Microalgae-based advanced municipal wastewater treatment for reuse in water bodies

Long-term outdoor operation of microalgae-based digestate treatment: impact of external drivers on process performances and techno-economic assessment

A pilot-scale algal pond for liquid digestate treatment (5.8 m2, 0.75-1.83 m3) was operated outdoor for 1145 cumulative days. Key performance indicators were correlated with relevant external drivers including influent characteristics, weather conditions, and operational parameters, gaining information for management and optimisation. A techno-economic assessment allowed to calculate and validate process costs for a scaled-up plant (2-ha). Despite external factors, thanks to consistent nitrification, ammonium removal was high and stable during all trials (85 ± 8%). Environmental and operational conditions strongly influenced phosphate removal (51 ± 21%) and biomass productivity (10.4 ± 6.6 g TSS/m2/d). High biomass productivities were mostly associated to global radiation, while phosphate removal was influenced by digestate characteristics. The relatively low biomass production costs (6.9 €/kg or 7.4 $/kg) and substantial savings for liquid digestate bioremediation (3.4 €/m3 or 3.7 $/m3) and N removal (15.1 €/kg N or 16.2 $/kg N) confirmed the high potential of microalgae-based technologies as a cost-effective and sustainable alternative to conventional approaches for wastewater bioremediation and resource recovery.

Acid-tolerant microalgae-based winery wastewater treatment: performance evaluation and techno-economic analysis

This study evaluates the potential of two acid-tolerant microalgal strains, Desmodesmus sp. MAS1 and Heterochlorella sp. MAS3, for winery wastewater treatment (WWT) by including laboratory- and pilot-scale investigations, and techno-economic analysis (TEA). Laboratory experiments revealed significant growth in strain MAS1, especially in photobioreactors (PBRs), outpacing strain MAS3 with >125 % increase in biomass after four days. Both the strains showed lower but consistent growth in aquaria whereas biomass production was significantly higher in PBR. Nutrient removal in PBR was superior for organic carbon while it varied for nitrogen and phosphates, indicating system-specific advantages. Transitioning to pilot-scale high-rate algal pond with semi-continuous operations showcased higher growth rates in strain MAS1, achieving 1110 mg L-1 biomass yields and improved specific nutrient removal rates for organic carbon, nitrogen, and phosphates as compared to strain MAS3. When considered three processing capacities of 10, 50, and 100 m3 over 15 years, TEA revealed a positive net present value and internal rate of return based on the revenue generated from biomass, thus confirming the economic viability of microalgae-based WWT. Our study highlights the potential of microalgal systems for sustainable wastewater management, stressing the importance of system design and scaling for environmental and economic efficiency.

Less toxic combined microplastics exposure towards attached Chlorella sorokiniana in the presence of sulfamethoxazole while massive microalgal nitrous oxide emission under multiple stresses

Microalgae-based wastewater treatment could realize simultaneous nutrients recovery and CO2 sequestration. However, impacts of environmental microplastics (MPs) and antibiotic co-exposure on microalgal growth, nutrients removal, intracellular nitric oxide (NO) accumulation and subsequent nitrous oxide (N2O) emission are unclarified, which could greatly offset the CO2 sequestration benefit. To reveal the potential impacts of environmental concentrations of MPs and antibiotic co-exposure on microalgal greenhouse gas mitigation, this study investigated the effects of representative MPs (PE, PVC, PA), antibiotic sulfamethoxazole (SMX), and nitrite (NO2--N) in various combinations on attached Chlorella sorokiniana growth, nutrients removal, anti-oxidative responses, and N2O emission originated from intracellular NO build-up. Microalgal biofilm growth was more inhibited under 10 μg/L MPs than 100 μg/L SMX, and MPs+SMX co-exposure displayed toxicity antagonism while MPs+MPs co-exposure caused toxicity synergism (up to 66 % growth inhibition). Extracellular polysaccharides content correlated well with microalgal biofilm density under various stresses, while SMX involved stresses displayed chlorophyll a content reduction. Microalgal assimilation and MPs adsorption contributed to nutrients removal, and phosphorus removal displayed less variance among different stresses (residual phosphorus <0.5 mg/L) than nitrogen. Intracellular NO conversion to N2O almost doubled during the co-exposure processes, and N2O emission under NO2--N + PE+PVC co-exposure could offset the contribution of microalgal CO2 sequestration by as high as 176.2 %. Results of this study appealed for urgent concern regarding environmental MPs and antibiotic co-exposure on primary producers' growth characteristics and their greenhouse gas mitigation properties.

Impacts of tropical climate on outdoor treatment of anaerobically digested sanitary wastewater using native microalgae

Microalgal technologies interact with other processes, enabling treatment systems to remove nutrients and pollutants while facilitating the reuse of final effluents. However, the development of these systems in various climates and their specific characteristics has been poorly studied. The objective of this work was to evaluate the influence of different tropical seasons within the Cerrado Biome on effluent treatment in a closed photobioreactor using a consortium of native microalgae. Cultivation was performed on a bench scale using anaerobically digested sanitary wastewater processed by two mechanisms from the wastewater treatment plant in Bauru, Brazil: the Upflow Anaerobic Filter (UAF) and the Anaerobic Baffled Reactor (ABR). The cultivation took place in an outdoor area with constant aeration and under natural climatic conditions during two seasons: summer and winter. Each season's cultivation occurred in triplicate over 7 days. After cultivation, the effluent and microalgal biomass were separated using a tannin-based organic coagulant (Tanfloc SG) to evaluate effluent quality for non-potable reuse. During summer, the treatment of anaerobically digested sanitary wastewater from UAF achieved 76 ± 3 % and 84 ± 1.2 % phosphorus and nitrogen removal, respectively, while the treatment of anaerobically digested sanitary wastewater from ABR achieved 83 ± 4 % and 85 ± 3 % phosphorus and nitrogen removal, respectively. In winter, 86 ± 2 % and 89 ± 5 % of phosphorus and nitrogen, respectively, were removed from the anaerobically digested sanitary wastewater from UAF, while 68 ± 6 % and 93 ± 3 % were removed from the anaerobically digested sanitary wastewater from ABR. The removal of generic bacteria exceeded 3.0 log for most conditions, with the summer experiments showing absolute values of E. coli below 100 CFU·100 ml⁻1, indicating that the effluent could be used for unrestricted irrigation. Microalgal technology can serve as a tertiary treatment in countries with tropical climates, promoting the reintegration of water into the production cycle, which aligns with circular economy principles.

Modulation of Zn Ion Toxicity in Pisum sativum L. by Phycoremediation

Microalgae offer a promising alternative for heavy metal removal, and the search for highly efficient strains is ongoing. This study investigated the potential of two microalgae, Coelastrella sp. BGV (Chlorophyta) and Arthronema africanum Schwabe & Simonsen (Cyanoprokaryota), to bind zinc ions (Zn2⁺) and protect higher plants. Hydroponically grown pea (Pisum sativum L.) seedlings were subjected to ZnSO4 treatment for 7 days in either a nutrient medium (Knop) or a microalgal suspension. The effects of increasing Zn2⁺ concentrations were evaluated through solution parameters, microalgal dry weight, pea growth (height, biomass), and physiological parameters, including leaf gas exchange, chlorophyll content, and normalized difference vegetation index (NDVI). Zinc accumulation in microalgal and plant biomass was also analyzed. The results revealed that microalgae increased pH and oxygen levels in the hydroponic medium while enhancing Zn accumulation in pea roots. At low ZnSO4 concentrations (2-5 mM), microalgal suspensions stimulated pea growth and photosynthetic performance. However, higher ZnSO4 levels (10-15 mM) caused Zn accumulation, leading to nutrient deficiencies and growth suppression in microalgae, which ultimately led to physiological disturbances in peas. Coelastrella sp. BGV exhibited greater tolerance to Zn stress and provided a stronger protective effect when co-cultivated with peas, highlighting its potential for phycoremediation applications.

Design, construction and application of algae-bacteria synergistic system for treating wastewater

The wastewater treatment technology of algae-bacteria synergistic system (ABSS) is a promising technology which has the advantages of low energy consumption, good treatment effect and recyclable high-value products. In this treatment technology, the construction of an ABSS is a very important factor. At the same time, the emergence of some new technologies (such as microbial fuel cells and bio-carriers, etc.) has further enriched constructing the novel ABSS, which could improve the efficiency of wastewater treatment and the biomass harvesting rate. Thus, this review focuses on the construction of a novel ABSS in wastewater treatment in order to provide useful suggestions for the technology of wastewater treatment.

Enhancing Wastewater Depollution: Sustainable Biosorption Using Chemically Modified Chitosan Derivatives for Efficient Removal of Heavy Metals and Dyes

Driven by concerns over polluted industrial wastewater, particularly heavy metals and dyes, this study explores biosorption using chemically cross-link chitosan derivatives as a sustainable and cost-effective depollution method. Chitosan cross-linking employs either water-soluble polymers and agents like glutaraldehyde or copolymerization of hydrophilic monomers with a cross-linker. Chemical cross-linking of polymers has emerged as a promising approach to enhance the wet-strength properties of materials. The chitosan thus extracted, as powder or gel, was used to adsorb heavy metals (lead (Pb2+) and copper (Cu2+)) and dyes (methylene blue (MB) and crystal violet (CV)). Extensive analysis of the physicochemical properties of both the powder and hydrogel adsorbents was conducted using a range of analytical techniques, including Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), and scanning electron microscopy (SEM), as well as 1H and 13C nuclear magnetic resonance (NMR). To gain a comprehensive understanding of the sorption process, the effect of contact time, pH, concentration, and temperature was investigated. The adsorption capacity of chitosan powder for Cu(II), Pb(II), methylene blue (MB), and crystal violet (CV) was subsequently determined as follows: 99, 75, 98, and 80%, respectively. In addition, the adsorption capacity of chitosan hydrogel for Cu(II), Pb(II), MB, and CV was as follows: 85, 95, 85, and 98%, respectively. The experimental data obtained were analyzed using the Langmuir, Freundlich, and Dubinin-Radushkevich isotherm models. The isotherm study revealed that the adsorption equilibrium is well fitted to the Freundlich isotherm (R2 = 0.998), and the sorption capacity of both chitosan powder and hydrogel was found to be exceptionally high (approximately 98%) with the adsorbent favoring multilayer adsorption. Besides, Dubinin has given an indication that the sorption process was dominated by Van der Waals physical forces at all studied temperatures.

Probing interspecies metabolic interactions within a synthetic binary microbiome using genome-scale modeling

Aim: Metabolic interactions within a microbial community play a key role in determining the structure, function, and composition of the community. However, due to the complexity and intractability of natural microbiomes, limited knowledge is available on interspecies interactions within a community. In this work, using a binary synthetic microbiome, a methanotroph-photoautotroph (M-P) coculture, as the model system, we examined different genome-scale metabolic modeling (GEM) approaches to gain a better understanding of the metabolic interactions within the coculture, how they contribute to the enhanced growth observed in the coculture, and how they evolve over time. Methods: Using batch growth data of the model M-P coculture, we compared three GEM approaches for microbial communities. Two of the methods are existing approaches: SteadyCom, a steady state GEM, and dynamic flux balance analysis (DFBA) Lab, a dynamic GEM. We also proposed an improved dynamic GEM approach, DynamiCom, for the M-P coculture. Results: SteadyCom can predict the metabolic interactions within the coculture but not their dynamic evolutions; DFBA Lab can predict the dynamics of the coculture but cannot identify interspecies interactions. DynamiCom was able to identify the cross-fed metabolite within the coculture, as well as predict the evolution of the interspecies interactions over time. Conclusion: A new dynamic GEM approach, DynamiCom, was developed for a model M-P coculture. Constrained by the predictions from a validated kinetic model, DynamiCom consistently predicted the top metabolites being exchanged in the M-P coculture, as well as the establishment of the mutualistic N-exchange between the methanotroph and cyanobacteria. The interspecies interactions and their dynamic evolution predicted by DynamiCom are supported by ample evidence in the literature on methanotroph, cyanobacteria, and other cyanobacteria-heterotroph cocultures.

Competence of algal consortia under municipal wastewater: remediation efficiency, photosynthetic performance, antioxidant defense mechanisms and biofuel production

Utilizing monoalgal species for wastewater treatment is facing tremendous challenges owing to changing wastewater complexity in terms of physico-chemical characteristic, nutrient and metal concentration. The environmental conditions are also fluctuating therefore, the formation of robust system is of utmost importance for concomitant sustainable wastewater treatment and bioenergy production. In the present study, the tolerance and adaptability potential of algal consortia-1 (Chlorococcum humicola and Tetradesmus sp.) and consortia-2 (Chlorococcum humicola, Scenedesmus vacuolatus and Tetradesmus sp.) treated with municipal wastewater were examined under natural environmental conditions. The results exhibited that consortia-2 was more competent in recovering nitrate-nitrogen (82.92%), phosphorus (70.47%), and heavy metals (31-73.70%) from municipal wastewater (100%) than consortia-1. The results further depicted that total chlorophyll, carbohydrate, and protein content decreased significantly in wastewater-treated consortia-1 as compared to consortia-2. However, lipid content was increased by 4.01 and 1.17 folds in algal consortia-1 and consortia-2 compared to their respective controls. Moreover, absorption peak at 1740.6 cm-1 reflected higher biofuel-producing potential of consortia-1 as compared to consortia-2 as confirmed through FTIR spectroscopy. The results also revealed that consortia-2 showed the highest photosynthetic performance which was evident from the increment in the active photosystem-II reaction center (1.724 ± 0.068), quantum efficiency (0.633 ± 0.038), and performance index (3.752 ± 0.356). Further, a significant increase in photosynthetic parameters was observed in selected consortia at lag phase, while a noteworthy decline was observed at exponential and stationary phases in consortia-1 than consortia-2. The results also showed the maximum enhancement in ascorbic acid (2.43 folds), proline (3.34 folds), and cysteine (1.29 folds) in consortia-2, while SOD (1.75 folds), catalase (2.64 folds), and GR (1.19 folds) activity in consortia-1. Therefore, it can be concluded that due to remarkable flexibility and photosynthetic performance, consortia-2 could serve as a potential candidate for sustainable nutrient resource recovery and wastewater treatment, while consortia-1 for bio-fuel production in a natural environment. Thus, formation of algal consortia as the robust biosystem tolerates diverse environmental fluctuations together with wastewater complexity and ultimately can serve appropriate approach for environmental-friendly wastewater treatment and bioenergy production.

Effects of staged multiple phytohormones application on capillary-driven attached Chlorella sp. biofilm

Comparing with single phytohormone application, applying multiple phytohormones to microalgae-based wastewater treatment systems can offer more extensive growth-promoting and stress-protecting effects for microalgae, yet the advantage of stress-relieving salicylic acid (SA) under combined phytohormones application scenario has not been exploited. Employing the improved capillary-driven attached microalgae culturing device (CD-PBR) previously used for single phytohormone application, this study compared the effects of mixed and single phytohormone(s) addition under as low as 10-7 M dosage. In order to make the best of SA for its stress-relieving property, postponed SA addition combined with applying other phytohormone(s) at the beginning of microalgae cultivation was also investigated. Combination of 10-6 M 6-benzylaminopurine (6-BA) with 10-7 M SA was sufficient for enhancing growth-promoting effects and anti-oxidative responses for attached Chlorella sp., while indole-3-acetic acid (IAA) addition was unnecessary. Combination of 6-BA addition at the beginning while postponed SA addition on Day 4 could further sustain such beneficial effects, while removing up to 99.7% total nitrogen (TN) and 97.9% total phosphorus (TP) from the bulk liquid. These results provided innovative strategies on mixed phytohormones addition for microalgae.

Potential of Porous Substrate Bioreactors for Removal of Pollutants from Wastewater Using Microalgae

Porous substrate bioreactors (PSBRs) are a new technology to grow microalgae immobilized in a dense culture and solve some problems linked to suspended cultivation. During recent years, this technology has been used in laboratory and pilot setups in different fields of environmental biotechnology, such as wastewater treatment. The aim of this short review is to introduce the PSBR technology, summarize the results obtained in removing some pollutants from wastewater, provide an assessment of the potential of PSBRs for wastewater treatment, and the subsequent use of the algal biomass for other purposes.

Wastewater treatment by using microalgae: Insights into fate, transport, and associated challenges

The remediation of wastewater with microalgae is a new topic that concentrates on devising a cost-effective and environmentally beneficial method. Multiple microalgae and bacterial consortiums have recently been evaluated to determine if they can purify effluent from various sources. Critical to a system's efficacy is its ability to remove nutrients such as nitrogen (N) and phosphorus (P) and heavy metals such as arsenic (As), lead (Pb), and copper (Cu). This study compared traditional wastewater treatment systems to microalgae-based systems for treating different types of wastewater. The research investigates the potential for microalgae to cleanse wastewater. The research also evaluates wastewater parameters, methods, and scientific techniques for extracting nutrients and heavy metals from polluted water. According to the literature, Microalgae can remove between 98.7% and 100% of nitrogen (N), phosphorous (P), and heavy metals from various effluents. The paper concludes by discussing the difficulties of using microalgae to remediate wastewater. The elimination of nutrients from the effluent is influenced by biomass production, osmotic capacity, temperature, pH, and O2 concentration. Therefore, a "pilot" study is recommended to investigate contaminants.

Biodiesel production and simultaneous treatment of domestic and livestock wastewater using indigenous microalgae, Chlorella sorokiniana JD1-1

In this study, the potential of Chlorella sorokiniana JD1-1 for biodiesel production was evaluated using domestic wastewater (DWW) as a diluent for locally-generated livestock wastewater (LWW). This strategy aimed to provide sustainable wastewater treatment, reduce environmental impacts, enhance cost-effectiveness, and promote biodiesel production. LWW was diluted with tap water and DWW at ratios of 75%, 50%, and 25% (v/v), and the effects on microalgal growth, nutrient removal efficiency, and lipid yield were evaluated. Although the maximum biomass concentration was observed in the artificial growth medium (BG-11) (1170 mg L-1), 75% dilution using tap water (610 mg L-1) and DWW (780 mg L-1) yielded results comparable to the exclusive use of DWW (820 mg L-1), suggesting a potential for substitution. Total nitrogen (TN) removal rates were consistently high under all conditions, particularly in samples with higher concentrations of LWW. Conversely, total phosphorus (TP) concentrations decreased under most conditions, although some displayed large increases. Further studies are necessary to optimize the nutrient balance while maintaining economic feasibility and maximizing biodiesel production.

Growth Efficiency of Chlorella sorokiniana in Synthetic Media and Unsterilized Domestic Wastewater

Incorporating a variety of microalgae into wastewater treatment is considered an economically viable and environmentally sound strategy. The present work assessed the growth characteristics of Chlorella sorokiniana during cultivation in balanced synthetic media and domestic wastewater. Increasing the NH4+-N concentration to 360 mg L-1 and adding extra PO43--P and SO42--S (up to 80 and 36 mg L-1, respectively) contributed to an increase in the total biomass levels (5.7-5.9 g L-1) during the cultivation of C. sorokiniana in synthetic media. Under these conditions, the maximum concentrations of chlorophylls and carotenoids were 180 ± 7.5 and 26 ± 1.4 mg L-1, respectively. Furthermore, when studying three types of domestic wastewaters, it was noted that only one wastewater contributed to the productive growth of C. sorokiniana, but all wastewaters stimulated an increased accumulation of protein. Finally, the alga, when growing in optimal unsterilized wastewater, showed a maximum specific growth rate of 0.73 day-1, a biomass productivity of 0.21 g L-1 day-1, and 100% NH4+-N removal. These results demonstrate that the tested alga actively adapts to changes in the composition of the growth medium and accumulates high levels of protein in systems with poor-quality water.

Microalgae systems - environmental agents for wastewater treatment and further potential biomass valorisation

Water is the most valuable resource on the planet. However, massive anthropogenic activities generate threatening levels of biological, organic, and inorganic pollutants that are not efficiently removed in conventional wastewater treatment systems. High levels of conventional pollutants (carbon, nitrogen, and phosphorus), emerging chemical contaminants such as antibiotics, and pathogens (namely antibiotic-resistant ones and related genes) jeopardize ecosystems and human health. Conventional wastewater treatment systems entail several environmental issues: (i) high energy consumption; (ii) high CO₂ emissions; and (iii) the use of chemicals or the generation of harmful by-products. Hence, the use of microalgal systems (entailing one or several microalgae species, and in consortium with bacteria) as environmental agents towards wastewater treatment has been seen as an environmentally friendly solution to remove conventional pollutants, antibiotics, coliforms and antibiotic resistance genes. In recent years, several authors have evaluated the use of microalgal systems for the treatment of different types of wastewater, such as agricultural, municipal, and industrial. Generally, microalgal systems can provide high removal efficiencies of: (i) conventional pollutants, up to 99%, 99%, and 90% of total nitrogen, total phosphorus, and/or organic carbon, respectively, through uptake mechanisms, and (ii) antibiotics frequently found in wastewaters, such as sulfamethoxazole, ciprofloxacin, trimethoprim and azithromycin at 86%, 65%, 42% and 93%, respectively, through the most desirable microalgal mechanism, biodegradation. Although pathogens removal by microalgal species is complex and very strain-specific, it is also possible to attain total coliform and Escherichia coli removal of 99.4% and 98.6%, respectively. However, microalgal systems' effectiveness strongly relies on biotic and abiotic conditions, thus the selection of operational conditions is critical. While the combination of selected species (microalgae and bacteria), ratios and inoculum concentration allow the efficient removal of conventional pollutants and generation of high amounts of biomass (that can be further converted into valuable products such as biofuels and biofertilisers), abiotic factors such as pH, hydraulic retention time, light intensity and CO₂/O₂ supply also have a crucial role in conventional pollutants and antibiotics removal, and wastewater disinfection. However, some rationale must be considered according to the purpose. While alkaline pH induces the hydrolysis of some antibiotics and the removal of faecal coliforms, it also decreases phosphates solubility and induces the formation of ammonium from ammonia. Also, while CO₂ supply increases the removal of E. coli and Pseudomonas aeruginosa, as well as the microalgal growth (and thus the conventional pollutants uptake), it decreases Enterococcus faecalis removal. Therefore, this review aims to provide a critical review of recent studies towards the application of microalgal systems for the efficient removal of conventional pollutants, antibiotics, and pathogens; discussing the feasibility, highlighting the advantages and challenges of the implementation of such process, and presenting current case-studies of different applications of microalgal systems.

Phycobiliprotein recovery coupled to the tertiary treatment of wastewater in semi-continuous photobioreactors. Tracking contaminants of emerging concern

This study evaluated a tertiary wastewater treatment technology using cyanobacteria to recover value-added phycobiliproteins. The presence of contaminants of emerging concern (CECs) in wastewater, cyanobacteria biomass and pigments recovered were also analyzed. For this, a wastewater-borne cyanobacterium (Synechocystis sp. R2020) was used to treat secondary effluent from a municipal wastewater treatment plant, with and without nutrients supplementation. Then, the stability of phycobiliprotein production was assessed by operating the photobioreactor in semi-continuous mode. Results showed similar biomass productivity with and without nutrients supplementation (153.5 and 146.7 mg L^-1 d^-1, respectively). Upon semi-continuous operation, the phycobiliprotein content was stable and reached up to 74.7 mg gDW^-1. The phycocyanin purity ratio ranged from 0.5 to 0.8, corresponding to food grade (> 0.7). Out of 22 CECs detected in secondary effluent, only 3 were present in the phycobiliprotein extract. In order to identify applications, prospective research should focus on CECs removal during pigment purification.

Improving the Quality of Reclaimed Water via Applying Spirulina platensis to Eliminate Residual Nitrate

The application of reclaimed water has been recognized as the key approach for alleviating water scarcity, while its low quality, such as high nitrogen content, still makes people worry about the corresponding ecological risk. Herein, we investigated the feasibility of removing residual nitrate from reclaimed water by applying Spirulina platensis. It is found that 15 mg/L total nitrogen could be decreased to 1.8 mg/L in 5 days, equaling 88.1 % removal efficiency under the optimized conditions. The deficient phosphorus at 0.5-1.0 mg/L was rapidly eliminated but was already sufficient to support nitrate removal by S. platensis. The produced ammonia is generally below 0.2 mg/L, which is much lower than the standard limit of 5 mg/L. In such a nutrient deficiency condition, S. platensis could maintain biomass growth well via photosynthesis. The variation of pigments, including chlorophyll a and carotenoids, suggested a certain degree of influences of illumination intensity and phosphorus starvation on microalgae. The background cations Cu²⁺ and Zn²⁺ exhibited significant inhibition on biomass growth and nitrate removal; thus, more attention needs to be paid to the further application of microalgae in reclaimed water. Our results demonstrated that cultivation of S. platensis should be a very promising solution to improve the quality of reclaimed water by efficiently removing nitrate and producing biomass.

Development of economical and sustainable cultivation system for biomass production and simultaneous treatment of municipal wastewater using Tetraselmis indica BDUG001

Microalgal-based bioprocess offers several advantages including wastewater reclamations, therefore present study assessed the usability of the combination of untreated municipal sewage wastewater (UTMSWW) and secondary treated municipal sewage wastewater (STSWW) for nutrient removal and recovery by Tetraselmis indica (T. indica) BDUG001. The present study optimized the additional nutrient supplementations (e.g. ASN-III) percentage and day-night cycle, pH and pH with aeration for monitoring high-rate biomass production and nutrient recovery. The study results showed that the combination of 75% UTMSWW + 25% ASN-III supported maximum biomass production (2.65 ± 0.07 g/L). In the optimized day-night cycle (12:12 h), T. indica BDUG001 showed improved biomass production (2.75 ± 0.07 g/L), biomass productivity (165.63 ± 4.42 mg/L/d), and photosynthetic pigments production. Under optimized pH∼ 7.0 with aeration, maximum total nitrate (TN) removal efficiency (87.67 ± 3.08-91.55 ± 1.92%) was observed, while COD and TP removal was maximum at pH ∼ 9.0. The maximum biomass production (2.35 ± 0.07-2.77 ± 0.04 g/L) with biomass productivity (93.75 ± 167.19 ± 2.21 mg/L/d) and lipid content (42.98 ± 1.86-47.85 ± 0.21% DCW) were also at pH 7.0. with aeration. The present study verified the utilization of UTMSWW with the combination of conventional medium, optimized day-night cycle, pH with aeration along with designing low-cost PBR. It was the ideal system for the cultivation of T. indica BDUG001 for the recovery of nutrients from wastewater, production of biofuels and value-added feedstock.

The long overlooked microalgal nitrous oxide emission: Characteristics, mechanisms, and influencing factors in microalgae-based wastewater treatment scenarios

Microalgae-based wastewater treatment is particularly advantageous in simultaneous CO₂ sequestration and nutrients recovery, and has received increasing recognition and attention in the global context of synergistic pollutants and carbon reduction. However, the fact that microalgae themselves can generate the potent greenhouse gas nitrous oxide (N₂O) has been long overlooked, most previous research mainly regarded microalgae as labile organic carbon source or oxygenic approach that interfere bacterial nitrification-denitrification and the concomitant N₂O production. This study, therefore, summarized the amount and rate of N₂O emission in microalgae-based systems, interpreted in-depth the multiple pathways that lead to NO formation as the key precursor of N₂O, and the pathways that transform NO into N₂O. Reduction of nitrite could take place in either the cytoplasm or the mitochondria to form NO by a series of enzymes, while the NO could be enzymatically reduced to N₂O at the chloroplasts or the mitochondria respectively under light and dark conditions. The influences of abiotic factors on microalgal N₂O emission were analyzed, including nitrogen types and concentrations that directly affect the nitrogen transformation routes, illumination and oxygen conditions that regulate the enzymatic activities related to N₂O generation, and other factors that indirectly interfere N₂O emission via NO regulation. The uncertainty of microalgae-based N₂O emission in wastewater treatment scenarios were emphasized, which would be particularly impacted by the complex competition between microalgae and ammonia oxidizing bacteria or nitrite oxidizing bacteria over ammonium or inorganic carbon source. Future studies should put more efforts in improving the compatibility of N₂O emission results expressions, and adopting consistent NO detection methods for N₂O emission prediction. This review will provide much valuable information on the characteristics and mechanisms of microalgal N₂O emission, and arouse more attention to the non-negligible N₂O emission that may impair overall greenhouse gas reduction efficiency in microalgae-based wastewater treatment systems.

Microalgae-based wastewater treatment: Mechanisms, challenges, recent advances, and future prospects

The rapid expansion of both the global economy and the human population has led to a shortage of water resources suitable for direct human consumption. As a result, water remediation will inexorably become the primary focus on a global scale. Microalgae can be grown in various types of wastewaters (WW). They have a high potential to remove contaminants from the effluents of industries and urban areas. This review focuses on recent advances on WW remediation through microalgae cultivation. Attention has already been paid to microalgae-based wastewater treatment (WWT) due to its low energy requirements, the strong ability of microalgae to thrive under diverse environmental conditions, and the potential to transform WW nutrients into high-value compounds. It turned out that microalgae-based WWT is an economical and sustainable solution. Moreover, different types of toxins are removed by microalgae through biosorption, bioaccumulation, and biodegradation processes. Examples are toxins from agricultural runoffs and textile and pharmaceutical industrial effluents. Microalgae have the potential to mitigate carbon dioxide and make use of the micronutrients that are present in the effluents. This review paper highlights the application of microalgae in WW remediation and the remediation of diverse types of pollutants commonly present in WW through different mechanisms, simultaneous resource recovery, and efficient microalgae-based co-culturing systems along with bottlenecks and prospects.

Growth-promoting effects of phytohormones on capillary-driven attached Chlorella sp. biofilm

Using low strength wastewater for microalgae cultivation is challenged by slow growth and biomass harvesting issue in suspended systems, and growth-promoting effects of phytohormones at currently recommended dosages could neither obtain high enough biomass concentrations nor economic feasibility. This study aims to solve the issues of slow growth, biomass harvest, and phytohormone costs altogether by supplementing low dosage phytohormones in an improved capillary-driven attached cultivation device. The device displayed nutrients-condensing properties, and dosages of indole acetic acid (IAA), 6-benzylaminopurine (6-BA), and salicylic acid (SA) for highest microalgal growth were respectively 10-6 M, 10-6 M, and 10-7 M, being at least one order of magnitude lower than in suspended cultures. SA was most effective in growth-promoting (up to 7.0 g/m2 biomass density) and nutrients uptake (up to 98.6 % from the bulk environment), while IAA was most effective in antioxidative defenses. These results provided new insights in cost-effective and harvesting-convenient microalgae production.

Isolation and screening of indigenous microalgae species for domestic and livestock wastewater treatment, biodiesel production, and carbon sequestration

The use of indigenous microalgae strains for locally generated domestic (DWW) and livestock wastewater (LWW) treatment is essential for effective and economical applications. Phototrophic microalgae-based biofuel production also contributes to carbon sequestration via CO₂ fixation. However, simultaneous consideration of both isolation and screening procedures for locally collected indigenous microalgae strains is not common in the literature. We aimed to isolate indigenous microalgae strains from locally collected samples on coastlines and islands in South Korea. Among five isolated strains, Chlorella sorokiniana JD1-1 was selected for DWW and LWW treatment due to its ability to grow in waste resources. This strain showed a higher specific growth rate in DWW than artificial growth medium (BG-11) with a range of 0.137-0.154 d^-1. During cultivation, 96.5%-97.1% of total nitrogen in DWW and 89.2% in LWW was removed. Over 99% of total phosphorus in DWW and 96.4% in LWW was also removed. Finally, isolated C. sorokiniana JD1-1 was able to fix CO₂ within a range of 0.0646-0.1043 g CO₂ L^-1 d^-1. These results support the domestic applications of carbon sequestration-efficient microalgae in the waste-to-energy nexus.

Microalgae's prospects in attaining sustainable economic and environmental development

Sustainable Development Goals (SDGs) have been part of much worldwide cooperation in engineering design, nutrients production that contributes towards a better and more sustainable future. This review intends to uncover a potential renewable source that could significantly contribute to various goals under the SDGs. The prospects of algae tackling the socio-ecological, economic, and environmental issues faced globally are discussed, along with approaches of algae that can be utilized to achieve many of the SDGs are reviewed and discussed. Moreover, the recent trends in terms of engineering application that co-relate to novel algae-based technology has also been included. Apart from that, algae have high oil content which is suitable for producing affordable and clean energy, which can be used for biofuels or electricity generation. The promising characteristics of algae will lead to its global acceptance and utilization for sustainability to help create a better world.

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

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

Occurrence of antibiotics in waters, removal by microalgae-based systems, and their toxicological effects: A review

Global antibiotics consumption has been on the rise, leading to increased antibiotics release into the environment, which threatens public health by selecting for antibiotic resistant bacteria and resistance genes, and may endanger the entire ecosystem by impairing primary production. Conventional bacteria-based treatment methods are only moderately effective in antibiotics removal, while abiotic approaches such as advanced oxidation and adsorption are costly and energy/chemical intensive, and may cause secondary pollution. Considered as a promising alternative, microalgae-based technology requires no extra chemical addition, and can realize tremendous CO₂ mitigation accompanying growth related pollutants removal. Previous studies on microalgae-based antibiotics removal, however, focused more on the removal performances than on the removal mechanisms, and few studies have concerned the toxicity of antibiotics to microalgae during the treatment process. Yet understanding the removal mechanisms can be of great help for targeted microalgae-based antibiotics removal performances improvement. Moreover, most of the removal and toxicity studies were carried out using environment-irrelevant high concentrations of antibiotics, leading to reduced guidance for real-world situations. Integrating the two research fields can be helpful for both improving antibiotics removal and avoiding toxicological effects to primary producers by the residual pollutants. This study, therefore, aims to build a link connecting the occurrence of antibiotics in the aquatic environment, the removal of antibiotics by microalgae-based processes, and the toxicity of antibiotics to microalgae. Distribution of various categories of antibiotics in different water environments were summarized, together with the antibiotics removal mechanisms and performances in microalgae-based systems, and the toxicological mechanisms and toxicity of antibiotics to microalgae after either short-term or long-term exposure. Current research gaps and future prospects were also analyzed. The review could provide much valuable information to the related fields, and provoke interesting thoughts on integrating microalgae-based antibiotics removal research and toxicity research on the basis of environmentally relevant concentrations.

A Review on the Reliability and the Readiness Level of Microalgae-Based Nutrient Recovery Technologies for Secondary Treated Effluent in Municipal Wastewater Treatment Plants

Algae-based wastewater treatment technologies are promising green technologies with huge economical potential and environmental co-benefits. However, despite the immense research, work, and achievement, no publications were found wherein these technologies have been successfully applied in an operational environment for nitrogen and phosphorus removal of secondary treated effluent in municipal wastewater treatment plants. Based on a literature review and targeted comprehensive analysis, the paper seeks to identify the main reasons for this. The reliability (considering inlet wastewater quality variations, operating conditions and process control, algae harvesting method, and produced biomass) as well as the technology readiness level for five types of reactors are discussed. The review shows that the reactors with a higher level of control over the technological parameters are more reliable but algal post-treatment harvesting and additional costs are barriers for their deployment. The least reliable systems continue to be attractive for research due to the non-complex operation and relieved expenditure costs. The rotating biofilm systems are currently undertaking serious development due to their promising features. Among the remaining research gaps and challenges for all the reactor types are the identification of the optimal algal strains, establishment of technological parameters, overcoming seasonal variations in the effluent’s quality, and biomass harvesting.

Progress and challenges of contaminate removal from wastewater using microalgae biomass

The utilization of microalgae in treating wastewater has been an emerging topic focussed on finding an economically sustainable and environmentally friendly approach to treating wastewater. Over the last several years, different types of con microalgae and bacteria consortia have been experimented with to explore their potential in effectively treating wastewater from different sources. The basic features considered while determining efficiency is their capacity to remove nutrients including nitrogen (N) and phosphorus (P) and heavy metals like arsenic (As), lead (Pb), and copper (Cu). This paper reviews the efficiency of microalgae as an approach to treating wastewater from different sources and compares conventional and microalgae-based treatment systems. The paper also discusses the characteristics of wastewater, conventional methods of wastewater treatment that have been used so far, and the technological mechanisms for removing nutrients and heavy metals from contaminated water. Microalgae can successfully eliminate the suspended nutrients and have been reported to successfully remove N, P, and heavy metals by up to 99.6 %, 100 %, and 13%-100 % from different types of wastewater. However, although a microalgae-based wastewater treatment system offers some benefits, it also presents some challenges as outlined in the last section of this paper. Performance in eliminating nutrients from wastewater is affected by different parameters such as temperature, biomass productivity, osmotic ability, pH, O₂ concentration. Therefore, the conducting of pilot-scale studies and exploration of the complexities of contaminants under complex environmental conditions is recommended.

Removal of nitrate and phosphate from simulated agricultural runoff water by Chlorella vulgaris

Microalgae such Chlorella vulgaris can effectively absorb nitrate and phosphate from contaminated water. This work characterized nitrate and phosphate removal from simulated agricultural runoff using C. vulgaris. Statistically designed experiments were used to model the following responses: (1) algal growth; (2) nitrate removal; (3) phosphate removal; (4) protein in the algal biomass; (5) chlorophyll content of the biomass; (6) the biomass phenolics content; and (7) the free radical scavenging antioxidant activity of the biomass. These response were modelled for the following key experimental factors: initial nitrate concentration in the simulated runoff (1080-3240 mg L^-1, as NaNO₃), initial phosphate concentration (20-60 mg L^-1, as K₂HPO₄), photoperiod (8-24 h of light/day) and culture duration (5-15 days). The validated models were used to identify the factor levels to maximize the various responses. Nitrate removal was maximized at 85.6% when initial nitrate and phosphate concentrations were 2322 mg L^-1 and 38 mg L^-1 (N:P atom ratio ≈ 125:1), respectively, with a 17.2 h daily photoperiod in a 13-day culture. Phosphate removal was maximized at 95% when the initial nitrate and phosphate concentrations were 1402 mg L^-1 and 56.7 mg L^-1 (N:P ≈ 51:1), respectively, with a 15.7 h daily photoperiod in a 14.7-day culture. At least ~14 h of a daily photoperiod and a ~11-day culture period were required to maximize all the studied responses. C. vulgaris is edible and may be used as animal feed. Nutritional aspects of the biomass were characterized. Biomass with more than 24% protein could be produced. Under the best conditions, the chlorophyll (potential food colorants) content of the biomass was 8.5% and the maximum level of total phenolics (antioxidants) in the biomass was nearly 13 mg gallic acid equivalent g^-1.

Insights into the potential impact of algae-mediated wastewater beneficiation for the circular bioeconomy: A global perspective

Algae-based technologies are one of the emerging solutions to societal issues such as accessibility to clean water and carbon-neutral energy and are a contender for the circular bioeconomy. In this review, recent developments in the use of different algal species for nutrient recovery and biomass production in wastewater, challenges, and future perspectives have been addressed. The ratio and bioavailability of nutrients in wastewater are vital parameters, which significantly impact nutrient recovery efficiency and algal biomass production. However, the optimum nutrient concentration and ratio may vary depending upon the microalgal species as well as cultivation conditions. The use of indigenous algae and algae-based consortia with other microorganisms has been proved promising in improving nutrient recovery efficiency and biomass production in pilot scale operations. However, environmental and cultivation conditions also play a significant role in determining the feasibility of the process. This review further focused on the assessment of the potential benefits of algal biomass production, renewable biofuel generation, and CO₂ sequestration using wastewater in different countries on the basis of available data on wastewater generation and estimated nutrient contents. It was estimated that 5-10% replacement of fossil crude requirement with algal biofuels would require ~952-1903 billion m³ of water, 10-21 billion tons of nitrogen, and 2-4 billion tons of phosphorus fertilizers. In this context, coupling wastewater treatment and algal biomass production seem to be the most sustainable option with potential global benefits of polishing wastewater through nutrients recycling and carbon dioxide sequestration.

Mixotrophic cultivation of microalgae coupled with anaerobic hydrolysis for sustainable treatment of municipal wastewater in a hybrid system of anaerobic membrane bioreactor and membrane photobioreactor

This study investigated the possibility of coupling anaerobic hydrolysis in an anaerobic membrane bioreactor (AnMBR) with mixotrophic microalgae cultivation in a membrane photobioreactor (MPBR) for the sustainable treatment of municipal wastewater. Using the hydrolyzed wastewater discharged from AnMBR, Chlorella pyrenoidosa in MPBR grew in a mixotrophic mode and realized rapid growth. During the stable operation, MPBR achieved average carbon capture rate of 42.82 mg L^-1 d^-1 and algal lipid production rate of 19.66 mg L^-1 d^-1. The average reduction in TN, TP, and TOC during stable operation was 96.7%, 98.0%, and 95.9%, respectively. Mass balance analysis showed that the overall system captured 14.76 mg of carbon from the atmosphere per liter of wastewater treated. Therefore, this AnMBR-MPBR hybrid system simultaneously realized advanced treatment of municipal wastewater, efficient production of algal lipid, and carbon capture from atmosphere, and thus has a good potential in the sustainable treatment of municipal wastewater.

Sustainable resource production for manufacturing bioactives from micro- and macroalgae: Examples from harvesting and cultivation in the Nordic region

Micro- and macroalgae are a great and important source of raw material for manufacturing of bioactives and ingredients for food, feed, cosmetics, or pharmaceuticals. Macroalgae (or seaweeds) have been harvested locally from wild stocks in smaller volumes for a long time, and a production chain based on cultivated seaweed for the harvest of considerably larger amounts is in progress for several species. Microalgae and cyanobacteria such as Spirulina have been produced in "backyard ponds" for use in food and feed also for a long time, and now we see the establishment of large production plants to control the cultivation process and increase the production yields. There is also a shift from harvesting or cultivation centered in warmer, sunnier areas to increasing exploitation of natural resources in temperate to boreal regions. In locations with strong seasonal variations in solar irradiance and temperatures, we need to develop procedures to maximize the biomass production in the productive seasons and ensure efficient stabilization of the biomass for year-round processing and product manufacturing. Industrialized biomass production and large-scale manufacturing of bioactives also mean that we must employ sustainable, cost-effective, and environmentally friendly processing methods, including stabilization and extraction methods such as ensiling and subcritical water extraction (SWE) and advanced analytic tools to characterize the products. These topics are focus areas of the Nordic Centre of Excellence (NCoE) NordAqua, and here we present a review of current activities in the field of micro- and macroalgae biomass production sectors illustrated with some of our experiences from the NordAqua consortium.

The efficiency of microalgae biofilm in the phycoremediation of water from River Kaduna

This study is aimed at investigating the efficiency of microalgae biofilm in the phycoremediation of water from a polluted river. Freshwater microalgae biofilm inherent in a contaminated petrochemical stream was employed to remediate water from the River Kaduna, which is the largest river in Kaduna town, Kaduna State, Nigeria, and serves as the primary water source in Kaduna town. The results indicate high reduction efficiency of some physicochemical parameters and pollutants (turbidity (71%), conductivity (9.8%), sulfate (37.5%), alkalinity (62.5%), chloride (11.5%), TDS (9.9%), TSS (66.7%), nitrate (42.9%), COD (24%), and BOD (33%), Cd (70.0%), Ni (74.0%) and Pb (71.0%)), indicating the effectiveness of microalgae biofilm in the phycoremediation of water from River Kaduna. According to scanning electron microscope (SEM) observation, the microalgae biofilm has rough surface morphology after the treatment of the river water, which implies that the biofilm was capable of removing the pollutants in water via biosorption. Other characterizations such as XRF, XRD, and FTIR also buttressed that biosorption was the primary removal mechanism of pollutants by microalgae biofilm. Besides, the results also show the production of ROS during the treatment of water from the River Kaduna by the microalgae biofilm. This high concentration of ROS produced during the treatment correlates significantly with pollutant degradation. The GC-MS analysis of the microalgae biofilm shows the involvement of some phytochemicals in the process of pollutant degradation. As a result, microalgae biofilm is a simple and cost-effective method of polluted water phycoremediation with promising applications and future prospects.

Bioremediation of ossein effluents using the filamentous marine cyanobacterium Cylindrospermum stagnale

Wastewater containg proteinaceous ossein effluents are problematic to be treated. We studied the possibility to treat ossein effluents with the marine cyanobacterium strain Cylindrospermum stagnale. After optimizing the culture conditions of the bacterium, three different types of ossein effluents were tested: dicalcium phosphate (DCP), high total dissolved solids (HTDS) and low total dissolved (LTDS). The effluents were diluted with sea water at the following ratios 1:1, 2:1 and 3:2. The optimum operating conditions were at 3000 lux light intensity and 37 °C temperature. The highest degradation of ossein effluens by C. stagnale was attained for a dilution ratio of 1:1. However, less diluted ossein effluents reduced the growth of C. stagnale drastically. The degradation was shown by measuring the chlorophyll a content and the dry weight of bacterial cells during a seven-day incubation period degradation. Fourier Transform Infrared Spectroscopy (FT-IR) analysis verified the degradation showing the presence of the degradation products of ossein (i.e. calcium carbonate and calcite) in the culture medium. Lipid composition in fatty acids appeared to be suitable for biofuel production. The results showed that the marine cyanobacterium C. stagnale can be used to treat ossein effluents, and at the same time, to produce biofuel in a sustainable way.

Microalgal culture in animal cell waste medium for sustainable 'cultured food' production

'Cultured food' has tremendous potential as a sustainable meat alternative. Increased cultured food production is increasing the amount of waste medium from cell culture. Nitrogen- and phosphorus-containing compounds in waste medium can cause eutrophication of water bodies. Currently, microalgae are used in energy production, environmental protection, agriculture and pharmaceutical and health food industries. Here, we used the microalgae, Chlorococcum littorale and Chlorella vulgaris and the waste medium of C2C12 cells for a case study. We found that 80% and 26% of ammonia and 16% and 15% of phosphorus in the waste medium were consumed by C. littorale and C. vulgaris, respectively. In addition, C. littorale and C. vulgaris proliferated 3.2 folds and 1.6 folds, respectively, after seven days in the waste medium that was enhanced by adjusting medium salt concentration. This report demonstrates the potential of sustainability for solving the issue of waste medium production during the production of cultured food.

Analysis of the Status and Improvement of Microalgal Phosphorus Removal from Municipal Wastewater

Phosphorus, as one of the main pollutants in municipal sewage, has received increasing attention recently. Phosphorus recovery also increases the sustainable development of municipal wastewater. Since algae have the ability to effectively redirect nutrients, including phosphorus, from municipal sewage to algae biomass, municipal sewage treatments involving microalgae have piqued the interest of many researchers. The phosphorus removal depends on the potential of the microalgae to absorb, preserve, or degrade phosphorus in municipal wastewater. It is, therefore, of great interest to study the mechanisms underlying the absorption, storage, and degradation of phosphorus by microalgae to ensure the viability of this phosphorus removal process in wastewater. The objectives of this review were to summarize phosphorus metabolism in microalgae, examine key external and internal factors impacting phosphorous removal by microalgae from wastewater, and examine the status of phosphorous-metabolism-related research to improve our understanding of microalgae-based municipal wastewater treatments. In addition, the methods of recovery of microalgae after phosphorous removal were summarized to ensure the sustainability of municipal wastewater treatment. Finally, a potential approach using nanomaterials was proposed to enhance the overall phosphorous removal performance in municipal wastewater through the addition of nanoparticles such as magnesium and iron.

Potential of Microalgae in Bioremediation of Wastewater

The increase in global pollution, industrialization and fast economic progress are considered to inflict serious consequences to the quality and availability of water throughout the world. Wastewater is generated from three major sources, i.e. industrial, agricultural, and municipal which contain pollutants, such as: xenobiotics, microplastics, heavy metals and augmented by high amount of carbon, phosphorus, and nitrogen compounds. Wastewater treatment is one of the most pressing issues since it cannot be achieved by any specific technology because of the varying nature and concentrations of pollutants and efficiency of the treatment technologies. The degradation capacity of these conventional treatment technologies is limited, especially regarding heavy metals, nutrients, and xenobiotics, steering the researchers to bioremediation using microalgae (Phycoremediation). Bioremediation can be defined as use of microalgae  for removal or biotransformation of pollutants and CO2 from wastewater with concomitant biomass production. However, the usage of wastewaters for the bulk cultivation of microalgae is advantageous for reducing carbon, nutrients cost, minimizing the consumption of freshwater, nitrogen, phosphorus recovery, and removal of other pollutants from wastewater and producing sufficient biomass for value addition for either biofuels or other value-added compounds. Several types of microalgae like Chlorella and Dunaliella have proved their applicability in the treatment of wastewaters. The bottlenecks concerning the microalgal wastewater bioremediation need to be identified and elucidated to proceed in bioremediation using microalgae. This objective of this paper is to provide an insight about the treatment of different wastewaters using microalgae and microalgal potential in the treatment of wastewaters containing heavy metals and emerging contaminants, with the specialized cultivation systems. This review also summarizes the end use applications of microalgal biomass which makes the bioremediation aspect more environmentally sustainable. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0). 

Interaction between CO2-consuming autotrophy and CO2-producing heterotrophy in non-axenic phototrophic biofilms

As the effects of climate change become increasingly evident, the need for effective CO2 management is clear. Microalgae are well-suited for CO2 sequestration, given their ability to rapidly uptake and fix CO2. They also readily assimilate inorganic nutrients and produce a biomass with inherent commercial value, leading to a paradigm in which CO2-sequestration, enhanced wastewater treatment, and biomass generation could be effectively combined. Natural non-axenic phototrophic cultures comprising both autotrophic and heterotrophic fractions are particularly attractive in this endeavour, given their increased robustness and innate O2-CO2 exchange. In this study, the interplay between CO2-consuming autotrophy and CO2-producing heterotrophy in a non-axenic phototrophic biofilm was examined. When the biofilm was cultivated under autotrophic conditions (i.e. no organic carbon), it grew autotrophically and exhibited CO2 uptake. After amending its growth medium with organic carbon (0.25 g/L glucose and 0.28 g/L sodium acetate), the biofilm rapidly toggled from net-autotrophic to net-heterotrophic growth, reaching a CO2 production rate of 60 μmol/h after 31 hours. When the organic carbon sources were provided at a lower concentration (0.125 g/L glucose and 0.14 g/L sodium acetate), the biofilm exhibited distinct, longitudinally discrete regions of heterotrophic and autotrophic metabolism in the proximal and distal halves of the biofilm respectively, within 4 hours of carbon amendment. Interestingly, this upstream and downstream partitioning of heterotrophic and autotrophic metabolism appeared to be reversible, as the position of these regions began to flip once the direction of medium flow (and hence nutrient availability) was reversed. The insight generated here can inform new and important research questions and contribute to efforts aimed at scaling and industrializing algal growth systems, where the ability to understand, predict, and optimize biofilm growth and activity is critical.

Interactions of microalgae-bacteria consortia for nutrient removal from wastewater: A review

Nitrogen and phosphorus pollution can cause eutrophication, resulting in ecosystem disruption. Wastewater treatment systems employing microalgae-bacteria consortia have the potential to enhance the nutrient removal efficiency from wastewater through mutual interaction and synergetic effects. The knowledge and control of the mechanisms involved in microalgae-bacteria interaction could improve the system's ability to transform and recover nutrients. In this review, a critical evaluation of recent literature was carried out to synthesize knowledge related to mechanisms of interaction between microalgae and bacteria consortia for nutrient removal from wastewater. It is now established that microalgae can produce oxygen through photosynthesis for bacteria and, in turn, bacteria supply the required metabolites and inorganic carbon source for algae growth. Here we highlight how the interaction between microalgae and bacteria is highly dependent on the nitrogen species in the wastewater. When the nitrogen source is ammonium, the generated oxygen by microalgae has a positive influence on nitrifying bacteria. When the nitrogen source is nitrate, the oxygen can have an inhibitory effect on denitrifying bacteria. However, some strains of microalgae have the capability to supply hydrogen gas for hydrogenotrophic denitrifiers as an energy source. Recent literature on biogranulation of microalgae and bacteria and its application for nutrient removal and biomass recovery is also discussed as a promising approach. Significant research challenges remain for the integration of microalgae-bacteria consortia into wastewater treatment processes including microbial community control and process stability over long time horizons.

Integrated Approach for Wastewater Treatment and Biofuel Production in Microalgae Biorefineries

The increasing world population generates huge amounts of wastewater as well as large energy demand. Additionally, fossil fuel’s combustion for energy production causes the emission of greenhouse gases (GHG) and other pollutants. Therefore, there is a strong need to find alternative green approaches for wastewater treatment and energy production. Microalgae biorefineries could represent an effective strategy to mitigate the above problems. Microalgae biorefineries are a sustainable alternative to conventional wastewater treatment processes, as they potentially allow wastewater to be treated at lower costs and with lower energy consumption. Furthermore, they provide an effective means to recover valuable compounds for biofuel production or other applications. This review focuses on the current scenario and future prospects of microalgae biorefineries aimed at combining wastewater treatment with biofuel production. First, the different microalgal cultivation systems are examined, and their main characteristics and limitations are discussed. Then, the technologies available for converting the biomass produced during wastewater treatment into biofuel are critically analyzed. Finally, current challenges and research directions for biofuel production and wastewater treatment through this approach are outlined.

Comparing the use of a two-stage MBBR system with a methanogenic MBBR coupled with a microalgae reactor for medium-strength dairy wastewater treatment

Two systems were compared for medium-strength dairy wastewater treatment. The first comprised a methanogenic Moving Bed Biofilm Reactor (AnMBBR) and an aerobic MBBR (AeMBBR), while the second an AnMBBR and a sequencing batch reactor (SBR) with Chlorella sorokiniana. The AnMBBR, under ambient conditions, achieves biogas production sufficient enough to attain energy autonomy. The produced energy was 0.538 kWh m^-3, whereas the energy consumption 0.025 kWh m^-3. Its coupling with the AeMBBR removed COD, NH₄-N TKN, and PO₄-P by 93 ± 4%, 97 ± 3%, 99 ± 1% and 49 ± 15%, respectively, while the use of the SBR as a second step eliminated totally COD but partially the other pollutants. The higher nitrogen removal in the first system was due to nitrification occurring in the AeMBBR. The acclimatization of microalgae to dairy wastewater enhanced their growth. Their protein content was 54.56%, while starch and lipids were 3.39% and 23.1%, respectively.

Alternative Management Systems of Beef Cattle Manure for Reducing Nitrogen Loadings: A Case-Study Approach

Livestock manure is one of the main sources of agricultural nonpoint source pollution and poses a great threat to the environment and human health. Sustainable management of manure via recycling is an effective means to tackle the problem. Based on field interviews in China, multiple case studies were employed to investigate alternative manure management systems. Four conclusions arose. First, compost-based systems, product-based systems, substrate-based systems, and biogas-based systems were identified as four main types of manure management systems, with each possessing its success factors and risk factors. The adoption of a system was driven by various factors. Second, market-oriented operation was the dominant operation mechanism of all the manure management systems. Third, compared to direct application of manure to croplands, all the four manure management systems could reduce nitrogen loadings from livestock farms and lower their environmental effects. Among the systems, biogas-based systems could reduce nitrogen loadings to the greatest extent, followed by product-based systems and substrate-based systems, and then by compost-based systems. Lastly, integrated management of manure with mixed recycling systems is imperative for reducing its environmental effects, which can benefit from the increasing role of third-party entities in manure recycling. Policy implications were also discussed.

Kinetics Growth and Recovery of Valuable Nutrients from Selangor Peat Swamp and Pristine Forest Soils Using Different Extraction Methods as Potential Microalgae Growth Enhancers

Soil extracts are useful nutrients to enhance the growth of microalgae. Therefore, the present study attempts for the use of virgin soils from Peninsular Malaysia as growth enhancer. Soils collected from Raja Musa Forest Reserve (RMFR) and Ayer Hitam Forest Reserve (AHFR) were treated using different extraction methods. The total dissolved nitrogen (TDN), total dissolved phosphorus (TDP), and dissolved organic carbon (DOC) concentrations in the autoclave methods were relatively higher than natural extraction with up to 132.0 mg N/L, 10.7 mg P/L, and 2629 mg C/L, respectively for RMFR. The results of TDN, TDP, and DOC suggested that the best extraction methods are autoclaved at 121 °C twice with increasing 87%, 84%, and 95%, respectively. Chlorella vulgaris TRG 4C dominated the growth at 121 °C twice extraction method in the RMRF and AHRF samples, with increasing 54.3% and 14%, respectively. The specific growth rate (µ) of both microalgae were relatively higher, 0.23 d^-1 in the Ayer Hitam Soil. This extract served well as a microalgal growth promoter, reducing the cost and the needs for synthetic medium. Mass production of microalgae as aquatic feed will be attempted eventually. The high recovery rate of nutrients has a huge potential to serve as a growth promoter for microalgae.