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• Natural plant extracts were utilized as flocculants to harvest algal consortium. • 8 mg ml−1 of M. oleifera showed 75.5% biomass harvesting efficiency after 100 min. • Natural coagulants have lower costs with lesser environmental and energy impacts. • 95.76% efficiency was obtained in 20 min on combining M. oleifera with chitosan. • Synergistic influence of natural and chemical coagulant formed bigger flocs. The study investigated the ability of plant based natural coagulants from Azadirachta indica; Ficus indica ; Moringa oleifera ; Citrus sinensis ; Punica granatum and Musa acuminata to harvest the microalgal biomass. Influence of eluent type (water and NaCl) and concentration (1–5 N) on coagulant extraction; coagulant dosage (1–5 g) and volume (20–100 ml); pH (6–12) and algal concentration (0.1–1 g l−1) on harvesting were analyzed. The results obtained were compared with alum and chitosan. FTIR and biochemical analysis confirmed the presence of bioactive compounds to aid coagulation. Biomass removal efficiency of 75.50% was obtained with M. oleifera extracts (8 mg ml−1) at pH 7.5–7.8, within 100 min. The harvesting efficiency increased to 95.76% when 4 mg ml−1 M. oleifera extracts was combined with 0.75 mg ml−1 chitosan. The life cycle and cost analysis acknowledged the eco-friendly coagulants as strong alternative for conventional coagulants used in microalgal harvesting, thereby improvising the overall bioprocess. [ABSTRACT FROM AUTHOR]

Abstract The present study evaluated the carbon dioxide (CO 2) sequestration potential of Thalassiosira pseudonana using the influencing key climatic datasets of an Indian coal based power plant through a biophysical energy balance model developed on MATLAB platform with ODE45s solver. The maximum average areal biomass productivity was predicted to be 111.39 kg ha−1d−1 with maximum growth in February, leading to the CO 2 capture of 147.03 kg ha−1d−1. Further, techno-economic analysis was performed utilizing the results of the formulated biophysical model to access the process feasibility using SuperPro designer. Results revealed the technical viability and economic feasibility of the process, with the unit production price of 58.41 $ per ton with a reasonable rate of returns and acceptable short payback time of 2.81 years. A yearly carbon credits of 52 M$ could be earned by the company. Sensitivity analysis showed the process to be highly dependent over the raw material prices and facility operation. The integrated site-specific biophysical model and economic analysis are essential to provide reliable realistic estimates of the algal carbon sequestration technology that could drive the energy policies towards sustainable development. Graphical abstract Image 1 Highlights • Biophysical model formulated to assess algal productivity at thermal power plant. • Microalgal productivity of 111.39 kg ha−1 d−1 could be used as a resource. • Waste flue gas containing CO 2 at the rate of 147.03 kg ha−1 d−1 could be captured. • Unit cost of 58.41 $ ton−1 (algae) with 52 M$ yr−1 carbon credits were projected. • Raw material costs and operational parameters are major economic drivers. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Biophysical model for assessing algal performance in open ponds was formulated. • Model was based on site-specific light and temperature dynamics. • Light attenuation effects over the microalgal productivity were evaluated. • Location specific optimal pond depth and operating conditions were presented. • Photoinhibition effects declines the algal productivity by 19% at the study site. Abstract Microalgal biomass is currently recognized as a promising sustainable source for biofuel production and carbon dioxide (CO 2) sequestration. Utilization of biophysical models are emerging to access the real-time feasibility of microalgal technology. In this present work, a comprehensive mathematical model based on the site-specific meteorological variables is formulated using MATLAB ODE 45 s solver to estimate the microalgal productivity. The predictive model framework utilized material balance equations with basic laws of physics, known constants and conservative assumptions to evaluate the water temperature that influences the microalgal viability. The dynamic behaviour of algal ponds considering the operating variables like light intensity (including the effects of photoinhibition), water temperature, and design criteria like pond depth, microalgal concentration, was used to estimate the performance of T. pseudonana in open ponds. Maximum growth was projected in September accounting to the biomass and lipid productivity of 170.28 kg (dry mass) ha−1 d−1 and 39.42 l ha−1 d−1 respectively with a CO 2 capture potential of 224.77 kg (CO 2) ha−1 d−1 based on the influence of water temperature. Optimal pond depth and operational conditions to achieve the desired productivity for the specific site were estimated. The maximum annual areal productivity dropped by 19% from 62.18 tons (dry mass) ha−1 yr−1 due to photoinhibition. The simulated biophysical model as a tool could be used to evaluate the biokinetic processes affecting the algal pond performance for further facilitation of effective decision making on scaling up of microalgae cultivation. [ABSTRACT FROM AUTHOR]

[Display omitted] • Research trends and future prospects of microalgal biorefinery was explored. • Integrated zero-waste microalgal biorefinery are major identified research hotspots. • Microalgae based products have enormous market opportunities for commercialization. • Enviro-economic constraints need to be addressed by process optimization. • Reframing policies with government subsidies will increase market credibility. Microalgae as an alternative feedstock for sustainable bio-products have gained significant interest over years. Even though scientific productivity related to microalgae-based research has increased in recent decades, translation to industrial scale is still lacking. Therefore, it is essential to understand the current state-of-art and, identify research gaps and hotspots driving industrial scale up. The present review through scientometric analysis attempted to delineate the research evolution contributing to this emerging field. The research trends were analysed over the last decade globally highlighting the collaborative network between the countries. The comprehensive knowledge map generated confirmed microalgal biorefinery as a scientifically active field, where the present research interest is focussed on synergistically integrating the unit processes involved to make it enviro-economically feasible. Market opportunities and regulatory policy requirements along with the consensus need to adopt circular bio-economy perspectives were highlighted to facilitate real-time implementation of microalgal biorefinery. [ABSTRACT FROM AUTHOR]

[Display omitted] • Algal drying characteristics of convective hot air and microwave oven were discussed. • Microwave drying is faster with relatively lower specific energy requirements. • Convective and microwave drying followed thin layer drying models. • 14.4% higher lipid content was obtained in microwave dried algal biomass. • Uniform volumetric heating in microwave drying preserves biochemical content. Conventional microalgal drying consumes huge time and contributes to 60–80% of downstream process costs. With the aim to develop an effective and rapid drying process, the present study evaluated the performance of microwave based drying (MWD) with a power range of 360–900 W and compared with the conventional oven drying (OD) at 40–100 °C. MWD was found to be efficient due to uniform and volumetric heating because of dipolar interaction, with an effective diffusivity of 0.47 × 10−9−1.63 × 10−9 m2 s−1, comparatively higher than OD. Activation and specific energy of 32.43 W g−1 and 42.9–56.07 kWh kg−1 was projected respectively, and a falling rate period with best fit for Newton and Henderson-Pabis model was observed for MWD. Uniform heating from internal sub-surface avoided cell distress, resulting in 14.4% higher lipid yield and significant preservation of biochemical components that can be processed into bioenergy and valuable products in microalgal biorefinery. [ABSTRACT FROM AUTHOR]

[Display omitted] • Microalgae offers rich source of multifunctional bioactive compounds. • Microalgal biostimulants elicit signaling pathways providing systemic resistance. • Market opportunities and criticalities for commercialization are described. • Enviro-economic constraints can be addressed by integrated algal biorefineries. • Circular bioeconomy concepts increase algal biostimulants market credibility. Adverse detrimental impacts of environmental pollution over the health regimen of people has driven a shift in lifestyle towards cleaner and natural resources, especially in the aspects of food production and consumption. Microalgae are considered a rich source of high value metabolites to be utilized as plant growth biostimulants. These organisms however, are underrated compared to other microbial counterparts, due to inappropriate knowledge on the technical, enviro-economical constrains leading to low market credibility. Thus, to avert these issues, the present review comprehensively discusses the biostimulatory potential of microalgae interactively combined with circular bio-economy perspectives. The biochemical content and intracellular action mechanism of microalgal biostimulants were described. Furthermore, detailed country-wise market trends along with the description of the existing regulatory policies are included. Enviro-techno-economic challenges are discussed, and the consensus need for shift to biorefinery and circular bio-economy concept are emphasized to achieve sustainable impacts during the commercialization of microalgal biostimulants. [ABSTRACT FROM AUTHOR]

Algal biofuels have received wide attention in recent years for its potential to reduce the dependence on conventional fossil fuels. Despite the portrayed advantages of high growth rate, carbon sequestration and waste remediation; large scale application of microalgal biofuels is still lacking because of the lower percentage of extractable lipids obtained from the harvested biomass. Thus, there is a substantial impetus to analyse the strategies for enhancing the lipid profile and yield to improve the microalgal biofuel quality as well as to reduce the costs incurred at field scale. Several biochemical and molecular strategies to increase the algal lipid accumulation has gained huge scientific interest in recent years and have opened up new avenues for algal biorefinery. However, the time and cost involved as well as the ecological risks associated with real-time applications often restricts their utilization. The present review gathers a compendium of the key milestones associated with the recent approaches of biochemical, genetic and metabolic engineering for lipid quantity and quality enhancement. Biochemical and engineering aspects of coercing the cells to environmental stress and altering the mode of nutrition has been elucidated. The advancements in genetic and metabolic engineering, the associated risk factors and the future perspectives have been highlighted. Strategic integration of the bioprocess and biomolecular techniques to explore its synergistic impact to rationally engineer microalgae with improved triacylglycerols has been emphasized. Assessment of the long term risks associated herewith can be used to avert the challenges, making algal biofuels a commercial reality in future. [Display omitted] • System biology and molecular techniques facilitate strain improvement. • Key milestones of algal genomics for lipid enhancement were discussed. • Biochemical and engineering strategies for enhancing lipid accumulation were outlined. • Understanding the underlying challenges and advances aid their application. • Synergistic coupling of the approaches are expected to provide better results. [ABSTRACT FROM AUTHOR]

• Microalgal growth and nutrient depletion kinetics were simulated in bubble column PBR. • CFD simulations highlight the dependence of microalgal growth on PBR hydrodynamics. • Microalgal productivity of 0.14 g L−1 d-1 achieved with 5.5 % human urine and 4% CO 2. • Phosphate and ammonium removal of 90 % and 84 % was attained in PBR respectively. • Contours of water velocity and turbulent energy gave insights on PBR hydrodynamics. Optimization of microalgae cultivation to reduce the associated costs is one of the major objectives in a biorefinery model. The present study optimized the microalgal cultivation using 4.5–8.5 % v/v of DHU (diluted human urine) as a cost-effective resource, in a bubble column photobioreactor (BCPBR) under real-time conditions. Media with 5.5 % DHU and 4% CO 2 showed maximal biomass productivity of 0.14 g L−1 day−1 with a final concentration of 1.06 g L−1. Phosphate and ammonium removal of 90.70 % and 84.10 % respectively was achieved. The biofixation of CO 2 obtained for 6 days, during the cultivation in 5.5 % DHU, by supplying 4% of CO 2 enriched air was 0.29 g L-1 d-1. Computational fluid dynamics (CFD) simulations were used to study the effects of velocity magnitude, shear stress, turbulent kinetic energy, and irradiance on the microalgal growth inside the BCPBR. The developed kinetic model predicted the biomass concentration and phosphate removal up to 98 % and 82 % accuracy respectively. Such studies would aid in comprehending the large scale commercial cultivation and thereby facilitate the application of microalgae in the future. [ABSTRACT FROM AUTHOR]

Conventional hydroponic units producing disease free plants with 30–40% faster growth rate are regarded as a promising agricultural alternative in case of unfertile/metal contaminated soil. However, most nutrients in hydroponics medium remain unused and are drained off creating environmental pollution. To reduce the nutrient load and eutrophication effects, cocultivation of plant and microalgae in the hydroponic units has attracted the attention of researchers over the past few years. The present study aims to explore the influence of initial inoculum concentration of a native algal consortium over the performance of hydroponic system. Cocultivation of tomato plant with varying initial inoculum algal concentration of 0.2–0.8 mg/ml, showed that 0.8 mg/ml concentration resulted in positive interactions between microalgae and plant, with algal and plant productivity of 0.149 ± 0.024 and 0.328 ± 0.087 g/m2/d respectively after 42 days. Higher chlorophyll accumulation, along with nutritionally rich algal and plant biomass revealed lack of unwanted competition during cocultivation. Increase in dissolved oxygen during cocultivation was corroborated with efficient root respiration. Highly developed roots also provided adequate metabolic energy thus significantly increasing the nutrient uptake and accumulation, thereby reducing the nutrient load during drainage. The supernatant after algal harvesting (recycled media) supported plant growth for 24 days due to limited nutrients. Such system with microalgae symbiotically favouring plant productivity and simultaneous nutrient load reduction would ultimately support in attaining sustainable agriculture. Image 1 • Mixed algal consortium and plants showed symbiotic interaction during coculture. • Initial microalgal inoculum influences the performance of cocultivation in hydroponics. • Dissolved oxygen level in hydroponics was higher due to microalgal cultivation. • Higher algal and plant productivity was obtained with 0.8 mg/ml of algal inoculum. • 85% of N, P and K uptake efficiency was achieved during hydroponic cocultivation. [ABSTRACT FROM AUTHOR]

• Microalgal extracts as biostimulants were evaluated for tomato plants. • 20-100 % of algal extracts showed positive effects on tomato growth. • Seed treatment with 40-80% algal extracts showed 100% germination in 3 days • Foliar spray of 60% algal extracts showed better yield. • Increased plant growth rate was also correlated with higher chlorophyll content • Lower concentration of algal extracts showed better stimulatory effects. A probable strategy for increasing the economic sustainability of algal technology would involve the utilization of microalgal biomass as biofertilizer by off-setting the high production costs. The present study focusses on the utilization of mixed algal consortia as biofertilizer for analysing the growth rate of tomato plant. Algal extracts (20-100%) in the form of seed primer and foliar spray were used as biostimulants for the growth of tomato plant. Characterization of algal consortium showed the presence of 40.90% carbohydrates and 26.18% proteins that could potentially act as precursors for bioactive compounds to stimulate plant growth. Faster germination percentage was found with extract concentrations ranging from 20-60% in 3 days compared to the untreated seeds. Seeds treated with cellular extracts of 40% concentration also showed faster plant growth rate after sowing in terms of increase in shoot length 19.86 ± 0.51 cm and root length of 14.87 ± 0.63 cm with a fresh and dry weight of 3.47 ± 0.04 g and 0.389 ± 0.036 g respectively after 20 days. Foliar spraying of 60% algal extracts resulted in total plant height of 7.98 ± 0.19 cm with root length of 5.8 ± 0.16 cm, 46% higher compared to the control. 11 ± 0.35 leaves with chlorophyll content of 13.45 ± 0.307 mg g-1 were also obtained after 20 days, with fresh and dry biomass content of 0.416 ± 0.015 g and 0.062 ± 0.005 g respectively. Thus, microalgal cellular extracts could act as an environmental-friendly and economical alternative to synthetic liquid fertilizer for promoting sustainable agriculture. [ABSTRACT FROM AUTHOR]

• Efficiency of acidified biochar catalyst for transesterification was evaluated. • Peanut shell biochar pyrolyzed at 400 °C showed best catalytic efficiency. • Yield of biodiesel was influenced by the operational parameters. • Maximal biodiesel yield of 94.91% was obtained with significant FAMEs. • 16.83% of oleic acid and 12.15% palmitic acid was present in algal biodiesel. This study evaluates the use of engineered biochar as a heterogeneous solid acid catalyst for transesterification of algal oil derived from a native microalgal consortium. Biochar derived from sugarcane bagasse, coconut shell, corncob and peanut shell were evaluated for catalytic activity following surface modification. Peanut shell pyrolyzed at 400 °C with the sulfonic acid density of 0.837 mmol/g having 6.616 m2/g surface area was selected for efficient catalysis. The efficiency of transesterification was evaluated with 1–7 wt% catalyst loading, methanol: oil ratio of 6:1 to 30:1 at 55–85 °C over 2–8 h. Biodiesel yield of 94.91% was obtained with 5 wt% catalyst loading, MeOH: oil ratio of 20:1 at 65 °C after 4 h. Spectral analysis of algal biodiesel showed the presence of functional groups corresponding to esters. GC–MS analysis revealed the prominent presence of palmitic and oleic acids, further advocating the suitability of the technology for commercial application. [ABSTRACT FROM AUTHOR]

Utilization of waste resources is necessary to harness the long-term sustainability of algal technology. The study focused on the use of human urine as the basic nutrient source for culturing native microalgal consortium and further optimized the process parameters using response surface methodology. A full factorial, central composite rotatable design (CCRD) with three variables: urine concentration (1–10% vol of urine/vol of distil water [%v/v]), pH (6.5–9) and light intensity (50–350 μ m o l p h o t o n s m − 2 s e c − 1 ) was used to evaluate the microalgal biomass and lipid content. Results indicated that at 95% confidence limits, the selected factors influence the biomass and lipid productivity. The maximum biomass productivity of 211.63 ± 1.40 mg l−1 d−1 was obtained under optimized conditions with 6.50% v/v of urine, pH of 7.69 and at light intensity of 205.40 μ m o l p h o t o n s m − 2 s e c − 1 . The lipid content was found to increase from 18.96 ± 1.30% in control media to 26.27 ± 1.94% under optimal conditions. The interactive effect of variables over the microalgal biomass and lipid content has also been elucidated. The data obtained were comparable to the BG11 media (control). Optimized diluted urine media in the presence of ammonium ions and under limited nitrate showed better lipid yields. Significant lipid biomolecules were detected in the algal oil extracts obtained from the diluted urine media characterized by Fourier transform infrared spectroscopy (FTIR) and Nuclear magnetic resonance (NMR). Gas chromatography-mass spectrometry (GCMS) revealed the presence of several monounsaturated and polyunsaturated fatty acids in the transesterified algal oil. Such studies would aid in technically realizing the field scale cultivation of microalgae for biofuels. Image 1 • Potential of diluted human urine for microalgal cultivation was highlighted. • Urine concentration, light intensity and pH interactively influences algal growth. • Maximum biomass productivity of 211.63 ± 1.4 mg l−1 d−1 was achieved. • Lipid content was found to increase by 7.31% compared to the control. • Diluted urine media with ammonium and limited nitrate showed better lipids. [ABSTRACT FROM AUTHOR]