ABSTRACT

The expansion of human population and anthropogenic activities include mining, release of industrial waste, smelting of as-ore, incineration of fossil fuel, particularly coal, utilization of as-loaded water for irrigation, as-based pesticides, herbicides, and fertilizers have affected the availability of water resources for human consumption. Urban wastewater includes the presence of contaminants which are toxic to micro and macro-organisms and thus requires treatment. Microalgae are studied extensively for their efficiency to remediate the contaminants present in industrial and domestic effluents. Scientists focus on microalgae-based remediation of wastewater because of the ability of microalgae to grow and survive under diversified harsh environmental conditions. Carbon dioxide mitigation is a major role played by microalgae indicating an added benefit to the environment. Introduction of microalgae in remediation is much involved due to their contribution to the circular bio-economy approach which includes generation of integral bio-products during the traditional wastewater process. This systematic review focusses on the application of microalgae in urban wastewater treatment with discussion on remediation strategies and mechanisms followed for pollutants present in the wastewater. Additionally, the paper dwells on the future prospect of microalgae in incorporation of circular bioeconomy along with the wastewater remediation.

1. INTRODUCTION

Increasing industrialization and human dwelling has extensively enhanced waste generation, which in turn drastically affects the natural ecosystem and climate. Uncontrolled release of wastewater from industries is one of the major reasons for deposition of contaminants like heavy metals, pesticides, and pharmaceutical components, as well as petroleum in the environment. Studies have revealed that apart from these contaminants, organic constituents, like amino acids, carbohydrates, fats, and volatile acids are also released from the industrial wastewater. Industrial wastewater also adds to the chlorine, magnesium, potassium, sulphur, and sodium concentration in the environment (Ilyas et al. 2019). Wastewater contains amino acids, carbohydrates, fats, and proteins, as well as volatile acids along with inorganic components like ammonium salts, arsenic, bicarbonate, calcium, chlorine, magnesium, phosphate, and sodium (Goswami et al. 2021). A wide range of microorganisms, both harmless and pathogenic types, are also present in wastewater. Certain bacteria, fungi, and protozoa are even able to treat the wastewater. The presence of pathogenic microbes results in waterborne diseases, such as cholera, dysentery, hepatitis, tuberculosis, and typhoid (Wen et al. 2020). The untreated wastewater, when released in the environment, negatively impacts the aquatic bodies by increasing toxicity and biological oxygen demands (Siddiki et al. 2021).

Wastewater treatment is crucial in eliminating toxic pollutants. Several wastewater treatment plants are established in urban regions specifically focusing on degradation of organic components and declination of pathogenic counts (Hashmi et al. 2019). Current wastewater treatment plants are designed based on the quality of the industrial effluents, properties of wastewater, availability of the lands, and economic feasibility. Traditional wastewater management processes include only primary and secondary treatment, but not tertiary treatment where microorganisms are involved. According to a recent report, wastewater treatment plants following traditional mechanisms were unable to completely remove the toxic heavy metals, pharmaceutical products, and pesticides from the wastewater (Vakili et al. 2019). The high amount of sludge produced during the conventional management of wastewater acts as a further risk to the environment and further techniques are required to effectively eliminate the sludge (Ferronato and Torretta 2019).

Algal-dependent mechanisms for wastewater treatment have been developed in recent years as primarily eco-friendly and cost-effective methods. Nitrogen, phosphorus, and other inorganic and organic components are used for the growth of microalgae, thereby resulting in a reduction of their concentration. The application of microalgae in wastewater treatment is beneficial due to their oxygen generation (Mohsenpour et al. 2021). In this review, the importance of microalgae in urban wastewater management and the impact of the process of generation of circular bioeconomy will be discussed in detail.

2. COMPOSITION AND IMPACT OF MICROALGAE

Microalgae, microscopic photosynthetic organisms, exist in freshwater and marine aquatic ecosystems. The photosynthetic potential of the microalgae is almost equal to the terrestrial plants due to their cellular components and habitat. Microalgae produce fatty acids which direct innovative interests regarding microalgal based techniques. The generation of polyunsaturated fatty acids by the microalgal strains are improved under stress conditions (Fernández and Albentosa 2019). Microalgae, such as, Nannochloropsis oculata and Porphyridium purpureum are studied to produce increased quantity of polyunsaturated fatty acids when in a starved condition. Phosphate stressed environmental conditions result in the altering of the production rate of unsaturated fatty acids by enhancing the activity of D6- desaturase activity (Amaral-Zettler et al. 2020).

The cell wall of microalgae consists of cellulose in conjunction with proteins, polysaccharides, and lipids. The important functional groups, like amino, hydroxyl, carboxyl, carbonyl, phosphate, and sulfhydryl, present in the cell wall attributes to the negative charge of the cellular surface and are responsible for binding with the metal ions. The chemical structures of the cell wall influence the uptake of heavy metals in the microalgal cell. Microalgae target the heavy metals present in the urban wastewater and are not affected by the toxic nature of the heavy metals due to their extreme adaptability. Inclusion of certain genetic alteration or regulations, chelation, immobilization, and exclusion mechanisms help the microalgae to survive in the harsh wastewater environment (Leong and Chang 2020). The entry of metal ions in the microalgal cells enhances the involvement of the protein-metal ions complex formation followed by secretion of various pyochelin’s and enzymes which are antioxidants in nature. The formation of phytochelatins helps in the reduction of metals (Salam 2019). Microalgae synthesize extracellular polymeric substances (EPS) which plays an important role in bioremediation. The presence of polysaccharides and proteins with certain functional groups such as carboxyl group, hydroxyl group, and phosphoric amines can bond with the metal ions. Chlorella pyrenoidosa, a freshwater microalga, secretes an extracellular polymeric substance which can interact with arsenic ions (Zhang et al. 2020). Table 1 discusses the advantages and disadvantages of using microalgae to treat urban wastewater.

Table 1 Benefits and drawbacks of microalgae utilization in urban wastewater treatment.

3. ENVIRONMENTAL APPLICATIONS OF MICROALGAE

The extended photosynthetic ability of the microalgae helps them in sequestration and recycling of atmospheric carbon dioxide, further generating biofuels and bioproducts. Microalgae, when compared to higher plants, exhibit a higher efficiency of carbon dioxide sequestering ability. Carbon dioxide produced and released during the industrial processes are successfully eliminated by the microalgal interference and used for transformation to certain organic molecules, such as carbohydrates, proteins, lipids, etc. (Orejuela-Escobar et al. 2021; Singh and Dhar 2019).

The utilization of microalgae has been recently gaining intense importance in the field of environmental biotechnology. The bioremediation capacity of microalgae is more enhanced than the chemical processes of remediation. The utilization of contaminants as nutrients by the microalgae plays an important role in the remediation technique. The advantages of microalgae in bioremediation are their ability to survive in extreme environmental conditions maintaining strengthened symbiotic relationships with microorganisms and plants. The symbiosis certainly helps in adjusting to fluctuating environmental conditions, which enhances the sustainability of the microalgae in industrial sectors. Though the treatment time of remediation using microalgae is longer than chemical processes, the cost-advantage ratio of the microalgal bioremediation is highly effective (Abubakar et al. 2023).

Microalgae use the mechanism of bioaccumulation and biosorption for the removal of toxic metal components from contaminated sources. Bioaccumulation is an active process of heavy metal removal using live microbial cells where a combination of several strategies, such as binding of ions on intracellular parts, methylation or intracellular precipitation is attempted for reducing the toxic components from the surrounding area. On the other hand, biosorption is a metabolically inactive mechanism of eliminating contaminants where microalgal cells are converted into bio sorbent which efficiently adsorbs the heavy metal ions (Chatterjee and Abraham 2019). The techniques involved are proven to be eco-friendly in nature. Earlier studies have reported that Spirulina platensis effectively eliminated copper and cadmium within a range of 91–98%. Chlorella sp. and Scenedesmus sp. exhibits the ability to remove calcium, cadmium, copper, zinc, manganese, and magnesium from heavy metal contaminated areas (Salama et al. 2019). pH plays a major role in the uptake of heavy metals by microalgae as the process of changing the charge of algal cell surface and metal ion. However, contact time and temperature affect the process in certain cases, mostly, when live cells are involved (Furuhashi et al. 2019).

Dye removal from industrial effluents is a major threat that has been targeted using microalgal biomass in wastewater treatment plants. The dyes released from the industries further reach the environment and cause serious implications. Thus, the removal of toxic dyes is required. Factors affecting the process include temperature, pH, time, concentration of the dye, and most importantly the microalgae species, as well as the concentration of the microbe used (Ayele et al. 2021). Wastewater polluted with dyes are generally treated using physicochemical, chemical, and biological methods. Common approaches include adsorption, flocculation, odorization, ultrafiltration, coagulation, and oxidation, etc. However, ineffective drawbacks observed while attempting these processes indicated the requirement of innovative microbial intervention in abatement of dye from environment (Ayele et al. 2021). The advantages of using microalgae as an agent of reducing dye are the simple process of execution, cost-effectiveness, elevated rate of adsorption, and the application of available materials in the market (Naji and Salman 2019). Microalgae are mostly solar powered, and thus largely reduce the operational cost. Similarly, pharmaceutical compounds released along with the hospital and domestic effluents are also degraded by microalgae in the treatment plants.

4. APPLICATION OF MICROALGAE IN URBAN WASTEWATER TREATMENT

Wastewater management includes three major treatments, namely, primary, secondary, and tertiary. Primary treatment aims to eliminate the solid components present in urban wastewater, whereas secondary treatment involves the application of microbial approaches in breakdown of organic components. The treatment methods involve relatively higher operational costs than conventional techniques (Crini and Lichtfouse 2019). Conventional chemical methods of wastewater treatment involve the application of heavy metals like aluminium and iron for precipitation resulting in the release of sludge which is required to be further treated. Biological techniques of treatment, on the other hand, require strong infrastructures with high capital for the execution of the process of removal of activated sludges and incorporate complexity in the processes. However, the disadvantages of both chemical and biological techniques indicated the requirement of an innovative and efficient wastewater management plan. Microalgae-based remediation processes have recently been studied as an effective alternative for conventional treatment processes due their ability to treat both liquid and solid wastes, efficiently converting them into value-added byproducts. Harvesting microalgae is often clubbed with wastewater treatment to reduce the cost of execution (Kalra et al. 2021). Microalgae have a high capacity to uptake nutrients from the urban wastewater and produce biomass to be used in commercial applications. Microalgae have a high rate of growth as compared to vascular plants, and harvesting microalgae in the wastewater system does not expose any negative or toxic effects on the environment (Mohsenpour et al. 2021). It is reported that microalgal biomass doubles the initial biomass with 13 h of harvesting process (Aron et al. 2021). The ability of microalgae to survive in adverse conditions helps them to grow in wastewater and thus exhibits nil or less land competition. Microalgae harvesting can be conducted in autotrophic, heterotrophic, and mixotrophic nature of growth. Figure 1 represents the process involved in wastewater management using a microalgal approach. In autotrophic mechanisms, microalgae utilize carbon dioxide as a carbon source, thereby in the process decrease the level of carbon dioxide in the environment. Studies show that individual microalgae species can effectively reduce around 1.8 pounds of carbon dioxide (Chai et al. 2021). Obligate heterotrophic types of microalgae are species that source their carbon from industrial waste, such as acetate, ethanol, glucose, and glycerol. Mixotrophic microalgae use carbon dioxide as well as organic sources of carbon for their survival. The carbon dioxide, nitrogen, and phosphorus assimilated by the microalgae are transformed into carbohydrates, proteins, and lipids, along with several other byproducts (Mastropetros et al. 2022). The nutrient rich nature of wastewater enables the application of microalgae in wastewater treatment plants at a much lower cost and decrease carbon footprints with much efficiency.

Figure 1: The water can be contaminated by industrial, agricultural, and domestic waste. The water can be treated by autotrophic, heterotrophic, and mixotrophic microalgae. These microalgae can consume carbon dioxide as a fuel.

Figure 1 Process involved in wastewater management using microalgal approach.

The application of microalgae has been tested on wastewater sources released from agricultural, municipal, and industrial industries. The major factors that determine the efficacy of the removal of contaminants from wastewater include the supply of nutrients and carbon dioxide along with optimum pH, temperature, and salinity for the growth of microalgae and the production of important biomolecules (Aron et al. 2021). For instance, the co-cultivation of Chlorella vulgaris and Phormidium keutzingium with activated sludge greatly increases FOG removal, biomass production, and nutrient recovery in municipal wastewater (Javed and Al-Zuhair 2025). The activated carbon can be manufactured from wastewater-cultivated microalgae, and the combination of Tanfloc® flocculation with thermochemical pretreatment resulted in high surface area and heat-stable activated carbon, providing a sustainable method of activated carbon production. Waste-to-resource strategy that creates high-quality activated carbon from microalgae cultivated in wastewater. Researchers optimized harvesting and pretreatment to significantly improve carbon performance, contributing to sustainable material development and economic goals (Estevam et al. 2025; Nápoles-Armenta et al. 2025). Using a Chlorella sp.–bacteria consortia is sustainable for wastewater treatment. The mathematical modeling and laboratory studies have shown that light intensity and municipal wastewater greatly improved biomass growth and nutrient removal, removing 87% of nitrogen and 94% of phosphorus. To simulate biomass production and nutrient intake, a mathematical model was developed, and it demonstrated a high degree of accuracy. For treating wastewater in rural and developing areas, the system provides an affordable and environmentally responsible option (Ortega-Blas et al. 2025). However, utilization of these methods is cost-effective and provides a low-cost substitute for traditional wastewater treatment, particularly in environments with limited resources. In an industrial aspect, the evaluation of nutrient removal efficiency of a pilot-scale microalgae-based wastewater treatment system in real environmental conditions by conducting experiments in three phases using pre-settled municipal wastewater, with and without sodium addition, and anaerobic digester liquor. This achieved up to 99.36% ammoniacal nitrogen and 21.41% phosphate removal, the highest in Phase 2 (with sodium), showing significant seasonal variations affecting treatment efficiency (Magna et al. 2025). Utilization of microalgae in urban wastewater treatment endures difficulties in various levels, such as recovering of the biomolecules from the wastewater post, the harvest of microalgal biomass, as well as chances of the existence of toxic pollutants like pesticides and antibiotics (Maryjoseph and Ketheesan 2020; Ahmed et al. 2022). In certain cases, self-flocculation of the microalgae biomass is noticed due to the formation of cell aggregates by the function of EPS (Bernaerts et al. 2018). However, the low energy requirement, ability of microalgae to grow in any condition, and the rapid conversion of contaminants into high-value byproducts are important advantages which are not ignored by the scientific community. Table 2 suggests the utilisation of microalgae to remove pollutants from urban wastewater.

Table 2 Application of microalgae in remediation of contaminants from urban wastewater.

5. REMEDIATION STRATEGIES OF MICROALGAE

Remediation processes using microalgae primarily include three techniques, namely, bioaccumulation, biosorption, and biodegradation. The strategies are explained in detail below.

5.1 Bioaccumulation

This process is considered as an active metabolic mechanism occurring within the microbial cells. Requirement of energy is a major factor in the implementation of bioaccumulation. The process may be slower compared to the other techniques. Living microalgal cells are predominantly used for the reduction of contaminants in wastewater. The pollutants are taken up by the microalgae cells and either aggregated within the cells or metabolized during the cellular metabolism of the cells. Various organic and inorganic contaminants such as, heavy metals, phosphates, nitrates, sulphates, and phosphates are taken up by the microalgal cells. The microalgae allow entry of these contaminants in the cells along with the nutrients and microelements (Rempel et al. 2021; Mustafa et al. 2021). Even contaminants present at low concentrations are proficiently exempted by the microalgae due to their adapting characteristics to harsh environments. The microalgae can tolerate a range of pollutants present in urban wastewater and thus have an elevated rate of bioremediation (Mojiri et al. 2020). The effect of microbial cell in bioaccumulation is well detected by the ratio of concentration of pollutant taken up to the medium, which is termed as the bioconcentration factor. The first step of the bioaccumulation involves bio adsorption, where the molecules are attached to the cell surface of the microalgae and thereby taken within the cells (Hena et al. 2021). Cladophora sp. is reported to accumulate triclosan and triclocarban at concentrations of around 50–400 ng/g. However, it is important to notice that in the environment, the concentration of triclosan and triclocarban via effluents does not exceed 200 ng/kg. Spirogyra is also studied to take up around 8.5 x 10⁵ times of radio phosphorus when present in the wastewater system (Abdelfattah et al. 2023). The contaminants are transported from higher to lower concentration, that is, from the external environment to microalgal cells without requiring any energy (Sutherland and Ralph 2019). The cell membrane of the microalgae exhibits an hydrophobic nature due to which non-polar, lipid soluble, low molecular weight compounds can penetrate through the cell membrane following a passive diffusion process. High molecular weight and polar components, however, are unable to undergo diffusion. Carbamazepine, florfenicol, sulfamethoxazole, and trimethoprim have been studied to follow passive diffusion to enter the microalgal cells, followed by their utilization in metabolism (Song et al. 2019). Contaminants cause an alteration in the permeability of the cell membrane, leading to hyperpolarization or depolarization.

5.2 Biosorption

Biosorption, the passive process of remediation, usually involves a sorbent which is prepared using any biological substance. The sorbent is used to attach to the pollutants present in the wastewater. This process is dependent on mass transfer, where one component is dislocated from the liquid phase and attached to the sorbent’s surface. The involvement of independent physical, chemical, and metabolic techniques are observed in biosorption. The mechanisms mostly followed during the process include absorption, adsorption, electrostatic cooperation, ion exchange, precipitation, and surface complexation (Chia et al. 2020). The process of biosorption continues until the equilibrium is attained between the concentration of pollutants in the medium/surrounding (liquid) and the bio sorbent (El-Sheekh et al. 2020; Mustafa et al. 2021). Heavy metal pollution has been targeted for remediation following the biosorption process in several studies. The presence of heavy metals in trace levels eventually enhances the growth and metabolic rate of microalgae. Boron, copper, cobalt, iron, manganese, molybdenum, and zinc are responsible for the cellular metabolic functions and enzymatic activity of microbial cells. Microalgae serve as an important material for the preparation of bio sorbent. The microalgal cells consist of active sites on the cell’s surface for binding to the contaminants in the wastewater and forming complexes. The generation of the complexes will result in flocculation, eventually reducing the total suspended and dissolved solid particles (Erofeeva 2022; Al-Tohamy et al. 2022). The uptake of heavy metals by microalgae bio sorbents follows two preliminary steps. The process begins with the accelerated reversible attachment of sorbates (heavy metals) in this case, on the active sites of the binding of microalgal bio sorbent surface. The second step is a much slower process, which includes intracellular diffusion. The existence of exopolysaccharides, peptides, and uranic groups, as well as the cell wall components such as, alginates, cellulose, proteins, and lipids, are responsible for the availability of functional groups on the bio sorbent surface. Due to the presence of a wide range of carboxyl groups, sulphate groups, and monomeric alcohols, microalgae can adsorb both anionic and cationic heavy metals. The extracellular polymeric substance (EPS) acts as an advantage due to its ability to increase the rate of biosorption based on the structure of EPS. Other factors, such as properties of the wastewater, types of metals present, and operational circumstances, also determine the rate of biosorption (Liu et al. 2021).

5.3 Biodegradation

Biodegradation is one of the common methods for the removal of pollutants from industrial effluents. The technique simply involves the breakdown of complex and toxic compounds to simple and non-toxic end products. The difference between biodegradation with the above-mentioned processes is that in this technique, cleavage of the contaminant in conducted either by mineralization or transformation, whereas, in the case of biosorption and bioaccumulation, the involvement of microorganisms plays a major role as biological filters. The mineralization in biodegradation indicates a complete breakdown of the compound to either carbon dioxide or water. On the other hand, transformation is about the conversion of the parent contaminant to non-toxic or less toxic end-products through an array of enzymatic interactions synthesizing certain intermediates (Sutherland and Ralph 2019). Biodegradation mechanisms, when studied, can be categorized into two grades, which are metabolic degradation and metabolism. In metabolic degradation, the source of carbon as well as electron acceptors/donors to the microalgae, is served by the contaminants. Metabolism, on the other hand, involves a non-living matter, such as any enzymes for degradation of the contaminants (Leng et al. 2020). In the case of biodegradation using microalgae, both intracellular and extracellular breakdowns are observed. Extracellular polymeric substances play a role in the extracellular degradation of the contaminants. The presence of EPS bordering the cell wall of microalgae allows them to mineralize the contaminants into simple substances. EPS acts as an emulsifier as well, which escalates the contaminants bioavailability in the wastewater environment. A detailed study must be further conducted to analyze the interactions occurring between the EPS of the microalgal cells and the environmental pollutants. The process of microalgal degradation of contaminants involves three predominant stages, which are:

• Stage 1: In this stage, a requirement of enzymes belonging to cytochrome P450 group, such as, carboxylase, decarboxylase, hydrolase and monooxygenase is observed. The enzymes help enhance the hydrophilic nature of the contaminants by either inclusion or exclusion of hydroxyl group via redox reactions or hydrolysis (Priyadharshini et al. 2021).
• Stage 2: This phase involves the requirement of glucotransferases and glutathione-S-transferases which activate glutathione conjugation with several electrophilic centres containing compounds and thus protects the microalgal cell from oxidative damage (Xiong et al. 2019).
• Stage 3: The last phase is about the transformation of the components into lesser or non-toxic end products using the enzymes laccase, dehydratase, carboxylase, hydrolase, transferase, dehydrogenase, etc.

6. IMPACT OF CO-CULTURING STRATEGY FOR WASTEWATER TREATMENT

Co-culturing, or co-cultivation strategy, in wastewater treatment represents the harvest of two microbial species in the same environment functioning in combination as symbiotic association. The implementation of filamentous fungi in co-cultivation mechanism with microalgae has been studied by several researchers. It is observed that the process can be conducted in two significant steps: formation of the pellets and collection of the pellets. The co-cultivation of fungal assisted microalgae can be conducted in two methods which are: (a) inclusion of fungal spores in the microalgae harvest, and (b) inclusion of fungal pellets in the microalgae harvest (Chen et al. 2018). Figure 2 depicts both the mechanisms of co-cultivation of fungal assisted microalgal harvest. Apart from fungi, studies have also proven that bacterial assisted harvest of microalgae is successful in wastewater treatment. Amalgamation of bacteria, fungi, and microalgae are also tried for wastewater treatment, and thereby the harvest of microalgal biomass. In a recent study, Citrobacter freundii was combined with Chlorella pyrenoidosa resulting in the formation of bacterial-algal floc and further proceeded for the net trapping interaction by the application of Mucor circinelloides. The combination of the three efficiently increased the biomass harvest in the wastewater environment. The approach is much more practical than the individual application of microalgae. Since it is for industrial wastewater management plants, the wastewater is not sterilized before the treatment, and thus chances of growth of other microbial species are much more evident (Jiang et al. 2021). Microbial cellular surface components, alteration in the surface change and hydrophobic interaction, play a major role in co-cultivation techniques. Hydrophobic interaction is responsible for the flocculation of floating microalgae in the wastewater by the filamentous fungal cells. Hydrophobic interaction is exhibited by the hydrophobic proteins secreted primarily by filamentous fungi (Li et al. 2017).

Figure 2 Inclusion of fungal spores in the microalgae harvest.

Wastewater, which is rich in nutrients, acts as a great source of alternative media for the harvest of algal biomass. Co-culturing strategy or co-calibration of microalgae has been studied mostly along with fungi to eliminate the pollutants present in the wastewater. It has been observed that a co-cultivation strategy is advantageous in various ways, such as being environmentally-friendly, having a lower execution cost, and the rapid removal of the contaminants. The application of the combination of microalgae with fungi has been studied for the uptake of nutrients from wastewater as well as elimination of heavy metals. Nutrients, such as carbon, phosphorus, and nitrogen are present in wastewater in high concentration. The higher amount of these nutrients might cause eutrophication of natural water bodies (Han et al. 2019). The aggregation of microalgae and fungal cells in the wastewater results in a reduction of the nutrients. It has also been reported that the utilization of microalgae in combination with fungal spores efficiently increases the remediation rate when compared to individual application of microalgae. With the combination, microalgae can captivate the carbon dioxide and release oxygen via photosynthetic metabolism whereas the filamentous fungal mycelium, being heterotrophic in nature, utilized that oxygen and released carbon dioxide. Thus, during the process of treatment of wastewater by the combination, an exchange of carbon dioxide and oxygen is observed between microalgal and fungal cells, which indicates that microalgae support the survival and growth of fungi, and maintain the aerobic environmental condition, whereas fungal mycelium serves the microalgae with inorganic carbon (Yang et al. 2019). Filamentous fungi can degrade and decompose the organic matter in wastewater and release certain extra-cellular enzymes. These extra-cellular enzymes can further cleave the high molecular weight organic matter and convert them to low molecular weight organic matter, followed by the activity of microalgae which can assimilate the low molecular weight organic matter in the end. This process shows the effective removal of nutrients from the wastewater environment (Lin et al. 2022).

Co-cultivation also affects the heavy metals present in the wastewater system. Heavy metals are extensively present in various wastewater systems, such as sewage wastewater, metal mining wastewater, petrochemical effluent wastewater, paint industries effluents, etc. Algal cells, when administered individually for wastewater treatment, uptake the heavy metals by adsorption or assimilation techniques. However, recycling those heavy metals from the microalgal cells once uptaken is considered a disadvantage in industry. The higher cost of harvesting the microalgal cells is also a drawback of utilizing microalgae individually in the process. Thus, in this case, the co-culturing of microalgae with another microbial strain is considered advantageous and is gaining importance in the scientific field (Urrutia et al. 2019; Chatterjee and Abraham 2019). Certain filamentous fungi, such as Aspergilus nomius, Funalia trogii, etc. are highly capable of the adsorption of heavy metals induvial while using both live cells and dead bio sorbents (Chatterjee et al. 2020). The high affinity of filamentous fungi towards heavy metals provides a strong reason for using the co-culturing method of microalgae and filamentous fungi in wastewater remediation. In a recent study, the co-cultivation of Synechocystis sp. and Aspergillus fumigatus for wastewater treatment was studied. The analysis reported that the association was significantly effective in adsorption of heavy metals from the wastewater environment as compared to the individual type of microbes (Wang et al. 2021). Performance of live microalgae-fungal pellets have been compared with lyophilized pellets of the symbiotic combination in gold elimination. The study showed that lyophiles combination exhibited a better result than the live cells combination (Shen and Chirwa 2020). The functional groups, such as, OH and NH stretch, CH stretch, CN stretch, C-OH stretch present in the algal and fungal pellets are equally important for binding to the heavy metal’s ions (Li et al. 2019). More impactful studies are required to be done in this field. An important study that should be conducted is determining the nutritional values of the algal-fungal biomass combined to be used for animal feed for extending their immune response. However, a co-cultivation strategy is a demanding and profitable technology which can be industrialized and brought into the market.

7. OUTLOOK TOWARDS CONCEPT OF CIRCULAR BIO-ECONOMY

Conventional refinery is designated to convert certain raw materials to important value-added products, which enhances the traditional values as well as the economic aspect of the product. Microalgal biorefinery is a cutting-edge concept in which algal biomass is used in the generation of the product. The application of microalgae helps in carbon dioxide mitigation, wastewater management, and in the process generates alternative biofuels and byproducts which are used as chemicals, fertilizers, and food supplements (Yadav and Sen 2018). Utilization of microalgae is beneficial since they are not used as food crops, and neither are they competitive with terrestrial plants. Apart from these, the accelerated growth rate and efficient rate of photosynthesis adds to the interest of the scientific community as well as industrial sectors to apply microalgae in an integrated biorefinery mechanism. The importance of microalgae has been studied in various zero waste biorefinery concepts where industrial effluents (both solids and liquids) are used for the generation of value-added products. The application of microalgae with solar cells functioning as a renewable energy source would help in developing an industrial approach with zero carbon generation (Bera et al. 2021). As a result of this approach, more energy and capital will be generated, as well as a higher yield of product will be achieved from biorefineries (Chandra et al. 2019). It is possible to strengthen the cost efficiency of the algal biorefineries by the appropriate application of all the end products of the processes which will enable them to fetch the maximum return on the capital invested during the downstream processes. Various research is still ongoing to understand the ability of microalgae to contribute to the addition of nutritious biomass and the economy involved in completion of the process. Spirulina sp., a well-known cyanobacteria, contains about 60-70% protein by weight and serves as great food for weight loss treatment for the human body. It also exhibits anticancer and antiviral characteristics as well as helps in the reduction of diabetes. Spirulina sp., if harvested in an extremely nutritious wastewater environment, can be utilized as an important source of calcium and iron. Alterations in the genetic makeup of Spirulina sp. are reported to be even more nutritious (Palanisamy et al. 2019). Chlorella sp., Dunaliella sp., and Haematococcus sp. can secrete beta-carotene and astaxanthin, which are already proven to be beneficial for human health (Harvey and Ben-Amotz 2020). The nutritional benefits of microalgae are direct to their application in aquafeed and animal husbandry fodder generation and might also be diverted towards sustainable feedstock. The application of microalgae as agricultural crops should be studied and details focusing on their genetical advances and bioprocessing mechanisms.

Microalgal based chemicals can be used in generating alternative source to fossil fuel-based chemicals. The Petro-chemical dependent production of plastic is not accepted globally and is a point of concern for their hazardous impacts, thus, utilization of microalgae to convert the components of waste biomass, such as carbohydrates, lipids and starches into bioplastics are gaining huge importance in scientific societies (Noreen et al. 2016). The production of products using microalgae is considered as a feasible alternative due to lesser cost of algal biomass (Beckstrom et al. 2020). The elevated concentration of lipid content in the waste biomass aids in synthesis of better chemicals which suggests that along with microalgal species, the management of the process and assessment of the products produced are much required (Seon et al. 2020; Takahashi 2021).

Microalgae have grabbed attention in the industrial field due to their ability to produce a high number of biofuels and biofertilizers. Microalgal species add bio stimulating properties to biofertilizers which helps in the germination of seeds and growth of plants (Navarro-López et al. 2020). Although bio stimulating the activity of microalgae has been proven, the market is not quite open for the products, and thus appropriate species screening and integration of certain metabolites might help the biorefineries to venture the products into the market (Behera et al. 2021). The harvest of microalgal biomass results in the production of almost 760 ton/ha/year of biodiesel provided the harvest is conducted in the presence of optimum nutrients. Apart from biodiesel, growth of microalgal cells in a wastewater environment also results in the production of biohydrogen by the process of bio-photolysis. Several techno-economic examinations are conducted on microalgal biorefineries using certain species, such as, Anabaena sp., Spirulina sp., Spirogyra sp., Chlorella vulgaris, Scenedesmus obliquus, etc. There is an extreme potential in the growth of microalgal biorefineries due to resources and value-added byproducts along with wastewater management. Although, theoretically, typical biorefinery is not very successful in gathering capital investments due to a lack of pilot, as well as industrial scale observations. The studies related to the enhancement of a circular bioeconomy using microalgal biorefinery during wastewater management must be focused on the trials of industrial scale data to understand the real-life conditions, rather than experimental laboratory analyses (Gifuni et al. 2019).

8. CONCLUSIONS

The extensive exposure of environmental pollutants to wastewater is a tremendous threat to the ecosystem and living habitat in the environment. The application of microalgae in wastewater treatment has been proven to be more advantageous than the traditional techniques in various ways, as studied in many research works. Large scale implementation of microalgae-based wastewater treatment should be implemented across the globe to eliminate contaminants. The role of microalgae in wastewater treatment plants can be exciting based on their benefits. A range of pollutants are remediated by the microalgae from various sources of wastewater. The most important aspect to notice is that microalgae treatment of urban wastewater does not produce any activated sludge or secondary pollution. Moreover, the introduction of circular bioeconomy is an approachable concept which is interlinked with the management plan. The disposed microalgae after the reduction of contaminants are efficient enough to be used as raw materials for production of biofuels, biofertilizers, medicines and nutritional foods which can benefit economically as well as in saving energy. However, wastewater treatment plants merging applications of microalgae and their biorefinery purpose during the treatment of pollutants requires further study to eliminate the challenges faced during the process.

ACKNOWLEDGMENTS

The authors would like to thank their respective organizations for their overall support of this work.

References

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