An Integrated Algal Biorefinery Approach for Wastewater Treatment and Biomass Valorisation

Abstract

Biological wastewater treatment methods are considered suitable due to several advantages, such as fast processing, low operating cost, less secondary pollution, and overall, environmentally friendly. Microalgae-based wastewater treatment has promising potential, as it not only removes pollutants but also produces valuable biomass, which can be further utilised for various applications. In such systems, microalgae bacterial consortia enhance overall treatment efficiency by promoting symbiotic relationships that improve microbial activity, environmental resilience and enhance pollutant removal efficiency. The current review provides an overview of microalgae cultivation in various wastewater streams, CO₂ sequestration and the utilisation of produced microalgal biomass for multiple applications. The manuscript also focuses on the current role of molecular tools in optimisation and the integration of artificial intelligence to enhance microalgae-based wastewater treatment and management. The manuscript highlights recent progress in wastewater treatment, resource recovery, and the contribution of microalgal biomass to the emerging bioeconomy. To address the identified research gaps and promote the practical implementation of integrated algal systems, future research should focus on the combined approach of algae-based wastewater treatment and the concurrent utilisation of algal biomass. Such research should aim to optimise cultivation conditions and operational strategies to improve nutrient removal efficiency, enhance biomass valorisation for biochar, bioplastics, or feed applications, and ensure sustainable economics. This integrated perspective will help bridge the gap between laboratory-scale studies and integration at a larger scale. Overall, this review aims to guide the effective use of microalgae for treating diverse wastewater streams while supporting efforts to mitigate greenhouse gases and reduce pollution.

1. Introduction

The biotechnological potential of algae has been extensively acknowledged by the global scientific community due to its profound significance for environmental sustainability, economic development, and human welfare. Algae, broadly defined as photosynthetic aquatic organisms, utilise light energy to convert carbon dioxide (CO₂) into organic matter, thereby naturally contributing to the mitigation of greenhouse gas emissions [1,2]. These organisms exist in both macroscopic and microscopic forms: the former are commonly referred to as ‘seaweeds’, while the latter are termed ‘microalgae’. Both categories have been intensively investigated, with their biomass serving as a versatile substrate for the production of a wide range of valuable bioproducts. Applications of algae and algal-derived compounds span several different industries, such as medicine, food, biofuels, etc. [1,3,4]. Beyond product generation, algae offer several intrinsic advantages throughout their life cycle; they act as efficient biological systems for CO₂ capture, require no arable land for cultivation, represent a renewable and sustainable feedstock/raw material, and yield biomass that can be harnessed for multiple industrial and societal purposes [1,2,5].

The two algae groups can be subdivided into further classifications depending on the properties of the algae, such as colour, habitat, cell characteristics, etc. In recent decades, specifically microalgae have been a focal research point, because in conjunction with their inherent benefits, it also permits cultivation in waste (water) streams, allowing for concurrent bioremediation [3,4,6]. Furthermore, compared to macroalgae, microalgae have faster growth rates, can grow in various modes (phototrophic, heterotrophic, and mixotrophic), tend to be more compatible/efficient with wastewater treatment/growth, and are more adaptive to stressful conditions [1,7]. The pairing of wastewater-grown microalgae holds great promise toward resolving applicable sustainable development goals (SDGs), reducing the earth’s water footprint, reusing/recycling waste (water), and generating responsibly sourced biomass [2].

This review focuses on the potential of microalgae-based systems for sustainable wastewater treatment and resource recovery. It highlights the role of microalgae in improving pollutant removal efficiency, producing biomass, and converting biomass into biochar, bioplastics, fertilisers, and other high-value products. The review also emphasises the application of molecular tools and artificial intelligence for process optimisation, offering insights into strategies that enhance system performance and scalability. By integrating wastewater remediation with resource valorisation, microalgae-based approaches not only mitigate environmental pollution but also contribute to reducing greenhouse gases and the development of a circular bioeconomy. Furthermore, this current review is presented as a general/narrative review rather than a systematic review; therefore, it does not follow a structured bibliographic search strategy with predefined databases, keywords, or strict inclusion/exclusion criteria.

2. Algal Biotechnology and Cultivation

The definition of algal biotechnology is “the technological application of algae (both microalgae and macroalgae) or their derivatives to make or modify products or processes for specific use. This definition comprises various aspects, broadly related to all types of (i) algal farming, (ii) algae-based/incorporated product development, and (iii) phycoremediation (including all kinds of wastewater streams), and it is implicit that these processes will ultimately materialise on a large/industrial scale [8]. For ease of understanding, algae are typically categorised by size into two groups: macroalgae (also known as seaweeds) and microalgae. The two types of algae have diverse applications in many fields, including, but not limited to, food (human consumption), feed (for animal consumption), bioenergy, pharmaceuticals, nutraceuticals, biomaterials, cosmetics, bioactive compounds, agriculture, and bioremediation [1].

The uses of algae are wide-ranging and are dependent on several important factors, one of which is cultivation methods. Microalgae cultivation methods include two dominant systems: open raceway ponds and closed photobioreactors. Both cultivation practices have been extensively researched and well-documented to demonstrate the advantages and disadvantages of each technique [1]. A distinct feature of microalgae during cultivation is their ability to grow in wastewater streams, as well as in freshwater and seawater. This allows the microalgae to obtain their nutrients for growth from the ‘pollutants’ in the wastewater whilst simultaneously purifying the water. This attribute of microalgae enables low-cost cultivation in wastewater streams (from agricultural/food, domestic, or industrial sources), as the need for nutrients is eliminated, contributing to a circular process. However, the drawback of this cultivation strategy is that biomass has limited applications due to the cultivation conditions [7,9]. Wastewater-grown microalgal biomass finds suitable applications in bioenergy production as a promising feedstock. In addition, this biomass has also been extensively exploited for its lipids, a biopolymer commonly used in biodiesel production [10].

Macroalgae are typically grown near coastal regions, where they obtain the essential elements required for growth. Seaweed naturally grows along the seashore in specific areas; however, the majority (~95%) of macroalgae utilised by the human population is obtained through farmed activities [11]. There are also only four main species of macroalgae that are commonly cultured, namely Gracilaria, Laminaria, Porphyra, and Undaria. The cultivation of seaweed from inception to final product takes ~6–7 months. Macroalgae are mainly used in food, feed, pharmaceutical, nutraceutical, fertiliser, and cosmetic industries as opposed to other sectors due to their chemical composition containing high carbohydrate and hydrocolloid contents, as well as unique compounds (e.g., carrageenan, alginate, fucoidan, etc.) [12].

3. Integrated Algal Systems for Waste Valorisation

Addressing waste management is a rising, worldwide concern. The concept of ‘waste valorisation’ broadly means converting/recycling waste materials through various processing technologies into useful and valuable products. These products can range from energy fuels to materials to industrial chemicals. Valorisation of waste aligns with the principles of the circular economy and resource recovery by reusing or repurposing materials that would have otherwise been discarded, thereby reducing disposal loads and protecting the environment. Furthermore, it also contributes to the sustainable development of societies and economies, as well as mitigating greenhouse gas emissions and achieving zero waste [13,14]. According to Arora et al. [15], integrated algal systems are a complex, advanced fusion of open and closed culture operations, forming an improved hybrid system that collectively overcomes constraints.

In terms of utilising algal systems for waste valorisation, a consensus among many researchers/scientists holds that biorefinery is evidently the best path forward. Briefly, a biorefinery is an industrial process/facility where algal biomass is transformed into a diverse range of bioproducts (such as biochemicals, biofuels, and biomaterials), leaving no component of the biomass unused [16]. In this sense, microalgae cultivation in wastewater or waste streams plays a dual role in waste valorisation. Firstly, it serves as a bioremediation step for water treatment. At the same time, the second involves utilising the produced biomass as a feedstock for processes or as an input raw material for various products. Additionally, wastewater is diverted from wastewater treatment plants, which reduces energy costs associated with treating the water. This kind of integrated microalgal system for waste/wastewater upcycling is a promising and widely adopted approach [15,17]. Whilst this practice shows great potential, there are still some constraints that persist related to inflated costs, high energy consumption and the issue of safety of the biomass since it is generated, mainly if intended for food/feed usage [7,14,17].

It is also imperative that the microalgal biorefinery is financially feasible, where the primary bioproduct is produced in large volumes and is of high market value, ensuring a profitable operation. High-value bioproducts encompass a diverse array of items with various applications, including biofuels, biochemicals (such as proteins, vitamins, carbohydrates, and lipids), bioactive compounds (e.g., pigments, docosahexaenoic acid, eicosapentaenoic acid, and antioxidants), among others. The byproducts from the microalgae biorefinery can be used for animal feed and fertilisers. In addition to the various benefits of microalgae, it is considered a sustainable and renewable biomass resource, which is desirable for both the future and the green economy (Figure 1). Nonetheless, despite the benefits of a microalgal biorefinery, ongoing efforts are still being made to mitigate the challenges associated with this activity [18].

4. The Algal Biorefinery: From Waste to Value

4.1. Microalgae Cultivation in Domestic Wastewater

Domestic wastewater (also known as ‘municipal/grey wastewater’) is one of the most commonly used wastewater streams for microalgae cultivation, as it is readily available in large quantities and contains all the required nutrients [4]. In general, domestic wastewater for large-scale microalgae cultivation has been extensively investigated and proves to be a worthwhile culture medium. Municipal sewage contains high concentrations of nutrients, particularly nitrogen and phosphorus. Microalgal-based wastewater treatment can effectively reduce nutrient loads and other contaminants through mechanisms such as biosorption, bioaccumulation, and/or biodegradation (Figure 2). In addition to the biomass produced during cultivation, microalgae remove and reduce the nutrient load in wastewater, a process known as ‘phycoremediation’. Hence, microalgae cultivation in domestic wastewater serves a dual function, in line with circular economy principles [7,19].

Several factors influence the nutrient removal rate and efficiency of microalgae in domestic wastewater. Some of these factors include the point of collection of municipal wastewater/wastewater characteristics (e.g., influent, secondary effluent, post-chlorinated effluent), microalgal strain, cultivation conditions, cultivation system (open or closed), etc. It is noteworthy to mention that the efficiency of wastewater treatment improves whilst deploying a microalgae and bacteria consortium. This is due to their mutualistic interaction, whereby the combined organisms coexist and support each other, thus enhancing the overall efficiency of the system. Therefore, exploiting growing microalgae-bacteria consortia in wastewater has become an increasingly important area of research, although careful selection of both strains is required. Typical nitrogen, phosphorus, and chemical oxygen demand (COD) removal efficiencies of microalgae/microalgal-bacterial consortia from municipal wastewater usually range between 70 and 90%, even often approaching 100%; although, it is highly dependent on the factors listed above, and can vary considerably [2,19]. However, scaling up the use of microalgae for primary treatment is challenging due to the longer hydraulic retention times required compared to conventional wastewater treatment technologies. Additionally, the presence of high solids and variability in nutrient concentrations further complicates large-scale implementation. Consequently, most integrated approaches employ microalgae primarily for tertiary treatment or polishing to remove residual nutrients from wastewater and prevent eutrophication.

4.2. Dairy Wastewater

The dairy industry is one of the largest sectors in the global food market, requiring substantial volumes of water for cleaning, washing, sterilisation, and general facility maintenance [19]. As a result, it produces large quantities of wastewater. Dairy wastewater (DWW) commonly contains detergents, sanitising chemicals, fats, lactose, acetate, lactate, nutrients, spilt milk, and soluble proteins [20,21,22]. On average, dairy facilities use around 10 m³ of water for every 1 m³ of milk processed, generating approximately 0.2–10 L of DWW per litre of milk [23]. This wastewater is typically rich in organic matter and exhibits high COD and BOD, which make it unsuitable for direct discharge into the environment. Apart from high pollutant load, DWW contains nutrients, such as sugars, amino acids, ammonium, and phosphates. Furthermore, it has been estimated that 2.25 billion tonnes of DWW are produced annually worldwide, and inadequate treatment or direct disposal can negatively impact the environment [24]. In addition, ~80% of the world’s wastewater is released into the environment without any treatment [25].

Several countries implement regulations that require DWW to be treated before discharge, and dairy industries adopt a variety of physical, chemical, and biological treatment systems [26,27]. However, poorly designed or poorly maintained facilities of treatment technologies may still discharge effluent that exceeds permissible limits. Most of the existing treatment technology is based on conventional treatment methods, which are also costly, energy-intensive, and not sustainable and environmentally friendly. Identifying sustainable and eco-friendly solutions that are easily adaptable to existing DWW treatment facilities is crucial.

Different types of technology have been employed to treat DWW, such as anaerobic digestion, which is hindered by the high nitrogen content [28]. In the case of the denitrification process, due to the high levels of nitrogen in DWW, the overall process removes nitrogen inefficiently, requiring a high investment cost and producing no value-added products [28,29]. The other drawback of anaerobic processes is that they are sensitive to pH, temperature, and the presence of fats/oils may inhibit performance.

DWW treatment using reverse osmosis (RO), nanofiltration (NF), and ultrafiltration (UF) has also been reported [30,31,32,33]. A RO membrane was used to treat high-strength dairy industry wastewater (5000 and 10,000 mg/L COD), achieving a COD removal rate of 99.7%. The nanofiltration membrane was used for low-strength wastewater (40 and 450 mg/L COD) and showed a COD removal efficiency of 98%. The fouling of nanofiltration and reverse osmosis is challenging, and DWW treatment using filtration and ultrafiltration further requires conventional treatment technology [34]. Apart from fouling operation costs, the requirement of chemicals for cleaning and production of concentrated brine, which require proper disposal process, are the other major challenges with the filtration method.

Dairy wastewater creates a nutrient-rich environment that supports diverse microbial communities, within which microalgae closely interact with other microorganisms to facilitate nutrient cycling [35]. The microenvironment surrounding algal cells, known as the phycosphere, plays a critical role in algal growth, metabolism, and ecological interactions. Microorganisms inhabiting this zone can stimulate algal development by enhancing nutrient availability, such as through the degradation of organic matter or biological nitrogen fixation [36]. These synergistic interactions between microalgae and phycosphere-associated microbiota are essential for the ecological performance and functional efficiency of algal-based systems [37].

Microalgae have recently emerged as a promising, sustainable and environmentally friendly technology for treating dairy wastewater. Integrated or co-cultivation strategies where microalgae are grown in association with complementary microbial species have demonstrated significant enhancements in biomass productivity and treatment efficiency. For instance, a cyanobacteria–microalgae consortium achieved nutrient removal efficiencies exceeding 90%. Likewise, the co-cultivation of C. pyrenoidosa, C. vulgaris, and T. obliquus resulted in high removal efficiencies for total nitrogen (80.23%), ammonia nitrogen (80.40%), and total phosphorus (68.54%), while simultaneously promoting increased production of extracellular polymeric substances (EPS). Elevated EPS levels improved microalgal tolerance to biogas slurry and further enhanced overall nutrient removal performance [37]. Pandey et al. [38] cultivated Scenedesmus sp. ASK22 is using dairy wastewater under both indoor and outdoor conditions. Biomass productivity was higher in indoor cultivation (3.44 g/L) compared to outdoor cultivation (2.09 g/L). Similarly, nitrate, phosphate, and COD removal efficiency were higher under indoor conditions. The study also showed high lipid yield in the produced biomass, which can be used for biodiesel production.

Through continued optimisation of the treatment process and system integration, microalgal technologies have the promising potential to transform dairy wastewater from a waste product into a valuable resource. The products that produce biomass can be used either as animal feed or fertiliser. The technology has some limitations, such as high harvesting cost, light requirement, turbidity, variable DDW composition, large land requirement and harvesting and extraction of useful product from wet biomass, etc.

4.3. Sugar Mill Wastewater

The sugar industry generates substantial volumes of heavily polluted wastewater characterised by a high organic load. On average, approximately 1000 L of effluent are produced per ton of sugarcane processed [39]. This wastewater typically exhibits elevated chemical oxygen demand (COD; 1752–8339 mg/L) and biological oxygen demand (BOD; 1052–4641 mg/L), while containing comparatively low levels of minerals and nutrients [40,41]. The effective collection, treatment, and environmentally safe disposal of such effluents remain significant challenges for agro-processing industries. Consequently, biological treatment approaches are often preferred, given the high biodegradability of these waste streams [42]. By 2023, global ethanol production was estimated at approximately 119 billion litres, with nearly 36.5 billion litres derived from sugarcane distillation, highlighting sugarcane-based ethanol production as one of the most extensive biotechnological processes worldwide in terms of processing volume [43]. In the sugar mill industry, ethanol production is commonly implemented as an integrated process. However, ethanol production generates substantial volumes of a liquid effluent known as vinasse, with approximately 12–15 L produced per litre of ethanol [44]. This by-product is characterised by a high organic load, with a biological oxygen demand (BOD) ranging from 50,000 to 270,000 mg/L and a chemical oxygen demand (COD) between 111,000 and 658,000 mg/L. In addition, sugarcane vinasse contains high concentrations of nitrogen (843–1025 mg/L), phosphorus (31–850 mg/L), sulphate (308–6400 mg/L), and potassium (1735–4451 mg/L), and is typically acidic, with a pH of around 3 [45,46]. When discharged without proper treatment, vinasse poses serious environmental risks, underscoring the urgent need for efficient and sustainable management strategies [46,47].

Several biotechnological approaches have been investigated for treating sugarcane vinasse, including fertigation, anaerobic digestion, and microalgae-assisted remediation [44,48]. Among these strategies, microalgal cultivation has emerged as an up-and-coming option due to the metabolic versatility of microalgae and their high capacity for nutrient uptake. This enables efficient nutrient recovery from vinasse while simultaneously producing biomass with potential commercial value [49]. The resulting biomass can be further valorised into high-value products such as proteins, lipids, pigments, and carbohydrates, supporting the development of a sustainable biorefinery framework [50,51,52]. However, the successful implementation of microalgae-based vinasse treatment systems requires careful optimisation of cultivation conditions, including vinasse dilution or pretreatment to mitigate toxicity, as well as the appropriate selection of photobioreactor design and operational parameters [49].

For instance, Ramirez et al. [52] evaluated sugarcane vinasse as a potential cultivation medium for Scenedesmus sp., examining the effects of temperature, light intensity, and vinasse concentration on algal growth. Their results demonstrated that vinasse concentrations of up to 40% (v/v) significantly enhanced Scenedesmus biomass production. Sibisi et al. [42] demonstrated that locally isolated consortia of microalgae with bacteria and microalgae with yeast were capable of removing up to 86% and 71% of COD, respectively, from sugar mill effluent. These findings show the need for low-cost treatment methods by using local (indigenous) consortia of algae, bacteria, and yeast, which can improve nutrient removal and increase biomass production at the same time.

In addition, the practical application of sugar mill effluent is often constrained due to dark colour, high turbidity, and fluctuating organic load, which negatively affects algal productivity unless further dilution or pretreatment is employed. At demonstration scale implementation faces more challenges, such as seasonal variation of sugar mill effluent, contamination, and the higher harvesting cost. Further research is required to find an economical and sustainable solution to utilise sugar mill effluent for microalgae growth media, and proper utilisation of the biomass.

4.4. Slaughterhouse Wastewater

The slaughterhouse sector is a significant global industry, as meat remains a primary food source in many parts of the world [53]. Over the past few decades, global meat production has consistently increased [54]. Producing one tonne of meat requires a significant amount of water, such as ~15,500 m³ for cattle, 6100 m³ for sheep, 4800 m³ for pigs, and 4000 m³ for poultry. As a result, the expansion of meat production directly leads to higher volumes of slaughterhouse wastewater [55].

Slaughterhouse wastewater is generally characterised by high levels of BOD, COD, total suspended solids (TSS), ammoniacal nitrogen, and phosphates [56]. This is primarily due to the large amounts of blood, intestinal mucus, proteins, carbohydrates, and various lipids such as long-chain fatty acids contained in the effluent, which make it highly rich in organic matter. Moreover, the presence of organic stabilisers, disinfectants, detergents, and veterinary or sanitary pharmaceuticals further elevates the COD levels in this wastewater [57]. The direct discharge of slaughterhouse effluents, which are rich in organic matter and pathogenic microorganisms, poses a significant risk to aquatic ecosystems. Direct release of this wastewater into water bodies reduces DO levels, disrupts ecological balance, and can negatively impact aquatic biodiversity [58].

The high organic content of slaughterhouse wastewater necessitates energy-intensive treatment processes, resulting in increased electricity consumption and higher operational costs. Biological treatment methods, including activated sludge systems and anaerobic digestion, rely on constant energy input to ensure proper mixing and maintain optimal conditions for microbial activity. Additionally, the effluent’s variable and often acidic characteristics caused by the buildup of organic and volatile acids pose further challenges to maintaining stable system performance [59].

However, phycoremediation involves the use of algae, including both microalgae and macroalgae, to remove pollutants from waste and wastewater. This sustainable biotechnology leverages the inherent metabolic capabilities of algae to uptake nutrients, break down organic contaminants, and enhance overall water quality [60]. In addition, algae can capture CO₂ through photosynthesis while efficiently and economically removing excess nutrients from wastewater. For instance, Abirama et al. [61] investigated the phycoremediation potential of Botryococcus sp. for treating wastewater from meat processing plants using a bio-kinetic modelling approach. Their study demonstrated that this alga could efficiently remove nutrients, achieving reductions of 99.03% for ammonia and 99.93% for phosphorus. Likewise, Saleh et al. [62] reported that C. sorokiniana cultivated in secondary effluent from slaughterhouses not only effectively removed pollutants but also showed increased lipid accumulation. These results highlight the promising dual role of algae in both remediating slaughterhouse wastewater and producing biodiesel, as well as utilising residual biomass for fertiliser applications. In addition, it is important to consider economic and operational conditions such as harvesting, extraction, cell disruption, and overall process cost when evaluating biodiesel or fertiliser from microalgae grown in slaughterhouse wastewater.

4.5. Paper/Pulp Wastewater

Global consumption of paper and paperboard was approximately 399 million tons in 2020 and is projected to continue rising, potentially reaching about 466 million tons by 2030 [63]. In parallel, the global pulp and paper market was valued at USD 351.51 billion in 2021 and USD 354.39 billion in 2022, and is expected to expand further to USD 372.72 billion by 2029 [64]. However, the pulp and paper industry is highly water-intensive, consuming large volumes of freshwater across multiple processing stages, including raw material washing, chemical pulping, and bleaching, and consequently generating substantial quantities of wastewater. For instance, the production of one ton of paper typically requires ~190–200 m³ of freshwater [65]. Furthermore, in the pulping process, only 40–45% of the raw material is converted into usable pulp, while the remaining 55–60% of the lignocellulosic content is discharged as waste. As a result, paper pulp effluents are characterised by high concentrations of organic matter, elevated nutrient levels, and suspended solids. These pollutants include naturally occurring plant-derived compounds such as lignin, tannins, and resin acids, as well as xenobiotic substances formed during chemical pulping and bleaching operations. If discharged without adequate treatment, this wastewater can significantly deteriorate water quality, stimulate eutrophication, and deplete dissolved oxygen concentrations, thereby posing serious risks to aquatic ecosystems. Microalgae have demonstrated strong potential for the biological treatment of pulp and paper wastewater, offering a dual benefit of effective wastewater remediation while simultaneously generating valuable biomass suitable for various applications. For instance, Satiro et al. [66] investigated the use of microalgae–bacterial consortia in bioreactor systems for treating pulp and paper wastewater. Four batch-operated bioreactors (BR1–BR4) were employed, each with a different microalgae-to-bacteria ratio expressed as mg VSS/L. All reactors demonstrated effective nitrogen removal. The ammonia nitrogen removal efficiencies reached 91.55 ± 9.99% in BR1, 72.13 ± 19.18% in BR2, 64.04 ± 21.34% in BR3, and 86.15 ± 30.10% in BR4. Additionally, the resulting biomass exhibited a relatively high lipid content (7–22%), indicating its potential for subsequent biodiesel production. Similarly, Bagchi et al. [67] examined the growth performance of Tetradesmus obliquus in primary- and secondary-treated effluents from the pulp and paper industry. Cultivation in a medium containing 50% secondary-treated wastewater resulted in a nearly twofold increase in biomass, reaching approximately 2.5 g/L within 15 days, with a lipid yield of around 390 mg/L. When scaled up to 200 L raceway ponds, the system achieved an aerial biomass productivity of about 26.8 g/m²/day and a lipid productivity of 3.9 g/m²/day at a depth of 30 cm. In addition, the algal treatment led to substantial pollutant removal, reducing COD by nearly 80%, TOC by approximately 91%, and ammonia by 92%.

Due to the high COD load, turbidity, and nutrient concentrations, pulp and paper wastewater often requires dilution or pretreatment to meet the optimal conditions for microalgal cultivation. The dark colour and high turbidity of paper/pulp wastewater reduce light availability, which limits algal growth. The dilution and pretreatment of paper/pulp wastewater potentially improve the optical density and suitable growth conditions. However, this can add additional cost, which should be considered when evaluation large scale feasibility. Overall, this approach represents a sustainable and environmentally friendly technology that simultaneously removes pollutants and generates valuable biomass. These combined advantages make microalgae-based treatment a compelling alternative to conventional treatment methods, while also promoting circular economy principles and enhancing the environmental sustainability of the pulp and paper industry.

4.6. Aquaculture Wastewater

Since the last few decades, aquaculture has been growing continuously with an average annual growth rate of more than 8% [68]. In 2018, aquaculture contributed 46% of global aquatic food production and provided ~52% of the fish consumed by humans [69]. This growth is largely driven by freshwater production systems, which account for 62.5% of total aquaculture output [69]. The aquaculture industry continuously generates a huge amount of aquaculture wastewater. This wastewater contains suspended and dissolved solids, nitrogen-based compounds, total dissolved phosphorus, as well as various contaminants of emerging concern (CECs) [68,70,71]. These substances primarily come from the degradation of uneaten food and the metabolic processes of fish, as well as respiration and digestion. However, these pullulated wastewater needs to be treated before discharge into the environment to prevent any negative impact on the environment [72].

Microalgae have emerged as a promising approach for treating aquaculture wastewater. They can sequestrate CO₂ from the environment and utilise nutrients present in aquaculture effluents, converting them into valuable biomass. This dual functionality minimises environmental pollution load and promotes the development of a circular and sustainable production system [73]. For example, Borg-Stoveland [74] evaluated the potential of aquaculture wastewater obtained from a salmon recirculating aquaculture system for cultivating three marine microalgae species, Isochrysis galbana, Sketetonema marinoi, and Phaeodactylum tricornutum. All species were able to grow across different aquaculture wastewater concentrations, with optimal performance at 75% wastewater concentration. These algal species demonstrated high nutrient removal efficiency, removing ~100 nitrate, nitrite, and phosphate, and ~90% of ammonium by ~90%. Similarly, Ansari et al. [75] cultivated S. obliquus, C. sorokiniana, and Ankistrodesmus falcatus using aquaculture wastewater, reporting high nutrient removal efficiencies (Table 1). The three microalgal species achieved ammonia removal rates of 86.45–98.21%, nitrate removal rates of 75.76–80.85%, phosphate removal rates of 98.52–100%, and COD removal rates of 42–69%. In another study, Guldhe et al. [76] heterotrophically grew C. sorokiniana in aquaculture wastewater, observing removal efficiencies of 75.56% for ammonia, 84.51% for nitrate, 73.35% for phosphate, and 71.88% for COD. To enhance biomass productivity and biochemical composition yield, the authors employed a nutrient (nitrogen) supplementation strategy. They concluded that the addition of 400 mg/L sodium nitrate as a nitrogen source yielded a biomass productivity of 498.14 mg/L/d, along with lipid, carbohydrate, and protein productivities of 150.19, 172.19, and 141.57 mg/L/d, respectively.

The higher nutrient, COD removal efficiency, and biomass production are possibly due to algae cultivated in a laboratory scale under control conditions. Changes in cultivation conditions might change pollutant removal efficiency and biomass productivity. Nutrient removal efficiency and biomass productivity vary among microalgal strains and are strongly influenced by the cultivation conditions employed. To enhance overall process performance, integrating algal cultivation with fish farming is highly recommended rather than operating them as separate systems. Moreover, utilising the produced algal biomass as a feed ingredient offers additional benefits to the aquaculture industry, as it can serve as a valuable protein source or a sustainable alternative to fishmeal in aquafeeds.

In addition, the integration of laboratory-based result to commercial aquaculture systems remains challenging. The main challenges are fluctuation of nutrient concentration, light intensity, photoperiod, contamination risk, harvesting, etc. Therefore, the treatment of aquaculture wastewater using microalgae at laboratory scale/control conditions has shown promising potential. To translate the same finding at the pilot scale condition, further studies are vital to address cost effectiveness, and long-term performance under a real aquaculture operating environment.

4.7. Produced Water

Produced water is the wastewater produced during oil and gas extraction that represents ~70–80% of the total wastewater produced during oil recovery operations [77]. This effluent contains a mixture of hazardous constituents, such as heavy metals, residual hydrocarbons, naturally occurring radioactive materials, and various organic pollutants such as benzene, toluene, ethylbenzene, xylene (BTEX), and phenols [78]. Produced water often includes trace concentrations of numerous chemical contaminants in the effluent matrix [79].

Biological treatment of produced water is widely recognised as a more sustainable and cost-effective technology than conventional (Table 1). This technology employs microalgae and other microorganisms, which break down complex organic contaminants to small and non-toxic compounds, thereby enhancing overall water quality. Nevertheless, the high salinity and complex mixture of organic compounds in produced water further limit or slow microbial performance and overall treatment efficiency. For example, in extreme cases, salt concentrations go up to 300,000 mg/L, posing challenges for enhanced biological remediation [80]. To alleviate these challenges, the use of halophilic microorganisms is particularly advantageous as they are naturally adapted and treat efficiently under high-salinity conditions [81]. Furthermore, the toxicity of certain compounds present in produced water can be reduced by diluting it with seawater, thereby reducing the inhibitory effect on microbial growth and activity [82].

Microalgae are capable of growing under a wide range of environmental conditions, including extreme temperatures, elevated CO₂ levels, acidic or alkaline pH levels, high ammonia concentrations, and varying salinity, due to their vast species diversity [83]. However, microalgae cultivated in produced water their growth rate is generally have a lower growth rate than under optimal conditions. This is due to the limited availability of essential nutrients such as nitrogen and phosphorus. To alleviate this limitation, produced water can be supplemented with nutrient-rich wastewater to increase macronutrient availability and promote algal biomass production. Supplementation of wastewater as a nutrient source poses an advantage compared to conventional culture media [84].

For example, Parsy et al. [85] cultivated Nannochloropsis oculata in a medium composed of saline produced water with a total salt concentration of 114 g/L. The growth media were formulated using seawater and liquid digestate obtained from anaerobic digestion. The growth medium contained produced water at concentrations ranging from 0 to 50% (v/v). The findings show that the specific growth rates of N. oculata were 0.35, 0.27, and 0.16 day⁻¹ at produced water loadings of 10%, 20%, and 30% (v/v), respectively. Approximately 100% nitrogen removal was achieved across all experimental conditions, while ~40% of the organic carbon was removed within a single acclimation cycle [85]. Khairuddin et al. [86] utilised S. obliquus to evaluate growth in varying concentrations of produced water and synthetic growth media. Their findings indicated that algal growth was hampered at a higher ratio of produced water. They also found that low levels of essential nutrients, as well as the complex chemical composition of real produced water, further inhibit the algal growth. Similarly, Al Subaie et al. [87] investigated the cultivation of Scenedesmus obliquus in four types of synthetic produced water (formulated using crude oil, sodium dodecyl sulphate, and xanthin) as well as in real produced water. No algal growth was observed in 100% real produced water, even after supplementation with essential nutrients. The authors concluded that S. obliquus is capable of growing in 50% diluted real produced water when supplemented with essential nutrients. However, growth under same conditions remained lower than that achieved in all four synthetic produced water media at the same dilution level (50%).

Microalgae-based treatment of produced water remains highly challenging due to the complex nature of the wastewater and the presence of toxic constituents. Further research is needed to focus on the screening and isolation of microalgal strains from local environments or from produced water systems. Such locally adapted strains are most likely to show greater tolerance to high salinity, BTEX compounds, and other minerals, thereby improving treatment efficiency and process robustness.

4.8. Tannery Wastewater

The leather tanning industry remains a major source of pollution. Tannery effluents contained a complex mixture of contaminants, heavy metals, particularly hexavalent chromium (Cr VI), and various organic and inorganic compounds. These pollutants pose substantial challenges for wastewater treatment due to high toxicity and persistence. The conventional treatment methods are widely used, offering partial remediation, but these fail to meet the increasingly stringent environmental regulations [88].

Microalgae have shown significant attention in environmental biotechnology due to their ability to remove various types of heavy metals and degrade organic pollutants commonly found in industrial wastewater (Table 1). Through processes such as bioadsorption and bioconversion, microalgae successfully remove nutrients and toxic metals, providing a sustainable and eco-friendly treatment technology. This technology can be integrated into existing technology at various stages to reduce the pollutant load from wastewater. Microalgae applied as a pre-treatment step for tannery effluents, it significantly reduces pollutant loads before subsequent secondary treatment [89,90].

In comparison with conventional remediation methods, microalgae-based treatment presents multiple advantages, positioning it as an ecological and efficient alternative [91]. Microalgae can assimilate a wide range of organic and inorganic components, present in tannery wastewater, which serve as essential nutrients for their metabolic process. During the metabolic process, microalgae not only reduce pollutant load and purify the wastewater but also lead to the production of valuable biomass that are utilised for other products [92]. Microalgae effectively immobilise heavy metals, including Cu, Cd, Cr, Hg, Zn, Pb, and Ni, via processes such as adsorption, ion exchange, covalent bonding, and precipitation [93] (Figure 2). Nambukrishna and Singaram [94] assessed the feasibility of using marine microalgae, including Nannochloropsis marina, Chlorella marina, Thalassiosira sp., and Dunaliella salina, for tannery wastewater treatment in conjunction with lipid production. Among the tested species, Chlorella marina demonstrated superior performance, achieving a 78% reduction in COD, along with biomass and lipid yields of 1.92 g/L and 0.7 g/L, respectively. The extracted lipid fraction was subsequently converted into 0.59 g/L of biodiesel through direct transesterification. Similarly, Rajalakshmi et al. [95] demonstrated that Chlorella sp. cultivated in tannery wastewater for 20 days efficiently removed multiple heavy metals, including Cr, Pb, Ni, Cd, Co, Zn, and Cu, with removal concentrations of 81.36, 70.53, 82.15, 63.29, 58.92, 83.43, and 64.83 µg/mL, respectively. In addition to metal removal, the culture accumulated 0.95 g/L of lipids, 250 µg/mL of carbohydrates, and 160 µg/mL of proteins, while utilising 60.5% of the supplied CO₂. Algal-based technology shows strong potential for application in tannery wastewater treatment. The process operates without generating secondary waste streams, and the algal biomass produced during treatment can be further valorised for a wide range of applications (Figure 3).

Furthermore, the use of algae biomass cultivated in tannery wastewater may limit its potential application. This is because the biomass can accumulate heavy metals and other toxic compounds present in the wastewater, which not only pose a risk to human and animal health but also raise significant safety and regulatory concerns. Therefore, thorough characterisation of the biomass, such as heavy metal content, toxicity, and suitability for potential applications (biofertilizers, animal feed, or bioenergy production), is vital before any further use.

Table 1. Cultivation of microalgae in various types of wastewaters and nutrient removal efficiency and biochemical composition.

Wastewater	Microalgae	Removal Efficiency (%)				References
		N (%)	P (%)	COD (%)	BOD (%)
Aquaculture WW	S. obliquus	NO3− = 77.77 NH4+ = 68.09 NO2− = 73.83 TON = 68.09	PO43− = ~100	42	-	[68]
	C. sorokiniana	NO3− = 75.76 NH4+ = 67.89 NO2− = 81.79 TON = 67.89	PO43− = ~100	69	-
	A. falcatus	NO3− = 80.85 NH4+ = 75.029 NO2− = 99.73 TON = 75.03	PO43− = 98.52	61	-
75%Raw WW+ 25% algae harvested effluent	T. obliquus	NO3− = 72.63 NH4+ = 93.59	PO43− = 97.59	75.18	-	[2]
Paper pulp industrial WW	T. obliquus	NH4+ = 92.81 NO2− = 92.37 NO3− = 89.37	PO43− = 72.87	80		[67]
Aquaculture and Pulp WW	C. vulgris	TN = 76.5	TP = 92.7	75.5		[96]
Tannery WW	Chlorella sp.	NO3− = >90 NH4+ = >90	PO43− = 77.5	>90	>90	[97]
Cattle WW	S. obliquus	NH4+ = 98–99	PO43− = 69–77.65	65–70	-	[98]
Brewery effluent	S. obliquus	TN = 88	TP = 30	71	-	[99]
Brewery WW	S, obliquus	TN = 20.8	-	57.7		[100]
Dairy WW	C. vulgaris	TN = 77	TP = 78	92	77	[101]
Real Textile WW	C. vulgaris	NO3− = 60	TP = 42	45		[102]
Raw WW	C. vulgaris	NH4+ = 94.36	PO43− = 88.37	-	-	[103]
Domestic WW	S. obliquus	TN = 98.54	PO43− = 97.9	76.3	-	[4]
Slaughterhouse WW	Chlorella and Scenedesmus	TN = 99 NO3− = 90–95 NH4+ = 98–99	TP = 90–95 PO43 = 98–99	99	99	[104]
Agricultural runoff	Synechocystis sp., Cf Oocystis sp., and Ulothrix sp.	NH4+ = 93 NO3− = 54	PO43− = 100	-	-	[105]
Poultry abattoir WW	Tertaselmis suecica	NO3− = 98	PO43− = 79.9	94.5	94.3	[106]
	Micractinium reisseri	NO3− = 95.4	PO43− = 64.6	86.2	84.7

Table 1. Cultivation of microalgae in various types of wastewaters and nutrient removal efficiency and biochemical composition.

Wastewater	Microalgae	Removal Efficiency (%)				References
		N (%)	P (%)	COD (%)	BOD (%)
Aquaculture WW	S. obliquus	NO3− = 77.77 NH4+ = 68.09 NO2− = 73.83 TON = 68.09	PO43− = ~100	42	-	[68]
	C. sorokiniana	NO3− = 75.76 NH4+ = 67.89 NO2− = 81.79 TON = 67.89	PO43− = ~100	69	-
	A. falcatus	NO3− = 80.85 NH4+ = 75.029 NO2− = 99.73 TON = 75.03	PO43− = 98.52	61	-
75%Raw WW+ 25% algae harvested effluent	T. obliquus	NO3− = 72.63 NH4+ = 93.59	PO43− = 97.59	75.18	-	[2]
Paper pulp industrial WW	T. obliquus	NH4+ = 92.81 NO2− = 92.37 NO3− = 89.37	PO43− = 72.87	80		[67]
Aquaculture and Pulp WW	C. vulgris	TN = 76.5	TP = 92.7	75.5		[96]
Tannery WW	Chlorella sp.	NO3− = >90 NH4+ = >90	PO43− = 77.5	>90	>90	[97]
Cattle WW	S. obliquus	NH4+ = 98–99	PO43− = 69–77.65	65–70	-	[98]
Brewery effluent	S. obliquus	TN = 88	TP = 30	71	-	[99]
Brewery WW	S, obliquus	TN = 20.8	-	57.7		[100]
Dairy WW	C. vulgaris	TN = 77	TP = 78	92	77	[101]
Real Textile WW	C. vulgaris	NO3− = 60	TP = 42	45		[102]
Raw WW	C. vulgaris	NH4+ = 94.36	PO43− = 88.37	-	-	[103]
Domestic WW	S. obliquus	TN = 98.54	PO43− = 97.9	76.3	-	[4]
Slaughterhouse WW	Chlorella and Scenedesmus	TN = 99 NO3− = 90–95 NH4+ = 98–99	TP = 90–95 PO43 = 98–99	99	99	[104]
Agricultural runoff	Synechocystis sp., Cf Oocystis sp., and Ulothrix sp.	NH4+ = 93 NO3− = 54	PO43− = 100	-	-	[105]
Poultry abattoir WW	Tertaselmis suecica	NO3− = 98	PO43− = 79.9	94.5	94.3	[106]
	Micractinium reisseri	NO3− = 95.4	PO43− = 64.6	86.2	84.7

5. Microalgae for Carbon Neutrality

The escalating concentration of atmospheric CO₂ is a primary driver of global climate change, necessitating urgent strategies to mitigate and achieve carbon neutrality. Microalgae have emerged as a promising solution thanks to their exceptional photosynthetic efficiency, rapid growth rates, and ability to capture and convert CO₂ into valuable biomass. Unlike terrestrial plants, microalgae exhibit superior carbon fixation capabilities, with some species attaining up to 50 times higher productivity per unit area, making them an attractive candidate for integration into carbon-neutral frameworks [107]. Microalgae utilise CO₂ during photosynthesis, converting it into carbohydrates, proteins, and lipids. This process not only mitigates greenhouse gas emissions but also produces biomass for biofuels and bioproducts. A global projection estimates that cultivating microalgae on 13 million acres (~5.3 million hectares) sequesters approximately 0.5 Gigatons of CO₂ per Year, while producing more than 300 million tonnes of biomass [108]. Algae systems could contribute significantly to negative emissions, with global targets aiming for 500 Mt CO₂/year by 2030 and 8 Gt/year by 2050 [109].

Microalgae-based systems align well with circular economy principles by transforming waste streams into resources. Life cycle assessments (LCA) reveal both opportunities and challenges in microalgae-based carbon mitigation. A national-scale example from Thailand demonstrates the potential impact: using 0.4 Mt/year algal fish feed offsets −1.1 Mt CO₂/year, biodiesel production of 4015 million L/year offsets −30 Mt CO₂/year, and biofertilizer production of 5 Mt/year offsets −6 Mt CO₂/year. Combined, these measures achieve a reduction of approximately −37 Mt CO₂/year, equivalent to approximately 14% of Thailand’s annual emissions [110]. Industrial pilot findings indicate that one hectare of algae for biodiesel production generates 0.148 kg CO₂-eq/MJ, compared to 0.088 kg CO₂-eq/MJ for fossil diesel, highlighting the importance of integrating renewable energy for achieving net carbon benefits [108]. The biomass generated through CO₂ sequestration can be transformed into biofuels such as biodiesel, bioethanol, and biogas, and high-value products like pigments, nutraceuticals, and bioplastics. Residual biomass can serve as fertiliser or biochar, contributing to long-term carbon storage. These valorisation pathways not only offset fossil fuel dependency but also create revenue streams, improving the feasibility of large-scale algal systems.

Several factors govern the efficiency of CO₂ fixation in microalgae (

Table 2. Key Factors Influencing CO₂ Fixation by Microalgae.

Factor	Description	Impact on CO2 Fixation	References
Temperature	Species-specific optimal range for enzymatic activity.	Deviations reduce photosynthetic performance.	[107]
Species Selection	Different strains vary in photosynthetic efficiency and CO2 tolerance.	Robust and indigenous strains (e.g., Chlorella, Scenedesmus) perform better.	[107]
Light Intensity & Quality	Adequate light drives photosynthesis; the blue/red spectrum is most effective.	Too low → reduced growth; too high → photoinhibition.	[111]
CO2 Concentration & Delivery	Optimal CO2 supply enhances assimilation; excessive CO2 causes acidification.	Balanced supply maximises fixation efficiency.	[107,112]
Nutrient Availability	Nitrogen, phosphorus, and trace elements are essential for biomass synthesis.	Limitation reduces CO2 uptake; excess improves growth.	[112]
Cultivation System Design	Open ponds vs. photobioreactors: effects of control over conditions.	Closed systems → higher fixation; open ponds → lower cost but less efficient.	[111]
Mixing & Hydrodynamics	Ensures uniform light exposure and gas transfer.	Poor mixing leads to localised depletion and inhibits fixation.	[112]
Gas Composition	Flue gas impurities such as NOx and SOx can inhibit growth.	Pretreatment or tolerant strains are vital for industrial integration.	[111,112]
Genetic & Metabolic Engineering	Improves photosynthetic pathways and stress tolerance.	Improves carbon fixation and biomass productivity.	[113]

). Light intensity and spectrum are critical, with blue and red wavelengths being most effective for photosynthesis. Optimal CO₂ concentration and delivery enhance assimilation, while excessive levels cause acidification [111]. Temperature, nutrient availability, and species selection also play vital roles, with robust strains such as Chlorella and Scenedesmus commonly used for flue gas capture. Cultivation system design (open ponds versus photobioreactors) affects productivity and cost. Flue gas impurities like NO_x and SO_x may inhibit growth, necessitating pretreatment or the use of tolerant strains. However, microalgae-based systems face challenges including high capital costs, energy-intensive harvesting, and scalability issues. Life cycle analyses emphasise that sustainability depends on optimising energy inputs and integrating renewable energy sources [112]. Policy support, carbon credit mechanisms, and technological innovations like low-cost harvesting and hybrid cultivation systems are essential for widespread adoption. Microalgae could develop a cornerstone of carbon-neutral infrastructure, bridging environmental sustainability and industrial productivity. Microalgae cultivation for carbon neutrality synergistically combined with wastewater treatment, establishing a multi-functional technique that addresses both carbon and nutrient pollution. Wastewater provides essential nutrients (nitrogen and phosphorus), reduces the need for synthetic fertilisers and lowers operational costs. Concurrently, microalgae remove contaminants while sequestering CO₂, producing biomass that is valorised into bioenergy or bioproducts. The integrated approach exemplifies a closed loop, minimising environmental impact and enhancing economic viability. The biomass generated through CO₂ sequestration is processed into various products, aiding in carbon neutrality beyond the cultivation stage. Lipid-rich strains are superior for biodiesel production, while carbohydrate-rich species are converted into bioethanol or biogas. Residual biomass serves as feedstock for bioplastics, fertilisers, and biochar, which offers long-term carbon storage when applied to soils. These valorisation pathways not only offset fossil fuel dependency but also create revenue streams that improve the feasibility of large-scale algal systems.

6. Valorisation of Microalgae Biomass Cultivated in the Waste Stream

6.1. Bioplastics

Microalgae are high in proteins, which significantly contribute to the beneficial characteristics of the polymers derived from them [114]. Among different biopolymers, polyhydroxyalkanoates (PHAs) have attracted significant attention due to their remarkable properties, such as biodegradability, biocompatibility, thermal stability, and hydrophobicity. The current market potential for PHAs is expected to grow from 2.4 million tons in 2025 to 7.5 million tons by 2026 [115,116]. There are three types of PHAs, which include short-chain-length (scl-PHAs), medium-chain-length (mcl-PHAs), and long-chain-length (lcl-PHAs) polymers [117]. They are primarily aliphatic polyesters that are naturally synthesised and accumulated as intracellular granules by various types of bacteria (Gram-positive and Gram-negative species) [118]. The biosynthesis of PHAs is mediated by three key enzymes: acetyl-CoA acetyltransferase (PhaA), acetoacetyl-CoA reductase (PhaB), and polyhydroxyalkanoate synthase (PhaC) [119]. Apart from bacteria, several microalgae and cyanobacteria are also capable of producing PHAs under diverse cultivation conditions, using CO₂ and sunlight as energy sources. The PHAs can be extracted from the algal and cyanobacterial biomass and subsequently used to produce bioplastics for different applications [117]. In addition, selecting the right microalgae strain and cyanobacteria is crucial for optimising processes and scale-up. Afreen et al. [120] emphasised the advantages of these organisms, noting their low metabolic demands and ability to thrive under diverse environmental conditions. Chong et al. [121] researched the use of microalgal genera such as Chlamydomonas, Chlorella, Spirulina, and Botryococcus for PHA production, underscoring the importance of efficient cultivation systems. Kavitha et al. [122] reported a PHA content of 16% when Botrycoccus braunii was cultivated in synthetic BG11 medium. Kumari et al. [123] reported a higher PHA accumulation (30%) in C. Sorokiniana under two-phase (growth and stress phase) cultivation. Likewise, García et al. [124] achieved a 30% PHA yield by cultivating Scenedesmus sp. under phosphate-limited conditions. In contrast, Pezzolesi et al. [125] obtained a substantially higher PHA yield (54%) by applying a two-phase cultivation strategy involving phototrophic and mixotrophic growth. The variation in PHA yield directly depends on the algae strains and growth conditions provided. Although microalgae-based bioplastics are a promising substitute for conventional plastics, several critical challenges must be addressed. The primary limitation in producing bioplastics or other algal-derived raw materials lies in the overall economics of algal processes [126]. Selecting an appropriate algal strain, optimising the cultivation mode using waste substrates as carbon sources and developing economical and sustainable PHA extraction methods are all crucial for advancing algae-based bioplastic production.

However, the product cost is strongly influenced by the yield of the target compound, which depends on both biomass productivity and the intracellular concentration of the desired component. The use of natural feedstocks, like starch, is constrained by high production costs. Cultivation conditions that favour PHA accumulation, particularly nutrient limitation, often suppress biomass growth, rendering this strategy unsuitable for large-scale production. Additionally, co-producing bioplastics alongside high-value products is frequently proposed to enhance economic feasibility; this approach may reduce the yield of the target bioplastic. Furthermore, when PHA production is integrated with wastewater treatment, key challenges include culture contamination, reduced polymer yields, and limited production scale constrained by wastewater flow rates. Even advanced strategies such as genetic engineering require further development to establish time- and cost-efficient methods, assess strain stability at scale, and strengthen environmental regulatory frameworks [127]. Addressing these challenges highlights significant research gaps, as comprehensive economic and technical evaluations of chemical or bacterial synthesis routes are scarce. Many existing analyses of PHA production rely on hypothetical scenarios that still require technical validation.

6.2. Food/Feed

Due to their high growth rates, adaptability to various environments, including inhospitable conditions, and ability to utilise waste streams (industrial as well as agricultural), microalgae are often considered as a “cell factory” for food-feed production (Figure 3). Microalgae offer a renewable source of bioactive compounds, including proteins, carbohydrates, and essential fatty acids, such as omega-3 and omega-6 long-chain polyunsaturated fatty acids. Interestingly, the nutritional content of microalgae can compete with that of marine fish in terms of lower chemical contamination and higher purity [128]. Notably, due to their high nutritional potential, microalgae species such as C. vulgaris, Auxenocholella protothecoides, Dunaliella bardawil, Chlamydomonas reinhardtii, Euglena gracilis, and Arthrospira platensis have received FDA approval for consumption as edible algae worldwide [3,128]. The availability of pigments in microalgae that act as antioxidants, natural colourants and a nutritional powerhouse cannot be overlooked. Due to their rich dietary abundance, many algal species have been used as a source of food for humans in China, Japan, Africa, and Mexico [129]. This approach is particularly pertinent given the projected global population increase to 9.7 billion by 2050, which necessitates innovative strategies to meet the escalating demand for protein and other essential nutrients amidst diminishing arable land and overfished oceans [130]. The ability of microalgae to utilise several forms of water-soluble nitrogen in protein-rich biomass makes them a potential choice for the inclusion in pretreatment steps of agricultural wastewater streams, resulting in the nitrogen and other elements being made available for microalgal consumption [131]. However, the type of strain, availability of light, and temperature not only affect biomass production but also impact the quality of the byproduct [132].

Cultivating microalgae in wastewater offers a dual benefit, effectively removing pollutants and recovering valuable nutrients from these streams, thereby transforming waste into a resource. Integrating algal cultivation into wastewater treatment processes significantly reduces the carbon footprint of these facilities by replacing carbon-intensive inputs with biomass-derived alternatives and producing biomass that can be used as feed [133]. These characteristics make them suitable for integration into the food and feed industries, potentially reducing environmental impact and lowering land use and production costs. Specifically, algae such as Chlorella vulgaris and Phormidium laminosum, Tetraselmis sp., Isochrysis sp., Pavlova sp., Phaeodactylum sp., Chaetoceros sp., Nannochloropsis sp., Skeletonema sp. and Thalassiosira sp are notable for their capacity to accumulate high protein and lipid content, making them suitable for both bioremediation and subsequent application as animal feed or bioenergy sources [3,134]. This cultivation method also drastically reduces the water footprint associated with biomass production, as it reuses a resource that would otherwise require energy-intensive treatment.

Thoré et al. [135] cultivated Chlorella sorokiniana on poultry wastewater and reported an 83% and 113% increase in productivity when the wastewater was first diluted (50%) with tap water or standard growth medium, respectively. They also reported that wastewater sterilisation before use enhanced algal growth by 36–118%, provided the wastewater was diluted 25–50% with standard medium [135]. Furthermore, supplementation of algae in animal feed offers numerous benefits, including improved growth and body weight, reduced feed intake, enhanced immune response and resistance to illness, antibacterial and antiviral action, as well as enrichment of livestock products with bioactive compounds [128]. More than 40 varieties of microalgae are widely used in aquaculture. In general, algal biomass must be dewatered and dried before feeding it to fish, an energy-intensive. Researchers have proposed feeding the wastewater-grown algae to large-bodied zooplankton (small crustaceans). This has eliminated the need for drying and become a successful means of upcycling nutrients and protein into fish feed, as well as for small crustaceans (e.g., Daphnia and Moina), for which microalgae are a natural food source [136]. Gorzelnik et al. [137] found that the application of a two-step multitrophic process to assimilate residual nutrients into live feeds is suitable for fish. Researchers utilised unsterilized aquaculture wastewater for nutrient removal with microalgae Chlorella vulgaris, Scenedesmus dimorphus, and Haematococcus pluvialis. The first two algae were subsequently harvested using D. magna (a planktonic crustacean) as a grazer, with protein accumulation at 20–30% of dry weight, and an amino acid profile favourable for use as high-value fish feed. Khatoon et al. [138] also reported that N. maculate and T. chuii had significantly higher (p < 0.05) protein and lipid content when cultured in wastewater medium.

The additional benefits of microalgal cultivation come with its own limitations and challenges. The presence of toxic emerging pollutants, heavy metals, organic dyes, pathogens, and high nutrient concentrations in wastewater can limit photosynthetic activity and microalgae growth. Additionally, the carbon and nitrogen sources in the culture medium can significantly impact the physiological and biochemical characteristics of microalgae, affecting the protein, carbohydrate, chlorophyll, and fatty acid compositions [139]. Potential safety risks associated with pathogenic microorganisms in algal biomass obtained from sewage/industrial/food wastewater treatment may occur, potentially raising safety concerns. Wastewater contains various pathogenic microorganisms, organic and inorganic pollutants, contaminants, and microbial toxins, which may lead to the bioaccumulation process of heavy metals. Using microalgae biomass contaminated with pathogens as aquafeed can introduce harmful microorganisms or toxins, posing severe risks to aquatic animal health and potentially leading to failure of the aquaculture system. Research is underway, and the scientific community is working to resolve the issue effectively at the technical level. Microalgae cultivated in wastewater for feed/food application can pose significant safety risks due to the potential accumulation of heavy metals, pathogens and other contaminants. It is important to ensure strict monitoring, selection of the right downstream processing (harvesting, drying, cell disruption, product extraction, etc.) and compliance with regulatory standards are vital to ensure safety and public acceptance. Otherwise, microalgae biomass harvested from wastewater treatment at present is primarily used to produce biofuel or fertiliser instead of feed or food application.

6.3. Biochar

The use of microalgae grown on wastewater/waste or industrial streams as the feedstock for biochar synthesis is an excellent approach (Figure 3). The nutrients from waste streams harness microalgae’s rapid growth and high photosynthetic efficiency to simultaneously treat wastewater and produce a valuable carbonaceous adsorbent, known as biochar, when treated through thermochemical processes. Biochar is a carbon-rich material with enhanced adsorptive properties due to its high specific surface area, honeycomb-like microporous structure, and diverse oxygen- and nitrogen-containing functional groups [140]. This dual functionality underscores the potential for a circular bioeconomy, where waste products are transformed into resources for environmental remediation and carbon sequestration, providing a range of valuable products, including biochar, bio-oil, and syngas, which are key outputs. Microalgae species have been cultivated in municipal wastewater, industrial effluents, and agricultural runoff to take up nitrogen, phosphorus, and organic carbon while generating biomass feedstock for downstream conversion [111]. Thermochemical conversion of microalgal biomass to biochar principally employs slow or fast pyrolysis, hydrothermal carbonisation (HTC), and gasification, each yielding solids (biochar/hydrochar) of differing properties depending on temperature, residence time, and heating rate [140]. Pyrolysis at intermediate to high temperatures enhances pore development and surface chemistry. For example, nanoporous microalgae (Chlorella pyrenoidosa) biochar prepared by pyrolysis at ~700 °C produced a high specific surface area and ammonia adsorption capacity, whereas excessive temperature degraded textural features [141].

HTC produces hydrochar with distinct physicochemical properties and generally higher yields at lower temperatures compared to pyrolysis; however, differences in phytotoxicity and amendment suitability have been reported between hydrochar and pyrochar derived from wastewater microalgae [142]. Processing parameters, such as temperature and type of feedstocks, strongly determine the yield and functionality of biochar. Higher temperatures typically reduce char yield but increase fixed carbon content, porosity, and surface functionality, which are beneficial for adsorption or electrochemical uses [143]. Compared with terrestrial feedstocks, microalgae-derived chars often contain higher heteroatom contents (N, S, minerals) and distinct morphologies that can be tailored via activation or electrochemical modification for targeted nutrient management (e.g., slow-release N/P), struvite seeding, or soil conditioning. However, application-specific stabilisation may be required [143,144]. Microalgae-derived biochar has demonstrated multifunctional applications, including soil amendment, long-term carbon sequestration, pollutant adsorption, nutrient recovery, and electrochemical materials for energy storage [141,144]. For soil use, biochars from wastewater microalgae can supply nutrients and improve soil properties, though careful assessment of phytotoxicity and contaminants is required [142]. Lu et al. [145] cultivated Chlorella vulgaris in sewage wastewater and used it to produce biochar through microwave-assisted pyrolysis. Researchers not only found that growing microalgae in wastewater reduces the concentration of ammonia and total phosphorus (complete absorption within 60 days of cultivation) but also reported that mixing prepared biochar with agro-waste to form fuel pellets enhanced the calorific value of the fuel [145]. Nageshwari et al. [146] used an integrated electrocoagulation-flotation (ECF) process for algal biomass recovery and, at the same time, modified the algal surface with co-deposit magnesium onto the biomass, producing Mg-laden biochar that acts as a seed for struvite (MgNH₄PO₄·6H₂O) crystallisation, facilitating phosphorus recovery and creating fertiliser-ready products [146]. Electrochemical modifications have improved capacitance and energy-storage performance of algal biochars, indicating potential in circular energy-materials value chains [144]. Life-cycle and technoeconomic analyses emphasise environmental benefits from coupling wastewater remediation with biochar production, but outcomes depend on process optimisation, energy inputs, and market valorisation pathways [143,147]. Several adsorption studies have shown the use of algal biochars for the abatement of ammonia, heavy metals, textile dyes, and organic pollutants from wastewater, as well as soil [148]. Field and lab-scale studies demonstrate multifunctional benefits of wastewater microalgae biochars used as fertilisers or soil amendments, including improved soil physicochemical properties, nutrient retention, and pollutant adsorption, alongside potential co-benefits such as electrochemical utility; however, phytotoxicity assessments and contaminant screening are essential, particularly when feedstocks are drawn from municipal or industrial effluents. With numerous positive aspects, some limitations require attention, including scale-up barriers, technical challenges such as the variability of feedstock composition from waste streams, quality control during processing and algal cultivation, energy-intensive harvesting and conversion steps, and emissions control during thermochemical processing for biochar production. Additionally, regulatory acceptance for soil or reclamation uses, particularly when utilising wastewater-derived algal biomass, is another challenge [149]. Future research should prioritise process integration, low-energy harvesting, tailored activation routes for targeted applications, standardised safety testing for wastewater-derived biochars, and comprehensive life-cycle assessments to quantify net environmental benefits [142,143]. Studies recommended that wastewater microalgae can be transformed into safe, effective fertilisers and soil amendments in the form of biochar, hydrochar, if feedstock characterisation, process optimisation, and regulatory compliance are embedded across the value chain, enabling scalable deployment that delivers triple benefits: wastewater treatment, carbon sequestration, and agricultural productivity.

6.4. Fertiliser

Wastewater-grown microalgae offer a compelling, circular pathway to produce biofertilizers by coupling nutrient removal from effluents with the generation of nutrient-rich biomass that can be transformed into soil amendments and fertiliser precursors (Figure 3). Wastewater typically supplies nitrogen, phosphorus, and trace elements that microalgae assimilate rapidly, lowering treatment costs while yielding biomass suitable for agricultural use, thereby aligning environmental remediation with resource recovery in a closed-loop system. Nitrogen- and Phosphorus-rich microalgae biomass is considered a slow-release fertiliser, preventing nutrient losses from the soil through the gradual release of macro- and micronutrients [150]. Álvarez-González et al. [150] demonstrated that the nitrogen released by soil microalgae remains within the levels required by plants, helping to prevent excess nutrient runoff and reducing the risk of eutrophication in groundwater and surface water caused by the overapplication and loss of traditional fertilisers [150,151]. Several studies conducted in the past on different crops have indicated higher yields and plant growth when algal biomass from wastewater was used as a fertiliser [152]. Microalgae such as Chlorella minutissima, Scenedesmus spp., Chlorella spp., Chlorella and Scenedesmus consortium, and Nostoc muscorum are among the most extensively studied algal species for use as biofertilizers. The use of wastewater-treated algal biomass not only reduces the water footprint but also improves the soil’s organic and inorganic nutrient content, as it contains sufficient N and P that are required for plant growth. Moreover, microalgae are known to contain various biostimulatory compounds, including phytohormones mimicking compounds, terpenoids, polysaccharides and AAs [153].

Extracts and metabolites obtained from microalgae species such as Chlorella spp., Spirulina platensis, Acutodesmus spp., Scenedesmus spp., Dunaliella spp., Calothrix elenkini, etc., are commonly used as biostimulants [154]. Pooja et al. [155] used Chlorella vulgaris to remove toxic pollutants and nutrients from sewage wastewater and used the treated water as a bio-fertiliser to grow tomato plants. The study reported a significant reduction in the concentrations of nitrates (93%), COD (95%), and BOD (92%) in wastewater, with efficient growth and productivities comparable to those of chemical fertilisers in tomato plants [155].

Life-cycle and technoeconomic analyses consistently highlight that coupling wastewater remediation with algal biomass valorisation for fertiliser production can deliver net environmental benefits when energy inputs are optimised, and market pathways for biochar/fertiliser products are established; yet outcomes depend on careful control of harvesting energy, conversion efficiency, and product quality assurance [156]. Despite these advantages, several constraints need attention for scale-up. Variability in wastewater composition can alter biomass quality and downstream conversion performance. Energy-intensive dewatering and harvesting, as well as the regulatory acceptance of wastewater-derived fertilisers, hinge on standardised safety testing (e.g., heavy metals, persistent organic pollutants, pathogens) and robust certification frameworks. Emerging integrated approaches such as phycoremediation, utilisation of remediated wastewater for agriculture, use of treated algal biomass as biofertilisers/biostimulants of biochar for tailored release of nutrients, illustrate how process design can simultaneously solve wastewater nutrient burdens (N/P removal), recover valuable fertiliser components, and enhance soil performance, thereby strengthening the business case and policy rationale in a circular bioeconomy.

Table 3. Microalgae cultivation and impact on the environment.

Microalgae	Growth Medium	Application	Comment	References
S. obliquus	BG11	Fish feed supplement	Adding less than 7.5% microalgae improves fish growth and nutritional content.	[3]
A. obliquus	Poultry litter and domestic wastewater	fertiliser	Algae residual used as a fertiliser for mung bean crops showed improvement in plant growth and soil microbial activity.	[157]
Microalgae	Wastewater	fertiliser	Microalgae-based fertiliser demonstrated positive impacts in 10 out of 11 impact categories. Wastewater-grown microalgal biomass has potential as a sustainable alternative to mineral fertilisers, potentially contributing to greener agriculture.	[150]
Chlorella sp.	-	Biochar for plant growth activator	Biochar from Chlorella sp. acts as a seed growth stimulant, with potential for sustainable agriculture and environmental protection.	[158]
T. obliquus	75% raw wastewater 25% recycled effluent	CO2 sequestration, biomethane potential, and HHV	Highest CO2 fixation rate of 0.19 gCO2/L/d, theoretical biochemical methane potential of 471.54 mL CH4/g vs. and high heating value of 21.52 Kg/J were obtained	[2]
T. obliquus	Paper pulp industrial Wastewater	Biodiesel	Utilising paper-pulp industrial wastewater for T. obliquus growth provides a sustainable solution for both energy generation and wastewater treatment.	[67]
Chlorella sp. and Sargassum	Municipal wastewater	Biodiesel and biochar	The lipid was converted to biodiesel, and the residual biomass left after lipid extraction was used for biochar application. This illustrates a low-cost microalgae-based biorefinery approach for producing bioenergy and biochar residues.	[159]
Marvania coccoides	Optimised wastewater	Biodiesel	The ex situ and in situ transesterification methods using immobilised lipase showed significantly higher FAME yields of 91.95% and 72.5%.	[160]

presents the implications of algae biomass for various applications and their associated impacts.

7. Use of Molecular Techniques for Enhancing Algal Systems for Wastewater Management

The global challenge of wastewater remediation, particularly for nutrients (such as nitrogen and phosphorus) and heavy metals (HMs), requires innovative and sustainable solutions. Algal-based systems present a promising alternative to energy-intensive conventional methods, offering the benefits of phycoremediation and the production of valuable biomass for biofuels, feed, and bioproducts [161,162]. Despite this potential, limitations such as suboptimal biomass productivity, inefficient nutrient assimilation, poor resilience to toxicants like HMs, persistent organic pollutants (POPs), and costly harvesting methods hamper widespread adoption [163,164,165]. The biological complexity of these systems, often involving synergistic algae-bacteria partnerships, has been a “black box”, limiting controlled optimisation [162,166,167]. Advances in molecular biology are dismantling this barrier, providing a powerful toolkit to understand and enhance the biological mechanisms driving algal WT.

7.1. Genetic Engineering for Enhanced Algal Performance

Genetic engineering is a powerful tool to manipulate microalgal physiology, targeting traits critical for WT treatment. The primary strategies for improving these traits, including nutrient assimilation, tolerance to toxicants, harvesting efficiency, and consortium construction, are summarised in

Table 4. Target traits and molecular strategies for the genetic engineering of microalgae in wastewater treatment.

Target Trait	Molecular Strategy	Example Gene/Enzyme Target	Desired Outcome	References
Nutrient assimilation	Overexpression of transporters	Ammonium transporters, Phosphate permeases	Increased nitrogen/phosphorus removal efficiency and biomass yield.	[164]
Biofuel production	Metabolic engineering, gene knockout	Diacylglycerol acyltransferase (DGAT), Carbohydrate metabolism enzymes	Redirect carbon flux to enhance lipid (for biodiesel) or carbohydrate (for bioethanol) production.	[164,168]
Heavy metal tolerance and biosorption	Overexpression of chelators and cell wall modifiers	Metallothioneins (MTs), Phytochelatins (PCs), Alginate biosynthesis genes	Enhanced metal binding capacity, sequestration, and tolerance to HM-induced oxidative stress.	[169,170,171,172]
Oxidative stress tolerance	Overexpression of antioxidants	Superoxide dismutase, Catalase	Improved algal resilience and performance in HM streams containing HMs and POPs.	[173]
Harvesting efficiency	Heterologous expression of flocculation genes	FLO1, FLO5 (from S. cerevisiae)	Induction of self-flocculating phenotypes, reducing reliance on energy-intensive centrifugation and chemical flocculants.	[166]

HMs = Heavy Metals; POPs = Persistent Organic Pollutant.

.

7.1.1. Engineering Algal Metabolism for Remediation and Valorisation

The use of genetic engineering to redesign algal metabolism follows various strategies. A primary strategy involves increasing the innate capacity for nutrient assimilation by overexpressing genes encoding specific ammonium transporters and phosphate permeases. This approach has proven highly effective, with engineered strains of Chlorella and Scenedesmus demonstrating nitrogen and phosphorus removal efficiencies of up to 90% from municipal WT, while concurrently increasing overall biomass yield [164]. To transform this captured biomass and nutrient flux into valuable commodities, metabolic engineering is used to redirect carbon towards storage molecules. This involves targeting and amplifying key rate-limiting enzymes in biosynthetic pathways, such as diacylglycerol acyltransferase (DGAT) for lipid biosynthesis to enhance biodiesel production, and enzymes in carbohydrate metabolism to boost yields for bioethanol fermentation [164,168]. The precision of modern genome editing tools, particularly CRISPR-Cas9 and TALENs, is pivotal for this multi-trait engineering. These systems enable the targeted knock-out, knock-in, and transcriptional regulation of genes, allowing for the precise disruption of competitive metabolic pathways in organisms like Chlamydomonas, thereby enhancing carbon flux towards desired end-products, such as lipids or hydrogen [164,168].

7.1.2. Augmented Heavy Metal Biosorption and Tolerance

Heavy metal pollution is a critical concern, and algal biosorption is a cost-effective remediation strategy. Molecular techniques are used to amplify this natural capability. A prominent approach is the overexpression of metal-chelating proteins such as metallothioneins (MTs) and phytochelatins (PCs). For instance, expressing a bacterial metallothionein gene in Chlamydomonas reinhardtii significantly improved its cadmium tolerance and accumulation capacity [169,170]. Enhancing the algal cell wall, the primary site for metal biosorption, is another strategy. Engineering strains to overproduce metal-binding polysaccharides, such as alginate in brown algae or uronic acid-rich polymers in green algae, can increase the density of functional groups (e.g., carboxyl) for cation binding [171,172]. Additionally, improving tolerance by overexpressing antioxidant enzymes (e.g., superoxide dismutase, catalase) mitigates oxidative stress induced by HMs and POPs, ensuring robust algal performance in WT streams [173].

7.1.3. Improved Algal Harvesting and Productive Consortia

Microalgal harvesting is a significant economic bottleneck. Bio-flocculation, mediated by surface proteins, presents a low-energy solution. Computational and molecular dynamics studies have identified flocculation-enhancing proteins, such as FLO1 and FLO5, from Saccharomyces cerevisiae as promising candidates for transgenic expression in algae [166]. The expression of these FLO genes in model microalgae, such as Chlamydomonas reinhardtii, can induce self-flocculating phenotypes, thereby reducing reliance on energy-intensive centrifugation and chemical flocculants [166]. In addition to addressing challenges such as harvesting, genetic engineering can be utilised to create stable and productive consortia for upstream production. A prime example is the engineering of the bacteria Azotobacter vinelandii to over-excrete ammonium, which in turn cross-feeds and enhances the growth and lipid production of co-cultured, oil-accumulating microalgae [174]. Similarly, engineering cyanobacteria, such as Synechocystis sp., with the pdc and adhII genes from Zymomonas mobilis enables direct bioethanol production from WT-derived CO₂ [113].

7.2. Molecular Techniques for Consortia Analysis

A comprehensive understanding of algal community composition and dynamics is foundational for optimising wastewater treatment systems. DNA-based methods like utilising conserved genetic markers and primers, enable precise tracking of algal taxa within complex consortia. Commonly employed loci include the 18S ribosomal RNA (rRNA) gene for broad eukaryotic phylogenetics, the internal transcribed spacer (ITS) region for high-resolution species-level discrimination (particularly in green algae and diatoms), and the chloroplast genes rbcL and tufA [175]. Metabarcoding, which is the high-throughput sequencing of these marker genes from environmental samples, allows for detailed profiling of algal community shifts in response to operational parameters like nutrient loading, pH, and hydraulic retention time [176]. Furthermore, quantitative PCR (qPCR) assays using taxon-specific primers can monitor the abundance of key algal species or functional groups, providing a quantitative view of succession dynamics that is essential for assessing the stability and performance of engineered or native consortia [177].

Building on this taxonomic profiling, high-throughput multi-omics technologies decipher the functional interactions within algal consortia. Metagenomics (e.g., via NGS) successions the community’s taxonomic and functional gene repertoire. For instance, studies of photobioreactors have used this approach to identify dominant bacterial phyla like Proteobacteria and Bacteroidetes and to uncover novel genes involved in nutrient cycling [178]. Transcriptomics (e.g., RNA-Seq, qRT-PCR) moves beyond this genetic potential by profiling gene expression, revealing how consortia adapt to wastewater conditions. It has elucidated specific symbiotic mechanisms, such as the upregulation of a diatom transporter for 2,3-dihydroxypropane-1-sulfonate (DHPS), which is then utilised as a carbon source by the co-cultured bacterium Ruegeria pomeroyi [179,180]. Metabolomics (e.g., via GC-MS or LC-MS) profiles the complete set of metabolites, enabling the discovery that co-culturing Chlorella vulgaris with Pseudomonas sp. significantly enhances CO₂ fixation and nitrate removal, directly linking microbial interaction to valuable bioproduct synthesis [181]. Together, these integrated approaches provide a holistic view of consortium activity, from genetic potential to functional output, enabling targeted optimisation.

7.3. Prospects and Persistent Challenges

The future of algal systems lies in sophisticated and integrated molecular approaches. The combination of multi-omics data with machine learning and AI will enable predictive modelling of microbial consortia, enabling proactive optimisation of bioreactor conditions [182,183]. Advanced genome editing tools like CRISPR in the field of synthetic biology promise to create “smart” algal strains equipped with novel genetic circuits, empowering them to sense specific WT contaminants and respond by producing targeted degradative enzymes or initiating flocculation [184,185]. Despite the significant promise of genetically engineered microalgae, several key challenges impede their practical application.

The environmental release of genetically modified microalgae poses regulatory challenges and public scepticism, necessitating robust biocontainment measures [186]. Furthermore, scaling up lab-engineered strains to industrial levels in open systems is complex due to issues in maintaining their genetic stability, competitive edge, and other abiotic factors [187]. Finally, the high costs associated with advanced genetic engineering technologies currently limit economic viability, particularly in 3rd world countries and developing regions [160].

Furthermore, molecular techniques have fundamentally transformed the field of algal wastewater treatment, moving it from experimental practice to a predictive science. The ability to genetically engineer algal strains for optimal performance and to decipher and optimise the complex interactions within algal-bacterial consortia provides a robust toolkit for enhancing algal productivity. While challenges related to regulation, scale-up, and cost persist, the continued development of these molecular strategies is crucial to realising the full potential of algal systems as a cornerstone of global water security and the circular bioeconomy.

8. Use of AI in Circular Economy and Integrated Algal Systems for Wastewater Management

AI and ML-powered dynamic models and intelligent systems can optimise algal system performance. ML approaches offer a better grasp of biological process uncertainty than phenomenological or kinetic models [188]. AI/MLs can monitor, optimise, predict uncertainty, and identify faults in real-time in complex environmental systems. AI/ML models for algal wastewater treatment and optimising process parameters for resource recovery. Due to their dependability and longevity, AI/MLs are commonly used to automate, forecast, and make informed decisions in complex systems. The process of making machines think like humans is called AI. ML teaches computers to solve problems by using data from multiple sources, including time-series data and statistical analysis [189]. ML relies on inductive inference to generalise input-output correlations and guide decision-making in new contexts. End-to-end ML involves training, cross-validation, and testing. Hyperparameter tuning is utilised to find the optimal ML solution quickly and with fewer computational resources [190]. During testing, the best model is measured on a separate dataset. The optimised ML model can then be used for prediction [191]. ML is known for its excellent prediction accuracy and capacity to reduce time and resources by eliminating repeated testing in complex non-linear domains [192]. ML algorithms can rapidly analyse large datasets to identify the optimal predictor variable combinations and key patterns [193]. A three-layer feed-forward backpropagation artificial neural network model was used by Ansari et al. [194] to estimate algal dry cell weight in raceway ponds treating secondary wastewater with natural illumination and nutrient supplementation. Hossain et al. [195] employed soft computing methods, including MLP-ANN, RSM, and SVM, to investigate the impact of operational parameters such as N:P ratio, light-dark cycle, and temperature on municipal wastewater treatment using Chlorella kessleri. They discovered that the SVM-GA hybridised model predicted nitrogen and phosphorus removal efficiency better than RSM and MLP-ANN. Coşgun et al. [196] examined microalgal species, growth settings, CO₂ levels, reactor type, nutritional circumstances, and lipid extraction methods using the decision tree (DT) algorithm. Otálora et al. [197] created two ANN-based models to recognise Scenedesmus almeriensis and mixed-composition Chlorella vulgaris. The ANN models utilised the FlowCAM to collect sample particles and provide descriptive characteristics of these particles. The models were trained on pure species samples and validated on mixed cultures. Deep learning for microalgal culture classification improves image analysis. Harmon et al. [198] utilised an SVM model to classify six microalgal species from frequency division multiplexed fluorescence imaging flow cytometry data. For microalgal classification, use ANN. Single-excitation fluorescence spectroscopy and a BP-GA-optimised backpropagation neural network model were utilised by Liu et al. [199] to accurately monitor the Chlamydomonas reinhardtii algal cell content. The model input was fluorescence emission spectrum data, and the output was the algal concentration. The GA-optimised BP network prediction model outperformed the traditional model in the same investigation. The evolutionary method determines ideal initial weights and thresholds through a process of selection, crossover, and mutation, thereby reducing the prediction error of the BP neural network. Combining traditional prediction models with suitable optimisation procedures enhances accuracy. Traditional microalgal growth is typically measured by chlorophyll content. This method is occasionally wrong because carotenoids overlap with chlorophyll. Using linear regression (LR) and ANN-Multilayer perceptron, Tang et al. [200] estimated chlorophyll from colour models. The ANN model predicted chlorophyll content better than linear regression and spectroscopy. A good biomass parameter model, notably moisture content, can help regulate drying and reduce costs. Drying microalgae reduces the water content of cells after harvesting or dewatering. Drying microalgae accounts for 75–85% of the energy used in algal biorefineries [201]. To be profitable, microalgal products must reduce drying costs through efficient management. Practical models are needed to control algal drying. In 2020, Sonkar and Mallick used an ML algorithm, logistic regression, to optimise rotary drum dryer temperature and speed for drying Chlorella minutissima biomass. Ching et al. [202] modelled vacuum drying Chlorococcum infusionum for algal biofuel production using ANN networks, SVM, and XBG. They observed that XBG approximated extreme sample points better than competing models. Pilario et al. [203] estimated biomass vacuum drying moisture content using Gaussian process autoregressive (GPAR) models. GPAR models beat ANN, SVM, RF, and XBG for the same task. Kumar et al. [204] found that ANFIS predicted jatropha-algal oil blend transesterification biodiesel production better than RSM. Muhammad et al. [205] improved reaction duration, temperature, acid concentration, and solid-biomass ratio in acid-mediated direct transesterification of Chlorella pyrenoidosa biomass into biodiesel using the ANN model.

Microalgae can recover significant resources from wastewater using cutting-edge process control systems and biorefineries. Figure 4 shows an AI/ML innovative systems architecture for microalgal production and resource recovery. Microalgal biorefineries must incorporate microalgal productivity. Controlling light intensity, pH, nutrients, CO₂, and algal biomass is crucial for the growth and development of microalgae. Several sensors can measure these physicochemical characteristics in algal production. For optimal conditions, algal bioreactors can utilise several sensors and monitoring systems. AI/ML models can improve their parameters using the vast datasets provided by these monitoring systems. ML models from data and online sensors for real-time monitoring and automation could increase biomass productivity and treatment efficiency. These models can recommend cultivation settings for final products. Researchers are studying innovative control systems for microalgal cultivation. Zhu et al. [206] reduced energy input by 30% by controlling paddle mixing in Spirulina open pond production based on light intensity and temperature. The IoT-enabled up-scaled photobioreactor by Tham et al. [207] allows smartphone-based parameter monitoring. A 3D-printed real-time optical density monitoring device by Lee et al. [208] accurately predicted microalgal development kinetics. These literature examples demonstrate that AI/ML-enabled innovative microalgal cultivation systems can minimise resource consumption and enhance biorefinery decision-making. AI/ML models require a substantial amount of data for training and validation, making implementation challenging in any area. Extra data enhances ML models. However, obtaining detailed data in person is costly and time-consuming. With insufficient data, planning improves quality. Data augmentation (DA) can increase datasets to help trained models generalise invariances. In microalgal categorisation, Correa et al. [209]. found that data-augmented deep learning models performed better. Irrational data augmentation may mispredict. Most research uses a single ML model to make predictions. ANN/SVM/RF with GA outperforms solo models in prediction, reduces the risk of overfitting, and exhibits greater robustness. Also popular are Deep Learning and Reinforcement Learning, which forecast and optimise energy-producing processes. Standard machine learning methods include deep learning, also known as “Deep Structured Learning,” “Hierarchical Learning,” and “Deep Machine Learning” [210]. Deep learning captures specialised low-level characteristics and generalises high-level features to find new data features [210]. Genetic algorithms, particle swarm optimisation, differential evolution, and other naturalistic metaheuristics optimise microalgae reactors more efficiently than exact algorithms [211]. Optimise pH, temperature, CO₂ supply, and dissolved oxygen for reactor design and cost-effective output. It requires modelling and complex control. Traditional methods test design combinations using integrated physical models, CFD, and kinetic modelling. Using algorithms to manage process parameters, MLs can reduce costs and increase algal system efficiency, resulting in higher biomass output with the aid of online sensors. Automating algal culture and harvesting reduces the expenses of microalgal biorefineries. An AI/ML network of plug-and-play IoT sensors can monitor microalgal development and productivity in real-time [212]. For analysis, monitoring, and prediction, IoT sensor data trains application-specific deep learning algorithms. AI could automate the identification of microalgal strains and species faster, which currently requires microscopic images and spectroscopy. Reactor operators measure microalgae biomass daily. IoT sensors and ML-based optimisation can improve resource utilisation. Optimising culture conditions could increase microalgal biomass and reduce investment costs. Peter et al. [213] monitored the semi-batch development of C. vulgaris. They optimised the nutritional medium recycling for high biomass output using an AI-enabled IoT-based unique digital architecture framework. AI/ML microalgae productivity prediction models increase biorefinery production planning and operations. Microalgae conversion trials are laborious. Recent neural network designs can simulate temporal impacts, making them helpful in evaluating dynamic microalgae conversion systems. Neuro-evolutionary meta-learning forecasts microalgae heat. Microalgae promote SDGs 2 (zero hunger), 6 (clean water and sanitation), 7 (cheap and clean energy), 9 (industry, innovation, and infrastructure), 12 (responsible consumption and production), 14 (life below water), and 15. Microalgae biotechnology promotes many SDGs [214,215]. Microalgal biotechnology demands product optimisation, cost-effective large-scale cultivation, and significant investment to meet SDGs. But microalgal biotechnology is green. In microalgal biotechnology, AI/ML models enhance decision-making, reduce costs, and support the achievement of SDGs.

9. Challenges and Future Perspectives

The integrated cultivation of microalgae presents several challenges that must be addressed to facilitate its easy adoption globally. These include the capital required for scalability as well as economical harvesting techniques, such as drying algal biomass. Furthermore, the selection of suitable cultivation sites and indigenous microalgae strains is key for successful wastewater treatment and biomass valorisation. Despite several challenges associated with upstream and downstream processing, the future of integrated algal systems within the circular economy framework is bright. As technology optimises and matures, and the right economic conditions are achieved, an integrated microalgae system has promising potential to become a foundation of sustainable infrastructure worldwide. To address the mentioned research gap and implementation of an integrated algae system, it is recommended that future research be directed more towards integrating algae wastewater treatment and the simultaneous use of algae biomass.

Some key points that need to be taken into consideration for future consideration are:

• Most current research focuses on nutrient removal efficiency and biomass production at the laboratory scale using either real or synthetic wastewater. However, there is a critical need to translate these laboratory findings into real-world conditions to demonstrate practical wastewater treatment.
• Selection of an appropriate algal strain and determining the optimal inoculum concentration are crucial. An algal species that performs well in domestic wastewater may not necessarily be effective for treating other industrial wastewater.
• Application of molecular and bioinformatics tools to identify resilient algae–bacteria consortia that improve wastewater treatment and biomass production.
• Application of AI- and ML-based predictive models to simulate microalgae-driven wastewater treatment processes and optimise valuable biomass production across different wastewater treatment plants.
• Future studies should incorporate Life Cycle Assessment (LCA) and Techno-Economic Analysis (TEA) to provide a comprehensive understanding of the environmental impacts and economic feasibility of microalgae-based wastewater treatment systems. Integrating LCA &TEA will provide a realistic picture of the system’s sustainability, scalability, and potential for implementation under natural operational conditions.

10. Conclusions

This manuscript is presented as a narrative review rather than a systematic review; therefore, it does not incorporate a predefined, protocol-driven bibliographic search strategy or formal study screening and selection process. Nevertheless, it provides a comprehensive discussion of microalgae-based wastewater treatment, highlighting key advantages over conventional technologies, including enhanced nutrient removal efficiency, minimal sludge generation, and CO₂ sequestration. The adaptability of microalgae to various wastewater streams, the absence of arable land requirements, and their efficient operation throughout the year further enhance their potential for wide-scale application of algal biomass. The use of algal biomass in biochar, fertilisers, bioplastics, feed/food, and other products further improves overall process economics. However, microalgae harvesting and product extraction remain critical components of downstream processing, as they largely determine the overall efficiency, cost-effectiveness, and sustainability of the production process. Optimising these steps is essential to minimise energy and resource consumption, reduce operational costs, and ensure that microalgal products can be produced at a larger scale. Recent advances in molecular biology and tools have enhanced strain selection, allowing for a better understanding of metabolic processes and efficiency. The integration of AI and ML offers new opportunities for real-time monitoring, process optimisation, and improving productivity. Despite these advantages, seasonal variations, high risk of contamination and implementing laboratory-scale results on a large scale remain challenging. Overall, microalgae-based wastewater treatment and management pose promising potential to contribute to pollution control, greenhouse gas mitigation, and sustainable resource recovery.