Harvesting of <Emphasis Type="Italic">Scenedesmus obliquus</Emphasis> cultivated in seawater using electro-flotation

Seawater, when supplemented to a growth medium, appears to stimulate auto-flocculation of a certain microalgae species like Scenedesmus obliquus and thus renders its harvesting easy. To make use of this unique response for the purpose of biomass harvesting, S. obliquus was grown in a seawater-added medium and then collected in electrochemically- mediated ways. Significantly higher harvesting efficiency and energy saving were observed with electroflotation (EF) than with electro-coagulation-flotation (ECF) and the standard BG11 medium. An optimal EF condition, the highest recovery rate with least energy use, was found with a supply of 0.5 A. Seawater amendment was most beneficial in a level of 10%. All this clearly showed that applying EF to cells cultivated in the seawater-supplemented medium is a promising harvesting means that enables one to obtain algae biomass without interfering with the downstream process of biodiesel production.     

Cultivation and Hydrothermal Gasification of Microalgae Scenedesmus Obliquus – First Experimental Results

Microalgae (Scenedesmus obliquus) produced in flat panel photobioreactors were converted via supercritical water gasification. The long term aim is to gain higher process efficiency through recycling of byproducts of supercritical water gasification and waste water during the cultivation of microalgae. The first step of the project, the general feasibility of conversion of microalgae Scenedesmus obliquus via supercritical water gasification to a hydrogen‐rich combustible gas is presented. The product gas had a higher heating value and consisted of hydrogen, methane, carbon dioxide, and ethane.     

Growth and fatty acid composition of Acutodesmus obliquus under different light spectra and temperatures

The combined impact of temperature and light spectra on the fatty acid (FA) composition in microalgae has been sparsely investigated. The aim of this study was to investigate the interactions of light and temperature on the FA composition in Acutodesmus obliquus. For this purpose, A. obliquus was cultivated with different temperatures (20, 30, and 35°C), as well as broad light spectra (blue, green, and red light). Growth and FA composition were monitored daily. Microalgal FA were extracted, and a qualitative characterization was done by gas chromatography coupled with electron impact ionization mass spectrometry (GC-EI/MS). Compared to red light, green and blue light caused a higher percentage of the polyunsaturated fatty acids (PUFA) 16:4, 18:3, and 18:4, at all temperatures. The highest total percentage of these PUFA were observed at the lowest cultivation temperature and blue and green light. These data imply that a combination of lower temperatures and blue-green light (450–550 nm) positively influences the activity of specific FA-desaturases in A. obliquus. Additionally, a lower 16:1 trans/cis ratio was observed upon green and blue light treatment and lower cultivation temperatures. Remarkably, green light treatment resulted in a comparably high growth under all tested conditions. Therefore, a higher content of green light, compared to blue light might additionally lead to a higher biomass concentration. Microalgae cultivation with low temperatures and green light might therefore result in a suitable FA composition for the food industry and a comparably high biomass production.     

Microalgal biomass as a fermentation feedstock for bioethanol production

BACKGROUND: The increasing cost of fossil fuels as well as the escalating social and industrial awareness of the environmental impacts associated with the use of fossil fuels has created the need for more sustainable fuel options. Bioethanol, produced from renewable biomass such as sugar and starch materials, is believed to be one of these options, and it is currently being harnessed extensively. However, the utilization of sugar and starch materials as feedstocks for bioethanol production creates a major competition with the food market in terms of land for cultivation, and this makes bioethanol from these sources economically less attractive. RESULT: This study explores the suitability of microalgae (Chlorococum sp.) as a substrate for bioethanol production via yeast (Saccharomyces bayanus) under different fermentation conditions. Results show a maximum ethanol concentration of 3.83 g L^?1 obtained from 10 g L^?1 of lipid‐extracted microalgae debris. CONCLUSION: This productivity level (～38% w/w), which is in keeping with that of current production systems endorses microalgae as a promising substrate for bioethanol production. Copyright © 2009 Society of Chemical Industry     

Pyrolysis of microalgae biomass over carbonate catalysts

Microalgae are seen as potential biomass to be used in a biorefinery concept. Several technologies can be used to convert microalgal biomass, but pyrolysis is viewed as a unique pathway to obtain valuable chemicals distributed in three phases: liquid (bio-oil), gas (bio-gas) and solid (bio-char). The liquid phase, bio-oil, usually presents higher heating value than raw biomass, but acidity and oxygen content are major drawbacks. In situ catalyzed pyrolysis can help to decrease the oxygen content and acidity of pyrolytic bio-oils. Chlorella vulgaris and Scenedesmus obliquus were pyrolyzed in a fixed-bed reactor using commercial carbonate catalysts (Li₂CO₃, Na₂CO₃, K₂CO₃, MgCO₃, SrCO₃ and MnCO₃). The catalysis pyrolysis temperature (375 °C) was selected from thermal degradation profiles obtained using thermogravimetry under nitrogen flow and corresponds to the maximum degradation rate for both microalgae. In spite of similar volatile and fixed carbon contents, microalgae performed differentially during pyrolysis mainly due to the different contents of carbohydrates, oils and proteins. Chlorella vulgaris and Scenedesmus obliquus showed bio-oil yield in the range 26–38 and 28–50 wt%, respectively. Only sodium carbonate was able to decrease the bio-char yield, confirming that carbonate catalysts prompt simultaneously gasification and carbonization reactions. Fourier transform infrared spectra of produced bio-oils showed a net decrease of acidity, associated with carbonyl species when carbonate catalysts were used. Bio-char morphology, for both microalgae, showed evidence of melting and resolidification of cell structures, which might be due to the lower melting points of the pyrolysis products obtained from proteins and lipids. © 2020 Society of Chemical Industry     

Dewatering of microalgal culture for biodiesel production: exploring polymer flocculation and tangential flow filtration

BACKGROUND: Conventional biodiesel production relies on trans‐esterification of lipids extracted from vegetable crops. However, the use of valuable vegetable food stocks as raw material for biodiesel production makes it an unfeasibly expensive process. Used cooking oil is a finite resource and requires extra downstream processing, which affects the amount of biodiesel that can be produced and the economics of the process. Lipids extracted from microalgae are considered an alternative raw material for biodiesel production. This is primarily due to the fast growth rate of these species in a simple aquaculture environment. However, the dilute nature of microalgae culture puts a huge economic burden on the dewatering process especially on an industrial scale. This current study explores the performance and economic viability of chemical flocculation and tangential flow filtration (TFF) for the dewatering of Tetraselmis suecica microalgae culture. RESULT: Results show that TFF concentrates the microalgae feedstock up to 148 times by consuming 2.06 kWh m^?3 of energy while flocculation consumes 14.81 kWh m^?3 to concentrate the microalgae up to 357 times. Economic evaluation demonstrates that even though TFF has higher initial capital investment than polymer flocculation, the payback period for TFF at the upper extreme of microalgae revenue is ～1.5 years while that of flocculation is ～3 years. CONCLUSION: These results illustrate that improved dewatering levels can be achieved more economically by employing TFF. The performances of these two techniques are also compared with other dewatering techniques. Copyright © 2009 Society of Chemical Industry     

Studying the drying mechanism of microalgae Chlorella vulgaris and the optimum drying temperature to preserve quality characteristics

Drying harvested microalgae from an average moisture content of 80% wet basis to a safe moisture content of 10% is challenging. Removing this high amount of water from microalgal biomass is time-consuming and is not as easy as agricultural crop dehydration. The long drying time results in large drying costs. Although drying is a suitable technique for algal-based fuel production, it has not been commercialized due to its associated challenges. This study was performed to fulfill the knowledge gap in the microalgae drying mechanism and to understand the reason for long drying times. For this purpose, the thin-layer drying of microalgae Chlorella vulgaris at the temperature range of 40 to 140°C was studied in a convective oven. The effect of drying air temperature on Chlorella elemental and chemical composition, surface color, and surface structure in the aforementioned temperature range was also analyzed. The results revealed that the dominant mechanism in Chlorella drying is diffusion, which is attributed to the collapse of microalgal cells structure with increased drying temperature. In fact, moisture is entrapped in the Chlorella cells and it takes a long time for them to reach the biomass surface and evaporate. The result was that both low and high drying temperatures have adverse effects on Chlorella surface color, structure, and carbohydrate and lipid composition. This suggests that microalgae should be dried at an optimum medium (60–80°C) temperature.     

Sustainable Bioactive Packaging Based on Thermoplastic Starch and Microalgae

This study combines the use of corn starch and Tetradesmus obliquus microalgae for the production of antioxidant starch films as flexible packaging material. Starch was plasticized with glycerol and blended with 1 w% polyallylamine chosen as an agent to modify the film physical properties. The addition of polyallylamine improved film water stability and water vapor transmission rate as well as mechanical stiffness and tenacity. The dried Tetradesmus obliquus microalgae, which showed an EC₅₀ value of 2.8 mg/mg DPPH (2.2-Diphenyl-1-picrylhydrazyl radical), was then used as antioxidant filler. The addition of microalgae provided the films with good antioxidant activity, which increased with microalgae content increasing. To our knowledge, this is the first study reporting the development of sustainable bioactive packaging films composed of almost 100% starch, and follows the European union’s goals on plastics strategy concerning the promotion of bio-based, compostable plastics and the setting up of approaches to prevent food waste with a simple plastic packaging.     

Isolation and characterization of autoflocculating mutants of cyanobacterium Arthrospira platensis

Harvesting microalgae is a major concern for mass culture in industry. Flocculation is an easy and effective way to harvest microalgae. However, flocculation using chemical flocculants is not feasible for scaling-up due to their toxicity. As an alternative technique, mutation breeding of autoflocculating microalgae strain has been reported in this study. We characterized autoflocculating mutants of Arthrospira platensis (A. platensis) by ethyl methane sulfonate (EMS). The cells of mutants were aggregated during the culture and dry cell weight increased 1.2- to 1.8-fold compared to the wild type. Autoflocculation was induced highly at an optimal pH level of 9 and the flocculation efficiency reached almost 90%. Mutants showed higher flocculation efficiency irrespective of the addition of chemical flocculants. Thus, it is definitely useful to harvest microalgae using autoflocculating mutants in large-scale culture without any drawbacks of harvested algal biomass.     

Daily doses of light in relation to the growth of Scenedesmus obliquus in diluted three‐phase olive mill wastewater

BACKGROUND: The use of olive‐oil mill wastewater (OMW) from a three‐phase centrifugation process used in the olive‐oil industry, has been studied in relation to the production of the microalga Scenedesmus obliquus CCAP 276/3A. The chemical characteristics of OMW indicated nitrogen deficiency. RESULTS: S. obliquus is able to assimilate nutrients present in a culture medium (water‐OMW 5%) and grow at its maximum specific growth rate of 0.026 h^?1, both under mixotrophic as well as heterotrophic conditions. The different daily doses of light (DDL) used, in the range 0–36 E m^?2 d^?1, determined light‐limited and light‐inhibited cultures. The light‐inhibited mixotrophic cultures bore characteristics similar to those of the heterotrophic cultures, and became more so when the dose of light received was higher. The low protein yield (258 mg g^?1) and high percentage of carbohydrates of the biomass (65.8%) confirmed a nutritional‐stress situation associated with nitrogen limitation. CONCLUSION: The similarity between the fatty‐acid composition of the heterotrophic and mixotrophic cultures strongly inhibited by light appeared to indicate the cancelling of the photosynthetic behaviour of the cells at high DDL values. The biomass generated can be used for biofuels. The maximum elimination of biological oxygen demand (BOD₅) per unit of biomass was achieved in the heterotrophic cultures. Copyright © 2009 Society of Chemical Industry     

Simultaneous harvesting and cell disruption of microalgae using ozone bubbles: optimization and characterization study for biodiesel production

In the present study, ozone was introduced as an alternative approach to harvest and disrupt microalgae cells (Chlorella vulgaris) simultaneously for biodiesel production. At the optimum ozonation conditions (6.14 g·h^–1 ozone concentration, 30 min ozonation time, 1 L·min^–1 of ozone flowrate at medium pH of 10 and temperature of 30 °C), the sedimentation efficiency of microalgae cells increased significantly from 12.56% to 68.62%. It was observed that the microalgae cells aggregated to form flocs after pre-treated with ozone due to the increment of surface charge from –20 to –6.59 mV. Besides, ozone had successfully disrupted the microalgae cells and resulted in efficient lipid extraction, which was 1.9 times higher than the control sample. The extracted microalgae lipid was mainly consisted of methyl palmitate (C16:0), methyl oleate (C18:1) and methyl linolenate (C18:3), making it suitable for biodiesel production. Finally, utilization of recycled culture media after ozonation pre-treatment showed robust growth of microalgae, in which the biomass yield was maintained in the range of 0.796 to 0.879 g·h^–1 for 5 cycles of cultivation.     

Modified Photobioreactor for Biofixation of Carbon Dioxide by Chlorella vulgaris at Different Light Intensities

The performance of a modified bioreactor inside a light enclosure for carbon dioxide biofixation by Chlorella vulgaris was investigated. The influence of different light intensities on the CO₂ biofixation and biomass production rates was evaluated. The results showed that the photon flux available to the microalgal cultures can be a key issue in optimizing the microalgae photobioreactor performance, particularly at high cell concentrations. Although the optimal pH values for C. vulgaris are in the range of 6–8, cell growth can take place even at pH 4 and 10. Batch microalgae cultivation in the photobioreactor was used to investigate the effect of different light intensities. The maximum biomass concentration of 1.83 g L^?1 was obtained at a light intensity of 100 μmol m^?2s^?1 and under aeration with 2 L min^?1 of 2 % CO₂‐enriched air.     

Dewatering and Drying Methods for Microalgae

Microalgae can efficiently fix carbon dioxide through their phototropic metabolism, and have been recognized as a promising bioresource for animal feed, health food, fuel, cosmetic, and pharmaceutical products. However, since microalgae in cultivated medium have a low biomass concentration (0.1–1% w/w), both harvesting and concentration of microalgal biomass are often required prior to the production of commercial products. Efficient and cost-effective dewatering and drying methods for microalgae heavily affect the overall energy consumption and production cost of microalgal products. This review describes the characteristics of commonly used dewatering and drying technologies, and critically evaluates the feasibility for their use to treat microalgal biomass. No single dewatering or drying method can satisfactorily handle all types of microalgae. The suitability of each method depends on the properties of the microalgae suspension, the required process design, the quality of the end product, and the related capital and production costs.     

Ozone for Microalgae Biomass Harvesting from Wastewater

The use of ozone has been investigated as a harvesting technique (ozone-flotation) for microalgae recovery from wastewater. This document summarizes the most outstanding results achieved to date by our working group, including the ozone effect on the lipid and fatty acid methyl ester (FAME) content. Additionally, the effect of the initial concentration of microalgal biomass on ozone-flotation was investigated using microalgae Scenedesmus sp. cultured in wastewater. Ozone-flotation required specific conditions to separate biomass from wastewater, depending on the type of microalgae strain, culture (single or mixed) and concentration. The effect of ozone-treatment improved lipid extractability and increased degree of FAME saturation.     

Growth and Biomass Characteristics of Picochlorum oklahomensis and Nannochloropsis oculata

Biofuels derived from microalgae are considered a viable alternative energy resource that eliminates the major drawbacks associated with biofuels produced from land crops. Strain selection is a critical factor for the success of microalgal production systems. This study evaluates Picochlorum oklahomensis (PO) as a potential microalga strain for biofuel and bioproduct applications. Nannochloropsis oculata (NO) was included in the study as a well-studied reference strain with biomass properties suitable for commercial applications. Growth pattern including specific growth rates, generation time and maximum biomass concentrations in culture media were determined for both PO and NO. Oil and protein contents and fatty acid composition of both strains were analyzed. PO had a higher biomass concentration in the medium and shorter generation time than NO. Both strains were rich in oil, protein and polyunsaturated fatty acids. This study indicates that PO can be a viable strain for bioproduct manufacturing because of its high oil and protein content and biomass productivity.     

Effect of Processing Parameters on Flocculation of Picochlorum oklahomensis

The flocculation process is commonly used to separate suspended solids from water. The microalgae strain, Picochlorum oklahomensis (PO), was investigated for its flocculation characteristics. Efficiencies of biopolymer addition, pH adjustment and electroflocculation for biomass recovery from the culture medium were examined. Flocculation efficiency of PO increased sharply above pH 11 and reached 97 % at pH 13. Chitosan was more effective in flocculating PO cells than sodium alginate and cationic starch. A generalized linear mixed model using a beta distribution for response was utilized for optimization of the chitosan flocculation process variables. Biomass: chitosan ratio, pH and settling time (ST) were the independent variables. There were significant 3-way interactions among the variables. The highest PO flocculation efficiency, 98.4 %, was obtained at biomass: chitosan ratio of 2.78, pH 9 and ST of 12 h. The electroflocculation efficiency improved with increasing current, operation time (OT) and ST. The highest electroflocculation efficiency, 99.74 %, was obtained under the following conditions; 0.8 A (ampere) and 15 min and 12 h, OT and ST, respectively. This study demonstrated that pH adjustment, chitosan addition and electroflocculation were all technically viable methods to flocculate PO cells. However, selection of the most suitable technique and the optimum treatment conditions needs to be based not only on the application of algal biomass, but also on an economic feasibility study.     

Biodiesel production from Stichococcus strains at laboratory scale

BACKGROUND: This paper reports the results of an experimental campaign of autotrophic cultures of Stichococcus strains aiming at selecting the most promising strain for biofuel production. The strain selected—S. bacillaris 158/11—was cultivated in 1 L lab‐scale bubble column photobioreactors under fed‐batch and semi‐continuous conditions. A Bold basal medium supplemented with NaNO₃ as nitrogen source was adopted. Tests were carried out at 23 °C, 140 µE m^?2 s^?1, and air flow rate ranging between 0.4 and 4 vvm. Cultures were characterized in terms of pH, concentration of total nitrogen, total organic carbon, total inorganic carbon, biomass, lipid fraction and methyl‐ester distribution of transesterified lipids. RESULTS: S. bacillaris 158/11 proved to be the best strain to produce biodiesel. Methyl‐ester distribution was characterized by a large fraction of methyl palmitate, methyl linolenate, methyl linoleate, and methyl oleate along with phytol. The process photosynthetic efficiency—fraction of available light stored as chemical energy ‐ was about 1.5%. Specific biomass productivity was ～60 mg_DM L^?1 day^?1 under the semi‐continuous conditions tested. Total lipid productivity was 14 mg L^?1 day^?1 at a dilution rate of 0.050 L day^?1. CONCLUSION: S. bacillaris 158/11 is a potential strain for massive microalgae cultures for biofuel production. Higher biomass/total‐lipid productivity could be obtained in sunlight. Copyright © 2011 Society of Chemical Industry     

Process Parameter Screening for the Microalga Chlamydomonas asymmetrica in Batch and Turbidostat Cultivations

Algae offer tremendous potential for the production of various high-value products (HVPs). Besides the light quantity and quality, factors such as temperature, medium composition, pH, and aeration are important algae-specific process parameters that influence growth behavior and their chemical composition. Screenings for algae and their HVPs are ongoing, but the available data for their cultivability and suitability as production organisms are limited. Turbidostat processes enable a simple and fast screening for light-specific influences, as constant biomass concentrations will result in constant self-shading. Here, the microalgae Chlamydomonas asymmetrica was cultivated in cylindrical photobioreactor screening modules. A detailed characterization the light and temperature dependency of the organisms' HVPs is provided.     

Multistage continuous cultivation of blue-green alga spirulina maxima in the flat tank photobioreactors with recycle

Spirulina maxima was continuously cultivated in four 64-liter flat tank photobioreactors in cascade, under continuous 30 klx fluorescent light and nonaseptic condition, in an industrial grade synthetic medium which bubbled with atmospheric air. The increase in the number of stages resulted in the higher biomass concentration and allowed operation with a dilution rate D higher than the alga specific growth rate μ. However, multistage operation does not permit a significant increase in productivity. The recycling of the all culture medium with biomass increased the net productivity of the first two stages of the multistage system. The flat tank design offered great advantages over other geometrical configurations by occupying less space for indoor cultivation, minimizing the water loss by evaporation, and permitting a greater flexibility in construction, installation, and operation. This system was operating continuously for two years in our laboratory for indoor mass production of S. maxima biomass. The flat tank configuration gave the highest areal productivity Pa = 60.5 g/m²·d and highest volumetric productivity Pv = 1.17 g/L·d when operated as a single stage continuous cultivation with high solid recycle. These values are the highest ever reported for any indoor or outdoor mass production system for S. maxima.     

Process for production of arachidonic acid concentrate by a strain of mortierella alpina

Various Mortierella alpina fungi were screened for their capacities to produce arachidonic acid. A strain of M. alpina was found to show the highest productivity. Arachidonic acid content of biomass and overall yield per litre of culture was highest in soya flour supplemented medium which produced dispersed mycelium. When the glucose concentration in the medium was varied from 30 to 100 g/L, biomass, lipid, arachidonic acid content of biomass and arachidonic acid yield increased with increasing glucose concentration. Several natural oils, when added to the growth medium, stimulated arachidonic acid production. After fermentation in a 20-L fermenter under optimal culture conditions, the arachidonic acid yield was 5.3 g/L, representing 34.2% w/w of total fatty acids and 13.7% w/w of biomass. An extract containing 72.5% w/w arachidonic acid was prepared from the recovered mycelium.