Microalgae-based biodiesel: economic analysis of downstream process realistic scenarios

Multicriteria analysis of sewage sludge-based biodiesel production

There is increasing attention on developing efficient processes including circular economy principles, and obtaining fuels from wastewater treatment feedstocks is among the most promising. As a wastewater treatment byproduct, sewage sludge is a source of lipids that can be converted to biodiesel in a transesterification process. Economic and environmental analysis have been applied to a 60 m3/h sewage sludge plant, exploring 32 process alternatives. Using solvent extraction from wet sewage sludge, the high cost associated with the drying step is skipped. The wet alternatives with low amounts of solvent and acid usage depicted higher performance compared to the dry ones. Incorporating additional extraction stages increases both the financial gains and environmental impacts. As a result, a multicriteria analysis is implemented to ascertain the optimum process based on different priorities. The case with 0.5:1 (v/v) of hexane to biomass ratio, 3-stage extractor, 60 min residence time and pH 4 was the optimum alternative in most criteria.

Influence of culture media composition on the rheology of microalgae concentrates on a large scale

The role of microalgae in the production of bioproducts and biofuels, along with their ability to provide a sustainable pathway for wastewater treatment, makes them promising alternatives to conventional processes. Nevertheless, large-scale downstream processing requires an understanding of biomass rheology that needs to be addressed further. This study aimed to characterize microalgal concentrates rheologically in different culture media. The presence of bacteria was quantified by photorespirometry and plate counting techniques. The culture medium was found to significantly influence viscosity, with primary wastewater exhibiting the highest viscosity and seawater plus pig slurry the lowest. The concentration of heterotrophic bacteria was directly related to the viscosity. Extracellular polysaccharides (EPS) in supernatant exhibited an inverse viscosity trend compared to biomass concentrates, with pig slurry cultures having higher concentrations. These findings emphasize the profound influence of culture medium and EPS on the rheology of microalgal biomass, underscoring the need for continued research aimed at facilitating and optimizing large-scale downstream processes within the framework of a circular economy and the attainment of the Sustainable Development Goals (6,8, and 12).

Factors Influencing the Production of Extracellular Polysaccharides by the Green Algae Dictyosphaerium chlorelloides and Their Isolation, Purification, and Composition

The freshwater green microalgae, Dictyosphaerium chlorelloides (CCALA 330), has the ability to produce extracellular polysaccharides (EPS). Conditions for optimum growth and EPS overproduction were determined in laboratory-scale tubular photobioreactors (PBR) with a working volume of 300 mL. Multiple limitations in nutrient supply were proven to be an effective method for EPS overproduction. Salinity stress was also applied to the culture, but no significant increase in EPS production was observed. The effects of different nitrogen sources were examined and the microalgae exhibited the fastest growth and EPS production in medium containing ammonium nitrate. Under determined optimal conditions, EPS concentration reached 10 g/L (71% of the total biomass) and a total biomass of 14 g/L at the end of 17 days cultivation. Pilot-scale cultivation was also carried out in a column type airlift photobioreactor (PBR) with a working volume of 60 L. A new and efficient methodology was developed for separating cells from the EPS-containing culture broth. Due to the strong attachment between cells and EPS, high-pressure homogenization was carried out before a centrifugation process. The EPS in the supernatant was subsequently purified using ultrafiltration. The green microalgae Dictyosphaerium chlorelloides may therefore be appropriate for the commercial production of EPS.

Microalgae proteins: production, separation, isolation, quantification, and application in food and feed

Many countries have been experienced an increase in protein consumption due to the population growth and adoption of protein-rich dietaries. Unfortunately, conventional-based protein agroindustry is associated with environmental impacts that might aggravate as the humankind increase. Thus, it is important to screen for novel protein sources that are environmentally friendly. Microalgae farming is a promising alternative to couple the anthropic emissions with the production of food and feed. Some microalgae show protein contents two times higher than conventional protein sources. The use of whole microalgae biomass as a protein source in food and feed is simple and well-established. Conversely, the production of microalgae protein supplements and isolates requires the development of feasible and robust processes able to fractionate the microalgae biomass in different value-added products. Since most of the proteins are inside the microalgae cells, several techniques of disruption have been proposed to increase the efficiency to extract them. After the disruption of the microalgae cells, the proteins can be extracted, concentrated, isolated or purified allowing the development of different products. This critical review addresses the current state of the production of microalgae proteins for multifarious applications, and possibilities to concatenate the production of proteins and advanced biofuels.

Emerging prospects of mixotrophic microalgae: Way forward to sustainable bioprocess for environmental remediation and cost-effective biofuels

Algal bioremediation becoming most fascinating to produce biomass as biofuels feedstock while remediating wastes, also improving carbon-footprint through carbon capturing and utilization (CCU) technology. Non-algae process however offers effective treatment but metabolic CO₂ emission is major drawback towards sustainable bioprocess. Mixotrophic cultivation strategy (MCS) enables to treat organic and inorganic wastes which broadly extend microalgae application towards cleaner and sustainable bioeconomy. Latest focus of global think-tanks to encourage bioprocess holding promise of sustainability via CCU ability as important trait. Several high CO₂ emitting industries forced to improve their carbon-footprints. MCS driven microalgae treatment could be best solution for those industries. This review covers recent updates on MCS applications for waste-to-value (biofuels) and environment remediation. Moreover, recommendations to fill knowledge gaps, and commercial algal biofuel could be cost-effectiveness and sustainable technology for biocircular economy if fuelled by waste streams from other industries.

The effect of medium and light wavelength towards Stichococcus bacillaris fatty acid production and composition

Introduction of novel species will highlight technical feasibility of microalgae-based biofuels for commercial applications. This paper reports the effect of culture medium and light wavelength on biomass and fatty acid production of S. bacillaris which holds some advantages as short life cycle, easy cultivation, high lipid content, diversity of fatty acids and stability under harsh environmental conditions. The results displayed that, soil extract (SE) greatly enhance growth rate of cultures. Maximum biomass and lipid productivity were achieved in TAP medium as 81 mg/L·day, 19.44 mg/L·day; respectively. Light wavelength didn't significantly change growth kinetics but played a critical role on chlorophyll-a accumulation. C14:0, C16:0 and C18:0 fatty acids were abundant which are suitable for biodiesel conversion. Interestingly, blue and red light increased longer chain fatty acids content. These results indicated that; S. bacillaris holds potential for further development of biodiesel production and feasibility of algal biodiesel for fundamental and applied sciences.

Significance and Challenges of Biomass as a Suitable Feedstock for Bioenergy and Biochemical Production: A Review

Fossil fuels have been a major contributor to greenhouse gases, the amounts of which could be reduced if biofuels such as bioethanol and biodiesel were used for transportation. One of the most promising biofuels is ethyl alcohol. In 2015, the world production of ethanol was 25.6 billion gallons and the USA, Brazil, China, the European Union, and 28 other countries have set targets for blending ethanol with gasoline. The two major bio-source materials used for ethanol production are corn and sugarcane. For 1st generation biofuels, sugarcane and corn feedstocks are not able to fulfill the current demand for alcohol. Non-edible lignocellulosic biomass is an alternative bio-source for creating 2nd generation biofuels and algae biomass for 3rd and 4th generation biofuels. This review discusses the significance of biomass for the different generations of biofuels, and biochemical and thermochemical processes, and the significance of biorefinery products.

Microalgae fractionation using steam explosion, dynamic and tangential cross-flow membrane filtration

In this study, the microalga Nannochloropsis gaditana was subjected to acid catalysed steam explosion treatment and the resulting exploded material was subsequently fractionated to separate the different fractions (lipids, sugars and solids). Conventional and vibrational membrane setups were used with several polymeric commercial membranes. Two different routes were followed: 1) filtration+lipid solvent extraction and 2) lipid solvent extraction+filtration. Route 1 revealed to be much better since the used membrane for filtration was able to permeate the sugar aqueous phase and retained the fraction containing lipids; after this, an extraction required a much lower amount of solvent and a better recovering yield. Filtration allowed complete lipid rejection. Dynamic filtration improved permeability compared to the tangential cross-flow filtration. Best membrane performance was achieved using a 5000Da membrane with the dynamic system, obtaining a permeability of 6L/h/m²/bar.

Evaluating the relative impacts of operational and financial factors on the competitiveness of an algal biofuel production facility

Algal biofuels are becoming more economically competitive due to technological advances and government subsidies offering tax benefits and lower cost financing. These factors are linked, however, as the value of technical advances is affected by modeling assumptions regarding the growth conditions, process design, and financing of the production facility into which novel techniques are incorporated. Two such techniques, related to algal growth and dewatering, are evaluated in representative operating and financing scenarios using an integrated techno-economic model. Results suggest that these techniques can be valuable under specified conditions, but also that investment subsidies influence cost competitive facility design by incentivizing development of more capital intensive facilities (e.g., favoring hydrothermal liquefaction over transesterification-based facilities). Evaluating novel techniques under a variety of operational and financial scenarios highlights the set of site-specific conditions in which technical advances are most valuable, while also demonstrating the influence of subsidies linked to capital intensity.

In-situ pyrogenic production of biodiesel from swine fat

In-situ production of fatty acid methyl esters from swine fat via thermally induced pseudo-catalytic transesterification on silica was investigated in this study. Instead of methanol, dimethyl carbonate (DMC) was used as acyl acceptor to achieve environmental benefits and economic viability. Thermo-gravimetric analysis of swine fat reveals that swine fat contains 19.57wt.% of water and impurities. Moreover, the fatty acid profiles obtained under various conditions (extracted swine oil+methanol+NaOH, extracted swine oil+DMC+pseudo-catalytic, and swine fat+DMC+pseudo-catalytic) were compared. These profiles were identical, showing that the introduced in-situ transesterification is technically feasible. This also suggests that in-situ pseudo-catalytic transesterification has a high tolerance against impurities. This study also shows that FAME yield via in-situ pseudo-catalytic transesterification of swine fat reached up to 97.2% at 380°C. Therefore, in-situ pseudo-catalytic transesterification can be applicable to biodiesel production of other oil-bearing biomass feedstocks.

Biodiesel production potential of mixed microalgal culture grown in domestic wastewater

In this study, a mixed microalgal culture grown in secondarily treated domestic wastewater effluent was investigated for biodiesel production using in situ transesterification method with conventional heating. The total lipid content of the mixed culture was found as 26.2% ± 0.6 by weight of dry biomass, and 74% of the lipids were contributed by total glycerides. In situ transesterification with conventional heating process under acidic conditions produced higher biodiesel yield with chloroform as the co-solvent (82.1% ± 3.9) compared to hexane (55.3% ± 3.9) under the same reaction conditions. The gas chromatography analysis showed that FAME composition was mainly composed of palmitic, palmitoleic, stearic, oleic, linoleic and linolenic acid methyl esters., and thus the mixed microalgal culture fed by domestic wastewaters has had comparable biodiesel conversion yields and FAME composition to mono-culture and pure cultures fed by synthetic culture media. Hence, this study showed that secondarily treated domestic wastewater could potentially be a suitable and sustainable medium for microalgae grown to be used as biodiesel feedstock.

Scale-up and economic analysis of biodiesel production from municipal primary sewage sludge

Municipal wastewater sludge is a promising lipid feedstock for biodiesel production, but the need to eliminate the high water content before lipid extraction is the main limitation for scaling up. This study evaluates the economic feasibility of biodiesel production directly from liquid primary sludge based on experimental data at laboratory scale. Computational tools were used for the modelling of the process scale-up and the different configurations of lipid extraction to optimise this step, as it is the most expensive. The operational variables with a major influence in the cost were the extraction time and the amount of solvent. The optimised extraction process had a break-even price of biodiesel of 1232 $/t, being economically competitive with the current cost of fossil diesel. The proposed biodiesel production process from waste sludge eliminates the expensive step of sludge drying, lowering the biodiesel price.

Pyrogenic transformation of Nannochloropsis oceanica into fatty acid methyl esters without oil extraction for estimating total lipid content

This study fundamentally investigated the pseudo-catalytic transesterification of dried Nannochloropsis oceanica into fatty acid methyl esters (FAMEs) without oil extraction, which was achieved in less than 5min via a thermo-chemical pathway. This study presented that the pseudo-catalytic transesterification reaction was achieved in the presence of silica and that its main driving force was identified as temperature: pores in silica provided the numerous reaction space like a micro-reactor, where the heterogeneous reaction was developed. The introduced FAME derivatization showed an extraordinarily high tolerance of impurities (i.e., pyrolytic products and various extractives). This study also explored the thermal cracking of FAMEs derived from N. oceanica: the thermal cracking of saturated FAMEs was invulnerable at temperatures lower than 400°C. Lastly, this study reported that N. oceanica contained 14.4wt.% of dried N. oceanica and that the introduced methylation technique could be applicable to many research fields sharing the transesterification platform.

Biotechnological exploitation of microalgae

Microalgae are a diverse group of single-cell photosynthetic organisms that include cyanobacteria and a wide range of eukaryotic algae. A number of microalgae contain high-value compounds such as oils, colorants, and polysaccharides, which are used by the food additive, oil, and cosmetic industries, among others. They offer the potential for rapid growth under photoautotrophic conditions, and they can grow in a wide range of habitats. More recently, the development of genetic tools means that a number of species can be transformed and hence used as cell factories for the production of high-value chemicals or recombinant proteins. In this article, we review exploitation use of microalgae with a special emphasis on genetic engineering approaches to develop cell factories, and the use of synthetic ecology approaches to maximize productivity. We discuss the success stories in these areas, the hurdles that need to be overcome, and the potential for expanding the industry in general.

Optimization of process configuration and strain selection for microalgae-based biodiesel production

A mathematical model was developed for the design of microalgae-based biodiesel production system by systematically integrating all the production stages and strain properties. Through the hypothetical case study, the model suggested the most economical system configuration for the selected microalgae strains from the available processes at each stage, thus resulting in the cheapest biodiesel production cost, S$2.66/kg, which is still higher than the current diesel price (S$1.05/kg). Interestingly, the microalgae strain properties, such as lipid content, effective diameter and productivity, were found to be one of the major factors that significantly affect the production cost as well as system configuration.

Strategies of preserving lipids in microorganism after fermentation

Microbial oil is accumulated by microorganisms as stored energy during cultivation, and will be degraded to generate energy when they are not able to obtain external energy source. As the lipid is the desired production, the in situ degradation of oil by microorganisms after fermentation or during downstream processing has to be prevented. This study investigated the effect of pH, thermal, salinity, bead milling, ultrasonication, and microwave treatment on the viability of Trichosporon oleaginosus when it was used for lipid production. The cells in broth were completely inactivated with the treatment of pH 1 and 2 for 1h, temperature 80 °C for 10 min, ultrasonication for 8 min, and microwave for 6 min, respectively. It was observed that these treatments had no impact on final product (biodiesel) composition and were considered as safe and efficient methods to preserve lipid in cells.

The use of fluorescent Nile red and BODIPY for lipid measurement in microalgae

Microalgae are currently emerging as one of the most promising alternative sources for the next generation of food, feed, cosmetics and renewable energy in the form of biofuel. Microalgae constitute a diverse group of microorganisms with advantages like fast and efficient growth. In addition, they do not compete for arable land and offer very high lipid yield potential. Major challenges for the development of this resource are to select lipid-rich strains using high-throughput staining for neutral lipid content in microalgae species. For this purpose, the fluorescent dyes most commonly used to quantify lipids are Nile red and BODIPY 505/515. Their fluorescent staining for lipids offers a rapid and inexpensive analysis tool to measure neutral lipid content, avoiding time-consuming and costly gravimetric analysis. This review collates and presents recent advances in algal lipid staining and focuses on Nile red and BODIPY 505/515 staining characteristics. The available literature addresses the limitations of fluorescent dyes under certain conditions, such as spectral properties, dye concentrations, cell concentrations, temperature and incubation duration. Moreover, the overall conclusion of the present review study gives limitations on the use of fluorochrome for screening of lipid-rich microalgae species and suggests improved protocols for staining recalcitrant microalgae and recommendations for the staining quantification.

Biorefinery concept in a microalgae pilot plant. Culturing, dynamic filtration and steam explosion fractionation

A 53 m(3) microalgae pilot plant with semi-closed photobioreactors has been operated in batch mode at Tarragona, where proper climatic conditions exist. Operations carried-out were microalgae culturing, concentration, cell disruption and lipid extraction. Culturing was performed with and without CO2 fertilization. pH was used to control CO2 dosing. With CO2 fertilization best microalgae concentrations were obtained. Productivities up to 19.9 g/m(2)/day were reached. Although sedimentation and centrifugation was performed, studies focused on dynamic membrane filtration as improved technique compared with conventional one. Significant fouling reduction was obtained by using a vibrational membrane setup. Cell disruption including product pre-extraction was performed by using steam explosion at moderate conditions, as novel technique for this application. Lipid extraction was performed by using conventional techniques with and without exploded material. The amount of lipids obtained from exploded material was higher than with non-exploded material, which shows that steam explosion provides a clear enhancement.

Sources and resources: importance of nutrients, resource allocation, and ecology in microalgal cultivation for lipid accumulation

Regardless of current market conditions and availability of conventional petroleum sources, alternatives are needed to circumvent future economic and environmental impacts from continued exploration and harvesting of conventional hydrocarbons. Diatoms and green algae (microalgae) are eukaryotic photoautotrophs that can utilize inorganic carbon (e.g., CO2) as a carbon source and sunlight as an energy source, and many microalgae can store carbon and energy in the form of neutral lipids. In addition to accumulating useful precursors for biofuels and chemical feed stocks, the use of autotrophic microorganisms can further contribute to reduced CO2 emissions through utilization of atmospheric CO2. Because of the inherent connection between carbon, nitrogen, and phosphorus in biological systems, macronutrient deprivation has been proven to significantly enhance lipid accumulation in different diatom and algae species. However, much work is needed to understand the link between carbon, nitrogen, and phosphorus in controlling resource allocation at different levels of biological resolution (cellular versus ecological). An improved understanding of the relationship between the effects of N, P, and micronutrient availability on carbon resource allocation (cell growth versus lipid storage) in microalgae is needed in conjunction with life cycle analysis. This mini-review will briefly discuss the current literature on the use of nutrient deprivation and other conditions to control and optimize microalgal growth in the context of cell and lipid accumulation for scale-up processes.

Mixotrophic cultivation of microalgae for biodiesel production: status and prospects

Biodiesel from microalgae provides a promising alternative for biofuel production. Microalgae can be produced under three major cultivation modes, namely photoautotrophic cultivation, heterotrophic cultivation, and mixotrophic cultivation. Potentials and practices of biodiesel production from microalgae have been demonstrated mostly focusing on photoautotrophic cultivation; mixotrophic cultivation of microalgae for biodiesel production has rarely been reviewed. This paper summarizes the mechanisms and virtues of mixotrophic microalgae cultivation through comparison with other major cultivation modes. Influencing factors of microalgal biodiesel production under mixotrophic cultivation are presented, development of combining microalgal biodiesel production with wastewater treatment is especially reviewed, and bottlenecks and strategies for future commercial production are also identified.

Perspectives and advances of microalgal biodiesel production with supercritical fluid technology

Biodiesel, a sustainable and clean energy source, has been greatly attracting interest to compete against serious challenges like energy crisis and environmental pollution.

Microalgae-based biodiesel: a multicriteria analysis of the production process using realistic scenarios

Microalgae-based biodiesel has several benefits over other resources such as less land use, potential cultivation in non-fertile locations, faster growth and especially a high lipid-to-biodiesel yield. Nevertheless, the environmental and economic behavior for high scale production depends on several variables that must be addressed in the scale-up procedure. In this sense, rigorous modeling and multicriteria evaluation are performed in order to achieve optimal topology for third generation biodiesel production. Different scenarios and the most promising technologies tested at pilot scale are assessed. Besides, the sensitivity analysis allows the detection of key operating variables and assumptions that have a direct effect on the lipid content. The deviation of these variables may lead to an erroneous estimation of the scale-up performance of the technology reviewed in the microalgae-based biodiesel process. The modeling and evaluation of different scenarios of the harvesting, oil extraction and transesterification help to identify greener and cheaper alternatives.