Nanobiotechnology Advances in Bioreactors for Biodiesel Production
The global demand for energy has increased drastically along with the rapid increase in population and urbanization. The energy consumption is proportionally increasing with high economic growth. The worldwide trend resonates the manufacturing and servicing sector growth is increasing every year. The overall development in these two sectors pushes the growth index, which subsequently leads to higher energy consumption as per the reports of International Energy Outlook 2018 (IEO 2018). The energy consumption over the world is shown to increase from 1990 to 2040 in Figure 1. All countries over the world are classified into two categories based on their membership in the Organization for Economic and Cooperation Development (OECD). It was reported that population and economic growth rapidly growing in non-OECD countries had resulted in higher energy consumption. The sector-wise consumption of the world showed that the industrial sector accounts for higher energy consumption compared to transportation, building and other end-use sectors. As per the reports of IEO 2018, the energy consumption will increase drastically in all the sectors from 2020 to 2040. The fossil fuel depletion throughout the world is questing for alternative energy sources which primarily include energy obtained from water and wind, solar energy and biomass-derived energy. This has led to a focus on the tliud-generation of biofuels from algae biomass.
Figure 1. Global energy consumption trend for different time (A) Non-OECD and OECD energy consumption; (B) Sector-wise energy consumption. (The data reproduced using the statistics from the below mentioned website (https://www.eia.gov/pressroom/presentations/capuano_07242018.pdf)).
The biological agents, such as microorganisms and plants, were playing a pivotal role in biofuel production. Photosynthetic microorganisms were found to be the potential candidatures for biofuel production. Thus, significant research had taken place on biofuel production from algae biomass. However, developing a sustainable, cost-effective and safe method for bioenergy production from biological agents is quite essential. Nanotechnology is a branch of science found to be prominent in addressing the new challenges in the field of science and engineering. This branch of science deals with the fabrication and use of nanoscale materials for different applications (Verma et al. 2013). Numerous nanoscale materials have been used fox- biological applications. Mostly, the importance of nanotechnology resulted in driving the research in the field of bioenergy and biofuel production on par with different branches of science and engineering. Thus, the use of nanotechnology for biological application emerged as a nanobiotechnology (Verma et al. 2016). Nanomaterials have the ability to recognize the biological molecule and also improve the rate of reaction. The fimctionalization of nanomaterial is the crucial step to make it a fimctional material for real-tune applications. Numerous nanomaterials have been developed and tested as biocatalysts. Nanobiotechnology applied for biofuels production from algae was intended to reduce the greenhouse gas emission which was found to be higher in the case of conventional approach for biofiiel production (Alnnadi 2018). Nanocatalyst was used to generate the electricity by the breakdown of methane hito carbon and hydrogen. Many studies have been conducted on nanocatalysts for biodiesel production from edible and non-edible oils. The introduction of nanocatalyst is highly potential and a novel approach to enhance the efficiency of biodiesel production.
The lower greenhouse gas emission is achievable with the use of biodiesel as a renewable energy resource and it is derived from fatty acids and oil. To increase the yield of biodiesel, fatty acid, lipid synthesis and metabolic engineering approach are inevitable. The initial step hi the fat synthesis is the conversion of actyl-CoA hito Malonyl-CoA, catalyzed by acetyl-CoA carboxylase. The soxidation of NADPH requires fat synthesis, which occurs in the cytosol. The pyruvate dehydrogenase (PDH) and fatty acid oxidation pathways generate acetyl-CoA in mitochondria; it caimot be transported hito the cytosol. Kerb’s cycle produces citrate in mitochondria, which is transported into the cytoplasm. Citrate breaks down into acetyl-CoA and oxalo acetate (OAA); this is catalyzed by ATP citrate lyase. The OAA converts into malate by malate dehydrogenase enzyme and NAD". Subsequently, pyruvate is synthesized from malate and then it enters into mitochondria. This pyruvate is also produced through the glycolytic pathway. Transesterification of triacyl glycerol (TAG) produces biodiesel. TAG serves as energy storage in all cells and easily catabolized to provide metabolic energy. TAG contains three fatty acids and the glycerol molecule (Vuppaladadiyam et al. 2018).
The present chapter discusses on microalgae cultivation, harvesting, biodiesel production, properties of biodiesel, bioreactors developed for biodiesel production, the concept of green building for self-sustained infrastructure, characterization techniques for biodiesel analysis, nauobiocatalysts for biodiesel production and microstructured devices for biodiesel production. The need for microscale technology for biodiesel production is the priority of this chapter and discusses hi detail the bioreactors used for biodiesel production. It also briefly addresses the variety of microreactors used in biodiesel production, and then advantages and disadvantages are summarized. The importance of an integrated microscale system for biodiesel production and purification and then role in large scale production are presented in this chapter.
Nutritional Modes of Microalgae and Substrates for Microalgae Growth and Lipid Production (CO, and Wastewater)
Microalgae biomass is a potential feedstock for biodiesel production over other microbial biomass due to its higher biomass productivity and accumulation of lipid content (Lam and Lee 2012, Leong et al. 2018, Chen et al. 2018). Microalgae can survive in three nutritional modes: autotrophic, heterotrophic and mixotrophic. However, it has the ability to adapt and switch the metabolism according to any kind of nutritional mode. Autotrophic microalgae can generate energy and synthesize the essential molecules for cellular sustainability by using sunlight, CO, and H,0. hi contrast to tins, heterotrophs’ primary energy source is organic carbon substrate produced by autotrophs. Heterotrophic cultivation is advantageous over autotrophic because of their survival in the absence of sunlight, effective monitoring of the cultivation and the cost-effective biomass harvesting. Besides, this cultivation aids for wastewater treatment along with lipid synthesis which is a quite suitable approach to find out the solutions for the current environmental challenges.
In the case of mixotrophic cultivation, autotrophic and heterotrophic mechanism jointly aids the microalgae survival and growth and here CO, and organic carbon substrate are used for metabolism. It was reported that mixotrophic cultivation enhanced the growth rate of microalgae compared to autotrophic and heterotrophic cultures (Perez-garcia et al. 2010, Mohan et al. 2011, Devi et al. 2013). Microalgae biomass production using autotrophic conditions (light, CO„ inorganic salts, water and optimal temperature of 20-30°C) is not economically viable. The best cost-effective strategy is using wastewater for nutrients, available sunlight and atmospheric CO,
(Molian et al. 2011). Also, the substrates in wastewater can be degraded and the same can be used as a carbon intake by the microalgae for survival and growth.
The cultivation system highly influences the growth rate and cellular content of microalgae due to the change in nutrient availability and energy source. Besides, it also determines microalgae biomass production. The cultivation systems include open pond, closed, dark and offshore cultivation have chosen for algal culture on a larger scale (Wayne et al. 2018). The selection of cultivation is dependent on the desired product, algal strain, cost estimation and nutrient source. Numerous cultivation systems have been reported so far that are mainly categorized into two systems: open and closed cultivation systems (Klintliong et al. 2015).
Open Pond Cultivation Systems
Open ponds were the best choice for microalgae cultivation on a larger scale, easy to monitor the cultivation and are cost-effective. It can be done in two ways: namral water and artificial water systems. Natural water sources including ponds, lakes and lagoons can be used for open system cultivation. In the case of artificial water cultivation, containers, tanks and ponds were developed for algal cultivation. The area, shape and types (agitated and inclined) were chosen based on the product interest and its application (Klintliong et al. 2015). Unstirred, raceway and circular ponds are being used for cultivation and are shown in Figure 2. Unstirred ponds are the potential substimte for cultivation over raceway and circular ponds because their construction is easier and economical and have larger-scale cultivation. However, microalgal growth is found to be lower due to the lack of competitive growth in the presence of bacteria and protozoal namral habitats. Also, it has been reported that it is limited to some microalgal species (Chaumont 1993). The rotator is provided in circular ponds for agitation. Higher power consumption and construction cost and inadequate CO, supply are the major limitations of circular ponds. These can
Figure 2. Different types of open pond systems for microalgae cultivation. Adapted from (Sreekumar et al. 2016) with permission of AIP Publishing © 2016.
mitigate by the raceway pond open cultivation system. In this, a paddlewheel is used to mix the algal culture and water. The limitations of open pond systems are uncontrolled process parameters, such as temperature, lighting, CO,, evaporation and getting a chance of having contaminated bacterial or algal growth. To address these limitations, closed cultivation systems using photobioreactors have been introduced for algal cultivation. However, the closed system is expensive and scale-up is difficult compared to open pond systems. Closed cultivation system gives higher productivity, employs cloned microalgal culture, has long term culture maintenance and aids in higher surface-to-volume ratios (Hallmaun 2016).
-  1 Department of Biosciences and Bioengineering, Indian Institute of Technology Guwahati,Assam-781039, India.