nm, among which only 390 nm to 700 nm are utilized during photosynthesis. According to the energy point of view, 42.3% of the solar energy is absorbed. Further, the efficiency for the conversion of light energy to chemical energy is 27%. Combining this, the PE is calculated as 11.3%, whereas, experimentally, highest PE is estimated nearly as 13%, but the global average is quite below around 1–2%. This is due to some other effects such as photorespiration, photo-saturation, poor light absorption, etc. (Vasudevan and Briggs 2008).

9.4.4.2 Impact of Strain Selection

Selecting the suitable strain is one of the most important factors for algal biodiesel production. The factors which need to be considered are as follows:

  • High reproducing cycle.

  • Cell body should contain high lipid.

  • Should be able to survive in temperature and pH fluctuations.

  • Should be able to tolerate the shear stress generated in the photobioreactors.

  • Ability to fix high CO2 concentration.

  • Can grow in low nutrient condition.

  • Produce valuable co-products.

  • Have a high PE.

  • Have self-flocculation characteristics.

But, no such strain is available, which shows all these characteristics. Experts say selecting algae by screening of locally available strains is the best option (Sheehan et al. 1998; Borowitzka 2013; Borowitzka and Moheimani 2013). It is due to the habituation of the local strains with the climate of that place. But, obtained robust local strains may not be able to generate high lipid, so transgenic cells (a transgene is a genetic material that has been transferred naturally or by any of a number of genetic engineering techniques from one organism to another.) may be needed. Transgenic organisms are not only a better choice for biofuel generation; they are viable for usable by-products also. But, this method is not much popular due to the cost factor and lack of expert people available (León-Bañares et al. 2004).

9.4.4.3 Lipid Productivity

The lipid content of some of the microalgae is naturally high (i.e., up to 50% dry weight) (Hu et al. 2008). But, percentage of lipid can be increased by controlling several factors such as intensity of light (Weldy and Huesemann 2007), optimum temperature (Qin 2005), nitrogen-deficient nutrients (Widjaja et al. 2009), and CO2 concentration (de Morais and Costa 2007a, b; Chiu et al. 2009) or even by harvesting method (Widjaja et al. 2009; Chiu et al. 2009). But it is necessary to understand that the lipid content and lipid productivity are not linearly correlated. Lipid content considers the total lipid inside the cellular body; it doesn’t consider the total biomass production, but lipid productivity considers the lipid inside the total biomass. So, it’s a more suitable quantification for the cost factor of biofuel production (Brindhadevi et al. 2021; Kiran et al. 2016).

Focused research has been conducted on strains which are able to grow in very high CO2 conditions (such as flue gas of coal fired plant, ethanol biorefinery, etc.) as well as those which consist of high lipid. The bottomline of the outputs are as follows:

  • Nitrogen deficiency strategy for increasing the lipid level in cells sometimes go wrong due to the decrease of the overall biomass yield, thus decreasing the overall lipid production (Weissman and Tillett 1992; Laws et al. 1986).

  • Nitrogen deficient approach is very useful when conducted in well-researched and precise manner. It not only increases the total lipid content but also changes the free fatty acids to triglycerol. Triglycerol is more useful for the production of biodiesel (Widjaja et al. 2009; Meng et al. 2009).

  • Open pond production for large scale can be considered as cheap, but, at the same time, maintaining monoclonal culture in open pond is very difficult (Weissman et al. 1988; Chelf et al. 1993).

Except this, genetic manipulations are also done to increase lipid productivity. The US National Renewable Energy Laboratory (NREL) developed new modified diatom species which are able to develop above 60% and 40% lipid in laboratory conditions and outdoor cultivation. The acetyl-coA carboxylase gene is responsible for this high level of lipid accumulation (Huang et al. 2010).

9.4.5 Useful Co-processes During the Production of Microalgae

Up to this point of discussion, it is clear to us that algae absorb CO2 during photosynthetic growth; at the same time, they need various nutrients during the growth cycles. Thus, researchers tried to remove two main polluting resources, viz., industrial flue gases and wastewater generated from different sources, simultaneously generating biomass. The underlying concept is that; flue gases contain high amount of CO2 which could be a very good carbon source for algae, and the wastewater contains different macro- and micro-nutrients which could be used as growth medium. Another effort is that utilization of valuable intracellular and extracellular products which are automatically generated during the production of algal biomass.

9.4.5.1 Utilization of CO2 from Flue Gases

Detention of CO2 is possible in three different ways, i.e., from the atmosphere, from flue gas, and from soluble carbonates (Wang et al. 2008). Due to the low concentration of CO2 in the atmosphere (around 0.03–0.04%) and dependency on the mass transfer for mixing in the water, it is not viable for commercial production (Stepan et al. 2002). Flue gases solve this problem as they contain high amount of CO2 (up to 20%) (Bilanovic et al. 2009). But flue gas also contains SOx and NOx gases which must be removed before feeding to the bioreactor. Some algae could grow by absorbing carbon from NaHCO3 and Na2CO3. But these salts change the pH to a huge extent which could be a limiting factor for some species (Wang et al. 2008).

Again, a suitable strain for utilizing flue gases needs several criteria, viz., ability to absorb CO2 from high concentration, ability to tolerate SOx and NOx, should contain high extent of lipid, possibility of valuable by-products, bio-flocculating nature for easy harvesting, toleration to temperature fluctuation during the injection of hot flue gas, etc. No single strain can show all these characteristics; thus, choice of stains are based on specific requirements (Huntley and Redalje 2007; Chiu et al. 2008; de Morais and Costa 2007a, b).

As example of successful commercial application, a combined unit for the production of microalgal biomass using the sequestration of CO2 exhauster from an ethanol biorefinery is established in Iowa, USA. Around 60% of produced CO2 (annually 143,000 tons) are sequestrated using a 1000 acres of land. Annually, 2.4–7.2 million gallons of biodiesel production is estimated depending upon the strains used (Rosenberg et al. 2011). Except this, Zhu et al. have used 8000 L open raceway ponds for the growth of Nannochloropsis strains using flue gas exhausted from thermal power plant (coal fired) (Zhu et al. 2014).

9.4.5.2 Treatment of Wastewater

Everyone agreed that utilization of wastewater during the growth of algae is one of the most successful and credible steps for commercial applications. Because the organic, inorganic, and pathogens from the wastewater are removed and at the same time the pollutants are used by algae as their macro- and micro-nutrients for growth, a high degree of growth in biomass is obtained (Munoz and Guieysse 2006; Salama et al. 2017). If CO2 is available in wastewater, Redfield ratio (C:N:P = 106:16:6) can be maintained by adding some extra inorganic material from the outside. This ratio also helps in the production of high lipid and reducing harvesting cost. Some researchers also used supporting bacterial culture parallelly with the growth of phototrophic algae. These bacteria could break down poisonous cyclic compounds (such as phenolic or polycyclic aromatic hydrocarbons, etc.) in the presence of oxygen. Thus, phototrophic algae are required (Lundquist 2008).

9.5 Recovery of Microalgal Biomass

Recovery of algal biomass and further treatment is important prior to extraction of oil. The recovery of microalgae biomass generally requires one or more solid-liquid separation steps followed by concertation and drying processes. The recovery and harvesting is a challenging phase of the algal biomass production process and accounts for 20–30% of the total costs of production. The process involved two steps, i.e., thickening and dewatering. The process includes flocculation, filtration, flotation, and centrifugal sedimentation, some of which are highly energy intensive. The selection of harvesting technology is crucial for economical sustainable production of microalgae biomass on a large scale. A major factor includes size density and type of microalgae, salt concentration, and moisture content for effective harvesting techniques.

9.5.1 Harvesting Methods

The efficient harvesting technique relies upon the higher biomass concentration output achievement at lower energy and operation cost. Harvesting process includes dewatering and drying of biomass followed by extraction of lipids and transesterification of biomass to biodiesel. Different harvesting methods used are chemical, mechanical, biological, and electrical. The harvesting techniques should pass six important criteria, i.e., biomass quality, quantity, cost, processing time, type, and toxicity of biomass (Culaba et al. 2020). The goal of efficient harvesting method is to achieve a high biomass concentration under low operating cost. Apart from cell damage, density has to be considered. Generally, microalgae harvesting step can be broadly classified into two categories:

  • Thickening—the aim is to increase biomass concentration and obtain a slurry-like consistency. The techniques used for this purpose are centrifugation, filtration, and ultrasonic aggregation.

  • Bulk harvesting: Dewatering of algae biomass from bulk suspension to obtain 15–25% solid in concentration. The techniques used are filtration and centrifugation.

9.5.1.1 Flocculation Aggregation

Flocculation process is intended to aggregate the microalgal biomass to form large size precipitate. Flocculation occurs in different mechanisms which can be a single or combined mechanism. The different mechanisms to form an aggregate are listed below:

  1. (a)

    A negative charge on microalgae cells prevent aggregation of cells. The addition of multivalent cations and cationic polymers (i.e., flocculants) neutralizes the negative charge and helps the microalgae to form aggregation.

  2. (b)

    Microalgae may also be linked to one another by the process called bridging, to facilitate the aggregation.

  3. (c)

    Microalgal cells can be entrapped by the precipitation mineral called as sweeping flocculation process.

  4. (d)

    Patch mechanism: A low molecular polymer is added to the solution which neutralizes the small surface of negatively charged suspended particle to create a patch of positive surface by which flocs are formed.

  5. (e)

    Ultrasonic vibrations are also tried which have advantages of continuous operation, suitable for large-scale usage and applied for vast range of algal species.

The flocculation process differs with types of flocculants used for microalgae harvesting. The various flocculants are chemical flocculants (organic, synthetic organic, natural organic flocculants), bioflocculation, organoclay flocculation, auto-flocculation, and electrolytic flocculation (Ananthi et al. 2021; Pandey et al. 2020).

9.5.1.2 Flotation

Flotation methods are based on the separation of algae cells using dispersed air or gas bubbles. It is also called as inverted sedimentation as air bubbles attached to destabilized algae particles arise to the surface with lifting force provided by air and accumulate concentrate to form flocculants (Coward et al. 2013). The flotation process is performed in two ways: dissolved air flotation (DAF) and froth flotation. Flotation process does not require any addition of chemicals except some surfactants (e.g., potassium amyl xanthate, potassium ethyl xanthate, xanthogen formates, thiocarbanilide). Surfactants are added to decrease the size and coalescence of air bubbles. The self-floating nature of microalgal makes the separation more fast and efficient. Flotation process is suitable for large-scale harvesting, and it is considered to be more economical and requires less space (Sharma et al. 2013). Still there is very limited evidence of its technical or economic viability for different strains of microalgae as process needs surfactants. The most common requirement for successful flotation process is high-molecular-weight microalgal cells with hydrophobicity nature; both can be achieved by adding surfactant and coagulants (Sharma et al. 2013).

9.5.1.3 Gravity and Centrifugal Sedimentation

Gravity sedimentation is a common process in wastewater treatment to separate algae. It is the slowest process for the recovery of biomass. This method is only suitable for large (>70 μm) microalgae such as Spirulina. The sedimentation of algae depends upon size, density temperature, time, and type of microalgae. To speed up the settling process, a centrifugation process is usually applied. The advantage of centrifugation process is that it doesn’t require any additive agent to separate microalgae. Though the centrifuge process is very effective, it includes high energy and maintenance cost. The high shear and gravitational forces can damage microalgal structure. The most common centrifuge employed in algal harvesting are scroll centrifuge, decanter, and specially designed hydro cyclones with consideration that microalgae are sticky in nature and hard to move from one point to another.

9.5.1.4 Biomass Filtration

The conventional filtration process is the most appropriate and traditional method that has high recovery efficiency for harvesting relatively large (>70 μm) microalgae such as Coelastrum and Spirulina. It suitable to use small-size algae such as Scenedesmus, Dunaliella, and Chlorella as they result in a gelatinous layer on the filter medium, also causing plugging of the filters. Conventional filtration operates under pressure or suction, and filtration aids such as diatomaceous earth or cellulose can be used to improve efficiency. The most common convention filters are screening, micro-strainers, vacuum operated, and pressure-based type.

For harvest of smaller algal cells (<30 μm), membrane-based filtration processes are quite effective as the recovery of algal biomass is high based on the membrane pore size and algal size. Membrane-based harvest technology is well suited for fragile cells that required low operating pressure and flow rate to avoid cell damage. Pressure-driven micro- and ultrafiltration are mainly used in harvesting of microalgae. The drawback of membrane-based filtration is fouling and clogging of membrane pores due to deposition of algal matters on the membrane surface. Antifouling strategies such as back washing, air scouring, and removal of algal deposited layer on regular short interval are much needed for efficient use of membrane technology. Technologies like submerged membrane with centrifugation and membrane photobioreactor are also used to improve harvesting efficiency.

9.5.2 Recovery of Microalgal Biomass

9.5.2.1 Dehydration Process

The presence of moisture in algal biomass possesses the negative effect on harvesting and transesterification of algal biomass to biodiesel. The slurry harvested can be spoiled within a few hours at room temperature if there is a presence of excess water in biomass. The harvested biomass slurry (typical 5–15% dry solid content) must be in low water content to obtain targeted products. To get the maximum yield of oil, the moisture content of biomass should be less than 10% (Al Rey et al. 2016). The drying process accounts for 85% of the total energy consumed during algal biodiesel production. Drying processes such as sun drying, spray drying, freeze drying, microwave drying, and fluidized bed drying are usually used as per the finished product requirements. Spray drying is generally used for high-value products, but it is relatively expensive and can cause significant damage to some algal pigments. Sun drying is the cheapest dehydration method; but the main disadvantages include long drying times due to high moisture content in the algal biomass and large drying surface area. The risk of dissipation of nutrient value in the species is high due to direct sunlight as temperature affects the concertation of triglycerides and lipid extraction from algal biomass (Behera et al. 2015).

For efficient dewatering process, it is important to establish a balance between the selected drying technology and its cost-effectiveness in order to achieve the final product specification. Operating conditions such as temperature during drying process affect both the composition and yield of the targeted extracted product from algal biomass. Some valuable products such as astaxanthin, lutein and C-phycocyanin are easily degraded by excess heat and light. For such heat-sensitive products, advance spray drying or freeze drying methods are necessary. For other products like biofuel or fermentation, low-cost conventional drying (e.g., flotation, sedimentation) methods are more suitable.

9.5.2.2 Recovery of Algal Metabolites

The synthetization of microalgae biomass has several applications. Microalgae are a nonconventional source of proteins (e.g., spirulina and chlorella), vitamins, lipids, carbohydrates (e.g., glucose, starch, and sugar), fatty acids, metabolites, etc. Depending on the microalgal species, these compounds can be extracted. Mostly, these compounds are found within the cell and its membrane wall (Grima et al. 2003). Cell disruption is often required for recovering intracellular products from microalgae. The extraction process becomes easy and efficient if cell wall is disrupted. Cell disruption methods are of two types, namely, mechanical (based on mechanical forces, bead milling, high-speed and high-pressure homogenization, ultrasound, and microwave) and non-mechanical (using chemical and biochemical agents like solvents, detergents, chelating, alkalis, and enzymes). Among mechanical and non-mechanical methods, the non-mechanical method is less destructive as it increases the permeability of the cell wall by perforation rather than breaking it. Mechanical methods cause the damage to the algal cell by heat generated during processes which affect the quality of the product (Günerken et al. 2015).

9.5.2.3 Solvent Extraction

Microalgal biomass contains metabolites such as astaxanthin, β-carotene, and fatty acids which are considered high-value or value-added products. These components would contribute to the economics of the algal-based biorefinery. The most common techniques for extraction of these valuable metabolites are by organic solvents or supercritical fluids. The process solvent extraction from microalgae involves two main steps, i.e., washing and diffusion. For example, for lipid extraction, washing step dissolves all the lipids into an extraction solution, and diffusion step helps extract lipid from microalgae cells. Diffusion is a slow process, and generally rupturing the cells before washing step is often done to microalgal biomass for higher solvent extraction efficiency (Belarbi et al. 2000).

9.6 Algal Biomass to Biodiesel

After the extraction of algal oil, it is converted to biodiesel with the help of transesterification. Transesterification is the chemical reaction by which oil/triglycerides are reacted with alcohol to form biodiesel and glycerol. The process can be done by two different methods. Traditional transesterification, which is specifically applied for the conversion of plant oil and animal fats, could also be applied to algal oil after extraction. Additionally, direct transesterification is an advanced process which does not need the extraction of oil from algae. A solvent (or a mixture of solvents) is used in oil extraction as well as an acyl acceptor for the transesterification process. This wet biomass directly could be converted to biodiesel (Hidalgo et al. 2013). Further, biodiesel and glycerol are separated by gravity separation (Brennan and Owende 2010; Ma and Hanna 1999; Hariram et al. 2019).

9.6.1 Traditional Transesterification (TT) Method

The transesterification occurs in stepwise, from triglycerides (TG) to diglycerides (DG) then to monoglycerides (MG) and finally to glycerol in the presence of acid, base, or enzymatic catalyst. Reactions are reversible in nature, but the equilibrium favors the forward direction (Ma and Hanna 1999). The reactions are as follows (Eckey 1956):

$$ \mathrm{TG}+\mathrm{R}-\mathrm{OH}\leftrightarrow \mathrm{DG}+\mathrm{R}-{\mathrm{COOR}}_1\left(\mathrm{Biodiesel}\right) $$
$$ \mathrm{DG}+\mathrm{R}-\mathrm{OH}\leftrightarrow \mathrm{MG}+\mathrm{R}-{\mathrm{COOR}}_2\left(\mathrm{Biodiesel}\right) $$
$$ \mathrm{MG}+\mathrm{R}-\mathrm{OH}\leftrightarrow \mathrm{Glycerol}+\mathrm{R}-{\mathrm{COOR}}_3\left(\mathrm{Biodiesel}\right) $$

There are several factors which regulate the reaction.

9.6.1.1 Molar Ratio of Reactants

Theoretical ratios between triglycerides and alcohol are 1:3, but higher ratio of alcohol is favorable for the forward reaction (Freedman et al. 1984). Up to 1:30 ratio is reported for the TT.

9.6.1.2 Effect of Catalyst

Base-catalyzed reactions are faster compared to acid-catalyzed reactions. But, if the oil contains high amount of free fatty acids, alkaline catalyst will generate soapy by-products. Sulfuric acid, sulfonic acid, and hydrochloric acid are reported to be used as acid catalysts. Several alkaline materials are used as catalyst such as NaOH, KOH, NaNH2, NaH, KH, potassium methoxide, etc. (Sridharan and Mathai 1974). Pandit and Fulekar used CaO catalyst (prepared from chicken eggshell) for the transesterification of Chlorella vulgaris biomass (Pandit and Fulekar 2019). Except this, Fe2O3 nano-catalysts were reported to be use for biodiesel preparation from Neochloris oleoabundans biomass. Yield of biodiesel is reported as 81% (Banerjee et al. 2019).

9.6.1.3 Effect of Time and Temperature

For TT, conversion of glycerides to biodiesel increases with respect to time. Optimum temperature is reported between 20 °C and 60 °C, depending on the types of raw materials and catalyst used (Ma and Hanna 1999).

9.6.2 Direct Transesterification (DT)

In DT, wet biomass is directly converted to biodiesel in a single-step process. Thus, the energy requirement for drying the biomass is eliminated, which consequently reduces the cost of production up to 70%. Although the first research on DT was carried out around 1985, it gains attention of researchers in recent days. The process is mainly done either with the microwave assisted method or with the help of ultrasonication.

9.6.2.1 Microwave-Assisted Method

The generated dipole moments under the microwave radiation help the migration of solvent ions through the wet microalgal biomass. The most commonly used solvent is methanol. Several researches have been conducted under basic and acid catalysts with 80–97% yield. A mixture of ionic liquid (1-ethyl-3-methylimmidazolium methyl sulfate) and methanol is used as the solvent for the DT of Nannochloropsis sp. biomass has shown yield of around 40% (Wahidin et al. 2018).

9.6.2.2 Ultrasonication

Engendered micro-bubbles and shear stress during the ultrasonication enhance the mass transfer between the immiscible phases. Due to this, ultrasonication is an acknowledged method for the DT of wet microalgal biomass. The process can reduce the time of transesterification to a huge extent compared with traditional method. Up to 99% yield of biodiesel is reported by Ji et al. (2006).

Except this, DT was reported to be conducted just by heating and stirring in magnetic stirrer. Cultivated biomass of Nannochloropsis gaditana is converted to biodiesel by DT at an optimum temperature and time of 100 °C and 105 min in acid catalyzed condition (Macías-Sánchez et al. 2015). An enzymatic method consisting of two steps is conducted for treating wet microalgal biomass. Oil is extracted as an intracellular product by breaking down the cell wall with cellulase. Further lipase is used for the transesterification (He et al. 2018).

9.6.2.3 Parameters Affecting Direct Transesterification

The discussion won’t be complete unless the important parameters and conditions for DT are mentioned. A detiled description of this topic is beyond the scope of this chapter, thus, a brief picture is tried to portrayed here:

  • Type of solvent: Methanol is the most common solvent used by researchers, whereas ethanol, propanol, and butanol are the other options available.

  • Alcohol to lipid molar ratio: Quite higher quantity of alcohol is used compared to the traditional transesterification. The reported rations are between 100:1 and 550:1.

  • Used catalyst: Although H2SO4 enjoys the most relevant catalyst for DT, KOH and SrO are also reported to produce comprehensive yields.

  • Reaction time and temperature: similar to traditional transesterification (Hidalgo et al. 2013).

  • Co-solvents: Hexane, diethoxymethane, tetrahydrofuran, etc. are used as co-solvents to enhance oil extraction from wet biomass (Zhang et al. 2015).

9.7 Conclusion and Future Direction of Research

To overcome the drawbacks of standard biodiesel production (such as higher cost, limited availability of raw materials, etc.), algal biodiesel has been considered which has massive possibility. Algae are grown by phototrophic, heterotrophic, and mixotrophic methods, among which phototrophs are most popular. Despite using the traditional transesterification (conversion of esters present in oil to biodiesel; by replacing the alkyl group) method, direct transesterification (drying and lipid conversion done in a single step) has been applied to deal with wet biomass. Many reports prove the applicability of algal biodiesel as the successful replacement of diesel.

The overall process will be more effective and profitable, if, the discarded biomass after oil extraction could be suitably used. With the thermochemical or biochemical conversion of the biomass, different useful products or utilities (such as syngas, bio-oil, specialty chemicals, methane, hydrogen, ethanol, or even electricity) could be prepared. Except this, currently/future research are conducted for:

  1. 1.

    Finding more suitable strains

  2. 2.

    Using naturally growing seaweed or microalgae in the ocean to obtain the lipid

The bottom-line is due to the rapid growth rate of algae, ability to absorb high concentration of CO2 from flue gases, and low space requirement, technologists and scientists found the method very efficient. Some more research on the abovementioned parallel applications and with some pilot plant/industrial-scale studies could lead to the final success.