Sustainable biodiesel production from microalgae Graesiella emersonii through valorization of garden wastes-based vermicompost

Highlights

• Vermicompost extract is found effective for Grasiella sp. biodiesel production.

• The 50:50 ~ vermicompost extract:BG11—suitable for quality biodiesel production.

• Technology for solid-waste management and lipid-rich biomass production.

Abstract

Biodiesel production from microalgae has gained significant interest recently due to the growing energy demand and non-renewable nature of petroleum. However, high cost of production and environmental health related issues like excess use of inorganic fertilizers, eutrophication are the major constraints in commercial-scale biodiesel production. Besides this, solid wastes (garden-based) management is also a global concern. In the present study, to overcome these limitations vermicompost extract was tested as nutrient source to enhance growth performance and lipid production from a freshwater microalga (Graesiella emersonii MN877773). Garden wastes were first converted into vermicompost manure and its extract (aerobic and anaerobically digested) was prepared. The efficacy of the extract was then tested in combination with BG11 medium. The mixotrophic cultivation of microalgae in anaerobically digested vermicompost extract at 50:50 combination with BG11 medium enhanced the cell biomass (0.64 g d. wt. L⁻¹) and lipid productivity (3.18 mg L⁻¹ day⁻¹) of microalgae by two times. Moreover, the combination also improved the saturated (methyl palmitate) and monounsaturated fatty acids (oleic acid) content in the test algae. The quality of biodiesel also complies with all the properties of biodiesel standard provided by India, the USA, and Europe except the cold filter plugging property. The combination was also found to improve the cell biomass (0.041 g L⁻¹) as compared to BG11 medium in mass-scale cultivation. Hence, the study proved that G. emersonii grown in media supplemented with garden waste-based vermicompost extract had significant potential for mass-scale biodiesel and bioproduct production.

Introduction

The global consumption of diesel by the transportation sector alone is about 1460 trillion liters and invariably the demand is likely to be increased by 40% in the coming decade (Leite et al., 2013). But this ever-growing demand for diesel can be seldom fulfilled by the petroleum resources due to its finite and non-renewable nature. Moreover, usage of fossil fuels is also reported to produce several environmental consequences such as greenhouse gas (GHG) emission and other climatic extremities. To tackle the issue and cater the future energy demands, globally there has been a lot of interest in developing alternative sustainable strategies. Researchers find renewable and clean energy resources as the alternative means to overcome the fuel shortage and climate change-related issues (Li et al., 2019). Biodiesel research has gained significant commendation since it has been proved as the viable alternative energy source due to its carbon neutrality, non-toxicity and biodegradability (Bennion et al., 2015). In addition, biodiesel is also found to enhance the engine performance and simultaneously curtail GHG emissions from vehicles (Manigandan et al., 2020). Though land-based oil crops are used for biodiesel production, the usage of cultivable land, water, and production costs hamper the technology to achieve sustainability (Li et al., 2020a).

Biodiesel produced from microalgae received greater attention due to its high lipid content (15–50%), rapid doubling time (3.5 h), year-round production irrespective of the season, non-usage of cultivable land, and atmospheric CO₂ fixation potential (Chisti, 2007; Li et al., 2020a; Zhu et al., 2017). At the same time, the selection of microalgal strain is pivotal for successful biodiesel production in addition to culture conditions such as temperature, light intensity, and nutrients (Islam et al., 2013a). High lipid productivity, growth rate, scale-up potential and valued by-products are the important characters that would be considered during strain selection (Leu and Boussiba, 2014). The Chlorophyceae class of green algae, with a doubling time of <24 h and about 50% lipid content, has been found suitable for biodiesel production (Murthy and Kumar, 2019). Graesiella spp. is a native freshwater microalga that belongs to the class Chlorophyceae has some desirable traits which make them more advantageous than other algal strains. First, the size of the cell is large (8–12 μm) which makes them easily harvestable by sedimentation. Second, the ability to grow in high pH and extreme conditions. Third, the presence of high-quality lipid (33.4% d. wt.; ~ 90% as storage triglycerides (TAGs) and a reasonable amount of protein content (Duong et al., 2015). Fourth, it is not easily predated by zooplankton and protozoan due to its large size. Finally, it is found promising for outdoor raceway and pond cultivation when compared to other microalgal strains (Wen et al., 2016). Therefore, G. emersonii was isolated and utilized for the present study.

The mass cultivation of microalgae requires a large amount of fertilizers, which could disturb ecosystem health and questions the sustainability of commercial microalgal biofuel industry (Lam and Lee, 2012). During algal cultivation, the fertilizer production also seeks >50% of energy consumption (Clarens et al., 2010). Hence, it requires a cheap and organic form of nutrients to achieve sustainability in microalgae biodiesel production. The utilization of waste resources as a culture medium could be the suitable strategy. Various waste resources have been used for microalgae growth, such as animal excreta (Kim et al., 2016; Zhu et al., 2017), raw chicken manure, municipal wastewaters (Calixto et al., 2016), domestic sewage (Kligerman and Bouwer, 2015), etc. However, the major uncertainties include more diverse quality of waste resources, selection of suitable species, the presence of pathogens and predators, adaptability of microalgae, and reduced lipid productivity (Calixto et al., 2016; Zhu et al., 2017).

On the other hand, solid waste management (SWM) is a major challenge in developing countries because of the rapid urbanization and industrialization, societal modernization, and changing food consumption pattern (Alshehrei and Ameen, 2021). Globally, about 44% of the total wastes generated include garden, agriculture residues, and food-based wastes (Luttenberger, 2020). Among these, garden wastes (i.e., grass clippings, leaves, other plant debris etc.) contributes significantly and left unutilized either dumped as landfills or incinerated which may cause serious environmental problems (Li et al., 2020b). Therefore, proper management of these garden wastes is of prime concern since it forms the highest budget item (about 20% of the municipal budget) in low-income countries (Luttenberger, 2020). Microbial composting is a suitable approach to manage the solid organic waste globally; excluding the garden wastes which are difficult to decompose by microbes due to high C/N ratio and lignocellulose content (Li et al., 2020b). Vermicomposting is an effective technique for composting solid garden wastes rich in lignin and cellulose content (Gong et al., 2019) and convert them into stable organic fertilizers. It contains more amount of carbon, nitrogen, and phosphorus (Shinde et al., 1992) which are beneficial for microalgae (Kings et al., 2017). However, the nutrient rich compost could not be directly used for microalgal growth due to its complex nature and difficulty in assimilation by the algae (de Medeiros et al., 2020). Thus, production of extracts from the vermicompost is necessary to transfer the soluble or particulate organic matter, nutrients, and other chemical components from the solid media into a liquid phase that enables the microalgae to absorb nutrient more efficiently (Hanc et al., 2017; de Medeiros et al., 2020). In earlier studies, agro-industrial residues, fruits, and vegetable waste-based microbial compost have been found as lucrative nutrient source for microalgal cultivation (de Medeiros et al., 2020). It is noteworthy to mention that the quality of raw materials used for vermicompost, and the process of extract (aerobic or anaerobic) production decides the nutrient quality of the compost and the extract produced thereof (Hanc et al., 2017). To the best of our knowledge, no studies have utilized garden wastes as raw material for vermicomposting and utilized the same for extract production and for microalgal cultivation.

Therefore, the present study utilized the garden wastes, converted them into vermicompost and the extract was prepared by two methods (i) aerobic (ii) anaerobic, then the nutrient quality was estimated. The extracts were then added with BG11 medium at different proportions and the growth, cell size, and lipid productivity of the Chlorophyceae microalgae (Graesiella emersonii MN877773) were estimated. Following this, the best performing media combinations have been selected for studying the mineral content, fatty acid profiling and biodiesel quality. The same combination was also utilized for outdoor cultivation.