**5. Microalgae as potential source of biofuels**

Microalgae has a potential to deliver renewable energy resources such as biofuels. Due to the problem of global warming (burning of fossil fuels) and day-to-day surge in petroleum, prices the role of microalgae has been rethink by various domains for using as a source of clean energy. The reason behind microalgal biomass as suitable feedstock for biofuel as the algae has high biomass productivity, high lipid content and high photosynthetic efficiency than terrestrial plant. Algae is considered as third generation biofuel and have advantage over first and second generation biofuel in terms of readily available, ability to grow throughout the year, water consumption is very less, can grow on wastewater, ability to grow under harsh condition, and high biomass production. The oil content of algae compared to first and second generation biofuel is given in **Table 2**. In this section we will discussed the important product obtained from algae as biodiesel, biofertilizer and biochar.

## **5.1 Biodiesel**

The recent research developments in microalgae reveals that microalgal biomass is one of the promising sources of biodiesel, which partially could met the demand of transportation sector. Using microalgae to produce biodiesel will not compromise production of food, fodder and other products derived from crops. The microalgae


#### **Table 2.**

*Comparison of algae with different crops for biofuel.*

*Algal Biorefinery: A Synergetic Sustainable Solution to Wastewater Treatment and Biofuel… DOI: http://dx.doi.org/10.5772/intechopen.104762*

species such as *Kirchneriella lunaris, Ankistrodesmus fusiformis, Chlamydocapsa bacillus*, and *Ankistrodesmus falcatus* are prominent species for biodiesel production as they contain high polyunsaturated FAME [31]. The comparison of various oil yielding crops is given in **Table 2**. Thus, considering the potential of the algal based biodiesel production, it can be concluded that the biodiesel can be used to displace fossil diesel partially/completely. The oil percentage in various algae are in the range of 20–50% (**Table 3**) and increase in oil content can be achieved >80% by weight of dry biomass in microalgae. The oil productivity of microalgae is the mass of oil produced per unit volume of the microalgal broth per day, depends on the algal growth rate and the oil content of the biomass.

#### **5.2 Bio-fertilizer**

The microalgae can assimilate excess N&P from the wastewater and convert it into the valuable biomass which has potential as a manure for the agricultural crops. Various researches have reported that %N content in the dry algae biomass is significantly higher than the available organic manure (cow dung, farmyard manure etc.) [10, 13, 36]. The NPK content of dry algae biomass ranged from 3 to 7%, 0.5–2% and 0.4–0.8%, respectively [13, 14, 36, 37]. The algal based fertilizers are composed of high OC which support to increase the moisture retention capacity and nutrient bioavailability than chemical fertilizers and other organic inputs such as farm yard manure [38]. Algal bio-fertilizer being rich in carbohydrates, soluble protein contents and other important plant organic nutrients, ensure higher vegetative yield [39, 40]. The algal-biofertilizer input also enhance the microflora of the soils along with the availability of inorganic nutrients [13]. Renuka et al. [41] confirms that the microalgae-based biofertilizer decreases the nutrient losses as nutrients are slowly release into the soil and available to the crop in longer periods than the synthetic fertilizers. In a leaching experiment conducted by Sharma et al. [5] the application algae biomass (*C. minutiisma*) after harvesting from sewage wastewater results in reduction of nitrate leaching in spinach crops as compared to application of chemical fertilizer, hence prevent eutrophication in water bodies. The immobilization and mineralization of any fertilizer depends on its C:N ratio. If the C:N ratio of any fertilizer is more than 20, it


#### **Table 3.** *Oil content of microalgae.*

promotes immobilization and therefore not advisable for application in soil. The C:N ratio of phycoremediated algae manure is around 9.16, hence its application promotes mineralization in the soil [13]. In addition, algae fertilizer also reported to reduce nitrate leaching from the agricultural fields than synthetic fertilizer [5, 21]. Therefore, it can be summarized that phycoremediation of sewage wastewater with biofertilizer production is a resource conservation approach and recycling of wastewater as well as nutrient for improvement in crop quality.

## **5.3 Biochar**

Algae biomass is potential feedstock for various value added products. Since last decades, interest has been raised in production of biochar from microalgae biomass. As biochar is rich in organic carbon, so its application enhances carbon sequestration and improving the soil quality [42–44]. Generally, carbon, hydrogen, nitrogen and sulfur content in biochar is 48.45, 1.78, 1.47, and 0.78 (wt%) and it varies with the feedstock [45]. The microalgae derived biochar (*Chlorella vulgaris* FSP-E) is slightly alkaline in nature having carbon, hydrogen, nitrogen, oxygen and sulfur content (% dry wt) is 61.32,3.55, 9.76, 11.92, and 0.02% [46]. Similarly, Chaiwong et al. [47] reported volatile matter 16.8%, carbon 62.4%, and nitrogen 2.1% in spirogyra microalgae derived biochar. Generally, compared to lignocellulosic biochar, algae derived biochar have low organic carbon content and CEC, but having high nitrogen, P, K, Ca and Mg content [48]. Due to its high nutrient content and ion exchange capacity, algae biochar can be utilized for agricultural inputs and adsorbents in wastewater remediation [42]. Being an alkaline in nature, algae biochar could be used as amendment in acidic soil. Due to high biosorption capacity of associated with the large amount of functional group, microalgae biochar results enhancing the efficiency for the removal of organic contaminants [49]. Producing algae biochar also results in sequestration of atmospheric carbon dioxide, hence prevent global warming. Biochar is the carbonenriched (coke) obtained after pyrolysis under temperatures (600–700°C) and under anaerobic conditions. The produce yield from pyrolysis is related to parameters, such as temperature, heating rate, and residence time [50]. The yield of biochar increased with decrease in pyrolysis temperature, and with increase in the duration. Chen et al. [45] showed that the yield of biochar algae in terrified microalgae residue at the temperature ranged from 200 to 300°C with a residence time of 15–60 min. Similarly, the yield of 50.8–95.7% in microalgae *Chlamydomonas* sp. JSC4 under the temperature of 200–300°C for 15–60 min [51]. Hence, it can be concluded that, production of algal biochar is expected to contribute to a further sustainable environment in the future.

## **5.4 Carbon dioxide sequestration**

Global climate is a challenging issue, and reason behind is increasing concentration of greenhouse gases in atmosphere. Currently, CO2 concentration in the atmospheres is around 400 ppm and it may reach to 750 ppm by the end of century [52]. CO2 is well known greenhouse gases contributing climate change and global warming. The industrialization, and population expansion is the main cause of greenhouse gases emission. Several technologies has develop for capturing CO2, although biological capture of CO2 is a potential and attraction alternative. The algae mediated CO2 fixation coupled with wastewater treatment is gaining attention as compared to terrestrial plants [53]. The microalgae that are effective in CO2 sequestration generally belongs to *Chlorococcum*, *Chlorella*, *Scenedesmus* and *Euglena* genus. The carbon dioxide

*Algal Biorefinery: A Synergetic Sustainable Solution to Wastewater Treatment and Biofuel… DOI: http://dx.doi.org/10.5772/intechopen.104762*

sequestration potential of microalgae is around 10–50 times higher than terrestrial plants [54]. The nutrients content in wastewater (N & P) can be utilized by microalgae for source of food and resulting biomass could be utilized for biofuel, biofertilzer, biochar and value added products. Microalgae can be grown in photobioreactor by carbon dioxide from the point sources such as industry, cement kiln, thermal power plant etc. Tang et al. [55] conducted a study on the impact of CO2 concentration on biomass productivity of algae *Chlorella pyrenoidosa* in a photobioreactor and found that at 10% CO2 concentration, biomass production was highest (1.8 g/L). However the process is cumbersome and faced problem in down streaming process (harvesting). Open pond system and closed PBR are generally suggested for the purpose of growing algae. Open pond system/raceway ponds are cost effective, but significant amount of CO2 loss to the atmosphere as compared to closed PBR. The CO2 sequestration with remediation of wastewater thorough algae is cost efficient, sustainable, and recycling approach.
