Microalgae Cultivation in Wastewater: How Realistic Is This Approach for Value-Added Product Production?

Abstract

Microalgae cultivation in wastewater is a production approach that combines wastewater treatment with biomass generation for various applications. This strategy aligns with the concept of a circular bioeconomy, which aims to transform waste into valuable resources. However, although this is true, this synergy’s potential bumps into obstacles that still limit the consolidation of the commercial cultivation of microalgae using wastewater. This review analyzed how close or far we are from achieving the successful integration of commercial microalgae cultivation with wastewater treatment for the production of value-added products. The analysis of the scientific literature highlighted that certain strains, such as Chlorella, Arthrospira, and Scenedesmus, can remove up to 90% of nitrogen and phosphorus from effluents while maintaining productivities of up to 45 g/m²/day. The techno-economic analyses presented here indicate that production costs range between 1.98 and 9.69 EUR/kg, depending on the effluent composition and biomass productivity. From an environmental perspective, replacing synthetic media with wastewater can significantly reduce input use, but the environmental impacts associated with energy consumption remain a challenge. This paper also discusses the technological readiness level (TRL), which currently remains between levels 4 and 6, concentrated on demonstration and pilot scales. By gathering and critically analyzing the current literature, this work seeks to answer how realistic and sustainable this integration is today.

1. Introduction

The increasing generation of waste shows that the more we advance, the more complex and voluminous the industrial, agricultural, and urban waste streams we produce become [1]. It is estimated that more than 380 trillion liters of wastewater are generated globally each year, of which only a fraction is treated adequately [2]. Besides that, it is worth highlighting that inadequate sanitation and the use of contaminated water are responsible for a high rate of waterborne morbidities—such as cholera, diarrhea, hepatitis A, and others [3]. The direct and indirect costs of waterborne diseases (WBD) amount to hundreds of millions of dollars per year, reflecting the impact of the lack of access to potable water and basic sanitation—one of the Sustainable Development Goals (SDG 6) [4].

As a result, disruptive measures aimed at treating wastewater and promoting reuse perspectives have gained prominence [5]. Thus, waste—previously considered an environmental liability—has been reinterpreted as a strategic input for generating value [6,7,8]. This paradigm shift aligns with the precepts of the circular economy, closing cycles in which material and energy flows are optimized to maximize value and minimize waste [9]. In this scenario, microalgae emerge as disruptive agents, capable of reconfiguring the linear logic of disposal. Studies show that some microalgae strains, such as Chlorella, Arthrospira, and Scenedesmus, not only remove nutrients such as nitrogen and phosphorus from wastewater with efficiencies higher than 80–90% but also produce biomass rich in proteins, lipids, and carbohydrates [10,11,12,13,14,15]. This phycoremediation approach is a high-value biotransformation technology for organic and inorganic residues present in wastewater.

Considering this, it is important to highlight that the scientific relevance of phycoremediation can be observed through bibliometric analyses in the Web of Science and ScienceDirect databases. By way of example, searching for the term “phycoremediation” reveals over 220 research and review articles indexed from 2010 to 2020 and more than 450 articles from 2021 to 2024, showing consistent annual growth. Besides that, combining the keywords “microalgae” and “wastewater” resulted in over 12,800 research articles and more than 4600 review articles from 2010 to 2024, with approximately 62% and 75% published in the last five years, respectively (Figure 1). These quantitative data illustrate the growing efforts of the scientific community to advance the integration of microalgae cultivation with wastewater.

However, despite the enthusiasm surrounding the integration of microalgae cultivation with wastewater, this approach still occupies a marginal space in waste treatment and valorization strategies. This is due, in part, to the fact that biomass cultivated under these conditions may not meet the rigorous standards required for high-value products—such as nutraceuticals—without an additional purification process. The additional costs of these processes can lead to a mismatch between cost and market price. Furthermore, even if biomass is suitable for applications without security requirements or with minimum security requirements—such as bioenergy, biofertilizer, biostimulant, biopesticide, bioplastic, and animal feed—the low added value of these applications becomes the main bottleneck. In both cases, the costs do not seem to be offset, calling into check not only the technical-economic viability but also the environmental sustainability of this approach, which remains on the margins of progress [16,17,18].

Thus, in a scenario where the potential is palpable but practical solutions are still uncertain, it is essential to revisit the microalgae role in wastewater phycoremediation and the limitations—technical, economic, or environmental—that keep this approach from achieving the highest levels of commercial readiness. This article offers a critical analysis of this topic, examining the technoeconomics involved, the associated environmental impacts, and the degree of maturity of this technology. Despite the increasing number of publications on microalgae cultivation in wastewater, most studies still focus on isolated proofs of concept [19,20,21,22,23,24], failing to progress toward integrated analyses that consider the technical, economic, environmental, and technological maturity aspects simultaneously [25,26,27,28,29,30]. Moreover, most research articles and reviews are restricted to discussing nutrient removal efficiency [31,32,33,34,35,36], phycoremediation mechanisms [37,38,39,40,41,42,43], or biomass valorization potential [44,45,46,47,48], neglecting the articulation between them and their implications for commercial viability. Therefore, an important gap in the literature is identified: the absence of works that systematically integrate acquired knowledge with an explicit focus on the feasibility of industrial-scale application today. By articulating knowledge about the potential of microalgae for removing and biotransforming pollutants from wastewater with technical-economic and environmental analyses, as well as the level of technological maturity achieved by this proposal, this article provides a holistic and realistic view of how close—or far—this strategy is to becoming a scalable solution within the scope of the circular bioeconomy. The originality of this work lies in consolidating scattered evidence into an analytical framework that enables a critical analysis of what remains as a promise and what is already translated into practice.

2. Microalgae-Based Phycoremediation

Phycoremediation with microalgae emerged as a promising alternative for industrial, agricultural, and urban effluent treatment in the 1960s and 1980s, with emphasis on the pioneering studies conducted by William J. Oswald and collaborators in the United States [49,50,51]. However, this approach only gained a great boost from the 2000s onwards, with research demonstrating the synergy between effluent treatment and biomass production for various value-added applications [52,53,54,55,56,57,58,59].

The advantages of phycoremediation with microalgae are multiple. First, microalgae have a remarkable metabolic capacity to capture and biotransform a wide range of organic and inorganic contaminants, including heavy metals [60,61,62], phenolic compounds [63,64,65], pesticides [66,67,68], textile dyes [69,70], radionuclides [71,72], polycyclic aromatic hydrocarbons [73], antibiotics [46,74,75,76], and pharmaceuticals [77]. This action occurs through photosynthesis and mechanisms, such as bioadsorption and bioabsorption (Figure 2) [37,78,79]. Furthermore, microalgae serve as significant biological carbon sinks, capturing atmospheric CO₂ during photosynthesis, which aids in strategies for mitigating climate change [80,81,82]. Additionally, compared to conventional wastewater treatment methods, such as activated sludge, microalgae-based phycoremediation offers a complementary and multifunctional alternative that is more aligned with circularity, waste valorization, and sustainability goals [83,84,85]. Another relevant aspect is that the biomass produced can be used in the production of multiple value-added products [17,30,86].

Another important advantage is the diversity of microalgae strains available for phycoremediation [89]. However, it is worth noting that, sometimes, unique efficiency in removing specific contaminants, such as heavy metals, pharmaceuticals, and persistent organic pollutants. This can lead to targeted selection of species with high affinity for certain compounds. For example, strains from the genera Chlorella and Scenedesmus are notable for their efficacy in eliminating nitrogenous compounds, such as ammonia and nitrate [10,90,91,92,93,94,95,96], while Arthrospira shows great potential for removing heavy metals [60,97,98,99]. However, it is important to recognize that the diversity of microalgae strains is a double-edged sword: on the one hand, it expands the application possibilities and process specialization; on the other, it complicates standardization, increases development costs, and may restrict scalability.

Furthermore, so that the wastewater can be used safely and efficiently for microalgae cultivation, some purification or pretreatment methods are commonly applied. Sieving or filtration is used to remove coarse solids that could compromise the cultivation systems [100,101,102]. Then, decantation or sedimentation can be used to separate sludge and settleable particles [103,104]. In some cases, partial disinfection, such as UV radiation or ozonation, is used to reduce the microbial load of competing organisms [7,15,105]. pH adjustment is often necessary, as is controlled effluent dilution, especially in cases of high ammonia concentrations [106]. Adsorption may be necessary and highly effective when contaminants, such as heavy metals, are present in wastewater intended for microalgae cultivation [101]. These treatments, applied alone or in combination, allow the safe use of wastewater as a culture medium. However, often, especially in circular bioeconomy approaches, the objective is to use the effluent without any treatment to minimize material and energy costs. However, minimal pretreatment, such as sieving or filtration, is almost always necessary.

But, regardless of this, it is important to highlight that research on microalgae cultivation in wastewater has evolved significantly, with results that reinforce the potential of integrating microalgae cultivation with wastewater. On a pilot scale, Morillas-España et al. [107] operated 3600 L and 2400 L thin-layer cascade photobioreactors using domestic wastewater collected from the sewerage of the University of Almería (Almeria, Spain) without any treatment other than solids removal. The average annual productivity of the microalgae Scenedesmus sp. was 24.8 g/m²/day (82.0 t/ha/year), with peaks of 32.8 g/m²/day in summer and significant nitrogen and phosphorus removals of up to 2383.4 mg N/m²/day and 111.8 mg P/m²/day. In parallel, Abraham et al. [108] cultivated natural consortia of microalgae and cyanobacteria in 1000 L open raceway ponds using wastewater from the ammunition industry, achieving an average biomass productivity of up to 23.9 g/m²/day. These biomass productivity data converge with those presented by Fernández et al. [109], who, when evaluating the production cost of biomass cultivated in wastewater—using average productivity of 20 g/m²/day—found values of 4.45 EUR/kg. This value, as discussed in the following topic, is well above the value necessary for competitive insertion in low-value markets, such as biofuels. However, these concrete and scalable results, obtained under real operational conditions, demonstrate that the field has already advanced beyond proof-of-concept studies.

However, despite the documented potential of microalgae-based phycoremediation, the implementation of this synergy on a commercial scale remains a promise, with challenges related to process optimization, separation, and valorization of biomass [109,110]. The advances made since the launch of this proposal have allowed the development of initiatives on a demonstration and pilot scale; however, these experiences have not yet translated into operationally robust and consolidated solutions on an industrial scale [107,108,111,112,113,114,115,116,117,118]. First, the heterogeneity in the wastewater composition directly affects the microalgae productivity, requiring the selection of tolerant strains. Second, the strains’ diversity, although advantageous, complicates standardization. Besides that, the biomass produced in these systems may require purification steps, especially when intended for applications for human consumption, such as food or cosmetics, which increases costs and reduces the competitiveness of the technology. Another relevant bottleneck is the harvesting of biomass, generally highly diluted, making the process technologically and economically onerous. These are some of the specific challenges that help explain why, despite its conceptual alignment with the circular bioeconomy, phycoremediation has not yet achieved the same degree of robustness, standardization, and technological consolidation observed in conventional wastewater debugging methods.

Despite this, given the obstacles, it is inevitable not to ask: why has an approach with so much potential— disseminated for more than half a century —not yet reached the highest levels of technological readiness? Are we overestimating its potential or underinvesting in disruptive innovations that could make it viable? This leads us to reflect on the path that is being taken—and what is really preventing us from moving forward.

3. Technoeconomics of Microalgae Production Using Wastewater

The use of wastewater in microalgae cultivation is promising, especially for low-value-added applications, such as bioenergy and biofertilizer—segments in which profit margins are traditionally narrow [119]. Beyond promoting the circular economy, microalgae cultivation in wastewater is an approach that aims to reduce operational costs associated with water and nutrient (fertilizer and CO₂) consumption. However, there are bottlenecks in both upstream and downstream processes, which compromise the technical and economic viability of this large-scale operation [120]. In upstream processes, obstacles, such as heterogeneity in wastewater quality and composition, the need to select tolerant microalgae strains and control biological contaminants, as well as ideal cultivation conditions, stand out [121,122,123]. On the other hand, in downstream processes, the challenges are concentrated in biomass harvesting—normally diluted in large volumes of water—and in biomass separation from contaminants present in wastewater, which is especially critical when biomass is destined for applications that require greater safety requirements, such as food ingredients [124,125].

Notably, although the integration of microalgae cultivation with wastewater use has not been validated on a commercial scale, demonstrative experiments at relevant scales have already provided real data on biomass productivity, cost savings, and nutrient removal efficiency. The FP7 All-Gas project, for example, demonstrates low-cost microalgae production using municipal wastewater, with a biomass productivity close to 100 t/ha/year [126]. In parallel, other pilot-scale studies, such as those by Morillas-España et al. [107], Abraham et al. [108], Rossi et al. [115], and Morillas-España et al. [117] have demonstrated the technical feasibility of using untreated or minimally treated wastewater to produce microalgae biomass at rates comparable to those obtained when produced using freshwater and commercial fertilizers. These efforts represent the current state of the art, in which techno-economic assessments have been based on relevant operational scales. Furthermore, it is worth emphasizing that there is a growing consensus in the literature that the integration of microalgae cultivation with wastewater represents one of the most promising strategies for the production of low-cost biomass—to promote applications with lower added value. However, although the technology has progressed beyond laboratory proofs of concept, its transposition to industrial contexts has not yet occurred [127].

A close look at the data extracted from technical-economic analyses also reveals a complex and, at times, contradictory scenario [128,129]. While some studies indicate that the use of effluents does not significantly compromise microalgae growth, maintaining productivities comparable to those obtained in synthetic media [130], others reveal considerable variations—both increases and decreases in productivity [128,129,131]. This disparity is strongly related to the intrinsic heterogeneity of wastewater, whose composition varies widely in terms of nutrient concentration, presence of heavy metals, toxic compounds, and microbiological agents [132,133]. These variations shape the physiological response of microalgae in a species-specific manner. Conclusively, the replacement of synthetic media by wastewater is not a linear equation, and converting an unstable substrate into a reliable input continues to represent a major challenge for this approach [134].

Besides that, it is important to emphasize that biomass productivity influences virtually all aspects of a techno-economic analysis. Improving—or at least maintaining—biomass productivity is what allows for the dilution of fixed costs, the optimization of resource use efficiency, and the reduction of the payback period of invested capital. In this sense, to provide an applied perspective, this paper presents, in

Table 1. Parameters for microalgae cultivation in a raceway pond using wastewater. Acronyms: WW, Wastewater. Adapted from [109,128].

Case Study		Base Case	5% WW	10% WW	20% WW
Biomass productivity	g/m2/day	20	20	12.97	9.19
CO2 usage	kg/kg biomass	2	2	2	2
Water evaporation	L/m2/day	7.5	7.5	7.5	7.5
Mixing power consumption	W/m3	10	10	10	10
Labor	people/ha	1	1	1	1
Production days	days	300	300	300	300
Land area	ha	5	5	5	5
Ratio V/S	m3/m2	0.2	0.2	0.2	0.2
CO2 fixation efficiency		0.9	0.9	0.9	0.9
Dilution rate	1/day	0.2	0.2	0.2	0.2
Total culture volume	m3	10,000	10,000	10,000	10,000
Total biomass production	t/ha/year	60	60	38.91	27.57
Total CO2 consumption	t/ha/year	120	120	77.82	55.14
Total water evaporation	t/ha/year	22,500	22,500	22,500	22,500
Water and nutrients cost	EUR/kg	1.1
Power cost	EUR/kWh	0.12	0.12	0.12	0.12
Power for harvesting and others	kWh/m3 harvest	0.1	0.1	0.1	0.1
Bioreactor cost	EUR/m3	61.87	61.87	61.87	61.87
Biomass sludge production cost	EUR/kg	5.57	4.45	6.87	9.69

¹ The biomass productivity of 20 g/m²/day is a conservative but representative value for raceway pond systems, which currently represent more than 90% of commercial microalgae production. ² Cost data have been corrected for inflation, when needed, and thus, provide a reasonable estimate of the current costs.

, a preliminary techno-economic analysis, elaborated by the authors, for the microalgae cultivation using wastewater collected from an oil and gas facility. This case study comprehensively represents the results of research on microalgae production using wastewater, clearly illustrating the negative impact that reductions in productivity—caused by effluent use—can have on the technical-economic viability of the process. This case directly dialogues with the evidence that biomass productivity is a key variable in any economic analysis of microalgae production in wastewater. The specific characteristics of the effluent, as well as the assumptions adopted for the techno-economic analysis, are detailed in references [109,128,135].

According to the case study data (Table 1) carried out following the methodology of Acién et al. [135] and the results of Acién et al. [109] and Rahman et al. [128], the biomass production cost reaches 5.57 EUR/kg when using freshwater and nutrients (fertilizers and CO₂). However, when replacing these inputs with wastewater (5% WW), the cost is reduced to 4.45 EUR/kg. This slightly lower value is due to the elimination of freshwater and nutrient costs and productivity comparable to the control (base case). In this regard, it is worth highlighting that the annual productivity of 60 t/ha/year adopted in the base case is not an idealized overestimation but rather a conservative value based on the technical literature. Fernández et al. [109] consider this value representative for raceway pond systems, and studies on a relevant scale, such as the one conducted by the European project FP7 All-Gas, demonstrated productivities close to 100 t/ha/year in ponds operating with municipal wastewater [126]. In parallel, Morillas-España et al. [107] reported an average annual productivity of 82 t/ha/year in thin-layer cascade photobioreactors operating with domestic effluent.

However, following the data in Table 1 and according to the data of Rahman et al. [128], at wastewater concentrations of 10% and 20%, there is a decrease in biomass productivity of 35% and 54%, respectively. Under these conditions, biomass production costs are increased to 6.87 and 9.69 EUR/kg, respectively. These values confirm that biomass productivity directly impacts the economic viability of the process.

These costs are also comparable to those reported by Norsker et al. [136], who when evaluating the cost of producing microalgae biomass in open ponds, reported a cost of 6.63 EUR/kg (data corrected for inflation). In parallel, Rossi et al. [115], when conducting a pilot-scale study under real conditions for wastewater treatment, identified a variation in the production cost between 1.9 and 10.3 EUR/kg.

However, all these values are above the estimated critical value of 1.24 EUR/kg for low-value applications, such as bioenergy [109]. The biomass production cost needs to reach values equal to or below 1.24 EUR/kg to compete with commodities, such as cereals and vegetable oils. To achieve this baseline, it would be necessary not only to maintain high biomass productivity but also to adopt other measures to reduce operational costs, such as direct manpower. However, for the case study (Table 1), even if it were possible to reduce from 1 to 0.1 person/ha, costs would remain between 3.22 and 7.05 EUR/kg—still far from the target.

Additionally, from a wastewater treatment perspective, the situation is also challenging. Average treatment costs with microalgae exceed those observed in optimized or medium-sized conventional systems (0.2 to 0.7 EUR/m³) [137,138]. For example, Rossi et al. [115] reported a cost for piggery wastewater treatment in the range of 6.3–34.2 EUR/m³, which is in line with the values reported for microalgae-based phycoremediation but still high compared to conventional processes. Despite that, it is reported in the literature that the costs of treating wastewater with microalgae can be competitive with smaller-scale or low-efficiency systems [25,109,139].

On the other hand, despite the data presented above, more encouraging results are also found in the literature.

Table 2. Parameters for the microalgae cultivation in a raceway pond using wastewater from the dairy industry. Acronyms: WW, Wastewater. Adapted from [109,129].

Case Study		Base Case	75% WW
Biomass productivity	g/m2/day	14.2	45
CO2 usage	kg/kg algae biomass	2	2
Water evaporation	L/m2/day	7.5	7.5
Mixing power consumption	W/m3	10	10
Labor	people/ha	1	1
Production days	days	300	300
Land area	ha	5	5
Ratio V/S	m3/m2	0.2	0.2
CO2 fixation efficiency		0.9	0.9
Dilution rate	1/day	0.2	0.2
Total culture volume	m3	10,000	10,000
Total biomass production	t/ha/year	42.6	135
Total CO2 consumption	t/ha/year	85.2	270
Total water evaporation	t/ha/year	22,500	22,500
Water and nutrients cost	EUR/kg	1.1
Power cost	EUR/kWh	0.12	0.12
Power for harvesting and others	kWh/m3 harvest	0.1	0.1
Bioreactor cost	EUR/m3	61.87	61.87
Biomass sludge production cost	EUR/kg	7.84	1.98

¹ Cost data have been corrected for inflation, when needed, and thus, provide a reasonable estimate of the current costs.

presents a techno-economic analysis for the microalgae cultivation using wastewater collected from a dairy industry. The case study comprehensively demonstrates the most positive results found in the literature for the cultivation of microalgae in wastewater, illustrating the impact that significant productivity increases—caused by effluent use—can have on the technical-economic viability. The specific characteristics of the effluent, as well as the assumptions adopted for the techno-economic analysis, are detailed in references [109,129,135].

In this new scenario (Table 2), when cultivating microalgae with synthetic media, a conservative biomass productivity of 14.2 g/m²/day was achieved, equivalent to 42.6 t/ha/year, resulting in a cost of 7.84 EUR/kg. With wastewater use, however, beyond the nutrient costs being zero, productivity has increased considerably, reducing the production cost to 1.98 EUR/kg. For the same surface area of 5 ha, up to 675 t/year of microalgae biomass are produced when using wastewater, whereas this production capacity decreases to 213 t/year when using synthetic media. With the manpower reduction, the cost can fall to 1.44 EUR/kg. This value approaches the target of 1.24 EUR/kg, providing greater competitiveness for lower value-added markets and effluent treatment in specific contexts (smaller-scale or low-efficiency systems).

In sum, although the integration of microalgae cultivation with wastewater use has proven to be technically feasible and economically promising, it is worth mentioning that its consolidation remains restricted to laboratory, semi-controlled, and, ultimately, pilot-scale environments [107,108,109,115,117,140]. Thus, transposing these advances to an industrial scale cannot be seen simply as a matter of incremental improvement but as a systemic challenge that requires a paradigm shift; it is necessary to design large-scale operations without off-season periods, breaking with the logic of seasonality and operational fragility. Posteriorly, from this basis, the true technological frontier lies in efficiency—not only in photosynthetic efficiency or the biomass volumetric productivity, but in the efficiency of the economic conversion of available inputs. In a scenario of tight margins, such as the markets for bioenergy, biofertilizers, biostimulants, biopesticides, bioplastics, and, ultimately, aquaculture feed, efficiency is not a differentiator but a condition for technological survival. Furthermore, at the state-of-the-art, the real challenge is no longer proving that the technology works but making it efficient enough to compete with conventional wastewater treatment processes and, in terms of value-added product production, efficient enough to compete with commodities from conventional agriculture and fossil fuel-based industries.

4. Environmental Impact of Microalgae Production Using Wastewater: Environmental Sustainability Metrics and Indicators

As studies on the feasibility of integrating microalgae production with wastewater intensify, the need to understand and qualify the environmental impacts of this technology through life cycle assessment (LCA) also grows [141,142]. The objective is to quantify and analyze metrics associated with several environmental impact indicators—such as global warming potential, fossil resource scarcity, human toxicity, soil and water eutrophication, soil, water, and air acidification, and soil and water ecotoxicity—in different operational scenarios to guide decisions and promote improvements in production systems [143]. Here, particular emphasis is placed on the influence of wastewater composition, biomass productivity, and energy consumption on the technical-environmental performance of the system. By documenting these factors and their interactions, this analysis seeks to clarify the environmental tradeoffs and opportunities involved in the phycoremediation strategy, supporting its alignment with the logic of the circular bioeconomy.

Many studies on the environmental sustainability of microalgae production find, at their core, two critical variables that strongly condition the performance of the systems evaluated: energy consumption and biomass productivity [144,145,146,147,148,149,150]. When related to cultivation with wastewater, a third variable emerges: wastewater composition, which can positively or negatively impact the indicators, especially due to its influence on biomass productivity [151,152,153]. Both are interconnected in an intricate way and decisively determine the metrics applied in environmental analyses [154].

Energy consumption during the harvesting and drying phases, and especially during the cultivation stage, is high, and when it comes from non-renewable sources, such as coal, the environmental impacts are significant [155] (

Table A1. Characterized values of environmental impact categories associated with the production of 1 kg of dry biomass using wastewater based on the energy source. Acronyms: WW, Wastewater; FPMF, Fine particulate matter formation; FRS, Fossil resource scarcity; FEC, Freshwater ecotoxicity; FEU, Freshwater eutrophication; GWP, Global warming potential; HCT, Human carcinogenic toxicity; HnCT, Human non-carcinogenic toxicity; IR, Ionizing radiation; LU, Land use; MEC, Marine ecotoxicity; MRS, Mineral resource scarcity; OFTE, Ozone formation affecting terrestrial ecosystems; SOD, Stratospheric ozone depletion; TAC, Terrestrial acidification; TEC, Terrestrial eco-toxicity; TWC, Total water consumption.

Base Case
	Coal	Photovoltaic	Onshore wind	Hydropower
FPMF (kg PM2.5 eq.)	3.00 × 10−1	2.64 × 10−2	7.41 × 10−3	2.51 × 10−3
FRS (kg oil eq.)	6.02 × 101	2.74 × 100	1.07 × 100	2.77 × 10−1
FEC (kg 1,4-DCB)	1.53 × 101	2.94 × 100	2.02 × 100	5.56 × 10−2
FEU (kg P eq.)	6.22 × 10−1	9.59 × 10−3	1.83 × 10−3	3.40 × 10−4
GWP (kg CO2 eq.)	2.74 × 102	1.12 × 101	3.93 × 100	1.66 × 100
HCT (kg 1,4-DCB)	3.07 × 101	1.06 × 100	5.94 × 100	8.07 × 10−1
HnCT (kg 1,4-DCB)	6.14 × 102	4.14 × 101	9.70 × 100	1.21 × 100
IR (kBq Co-60 eq.)	6.42 × 10−1	1.04 × 100	2.26 × 10−1	6.78 × 10−2
LU (m2a crop eq.)	4.85 × 10−1	3.12 × 10−1	4.59 × 10−1	5.36 × 10−2
MEC (kg 1,4-DCB)	2.11 × 101	3.88 × 100	2.49 × 100	7.77 × 10−2
MRS (kg Cu eq.)	3.15 × 10−2	1.41 × 10−1	9.32 × 10−2	1.83 × 10−2
OFTE (kg NOx eq.)	4.21 × 10−1	2.67 × 10−2	1.23 × 10−2	4.95 × 10−3
SOD (kg CFC11 eq.)	4.24 × 10−5	5.23 × 10−6	1.72 × 10−6	8.10 × 10−7
TAC (kg SO2 eq.)	9.75 × 10−1	5.61 × 10−2	1.35 × 10−2	4.11 × 10−3
TEC (kg 1,4- DCB)	9.01 × 101	2.97 × 102	2.11 × 101	5.66 × 100
TWC (m3)	5.51 × 10−1	3.43 × 10−1	4.87 × 10−2	7.44 × 100
10% WW
	Coal	Photovoltaic	Onshore wind	Hydropower
FPMF (kg PM2.5 eq.)	4.56 × 10−1	4.02 × 10−2	1.13 × 10−2	3.82 × 10−3
FRS (kg oil eq.)	9.16 × 101	4.17 × 100	1.62 × 100	4.21 × 10−1
FEC (kg 1,4-DCB)	2.33 × 101	4.48 × 100	3.08 × 100	8.47 × 10−2
FEU (kg P eq.)	9.47 × 10−1	1.46 × 10−2	2.78 × 10−3	5.18 × 10−4
GWP (kg CO2 eq.)	4.17 × 102	1.71 × 101	5.99 × 100	2.53 × 100
HCT (kg 1,4-DCB)	4.68 × 101	1.61 × 100	9.04 × 100	1.23 × 100
HnCT (kg 1,4-DCB)	9.35 × 102	6.30 × 101	1.48 × 101	1.84 × 100
IR (kBq Co-60 eq.)	9.78 × 10−1	1.58 × 100	3.44 × 10−1	1.03 × 10−1
LU (m2a crop eq.)	7.38 × 10−1	4.75 × 10−1	7.00 × 10−1	8.16 × 10−2
MEC (kg 1,4-DCB)	3.21 × 101	5.91 × 100	3.79 × 100	1.18 × 10−1
MRS (kg Cu eq.)	4.79 × 10−2	2.14 × 10−1	1.42 × 10−1	2.79 × 10−2
OFTE (kg NOx eq.)	6.42 × 10−1	4.06 × 10−2	1.87 × 10−2	7.54 × 10−3
SOD (kg CFC11 eq.)	6.46 × 10−5	7.96 × 10−6	2.61 × 10−6	1.23 × 10−6
TAC (kg SO2 eq.)	1.48 × 100	8.54 × 10−2	2.06 × 10−2	6.26 × 10−3
TEC (kg 1,4- DCB)	1.37 × 102	4.52 × 102	3.22 × 101	8.62 × 100
TWC (m3)	8.39 × 10−1	5.22 × 10−1	7.42 × 10−2	1.13 × 101
20% WW
	Coal	Photovoltaic	Onshore wind	Hydropower
FPMF (kg PM2.5 eq.)	6.40 × 10−1	5.64 × 10−2	1.58 × 10−2	5.36 × 10−3
FRS (kg oil eq.)	1.29 × 102	5.86 × 100	2.28 × 100	5.91 × 10−1
FEC (kg 1,4-DCB)	3.28 × 101	6.29 × 100	4.32 × 100	1.19 × 10−1
FEU (kg P eq.)	1.33 × 100	2.05 × 10−2	3.90 × 10−3	7.27 × 10−4
GWP (kg CO2 eq.)	5.86 × 102	2.40 × 101	8.41 × 100	3.55 × 100
HCT (kg 1,4-DCB)	6.56 × 101	2.26 × 100	1.27 × 101	1.72 × 100
HnCT (kg 1,4-DCB)	1.31 × 103	8.84 × 101	2.07 × 101	2.58 × 100
IR (kBq Co-60 eq.)	1.37 × 100	2.21 × 100	4.83 × 10−1	1.45 × 10−1
LU (m2a crop eq.)	1.04 × 100	6.67 × 10−1	9.82 × 10−1	1.14 × 10−1
MEC (kg 1,4-DCB)	4.50 × 101	8.30 × 100	5.31 × 100	1.66 × 10−1
MRS (kg Cu eq.)	6.72 × 10−2	3.00 × 10−1	1.99 × 10−1	3.92 × 10−2
OFTE (kg NOx eq.)	9.00 × 10−1	5.69 × 10−2	2.63 × 10−2	1.06 × 10−2
SOD (kg CFC11 eq.)	9.06 × 10−5	1.12 × 10−5	3.67 × 10−6	1.73 × 10−6
TAC (kg SO2 eq.)	2.08 × 100	1.20 × 10−1	2.89 × 10−2	8.79 × 10−3
TEC (kg 1,4- DCB)	1.93 × 102	6.34 × 102	4.52 × 101	1.21 × 101
TWC (m3)	1.18 × 100	7.32 × 10−1	1.04 × 10−1	1.59 × 101
75% WW
	Coal	Photovoltaic	Onshore wind	Hydropower
FPMF (kg PM2.5 eq.)	1.39 × 10−1	1.22 × 10−2	3.43 × 10−3	1.16 × 10−3
FRS (kg oil eq.)	2.78 × 101	1.27 × 100	4.93 × 10−1	1.28 × 10−1
FEC (kg 1,4-DCB)	7.09 × 100	1.36 × 100	9.35 × 10−1	2.57 × 10−2
FEU (kg P eq.)	2.88 × 10−1	4.44 × 10−3	8.46 × 10−4	1.57 × 10−4
GWP (kg CO2 eq.)	1.27 × 102	5.20 × 100	1.82 × 100	7.69 × 10−1
HCT (kg 1,4-DCB)	1.42 × 101	4.90 × 10−1	2.75 × 100	3.73 × 10−1
HnCT (kg 1,4-DCB)	2.84 × 102	1.91 × 101	4.49 × 100	5.59 × 10−1
IR (kBq Co-60 eq.)	2.97 × 10−1	4.79 × 10−1	1.05 × 10−1	3.14 × 10−2
LU (m2a crop eq.)	2.24 × 10−1	1.44 × 10−1	2.13 × 10−1	2.48 × 10−2
MEC (kg 1,4-DCB)	9.75 × 100	1.80 × 100	1.15 × 100	3.59 × 10−2
MRS (kg Cu eq.)	1.46 × 10−2	6.51 × 10−2	4.31 × 10−2	8.48 × 10−3
OFTE (kg NOx eq.)	1.95 × 10−1	1.23 × 10−2	5.70 × 10−3	2.29 × 10−3
SOD (kg CFC11 eq.)	1.96 × 10−5	2.42 × 10−6	7.94 × 10−7	3.75 × 10−7
TAC (kg SO2 eq.)	4.51 × 10−1	2.60 × 10−2	6.26 × 10−3	1.90 × 10−3
TEC (kg 1,4- DCB)	4.17 × 101	1.37 × 102	9.78 × 100	2.62 × 100
TWC (m3)	2.55 × 10−1	1.59 × 10−1	2.25 × 10−2	3.44 × 100

). Among the most reported impact categories with expressive values are freshwater eutrophication, marine eutrophication, terrestrial acidification, and human toxicity, with most of these impacts being strongly associated with intensive energy consumption, especially in systems powered by fossil matrices [150,156,157,158].

Additionally, the cultivation system, classified as open (such as a raceway pond) and closed (such as tubular photobioreactors), also exerts influence. Magalhães et al. [159] recently compared the techno-environmental feasibility of both systems for microalgae biomass production in agro-industrial wastewater and observed that although the closed system presented greater productivity and efficiency in nutrient removal, its environmental impacts were greater than those of the open system, whose lower productivity was offset by lower energy requirements. Although closed systems allow greater operational control, favoring both biomass productivity and nutrient removal from wastewater, they demand greater energy consumption and, consequently, can present more significant environmental impacts [149,160]. In this sense, to what extent is the efficiency of these systems compatible with environmental sustainability? In contexts in which energy matrices are predominantly fossil, this virtue can become ambiguous, but environmental sustainability will depend directly on the balance between biomass productivity and energy consumption [161].

In parallel, under the same cultivation system, such as a raceway pond, biomass productivity is a key variable capable of diluting resource use and environmental impacts. By way of example,

Table 3. Life cycle inventory for microalgae production using wastewater [109,128,129]. Acronyms: WW, Wastewater.

		Base Case	10% WW	20% WW	75% WW
Raceway pond	m3	10	15.41	21.76	4.44
Electric energy for paddle wheel	kWh	129.60	199.71	282.01	57.54
Electric energy for water pumping	kWh	24.00	36.98	52.22	10.66
Electric energy for CO2 injection	kWh	79.20	122.05	172.34	35.16
CO2 consumption	kg/kg biomass	2.00	2.00	2.00	2.00
Biomass productivity	g/m2/day	20.00	12.97	9.19	45
Energy consumption centrifugation	kWh	12.50	19.26	27.20	5.55
Spray-dryer	kWh	8.52	8.52	8.52	8.52
Output
Whole dried biomass	kg	1	1	1	1
Electricity	kWh	253.82	386.53	542.30	117.44

shows an analysis based on data extracted from the literature, structured in the form of a life cycle inventory for microalgae production using wastewater. The base case considers the use of standard culture media, and the 10% WW and 20% WW scenarios consider the use of wastewater from an oil and gas facility [128], while the 75% WW scenario considers the use of wastewater from a dairy industry [129]. As can be seen in Table 3, to achieve a standardized functional unit of 1 kg of dry biomass, as productivity decreases (10% WW and 20% WW) or increases (75% WW), the energy consumption is proportionally different. In summary, productivity and resource consumption are closely related, and the lower the productivity, the greater the impact on the consumption of these resources [162].

The greater or lesser impact on resource consumption is reflected in the metrics of the main environmental performance indicators (Table A1). Although the use of wastewater can reduce or eliminate the need for fertilizer inputs, the process continues to demand energy, and when production is sometimes limited, the impacts per functional unit increase. Thus, even well-intentioned processes—such as effluent reuse—can present unsatisfactory metrics if they fail to, at least, maintain productivity. More than a kinetic parameter, biomass productivity assumes the role of a structuring variable in the metrics of environmental sustainability indicators, as can be seen in Figure 3.

Figure 3 presents the compact and comparative contribution of the different categories of environmental impact associated with the production of 1 kg of microalgae biomass cultivated in wastewater (base case, 10% WW, 20% WW, and 75% WW scenarios), considering the use of energy from coal, according to the data in Table A1. Among the categories evaluated, the most impactful are human non-carcinogenic toxicity (HnCT) and global warming potential (GWP), followed by terrestrial ecotoxicity (TEC) and fossil resource scarcity (FRS) [150,156,157,158]. The HnCT varied between 284 and 1312 kg 1,4-DCB eq./kg of biomass, while the values of GWP, TEC, and FRS oscillated between 127 and 586 kg CO₂ eq./kg of biomass, 41.7 and 193 kg 1,4-DCB eq./kg of biomass, and 27.8 and 129 kg oil eq./kg of biomass, respectively. These results confirm that such impacts are strongly conditioned by the biomass productivity and the intensity of electricity consumption—non-renewable.

The nature of the energy matrix used to feed microalgae-based processes plays an important role, capable of amortizing environmental impacts by more than an order of magnitude (Table A1). The replacement of fossil energy sources by renewable ones emerges as the backbone for operations with significantly lower emissions. Arbour et al. [163] found in their study that electricity use was the main environmental critical point of microalgae-based wastewater treatment and proposed replacing the predominantly fossil-based electricity matrix with renewable sources. The replacement was shown to reduce the environmental impact in all impact categories, with reductions ranging from 90% to 99%. This finding is in full agreement with the results presented in Figure 4, which summarizes part of the data described in Table A1, reinforcing the centrality of the energy transition for the sustainability of the process.

Figure 4 shows the compact and comparative contribution of the different categories of environmental impact associated with the production of 1 kg of microalgae biomass grown in wastewater (scenario: 75% WW), considering the use of energy from coal, hydropower, photovoltaic, and wind, as per the data in Table A1. While the impacts considering the fossil matrix were high in all categories, the adoption of renewable sources—especially hydropower and wind—demonstrated substantial reductions. While the GWP drops from 127 kg CO₂ eq./kg of biomass (coal) to 0.77 kg CO₂ eq./kg of biomass with hydropower and 1.82 kg CO₂ eq./kg of biomass with wind power, the HnCT is reduced from 284 to only 0.56 kg 1,4-DCB/kg of biomass with hydropower. Likewise, FRS decreases from 27.8 to 0.13 kg oil eq./kg biomass, and TEC decreases from 41.7 to 2.6 kg 1,4-DCB/kg biomass.

However, it is worth mentioning that access to these sources is not always trivial, especially for those who depend on electricity supplied by the electrical grids without having autonomous power generation systems. But, in this scenario, it is noteworthy that the advancement in photovoltaic (PV) systems stands out as a concrete opportunity to mitigate the environmental impacts associated with energy consumption. The proposal to implement PV systems in microalgae installations has proven to be economically viable, capable of improving environmental sustainability metrics, and strengthening the long-term economic viability of facilities—reducing the vulnerability to electricity prices [164,165].

Finally, environmental sustainability does not depend on a single factor but on a balance between interdependent pillars—such as wastewater composition, productivity, and energy consumption. Connecting these pillars requires more than coexistence: it requires integration, intensification, and systemic articulation. However, in practice, they do not always converge spontaneously and, often, tense up mutually. For example, prioritizing circularity—such as wastewater use—can impose limitations on productivity and operational predictability. Furthermore, life cycle analysis—although a powerful tool—may not capture all the nuances of the system due to the multiple variables in constant interaction. Therefore, first, it is important to recognize that the fragility of sufficiently robust support systems to deal with the systemic complexity of microalgae-based biotechnological processes is also one of the main hidden bottlenecks that contribute to the inefficiency of innovative and potentially sustainable technologies.

5. Technological Readiness Level of Microalgae Cultivation Using Wastewater

This topic aims to close the discussion developed throughout the work, showing the Technological Readiness Level (TRL) of the integration between microalgae cultivation and wastewater use. Although the previous topics have already indicated, based on several studies, that most of the experiences are at the laboratory or pilot scale, here we seek to consolidate this evidence in a clear diagnosis of the technology’s development stage. The TRL scale, widely used to measure the evolution of technologies towards their commercial application [166,167], indicates that this approach is predominantly between levels 4 and 6, concentrated in validation stages in a laboratory environment and a relevant operational environment, such as pilot plants (Figure 5).

At TRL scale 1 to 4, which corresponds to hypotheses, proof of concept, and validation in a laboratory environment, studies, such as those by Matamoros et al. [39], Petrini et al. [168], Krishnamoorthy et al. [169], Moondra et al. [170], Xu et al. [171], Azam et al. [172], Noshadi and Nouripour [173], and Moondra et al. [174], can be cited. At TRL scales 5 and 6, which correspond to more advanced experiments and prototypes tested in a relevant operational environment, the studies by Morillas-España et al. [107], Abraham et al. [108], Matamoros et al. [111], Ovis-Sánchez et al. [112], Goswami et al. [113], Yu et al. [114], Rossi et al. [115], Mantovani et al. [116], Morillas-España et al. [117], Dalvi et al. [118], and Blanco-Vieites et al. [175] can be cited. In these studies, the experiments were conducted in pilot systems with volumes ranging from 10, 100, 1000, and even 10,000 L, demonstrating important advances towards scalability. However, the transition to higher levels (NRT 7-9), corresponding to the implementation of the technology in a real operational environment with proven success under the desired conditions and subsequent commercialization—the apex of the technological trajectory—does not yet count on consolidated reports or robust evidence in the literature to date [127,176,177].

In sum, the typical technological symbiosis of a microalgae-based biorefinery, which integrates nutrient capture, CO₂ sequestration, and biomass production, has already been identified as a key solution to consolidate the technical and economic viability of value-added products, whose competitiveness depends heavily on reducing operating costs [178,179,180]. However, beyond the technical obstacles related to the cultivation operation in heterogeneous wastewater that require highly adaptive strains [181], there are economic, environmental, logistical, and regulatory issues [83,182,183,184]. The production systems lack greater scalability and competitive costs compared to conventional technologies [185]. Integration with effluent-generating industries is incipient, and, associated with this, the lack of clear regulatory frameworks for wastewater use in biomass production, especially for high value-added applications, creates a gap that inhibits investments [186].

Furthermore, fragmented research—focused on proofs of concept—is also not enough to achieve a paradigm shift. In truth, the integration of microalgae cultivation with wastewater depends less on isolated new discoveries and more on the systemic articulation between science, technology, market, and policy. Strategic vision, targeted funding, and, above all, the breaking of obsolete industrial paradigms could crown this approach. However, the field remains, for the most part, in a repetitive cycle of identifying bottlenecks and promises that rarely translate into concrete solutions. This symptomatic stagnation may reflect how disconnected fundamental research is from the real dynamics of technological implementation.

Finally, the actions needed to break the cycle of stagnation and advance to higher levels of technological readiness are already widely discussed in the literature. Therefore, it is time to stop simply mapping obstacles and start structuring collaborative projects at relevant scales (TRL 6-8), guided by economic, environmental, and social performance metrics. Shaping the technological maturity of the integration of microalgae cultivation with wastewater requires an intersectoral commitment, as the challenges go beyond the technical field and involve issues that require coordination between academia, the productive sector, and the public power.

6. Conclusions

The integration of microalgae cultivation with wastewater is a tangible approach within the circular bioeconomy paradigm. The findings show that this strategy can reduce biomass production costs to as little as EUR 1.98/kg, especially when combined with effluents with a favorable composition and high productivity. However, economic viability remains strongly conditioned by the quality of the effluent and the stability of biomass productivity, which, in turn, is also a determining variable of the environmental performance of the system. Drops in productivity intensify energy consumption per biomass unit, compromising the environmental gains associated with the reuse of residual resources. On the other hand, the replacement of fossil sources with renewable energy has proven to be the backbone for significantly mitigating the environmental impacts of the process. From a technological point of view, the data indicate that most initiatives remain between technology readiness levels 4 and 6, which reveals a still incipient stage of transition from the bench to the industry. In summary, the results presented confirm that the technology has already surpassed the proof-of-concept stage, but high costs and limited technological maturity continue to stifle the potential of this approach. Applications on modest scales, but real, indicate that prioritizing products with medium added value—such as aquaculture feed—that emerge in an intermediate zone between complexity and viability is a viable alternative to promote the gradual consolidation of the technology. In conclusion, transit through small victories can attract investments in research that, in the future, will support applications with lower added value, such as bioenergy. Furthermore, disruptive leaps will not be achieved without the patient construction of a solid base.