The effects of salinity on the growth and biochemical properties of Chlamydomonas mexicana GU732420 cultivated in municipal wastewater

Tertiary Wastewater Treatment Technologies: A Review of Technical, Economic, and Life Cycle Aspects

The activated sludge process is the most widespread sewage treatment method. It typically consists of a pretreatment step, followed by a primary settling tank, an aerobic degradation process, and, finally, a secondary settling tank. The secondary effluent is then usually chlorinated and discharged to a water body. Tertiary treatment aims at improving the characteristics of the secondary effluent to facilitate its reuse. In this work, through a literature review of the most prominent tertiary treatment methods, a benchmarking of their technical efficiency, economic feasibility, and environmental impact was carried out. The photo-Fenton method proved to be the most technically efficient process, significantly reducing the microbial load and pharmaceutical content (by 4.9 log and 84%, respectively) of the secondary effluent. Chlorination and UV irradiation exhibited the lowest treatment costs (0.004 EUR/m−3) and the lowest global warming potential (0.04 and 0.09 kg CO2eq. m−3, respectively). After all the data were aggregated, a decision-making tool was constructed in the form of a ternary diagram, which indicates the most appropriate tertiary treatment method according to the weight-per-process aspect (technical, economic, and environmental) selected by the user, with chlorination, UV irradiation, ozonation, microalgae cultivation, and constructed wetlands prevailing in the final results.

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".

The impact of abiotic factors on the growth and lipid accumulation of some green microalgae for sustainable biodiesel production

Three species of freshwater planktonic green microalgae: Ankistrodesmus braunii, Ankistrodesmus falcatus, and Scenedesmus incrassatulus, were isolated from the Nile water in Upper Egypt. These microalgae were exposed to nutritional (nitrogen, phosphorus, and iron) limitations and salinity stress to study their effects on the algal growth and to elevate the lipid content within their cells. The results indicated that exposure to these conditions had a significant impact on the algal growth. The lipid content of the studied algae increased as a result of the salinity stress. The highest lipid content was recorded in A. braunii culture treated with 50 mM NaCl (34.4% of dry weight) and S. incrassatulus cultures treated with 100 mM NaCl (37.7% of dry weight) on the 6th day of cultivation, while the culture of A. falcatus treated with 100 mM NaCl recorded the maximum lipid content (53% of dry weight) on the 10th day of the experiment. The biodiesel quality parameters of the fatty acid methyl ester profile of S. incrassatulus appeared to be in agreement with the international criteria. S. incrassatulus could be regarded as a quite promising feedstock for the biodiesel production.

Regulation and remodeling of intermediate metabolite and membrane lipid during NaCl-induced stress in freshwater microalga Micractinium sp. XJ-2 for biofuel production

Microalgae can accumulate a large fraction of reduced carbon as lipids under NaCl stress. This study investigated the mechanism of carbon allocation and reduction and triacylglycerol (TAG) accumulation in microalgae under NaCl-induced stress. Micractinium sp. XJ-2 was exposed to NaCl stress and cells were subjected to physiological, biochemical, and metabolic analyses to elucidate the stress-responsive mechanism. Lipid increased with NaCl concentration after 0-12 hr, then stabilized after 12-48 hr, and finally decreased after 48-72 hr, whereas TAG increased (0-48 hr) and then decreased (48-72 hr). Under NaCl-induced stress at lower concentrations, TAG accumulation, at first, mainly shown to rely on the carbon fixation through photosynthetic fixation, carbohydrate degradation, and membrane lipids remodeling. Moreover, carbon compounds generated by the degradation of some amino acids were reallocated and enhanced fatty acid synthesis. The remodeling of the membrane lipids of NaCl-induced microalgae relied on the following processes: (a) Increase in the amount of phospholipids and reduction in the amount of glycolipids and (b) extension of long-chain fatty acids. This study enhances our understanding of TAG production under NaCl stress and thus will provide a theoretical basis for the industrial application of NaCl-induced in the microalgal biodiesel industry.

Isolation of a freshwater microalgae and its application for the treatment of wastewater and obtaining fatty acids from tilapia cultivation

In the present study, isolated strains of the microalgae Chlamydomonas sp. (CH) and Chlorella vulgaris (CV) were used to treat aquaculture wastewater and to obtain fatty acids and from a fattening culture of tilapia. The microalgae were cultivated for 11 days in tubular photobioreactors with an operating volume of 2 L, constant aeration and illumination. High removal rates of NO₃^- and PO₄^3- were achieved for both Chlamydomonas sp. (84.7% and 96%, respectively) and Chlorella vulgaris (94.6 and 97.9%, respectively). The maximum biomass productivity achieved by Chlamydomonas sp. was 0.06 and 0.10 gL^-1d^-1 for Chlorella vulgaris. Therefore, tilapia wastewater contained the necessary nutrient concentration for algal growth and development. Chlamydomonas sp. biomass lipid content was 69%, while that of Chlorella vulgaris was 40%. The lipid profile of both microalgae was abundant in palmitic acid (78% for Chlamydomonas sp. and 35% for Chlorella vulgaris). This fatty acid is suitable for biodiesel production. Tilapia wastewater is low-cost alternative culture medium as it contains the necessary nutrient concentration for microalgae development and growth.

Highest accumulated microalgal lipids (polar and non-polar) for biodiesel production with advanced wastewater treatment: Role of lipidomics

Microalgal lipids consist of non-polar and polar lipids. Triacyleglyceride (TAG), a non-polar lipid, is convertible to biodiesel, whereas glycolipids and phospholipids are polar and not convertible to biodiesel owing to their high degree of unsaturation (polyunsaturated fatty acids), which makes the production process insufficient and expensive. In this review, microalgal species that contain the highest lipid content (≥40%) in the literature till 2019 are highlighted. The differentiation between non-polar and polar lipids and the limitations in the conversion of polar lipids to biodiesel are reported. The basic and advanced factors contributing to the accumulation of lipids convertible to biodiesel is discussed. Microalgal species including Scenedesmus obliquus, Ourococcus multisporus, Chlamydomonas pitschmannii, Micractinium reisseri, and Botryococcus braunii with high lipid content are potential candidates for biomass/biodiesel production and nutrient removal from wastewater. Application of lipidomics and transcriptomics to manipulate the lipid associated gene acetyl-CoA synthetase in microalgae improves the accumulative lipid content.

Investigation of phyco-remediation of road salt run-off with marine microalgae Nannochloropsis gaditana

Phyco-remediation is an environmental-friendly method, which involves the application of beneficial microalgae to treat wastewater-containing pollutants for a diverse range of conditions. Several industrial processes generate hyper saline wastewater, which is a significant challenge for conventional wastewater treatment, and the disposal of saline waters also has a negative impact on the environment. Road salt run-off is one such saline wastewater stream not currently treated and one that contributes significantly to negatively impacting receiving bodies of water. In this study, Nannochloropsis microalgae were able to assimilate >95% of the nitrates within 8 days in road salt concentrations ranging from 2.6% to 4.4% under phototrophic cultivation mode. Biomass yields of 1-2 g/l of culture were obtained with the maximum lipid of 22% (g/g) biomass in the road salt media. The crude road salt media provided all the essential micronutrients needed for algal cultivation. The fatty acid composition analysis of the obtained lipid composed of C16 and C18 over 45% of FAME are suitable for biofuel. This study has established that the use of road salt containing nitrate and phosphate nutrients will support the growth of marine micro algae for remediation of a waste water system that are the concern at winter-prevalent regions.

The effect of NaCl stress on photosynthetic efficiency and lipid production in freshwater microalga-Scenedesmus obliquus XJ002

Rapid industrialization and population growth have increased the world's energy demands, resulting in a shortage of conventional fossil fuels. Thus, there is an urgent need to develop sustainable and renewable forms of energy. Microalgae have emerged as a potential feedstock for biofuel production. Under stress conditions, lipid production is enhanced in algal cells due to changes in the lipid biosynthetic pathways that produce neutral lipids. In this study, we examined the physiological and biochemical effects of salinity stress (0.00, 0.01, 0.10, 0.15, 0.20M) on the freshwater microalga Scenedesmus obliquus XJ002. We found that the biomass and the content of chlorophyll a, b and carotenoids decreased with increasing NaCl concentration. NaCl stress damaged the oxygen evolving complex (OEC) and the PSII (photosystem II) reaction center and subsequently suppressed electron transport at the donor and receptor sides of the reaction center, influencing the absorption, transfer, and application of light energy. Additionally, the total lipid content of cells was significantly increased under NaCl stress treatment. The highest lipid content (32.26%) was found in cells cultured in the presence of 0.20M NaCl, which was about 2.52-fold higher than that of cells grown in medium lacking NaCl (12.82%). In addition to providing insight into the physiological and biochemical responses of S. obliquus XJ002 to salinity stress, these findings show that lipid production, and hence biofuel feedstock production, can be boosted by adjusting salt levels in the growth medium.

Enhancement of microalgal growth and biocomponent-based transformations for improved biofuel recovery: A review

Microalgal biomass has received much attention as feedstock for biofuel production due to its capacity to accumulate a substantial amount of biocomponents (including lipid, carbohydrate, and protein), high growth rate, and environmental benefit. However, commercial realization of microalgal biofuel is a challenge due to its low biomass production and insufficient technology for complete utilization of biomass. Recently, advanced strategies have been explored to overcome the challenges of conventional approaches and to achieve maximum possible outcomes in terms of growth. These strategies include a combination of stress factors; co-culturing with other microorganisms; and addition of salts, flue gases, and phytohormones. This review summarizes the recent progress in the application of single and combined abiotic stress conditions to stimulate microalgal growth and its biocomponents. An innovative schematic model is presented of the biomass-energy conversion pathway that proposes the transformation of all potential biocomponents of microalgae into biofuels.

Effect of high-temperature stress on microalgae at the end of the logarithmic phase for the efficient production of lipid

Efficient production of microalgae lipid is significant for the production of renewable biodiesel. In the present study, the high temperature of 40°C as stress environment was tested for stimulating lipid accumulation after the microalgae (Scenedesmus quadricauda) cells in suitable conditions grew to the end of the logarithmic phase. Different stress cultivation times of 1, 2, 3, 4, 5 and 6 days were studied. Interestingly, the lipid content and productivity reached 33.5% and 23.2 mg/L d after one day stress cultivation, showing substantial improvements of 39.6% and 33.3% compared with that in the untreated (day 0) microalgae cells, respectively. Longer stress time led to the decrease of biomass and lipid content compared with the untreated microalgae. However, a maximum protein content of 58.7% was obtained after six days. The stress cultivation at the end of the microalgae exponential phase for one day at a high temperature of 40°C could be a very useful industrial approach for efficiently promoting lipid content and biodiesel production.

Feasibility of various carbon sources and plant materials in enhancing the growth and biomass productivity of the freshwater microalgae Monoraphidium griffithii NS16

In order to assess the feasibility of various carbon sources and plant materials in increasing the growth rate and biomass productivity of Monoraphidium griffithii, ten carbon sources as well as six plant materials were tested in mixotrophic cultures with or without aeration. It was found that glucose, fructose, maltose, sodium acetate and mannitol were potential carbon sources for growth enhancement of M. griffithii. Supplementation of culture medium with these carbon sources resulted in approximately 1-4-fold increase in cell density compared to control in a small scale culture. In a larger scale mixotrophic culture with aeration, 0.05% mannitol and 0.1% fructose resulted in a decent 1-1.5-fold increase in final cell density, approximately 2-fold increase in growth rate and 0.5-1-fold increase in dry biomass weight. Findings from this study suggests that glucose, fructose, maltose and mannitol were potential organic carbon sources for mixotrophic culture of M. griffithii.

Lipid accumulation by NaCl induction at different growth stages and concentrations in photoautotrophic two-step cultivation of Monoraphidium dybowskii LB50

NaCl induction in photoautotrophic two-step cultivation is very promising, but time node and concentration are critical to the entire production. In this study Monoraphidium dybowskii LB50 was subjected to different NaCl concentrations at different growth phases. Results showed that during the initial phase (IP), fixed carbon was used for sugar and lipid under 5gL(-1) NaCl induction, as well as for protein under 10gL(-1) NaCl induction. At late-exponential growth phase (LEGP), the highest lipid productivity was obtained at 20gL(-1) NaCl. At stationary phase (SP) the highest lipid productivity was also under 20gL(-1) NaCl but lower than that of LEGP. In summary, lipid content and quality were improved at all growth phases under NaCl induction. Therefore, cultivation scale can be sued to determine the time node and dosage of the inducer, thereby realizing the economic efficiency of the fundamental guarantee in photoautotrophic two-step cultivation.

Salinity induced oxidative stress enhanced biofuel production potential of microalgae Scenedesmus sp. CCNM 1077

Microalgal biomass is considered as potential feedstock for biofuel production. Enhancement of biomass, lipid and carbohydrate contents in microalgae is important for the commercialization of microalgal biofuels. In the present study, salinity stress induced physiological and biochemical changes in microalgae Scenedesmus sp. CCNM 1077 were studied. During single stage cultivation, 33.13% lipid and 35.91% carbohydrate content was found in 400 mM NaCl grown culture. During two stage cultivation, salinity stress of 400 mM for 3 days resulted in 24.77% lipid (containing 74.87% neutral lipid) along with higher biomass compared to single stage, making it an efficient strategy to enhance biofuel production potential of Scenedesmus sp. CCNM 1077. Apart from biochemical content, stress biomarkers like hydrogen peroxide, lipid peroxidation, ascorbate peroxidase, proline and mineral contents were also studied to understand the role of reactive oxygen species (ROS) mediated lipid accumulation in microalgae Scenedesmus sp. CCNM 1077.

Biomass, lipid content, and fatty acid composition of freshwater Chlamydomonas mexicana and Scenedesmus obliquus grown under salt stress

Two freshwater microalgae including Chlamydomonas mexicana and Scenedesmus obliquus were grown on Bold Basal Medium (BBM) with different levels of salinity up to 100 mM NaCl. The dry biomass and lipid content of microalgae were improved as the concentration of NaCl increased from 0 to 25 mM. Highest dry weight (0.8 and 0.65 g/L) and lipid content (37 and 34 %) of C. mexicana and S. obliquus, respectively, were obtained in BBM amended with 25 mM NaCl. The fatty acid composition of the investigated species was also improved by the increased NaCl concentration. At 50 mM, NaCl palmitic acid (35 %) and linoleic acid (41 %) were the dominant fatty acids in C. mexicana, while oleic acid (41 %) and α-linolenic acid (20 %) were the major fractions found in S. obliquus.