Pretreatment Of Microalgal Biomass Research Articles

Optimisation of Ultrasound Pretreatment of Microalgal Biomass for Effective Biogas Production through Anaerobic Digestion Process

The anaerobic digestion, AD, process presents a solution for sustainable waste management, greenhouse gas mitigation and energy production for growing population needs and requirements. Adopting a biorefinery approach that utilises different feedstock may enhance energy production and support optimisation of the anaerobic digestion process. Algae is a promising feedstock that could be used for energy production via the anaerobic digestion process. Microalgal biomass is rich in carbohydrates and lipids; however, many species of algae exhibit tough cell walls that could also be difficult to digest and may influence or inhibit the efficiency of the AD process. This study concentrated on the comparison of AD remediation of two marine algal biomass species, Tetraselmis suecica and Nannochloropsis oceanica. The two species were pre-treated with an ultrasound technique and compared for their methane production using biochemical methane potential tests. For Tetraselmis, a specific methane production of 0.165 LCH4/KgVS was observed; however, for Nannochloropsis, a value of 0.101 LCH4/KgVS was observed for the samples treated with ultrasound. The BMP results from this study show that among the two micro-algae species tested, Tetraselmis suecica is found to be a better substrate for methane production potential. Contrary to increasing the specific methane production, ultrasound cavitation caused a slight decrease in the specific methane production values for both Nannochloropsis oceanica and Tetraselmis suecica biomass residues. The pre-treatment of the biomass using ultrasound techniques provided comparable results and can be recommended for effective bioenergy production. However, further research is required for the optimisation of the pre-treatment of microalgae and for the integration of microalgal biorefineries for circular economy.

Emerging trends in the pretreatment of microalgal biomass and recovery of value-added products: A review

Microalgae are a promising source of raw material (i.e., proteins, carbohydrates, lipids, pigments, and micronutrients) for various value-added products and act as a carbon sink for atmospheric CO2. The rigidity of the microalgal cell wall makes it difficult to extract different cellular components for its applications, including biofuel production, food and feed supplements, and pharmaceuticals. To improve the recovery of products from microalgae, pretreatment strategies such as biological, physical, chemical, and combined methods have been explored to improve whole-cell disruption and product recovery efficiency. However, the diversity and uniqueness of the microalgal cell wall make the pretreatment process more species-specific and limit its large-scale application. Therefore, advancing the currently available technologies is required from an economic, technological, and environmental perspective. Thus, this paper provides a state-of-art review of the current trends, challenges, and prospects of sustainable microalgal pretreatment technologies from a microalgae-based biorefinery concept.

Alkali pretreatment and enzymatic saccharification of blue-green alga Nostochopsis lobatus for bioethanol production

Using various Sodium hydroxide (NaOH) concentrations (0, 1, 2, and 3%), we studied the alkaline pretreatment of microalgal biomass from the blue-green alga Nostochopsis lobatus in this work. In this pretreatment stage, the entrapped polysaccharides found within the microalgae cell walls are broken down to form fermentable subunits. The selected pretreatment procedure (2% NaOH) was then followed by enzymatic saccharification for the generation of bioethanol. This pretreatment step aims to release and break down entrapped polysaccharides in the microalgae cell walls into fermentable subunits. The best result of sugars after enzyme hydrolysis was obtained in total and reducing sugars 210.22 and 98.54 g/L, respectively. In addition, Saccharomyces cerevisiae was used to ferment the hydrolysate; the highest ethanol yield was 14 g/L by 48 hours of fermentation. The alkaline pretreatment method was a promising option for pretreating microalgal biomass for bioethanol production.

Microalgal fuels: Promising energy reserves for the future

Traditional methods of fuel production and consumption result in the emission of huge quantities of CO2, leading to a rise in the Earth’s surface temperature. This has driven the scientific community to look for alternate sources of fuels that could fulfil the current energy demands, while reducing CO2 emissions. Microalgae have garnered substantial interest as an excellent biological resource for biorefineries, given their abilities to accumulate diverse CO2-derived bioproducts, such as various forms of fuels, chemical building blocks, and pharmaceuticals; this could be interpreted as an efficient method for storing solar energy by biological means. Microalgal biomass is free from ethical debates because it does not compete with food production for fertile arable lands and clean water for their cultivation, which makes the entire process highly sustainable. Microalgal biomass-derived fuels for each end-use can be obtained via several processes, including drying, chemical, and physical lipid-extraction methods, anaerobic digestion, and thermochemical treatments (e.g., pyrolysis, torrefaction, hydrothermal carbonization, liquefaction, and gasification). This review aims to critically explore the process layouts, their operation conditions (including catalysts), and expected outputs related to fuel production. It also provides a comprehensive overview of the most notable and promising strategies for the producing energy resources from various species of microalgae. Pretreatment of microalgal biomass, followed by bioconversion strategies, such as hydrothermal carbonization and photo-fermentation, along with genomics-based approaches, can lead to higher yields of microalga-derived solid, liquid, and gaseous fuels.

Strategies and advances in the pretreatment of microalgal biomass

Modification of structural components, especially the cell wall, through adequate pretreatment strategies is critical to the bioconversion efficiency of algal biomass to biorefinery products. Over the years, several physical, physicochemical, chemical and green pretreatment methods have been developed to achieve maximum productivity of desirable by-products to sustain a circular bioeconomy. The effectiveness of the pretreatment methods is however, species specific due to diversity in the innate nature of the microalgal cell wall. This review provides a comprehensive overview of the most notable and promising pretreatment strategies for several microalgae species. Methods including the application of stress, ultrasound, electromagnetic fields, pressure, heat as well as chemical solvents (ionic liquids, supercritical fluids, deep eutectic solvents etc.) have been detailed and analyzed. Enzyme and hydrolytic microorganism based green pretreatment methods have also been reviewed. Metabolic engineering of microorganisms for product specificity and lower inhibitors can be a future breakthrough in microalgal pretreatment.

Screening of microalgal isolates of the Persian Gulf and evaluation of their potential as the promising bioethanol feedstock

ABSTRACT The high levels of carbohydrates in some microalgae make them a promising feedstock for bioethanol production. The pretreatment of microalgal biomass is one of the most important stages to improve cell disruption for increasing the accessibility to the intracellular compounds. The isolation and identification of newly isolated microalgae that produce high biomass and carbohydrate production were done in this research. Among six isolates, Chlorella S-4 and Picochlorum D-8 showed the highest specific growth rates of 0.189 and 0.215 (µ day−1), respectively, from which the former showed a suitable self-precipitation property. A combination of cell disruption methods, including acid-thermal, acid-thermal/sonication bath, and acid-thermal/freeze & thaw pretreatment was applied to disrupt the cell wall of isolated microalgae on a laboratory scale. The effectiveness of cell disruption methods was evaluated based on total carbohydrate measurement. The maximum carbohydrate content of 22% and 27% of dry weight with the productivity of 7 and 6 mg/L/d was obtained from Chlorella S-4 and Picochlorum D-8, respectively, using acid-high thermal pretreatment (121°C for 20 min). This study is trying to demonstrate the Persian Gulf native microalgae potential for possible bioethanol production.

The use of solar pre-treatment as a strategy to improve the anaerobic biodegradability of microalgal biomass in co-digestion with sewage

Sustainable sewage treatment plants (STPs) have been intensively investigated in search for low-cost, environmental-friendly options. Anaerobic-aerobic treatment solutions, as upflow anaerobic sludge blanket (UASB) reactors followed by high rate algal ponds (HRAP) have already proved to be efficient for pollutants and micropollutants removal, as well as for energy recovery from the co-digestion of raw sewage and microalgal biomass. Since microalgae cells have complex structures that make them resistant to anaerobic digestion, pre-treatment techniques may be applied to improve microalgal biomass solubilisation and methane yield. Among the thermal pre-treatments, the use of solar energy for biomass solubilisation has yet to be investigated. Therefore, this study aimed at evaluating the performance of a solar thermal microalgal biomass pre-treatment prior to the anaerobic co-digestion with raw sewage, comparing a UASB reactor feed only raw sewage and other UASB reactor feed with raw sewage and pre-treated microalgal biomass. The results showed that, the solar pre-treatment step reached an organic matter solubilisation of 32% (COD). Furthermore, the methane yield was increased by 45% (from 81 to 117 NL CH4 kg−1 COD), after the anaerobic co-digestion with pre-treated microalgae as compared to the mono-digestion of raw sewage, indicating significant difference between the evaluated UASB reactors. The energy assessment showed a positive energy balance, as the total energy produced was twice the energy consumed in the system.

Microwave-assisted pretreatment of wet microalgal biomass for recovery of biofuel precursors

Pretreatment of microalgal biomass is essential for the maximal recovery of biofuel precursors packed inside the complex microalgal cell wall. In the present work, microwaves were exploited for efficient pretreatment of wet microalgal biomass. A response surface methodology – Box Behnken design (RSM-BBD) was utilized to obtain the optimum conditions for biomass pretreatment, and a mathematical model was devised. Using the designed experimental setup, the optimum conditions were obtained as 5 min of microwave irradiation time, 700 W output power, and 2 gL-1 microalgal biomass concentration. The model was further validated experimentally. The pretreatment results were confirmed qualitatively through microscopic observations and quantitatively in terms of sugar release and lipid extraction. The optimum conditions were found promising to enhance the lipid extraction by 55.32% compared to the untreated condition. Additionally, the present design showed the importance of water as a green solvent for biomass pretreatment under microwave irradiation. Overall, the current attempt facilitates a cost-effective, green methodology for wet microalgal biomass pretreatment in a short duration.

Green extraction of value-added compounds form microalgae: A short review on natural deep eutectic solvents (NaDES) and related pre-treatments

Sustainability and renewability are demanding the most important challenges that need to be achieved. Therefore, microalgae and Natural deep eutectic solvents (NaDEs) are two classes that deserve particular attention that can allow to meet the sustainability. Microalgae, as raw materials, can produce a variety of value-added compounds, which have numerous applications in pharmaceutical, nutraceutical and cosmetics sector. While, NaDES have peculiar characteristics that allow the recovery of various value-added compounds from microalgae biomass in sustainable manner. Microalgal compounds can be extracted using organic acids, alcohols or sugars by employing the NaDES process. These solvents act as are non-toxic, non-volatile and renewable during NaDES process. Considering the properties shown by microalgae and NaDES separately, a combined microalgae-NaDES extraction is extremely promising for future industrial applications. However, this technology requires proper pre-treatment of microalgal biomass to attain the complete recovery of valuable compounds. In conclusion, this review summarize the potential application of NaDES for extraction of intracellular compounds from microalgal biomass and their related pre-treatments to improve the extraction efficiency.

Deep eutectic solvents and ionic liquid assisted hydrolysis of microalgal biomass: A promising approach towards sustainable biofuel production

Utilization of each biochemical component such as lipid or carbohydrate in a biorefinery is essential for commercializing microalgal-based biofuels. We have performed a comparative evaluation of ionic liquids (ILs) and deep eutectic solvents (DESs) for microalgal hydrolysis in the present study. Among the eight tested ionic liquids, ethyl ammonium nitrate (EAN) resulted in the highest saccharification yield of 95.5% whereas, among hydrophobic deep eutectic solvents, menthol: lactic acid (Me: LA) exhibited the highest saccharification yield 85.7% and also did not require any additional high temperature or other pretreatments for hydrolysis indicating as the potential solvent system to reduce the cost of microalgal based bioethanol production concerning the harsh conditions in a conventional method. Further analysis of pretreated biomass with SEM, FTIR, and XRD analysis indicates that EAN or Me: LA ruptures the microalgal cell wall and hydrolyzes the cytoplasmic starch to release the sugars in the solution. IL/DES was recycled four times without losing the efficiency. Overall, the present study results indicate that the identified IL and DES can be used as a green and sustainable alternative for the pretreatment of microalgal biomass for biofuel production.

Numerical Analysis of Convective Heat Transfer Characteristics in Microalgae Slurries in Tube Flow

AbstractWith the various distinctive applications of microalgae, microalgae slurries have been extensively used in biogas and biofuel production. During these processes, the continuous hydrothermal pretreatment of microalgal biomass in tubular reactors is a key procedure. This pretreatment involves heat transfer, and the complex nature of the microalgae slurries containing micron‐sized cells flowing through the tube poses a major challenge for industrialization. There is only a scarce amount of literature available on the modeling of microalgae slurries; hence, a computational fluid dynamics model is developed in this study to analyze their heat transfer characteristics. Good agreement was obtained between the model results and experimental values.

Anaerobic digestion of microalgal biomass for bioenergy production, removal of nutrients and microcystin: current status

Using renewable microalgal biomass as active feedstocks for biofuels and bioproducts is explored to substitute petroleum-based fuels and chemicals. In the last few years, the importance of microalgae biomass has been realized as a renewable feedstock due to several positive attributes associated with it. Biorefinery via anaerobic digestion (AD) of microalgal biomass is a promising and sustainable method to produce value-added chemicals, edible products and biofuels. Microalgal biomass pretreatment is a significant process to enhance methane production by AD. Findings on the AD microbial community's variety and organization can give novel in turn on digester steadiness and presentation. This review presents a vital study of the existing facts on the AD microbial community and AD production. Co-digestion of microalgal biomass with different co-substrates was used in AD to enhance biogas production, and the process was economically viable with improved biodegradability. Microcystins, which are produced by toxic cyanobacterial blooms, create a severe hazard to environmental health. Anaerobic biodegradation is an effective method to degrade the microcystins and convert into nontoxic products. However, for the cost-effective conversion of biomass to energy and other beneficial byproducts, additional highly developed research is still required for large-scale AD of microalgal biomass.

Enzymatic and Hydrothermal Pretreatment of Newly Isolated Spirulina subsalsa BGLR6 Biomass for Enhanced Biogas Production

The complex structure and biochemical composition of the cell wall of microalgae impede their anaerobic digestion. To enhance the microalgae anaerobic biodegradability, various pretreatment approaches have been utilized. In this study, the enzyme and hydrothermal pretreatment methods were evaluated for microalgal biomass pretreatment and biogas yield. The optical and scanning electron microscopy along with Fourier transform infrared spectroscopy analysis confirmed the efficient action of both the pretreatment methods. The hydrothermal pretreatment resulted in more structural changes, though the increase in enzymatic concentration was also found to have a pronounced effect on both structural and chemical changes. The FTIR spectra determined that mostly the protein and carbohydrate structures of the microalgal cells were affected. Further upon quantitative analysis, it was observed that 10% dose (w/w) for 24 h of exposure time released significantly more soluble chemical oxygen demand compared to others. The multi-enzyme 10% dose for 24 h resulted in significantly higher biogas production potential (P) of 768.92 mL g−1 VS at a maximum biogas production rate (Rm) of 32.16 mL g−1 d−1 with a very short lag phase (λ) of 0.09 days at the end of 30 days, in comparison to untreated and other pretretment conditions in this study. Both the pretreatment approaches in the present study enhanced the microalgal biomass disintegration, digestibility and biogas production. However, more research is required to optimize the process parameters of these pretreatment approaches to make them more reasonable and applicable.

Lipid extraction from wet Nannochloropsis biomass via enzyme-assisted three phase partitioning

A green and efficient enzyme assisted three phase partitioning (EA-TPP) process was firstly developed to extract microalgal lipids using wet Nannochloropsis sp. biomass. In the pretreatment of microalgal biomass by four hydrolytic enzymes, TPP obtained a higher TFAs lipid extraction efficiency by cellulase compared with the resting enzymes. After optimization by EA-TPP of the wet disrupted Nannochloropsis biomass (3 g), the maximum TFAs extraction yield (90.40%) was attained at 20% ammonium sulphate, 6–7 pH, 1:2 slurry/tert-butanol ratio and 70 °C for 2 h incubation time and two extraction cycles. Moreover, results also revealed that the lipidic species compositions of Nannochloropsis sp. biomass were greatly related with the EA-TPP parameters. In the laboratory scale for wet disrupted microalgae biomass, EA-TPP process achieved 88.70% TFAs extraction yield under the optimized conditions. In all, EA-TPP process could be a promising approach to extract microalgae lipids for food application using wet microalgae biomass.

Pretreatment of microalgal biomass to improve the enzymatic hydrolysis of carbohydrates by ultrasonication: Yield vs energy consumption

Microalgal biomass has been considered as a possible alternative source of carbohydrates and lipids in fermentative/reactional processes, called third generation of biofuels. Carbohydrates from microalgae are mostly composed by glucose and some pentose-derived polymers that must be hydrolyzed to be efficiently used. When enzymatic hydrolysis is applied a pretreatment is required. Sonication/ultrasonication is one of the most promising methods, and in this paper the influence of pretreatment time, sonication intensity and biomass concentration was validated, and the energy consumed in the process compared as well. Sonication intensity had the major role on the enzymatic accessibility. Pretreatment time can be used to decrease hydrolysis time. More than 90% of hydrolysis efficiency was reached when higher amplitude (sonication intensity) and pretreatment time were used. The applied energy influenced indirectly the hydrolysis process. The best saccharification/energy relation was reached when 50% of amplitude for 25 min was applied, obtaining 91% of hydrolysis yield and spending 2.4 MJ/kg of dry biomass.

Improvement of biohydrogen production by optimization of pretreatment method and substrate to inoculum ratio from microalgal biomass and digested sludge

Abstract Biohydrogen from microalgal biomass has shown particular advantage due to its high growth rate and high bioenergy production. As a representative of microalgae, Chlorella vulgaris was chosen as substrate along with digested sludge (DS) as inoculum in this research. In order to improve the hydrolysis of algal biomass and enhance biohydrogen production, pretreatment methods like acid and thermal pretreatment were employed. Thermal pretreatment showed better results than acid pretreatment of microalgal biomass. 100 °C for 60 min was identified as the optimum condition for the thermal pretreatment of C. vulgaris by response surface methodology (RSM) analysis. Experiments were also carried out to identify the optimum substrate to inoculum ratio (SIR) for the process. SIR of 8 generated the highest hydrogen yield of 190.90 mL H2/g-VS. Moreover, the overall energy balance of the process was evaluated and the results showed a positive energy balance of 1790.13 kJ/kg. The results indicated that optimization of pretreatment methods and substrate to inoculum ratio was effective in enhancing biohydrogen production from microalgal biomass and digested sludge.

Severity Factor as an Efficient Control Parameter to Predict Biomass Solubilization and Saccharification During Acidic Hydrolysis of Microalgal Biomass

In this paper, the acidic pretreatment of microalgal biomass is investigated, and the solubilized biomass and hydrolyzed sugars were evaluated. The process is analyzed through the severity factor approach (acidic combined severity factor (ACSF)). A suitable kinetic model is developed and applied, and it is shown that the severity factor theory works. A discussion and comparison are presented with respect to the literature methods, which are mainly related to lignocellulosic biomass. In the case of microalgae, reaction orders for biomass and acid are shown to be the main parameters, and no other assumptions are needed. Two regions of the acidic treatment process have to be evaluated: low and high reactivity regions. Furthermore, a suitable experimental design is required in order to provide an appropriate reaction spectrum to obtain a good estimation of the kinetic parameters. A logarithmic severity factor range (ln ACSF) between 5 and 6 is able to solubilize around 80% of biomass and to hydrolyze more than 90% of sugars present in the biomass.

Towards energy neutral microalgae-based wastewater treatment plants

The aim of this study was to assess the energy balance of a hypothetical microalgae-based wastewater treatment plant (10,000 PE) located in the Mediterranean Region, where harvested microalgal biomass and primary sludge would be co-digested to produce biogas and bioenergy. The assessment was based on experimental results obtained over one year in pilot high rate algal ponds followed by anaerobic digesters for biogas production from harvested microalgal biomass and primary sludge. The energy balance compared four scenarios: 1) anaerobic co-digestion of microalgal biomass and primary sludge, and cogeneration from biogas in a combined with heat and power (CHP) unit; 2) co-digestion with thermal pretreatment of microalgal biomass, and cogeneration from biogas in a CHP unit; 3) co-digestion and heat generation from biogas in a boiler; and 4) co-digestion with thermal pretreatment of microalgal biomass, and heat generation from biogas in a boiler. According to the results, when biogas was used to cogenerate electricity and heat (scenarios 1 and 2), the electricity balance was always positive, and the best results were obtained with pretreated microalgal biomass (scenario 2). Similarly, the heat balance was always positive when biomass was thermally pretreated (scenario 2). On the other hand, when biogas was only used to produce heat (scenarios 3 and 4), heat requirements were covered during the whole year. The sensibility analysis of the scenarios with pretreatment (2 and 4) confirmed that the microalgae-based WWTP would be energy neutral or even net energy producer.

A new approach of microalgal biomass pretreatment using deep eutectic solvents for enhanced lipid recovery for biodiesel production

The biomass of Chlorella sp. was pretreated with three different aqueous deep eutectic solvents (aDESs), i.e. aqueous choline chloride-oxalic acid (aCh-O), aqueous choline chloride-ethylene glycol (aCh-EG) and aqueous urea-acetamide (aU-A). The effect of aDESs pretreatment of microalgae biomass was evaluated in terms of lipid recovery rate, total carbohydrate content, fatty acid composition, and thermal chemical behavior of biomass. Results indicated that, lipid recovery rate was increased from 52.03% of untreated biomass to 80.90%, 66.92%, and 75.26% of the biomass treated by aCh-O, aCh-EG and aU-A, respectively. However, there were no major changes observed in fatty acid profiles of both untreated and treated biomass, specifically palmitic acid, palmitoleic acid and stearic acid under various pretreatments. Furthermore, characterizations of untreated and treated biomass were carried out using Fourier transform infrared (FTIR), thermogravimetry analysis (TGA) and scanning electron microscope (SEM) to understand the enhanced lipids recovery.

Pretreatment of microalgal biomass for enhanced recovery/extraction of reducing sugars and proteins

Microalgae species including Chlamydomonas mexicana, Micractinium reisseri, Scenedesmus obliquus and Tribonema aequale were cultivated in batch cultures, and their biochemical composition was determined. C. mexicana showed the highest carbohydrate content of 52.6% and was selected for further study. Sonication pretreatment under optimum conditions (at 40 kHz, 2.2 Kw, 50 °C for 15 min) released 74 ± 2.7 mg g(-1) of total reducing sugars (TRS) of dry cell weight, while the combined sonication and enzymatic hydrolysis treatment enhanced the TRS yield by fourfold (280.5 ± 4.9 mg g(-1)). The optimal ratio of enzyme [E]:substrate [S] for maximum TRS yield was [1]:[5] at 50 °C and pH 5. Combined sonication and hydrolysis treatment released 7.3% (27.1 ± 0.9 mg g(-1)) soluble protein of dry cell weight, and further fermentation of the dissolved carbohydrate fraction enhanced the soluble protein content up to 56% (228.4 mg g(-1)) of total protein content. Scanning and transmission electron microscopic analyses indicated that microalgae cells were significantly disrupted by the combined sonication and enzyme hydrolysis treatment. This study indicates that pretreatment and subsequent fermentation of the microalgal biomass enhance the recovery of carbohydrates and proteins which can be used as feedstocks for generation of biofuels.