The Environmental Biorefinery: Using Microalgae to Remediate Wastewater, a Win-Win Paradigm

Radionuclide biogeochemistry: from bioremediation toward the treatment of aqueous radioactive effluents

Civilian and military nuclear programs of several nations over more than 70 years have led to significant quantities of heterogenous solid, organic, and aqueous radioactive wastes bearing actinides, fission products, and activation products. While many physicochemical treatments have been developed to remediate, decontaminate and reduce waste volumes, they can involve high costs (energy input, expensive sorbants, ion exchange resins, chemical reducing/precipitation agents) or can lead to further secondary waste forms. Microorganisms can directly influence radionuclide solubility, via sorption, accumulation, precipitation, redox, and volatilization pathways, thus offering a more sustainable approach to remediation or effluent treatments. Much work to date has focused on fundamentals or laboratory-scale remediation trials, but there is a paucity of information toward field-scale bioremediation and, to a lesser extent, toward biological liquid effluent treatments. From the few biostimulation studies that have been conducted at legacy weapon production/test sites and uranium mining and milling sites, some marked success via bioreduction and biomineralisation has been observed. However, rebounding of radionuclide mobility from (a)biotic scale-up factors are often encountered. Radionuclide, heavy metal, co-contaminant, and/or matrix effects provide more challenging conditions than traditional industrial wastewater systems, thus innovative solutions via indirect interactions with stable element biogeochemical cycles, natural or engineered cultures or communities of metal and irradiation tolerant strains and reactor design inspirations from existing metal wastewater technologies, are required. This review encompasses the current state of the art in radionuclide biogeochemistry fundamentals and bioremediation and establishes links toward transitioning these concepts toward future radioactive effluent treatments.

Application of microbial resources in biorefineries: Current trend and future prospects

The recent growing interest in sustainable and alternative sources of energy and bio-based products has driven the paradigm shift to an integrated model termed "biorefinery." Biorefinery framework implements the concepts of novel eco-technologies and eco-efficient processes for the sustainable production of energy and value-added biomolecules. The utilization of microbial resources for the production of various value-added products has been documented in the literatures. However, the appointment of these microbial resources in integrated resource management requires a better understanding of their status. The main of aim of this review is to provide an overview on the defined positioning and overall contribution of the microbial resources, i.e., algae, fungi and bacteria, for various bioprocesses and generation of multiple products from a single biorefinery. By utilizing waste material as a feedstock, biofuels can be generated by microalgae while sequestering environmental carbon and producing value added compounds as by-products. In parallel, fungal biorefineries are prolific producers of lignocellulose degrading enzymes along with pharmaceutically important novel products. Conversely, bacterial biorefineries emerge as a preferred platform for the transformation of standard cells into proficient bio-factories, developing chassis and turbo cells for enhanced target compound production. This comprehensive review is poised to offer an intricate exploration of the current trends, obstacles, and prospective pathways of microbial biorefineries, for the development of future biorefineries.

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.

Biofertilizers from wastewater treatment as a potential source of mineral nutrients for growth of amaranth plants

Exploring alternative fertilizers is crucial in agriculture due to the cost and environmental impact of inorganic options. This study investigated the potential of sewage-derived biofertilizers on the growth and physiology of Amaranthus cruentus plants. Various treatments were compared, including control treatments with inorganic fertilizer and treatments with biofertilizers composed of microalgae, biosolids and reclaimed water. The following traits were investigated: photosynthetic pigments, gas exchange, growth, and leaf nutrient concentrations. The results showed that the concentrations of N, P, Cu, Fe Zn and Na nutrients, in the dry microalgae and biosolids, were quite high for the needs of the plants. The wet microalgae presented high concentration of Cu, Fe and Zn nutrients while reclaimed water contained high concentration of N, K, Ca and S. Na and Zn nutrients increased in the leaf of plants treated with dry microalgae and biosolid, respectively. At the beginning of the flowering phase, total chlorophyll and carotenoids contents were lower for plants grown with wet microalgae while for plants grown with higher doses of biosolid or reclaimed water total chlorophyll was increased, and carotenoids were not affected. Lower photosynthetic pigments under wet microalgae resulted in lower photosynthetic rates. On the other hand, amendments with dry microalgae and biosolid increased photosynthetic rates with the biosolid being the most effective. Higher applications of biosolid, wet and dry microalgae produced a considerable increase in shoot biomass of amaranth, with the dry microalgae being the most effective. Additionally, reclaimed water obtained after tertiary treatment of sewage with microalgae and biosolids applied alone showed promising effects on plant growth. Overall, these findings suggest that organic fertilizers derived from sewage treatment have the potential to enhance plant growth and contribute to sustainable agricultural practices.

Energy balance and life cycle assessments in producing microalgae biodiesel via a continuous microalgal-bacterial photobioreactor loaded with wastewater

Life cycle assessments of microalgal cultivation systems are often conducted to evaluate the sustainability and feasibility factors of the entire production chain. Unlike widely reported conventional microalgal cultivation systems, the present work adopted a microalgal-bacterial cultivation approach which was upscaled into a pilot-scale continuous photobioreactor for microalgal biomass production into biodiesel from wastewater resources. A multiple cradle-to-cradle system ranging from microalgal biomass-to-lipid-to-biodiesel was evaluated to provide insights into the energy demand of each processes making up the microalgae-to-biodiesel value chain system. Energy feasibility studies revealed positive NER values (4.95-8.38) for producing microalgal biomass but deficit values for microalgal-to-biodiesel (0.14-0.23), stemming from the high energy input requirements in the downstream processes for converting biomass into lipid and biodiesel accounting to 88-90% of the cumulative energy demand. Although the energy balance for microalgae-to-biodiesel is in the deficits, it is comparable with other reported biodiesel production case studies (0.12-0.40). Nevertheless, the approach to using microalgal-bacterial cultivation system has improved the overall energy efficiency especially in the upstream processes compared to conventional microalgal cultivation systems. Energy life cycle assessments with other microalgal based biofuel systems also proposed effective measures in increasing the energy feasibility either by utilizing the residual biomass and less energy demanding downstream extraction processes from microalgal biomass. The microalgal-bacterial cultivation system is anticipated to offer both environmental and economic prospects for upscaling by effectively exploiting the low-cost nutrients from wastewaters via bioconversion into valuable microalgal biomass and biodiesel.

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.

Challenges for Circular Economy under the EU 2020/741 Wastewater Reuse Regulation

Wastewater reuse is seen as an opportunity to support a circular economy and optimize water resources. However, the use of wastewater is limited by the need for the proper protection of health and the environment and demands a certain minimum quality of treated reclaimed water. The objective of this work is to evaluate the opportunities both for the agents in the water treatment chain (from municipalities to farmers) and for technology providers under the recently approved Regulation EU-2020/741. The new market and opportunities require new value chains, technology development, governance, risk assurance, and adapted local regulation. Bottlenecks also pose technological, environmental, institutional, economic, and social challenges. The identified needs and barriers must be properly addressed in order to accelerate the transformation of the water sector toward the circular economy. As a conclusion, Reg EU 2020/741 introduces minimum requirements for urban wastewater reuse and requires the definition of risk management and transparency. The real impact of regulation on circular-economy objectives is limited by water scarcity and crop profitability. Social acceptance is critical for success.

Life cycle assessment of microalgae systems for wastewater treatment and bioproducts recovery: Natural pigments, biofertilizer and biogas

The aim of this study was to assess the potential environmental impacts associated with microalgae systems for wastewater treatment and bioproducts recovery. In this sense, a Life Cycle Assessment was carried out evaluating two systems treating i) urban wastewater and ii) industrial wastewater (from a food industry), with the recovery of bioproducts (i.e. natural pigments and biofertilizer) and bioenergy (i.e. biogas). Additionally, both alternatives were compared to iii) a conventional system using a standard growth medium for microalgae cultivation in order to show the potential benefits of using wastewater compared to typical cultivation approaches. The results indicated that the system treating industrial wastewater with unialgal culture had lower environmental impacts than the system treating urban wastewater with mixed cultures. Bioproducts recovery from microalgae wastewater treatment systems can reduce the environmental impacts up to 5 times compared to a conventional system using a standard growth medium. This was mainly due to the lower chemicals consumption for microalgae cultivation. Food-industry effluent showed to be the most promising scenario for bioproducts recovery from microalgae treating wastewater, because of its better quality compared to urban wastewater which also allows the cultivation of a single microalgae species. In conclusion, microalgae wastewater treatment systems are a promising solution not only for wastewater treatment but also to boost the circular bioeconomy in the water sector through microalgae-based product recovery.

Proteomics unveil a central role for peroxisomes in butyrate assimilation of the heterotrophic Chlorophyte alga Polytomella sp

Volatile fatty acids found in effluents of the dark fermentation of biowastes can be used for mixotrophic growth of microalgae, improving productivity and reducing the cost of the feedstock. Microalgae can use the acetate in the effluents very well, but butyrate is poorly assimilated and can inhibit growth above 1 gC.L-1. The non-photosynthetic chlorophyte alga Polytomella sp. SAG 198.80 was found to be able to assimilate butyrate fast. To decipher the metabolic pathways implicated in butyrate assimilation, quantitative proteomics study was developed comparing Polytomella sp. cells grown on acetate and butyrate at 1 gC.L-1. After statistical analysis, a total of 1772 proteins were retained, of which 119 proteins were found to be overaccumulated on butyrate vs. only 46 on acetate, indicating that butyrate assimilation necessitates additional metabolic steps. The data show that butyrate assimilation occurs in the peroxisome via the β-oxidation pathway to produce acetyl-CoA and further tri/dicarboxylic acids in the glyoxylate cycle. Concomitantly, reactive oxygen species defense enzymes as well as the branched amino acid degradation pathway were strongly induced. Although no clear dedicated butyrate transport mechanism could be inferred, several membrane transporters induced on butyrate are identified as potential condidates. Metabolic responses correspond globally to the increased needs for central cofactors NAD, ATP and CoA, especially in the peroxisome and the cytosol.

Recovery of nitrogen and phosphorus from livestock slurry with treatment technologies: A meta-analysis

The livestock industry has developed rapidly in recent decades, but the improper treatment of livestock manure, especially slurry, causes environmental pollution. Treatment technologies are considered to be effective in alleviating nitrogen (N) and phosphorus (P) losses from livestock slurry. Here, we used published research data to conduct a meta-analysis of the recovery efficiencies of N and P of five mainstream treatment technologies, including ammonia stripping, air scrubbing, membrane filtration, microalgae cultivation and struvite crystallization. Additionally, the agronomic effects of the recovered products of these treatment technologies were evaluated. The results showed that all technologies exhibited clear recovery effects on N and P. The N recovery efficiencies ranged from 57% to 86%, and those of P ranged from 64% to 87%. Struvite crystallization was the most efficient treatment technology for both N and P recovery; moreover, the ammonia stripping and microalgae cultivation technologies were less efficient. The pH levels and temperatures are the main factors that influence ammonia stripping, struvite crystallization and microalgae cultivation, while membrane filtration and air scrubbing are mainly affected by the membrane types and properties. When the equal amount of N or P input to fields, the recovered products (ammonium sulfate and struvite crystals) may achieve a similar crop yield, relative to commercial N or P fertilizers. Our findings can provide deep suggestions and parameters for designing proper treatment technologies to reduce nutrient discharge from livestock slurry in regions with high livestock density and also for identifying the research gaps that should be paid more attention in the future.

Roles of Close Homologues SigB and SigD in Heat and High Light Acclimation of the Cyanobacterium Synechocystis sp. PCC 6803

Acclimation of cyanobacterium Synechocystis sp. PCC6803 to suboptimal conditions is largely dependent on adjustments of gene expression, which is highly controlled by the σ factor subunits of RNA polymerase (RNAP). The SigB and SigD σ factors are close homologues. Here we show that the sigB and sigD genes are both induced in high light and heat stresses. Comparison of transcriptomes of the control strain (CS), ΔsigB, ΔsigD, ΔsigBCE (containing SigD as the only functional group 2 σ factor), and ΔsigCDE (SigB as the only functional group 2 σ factor) strains in standard, high light, and high temperature conditions revealed that the SigB and SigD factors regulate different sets of genes and SigB and SigD regulons are highly dependent on stress conditions. The SigB regulon is bigger than the SigD regulon at high temperature, whereas, in high light, the SigD regulon is bigger than the SigB regulon. Furthermore, our results show that favoring the SigB or SigD factor by deleting other group 2 σ factors does not lead to superior acclimation to high light or high temperature, indicating that all group 2 σ factors play roles in the acclimation processes.

Removal of hydroxychloroquine from an aqueous solution using living microalgae: Effect of operating parameters on removal efficiency and mechanisms

Wastewater contaminated with hydroxychloroquine (HCQ) poses a serious threat to the environment and human life. This study aimed to evaluate the ability of living microalgae to remove HCQ from an aqueous solution. Batch mode experiments were conducted under different conditions to investigate the effect of operating parameters on HCQ removal efficiency and mechanisms. Equilibrium, kinetic and thermodynamic study was also carried out to better describe the interactions between HCQ and microalgae. The maximum HCQ removal was 92.10 ± 1.25% obtained under optimal pH of 9.9 ± 0.1, a contact time of 45 min, a stirring speed of 300 rpm, an initial HCQ concentration of 20 mg/L, and a microalgae dose of 100 mg/L. The Langmuir isotherm and the pseudo-second-order kinetic model were best suited for the biosorption experiments, and the maximum biosorption capacity was 339.02 mg/g. The thermodynamic study showed that the biosorption process was exothermic and spontaneous. Experiments on real wastewater showed that the HCQ removal was not significantly affected by the presence of other contaminants in the water. PRACTITIONER POINTS: The best HCQ removal was 92.10 ± 1.25% obtained under optimal conditions. The Langmuir isotherm and the pseudo-second-order kinetic model were best suited for the biosorption experiments. The maximum biosorption capacity was 339.02 mg/g. The thermodynamic study showed that the biosorption process was exothermic and spontaneous. The microalgae studied can be successfully used in HCQ removal from water.

A Review about Microalgae Wastewater Treatment for Bioremediation and Biomass Production—A New Challenge for Europe

Microalgae have received much attention in the last few years. Their use is being extended to different fields of application and technologies, such as food, animal feed, and production of valuable polymers. Additionally, there is interest in using microalgae for removal of nutrients from wastewater. Wastewater treatment with microalgae allows for a reduction in the main chemicals responsible for eutrophication (nitrogen and phosphate), the reduction of organic substrates (by decreasing parameters such as BOD and COD) and the removal of other substances such as heavy metals and pharmaceuticals. By selecting and reviewing 202 articles published in Scopus between 1992 and 2020, some aspects such as the feasibility of microalgae cultivation on wastewater and potential bioremediation have been investigated and evaluated. In this review, particular emphasis was placed on the different types of wastewaters on which the growth of microalgae is possible, the achievable bioremediation and the factors that make large-scale microalgae treatment feasible. The results indicated that the microalgae are able to grow on wastewater and carry out effective bioremediation. Furthermore, single-step treatment with mixotrophic microalgae could represent a valid alternative to conventional processes. The main bottlenecks are the large-scale feasibility and costs associated with biomass harvesting.

Cultivation of two Chlorella species in Open sewage contaminated channel wastewater for biomass and biochemical profiles: Comparative lab-scale approach

Open sewage contaminated channel wastewater (OSCCW) has high pollutant loads, responsible for eutrophication, when mixed with various channels of urban communities. But, these pollutants can be converted and recovered into useful end products with the help of algal species. In this study, two species of Chlorella (C. vulgaris and C. pyrenoidosa) were selected and investigated for the production of algal biomass and nutrient removal efficiencies with 50% concentration of OSCCW, in a comparative way at lab-scale. Chlorella sp. cultivated in OSCCW have removed nitrate (76.9-78.8%) and phosphate (67.6-79.7%) whereas COD (72.4-76.2%) and BOD (62.3-72.4%) respectively. Correlation analysis was investigated between physico-chemical parameters and biochemical profile of both species to analyze the positive and negative correlation between two variables. The bio-chemical profile and biomass productivity of both species of Chlorella were observed well on the basis of productivity of biomass (60.1, 56.5 mg/l/d), carbohydrate (15.71, 8.82 mg/l/d), protein (11.21, 15.82 mg/l/d), lipid (20.8, 17.5 mg/l/d) and chlorophyll (0.78, 0.67 mg/l/d) in OSCCW. The maximum lipid content (34.6%) was obtained with C. pyrenoidosa as compared to C. vulgaris. Findings also support that OSCCW is well-off with nutrient resources, which can be suitable alternative for algal biomass production and remediated wastewater can be used for animal and fish farming type activities.

Algae-mediated processes for the treatment of antiretroviral drugs in wastewater: Prospects and challenges

The prevalence of pharmaceuticals (PCs), especially antiretroviral (ARV) drugs in various aquatic ecosystems has been expansively reported, wherein wastewater treatment plants (WWTPs) are identified as the primary point source. Consequently, the occurrence, ecotoxicity and treatment of ARV drugs in WWTPs have drawn much attention in recent years. Numerous studies have shown that the widely employed activated sludge-based WWTPs are incapable of removing ARV drugs efficiently from wastewater. Recently, algae-based wastewater treatment processes have shown promising results in PCs removal from wastewater, either completely or partially, through different processes such as biosorption, bioaccumulation, and intra-/inter-cellular degradation. Algal species have also shown to tolerate high concentrations of ARV drugs than the reported concentrations in the environmental matrices. In this review, emphasis has been given on discussing the current status of the occurrence of ARV drugs in the aquatic environment and WWTPs. Besides, the current trends and future perspectives of PCs removal by algae are critically reviewed and discussed. The potential pathways and mechanisms of ARV drugs removal by algae have also been discussed.

Advancement of green technologies: A comprehensive review on the potential application of microalgae biomass

Microalgae have drawn significant interest worldwide, owing to their enormous application potential in the green energy, biopharmaceutical, and nutraceutical industries. Many studies have proved and stated the potential of microalgae in the area of biofuel which is economically effective and environmentally friendly. Besides the commercial value, the potential of microalgae in environmental protection has also been investigated. Microalgae-based process is one of the most effective way to treat heavy metal pollution, compared to conventional methods, it does not release any toxic waste or harmful gases, and the aquatic organism will not receive any harmful effects. The potential dual role of microalge in phytoremedation and energy production has made it widely explored for its capability. The interest of microalgae in various application has motivated a new focus in green technologies. Considering the rapid population growth with the continuous increase on the global demand and the application of biomass in diverse field, significant upgrades have been performed to accommodate green technological advancement. In the past decade, noteworthy advancement has been made on the technology involving the diverse application of microalgae biomass. This review aims to explore on the application of microalgae and the development of green technology in various application for microalgae biomass. There is great prospects for researchers in this field to delve into other potential utilization of microalgae biomass not only for bioremediation process but also to generate revenues from microalgae by incorporating clean and green technology for long-term sustainability and environmental benefits.

Potential for Biomass Production and Remediation by Cultivation of the Marine Model Diatom Phaeodactylum tricornutum in Oil Field Produced Wastewater Media

While oilfield produced water (PW) is one of the largest, unclaimed wastewater streams of the oil industry, it could potentially be used as a cultivation medium for microalgae. Microalgae could help with the remediation of this water while also delivering biomass that can be transformed into valuable byproducts such as biofuels. The coupling of these two purposes is expected to cut production costs of biofuels while aiding environmental protection. In this study, we compared the cultivation capacity of the marine model diatom Phaeodactylum tricornutum in media at varying salinities and in media composed of PW from two oilfields in the Central Valley of California that differed drastically in the concentration of inorganic and organic constituents. Specifically, we measured the carrying capacity of these media, the maximum growth rates of P. tricornutum, its cellular lipid accumulation capacity, and its capacity to remediate the most polluted PW source. Our study shows that P. tricornutum can successfully adjust to the tested cultivation media through processes of short-term acclimation and long-term adaptation. Furthermore, the cultivation of P. tricornutum in the most heavily polluted PW source led to significant increases in cell yield and improved photosynthetic capacity during the stationary phase, which could be attributed chiefly to the higher levels of nitrate present in this PW source. Chemical water analyses also demonstrated the capability of P. tricornutum to remediate major nutrient content and potentially harmful elements like fluorine and copper. Because P. tricornutum is amenable to advanced genetic engineering, which could be taken advantage of to improve its cultivation resilience and productivity in an economic setting, we propose this study as a step towards essential follow-up studies that will identify the genetic regulation behind its growth in oilfield PW media and its remediation of the PW constituents.

Bio-products from algae-based biorefinery on wastewater: A review

Increasing resource demand, predicted fossil resources shortage in the near future, and environmental concerns due to the production of greenhouse gas carbon dioxide have motivated the search for alternative 'circular' pathways. Among many options, microalgae have been recently 'revised' as one of the most promising due to their high growth rate (with low land use and without competing with food crops), high tolerance to nutrients and salts stresses and their variability in biochemical composition, in so allowing the supply of a plethora of possible bio-based products such as animal feeds, chemicals and biofuels. The recent raising popularity of Circular Bio-Economy (CBE) further prompted investment in microalgae, especially in combination with wastewater treatment, under the twofold aim of allowing the production of a wide range of bio-based products while bioremediating wastewater. With the aim of discussing the potential bio-products that may be gained from microalgae grown on urban wastewater, this paper presents an overview on microalgae production with particular emphasis on the main microalgae species suitable for growth on wastewater and the obtainable bio-based products from them. By selecting and reviewing 76 articles published in Scopus between 1992 and 2020, a number of interesting aspects, including the selection of algal species suitable for growing on urban wastewater, wastewater pretreatment and algal-bacterial cooperation, were carefully reviewed and discussed in this work. In this review, particular emphasis is placed on understanding of the main mechanisms driving formation of microalgal products (such as biofuels, biogas, etc.) and how they are affected by different environmental factors in selected species. Lastly, the quantitative information gathered from the articles were used to estimate the potential benefits gained from microalgae grown on urban wastewater in Campania Region, a region sometimes criticized for poor wastewater management.

Phycoremediation and valorization of synthetic dairy wastewater using microalgal consortia of Chlorella variabilis and Scenedesmus obliquus

Microalgae are known to grow on wastewater utilizing their available nutrients. The residual algal biomass thus obtained could be used for producing value-added products thereby making it an economically viable and sustainable option for the dairy industry. The present study evaluates the ability of the microalgal consortia composed of Chlorella variabilis and Scenedesmus obliquus to treat and valorize diluted synthetic dairy wastewater under controlled laboratory conditions. The effect of time, inoculum concentration and light intensity on five responses, namely phosphate removal, ammoniacal nitrogen removal, COD removal, biomass productivity and lutein content, are studied by response surface methodology utilizing central composite design. The quadratic models are found to be suitable for phosphate removal, ammoniacal nitrogen removal, COD removal and biomass productivity. At optimized experimental conditions, the microalgal consortia exhibited phosphate removal of 70.19%, ammoniacal nitrogen removal of 86.22%, COD removal of 54.72%, biomass productivity of 29.13 mg/L/day and lutein content of 12.59 mg/g respectively. This study is of high importance as the lutein content exhibited by the microalgal consortia is higher when compared to other microalgal species and could be considered in the future as a commercial source of lutein.

Treatment of wastewater reverse osmosis concentrate using alginate-immobilised microalgae: Integrated impact of solution conditions on algal bead performance

Alginate can be used for entrapment of microalgal cells in gel beads to achieve high-rate treatment of wastewater and can overcome the difficulties of cell separation that would occur in suspended microalgae treatment systems. The potential for alginate beads to disintegrate in the presence of high ion concentrations could limit the use of alginate entrapment for treating municipal wastewater reverse osmosis concentrate (ROC). The combined effect of the pH, alkalinity, and salinity of the ROC that impact the physical stability, chemical characteristics, biomass production, and nutrient removal performance of alginate-entrapped Chlorella vulgaris for treating the ROC was investigated. Water adsorption resulting from the loss of calcium from the alginate matrix was the initiating cause of reduction of the algal bead stability. The combination of alkalinity >400 mg/L and pH ≥9.5 led to a >65% reduction in compressive strength and thus disintegration of beads during ROC treatment. However, alginate beads of C. vulgaris were sufficiently stable and were capable of nutrient remediation (up to 100% TP and 85% TN per treatment cycle of 48 h over a 10-day period) and biomass production (up to 340 mg/L/d) when salinity, pH, and alkalinity levels were <8 g TDS/L, 7-9.5, and <400 mg/L, respectively. Empirical models that were developed and validated could enable the prediction of the performance of the algal beads for various ROC compositions. This study enhances the insight and decision-making regarding the feasibility of the alginate-immobilised microalgal system for treating municipal wastewater ROC streams.

Structural Characteristics and Environmental Applications of Covalent Organic Frameworks

Covalent organic frameworks (COFs) are emerging crystalline polymeric materials with highly ordered intrinsic and uniform pores. Their synthesis involves reticular chemistry, which offers the freedom of choosing building precursors from a large bank with distinct geometries and functionalities. The pore sizes of COFs, as well as their geometry and functionalities, can be pre-designed, giving them an immense opportunity in various fields. In this mini-review, we will focus on the use of COFs in the removal of environmentally hazardous metal ions and chemicals through adsorption and separation. The review will introduce basic aspects of COFs and their advantages over other purification materials. Various fabrication strategies of COFs will be introduced in relation to the separation field. Finally, the challenges of COFs and their future perspectives in this field will be briefly outlined.

Hydroponic Farm Wastewater Treatment Using an Indigenous Consortium

Hydroponic farms produce wastewater that need to be treated before being released into the environment. A three-step screening process (microplate, batch, and semi-continuous flasks experiments) initially designed to select an efficient microalgae strain allowed the isolation of a consortium that naturally developed in the hydroponic farm wastewater. During the non-optimized semi-continuous experiments, the best performing microalgae strain, Scenedesmus obliquus UTEX393 and the wastewater-born consortium cultures achieved good average linear growth rate (0.186 and 0.198/d, respectively) and high average nitrogen removal rates (23.5 mgN/L/d and 21.9 mgN/L/d, respectively). Phosphorus removal was very high probably due to precipitation. An integrated process was designed to treat the hydroponic farm wastewater using the wastewater-born consortium. Despite relatively low coagulation efficiencies in the preliminary tests, when integrated in a continuous process, chitosan was efficient to harvest the naturally wastewater-born consortium. The process was also efficient for removing nitrate and phosphate in less than seven days (average removal of 98.2 and 87.1% for nitrate and phosphate, respectively). These very promising results will help to define a pre-industrial pilot process.

Wastewater Refinery: Producing Multiple Valuable Outputs from Wastewater

A wastewater refinery is a multifunctional solution that combines different technologies and processing schemes to recover a spectrum of valuable materials from municipal or industrial wastewater. The concept of wastewater refinery introduces a new perspective on wastewater treatment and management. It aims at making the most of wastewater constituents by co-producing different worthful outputs, such as water, energy, nitrogen, sulfide, and phosphorous. This can turn the treatment of wastewater from a major cost into a source of profit. The wastewater refinery approach is well aligned with the concept of the circular economy. A case study on Qatar’s wastewater revealed the potential recovery of significant quantities of valuable resources embodied in the country’s wastewater. Valorization of organic constituents and the recovery of nitrogen, phosphorus, and sulfide should be given priority. To facilitate the adoption of the wastewater refinery concept, research is required to explore technical and economic bottlenecks.

Removal of pharmaceutical and personal care products (PPCPs) from wastewater using microalgae: A review

Pharmaceuticals and personal care products (PPCPs) are a group of emerging micro-pollutants causing detrimental effects on living organisms even at low doses. Previous investigations have confirmed the presence of PPCPs in the environment at hazardous levels, mainly due to the inefficiency of conventional wastewater treatment plants (CWWTPs). Their stable structure induces longer persistence in the environment. Microalgae are currently used to bioremediate numerous pollutants of different characteristics and properties released from the domestic, industrial, agricultural, and farm sectors. CO₂ mitigation during culture and the use of biomass as feedstock for biodiesel or biofuel production are, briefly, other benefits of microalgae-mediated treatment over CWWTPs. This review provides a comprehensive summary of recent literature, an overview of approaches and treatment systems, and breakthrough in the field of algal-mediated removal of PPCPs in wastewater treatment processes. The mechanisms involved in phycoremediation, along with their experimental approaches, have been discussed in detail. Factors influencing the removal of PPCPs from aqueous media are comprehensively described and assessed. A comparative study on microalgal strains is analyzed for a more efficient implementation of future processes. The role of microalgae to mitigate the most severe environmental impacts of PPCPs and the generation of antibiotic-resistant bacteria is discussed. Also, a detailed assessment of recent research on potential toxic effects of PPCPs on microalgae was conducted. The current review highlights microalgae as a promising and sustainable approach to efficiently bio-transform or bio-adsorb PPCPs.

Multifaceted roles of microalgae in the application of wastewater biotreatment: A review

Microalgae have become imperative for biological wastewater treatment. Its capability in biological purification of wastewaters from different origins while utilizing wastewater as the substrate for growth has manifest great potentials as a sustainable and economical wastewater treatment method. The wastewater grown microalgae have also been remarked in research to be a significant source of value-added bioproducts and biomaterial. This paper highlights the multifaceted roles of microalgae in wastewater treatment from the extent of microalgal bioremediation function to environmental amelioration with the involvement of microalgal biomass productivity and carbon dioxide fixation. Besides, the uptake mechanism of microalgae in wastewater treatment was discussed in detail with illustrations for a comprehensive understanding of the removal process of undesirable substances. The performance of different microalgae species in the uptake of various substances was studied and summarized in this review. The correlation of microalgal treatment efficacy with various algal strain types and the bioreactors harnessed for cultivation systems was also discussed. Studies on the alternatives to conventional wastewater treatment processes and the integration of microalgae with accordant wastewater treatment methods are presented. Current research on the biological and technical approaches for the modification of algae-based wastewater system and the maximization of biomass production is also reviewed and discussed. The last portion of the review is dedicated to the assertion of challenges and future perspectives on the development of microalgae-based wastewater treatment technology. This review serves as a useful and informative reference for readers regarding the multifaceted roles of microalgae in the application of wastewater biotreatment with detailed discussion on the uptake mechanism.

Phycoremediation of phenol-polluted petro-industrial effluents and its techno-economic values as a win-win process for a green environment, sustainable energy and bioproducts

The discharge of the toxic phenol-polluted petro-industrial effluents (PPPIE) has severe environmental negative impacts, thus it is mandatory to be treated before its discharge. The objective of this review was to discuss the sustainable application of microalgae in phenols degradation, with a special emphasis on the enzymes involved in this bioprocess and the factors affecting the success of PPPIE phycoremediation. Moreover, it confers the microalgae bioenergetic strategies to degrade different forms of phenols in PPPIE. It also points out the advantages of the latest application of bacteria, fungi and microalgae as microbial consortia in phenols biodegradation. Briefly, phycoremediation of PPPIE consumes carbon dioxide emitted from petro-industries for; valorization of the polluted water to be reused and production of algal biomass which can act as a source of energy for such integrated bioprocess. Besides, the harvested algal biomass can feasibly produce; third-generation biofuels, biorefineries, bioplastics, fish and animal feed, food supplements, natural dyes, antioxidants and many other valuable products. Consequently, this review precisely confirms that the phycoremediation of PPPIE is a win-win process for a green environment and a sustainable future. Thus, to achieve the three pillars of sustainability; social, environmental and economic; it is recommendable to integrate PPPIE treatment with algal cultivation. This integrated process would overcome the problem of greenhouse gas emissions, global warming and climate change, solve the problem of water-scarce, and protect the environment from the harmful negative impacts of PPPIE.

Treatment of Wastewaters by Microalgae and the Potential Applications of the Produced Biomass—A Review

The treatment of different types of wastewater by physicochemical or biological (non-microalgal) methods could often be either inefficient or energy-intensive. Microalgae are ubiquitous microscopic organisms, which thrive in water bodies that contain the necessary nutrients. Wastewaters are typically contaminated with nitrogen, phosphorus, and other trace elements, which microalgae require for their cell growth. In addition, most of the microalgae are photosynthetic in nature, and these organisms do not require an organic source for their proliferation, although some strains could utilize organics both in the presence and absence of light. Therefore, microalgal bioremediation could be integrated with existing treatment methods or adopted as the single biological method for efficiently treating wastewater. This review paper summarized the mechanisms of pollutants removal by microalgae, microalgal bioremediation potential of different types of wastewaters, the potential application of wastewater-grown microalgal biomass, existing challenges, and the future direction of microalgal application in wastewater treatment.

Forward Osmosis as Concentration Process: Review of Opportunities and Challenges

In the past few years, osmotic membrane systems, such as forward osmosis (FO), have gained popularity as "soft" concentration processes. FO has unique properties by combining high rejection rate and low fouling propensity and can be operated without significant pressure or temperature gradient, and therefore can be considered as a potential candidate for a broad range of concentration applications where current technologies still suffer from critical limitations. This review extensively compiles and critically assesses recent considerations of FO as a concentration process for applications, including food and beverages, organics value added compounds, water reuse and nutrients recovery, treatment of waste streams and brine management. Specific requirements for the concentration process regarding the evaluation of concentration factor, modules and design and process operation, draw selection and fouling aspects are also described. Encouraging potential is demonstrated to concentrate streams more than 20-fold with high rejection rate of most compounds and preservation of added value products. For applications dealing with highly concentrated or complex streams, FO still features lower propensity to fouling compared to other membranes technologies along with good versatility and robustness. However, further assessments on lab and pilot scales are expected to better define the achievable concentration factor, rejection and effective concentration of valuable compounds and to clearly demonstrate process limitations (such as fouling or clogging) when reaching high concentration rate. Another important consideration is the draw solution selection and its recovery that should be in line with application needs (i.e., food compatible draw for food and beverage applications, high osmotic pressure for brine management, etc.) and be economically competitive.

Efficient production of fatty acid methyl esters by a wastewater-isolated microalgae-yeast co-culture

Improving the competitiveness of biodiesel production by microalgae cultures requires the application of several strategies to obtain a high content of lipids, rapid biomass growth and a capacity to adapt to different kinds of environment, with the aim of using non-renewable nutrient sources. Therefore, the use of an individual indigenous microalgae strain or a consortium from natural or anthropogenic sites is now considered an alternative for biofuel production. This study examined the temporal behaviour of secondary metabolites produced by a native microalgae and yeast consortium isolated from wastewater, which was characterized by a genetic identification method based on the MiSeq system. The predominant species in the consortium was Scenedesmus obliquus, representing 68% of the organisms. In addition, the consortium contained a number of yeast species, including Candida pimensis (43%), Arthroderma vanbreuseghemii (23%), Diaporthe aspalathi/Diaporthe meridionalis (25%) and Hericium americanum (3%). This indigenous co-culture of microalgae and yeast showed biomass productivity of 0.06 g l^-1 day^-1, with a content of 30% (w/w) carbohydrates, 4% (w/w) proteins and 55% (w/w) lipids. Transesterification of the extracted lipids produced fatty acid methyl esters (FAMEs), which were analysed by gas chromatography (GC). The FAMEs included methyl pentadecanoate (1.90%), cis-10-pentanedecanoic acid methyl ester (1.36%), methyl palmitate (2.64%), methyl palmitoleate (21.36%), methyl oleate (64.95%), methyl linolenate (3.83%) and methyl linolelaidate (3.95%). This composition was relevant for biodiesel production based on the co-culture of indigenous microalgae and yeast consortia.

A review of biochemical and thermochemical energy conversion routes of wastewater grown algal biomass

Microalgae are recognized as a potential source of biomass for obtaining bioenergy. However, the lack of studies towards economic viability and environmental sustainability of the entire production chain limits its large-scale application. The use of wastewaters economizes natural resources used for algal biomass cultivation. However, desirable biomass characteristics for a good fuel may be impaired when wastewaters are used, namely low lipid content and high ash and protein contents. Thus, the choice of wastewaters with more favorable characteristics may be one way of obtaining a more balanced macromolecular composition of the algal biomass and therefore, a more suitable feedstock for the desired energetic route. The exploration of biorefinery concept and the use of wastewaters as culture medium are considered as the main strategic tools in the search of this viability. Considering the economics of overall process, direct utilization of wet biomass using hydrothermal liquefaction or hydrothermal carbonization and anaerobic digestion is recommended. Among the explored routes, anaerobic digestion is the most studied process. However, some main challenges remain as little explored, such as a low energy pretreatment and suitable and large-scale reactors for algal biomass digestion. On the other hand, thermochemical conversion routes offer better valorization of the algal biomass but have higher costs. A biorefinery combining anaerobic digestion, hydrothermal carbonization and hydrothermal liquefaction processes would provide the maximum possible output from the biomass depending on its characteristics. Therefore, the choice must be made in an integrated way, aiming at optimizing the quality of the final product to be obtained. Life cycle assessment studies are critical for scaling up of any algal biomass valorization technique for sustainability. Although there are limitations, suitable integrations of these processes would enable to make an economically feasible process which require further study.

Potential of Chlorella vulgaris and Nannochloropsis salina for nutrient and organic matter removal from municipal wastewater reverse osmosis concentrate

Municipal wastewater reverse osmosis concentrate (ROC) poses health and environmental risks on its disposal as it contains nutrients and harmful organic compounds at elevated concentrations. This study compared a freshwater microalga Chlorella vulgaris and a marine microalga Nannochloropsis salina in suspended and alginate-immobilised cultures for batch and semi-continuous treatment of the ROC. The immobilised algae gave comparable nutrient removal rates to the suspended cells, demonstrating immobilisation had no apparent negative impact on the photosynthetic activity of microalgae. Semi-continuous algal treatment illustrated that the microalgae could remove significant amounts of nutrients (> 50% and > 80% for TN and TP, respectively), predominantly through algal uptake (> 90%), within a short period (48 h) and generate 335-360 mg DCW L^-1 d^-1 of algal biomass. The treatment also removed a significant amount of organic matter (12.7-13.3 mg DOC L^-1 d^-1), primarily (> 65%) through the biotic pathway.

Production of Microalgal Slow-Release Fertilizer by Valorizing Liquid Agricultural Digestate: Growth Experiments with Tomatoes

Anaerobic Digestion (AD) is a process that is well-known and fast-developing in Europe. AD generates large amounts of digestate, especially in livestock-intensive areas. Digestate has potential environmental issues due to nutrients (such as nitrogen) lixiviation or volatilization. Using liquid digestate as a nutrient source for microalgae growth is considered beneficial because digestate could be valorized and upgraded by the production of an added value product. In this work, microalgal biomass produced using liquid digestate from an agricultural biogas plant was investigated as a slow-release fertilizer in tomatoes. Monoraphidium sp. was first cultivated at different dilutions (1:20, 1:30, 1:50), in indoor laboratory-scale trials. The optimum dilution factor was determined to be 1:50, with a specific growth rate of 0.13 d−1 and a complete nitrogen removal capacity in 25 days of culture. Then, outdoor experiments were conducted in a 110 dm3 vertical, closed photobioreactors (PBRs) in batch and semi-continuous mode with 1:50 diluted liquid digestate. During the batch mode, the microalgae were able to remove almost all NH4+ and 65 (±13) % of PO43−, while the microalgal growth rate reached 0.25 d−1. After the batch mode, the cultures were switched to operate under semi-continuously conditions. The cell densities were maintained at 1.3 × 107 cells mL−1 and a biomass productivity around 38.3 mg TSS L−1 d−1 during three weeks was achieved, where after that it started to decline due to unfavorable weather conditions. Microalgae biomass was further tested as a fertilizer for tomatoes growth, enhancing by 32% plant growth in terms of dry biomass compared with the control trials (without fertilization). Similar performances were achieved in tomato growth using synthetic fertilizer or digestate. Finally, the leaching effect in soils columns without plant was tested and after 25 days, only 7% of N was leached when microalgae were used, against 50% in the case of synthetic fertilizer.

Extremophile Microalgae: the potential for biotechnological application

Microalgae are photosynthetic microorganisms that use sunlight as an energy source, and convert water, carbon dioxide, and inorganic salts into algal biomass. The isolation and selection of microalgae, which allow one to obtain large amounts of biomass and valuable compounds, is a prerequisite for their successful industrial production. This work provides an overview of extremophile algae, where their ability to grow under harsh conditions and the corresponding accumulation of metabolites are addressed. Emphasis is placed on the high-value products of some prominent algae. Moreover, the most recent applications of these microorganisms and their potential exploitation in the context of astrobiology are taken into account.

Biodegradation of phenol by Chlamydomonas reinhardtii

The data presented in this particular study demonstrate that the biodegradation of phenol by Chlamydomonas reinhardtii is a dynamic bioenergetic process mainly affected by the production of catechol and the presence of a growth-promoting substrate in the culture medium. The study focused on the regulation of the bioenergetic equilibrium resulting from production of catechol after phenol oxidation. Catechol was identified by HPLC-UV and HPLC-ESI-MS/MS. Growth measurements revealed that phenol is a growth-limiting substrate for microalgal cultures. The Chlamydomonas cells proceed to phenol biodegradation because they require carbon reserves for maintenance of homeostasis. In the presence of acetic acid (a growth-promoting carbon source), the amount of catechol detected in the culture medium was negligible; apparently, acetic acid provides microalgae with sufficient energy reserves to further biodegrade catechol. It has been shown that when microalgae do not have sufficient energy reserves, a significant amount of catechol is released into the culture medium. Chlamydomonas reinhardtii acts as a versatile bioenergetic machine by regulating its metabolism under each particular set of growth conditions, in order to achieve an optimal balance between growth, homeostasis maintenance and biodegradation of phenol. The novel findings of this study reveal a paradigm showing how microalgal metabolic versatility can be used in the bioremediation of the environment and in potential large-scale applications.

The effect of hydraulic retention time on microalgae-based activated sludge process for Wupa sewage treatment plant, Nigeria

This study evaluated the efficiency of microalgae activated sludge (MAAS) for wastewater treatment by investigating the influence of hydraulic retention time (HRT) on MAAS using batch regime pilot scale photobioreactors at Wupa Wastewater Treatment Plant. The outcome of the study showed that MAAS has a comparably high wastewater treatment performance in comparison with the current Wupa Wastewater Treatment Plant (WWTPA) activated sludge (AS) method and is capable of treating wastewater to the defined Nigerian effluent standards. It was further revealed that treatment performance for most parameters were optimal from HRT₃ (6-day hydraulic retention time). Precisely, total nitrogen (TN), total phosphorus (TP), and BOD₅ had highest removal efficiency at HRT₃ with average total removal of 81.36%, 91.77% and 87.04% respectively. Correspondingly, the average percentage DO increment peaked at HRT₃ with a value of 269.7%. In addition, there was a general deterioration of SVI with increasing HRT, particularly after HRT₂ (4-day HRT). Notably, SVI₃₀ was significantly good at HRT₁ and HRT₂ with SVI values of 48.6 ml/g and 105.52 ml/g; however, from HRT₃ down to HRT₉, the SVI₃₀ became remarkably increases greater than that of HRT₁ and HRT₂, with values ranging from 685.61 to 1832.46 ml/g, which indicates a badly bulking sludge. The MAAS system is recommended as an attractive alternative to the conventional AS wastewater treatment in Nigeria and by extension West African subregion.

Testing the Potential of Regulatory Sigma Factor Mutants for Wastewater Purification or Bioreactor Run in High Light

It is shown that a freshly inoculated culture of the model cyanobacterium Synechocystis sp. PCC 6803 consumed almost all phosphate and 50% of nitrate within 6 days from the nutrient-rich BG-11 growth medium, indicating potential of cyanobacteria to purify wastewaters. Synechocystis sp. PCC 6803 control strain also collected nutrients efficiently from a landfill leachate wastewater KA2 (5.9-6.9 mM ammonium and 0.073-0.077 mM phosphate). Wastewaters might induce oxidative stress to microalgae, which prompted us to test growth of sigma factor inactivation strains, as ΔsigBCE and ΔsigCDE strains show superior growth in chemically induced oxidative stress. All cyanobacterial strains, including a stress-sensitive strain ΔsigBCDE, grew well in KA2 for four days, indicating that KA2 did not cause immediate oxidative stress. Completely arrested growth and bleaching of ΔsigBCDE cells after one week in KA2 wastewater point to the importance of group 2 sigma factor-mediated changes in gene expression during wastewater treatment. The growth of ΔsigBCD was arrested early in un-buffered and Hepes buffered (pH 7.5) KA2. In ΔsigBCD, all phosphate transporter genes are upregulated in standard conditions, and ΔsigBCD cells showed growth defects in low-phosphate BG-11 medium. ΔsigBCD cells removed phosphate slower from KA2 than the control strain, but phosphate supplementation of KA2 did not improve growth of ΔsigBCD. The ΔsigBCE strain showed superior growth in a laboratory-scale bioreactor in bright light and removed phosphate even slightly more efficiently than the control strain if KA2 was Hepes buffered although ΔsigBCE grew slowly in un-buffered KA2 and in low-phosphate BG-11 medium. The results indicate that engineering expression of regulatory group 2 sigma factor(s) might be useful for practical applications.

Resource recovery from wastewaters using microalgae-based approaches: A circular bioeconomy perspective

The basic concepts of circular bioeconomy are reduce, reuse and recycle. Recovery of recyclable nutrients from secondary sources could play a key role in meeting the increased demands of the growing population. Wastewaters of different origin are rich in energy and nutrients sources that can be recovered and reused in a circular bioeconomy perspective. Microalgae can effectively utilize wastewater nutrients for growth and biomass production. Integration of wastewater treatment and microalgal cultivation improves the environmental impacts of the currently used wastewater treatment methods. This review provides comprehensive information on the potential of using microalgae for the recovery of carbon, nitrogen, phosphorus and other micronutrients from wastewaters. Major factors influencing large scale microalgal wastewater treatment are discussed and future research perspectives are proposed to foster the future development in this area.

Techno-Economic Study of CO2 Capture of a Thermoelectric Plant Using Microalgae (Chlorella vulgaris) for Production of Feedstock for Bioenergy

A current concern is the increase in greenhouse gas emissions, mainly CO2, with anthropogenic sources being the main contributors. Microalgae have greater capacity than terrestrial plants to capture CO2, with this being an attraction for using them as capture systems. This study aims at the techno-economic evaluation of microalgae biomass production, while only considering technologies with industrial scaling potential. Energy consumption and operating costs are considered as parameters for the evaluation. In addition, the capture of CO2 from a thermoelectric plant is analyzed, as a carbon source for the cultivation of microalgae. 24 scenarios were evaluated while using process simulation tools (SuperPro Designer), being generated by the combination of cultivations in raceway pond, primary harvest with three types of flocculants, secondary harvest with centrifugation and three filtering technologies, and finally the drying evaluated with Spray and Drum Dryer. Low biomass productivity, 12.7 g/m2/day, was considered, achieving a capture of 102.13 tons of CO2/year in 1 ha for the cultivation area. The scenarios that included centrifugation and vacuum filtration are the ones with the highest energy consumption. The operating costs range from US $ 4.75–6.55/kg of dry biomass. The choice of the best scenario depends on the final use of biomass.

Control schemes for a complex biorefinery plant for bioenergy and biobased products

This work proposes innovative feedback control schemes for a complex biorefinery plant which contains two continuous bioreactors: an anaerobic digester and a photobioreactor. The anaerobic digester is used to decompose organic matter inside a wastewater treatment process, the most useful final product being biogas/methane. The photobioreactor is used for a microalgae photosynthetic growth process where some components with added value are produced, and bio-mitigation of the carbon dioxide emissions is achieved. By using realistic models of the anaerobic digester and of the photobioreactor, novel adaptive and robust control schemes are designed. These proposed structures contain linearizing controllers, state observers and parameter estimators for the bioprocess unknown kinetics. The control designs are validated via numerical simulations that consider several realistic restrictions and disturbances which act on the process: unavailability of some biological variables, unknown and time-varying reaction kinetics, uncertain and time-varying influent flow rates, noisy measurements.

Influence of microalgae wastewater treatment culturing conditions on forward osmosis concentration process

Forward osmosis is envisioned as a technology for microalgae concentration but fouling propensity during dewatering is currently a limiting factor that requires better understanding. The purpose of this study is to define the impact of microalgae culturing conditions on the downstream forward osmosis (FO) separation process-water recovery and microalgae harvesting. Chlorella vulgaris was cultivated in an outdoor lab-scale reactor fed with synthetic wastewater mimicking primary settled municipal influent under changing environmental conditions (temperature, solar radiation, nutrient balance) with varying hydraulic retention time. High efficiency of nutrient removal was achieved under all tested conditions but microalgae autoflocculation and lower rate of pollutant removal were observed with batches where culturing temperature (6.5-21 ^°C), solar irradiation rate (181 W/m²), and nitrogen/phosphorous ratio (2.9) were below the optimal range. Regarding FO concentration, high initial water fluxes (in the range of 18.2 to 19.5 L·m²·h^-1) and water extraction rate (60.1-83.9%) were observed in all subsequent FO concentration tests. Significant membrane fouling (microalgae deposition on surface) associated with poor biomass recovery from the FO cell was found to be dependent on exopolymeric substance accumulation, which was a response to non-optimal environmental culturing conditions.

Salt Tolerance and Desalination Abilities of Nine Common Green Microalgae Isolates

In recent years, decline of freshwater resources has been recognized as one of the main environmental problems on global level. In addition to the increasing extent of primary salinization due to climate change, secondary salinization caused by human interventions is also a significantly increasing problem, therefore, the development of various chemical-free, biological desalination and removal procedures will become increasingly important. In the present study, the salinity tolerance, salinity, and nutrient reducing ability of nine common freshwater microalgae species from the genera Chlorella, Chlorococcum, Desmodesmus, Scenedesmus, and Monoraphidium were investigated. Our results proved that the studied green microalgae species are halotolerant ones, which are able to proliferate in environments with high salt concentrations. Furthermore, most of the species were able to reduce conductivity and remove significant amounts of chloride (up to 39%) and nutrients (more than 90% nitrate). The results proved that nitrate removal of the studied species was not influenced by salt concentration, only indirectly via growth inhibition. However, the results also highlighted that N:P ratio of the medium has primarily importance in satisfactory phosphorous removal. It can be concluded that assemblages of the studied microalgae species could be able to adapt to changing conditions even of salt-rich wastewaters and improve water quality during bioremediation processes.

Potential applications of algae in the cathode of microbial fuel cells for enhanced electricity generation with simultaneous nutrient removal and algae biorefinery: Current status and future perspectives

Production of biofuels and other value-added products from wastewater along with quality treatment is an uttermost necessity to achieve environmental sustainability and promote bio-circular economy. Algae-Microbial fuel cell (A-MFC) with algae in cathode chamber offers several advantages e.g. photosynthetic oxygenation for electricity recovery, CO₂-fixation, wastewater treatment, etc. However, performance of A-MFC depends on several operational parameters and also on electrode materials types; therefore, enormous collective efforts have been made by researchers for finding optimal conditions in order to enhance A-MFC performance. The present review is a comprehensive snapshot of the recent advances in A-MFCs, dealing two major parts: 1) the power generation, which exclusively outlines the effect of different parameters and development of cutting edge cathode materials and 2) wastewater treatment at cathode of A-MFC. This review provides fundamental knowledge, critical constraints, current status and some insights for making A-MFC technology a reality at commercial scale operation.

Comparative Thermogravimetric Assessment on the Combustion of Coal, Microalgae Biomass and Their Blend

In this work, thermogravimetric analysis (TGA), differential thermogravimetry (DTG), and differential scanning calorimetric (DSC) were used to assess the combustion of microalgae biomass, a bituminous coal, and their blend. Furthermore, different correlations were tested for estimating the high heating value of microalgae biomass and coal, with both materials possessing similar values. TGA evidenced differences between the combustion of the studied fuels, but no relevant interaction occurred during their co-combustion, as shown by the DTG and DSC curves. These curves also indicated that the combustion of the blend mostly resembled that of coal in terms of weight loss and heat release. Moreover, non-isothermal kinetic analysis revealed that the apparent activation energies corresponding to the combustion of the blend and coal were quite close. Overall, the obtained results indicated that co-combustion with coal might be a feasible waste to energy management option for the valorization of microalgae biomass resulting from wastewater treatment.