Introduction and Literature Review
1.1 General Introduction
1.1.2 Microalgal cultivation
Table 1.2. Chemical composition of various microalgae suitable for biofuel production (% dry matter) (Kobayashi et al., 2013, Menetrez, 2012; Shuba and Kifle, 2018)
S. No. Microalgae Carbohydrate
(%)
Lipid (%) Protein (%) Bacilliariophyceae
1 Chaetoceros calcitrans 10 39 58
2 Chaetoceros muellerii 11–19 40–57 44–65
Chlorophyceae
3 Botryococcus braunii 20 25–75 4
4 Chlamydomonas rheinhardii
17 21 48
5 Chlorella protothecoides 12–17 14–56 15–58
6 Chlorella pyrenoidosa 25 2 57
7 Chlorella vulgaris 12–21 16–40 51–56
8 Dunaliella salina 32 6 57
9 Haematococcus pluvialis 15–40 20–37 17–45
10 Scenedesmus dimorphus 17–51 15–45 8–18
11 Scenedesmus obliquus 10–16 20–55 50–56
12 Spirogyra sp. 33–64 11–19 6–21
Cyanophyceae
13 Anabaena cylindrical 25–35 4–7 43–57
14 Spirulina maxima 13–16 6–7 60–71
Euglenophyceae
15 Euglena gracilis 7–25 21–38 30–45
Rhodophyceae
16 Porphyridium cruentum 40–58 11–20 29–40
al., 2018). Hasnain et al. (2018) suggested replacing pure CO2 with flue gas to reduce the cost. Hasnain et al. (2018) also reported that 1 kg microalgal biomass can be produced from 1.8 kg CO2. Light/illumination is the source of energy required for growth and photosynthesis. The pH of the growth media and temperature of the growth environment should be maintained at specific conditions to promote microalgal cell growth. The microalgal cells majorly require nitrogen and phosphorus for growth. These could be provided with both organic and inorganic sources, but inorganic source utilization can cause water pollution. Hence, wastewater management is important step in microalgae cultivation. An alternative approach can be utilization of wastewater for microalgal growth as they are rich in nitrogen and phosphorous. This approach is beneficial as not only the cost of microalgal cultivation reduces, it also helps in decontaminating the water (wastewater treatment). However, the hindrance in its use for microalgal growth are composition imbalance, and presence of toxic compounds. To consider wastewater as an alternative nutrient source, it is necessary to apply a systematic wastewater analysis (Tan et al., 2018).
The commercial production of microalgae–based biodiesel is a combination of various processes followed in a specific order. These processes are cultivation of microalgae, then, harvesting/dewatering, and lastly conversion of lipids to biodiesel (Roux et al., 2017). The microalgae can be cultivated in open raceway pond or in closed photo–bioreactor (PBR).
A comparison of the open and closed systems for microalgae cultivation is shown in Table 1.3. Raceway pond (open system) as compared to photo–bioreactor require low capital cost, but are prone to contamination. In addition, there is huge water loss due to evaporation.
This system does not utilize the carbon dioxide optimally due to no mixing; hence, require several improvements to the current system. A photo–bioreactor, on the other hand, is a
closed and controlled system, thus requires small area and is less contaminable, but involves higher production cost.
Table 1.3. Comparison of open and closed systems for culturing microalgae Sr.
No. Open systems (Ponds) Closed systems (PBRs) 1. The open systems are simple, easy
maintenance and low cost.
The closed systems are complex, require high maintenance and are expensive.
2. These systems require low energy inputs. Hence, making it easier to scale–up for industrialization.
Closed systems require high energy inputs for various functions, thus, very hard to scale–up.
3. Important growth factors such as temperature, pH, illumination, CO2
cannot be controlled.
The system works under controlled and monitored growth factors. This also helps in enhanced CO2 utilization efficiency. Temperature and light are uniformly distributed through the culture.
4. Open system do not have sterilized conditions which results in growth and cultivation of multiple species.
Closed systems work under sterilized conditions procuring single species cultivation. Hence, the contamination risk is low and could be controlled.
5. Uniform mixing is very difficult, thus the nutrients and other growth factors are not available in equal proportion to each organism. Thus, affecting the biomass productivity.
Uniform mixing can be achieved with help of agitators enabling proper availability of all nutrients to every organism. This helps in achieving high biomass productivity.
Microalgae can either be grown in pure or mixed cultures. The energy and carbon source for all heterotrophic cultures are same, but phototrophic cultures require different sources of each. Organic carbon (e.g. glucose) constitutes both energy and carbon source for heterotrophic cultures. On the other hand, light and CO2 are the energy and carbon sources, respectively, for the phototrophic cultures (Zhu et al., 2017). A combination of phototrophic and heterotrophic cultures, termed as mixotropic culture, can result in increased cell production rate and higher lipid productivity as compared to the phototrophic culture. However, the organic substrate for microalgal growth makes up about 80% of the
total cultivation cost (Tan et al., 2018). Hence, to reduce the biodiesel production cost, proper selection and optimization of the cultivation conditions is essential.
Harvesting/dewatering is the process of removing water after cultivation from microalgae. It is the second step or process in cultivation of microalgae for biodiesel production. It can be divided into two steps, i.e., bulk harvesting followed by thickening.
As the name suggest, bulk harvesting uses flocculation, flotation, or sedimentation processes to collect solid matter (microalgal biomass) from the bulk culture. The biomass slurry content can be enhanced by thickening (centrifugation and then filtration). The harvesting costs and energy requirements are important factors associated with microalgae–
based biodiesel production (Tan et al., 2018). 20%–30% of the total production cost is incurred in harvesting (Alessandro and Antoniosi, 2016). Hence, it is crucial to select microalgal species that ease the harvesting process after growth.
Though the lipid content in microalgae is high, the cell walls are rigid, thick and protected by glycoproteins and complex carbohydrates. Similarly to the contents of microalgae, the thickness of their cell wall is also associated with the growth factors. This complexity of the microalgal cell wall restricts the extraction of algal oil. To obtain high oil extraction efficiency, higher rate of cell disruption is required. Hence, oil extraction should be preceded by a pretreatment method for cell wall disruption with ease, making the selection of species and culture medium a vital step for obtaining a fragile cell wall (Lee et al., 2017). Additionally, the growth parameters like pressure and temperature also need much attention. The biomass conditions (e.g. dry/wet biomass, its concentration) and the biomass stage of harvesting are few other factors to be monitored first.
Harvesting is generally followed by a drying step that reduces water content of biomass from 60% to ~ 10%. This step is crucial as high water content in microalgal biomass has adverse impact on conversion of lipids (Sitthithanaboon et al., 2015). Steriti et
al. (2014) reported that higher lipid yield could be achieved from dried microalgae as compared to wet biomass, dried microalgal cells possess more susceptible cell wall.
Takisawa et al. (2013) supported this fact with a similar analysis. Takisawa et al. (2013) reported that wet microalgal biomass with 80% water content, when used directly (without drying) post harvesting for release of lipid/oil from the cells caused several difficulties. The drying step is an energy intensive process consuming up to 80% of the entire consumed energy (Dong et al., 2016). However, the elimination of this crucial step of drying can reduce the total energy consumption and eventually the total cost of production. Hence, it is necessary to develop new and advanced technologies which is capable of converting microalgal oil to biodiesel from wet biomass without hampering the capital cost and also achieve high conversion efficiency.
In summary, microalgae is a new alternate sustainable feedstock for biodiesel production with several distinct merits. However, commercial implementation of microalgal production would require significant study and optimization of the governing factors that would reduce the energy consumption and the associated capital and operating costs. Selection of proper microalgal species, optimization of growth conditions and proper extraction and conversion of lipids are the important features. Moreover, recovery of valuable byproducts from microalgal biomass is also crucially important as it forms another source of revenue for the microalgae refinery.