Biodiesel from Estuarine Microalgae - Diatoms

Abstract

Rapidly depleting fossil fuel reserves with the burgeoning demand, associated escalating GHG footprint, and changes in the climate necessitated the exploration of alternate sustainable energy resources that are renewable, economically viable, and environmentally sound. Among renewable energy feedstocks, microalgae possess an intrinsic ability to convert atmospheric CO2 into renewable bioproducts at a shorter cycling period. Microalgae-based biofuel is emerging as a viable feedstock as it possesses faster growth rates and higher lipid productivity. Sustainable biofuel production from microalgae entails i) appropriate strain selection, ii) economic harvesting, and iii) eco-friendly transesterification. However, attaining economic viability in the production of microalgae-derived biofuels as potential drop-in fuels depend on minimising energy costs involved in its production. Detailed literature review reveals multiple research gaps in i) strain selection, ii) optimal harvesting, and iii) sustainable and environmentally friendly transesterification for achieving economic viability with the technical feasibility of microalgal biodiesel. A significant drawback of the current approaches is inappropriate strain selection of either random isolation or repository procured microalgal strains and inducing variations in growth conditions. Estimates indicate out of 72,500 strains of microalgae identified, about 44,000 species have been investigated with characteristics. Commonly researched species from a biofuel perspective belong to the genera Chlorella sp., Scenedesmus sp., Botryococcus sp., Nanochloropsis sp., Tetraselmis sp. Haematococcus sp. Cyanobacteria (blue-green algae) and the model pennate (bilaterally symmetrical) diatom Phaeodactylum tricornutum (Bacillariophytes). Among the investigated algal groups, green microalgae (class: Chlorophyceae) predominates (64%) while diatoms (Bacillariophyceae), picoplankton (Esutigmatophyceae), and cyanobacteria constitute only 8% necessitating further exploration to prioritise microalgal species through inventorying, habitat mapping, and monitoring to understand the factors responsible for species occurrence and its variability, which would provide vital insights on tolerance and sensitivity levels of different microalgae at the local level leading to resilient strain selection. This would help address the problems frequently encountered in the open cultivation of microalgae, such as difficulty in acclimatisation of microalgae, resistance to contamination, and prevalence of invasive taxa (pests/pathogens). Biomass harvesting continues to be significant energy-intensive process among various downstream operations, which not only escalates production costs but also enhances carbon footprints. Estimates reveal an approximately 30 – 40 % of the total costs involved in microalgal biodiesel production are expended in the harvesting of the algal biomass. Thus, to realise cost-effective production of microalgal biodiesel, the implementation of optimal harvesting strategies to minimise fuel production costs is crucial. Substrate-based attached algal cultivation systems are gaining prominence as a viable alternative to the conventional algal cultivation in open ponds and photobioreactors (PBRs’) owing to ease of harvesting, minimal water requirement per unit biomass production, lesser footprint area requirement, efficient light transmission, utilisation without adverse effects of cell shading and light saturation, higher biomass productivity and also scope for the use of wastewater with the remediation potential. Conventional transesterification using acids and alkalis poses environmental pollution consequences during disposal with large-scale production. In this context, the objectives of the current research were: Inventorying and habitat mapping of benthic diatom in the Aghanashini estuary, situated on the central west coast of India covering all seasons, revealed the occurrence of 27 tolerant diatoms species belonging to genera Amphora, Cyclotella, Navicula, Nitzschia, and Pleurosigma. During monsoon, species such as Amphora, Nitzschia, and Navicula were found to be dominant, while Achnanthes sp., Cyclotella sp., and Melosira sp., were predominant in monsoon. The post-monsoon season was dominated by diatoms Achnanthes, Navicula and Pleurosigma. Assessment of habitat conditions with species occurrence highlights the role of nutrient composition on species richness. Out of the 27 species found to be predominant during field studies, Nitzschia sp., Amphora sp. and Navicula sp. were prioritised due to their dominance in terms of species richness recorded during field investigations. Further to understand their lipid responses to varying nutrient and salinity conditions, lab-scale experimental studies were carried out using water equivalent to field conditions. The biomass productivity and lipid accumulation results of this study revealed the scope of using aquaculture wastewater for diatom cultivation. nutrient removal efficiencies of diatoms (N: 89.1 ± 0.85 %, P: 90.8 ± 0.12 %) from aquaculture wastewater highlights the scope for bioremediation and biofuel production. Comparative assessment of lipid accumulation in the prioritised diatom Nitzschia sp. under different trophic modes of photo, hetero, and mixotrophy revealed significant morphological variations, biomass yield (95.2 to 139.1 mg/L), fatty acid profiles, and biodiesel quality was observed when diatom cells were subjected to mixotrophic nutrition mode. Elemental composition analysis using scanning electron microscopy revealed the highest C (~ 48 %) in diatoms under mixotrophy compared to other trophic modes. This exercise also involved the comparison of biocatalyst over the conventional acid catalyst in the effective transesterification of diatom lipids into biodiesel. The results revealed a higher FAME conversion efficiency of biocatalyst-based transesterification (~ 87 %) than that of a conventional acid catalyst (~ 83%), thus demonstrating biocatalyst’s potential in large-scale sustainable production of biodiesel. A prototype low-cost substrate based bioreactor deployed in the flood plains of the Aghanashini estuary, based on the insights obtained from field investigations and lab-scale experiments, showed the highest areal biomass yield (22.23 g m-2), demonstrating its technical feasibility under actual field conditions. Techno-economic assessment of prototype bioreactor scaled up to one hectare exhibited economic viability with an estimated yearly profit ranging between ₹ 35,296 INR/ha and 2,09,190 INR/ha based on the level and type of inputs during cultivation. The payback period was the least (0.98 years) with the lowest biodiesel production cost (30.1 – 31.8 INR/kg biodiesel). The production cost of biofuel in a hectare (bioreactor) varied between 30.08 INR/kg to 59.52 INR/kg of biodiesel. Lifecycle assessment of microalgal biofuel to assess environmental footprint under different scenarios (no nutrient input, wastewater input, and fertiliser input) revealed a fossil energy requirement variation between 3.6 – 5.7 MJ/kg and the greenhouse gas emission (as kg equivalent CO2 emissions) of 0.85 – 1.46 kg CO2eq.kg-1 of biodiesel. This highlights a reduction in fossil energy requirement of about 87.3 % in the pilot substrate-based microalgal bioreactor. Wastewater – biocatalyst scenario exhibited the highest net energy ratio (NER) of 18.8 with an additional benefit of low-cost remediation of wastewater along the coastal regions of India which would enhance the job opportunities for rural women, while ensuring the energy security of the nation.