Microalgae cultivation in a wastewater dominated by carpet mill effluents for biofuel applications

Coupling anaerobic co-digestion of winery waste and waste activated sludge with a microalgae process: Optimization of a semi-continuous system

Wine production represents one of the most important agro-industrial sectors in Italy. Wine lees are the most significant waste in the winery industry and have high disposal and storage costs and few applications within the circular economy. In this study, anaerobic digestion and a microalgae coupled process was studied in order to treat wine lees and waste activated sludge produced within the same facility, with the aim of producing energy and valuable microalgae biomass that could be processed to recover biofuel or biostimulant. Chlorella vulgaris was cultivated on liquid digestate in a semi-continuous system without biomass recirculation. The best growth and phytoremediation performance were achieved applying a hydraulic retention time (HRT) of 20 days with a stable dry weight, lipid and protein storage of 1.85 ± 0.02 g l-1, 33.48 ± 7.54 % and 57.85 ± 10.14 % respectively. Lipid characterization highlighted the potential use in high quality biodiesel production, according to EN14214 (<12 % v/v linolenic acid). The microalgae reactor's liquid output showed high removal of ammonia (95.72 ± 2.10 %), but low organic soluble matter reduction. Further semi-continuous process optimization was carried out by increasing the time between digestate feeding and biomass recovery at HRT 10. These operative changes avoided biomass wash-out and provided a stable phytoremediation of the digestate with 84.58 ± 4.02 % ammonia removal, 33.01 ± 1.44 % sCOD removal, 38.06 ± 2.65 % of polyphenols removal.

Removal efficiencies of seven frequently detected antibiotics and related physiological responses in three microalgae species

The main objective of this study is to investigate the effect of mixtures of seven widely used human antibiotics (ciprofloxacin, clarithromycin, erythromycin, metronidazole, ofloxacin, sulfamethoxazole, and trimethoprim) on the growth, pH, pigment production, and antibiotics removal of three microalgal species (Auxenochlorella protothecoides, Tetradesmus obliquus, and Chlamydomonas acidophila). Batch assays were conducted with media with antibiotic mixtures at 10, 50, and 100 μg L-1 for each antibiotic. The three microalgae species effectively removed the antibiotics without any growth inhibition, even when exposed to the highest antibiotic concentrations. Biosorption was reported as the primary mechanism for ciprofloxacin, clarithromycin, metronidazole, and ofloxacin, with up to 70% removal, especially in A. protothecoides and C. acidophila. A. protothecoides, a species never investigated for antibiotic removal, was the only microalgae exhibiting bioaccumulation and biodegradation of specific antibiotics, including sulfamethoxazole. Furthermore, in media with the highest antibiotic concentration, all three species exhibited increased chlorophyll (up to 37%) and carotenoid (up to 32%) production, accompanied by a pH decrease of 3 units. Generally, in the present study, it has been observed that physiological responses and the removal of antibiotics by microalgae are interlinked and contingent on the antibiotic levels and types.

Suitability of pre-digested dairy effluent for mixotrophic cultivation of the hydrogen-producing microalgae Tetraselmis subcordiformis

The costs associated with microalgal biomass production can be reduced by leveraging alternative and cheap growth media. Digestate from fermentation reactors is a particularly interesting candidate for use in cultivating mixotrophic species. The aim of the present study was to assess whether pre-digested milk-industry effluent can be harnessed to grow Tetraselmis subcordiformis and produce hydrogen. The experimental series with 25% and 50% effluent in the growth medium performed the best, producing more than 2000 mgVS biomass/dm3. The biogas produced in these variants contained over 60% hydrogen. Increasing the effluent in the medium to 75% led to significant deterioration of performance, both in terms of T. subcordiformis biomass growth and biohydrogen production. The highest efficiency of nitrogen and phosphorus removal, respectively 98.1 ± 1.9% and 97.1 ± 1.4%, was observed in the system to which 25% of sewage was introduced. Increasing the share of fermented wastewater directly reduced the efficiency of removing biogenic compounds. A very strong negative correlation was found between initial N-NH4 in the growth medium and T. subcordiformis biomass production rates (R2 = 0.9177).

Utilization of microalgal-bacterial energy nexus improves CO2 sequestration and remediation of wastewater pollutants for beneficial environmental services

Carbon dioxide (CO2) emissions from the combustion of fossil fuels and coal are primary contributors of greenhouse gases leading to global climate change and warming. The toxicity of heavy metals and metalloids in the environment threatens ecological functionality, diversity and global human life. The ability of microalgae to thrive in harsh environments such as industrial wastewater, polluted lakes, and contaminated seawaters presents new, environmentally friendly, and less expensive CO2 remediation solutions. Numerous microalgal species grown in wastewater for industrial purposes may absorb and convert nitrogen, phosphorus, and organic matter into proteins, oil, and carbohydrates. In any multi-faceted micro-ecological system, the role of bacteria and their interactions with microalgae can be harnessed appropriately to enhance microalgae performance in either wastewater treatment or algal production systems. This algal-bacterial energy nexus review focuses on examining the processes used in the capture, storage, and biological fixation of CO2 by various microalgal species, as well as the optimized production of microalgae in open and closed cultivation systems. Microalgal production depends on different biotic and abiotic variables to ultimately deliver a high yield of microalgal biomass.

A critical review on phycoremediation of pollutants from wastewater-a novel algae-based secondary treatment with the opportunities of production of value-added products

Though the biological treatment employing bacterial strains has wide application in effluent treatment plant, it has got several limitations. Researches hence while looking for alternative biological organisms that can be used for secondary treatment came up with the idea of using microalgae. Since then, a large number of microalgal/cyanobacterial strains have been identified that can efficiently remove pollutants from wastewater. Some researchers also found out that the algal biomass not only acts as a carbon sink by taking up carbon dioxide from the atmosphere and giving oxygen but also is a renewable source of several value-added products that can be extracted from it for the commercial use. In this work, the cleaning effect of different species of microalgae/cyanobacteria on wastewater from varied sources along with the value-added products obtained from the algal biomass as observed by researchers during the past few years are reviewed. While a number of review works in the field of phycoremediation technology was reported in literature, a comprehensive study on phycoremediation of wastewater from different industries and household individually is limited. In the present review work, the efficiency of diverse microalgal/cyanobacterial strains in treatment of wide range of industrial effluents along with municipal wastewater having multi-pollutants has been critically reviewed.

Removal of organic and inorganic contaminants from the air, soil, and water by algae

Rapid increases in human populations and development has led to a significant exploitation of natural resources around the world. On the other hand, humans have come to terms with the consequences of their past mistakes and started to address current and future resource utilization challenges. Today's primary challenge is figuring out and implementing eco-friendly, inexpensive, and innovative solutions for conservation issues such as environmental pollution, carbon neutrality, and manufacturing effluent/wastewater treatment, along with xenobiotic contamination of the natural ecosystem. One of the most promising approaches to reduce the environmental contamination load is the utilization of algae for bioremediation. Owing to their significant biosorption capacity to deactivate hazardous chemicals, macro-/microalgae are among the primary microorganisms that can be utilized for phytoremediation as a safe method for curtailing environmental pollution. In recent years, the use of algae to overcome environmental problems has advanced technologically, such as through synthetic biology and high-throughput phenomics, which is increasing the likelihood of attaining sustainability. As the research progresses, there is a promise for a greener future and the preservation of healthy ecosystems by using algae. They might act as a valuable tool in creating new products.

Biotic countermeasures that rescue Nannochloropsis gaditana from a Bacillus safensis infection

The natural assemblage of a symbiotic bacterial microbiome (bacteriome) with microalgae in marine ecosystems is now being investigated as a means to increase algal productivity for industry. When algae are grown in open pond settings, biological contamination causes an estimated 30% loss of the algal crop. Therefore, new crop protection strategies that do not disrupt the native algal bacteriome are needed to produce reliable, high-yield algal biomass. Bacteriophages offer an unexplored solution to treat bacterial pathogenicity in algal cultures because they can eliminate a single species without affecting the bacteriome. To address this, we identified a highly virulent pathogen of the microalga Nannochloropsis gaditana, the bacterium Bacillus safensis, and demonstrated rescue of the microalgae from the pathogen using phage. 16S rRNA amplicon sequencing showed that phage treatment did not alter the composition of the bacteriome. It is widely suspected that the algal bacteriome could play a protective role against bacterial pathogens. To test this, we compared the susceptibility of a bacteriome-attenuated N. gaditana culture challenged with B. safensis to a N. gaditana culture carrying a growth-promoting bacteriome. We showed that the loss of the bacteriome increased the susceptibility of N. gaditana to the pathogen. Transplanting the microalgal bacteriome to the bacteriome-attenuated culture reconstituted the protective effect of the bacteriome. Finally, the success of phage treatment was dependent on the presence of beneficial bacteriome. This study introduces two synergistic countermeasures against bacterial pathogenicity in algal cultures and a tractable model for studying interactions between microalgae, phages, pathogens, and the algae microbiome.

Design and modelling of photobioreactor for the treatment of carpet and textile effluent using Diplosphaera mucosa VSPA

The current study investigated the potential of one less explored microalgae species, Diplosphaera mucosa VSPA, for treating carpet and textile effluent in a conventionally designed 10 L bubble column photobioreactor. To the best of our knowledge, this is the first study to evaluate COD (chemical oxygen demand) removal efficiency by microalgae in carpet effluent. To evaluate D. mucosa VSPA's potential, its growth and bioremediation efficacy were compared to those of a well-known strain, Chlorella pyrenoidosa. D. mucosa VSPA outperformed C. pyrenoidosa in both effluents, with the highest biomass concentration reaching 4.26 and 3.98 g/L in carpet and textile effluent, respectively. D. mucosa VSPA also remediated 94.0% of ammonium nitrogen, 71.6% of phosphate phosphorus, and 91.9% of chemical oxygen demand in carpet effluent, approximately 10% greater than that of C. pyrenoidosa. Both species also removed more than 65% of colour from both effluents, meeting the standard set by governing bodies. Microalgae growth and substrate removal patterns in the photobioreactor were simulated using photobiotreatment and the Gompertz model. Simulation results revealed that photobiotreatment was the better-fit model, concluded based on the coefficient of regression value and the second-order Akaike information criterion test. Modelling studies can assist in increasing the performance and scale-up of the photobioreactor.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03655-3.

Microalgae: A green eco-friendly agents for bioremediation of tannery wastewater with simultaneous production of value-added products

Tannery wastewater (TWW) has high BOD, COD, TS and variety of pollutants like chromium, formaldehydes, biocides, oils, chlorophenols, detergents and phthalates etc. Besides these pollutants, TWW also rich source of nutrients like nitrogen, phosphorus, carbon and sulphur etc. that can be utilized by microalgae during their growth. Direct disposal of TWW into the environment may lead severe environmental and health threats, therefore it needs to be treated adequately. Microalgae are considered as an efficient microorganisms (fast growing, adaptability and strain robustness, high surface to volume ratio, energy saving) for remediation of wastewaters with simultaneous biomass recovery and generation of value added products (VAPs) such as biofuels, biohydrogen, biopolymer, biofertilizer, pigments, bioethanol, bioactive compounds, nutraceutical etc. Most microalgae are photosynthetic and use CO₂ and light energy to synthesise carbohydrate and reduces the emission of greenhouse gasses. Microalgae are also reported to remove heavy metals and antibiotics from wastewaters by bioaccumulation, biodegradation and biosorption. Microalgal treatment can be an alternative of conventional processes with generation of VAPs. The use of biotechnology in wastewater remediation with simultaneous generation of VAPs is trending. The validation of economic viability and environmental sustainability, life cycle assessment studies and techno-economic analysis is undergoing. Thus, in this review, the characteristics of TWW and microalgae are summarized, which manifest microalgae as potential candidates for wastewater remediation with simultaneous production of VAPs. Further, the treatment mechanisms, various factors (physical, chemical, mechanical and biological etc.) affecting treatment efficiency as well as challenges associated with microalgal remediation are also discussed.

Reformation of dairy effluent-a phycoremediation approach

Microalgae are a unique renewable resource utilized since ages, serving as a reservoir for the production of various metabolites. In this study, dairy waste water (DWW) is used as the nutrient media for the cultivation of microalgae. This study focuses on the phycoremediation process of converting rich nutrients in the effluent into biomass and removing contaminants using microalgae. The specific growth rate reached the maximum of 0.55 day^-1 in Desmococcus olivaceous, followed by 0.39 day^-1 for Scenedesmus dimorphus, 0.23 day^-1 in DCS (consortia composing all three strains in equal ratio), and lastly 0.22 day^-1 in Chlorella vulgaris. The biomass productivity was 1.44 g L^-1 day^-1, 1.06 g L^-1 day^-1, 0.88 g L^-1 day^-1, and 0.65 g L^-1 day^-1 in D. olivaceous, S. dimorphus, C. vulgaris, and DCS, respectively. The COD and BOD removal percentage was 82.85% and 45.40% in D. olivaceous, 81.98% and 44.25% in C. vulgaris, 80.73% and 53.45% in S. dimorphus, and 80.10% and 43.10% in DCS, respectively. These results emphasize the promising role of algae in dairy effluent treatment, highlighting the effluent as a suitable medium for microalgae cultivation. It verifies the circular bio-economy concept where the treated wastewater is converted into value-added products.

Chemical composition and species identification of microalgal biomass grown at pilot-scale with municipal wastewater and CO2 from flue gases

The production potential of a locally isolated Chlorella vulgaris strain and a local green-algae consortium, used in municipal wastewater treatment combined with CO₂ sequestration from flue gases, was evaluated for the first time by comparing the elemental and biochemical composition and heating value of the biomass produced. The microalgae were grown in outdoor pilot-scale ponds under subarctic summer conditions. The impact of cultivation in a greenhouse climate was also tested for the green-algae consortium; additionally, the variation in species composition over time in the three ponds was investigated. Our results showed that the biomass produced in the consortium/outdoor pond had the greatest potential for bioenergy production because both its carbohydrates and lipids contents were significantly higher than the biomasses from the consortium/greenhouse and C. vulgaris/outdoor ponds. Although greenhouse conditions significantly increased the consortium biomass's monounsaturated fatty acid content, which is ideal for biodiesel production, an undesirable increase in ash and chemical elements, as well as a reduction in heating value, were also observed. Thus, the placement of the pond inside a greenhouse did not improve the production potential of the green-algae consortium biomass in the current study infrastructure and climate conditions.

Interaction of Scenedesmus quadricauda and native bacteria in marine biopharmaceutical wastewater for desirable lipid production and wastewater treatment

Improving knowledge of the alga-bacterium interaction can promote the wastewater treatment. The untreated marine biopharmaceutical wastewater (containing native bacteria) was used directly for culturing microalgae. Unlike previous studies on specific bacteria in algal-bacterial co-culture systems, the effect of native bacteria in wastewater on microalgae growth was investigated in this study. The results showed that the coexistence of native bacteria greatly promoted the microalgae growth, ultimately producing biomass of 0.64 g/L and biomass productivity of 56.18 mg/L·d. Moreover, the lipid accumulation in the algae + bacteria group was 1.31 and 1.13 times higher than those of BG11 and pure algae, respectively, mainly attributed to the fact that bacteria provided a good environment for microalgae growth by using extracellular substances released from microalgae for their own growth, and providing micromolecules of organic matter and other required elements to microalgae. This study would lay the theoretical foundation for improving biopharmaceutical wastewater treatment.

Influence of inoculation ratio on the performance and microbial community of bacterial-algal symbiotic system for rural wastewater treatment

In this study, co-culture of microalgae and activated sludge in photobioreactors (PBRs) was investigated at different inoculation ratios (0:1, 0.3:1, 0.7:1, and 1.3:1 sludge wt./algae wt.) for rural domestic wastewater treatment under direct solar radiation. Effluent qualities (such as pH, NO₂ ^- , PO₄ ^3- , and NH₄ ⁺ -N concentrations) were assessed; bacterial and microalgal communities in co-culture system were compared. The microalgal and bacterial biomass fraction played a significant role in the performance and microbial community structure of the treatment system. In reactors with inoculation ratio of 0.3:1 and 0.7:1, the pH exceeded 9 or 10 under solar radiation, which led to some functional bacteria being missing. In the reactor with inoculation ratio of 1.3:1, activated sludge effectively prevented excessive increase in pH in the reactor. Similar observations were made for reactors with inoculation ratios below 1.3:1 by adding sludge halfway through the process. The results show that activated sludge can inhibit excessive increase in pH caused by algal photosynthesis, maintain the activity of nitrite-oxidizing bacteria in PBR, and reduce algae loss with the effluent. PRACTITIONER POINTS: Appropriate fraction of activated sludge can effectively inhibit the excessive increase in pH caused by algal photosynthesis in PBR. Adding activated sludge could maintain the activity of nitrite-oxidizing bacteria in PBR, and reduce microalgae loss with the effluent. Considering the stability of operation and biodiversity in PBR, a 1.3:1 inoculation ratio of activated sludge and microalgae is preferred.

Simultaneous bioremediation of Disperse orange-2RL Azo dye and fatty acids production by Scenedesmus obliquus cultured under mixotrophic and heterotrophic conditions

Several types of green photosynthetic microalgae can grow through the process of heterotrophic growth in the dark with the help of a carbon source instead of the usual light energy. Heterotrophic growth overcomes important limitations in the production of valuable products from microalgae, such as the reliance on light, which complicates the process, raises costs, and lowers the yield of potentially useful products. The present study was conducted to explore the potential growth of green microalga Scenedesmus obliquus under mixotrophic and heterotrophic conditions utilizing Disperse orange 2RL Azo dye as a carbon source to produce a high lipid content and the maximum dye removal percentage. After 7 days of algal growth with dye under mixotrophic and heterotrophic conditions with varying pH levels (5, 7, 9, and 11), KNO₃ concentrations (1, 1.5, 2, and 3 g/L), and dye concentrations (20, 40, and 60 ppm); dye removal percentage, algal dry weight, and lipid content were determined. The results showed that the highest decolorization of Disperse orange 2RL Azo dye (98.14%) was attained by S. obliquus in heterotrophic medium supplemented with glucose at the optimal pH 11 when the nitrogen concentration was 1 g/L and the dye concentration was 20 ppm. FT-IR spectroscopy of the dye revealed differences in peaks position and intensity before and after algal treatment. S. obliquus has a high concentration of oleic acid, which is enhanced when it is grown with Disperse orange 2RL Azo dye, making it ideal for production of high-quality biodiesel. In general, and in the vast majority of instances, heterotrophic cultivation is substantially less expensive, easier to set up, and requires less maintenance than mixotrophic cultivation. Heterotrophic cultivation allows for large-scale applications such as separate or mixed wastewater treatment along with biofuel production.

Maximizing Nitrogen Removal and Lipid Production by Microalgae under Mixotrophic Growth Using Response Surface Methodology: Towards Enhanced Biodiesel Production

The present study aimed to optimize synthetic wastewater composition as a mixotrophic medium for enhanced growth and lipid accumulation coupled with high nitrogen removal by the green microalga Chlorella sp. Individual effects of the three main independent variables (nitrate concentration, seawater ratio, and glycerol supplementation) were tested initially, then response surface methodology (RSM) was subsequently performed to explore the optimum combined conditions. The highest lipid productivity of 37.60 mg/L day was recorded at 25% seawater. Glycerol supplementation enhanced both lipid content and biomass production, which resulted in the highest recorded lipid productivity of 42.61 mg/L day at 4 g/L glycerol. Central composite design followed by numerical optimization was further applied which suggested NaNO3 concentration at 101.5 mg/L, seawater ration of 23.8%, and glycerol supplementation of 0.25 g/L as the optimum conditions for dual maximum lipid productivity and nitrogen removal of 46.9 mg/L day and 98.0%, respectively. Under the optimized conditions, dry weight and lipid content increased by 31.9% and 20.3%, respectively, over the control, which resulted in increase in lipid productivity by 71.5%. In addition, optimization process resulted in pronounced changes in fatty acid proportions where saturated fatty acids increased by 7.4% in the optimized culture with simultaneous reduction of polyunsaturated fatty acids. The estimated biodiesel characteristics calculated from the fatty acid methyl ester (FAMEs) profile showed agreement with the international standards, while optimized cultures showed an 8.5% lower degree of unsaturation, which resulted in higher cetane numbers and lower iodine values. This study provides economical approach for optimization and efficient nutrient recycling through cultivation of Chlorella sp. for further enhanced biodiesel production.

Enhancing Bioenergy Production from the Raw and Defatted Microalgal Biomass Using Wastewater as the Cultivation Medium

Improving the efficiency of using energy and decreasing impacts on the environment will be an inevitable choice for future development. Based on this direction, three kinds of medium (modified anaerobic digestion wastewater, anaerobic digestion wastewater and a standard growth medium BG11) were used to culture microalgae towards achieving high-quality biodiesel products. The results showed that microalgae culturing with anaerobic digestate wastewater could increase lipid content (21.8%); however, the modified anaerobic digestion wastewater can boost the microalgal biomass production to 0.78 ± 0.01 g/L when compared with (0.35-0.54 g/L) the other two groups. Besides the first step lipid extraction, the elemental composition, thermogravimetric and pyrolysis products of the defatted microalgal residues were also analysed to delve into the utilisation potential of microalgae biomass. Defatted microalgae from modified wastewater by pyrolysis at 650 °C resulted in an increase in the total content of valuable products (39.47%) with no significant difference in the content of toxic compounds compared to other groups. Moreover, the results of the life cycle assessment showed that the environmental impact (388.9 mPET2000) was lower than that of raw wastewater (418.1 mPET2000) and standard medium (497.3 mPET2000)-cultivated groups. Consequently, the method of culturing microalgae in modified wastewater and pyrolyzing algal residues has a potential to increase renewable energy production and reduce environmental impact.

Microalgae-based wastewater treatment for developing economic and environmental sustainability: Current status and future prospects

Over the last several decades, concerns about climate change and pollution due to human activity has gained widespread attention. Microalgae have been proposed as a suitable biological platform to reduce carbon dioxide, a major greenhouse gas, while also creating commercial sources of high-value compounds such as medicines, cosmetics, food, feed, and biofuel. Industrialization of microalgae culture and valorization is still limited by significant challenges in scaling up the production processes due to economic constraints and productivity capacities. Therefore, a boost in resource usage efficiency is required. This enhancement not only lowers manufacturing costs but also enhancing the long-term viability of microalgae-based products. Using wastewater as a nutrient source is a great way to reduce manufacturing costs. Furthermore, water scarcity is one of the most important global challenges. In recent decades, industrialization, globalization, and population growth have all impacted freshwater resources. Moreover, high amounts of organic and inorganic toxins in the water due to the disposal of waste into rivers can have severe impacts on human and animal health. Microalgae cultures are a sustainable solution to tertiary and quaternary treatments since they have the ability to digest complex contaminants. This review presents biorefineries based on microalgae from all angles, including the potential for environmental pollution remediation as well as applications for bioenergy and value-added biomolecule production. An overview of current information about microalgae-based technology and a discussion of the associated hazards and opportunities for the bioeconomy are highlighted.

Current perspective on wastewater treatment using photobioreactor for Tetraselmis sp.: an emerging and foreseeable sustainable approach

Urbanization is a revolutionary and necessary step for the development of nations. However, with development emanates its drawback i.e., generation of a huge amount of wastewater. The existence of diverse types of nutrient loads and toxic compounds in wastewater can reduce the pristine nature of the ecosystem and adversely affects human and animal health. The conventional treatment system reduces most of the chemical contaminants but their removal efficiency is low. Thus, microalgae-based biological wastewater treatment is a sustainable approach for the removal of nutrient loads from wastewater. Among various microalgae, Tetraselmis sp. is a robust strain that can remediate industrial, municipal, and animal-based wastewater and reduce significant amounts of nutrient loads and heavy metals. The produced biomass contains lipids, carbohydrates, and pigments. Among them, carbohydrates and lipids can be used as feedstock for the production of bioenergy products. Moreover, the usage of a photobioreactor (PBR) system improves biomass production and nutrient removal efficiency. Thus, the present review comprehensively discusses the latest studies on Tetraselmis sp. based wastewater treatment processes, focusing on the use of different bioreactor systems to improve pollutant removal efficiency. Moreover, the applications of Tetraselmis sp. biomass, advancement and research gap such as immobilized and co-cultivation have also been discussed. Furthermore, an insight into the harvesting of Tetraselmis biomass, effects of physiological, and nutritional parameters for their growth has also been provided. Thus, the present review will broaden the outlook and help to develop a sustainable and feasible approach for the restoration of the environment.

Phycoremediation of milk processing wastewater and lipid-rich biomass production using Chlorella vulgaris under continuous batch system

This study compiles the results of phycoremediation of milk processing wastewater (MPWW) and production of lipid-rich Chlorella vulgaris biomass using a continuous batch system operated for 12-wks. After a 4-wks interval, a new MPWW was loaded photobioreactor to provide appropriate nutrient supply to algae. Results indicated that MPWW supported the algal growth efficiently and the maximum algal growth was recorded in the ranges of 400.36 to 421.58 mg L^-1 during 4-wk's of the cultivation cycle. Average reduction in total nitrogen, TN (45.82-69.18%); nitrate, NO₃ (93.32-94.54%); total ammonium nitrogen, TAN (92.94-94.54%); sulphate, SO₄^-2 (85.13-87.34%); total phosphorus (75.09-78.78%); and biochemical oxygen demands, BOD (89.53-92.40%) was recorded during 12-wks phycoremediation of MPWW. Harvested algal biomass (dry weight basis, DW) exhibited a significant content of total sugar (45.5%) and total lipid (39.7%). The lipid profiling results indicated the presence of palmitic acid (39.9%), oleic acid (21.08%), linoleic acid (13.13%), and other C18 compounds in algal biomass, suggesting the suitability of MPWW for Chlorella vulgaris cultivations. Algal biomass exhibited a high heating value (MJ/Kg of DW) in the range of 17.3 to 25.1, comparable to other lignocellulose biomass to be used for bioenergy purposes. Results of this study indicate that MPWW could be utilized as a valuable medium for Chlorella vulgaris cultivation under a circular economy approach: wastewater treatment and bioenergy feedstock production. The effect of controlled environmental conditions on algal growth behavior and lipid composition in biomass, while using MPWW as a medium, could be investigated in future studies.

Identification and Characterization of a New Microalga Dysmorphococcus globosus-HI from the Himalayan Region as a Potential Source of Natural Astaxanthin

Synthesized astaxanthin (ASX), stereoisomers of 3S,3′R, 3R,3′R, and 3S,3′S, have over 95% market share and have relatively poor antioxidant and bioactivity properties, with persistent issues in terms of biological functions, health benefits, and biosafety if compared to natural ASX. Bioprospecting of new microalgal strains could be vital for a new source of powerful antioxidant (ASX). In this study, a new algal strain was isolated from the Indian foothills of the Himalayas. Its identity was discerned by morphological and DNA barcode studies. It is a unicellular spheroidal cell-shaped alga with 100–200 μm diameter. The isolate has 93.4% similarity to Dysmorphococcus globosus species based on 18S-rDNA phylogenetic analysis and named as D. globosus-HI (HI stands for Himalayan India). Its growth and major cellular components (carotenoids, carbohydrates, protein, lipids, fatty acid profile, and ASX) were optimized using the seven different culture media. The highest biomass (1.14 g L−1) was observed in the MBBM medium, with a specific growth rate (0.087 day−1), division/day (0.125), and cellular yield (6.16 x 106 cells/mL). The highest carotenoids (1.56 mg g−1), lipids (32.5 mg L−1), and carbohydrates (135.62 mg L−1) were recorded in the 3N-BBM medium. The maximum ω3-FAs (17.78%), ω6-FAs (23.11%), and ω9-FAs (7.06%) were observed in MBBM, JW, and BG-11 medium respectively. The highest amount of antioxidant ASX was accumulated in the 3N-BBM medium (391 mg L−1). It is more than any other known algal species used in the production of natural ASX. The optimized biochemical studies on the D. globosus-HI strain should fulfill the increasing demand for natural ASX for commercial application.

Simultaneous biological removal of nitrogen and phosphorus from secondary effluent of wastewater treatment plants by advanced treatment: A review

With the advancement of water ecological protection and water control standard, it is the general trend to upgrade the wastewater treatment plants (WWTPs). The simultaneous removal of nitrogen and phosphorus is the key to improve the water quality of secondary effluent of WWTPs to prevent the eutrophication. Therefore, it is urgent to develop the applicable technologies for simultaneous biological removal of nitrogen and phosphorus from secondary effluent. In this review, the composition of secondary effluent from municipal WWTPs were briefly introduced firstly, then the three main treatment processes for simultaneous nitrogen and phosphorus removal, i.e., the enhanced denitrifying phosphorus removal filter, the pyrite-based autotrophic denitrification and the microalgae biological treatment system were summarized, their performances and mechanisms were analyzed. The influencing factors and microbial community structure were discussed. The advanced removal of nitrogen and phosphorus by different technologies were also compared and summarized in terms of performance, operational characteristics, disadvantage and cost. Finally, the challenges and future prospects of simultaneous removal of nitrogen and phosphorus technologies for secondary effluent were proposed. This review will deepen to understand the principles and applications of the advanced removal of nitrogen and phosphorus and provide some valuable information for upgrading the treatment process of WWTPs.

Novel microalgae strains from selected lower Himalayan aquatic habitats as potential sources of green products

Microalgal biomass provides a renewable source of biofuels and other green products. However, in order to realize economically viable microalgal biorefinery, strategic identification and utilization of suitable microalgal feedstock is fundamental. Here, a multi-step suboptimal screening strategy was used to target promising microalgae strains from selected freshwaters of the study area. The resulting strains were found to be affiliated to seven closely-related genera of the family Scenedesmaceae, as revealed by both morphologic and molecular characterization. Following initial screening under upper psychrophilic to optimum mesophilic (irregular temperature of 14.1 to 35.9°C) cultivation conditions, superior strains were chosen for further studies. Further cultivation of the selected strains under moderate to extreme mesophilic cultivation conditions (irregular temperature of 25.7 to 42.2°C), yielded up to 74.12 mgL^-1day^-1, 19.96 mgL^-1day^-1, 48.56%, 3.34 μg/mL and 1.20 μg/mL, for biomass productivity, lipid productivity, carbohydrate content, pigments content and carotenoids content respectively. These performances were deemed promising compared with some previous, optimum conditions-based reports. Interestingly, the fatty acids profile and the high carotenoids content of the studied strains revealed possible tolerance to the stress caused by the changing suboptimal cultivation conditions. Overall, strains AY1, CM6, LY2 and KL10 were exceptional and may present sustainable, promising feedstock for utilization in large-scale generation of green products, including biodiesel, bioethanol, pigments and dietary supplements. The findings of this study, which exposed promising, eurythermal strains, would expand the current knowledge on the search for promising microalgae strains capable of performing under the largely uncontrolled large-scale cultivation settings.

Attached cultivation of microalgae on rational carriers for swine wastewater treatment and biomass harvesting

Attachment effects of six carrier materials on the cultivation of high-value microalga Scenedesmus sp. LX1 in diluted swine wastewater were investigated. The results showed that when the initial algal densities were 5×10⁵ cells/mL and 1×10⁶ cells/mL in sterilized wastewater with 20-fold dilution, the biomass of microalgae attaching to geotextile was the highest. The contact angle and surface structure of the material affected the attachment of microalgae. Further, among the four dilutions, the highest attached biomass on geotextile was 414.47 mg/L in the sterilized wastewater at 20-fold, but the pollutants removal rate and attached biomass were higher in the non-sterile wastewater in the other three dilutions (original wastewater, 5-fold, 10-fold). Next, the microalgae were able to remove pollutants with the highest removal rates of COD, TN, NH₄⁺-N and TP reaching to 86.92%, 60.75%, 71.81% and 96.13%. Moreover, the microalga was found to accumulate high-value products especially protein as high as 44.57%.

Potential of the microalgae Chlorella fusca (Trebouxiophyceae, Chlorophyta) for biomass production and urban wastewater phycoremediation

Abstract

The present work focuses on: (1) the evaluation of the potential of Chlorella fusca to grow and synthesize metabolites of biotechnological interest, after being exposed for fourteen days to urban wastewater (UW) from Malaga city (UW concentrations: 25%, 50%, 75%, and 100%); (2) the study of the capacity of C. fusca to bioremediate UW in photobioreactors at laboratory scale; and (3) the evaluation of the effect of UW on the physiological status of C. fusca, as photosynthetic capacity by using in vivo Chl a fluorescence related to photosystem II and the production of photosynthetic pigments. C. fusca cell density increased in treatments with 50% UW concentration, followed by the treatment with 100% UW, 75% UW, the control, and finally 25% UW. Protein content increased to 50.5% in 75% UW concentration. Stress induced to microalgal cultures favored the increase of lipid production, reaching a maximum of 16.7% in 100% UW concentration. The biological oxygen demand (BOD₅) analysis indicated a 75% decrease in 100% UW concentration. Dissolved organic carbon (DOC) levels decreased by 41% and 40% in 50% UW and 100% UW concentration, and total nitrogen (TN) decreased by 55% in 50% UW concentration. The physiological status showed the stressful effect caused by the presence of UW on photosynthetic activity, with increasing impact as UW concentration grew. In the framework of circular economy, we seek to deepen this study to use the biomass of C. fusca to obtain metabolites of interest for biofuel production and other biotechnological areas.

Graphical Abstract

Chlorella fusca grows and synthesizes metabolites of biotechnological interest in UW

Protein and lipid content increased to 50.5% and 16.7% in 75% UW and 100% UW concentrations

TN, DOC, and BOD₅ reduction efficiency increased with the exposure time.

Recovery of value-added products by mining microalgae

Microalgae blooms are always blamed for the interruption of the aquatic environment and pose a risk to the source of drinking water. Meanwhile, microalgae as primary producers are a kind of resource pool and could benefit the environment and contribute to building a circular economy. The lipid and polyhydroxybutyrate (PHB) in the cells of microalgae could be alternatives to fossil fuels and plastics, respectively, which are the culprits of global warming and plastic pollution. Besides, some microalgae are rich in nutrients, such as proteins and astaxanthin, which make themselves suitable for feed additives. As wastewater is rich in nutrients necessary for microalgae, thus, value-added product recovery via microalgae could be an approach to valorizing wastewater. However, a one-size-fits-all approach deploying various wastewater for the above products cannot be summarized. On the contrary, specific technical protocols should be tailored regarding each product in microalgae biomass with various wastewater. Thus, this review is to summarize the research effort by far on wastewater-cultivated microalgae for value-added products. Wastewater type, regulation methods, and targeted product yields are compiled and discussed and are expected to guide future extrapolation into a commercial scale.

Microalgae as a solution of third world energy crisis for biofuels production from wastewater toward carbon neutrality: An updated review

The boost of the greenhouse gases (GHGs, largely carbon dioxide - CO₂) emissions owing to anthropogenic activity is one of the biggest global threats. Bio-CO₂ emission reduction has received more and more attention as an environmentally sustainable approach. Microalgae are very popular in this regard because of excellent speed of growth, low costs of production, and resistance to extreme environments. Besides, most microalgae can undergo photosynthesis, where the CO₂ and solar energy can be converted into sugar, and subsequently become biomass, providing a renewable and promising biofuel strategy with a few outstanding benefits. This review focuses on presenting CO₂ sequestration by microalgae towards wastewater treatment and biodiesel production. First, the CO₂ fixation mechanism by microalgae viz., sequestration and assimilation of CO₂ in green microalgae as well as cyanobacteria were introduced. Besides, factors affecting CO₂ sequestration in microalgae, containing microalgae species and cultivation conditions, such as light condition, photobioreactor, configuration, pH, CO₂ concentration, temperature, and medium composition, were then comprehensively discussed. Special attention was given to the production of biodiesel as third-generation biofuel from various wastewater (CO₂ biofixation), including processing steps of biodiesel production by microalgae, biodiesel production from wastewater, and improved methods. Furthermore, current life cycle assessment (LCA) and techno-economic analysis (TEA) used in biodiesel production were discussed. Finally, the research challenges and specific prospects were considered. Taken together, this review provides useful and updated information to facilitate the development of microalgal "green chemistry" and "environmental sustainability".

Microalgal-based removal of contaminants of emerging concern

The presence of contaminants of emerging concern (CECs) in the environment has been recognized as a worldwide concern. In particular, water pollution by CECs is becoming a major global problem, which requires ongoing evaluation of water resources policies at all levels and the use of effective and innovative wastewaters treatment processes for their removal. Microalgae have been increasingly recognized as relevant for wastewater polishing, including CECs removal. These microorganisms are commonly cultivated in suspension. However, the use of planktonic microalgae for wastewater treatment has limitations in terms of microbiological contamination, process effectiveness and sustainability. The use of consortia of microalgae and bacteria represents a significant advance for sustainable wastewater polishing, particularly when the microorganisms are associated as biofilms. These immobilized mixed cultures can overcome the limitations of suspended-microalgae systems and improve the performance of the involved species for CECs removal. In addition, microalgae-bacteria based systems can offer a relevant combined effect for CECs removal and biomass production enhancement. This study reviews the advantages and advances on the use of microalgae for wastewater treatment, highlighting the potential on the use of microalgae-bacteria biofilms for CECs removal and the further biomass valorisation for third-generation biofuel production.

“Nature-like” Cryoimmobilization of Phototrophic Microorganisms: New Opportunities for Their Long-Term Storage and Sustainable Use

It was found that immobilization of cells in poly(vinyl alcohol) (PVA) cryogel can be successfully applied for concurrent cryoimmobilization, cryoconservation and long-term storage of the cells of various phototrophic microorganisms (green and red microalgae, diatoms and cyanobacteria). For the first time, it was shown for 12 different immobilized microalgal cells that they can be stored frozen for at least 18 months while retaining a high level of viability (90%), and can further be used as an inoculum upon defrosting for cell-free biomass accumulation. Application of cryoimmobilized Chlorella vulgaris cells as inocula allowed the loading of a high concentration of the microalgal cells into the media for free biomass accumulation, thus increasing the rate of the process. It was shown that as minimum of 5 cycles of reuse of the same immobilized cells as inocula for cell accumulation could be realized when various real wastewater samples were applied as media for simultaneous microalgae cultivation and water purification.

Algal-based system for removal of emerging pollutants from wastewater: A review

The bioremediation of emerging pollutants in wastewater via algal biotechnology has been emerging as a cost-effective and low-energy input technological solution. However, the algal bioremediation technology is still not fully developed at a commercial level. The development of different technologies and new strategies to cater specific needs have been studied. The existence of multiple emerging pollutants and the selection of microalgal species is a major concern. The rate of algal bioremediation is influenced by various factors, including accidental contaminations and operational conditions in the pilot-scale studies. Algal-bioremediation can be combined with existing treatment technologies for efficient removal of emerging pollutants from wastewater. This review mainly focuses on algal-bioremediation systems for wastewater treatment and pollutant removal, the impact of emerging pollutants in the environment, selection of potential microalgal species, mechanisms involved, and challenges in removing emerging pollutants using algal-bioremediation systems.

Feasibility of Utilizing Wastewaters for Large-Scale Microalgal Cultivation and Biofuel Productions Using Hydrothermal Liquefaction Technique: A Comprehensive Review

The two major bottlenecks faced during microalgal biofuel production are, (a) higher medium cost for algal cultivation, and (b) cost-intensive and time consuming oil extraction techniques. In an effort to address these issues in the large scale set-ups, this comprehensive review article has been systematically designed and drafted to critically analyze the recent scientific reports that demonstrate the feasibility of microalgae cultivation using wastewaters in outdoor raceway ponds in the first part of the manuscript. The second part describes the possibility of bio-crude oil production directly from wet algal biomass, bypassing the energy intensive and time consuming processes like dewatering, drying and solvents utilization for biodiesel production. It is already known that microalgal drying can alone account for ∼30% of the total production costs of algal biomass to biodiesel. Therefore, this article focuses on bio-crude oil production using the hydrothermal liquefaction (HTL) process that converts the wet microalgal biomass directly to bio-crude in a rapid time period. The main product of the process, i.e., bio-crude oil comprises of C16-C20 hydrocarbons with a reported yield of 50–65 (wt%). Besides elucidating the unique advantages of the HTL technique for the large scale biomass processing, this review article also highlights the major challenges of HTL process such as update, and purification of HTL derived bio-crude oil with special emphasis on deoxygenation, and denitrogenation problems. This state of art review article is a pragmatic analysis of several published reports related to algal crude-oil production using HTL technique and a guide towards a new approach through collaboration of industrial wastewater bioremediation with rapid one-step bio-crude oil production from chlorophycean microalgae.

Resource recovery through bioremediation of wastewaters and waste carbon by microalgae: a circular bioeconomy approach

Microalgal biomass-based biofuels are a promising alternative to fossil fuels. Microalgal biofuels' major obstacles are the water and carbon sources for their cultivation and biomass harvest from the liquid medium. To date, an economically viable process is not available for algal based biofuels. The circular bioeconomy is an attractive concept for reuse, reduce, and recycle resources. The recovery of nutrients from waste and effluents by microalgae could significantly impact the escalating demands of energy and nutraceutical source to the growing population. Wastewaters from different sources are enriched with nutrients and carbon, and these resources can be recovered and utilized for the circular bioeconomy approach. However, the utilization of wastewaters and waste seems to be an essential strategy for mass cultivation of microalgae to minimizing freshwater consumption, carbon, nutrients cost, nitrogen, phosphorus removal, and other pollutants loads from wastewater and generating sustainable biomass for value addition for either biofuels or other chemicals. Hence, the amalgamation of wastewater treatment with the mass cultivation of microalgae improved the conventional treatment process and environmental impacts. This review provides complete information on the latest progress and developments of microalgae as potential biocatalyst for the remediation of wastewaters and waste carbon to recover resources through biomass with metabolites for various industrial applications and large-scale cultivation in wastewaters, and future perspectives are discussed.

An approach for dairy wastewater remediation using mixture of microalgae and biodiesel production for sustainable transportation

The aim of this work is remediation of dairy wastewater (DWW) for biodiesel feedstock production using poly-microalgae cultures of four microalgae namely Chlorella minutissima (C. minutissima), Scenedesmus abundans (S. abundans), Nostoc muscorum (N. muscorum) and Spirulina sp. The poly-microalgae cultures were prepared as C. minutissima + N. muscorum (CN), C. minutissima + N. muscorum + Spirulina sp. (CNSS) and S. abundans + N. muscorum + Spirulina sp. (SNSS). Poly-microalgae culture CNSS cultivated on 70% DWW achieved 75.16, 61.37, 58.76, 84.48 and 84.58%, removals of biological oxygen demand (BOD), chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP), and suspended solids (SS), respectively, at 12:12 h photoperiod that resulted into total biomass and lipid yield of 3.47 ± 0.07 g/L and 496.32± 0.065 mg/L. However, maximum biomass and lipid yields of 5.76 ± 0.06 and 1152.37 ± 0.065 mg/L were achieved by poly-microalgae culture CNSS cultivated on 70% DWW + 10 g/L of glucose at 18:6 h photoperiod. Fatty acid methyl ester (FAME) analysis shown presence of C14:0 (myristic acid) C16:0 (palmitic acid), C16:1 (palmitoleic acid), C18:0 (stearic acid), C18:2 (linoleic acid) and C18:3 (linolenic acid), it indicates that the lipids produced from poly-microalgae cultures are suitable for biodiesel production. Thus, poly-microalgae cultures could be more efficient than mono-microalgae cultures in the remediation of DWW and for biodiesel feedstock production.

Biochemical wastewater from landfill leachate pretreated by microalgae achieving algae's self-reliant cultivation in full wastewater-recycling chain with desirable lipid productivity

Heightened awareness of additional pretreatment for wastewater, has driven studies towards building a full wastewater-recycling chain wherein the wastewater pretreatment is performed by microalgae themselves. We applied biochemical wastewater from landfill leachate with added K₂HPO₄ (BWLL + P) directly to microalgal cultivation. The results showed that the pretreatment provided by the 1st cultivation reduced suspended solids by nearly half, greatly boosting microalgal growth, which thus yielded 1.06 g/L of dry mass and 87.06 mg/L·d of biomass productivity. From the 2nd to the 4th cultivation, lipid accumulation in BWLL + P was 1.12-1.27 times and 1.95-2.36 times higher than in BG11 and BWLL, respectively, mainly attributed to the comfortable environment engendered by the microalgal pretreatment and the organic carbon in the wastewater. Strikingly, the biodiesel production fed with BWLL + P could save 99% of the cost compared with in BG11. In combination, our pioneering full wastewater-recycling chain achieved microalgae's self-reliant cultivation, with wastewater nourishment.

Wastewater treatment by microalgae

The growth of the world's population increases the demand for fresh water, food, energy, and technology, which in turn leads to increasing amount of wastewater, produced both by domestic and industrial sources. These different wastewaters contain a wide variety of organic and inorganic compounds which can cause tremendous environmental problems if released untreated. Traditional treatment systems are usually expensive, energy demanding and are often still incapable of solving all challenges presented by the produced wastewaters. Microalgae are promising candidates for wastewater reclamation as they are capable of reducing the amount of nitrogen and phosphate as well as other toxic compounds including heavy metals or pharmaceuticals. Compared to the traditional systems, photosynthetic microalgae require less energy input since they use sunlight as their energy source, and at the same time lower the carbon footprint of the overall reclamation process. This mini-review focuses on recent advances in wastewater reclamation using microalgae. The most common microalgal strains used for this purpose are described as well as the challenges of using wastewater from different origins. We also describe the impact of climate with a particular focus on a Nordic climate.

Microalgal Hydrogen Production in Relation to Other Biomass-Based Technologies—A Review

Hydrogen is an environmentally friendly biofuel which, if widely used, could reduce atmospheric carbon dioxide emissions. The main barrier to the widespread use of hydrogen for power generation is the lack of technologically feasible and—more importantly—cost-effective methods of production and storage. So far, hydrogen has been produced using thermochemical methods (such as gasification, pyrolysis or water electrolysis) and biological methods (most of which involve anaerobic digestion and photofermentation), with conventional fuels, waste or dedicated crop biomass used as a feedstock. Microalgae possess very high photosynthetic efficiency, can rapidly build biomass, and possess other beneficial properties, which is why they are considered to be one of the strongest contenders among biohydrogen production technologies. This review gives an account of present knowledge on microalgal hydrogen production and compares it with the other available biofuel production technologies.

A Comparative Study of Improvement of Phycoremediation Using a Consortium of Microalgae in Municipal Wastewater Treatment Pond Systems as an Alternative Solution to Africa’s Sanitation Challenges

The reuse of wastewater has been observed as a viable option to cope with increasing water stress in Africa. The present case studies evaluated the optimization of the process of phycoremediation as an alternative low-cost green treatment technology in two municipality wastewater treatment pond systems that make up the largest number of domestic sewage treatment systems on the African continent. A consortium of specific microalgae (Chlorella vulgaris and Chlorella protothecoides) was used to improve the treatment capacity of domestic wastewater at two operational municipality wastewater pond systems under different environmental conditions in South Africa. Pre- and post-phycoremediation optimization through mass inoculation of a consortium of microalgae, over a period of one year under different environmental conditions, were compared. It was evident that the higher reduction of total phosphates (74.4%) in the effluent, after treatment with a consortium of microalgae at the Motetema pond system, was possibly related to (1) the dominance of the algal taxa C. protothecoides (52%), and to a lesser extent C. vulgaris (36%), (2) more cloudless days, (3) higher air temperature, and (4) a higher domestic wastewater strength. In the case of the Brandwag pond treatment system, the higher reduction of total nitrogen can possibly be related to the dominance of C. vulgaris, different weather conditions, and lower domestic wastewater strength. The nutrient reduction data from the current study clearly presented compelling evidence in terms of the feasibility for use of this technology in developing countries to reduce nutrient loads from domestic wastewater effluent.

Self-Flocculation as an Efficient Method to Harvest Microalgae: A Mini-Review

The development of clean and renewable biofuels has been of wide concern on the topic of energy and environmental issues. As a kind of biomass energy with great application prospects, microalgae have many advantages and are used in the fields of environmental protection and biofuels as well as food or feed production for humans and animals. However, the high cost of microalgae harvesting is the main bottleneck of industrial production on a large scale. Self-flocculation is a cost-efficient and promising method for harvesting microalgal biomass. This article briefly describes the current commonly used technology for microalgae harvesting, focusing on the research progress of self-flocculation. This article explores the relative mechanisms and influencing factors of self-flocculation and discusses a proposal for the integration of algae cultivation and harvesting as well as the co-cultivation of algae and bacteria in an effort to provide a reference for microalgae harvesting with high efficiency and low cost.

A Recommendation for a Pre-Standardized Marine Microalgal Dry Weight Determination Protocol for Laboratory Scale Culture Using Ammonium Formate as a Washing Agent

Simple Summary

Microalgae are increasingly recognized as a source of valuable biomass with numerous health benefits. Cleaning of marine microalgal biomass is very crucial for microalgal studies as the salt on the microalgae cells will lead to overestimation of biomass determination. Incomplete washing of salt from microalgae could also interfere with the nutritional analyses. The biomass, especially dry weight, has been utilized for nutritional or compositional evaluation. Although standard methods of marine microalgal dry weight determination are available, these methods did not provide comprehensive details, and the parameters vary among themselves. Without a standard method, a comparison of results among previous studies can be misleading and unreliable. Therefore, the current study aimed to investigate and determine the ideal setting of several parameters in the marine microalgal dry weight determination for laboratory-scale culture. The present findings could assist in developing a standardized protocol to ensure a high quality of biomass for microalgal studies.

Abstract

Microalgal biomass is one of the crucial criteria in microalgal studies. Many reported methods, even the well-established protocol on microalgal dry weight (DW) determination, vary greatly, and reliable comparative assessment amongst published results could be problematic. This study aimed to determine the best condition of critical parameters in marine microalgal DW determination for laboratory-scale culture using four different marine microalgal species. These parameters included the washing process, grades of glass microfiber filter (GMF), GMF pretreatment conditions, washing agent (ammonium formate) concentrations, culture: washing agent ratios (v:v) and washing cycles. GMF grade GF/A with precombustion at 450 °C provided the most satisfactory DW and the highest ash-free dry weight (AFDW)/DW ratio. Furthermore, 0.05 M ammonium formate with 1:2 culture: washing agent ratio and a minimum of two washing cycles appeared to be the best settings of microalgal DW determination. The present treatment increased the AFDW/DW ratio of the four respective microalgae by a minimum of 19%. The findings of this study could serve as a pivotal reference in developing a standardized protocol of marine microalgal DW determination to obtain veracious and reliable marine microalgal DW.

Microalgal Production of Biofuels Integrated with Wastewater Treatment

Human civilization will need to reduce its impacts on air and water quality and reduce its use of fossil fuels in order to advance towards a more sustainable future. Using microalgae to treat wastewater as well as simultaneously produce biofuels is one of the approaches for a sustainable future. The manufacture of biofuels from microalgae is one of the next-generation biofuel solutions that has recently received a lot of interest, as it can remove nutrients from the wastewater whilst capturing carbon dioxide from the atmosphere. The resulting biomass are employed to generate biofuels, which can run fuel cell vehicles of zero emission, power combustion engines and power plants. By cultivating microalgae in wastewater, eutrophication can be prevented, thereby enhancing the quality of the effluent. Thus, by combining wastewater treatment and biofuel production, the cost of the biofuels, as well as the environmental hazards, can be minimized, as there is a supply of free and already available nutrients and water. In this article, the steps involved to generate the various biofuels through microalgae are detailed.

Microalgal growth, nitrogen uptake and storage, and dissolved oxygen production in a polyculture based-open pond fed with municipal wastewater in northern Sweden

Microalgal-based wastewater treatment and CO₂ sequestration from flue gases with subsequent biomass production represent a low-cost, eco-friendly, and effective procedure of removing nutrients and other pollutants from wastewater and assists in the decrease of greenhouse gas emissions. Thus, it supports a circular economy model. This is based on the ability of microalgae to utilise inorganic nutrients, mainly nitrogen and phosphorous, as well as organic and inorganic carbon, for their growth, and simultaneously reduce these substances in the water. However, the production of microalgae biomass under outdoor cultivation is dependent on several abiotic and biotic factors, which impact its profitability and sustainability. Thus, this study's goal was to evaluate the factors affecting the production of microalgae biomass on pilot-scale open raceway ponds under Northern Sweden's summer conditions with the help of a mathematical model. For this purpose, a microalgae consortium and a monoculture of Chlorella vulgaris were used to inoculate outdoor open raceway ponds. In line with the literature, higher biomass concentrations and nutrient removals were observed in ponds inoculated with the microalgae consortium. Our model, based on Droop's concept of macronutrient quotas inside the cell, corresponded well to the experimental data and, thus, can successfully be applied to predict biomass production, nitrogen uptake and storage, and dissolved oxygen production in microalgae consortia.

Insights into bioflocculation of filamentous cyanobacteria, microalgae and their mixture for a low-cost biomass harvesting system

Cyanobacteria and microalgae are considered as interesting feedstocks for either the production of high value bio-based compounds and biofuels or wastewater treatment. Nevertheless, the high costs of production, mainly due to the harvesting process, hamper a wide commercialization of industrial cyanobacteria and microalgae based products. Recent studies have found in autoflocculation and bioflocculation promising spontaneous processes for a low-cost and environmentally sustainable cyanobacteria and microalgae biomass harvesting process. In the present work, bioflocculation process has been studied for three different inocula: filamentous cyanobacteria, microalgae and their mixture. Their cultivation has been conducted in batch mode using two different cultivation media: synthetic aqueous solution and urban wastewater. The removal of nutrients and flocculation process performance were monitored during the entire cultivation time. Results have proved that bioflocculation and sedimentation processes occur efficiently for filamentous cyanobacteria cultivated in synthetic aqueous solution, whereas such processes are less efficient in urban wastewater due to the specific characteristics of this medium that prevent bioflocculation to occur. Besides different efficiencies associated to cultivation media, this work highlighted that bioflocculation of sole microalgae is not as effective as when they are cultivated together with filamentous cyanobacteria.

Towards high-quality biodiesel production from microalgae using original and anaerobically-digested livestock wastewater

In this study, we conducted proof-of-concept research towards the simultaneous treatment of livestock wastewater and the generation of high-quality biodiesel, through microalgae technology. Both original (OPE) and anaerobically-digested (DPE) piggery effluents were investigated for the culture of the microalgae, Desmodesmus sp. EJ8-10. After 14 days' cultivation, the dry biomass from microalgae cultivated in OPE increased from an initial value of 0.01 g/L to 0.33-0.39 g/L, while those growing in DPE only achieved a final dried mass of 0.15-0.35 g/L, under similar initial ammonium nitrogen (NH₄⁺-N) concentrations. The significantly higher microalgal biomass production achieved in the OPE medium may have been supported by the abundance of both macronutrient, such as phosphorus (P), and of micronutrients, such as trace elements, present in the OPE, which may not been present in similar quantities in the DPE. However, a higher lipid content was observed (19.4-28%) in microalgal cells from DPE cultures than those (18.7-22.3%) from OPE cultures. Moreover, the fatty acid compositions in the microalgae cultured in DPE contained high levels of monounsaturated fatty acids (MUFAs) and total C16-C18 acids, which would afford a superior potential for high-quality biodiesel production. The N/P ratio (15.4:1) in OPE was much closer to that indicated by previous studies to be the most suitable (16:1) for microalgae growth, when compared with that determined from the DPE culture medium. This may facilitate protein synthesis in the algal cells and induce a lower accumulation of lipids. Based on these findings, we proposed a new flowsheet for sustainable livestock waste management.

The scientometric analysis of the research on microalgae-based wastewater treatment

This study explores the characteristics of the literature on microalgae-based wastewater treatment during the past 20 years, based on the Web of Science Core Collection database and its scientometric techniques. The results reveal that the literature on microalgae-based wastewater treatment has grown rapidly with 2621 publications and 54,388 citations in total. Most of the document types are journal articles, constituting 80.7% of the total records. China and the USA are the two most active countries, regarding the publications and cooperation in this filed from the viewpoint of the number of publishing papers, total number of citations, and the number of multinational author papers. The Chinese Academy of Sciences is the largest institutional contributor, publishing 2.3% of the papers, followed by the Indian Institute of Technology (2.2%) and Council of Scientific & Industrial Research (2.1%). The most publishing author is Ruan (35 papers) with the highest number of citation (2460 times). "Bioresource Technology" is the most publishing journal with 365 published papers, while 36.2% of the total sample is published in the subject area of "Environmental Sciences Ecology." The most cited paper in the past 20 years is a review of the status of phosphorus removal in wastewater by de-Bashan in 2004. Bibliometric analysis has systematically combed the development system of microalgae-based wastewater treatment in the past 20 years and has a great potential to gain valuable insights for the future development, which provides a supplement to the common content analysis.

A novel nanoemulsion-based microalgal growth medium for enhanced biomass production

BACKGROUND

Microalgae are well-established feedstocks for applications ranging from biofuels to valuable pigments and therapeutic proteins. However, the low biomass productivity using commercially available growth mediums is a roadblock for its mass production. This work describes a strategy to boost algal biomass productivity by using an effective CO₂ supplement.

RESULTS

In the present study, a novel nanoemulsion-based media has been tested for the growth of freshwater microalgae strain Chlorella pyrenoidosa. Two different nanoemulsion-based media were developed using 1% silicone oil nanoemulsion (1% SE) and 1% paraffin oil nanoemulsion (1% PE) supplemented in Blue-green 11 media (BG11). After 12 days of cultivation, biomass yield was found highest in 1% PE followed by 1% SE and control, i.e., 3.20, 2.75, and 1.03 g L^-1, respectively. The chlorophyll-a synthesis was improved by 76% in 1% SE and 53% in 1% PE compared with control. The respective microalgal cell numbers for 1% PE, 1% SE and control measured using the cell counter were 3.00 × 10⁶, 2.40 × 10⁶, and 1.34 × 10⁶ cells mL^-1. The effective CO₂ absorption tendency of the emulsion was highlighted as the key mechanism for enhanced algal growth and biomass production. On the biochemical characterization of the produced biomass, it was found that the nanoemulsion-cultivated C. pyrenoidosa had increased lipid (1% PE = 26.80%, 1% SE = 23.60%) and carbohydrates (1% PE = 17.20%, 1% SE = 18.90%) content compared to the control (lipid = 18.05%, carbohydrates = 13.60%).

CONCLUSIONS

This study describes a novel nanoemulsion which potentially acts as an effective CO₂ supplement for microalgal growth media thereby increasing the growth of microalgal cells. Further, nanoemulsion-cultivated microalgal biomass depicts an increase in lipid and carbohydrate content. The approach provides high microalgal biomass productivity without altering morphological characteristics like cell shape and size as revealed by field emission scanning electron microscope (FESEM) images.

Algae utilization and its role in the development of green cities

With the rapid urbanisation happening around the world followed by the massive demand for clean energy resources, green cities play a pivotal role in building a sustainable future for the people. The continuing depletion of natural resources has led to the development of renewable energy with algae as the promising source. The high growth rate of microalgae and their strong bio-fixation ability to convert CO₂ into O₂ have been gaining attention globally and intensive research has been conducted regarding the microalgae benefits. The focus on potential of microalgae in contributing to the development of green cities is rising. The advantage of microalgae is their ability to gather energy from sunlight and carbon dioxide, followed by transforming the nutrients into biomass and oxygen. This leads to the creation of green cities through algae cultivation as waste and renewable materials can be put to good use. The challenges that arise when using algae and the future prospect in terms of SDGs and economy will also be covered in this review. The future of green cities can be enhanced with the adaptation of algae as the source of renewable plants to create a better outlook of an algae green city.