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In the past few decades, the role of nanotechnology has expanded into environmental remediation applications. In this regard, nanofibers have been reported for various applications in water treatment and air filtration. Nanofibers are fibers of polymeric origin with diameters in the nanometer to submicron range. Electrospinning has been the most widely used method to synthesize nanofibers with tunable properties such as high specific surface area, uniform pore size, and controlled hydrophobicity. These properties of nanofibers make them highly sought after as adsorbents, photocatalysts, electrode materials, and membranes. In this review article, a basic description of the electrospinning process is presented. Subsequently, the role of different operating parameters in the electrospinning process and precursor polymeric solution is reviewed with respect to their influence on nanofiber properties. Three key areas of nanofiber application for water treatment (desalination, heavy-metal removal, and contaminant of emerging concern (CEC) remediation) are explored. The latest research in these areas is critically reviewed. Nanofibers have shown promising results in the case of membrane distillation, reverse osmosis, and forward osmosis applications. For heavy-metal removal, nanofibers have been able to remove trace heavy metals due to the convenient incorporation of specific functional groups that show a high affinity for the target heavy metals. In the case of CECs, nanofibers have been utilized not only as adsorbents but also as materials to localize and immobilize the trace contaminants, making further degradation by photocatalytic and electrochemical processes more efficient. The key issues with nanofiber application in water treatment include the lack of studies that explore the role of the background water matrix in impacting the contaminant removal performance, regeneration, and recyclability of nanofibers. Furthermore, the end-of-life disposal of nanofibers needs to be explored. The availability of more such studies will facilitate the adoption of nanofibers for water treatment applications.

Microalgae are convincing materials to promote bio-based sustainable economy. Microalgae have the ability to perform ecological and environmental services. However, the scale-up application of microalgae cannot be realized without significant improvements in the economics of their bioprocessing. It triggers up the need for integrating microalgae with closely related environmental technologies and identify their new applications in the biorefinery. This chapter underlines the existing fate of microalgal technology as well as recent trends that can provide major break-through to qualify microalgae as a sustainable biorefinery feedstock. The particular focus of this chapter is to visualize the potential of microalgae for resource recovery from waste streams.

Hydrothermal liquefaction (HTL) of G. sulphuraria (GS) has been demonstrated to produce energy-dense biocrude oil, water-soluble chemicals, and biochar. The recycling of nutrients from HTL water-soluble chemicals for algae cultivation has been demonstrated. The current study evaluated the feasibility of using HTL biochar, a by-product to recover nutrients and use the nutrients to cultivate microalgae without inhibition. GS biomass was liquefied to produce biocrude oil, biochar, water phase, and gasses under subcritical water conditions (300 °C, 15 MPa & 30 min). The N and P content in the HTL biochar were ~5.27 wt.% and ~15.98 wt.%, respectively. Both nutrients were leached from the biochar at varied pH values with ~40.23% of phosphate and ~70.01% of ammoniacal nitrogen recovery obtained at a pH of 0.5. The leached phosphates and ammoniacal nitrogen were added to N and P free Cyanidium medium (CM) for cultivating GS. Growth experiments were carried out in microtiter plates and culture tube reactors at 40 °C and with CO₂ (2–3%) supplementation. Similar growth rates were observed in cultures supplemented with the leached nutrients when compared to controls in standard CM growth media proving the absence of inhibition in the former. Comprehensive characterization of HTL biochar by Inductively Coupled Plasma Optical Emission Spectrometry, bomb calorimeter, and Scanning Electron Microscope is also presented. This study confirmed the feasibility of recovering nutrients from GS biochar and using these nutrients to cultivate algae successfully without inhibition, thus supporting reduced steps for recycling, process optimization, and HTL technology commercialization.

Previous laboratory studies have demonstrated the ability of microalgae Galdieria sulphuraria (G. sulphuraria) in removing organic carbon and nutrients from filtered primary-settled urban wastewater via mixotrophic metabolism. An advantage of mixotrophic cultivation of G. sulphuraria over heterotrophic conditions is higher biomass yield that can potentially translate into higher energy recovery from the biomass. This study recorded a yield of 0.63 g biomass/g glucose under mixotrophic conditions while that under heterotrophic conditions was 0.42 g biomass/g glucose. These laboratory studies were extended to cultivate G. sulphuraria under field conditions in a 700 L photobioreactor (PBR) fed with primary-settled wastewater. Biomass growth and removal of dissolved organic carbon and nutrients in this PBR under batch mode were monitored over a range of influent and operating conditions. This field study confirmed that G. sulphuraria was able to grow well in primary-settled wastewater and reduce organic carbon (measured as BOD5), ammoniacal nitrogen, and phosphate levels to below the respective discharge standards; corresponding 3-day removal efficiencies ranged 46–72%; 63–89%; and 71–95%.

The cultivation efforts within the National Alliance for Advanced Biofuels and Bioproducts (NAABB) were developed to provide four major goals for the consortium, which included biomass production for downstream experimentation, development of new assessment tools for cultivation, development of new cultivation reactor technologies, and development of methods for robust cultivation. The NAABB consortium testbeds produced over 1500 kg of biomass for downstream processing. The biomass production included a number of model production strains, but also took into production some of the more promising strains found through the prospecting efforts of the consortium. Cultivation efforts at large scale are intensive and costly, therefore the consortium developed tools and models to assess the productivity of strains under various environmental conditions, at lab scale, and validated these against scaled outdoor production systems. Two new pond-based bioreactor designs were tested for their ability to minimize energy consumption while maintaining, and even exceeding, the productivity of algae cultivation compared to traditional systems. Also, molecular markers were developed for quality control and to facilitate detection of bacterial communities associated with cultivated algal species, including the Chlorella spp. pathogen, Vampirovibrio chlorellavorus , which was identified in at least two test site locations in Arizona and New Mexico. Finally, the consortium worked on understanding methods to utilize compromised municipal wastewater streams for cultivation. This review provides an overview of the cultivation methods and tools developed by the NAABB consortium to produce algae biomass, in robust low energy systems, for biofuel production.

Produced water (PW) is the largest waste stream generated by the oil and gas industry. Traditional treatment of PW burdens the industry with significant expenses and environmental issues. Alternatively, microalgal-based bioremediation of PW is often viewed as an ecologically safe and sustainable platform for treating PW. Moreover, the nutrients in PW could support algal growth. However, significant dilution of PW is often required in algal-based systems due to the presence of complex chemical contaminants. In light of these facts, the current work has investigated the potential of cultivating Galdieria sulphuraria and Chlorella vulgaris in PW using multiple dilutions, 0% PW, 5% PW, 10% PW, 20% PW, 50% PW and 100% PW. While both algal strains can grow in PW, the current results indicated that G. sulphuraria has a higher potential of growth in up to 50% PW (total dissolved solids of up to 55 g L−1) with a growth rate of 0.72 ± 0.05 g L−1 d−1 and can achieve a final biomass density of 4.28 ± 0.16 g L−1 in seven days without the need for additional micronutrients. Additionally, the algae showed the potential of removing 99.6 ± 0.2% nitrogen and 74.2 ± 8.5% phosphorus from the PW.

This study evaluates the potential of thermophilic mixotrophic alga, Galdieria sulphuraria, to efficiently perform on-site treatment of landfill leachate (LL). Nutrient removal and algae growth experiments using various dilutions (20%, 40%, 60%, 80%, 100%) of actual municipal LL were carried out in a 14-day lab scale set-up. The effect of N/P mass ratio (255:1; 100:1; 50:1; 25:1; 10:1; 4.55:1) was also tested. Based on the results, it was evident that the growth reactors with 20% LL outperformed the other growth reactors. The final algal biomass density in 20% LL was 2.16 ± 0.22 g L−1 (twice more than compare to standard media) and the observed growth rate was 159 mg L−1d−1. Fourteen-day removal efficiencies were 99.9% for ammoniacal-nitrogen and 34% for phosphate, with removal rate of 22.78 mg L−1d−1 and 2.91 mg L−1d−1, respectively. Addition of P increased NH4-N removal rate from 16.3 to 23.6 mg L−1 day−1 when N/P mass ratio was changed from 255:1 to 4.55:1. Furthermore, range of N/P mass ratio from 100:1 to 25: 1 has been found to achieve almost 100% removal of both NH4-N and PO4-P from 20% LL in 14 days. Overall, this study showed a promising algal-based treatment system for LL nutrient removal and producing algal-biomass for subsequent value-added product generation.

Historically, UWWs (urban wastewaters) that contain high levels of organic carbon, N (nitrogen), and P (phosphorous) have been considered an environmental burden and have been treated at the expense of significant energy input. With the advent of new pollution abatement technologies, UWWs are now being regarded as a renewable resource from which, useful chemicals and energy could be harvested. This study proposes an integrated, algal-based system that has the potential to treat UWWs to the desired discharge standards in a sustainable manner while recovering high fraction of its energy content as well as its N- and P-contents for use as fertilizers. Key embodiments of the system being proposed are: i) cultivation of an extremophile microalga, Galdieria sulphuraria, in UWW for removal of carbon, N, and P via single-step by mixotrophic metabolism; ii) extraction of energy-rich biocrude and biochar from the cultivated biomass via hydrothermal processing; and, iii) enhancement of biomass productivity via partial recycling of the nutrient-rich AP (aqueous product) from hydrothermal-processed biomass to the cultivation step to optimize productivity, and formulation of fertilizers from the remaining AP. This paper presents a process model to simulate this integrated system, identify the optimal process conditions, and establish ranges for operational parameters.

Temperature effect on hydrothermal liquefaction (HTL) of Nannochloropsis gaditana and Chlorella sp. was investigated with 10% biomass loading at HTL temperatures of 180–330 °C, and reaction time of 30 min. Maximum yields of 47.5% for Nannochloropsis sp. and 32.5% biocrude oil yields for Chlorella sp. were obtained at 300 °C. The higher heating values of biocrude oils produced in this work ranged between 34 and 39 MJ/kg. 79% of energy in the Nannochloropsis sp. was recovered at 300 °C and 62% of energy recovery from Chlorella sp. was achieved at 200 °C. Valuable nutrients (NH3–N and PO43−) produced during the HTL process were quantified from the aqueous phase for both strains of biomass. The aqueous phase samples obtained at all temperatures were also analyzed for amino acids and carbohydrates. The suitable temperatures for extraction of lipids, amino acids and carbohydrates have been identified. Sequential HTL experiments conducted have shown the prospect of recovering nutrients and other valuable byproducts along with biocrude oil. The experimental results and analysis indicate that sustainable biofuel production requires the development of strain based strategies for the hydrothermal liquefaction process.

Produced water (PW), the largest waste stream generated in oil and gas industries, has the potential to be a harmless product rather than being a waste. Biological processes using microorganisms have proven useful to remediate PW contaminated by petroleum hydrocarbons, complex organic chemicals, and solvents. In particular, the bioremediation of PW using algae is an eco-friendly and low-cost approach due to algae’s ability to utilize certain pollutants as nutrient sources. Therefore, the utilization of PW as an algal growth medium has a great potential to eliminate chemicals from the PW and minimize the large volumes of freshwater needed for cultivation. Although several reviews describing the bioremediation of PW have been published, to the best of our knowledge, no review has exclusively focused on the algae-based PW treatment. Therefore, the present review is dedicated to filling this gap by portraying the many different facets of the algae cultivation in PW. Several algal species that are known to thrive in a wide range of salinity and the critical steps for their cultivation in hypersaline PW have been identified. Overall, this comprehensive review highlights the PW bioremediation using algae and brings attention to utilizing PW to grow biomass that can be processed to generate biofuels and useful bioproducts.

With increasingly stringent environmental regulations, desulfurization for gasoline oil production has become an important issue. Nowadays, desulfurization technologies have become an integral part of environmental catalysis studies. It is also important for processing of fuel for fuel-cells, which has a strict requirement for sulfur content for internal combustion engines. In this study, we focused on the preparation and characterization of magnesium hydroxide/aluminum supported NiO, ZnO, ZrO2, NiO-ZnO, NiO-ZrO2, adsorbents for the adsorptive desulfurization of liquid fuels. These hydrotalcite adsorbents were prepared by co-precipitation method and used for adsorption of thiophene (in n-pentane, as model fuel) and dibenzothiophene at ambient temperature and pressure. The physicochemical behaviors of the fresh adsorbents such as structure, composition, and bonding modes were determined using X-ray diffraction (XRD), Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR), energy dispersive X-Ray analysis (EDAX), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA). The sulfur concentration in the mixture (thiophene and n-pentane) was measured by UV-Vis spectrophotometry. The percentages of thiophene removal and the adsorption capacity (mg of sulfur per g of adsorbent) of the five adsorbents were compared. The adsorption performance confirmed that NiO-ZrO2 and NiO-ZnO adsorbents are more efficient in removing thiophene/dibenzothiophene than that of three other adsorbents. The qualitative studies using XPS confirmed the efficient adsorption nature of modified hydrotalcite adsorbents on dibenzothiophene.

Hydrothermal liquefaction (HTL) provides a promising option for extracting bio-crude oil from wet algal biomass. One of the byproducts of HTL is an aqueous phase (AP) rich in organic carbon and nutrients. This study evaluated the hypothesis that recycling the AP to the cultivation step could enhance biomass productivity and net energy yield. Since the yields of bio-crude and nutrients post-HTL are functions of HTL reaction temperature, this study evaluated the impact of reaction temperature on net energy yield. Nutrient recycle experiments were conducted with a low-lipid, acidophilic strain, Galdieria sulphuraria, being developed for single-step removal of organic carbon, nitrogen and phosphorus from urban wastewater. G. sulphuraria was cultivated in different dilutions of the AP generated by HTL performed between 180 and 300 °C. Biomass productivity recorded in this study with recycled AP was greater than that in the control by a factor as much as 1.94. Estimates of net energy yields indicate the optimum temperature for the second-stage HTL bio-crude oil extraction from G. sulphuraria to be 280 to 300 °C.

Recent studies have proposed algal cultivation in urban wastewaters for the dual purpose of waste treatment and bioenergy production from the resulting biomass. This study proposes an enhancement to this approach that integrates cultivation of an acidophilic strain, Galdieria sulphuraria 5587.1, in a closed photobioreactor (PBR); hydrothermal liquefaction (HTL) of the wet algal biomass; and recirculation of the nutrient-rich aqueous product (AP) of HTL to the PBR to achieve higher biomass productivity than that could be achieved with raw wastewater. The premise is that recycling nutrients in the AP can maintain optimal C, N and P levels in the PBR to maximize biomass growth to increase energy returns. Growth studies on the test species validated growth on AP derived from HTL at temperatures from 180 to 300 °C. Doubling N and P concentrations over normal levels in wastewater resulted in biomass productivity gains of 20–25% while N and P removal rates also doubled.

Ecological studies of microalgae have revealed their potential to co-exist in the natural environment. It provides an evidence of the symbiotic relationship of microalgae with other microorganisms. The symbiosis potential of microalgae is inherited with distinct advantages, providing a venue for their scale-up applications. The deployment of large-scale microalgae applications is limited due to the technical challenges such as slow growth rate, low metabolites yield, and high risk of biomass contamination by unwanted bacteria. However, these challenges can be overcome by exploring symbiotic potential of microalgae. In a symbiotic system, photosynthetic microalgae co-exist with bacteria, fungi, as well as heterotrophic microalgae. In this consortium, they can exchange nutrients and metabolites, transfer gene, and interact with each other through complex metabolic mechanism. Microalgae in this system, termed as a binary culture, are reported to exhibit high growth rate, enhanced bio-flocculation, and biochemical productivity without experiencing contamination. Binary culture also offers interesting applications in other biotechnological processes including bioremediation, wastewater treatment, and production of high-value metabolites. The focus of the study is to provide a perspective to enhance the understanding about microalgae binary culture. In this review, the mechanism of binary culture, its potential, and limitations are briefly discussed. A number of queries are evolved through this study, which needs to be answered by executing future research to assess the real potential of binary culture.

Nutrient removal from primary wastewater effluent was tested using Galdieria sulphuraria, an acidophilic and moderately thermophilic alga. Biomass yield recorded in this study (27.42 g biomass per g nitrogen removed) is higher than the average reported in the literature (25.75 g g−1) while, the theoretical yield estimated from the empirical molecular formula of algal biomass is 15.8 g g−1. Seven-day removal efficiencies were 88.3% for ammoniacal-nitrogen and 95.5% for phosphates; corresponding removal rates were 4.85 and 1.21 mg L−1 d−1. Although these rates are lower than the average literature values for other strains (6.36 and 1.34 mg L−1 d−1, respectively), potential advantages of G. sulphuraria for accomplishing energy-positive nutrient removal are highlighted. Feasibility of growing G. sulphuraria outdoors at densities higher than in high-rate oxidation ponds is also demonstrated.

Photoautotrophic algal systems have been investigated as potentially greener and more sustainable alternatives to the traditional bacteria-based wastewater treatment (WWT) systems. This paper presents mixotrophic algal systems as better suited for WWT. Since the literature is void of mixotrophic WWT systems, a brief review of the laboratory results from the literature comparing the different types of algal metabolisms is presented first. Details of a mixotrophic WWT system driven by an extremophilic alga, Galdieria sulphuraria (pH = 1 to 4; temperature = 25 to 56 °C) are presented next. Results from pilot scale version of this mixotrophic system (700 L) are summarized to show that it can reduce biochemical oxygen demand (BOD) and nutrients in primary-settled wastewater in a single step to yield discharge-ready effluent in 5 log) and other pathogenic bacteria.

This work investigates the hydrothermal liquefaction (HTL) of Cyanidioschyzon merolae algal species under various reaction temperatures and catalysts. Liquefaction of microalgae was performed with 10% solid loading for 30min at temperatures of 180-300°C to study the influences of two base and two acid catalysts on HTL product fractions. Maximum biocrude oil yield of 16.98% was obtained at 300°C with no catalyst. The biocrude oil yield increased to 22.67% when KOH was introduced into the reaction mixture as a catalyst. The algal biocrude and biochar has a higher heating values (HHV) of 32.22MJkg-1 and 20.78MJkg-1 respectively when no catalyst was used. Gas chromatography time of flight mass spectrometry (GC/TOFMS) was employed to analyze the biocrude oil composition, and elemental analysis was performed on the algae, biocrude and biochar samples. Analysis of the HTL aqueous phase revealed the presence of valuable products.