Transforming Waste into Value: The Role of Physicochemical Treatments in Circular Water Management

Abstract

The growing global population and increasing water demand have intensified the urgency for efficient wastewater treatment strategies to address environmental pollution and water scarcity. Physicochemical treatment technologies remain among the most widely implemented solutions due to their high removal efficiency, operational simplicity, and relatively low cost. These processes effectively target a broad spectrum of contaminants—including suspended solids, heavy metals, recalcitrant organic compounds, and high salinity—through unit operations such as coagulation, flocculation, adsorption, and filtration. Nevertheless, they often generate concentrated waste streams that present significant disposal and environmental challenges. Applying these technologies within a circular economy framework enables wastewater reuse, resource recovery, and a reduced environmental impact. Circular strategies enable the recovery and reuse of water, energy, and materials, converting waste into valuable resources. Treated water can be safely reused, while by-products such as biogas and nutrients (e.g., phosphorus, nitrogen, and organic carbon) can be recovered and reintegrated into agricultural and industrial processes. Furthermore, advanced methods such as membrane separation and electrochemical treatments allow for the selective recovery of high-value metals. This review analyzes key physicochemical technologies for wastewater treatment and evaluates their integration into circular economy models, with a focus on waste valorization, resource recovery, and environmental impact reduction. By adopting circular approaches, wastewater treatment systems can enhance sustainability, improve economic performance, and contribute to achieving the global water and sanitation target.

1. Introduction

As the global population continues to grow, wastewater treatment has become essential to address the severe pollution of receiving bodies and the scarcity of drinking water [1]. Within the framework of the 2030 Agenda on Sustainable Development, of the 17 proposed Development Goals, Goal 6, which consists of guaranteeing the availability of water and its sustainable management and sanitation for all, aims for water to remain free of impurities in addition to being accessible to all human beings, since it is an indispensable natural resource in the environment in which life develops [2]. Achieving this objective directly impacts food security, education, poverty reduction, and gender equality. Conversely, water scarcity, poor quality, and inadequate sanitation reduce people’s quality of life and hinder national development [3,4].

In order to comply with wastewater sanitation, there are different technological wastewater treatments that seek to ensure that an effluent is deposited in an environment with such quality that it can be reincorporated into the receiving environment, through a treatment process line [5]. A treatment process line or treatment train is a set of unitary operations of a physical, chemical, or biological type whose purpose is the elimination of or reduction in contamination or undesirable characteristics of the water [6]. Of these technologies, physical–chemical treatments are the most widely used since they are economical and effective [7,8]. These treatments are designed based on the contaminants present in the effluents, which may contain one or more contaminants such as suspended solids, heavy metals, oils and greases, recalcitrant organic compounds, toxic compounds (hexavalent chromium, cyanides, pesticides, etc.), and even high concentrations of salts (brines). However, after carrying out these treatments, waste is generated with the contaminants removed from the wastewater [7]. One proposal being considered is to find a way to take advantage of the waste obtained after the physical–chemical treatments of wastewater through the circular economy [9,10].

The circular economy redefines production processes as closed systems that maximize resource efficiency, minimize waste, and promote reuse, repair, and recycling. Beyond resource management, it also encompasses principles such as eco-design and zero waste, offering a systemic alternative to the linear “take-make-dispose” model [9,11,12,13]. Circularity allows for more efficient management of resources and reduces the economy’s dependence on the use of finite resources, and even improves productivity and provides long-term resilience. Circular economy processes can be implemented in wastewater systems, since it is feasible to reuse treated wastewater so that it becomes part of the cycle again [10]. Likewise, instead of using energy from conventional sources in its treatment, it is possible to implement systems for the emission and capture of biogas, and cogeneration of heat and electricity, having as a direct effect the limitation of the use of fossil fuels, by recovering energy from the process. It has been reported that municipal wastewater treatment plants (WWTPs) allow the recovery of energy and valuable elements (phosphorus, nitrogen, and organic carbon) for the soil. In this article, they presented, in a schematic way, the cycle of carbon, nitrogen, and phosphorus in a WWTP with a load of 70,000 Equivalent Inhabitants, demonstrating that the generation of biogas allowed the recovery of 1126 Mg of organic carbon per year and generated 12.6 GWh of energy [10]. In a water treatment system, not only can energy and nutrients be used, but there are processes such as adsorption, retention by membrane filtration, or electrochemical processes where metals are retained and can also be recovered [14,15]. In this context, physicochemical technologies—such as coagulation–flocculation, adsorption, chemical precipitation, membrane separation, electrocoagulation, and advanced oxidation processes—are not only effective for removing pollutants but also play a central role in recycling schemes within circular water management. These methods enable the recovery of water, energy, nutrients, and valuable metals, which can be reintegrated into industrial, agricultural, or domestic cycles, reducing dependence on virgin raw materials. Therefore, the aim of this review is to analyze the main physicochemical treatment technologies for wastewater and evaluate their integration into circular economy models, with a particular focus on resource valorization, environmental benefits, and future perspectives for sustainable water management. In addition to this general analysis, the review includes the microelectronics industry as a case study to illustrate how physicochemical technologies can be applied in high-water-demand sectors within a circular economy framework.

2. Physicochemical Treatments in Circular Water Management

Circular water management is based on the transition from a linear “use and discharge” approach to closed systems that prioritize reuse, recycling, and resource recovery [16,17], as seen in Figure 1. In the linear model, water extracted from surface or groundwater is treated, distributed, and used for domestic, agricultural, or industrial purposes; after consumption, it is treated again and discharged into water bodies, generating waste. In contrast, the circular model incorporates recovery and valorization of resources throughout the cycle, integrating processes such as recycling, reuse, and conversion of by-products into valuable resources like biogas, nutrients, or materials [18]. From this perspective, wastewater treatment plants are conceived as biorefineries capable of producing reclaimed water and recovering nutrients, biopolymers, or energy, integrating physicochemical and biological processes [19,20,21,22]. The treatment of specific streams, such as landfill leachate, greywater, or industrial wastewater, requires considering the composition and final destination of the effluent, adopting combined strategies to maximize efficiency [16,21]. These practices not only reduce pressure on receiving bodies but also generate reusable inputs that replace virgin raw materials, thus contributing to the objectives of the circular economy [23,24].

Effective integration of physicochemical water treatment technologies into the circular economy requires a precise conceptualization of the circular economy and the establishment of a standardized definition, thereby preventing misconceptions and conflation with related paradigms such as sustainability [13,18]. Furthermore, this effort could expand its conceptual scope beyond resource management within production systems toward a more holistic and systemic interpretation of the circular economy. In some cases, the circular economy has been considered as zero waste, and its key principles are resource efficiency and eco-design [9]. Physicochemical water treatment systems can enable the recovery of valuable resources from wastewater—including nutrients, metals, energy, and water—that may be reintegrated into the circular economy. The degree of implementation in the wastewater sector can be assessed through various indicators, encompassing both technical and environmental dimensions. To better understand current research trends, a literature search was conducted in databases such as ScienceDirect over the past 10 years using the keywords “Physicochemical Treatments for Circular Water Management,” “Circular Economy Water,” and “Wastewater Treatments for water Circular Economy”. These phrases were selected to cover all relevant articles on the topic. Relevant articles were selected based on their relevance and quality. The eligibility criteria for this evaluation included academic articles published in English between 2015 and 2025.

The recent evolution of research into physicochemical treatments for circular water management reflects sustained growth in scientific output, especially since 2020. As shown in Figure 2a, the number of publications has grown from 120 in 2015 to 3869 in 2025, evidencing an exponential increase in academic and technological interest in this topic. This boom responds both to the urgency of addressing water scarcity and to the need to close material cycles within the framework of the circular economy. Regarding the distribution by type of treatment (Figure 2b), membrane separation accounts for the largest proportion of studies (32.13%), followed by advanced oxidation processes (26.93%) and chemical precipitation (21.76%). Technologies such as coagulation–flocculation, adsorbents, and electrocoagulation represent smaller percentages (2.80%, 11.76% and 4.60%, respectively), suggesting that, although they have relevant applications, recent research has been primarily oriented towards highly efficient methods and the ability to integrate with resource recovery systems. Physicochemical treatment technologies and their circular economy potential for different wastewater streams are summarized in

Table 1. Physicochemical Treatment Technologies applied to different Wastewater Streams and their Circular Economy Potential.

Type of Treated Stream	Physicochemical Technology	Circularity Potential	Reference
Landfill leachate	Adsorbents (natural zeolite), Coagulation–flocculation (polyferric sulfate), Chemical precipitation (struvite)	Recovery of N (89.4%), P (63.9%), and K (47.8%); sludge use as fertilizer	[21]
Dairy wastewater	Electrocoagulation (recycled Al electrodes)	COD * (91.67%) and BOD ** (95.36%) removal; treated water reuse; electrode recycling	[25]
Domestic greywater	Coagulation–flocculation, Adsorbents, Advanced oxidation, Membrane separation (MBR)	Reuse for irrigation and non-potable domestic purposes; use of solid residues	[16]
Textile wastewater	Coagulation–flocculation, Advanced oxidation (activated H2O2)	Color and aromatic compounds removal; process water recovery	[26]
Refinery wastewater	Dissolved air flotation and Advanced oxidation	Recovery of oils (>90%); treated water reuse in industrial processes	[23]
Agro-industrial wastewater	Advanced oxidation (ozone, photocatalysis)	Irrigation-grade water; pathogen reduction; valorization of residual biomass	[27]
Industrial brackish water	Membrane separation (electrodialysis)	Recovery of water and concentrated salts for reuse	[28]
Oily industrial effluents	Membrane separation (ceramic ultrafiltration)	Water reuse; oil recovery; high material durability	[13]
Mining wastewater	Adsorbents (modified industrial by-products)	Recovery of metals for reintegration into production processes	[29]
Hospital wastewater	Advanced oxidation (photocatalysis) and Filtration	Pharmaceuticals removal (>90%); reuse of water for non-potable purposes	[20,30]
Battery leachates	Chemical precipitation; selective adsorption; membrane separation	Recovery of critical metals (Li, Co, Ni) for battery manufacturing	[31,32]
Desalination brines	Nanofiltration; reverse osmosis; electrodialysis	Recovery of salts and critical minerals (Li, Mg, Sr, B)	[22,33]

* COD = Chemical Oxygen Demand; ** BOD = Biological Oxygen Demand.

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Table 1 summarizes and compares recent case studies that demonstrate that physicochemical technologies applied to different types of wastewater streams offer high potential for resource recovery and water reuse, aligning with the principles of the circular economy. In streams with high organic and nutrient loads, such as landfill leachate, the combination of natural zeolite adsorption, coagulation–flocculation, and chemical precipitation of struvite has allowed the recovery of up to 89.4% of nitrogen and 63.9% of phosphorus, in addition to generating sludge suitable for agricultural use [21]. In industrial effluents, such as dairy and refinery effluents, processes such as electrocoagulation with recycled electrodes or dissolved air flotation combined with advanced oxidation have achieved high COD and oil removal efficiencies, facilitating the reincorporation of treated water and oily fractions into production processes [23,25]. Greywater and agro-industrial waters benefit from hybrid schemes that integrate coagulation, adsorption, membranes, and advanced oxidation, obtaining effluents suitable for irrigation and non-potable domestic use, while in specific streams such as industrial brines or mining effluents, electrodialysis and the use of modified adsorbents allow the recovery of valuable salts and metals [28,29]. In addition to the streams already discussed, battery leachates and desalination brines represent increasingly important sources of critical metals. From battery leachates, lithium, cobalt, and nickel can be selectively recovered through chemical precipitation, adsorption, and advanced membrane processes, providing secondary raw materials for the battery industry [31,32]. Similarly, desalination brines, traditionally considered waste, are now exploited for the extraction of valuable elements such as lithium, magnesium, and strontium using nanofiltration, reverse osmosis, and electrodialysis [22,33]. Incorporating these streams into circular strategies strengthens the role of physicochemical technologies in securing critical resources while minimizing environmental impacts. This diversity of applications confirms that technology selection must be tailored to the composition and potential value of byproducts, prioritizing integrated configurations that maximize recovery efficiency and minimize waste generation.

2.1. Coagulation–Flocculation, a Cornerstone in the Removal of Colloidal and Particulate Contaminants

Coagulation–flocculation is widely applied in industrial wastewater treatment, drinking water purification, and municipal wastewater clarification to remove colloids and suspended solids [34]. In the coagulation stage, metal salts such as aluminum sulfate or ferric chloride release multivalent cations (Al³⁺, Fe³⁺) into the water, which hydrolyze to form metal hydroxide species that neutralize particle surface charges, reduce electrostatic repulsion, and initiate microfloc formation [35]. In the subsequent flocculation stage, slow mixing—often aided by synthetic or natural polymers—promotes the aggregation of microflocs into larger, denser flocs. These flocs are then separated from the water by sedimentation in clarifiers or by flotation processes, enabling the removal of suspended solids, heavy metals, organic matter, phosphorus, and pathogens from the effluent [36].

Electrostatically stabilized colloids in water present removal challenges, often requiring pre-treatment methods such as coagulation–flocculation before advanced filtration processes like ultrafiltration or reverse osmosis to mitigate environmental and health risks [37]. Coagulants such as polyaluminum chloride, used in combination with synthetic polyacrylamide flocculants (non-ionic, amphoteric, cationic, anionic), promote aggregation through mechanisms including charge neutralization, compression of the electric double layer, polymer bridging, and adsorption [38]. According to DLVO theory (Derjaguin–Landau–Verwey–Overbeek), colloidal stability results from the balance between van der Waals attraction and electrostatic repulsion arising from the electric double layer. The resulting total interaction potential typically features an energy barrier that prevents particle aggregation unless overcome. Coagulants reduce this barrier by neutralizing surface charges and increasing ionic strength, enabling attractive forces to dominate and allowing microfloc formation [39]. The coagulation process (Figure 3a) involves rapid mixing to evenly disperse the coagulant, facilitating surface charge neutralization and the initial formation of microflocs. During the subsequent flocculation stage (Figure 3b), destabilized colloids, polymer chains, and amorphous hydrated metal hydroxides aggregate into larger and denser flocs under gentle mixing conditions. In the sedimentation phase (Figure 3c), these densified flocs adsorb particulate matter and settle, dragging suspended contaminants to the bottom of the tank. The illustration depicts negatively charged colloids (blue), aluminum-derived coagulant species (gray), positively charged colloids (green), and polyacrylamide chains represented in a ball-and-stick model. DLVO theory (Derjaguin–Landau–Verwey–Overbeek) provides a framework for understanding colloidal stability and predicting the minimum coagulant dose needed to overcome the energy barrier to aggregation. However, because water composition factors such as pH, natural organic matter, and dissolved salts influence real-world performance beyond DLVO predictions, optimal dosing typically requires experimental validation through jar tests to ensure effective aggregation and sedimentation in industrial effluent treatment [39].

Treated water from processes such as coagulation–flocculation can be suitable for reuse in various industrial applications when complemented with additional polishing steps if needed [34]. However, the residual sludge from these treatments—particularly in high-turbidity chemical–mechanical polishing wastewater—poses a management challenge because it concentrates largely non-biodegradable solids (i.e., metal hydroxides, fine silicon/aluminum/cerium oxides, and polymer residues from coagulant aids). In such streams, optimized poly-aluminum chloride C/F at near-neutral pH effectively aggregates particles and clarifies the water, but primarily shifts the contaminant load into the sludge, requiring dedicated post-treatment and disposal strategies [40].

To address the challenge of such complex sludge, certain treatment approaches can transform it into value-added products. For example, Xu et al. (2024) applied co-pyrolysis of sewage sludge with biomass, producing biochar with enhanced surface area, pore structure, and adsorption capacity, while mitigating heavy metal risks [41]. Another study using a nature-based coagulant demonstrates a successful circular economy approach through an innovative microwave-assisted continuous-flow process to produce a coagulant derived from lentil waste. This technology repurposes agricultural waste as a sustainable feedstock, converting it into an effective, biodegradable, and non-toxic coagulant via microwave-assisted copolymerization, enhancing both efficiency and product quality. LENFLOC^TM achieved 99% turbidity removal with a lower dosage compared to untreated extracts, forming compact flocs that significantly reduce sludge volume. This approach exemplifies circular economy principles by reducing treatment costs and resource consumption while minimizing waste generation compared to conventional batch methods. The resulting biodegradable sludge, free of toxic residues, can be directly recycled as a soil amendment, thus closing the resource loop in water treatment [42]. These examples underscore the potential of integrating circular economy principles into wastewater treatment through innovative technologies like nature-based coagulants and advanced coagulation–flocculation processes, demonstrating how resource recovery, cost efficiency, and environmental sustainability can be achieved across diverse industrial and regional contexts. Within a circular framework, the clarified water can be reused for industrial or irrigation purposes, while the resulting sludge—when stabilized or co-processed—may be repurposed as a soil amendment, construction additive, or energy feedstock, transforming a disposal challenge into a valuable resource.

2.2. From Conventional to Green Adsorbents

Adsorption is a widely applied physicochemical process in water treatment in which contaminants—organic or inorganic—are transferred from the aqueous phase and retained on the surface of a solid adsorbent [43]. The process occurs via intermolecular and interfacial forces, which may involve physical adsorption (e.g., van der Waals forces, hydrophobic interactions, hydrogen bonding) or chemical adsorption (e.g., covalent bonding, surface complexation). The efficiency of adsorption is strongly influenced by the adsorbent surface area (e.g., m²/g), pore size distribution (e.g., nm), and surface chemistry (e.g., mmol/g of functional groups or mV for zeta potential [39]. In Figure 4, granular activated carbon (GAC) is used as the adsorbent; on the left, the column schematic shows water flow (black arrow) through the packed bed where contaminants are removed, while on the right, a magnified molecular representation of the adsorbent surface is shown, with differently colored regions representing adsorbed contaminants of varied chemical nature. Adsorption is particularly effective for removing dissolved compounds such as organic pollutants (e.g., dyes, phenols, pesticides) and inorganic species (e.g., heavy metals like Pb²⁺ and Cd²⁺). The affinity of these contaminants for the adsorbent is determined by their physicochemical characteristics—such as hydrophobicity, molecular size, solubility, and polarity—as well as by operational conditions like pH, temperature, and competing solutes [44].

Adsorbent materials in wastewater treatment must have a high surface area to maximize binding sites, a porous structure for efficient pollutant interaction, and chemical stability to withstand harsh conditions. They should also exhibit selectivity for specific contaminants based on properties like hydrophobicity or charge and be regenerable for cost-effective use. Strong affinity for pollutants through interactions such as van der Waals forces or hydrogen bonding is essential. Common types of adsorbents include natural mineral materials such as bentonite, kaolinite, zeolites, diatomaceous earth, and pumice; synthetic adsorbents like ion-exchange resins, metal–organic frameworks (MOFs), mesoporous silica materials such as SBA-15 and MCM-41, and synthetic zeolites; carbon-based materials including granular or powdered activated carbon, carbon nanotubes, graphene oxide, and biochar; metal oxides and hydroxides such as alumina (Al₂O₃), iron oxides (Fe₂O₃, Fe₃O₄, FeOOH), manganese oxides like birnessite, titanium dioxide (TiO₂), zirconia (ZrO₂), magnesium oxide (MgO), and cerium oxide (CeO₂); as well as biological adsorbents derived from peat, chitosan, bacterial cellulose and agricultural waste such as rice husk ash, sawdust, or fruit peels. These bio-adsorbents underscore the need for further study and justify continued research, as they may exhibit batch-to-batch compositional variability, a frequent need for activation or chemical modification to achieve adequate capacity/selectivity, and difficulties in standardizing synthesis and regeneration, which may affect performance and scalability. Selection depends on the contaminants and treatment goals, balancing efficiency with cost and sustainability [45,46,47].

While materials for adsorption are manufactured from various sources, waste materials offer a promising alternative due to their abundance, low cost, and environmental benefits. Biowaste from agricultural, industrial, and forestry activities is generated in large quantities and often holds little to no economic value, making it a viable and sustainable option compared to traditional commercial adsorbents, which are typically expensive to produce [48]. Moreover, utilizing these waste materials aligns with the principles of the circular economy, transforming what would otherwise be discarded into valuable resources, reducing landfill accumulation, and minimizing the need for incineration. Beyond their availability, waste materials possess inherent functional properties that make them suitable for adsorption. They often contain functional groups, such as hydroxyl, carboxyl, and amino groups, which enhance their ability to bind with contaminants like heavy metals or organic pollutants. With proper preparation and modification, these biowastes can achieve high adsorption capacities, offering an environmentally friendly and cost-effective solution for wastewater treatment [49].

A current approach in environmental remediation is the development of green adsorbents, adsorbent materials made from renewable or recycled resources that minimize environmental impact throughout their lifecycle. These materials aim to reduce toxicity, lower energy and chemical usage during production, and prioritize high regenerability and biodegradability. Commonly derived from sources such as natural polymers, agricultural waste, and industrial byproducts, green adsorbents are designed to efficiently remove contaminants while avoiding additional waste generation. This innovative approach aligns with sustainability and circular economy principles, transforming waste materials into valuable resources for pollutant remediation in soil and water systems [43].

The use of unusual materials, such as cellulose derivatives, chitosan, and other biopolymers obtained from agricultural, forestry, and maritime waste, demonstrates sustainability through its production process and application potential [50]. Another example is bacterial cellulose, obtained through microbial fermentation, utilizing renewable resources and addressing the environmental challenges of industrial waste disposal. This approach aligns with the principles of the circular economy by transforming low-value byproducts into high-performance adsorbents. BC has been effectively used in water treatment applications, particularly for the adsorption of phenolic compounds, dyes, and heavy metals. It has shown remarkable performance, with adsorption capacities of up to 26.96 mg of gallic acid equivalents per gram for phenolic compounds, achieving removal efficiency exceeding 50% for dyes like acid yellow 17 and around 96–97% for heavy metals such as zinc and cadmium, respectively. These results highlight its versatility and efficiency as a bio-adsorbent, offering a sustainable alternative to conventional materials while promoting resource recovery and environmental protection [51].

Green adsorbents, such as bacterial cellulose derived from waste streams, exemplify the integration of circular economy principles into environmental remediation. These materials, sourced from renewable or recycled resources, transform low-value byproducts into high-performance solutions for water treatment, achieving remarkable adsorption efficiencies for phenolic compounds, dyes, and heavy metals [50]. By aligning resource recovery with sustainability, green adsorbents offer an environmentally friendly, cost-effective alternative to conventional methods, reducing waste and promoting long-term ecological balance. In circular economy terms, adsorbents can be regenerated and reused in multiple treatment cycles, while exhausted materials—particularly those derived from biomass—can be converted into biochar, catalysts, or incorporated into composite materials, thus minimizing waste and promoting resource recovery.

2.3. Chemical Precipitation of Valuable Resources in Water

Chemical precipitation is a fundamental process in chemistry and environmental engineering, characterized by the formation of an insoluble solid, or precipitate, from ions dissolved in a solution [52]. This process occurs when the ionic product of the solution exceeds the solubility product constant (Ksp) of the resulting compound. The solubility product constant is a thermodynamic parameter defining the equilibrium concentrations of ions in a saturated solution. For example, the solubility of a generic salt AB in water is governed by:

$$K_{sp}=\left[A^{+}\right]\left[B^{-}\right]$$

(1)

Here, [A⁺] and [B⁻] denote the concentrations of the dissociated ions. Precipitation commences when the product of these concentrations surpasses Ksp, leading to the crystallization of the compound. This phenomenon plays a crucial role in processes such as water purification, the removal of heavy metals, and chemical synthesis. Precipitation not only separates specific ions but also allows for their safe disposal or recovery. The solubility product constant (Ksp) is an equilibrium constant that quantifies the solubility of a compound in water. The units and magnitude of Ksp depend on the stoichiometry of the dissolution reaction and the concentrations of the ions involved. This constant plays a critical role in predicting whether a substance will precipitate under given conditions. Importantly, the magnitude of Ksp provides a direct indication of the solubility of a compound: smaller values signify limited solubility and a greater tendency for precipitation, while larger values denote higher solubility.

Chemical precipitation is an effective method for removing heavy metals from wastewater and industrial effluents. Commonly targeted metals include lead $\left({Pb}^{2+}\right)$, cadmium $\left({Cd}^{2+}\right)$, and copper $\left({Cu}^{2+}\right)$. By introducing appropriate reagents, such as hydroxides, sulfides, or carbonates, these metals can be converted into their insoluble forms. For instance, ${Cu}^{2+}$ in water can be precipitated as copper hydroxide (${Cu\left(OH\right)}_2$) by adding sodium hydroxide $\left(NaOH\right)$. The reaction is driven by the following equilibrium:

$${Cu}^{2+}+{2OH}^{-}\to {Cu\left(OH\right)}_{2\left(s\right)}$$

(2)

${Cu\left(OH\right)}_2$ has a low solubility product, $K_{sp}=2.2\times {10}^{-20}$, ensuring effective precipitation even at low concentrations of ${Cu}^{2+}$. Coagulation–flocculation aggregates colloidal particles by destabilizing suspensions with chemical coagulants and promoting floc growth and densification via controlled mixing for efficient solid–liquid separation, whereas chemical precipitation transforms dissolved ions into insoluble compounds through thermodynamic equilibrium, enabling their separation from the liquid phase. Coagulation–flocculation, governed by electrostatic interactions and physical bridging mechanisms, contrasts with precipitation, which relies on solubility limits to form and separate solid phases. For example, in water treatment, removing arsenic ${As}^{3+}$ may involve coagulation with ferric chloride $\left(Fe{Cl}_3\right)$ to adsorb arsenic on iron hydroxide flocs, while lead removal could be achieved by precipitating lead sulfate $\left(PbS{O}_4\right)$ using sulfuric acid $\left(H_{2}S{O}_4\right)$. Alternatively, it is relevant that phosphate precipitation stood out for its versatility, effectively removing metals like barium, calcium, and chromium at a pH of 6. Beyond its removal efficiency, phosphate precipitation offered superior sludge dewatering characteristics, reducing sludge volume and improving filtration. These advantages make it particularly suitable for applications prioritizing efficient sludge management [53].

By converting dissolved ions into insoluble compounds, the chemical precipitation process enables the extraction and reuse of valuable metals from industrial effluents, reducing the need for virgin materials and mitigating waste. For instance, the recovery of phosphorus from wastewater through chemical precipitation not only addresses nutrient pollution but also supplies a critical resource for fertilizers, exemplifying a closed-loop system [54].

Chemical precipitation holds immense potential as a cornerstone technology in environmental engineering. By enabling the selective removal and recovery of valuable resources such as heavy metals and nutrients from wastewater, it not only mitigates pollution but also transforms waste streams into sustainable sources of raw materials. Techniques such as chemical precipitation and ammonia stripping/absorption have achieved full commercialization, enabling recovery rates up to 100% P and 99% N, though product quality may be affected by impurities. Recovered nutrient products can enhance biomass yields by up to 60% compared to synthetic fertilizers, with life cycle assessments confirming positive environmental impacts [55].

Its adaptability to various industrial effluents, including challenging streams such as acid mine drainage, coupled with its ability to produce manageable and often reusable by-products, positions chemical precipitation as a critical tool in advancing sustainable practices. In the mining sector, where wastewater contains dissolved metals of economic value, chemical precipitation—integrated within advanced treatment chains such as reverse osmosis for metals concentration and water reclamation, and electrodialysis with bipolar membranes for NaOH recovery—can selectively recover valuable metals (Al, Zn, Cu, Mn, Mg, and Ca) at rates above 60%, while reclaiming up to 97% of water and recovering up to 58% of reagents for reuse [31]. By overcoming key limitations such as low influent metal concentrations and high reagent consumption, this approach improves both the efficiency and circularity of precipitation processes. As industries increasingly prioritize resource efficiency and environmental stewardship, chemical precipitation emerges as a proven, scalable solution that aligns with circular economic principles and global sustainability goals [32,56].

2.4. Electrocoagulation as a Viable Candidate for Solar-Powered Water Treatment Processes

Electrocoagulation (EC) is an electrochemical-based technology that effectively removes contaminants such as grease, hydrocarbons, suspended solids, and heavy metals [57]. EC operates on the principles of coagulation and flocculation without requiring external coagulants. In electrocoagulation, a direct current potential applied between metallic electrodes in an electrolytic cell induces anodic oxidation of the electrode material (e.g., Fe or Al), generating metal cations (Fe²⁺, Fe³⁺, Al³⁺) in solution [58]. These cations hydrolyze to form metal hydroxides and polyhydroxy complexes, which, as mentioned earlier, act as coagulants by destabilizing colloidal suspensions and promoting aggregation. The system can operate in batch or continuous mode, typically applied at laboratory-pilot scale and industrial scale, respectively, although exceptions exist depending on process requirements. Other configurations involve electrode arrangements (monopolar or bipolar connections, in either series or parallel), which influence voltage distribution, current flow, and overall efficiency. Additional design variables include the number of electrodes and their active surface area, both of which directly affect current density, reaction kinetics, and energy consumption [59,60].

The electrocoagulation (EC) process involves several stages: (i) anodic oxidation of the sacrificial electrode (e.g., Fe or Al) to release metal cations that act as coagulants, (ii) cathodic reduction of water producing hydroxyl ions and hydrogen gas, (iii) destabilization of the colloidal system through charge neutralization and adsorption of contaminants onto in situ–formed metal hydroxides, and (iv) aggregation into flocs that are subsequently removed by sedimentation or by flotation assisted by hydrogen bubbles generated at the cathode [61,62]. The chemical reactions in the electrolytic cell vary depending on the electrode material, but can be represented by general reactions [63]. Reactions (1) and (2) take place at the anode, where water oxidation generates protons and oxygen gas, and the sacrificial metal electrode undergoes anodic dissolution to release metal cations. Reaction (3) occurs at the cathode, where water reduction produces hydrogen gas and hydroxyl ions. The in situ-generated metal cations react with OH⁻ to form amorphous metal hydroxides, which adsorb and destabilize contaminants, leading to floc formation. Furthermore, the evolution of O₂ and H₂ gases facilitates the separation of flocs that do not settle by promoting flotation [64].

2H₂O → 4H⁺(aq) + O₂(g) + 4e⁻

(3)

M(s) → M⁺(aq) + ne⁻

(4)

nH₂O + ne⁻ → (n/2)H₂(g) + nOH⁻(aq)

(5)

Electrocoagulation (EC) can outperform conventional coagulation in treating waters with organic and inorganic contaminants, particularly under optimized operational conditions. Key performance indicators include high contaminant removal rates, low energy consumption, favorable cost–benefit ratios, and minimal sludge generation. Operational parameters influencing EC efficiency include water pH and conductivity, contaminant concentration, electrode material, reaction time, and current density. Many of these parameters are interrelated and can be described by Faraday’s law, which quantitatively relates the electrical charge passed through the system to the mass of anode material dissolved and, consequently, to the amount of coagulant generated in situ [58].

Among the advantages of this technology are the low maintenance costs, simplicity in operation, capacity for treating large volumes of water in a continuous regime, effectiveness for diverse contaminants, elimination of external coagulants, less toxic generated sludge, and high removal efficiency [64]. The disadvantages of EC are the electrode passivation (which requires periodic cleaning or replacement) and the anode degradation during operation. The dissolved metal from the anode can be recovered from the generated flocs when treating large water volumes or when the electrode materials are costly. Recovery methods include calcination, chemical dissolution, electrowinning, or ion exchange processes [65]. Different applications of electrocoagulation in various types of wastewater are shown in

Table 2. Summary of electrocoagulation applications in different wastewater types.

Type of Wastewater	Main Contaminants	Removal Efficiency (%)	Technical Information	References
Textile	Dyes, COD, turbidity	85–99	20–60 min; Al or Fe electrodes; pH 6–8; 10–25 mA/cm2	[66]
Tannery	Chromium, dyes, organic matter	80–98	30–90 min; Fe/Al electrodes; pH 4–7	[64]
Mining/Metallurgy	Arsenic, Se, heavy metals	85–99	15–40 min; Fe or Al electrodes; pH 6–8	[67]
Food industry (dairy, oils)	Fats, oils, proteins	80–95	30–60 min; Al/Fe electrodes; NaCl as supporting electrolyte	[68]
Municipal wastewater	Turbidity, pathogens, antibiotics	>99	10–30 min; Al/Fe electrodes; <20 mA/cm2	[14,68]
Emerging contaminants	Micro-/nanoplastics, diclofenac, pesticides	80–99	10–90 min; Al > Fe electrodes; neutral pH	[69,70]

.

Table 2 summarizes representative applications of EC for different types of wastewater, including textile, tannery, mining, food industry, municipal, and streams containing emerging contaminants. Reported removal efficiencies range from 80 to 99%, with effective treatment of dyes, heavy metals, oils, pathogens, pharmaceuticals, and micro/nanoplastics. Most studies employ aluminum or iron electrodes, under operating times of 10–90 min and pH adjustments according to the target contaminants. Despite its versatility, EC is not exempt from limitations such as relatively high energy consumption, electrode passivation, and the generation of metal-rich sludge that requires appropriate management [68]. These constraints highlight the importance of coupling EC with complementary approaches and valorization strategies. Integrating EC with other unit operations offers a comprehensive solution for treating large volumes of contaminated water in continuous regimes—particularly relevant for industries managing metals and electronics [57]. Circular economy benefits can be enhanced by implementing sustainable practices such as reusing recovered metals from sludge to produce new electrodes and deploying photovoltaic panels for autonomous power generation. Recent literature supports this integrated paradigm, including the treatment and valorization of EC sludges, the development of renewable energy-powered EC systems, and the incorporation of artificial intelligence for process optimization [14]. Within this circular framework, EC also enables resource recovery by facilitating the reuse of treated effluents in agricultural or industrial applications, incorporating low-cost adsorbents synthesized from waste materials, and valorizing by-products into fertilizers, pigments, construction materials, or catalysts. Furthermore, the hydrogen gas generated at the cathode can be recovered as a clean energy vector, further strengthening the contribution of EC to sustainable water management [71].

EC is particularly suitable for integration with photovoltaic (PV) systems because the process operates with a low-voltage direct current (DC), typically in the range of 1–5 V, which matches the output characteristics of PV panels. This compatibility eliminates the need for DC–AC inverters, reducing capital costs, minimizing energy conversion losses, and simplifying system design and maintenance [72]. In off-grid or remote settings, solar-powered EC systems (SPEC) can be directly coupled to PV arrays through charge controllers to regulate current density, a critical operational parameter influencing coagulant generation rate, contaminant removal efficiency, and energy consumption [73].

Moreover, solar-driven EC not only reduces dependence on grid electricity but also strengthens circular economy strategies by enabling decentralized treatment, lowering operational costs, and expanding the valorization of by-products. These systems can be scaled from small-scale rural applications to industrial units, with configurations ranging from standalone PV–EC units to hybrid systems integrating battery storage, ensuring treatment continuity during periods of low irradiance. Recent field studies have demonstrated high removal efficiencies for heavy metals, dyes, and oil–water emulsions using direct PV–EC coupling without intermediary energy conversion stages, confirming both the technical feasibility and the sustainability advantages of this approach [72,73].

2.5. Advanced Oxidation Processes Within the Framework of the Circular Economy

These advanced oxidation processes (AOPs) involve the generation of the hydroxyl radical (OH^•), which facilitates the degradation of recalcitrant compounds that are resistant to conventional treatments. For this reason, AOP has been established as an effective alternative for the degradation or oxidation of organic substances, inorganic materials, metals, or pathogens [27]. Among the advantages of AOPs is the ability to act on complex matrices of various contaminants, thanks to the non-selective nature of the hydroxyl radical OH^• and its high oxidation potential of 2.80 V [74] in addition to its behavior and rapid reaction with numerous species, with the rate constants on the order of 108 to 1010 M⁻¹s⁻¹ [75].

The OH^• radical can be generated through photochemical means or other forms of energy, and the reaction with organic compounds can be classified into four different mechanisms: radical addition, hydrogen abstraction, electron transfer, and radical combination [76]. Organic compounds containing a carbon–carbon double bond react with OH^• under the addition reaction due to the rich π-electron cloud on the aromatic ring [77]. Some of the most widely used advanced oxidation processes for industrial wastewater treatment are those based on ozone (O₃) [74], ultraviolet (UV) radiation [76], Fenton-related reactions [78], and electrochemical reactions [14].

Ozone (O₃) is a strong oxidant with an oxidation potential of 2.07 V. It can directly react with organic matter, exhibiting reaction rates of 1.0 × 10⁰ to 10³ M⁻¹s⁻¹ [75] and under certain conditions, ozone can generate hydroxyl radicals (OH^•), initiating non-selective reactions [76].

$$3O_3+H_{2}O\to 2{OH}^{•}+4O_2$$

(6)

By coupling other oxidants with this process, the production of OH^• can be enhanced, with hydrogen peroxide (H₂O₂) being one of the most employed oxidants.

$$H_2{O}_2\to H{O}_2^{-}+H^{+}$$

(7)

$$H{O}_2^{-}+O_3\to {OH}^{•}+O_2^{-}+O_2$$

(8)

Another method to generate OH^• is through UV irradiation.

$$O_3+H_2+hv\to H_2{O}_2+O_2$$

(9)

$$H_2{O}_2+hv\to 2{OH}^{•}$$

(10)

In the Fenton process, Fe²⁺ reacts with H₂O₂ to produce stronger reactive species, traditionally recognized as hydroxyl radicals, alongside other substances such as ferryl ions [79]. Finally, a group of AOPs to consider are electrochemical processes, such as electrochemical oxidation or electro-oxidation, which require an electric current provided by an external power source and electrodes of different materials [77]. In this technology, the degradation of contaminants occurs directly and indirectly. In direct oxidation, the contaminants are oxidized when they encounter the anode, while in indirect oxidation, they are oxidized by free radicals, which are generated depending on the anode electrode used and the presence of substances such as chlorine and hydrogen peroxide [80]. For this reason, advanced oxidation processes have been coupled with biological treatments (activated sludge) for the degradation and/or removal of thin-film-transistor liquid crystal display (TFT-LCD), tetra-methyl ammonium hydroxide (TMAH), and many other organic and inorganic compounds from wastewater in the microelectronics industry, achieving a removal percentage greater than 98% [26]. These types of contaminants from the microelectronics industry, found in low concentrations but posing significant risks to health and the environment, require advanced treatment solutions [81].

Advanced oxidation processes (AOPs) are not only effective for degrading persistent pollutants but also support circular economy goals by enhancing water reuse and reducing environmental impact. According to Wang et al. (2023), coupling AOPs with other technologies can improve energy efficiency, reduce chemical use, and enable resource recovery [82]. Innovations such as catalytic ozonation and electrochemical oxidation demonstrate the potential of AOPs to align wastewater treatment with sustainability and carbon neutrality objectives [82].

The evolution of AOPs applied to water treatment demonstrates some transition toward more efficient and sustainable configurations, which fully fit within the framework of the circular economy of physicochemical processes [83]. Initially, ozonation and its catalytic variant improved the degradation of refractory organic compounds by up to 50% compared to simple ozonation, although with challenges associated with the control of byproducts such as bromates (>10 µg/L), underscoring the need to optimize reagents and conditions to minimize hazardous waste [84]. The subsequent integration of photocatalysis and ultrafiltration in industrial effluents not only allowed COD reductions greater than 90% and the removal of emerging contaminants by over 85%, but also extended the membrane lifetime by 40%, reducing resource consumption and replacement costs [85]. In pesticide treatment, electrochemical processes such as anodic oxidation, electro-Fenton, and photo-assisted variants achieved removal rates greater than 95%, with 25% less energy consumption when coupled with solar energy, reinforcing the energy efficiency of the treatment cycle [86]. The incorporation of sustainable catalysts such as modified biochar in AOPs has allowed degradations of more than 90% in dyes and pharmaceuticals with low oxidant doses (1–3 mM) and a loss of activity of less than 10% after five cycles, facilitating their reuse and reducing waste generation [87]. Similarly, the use of visible-light photocatalysis, thanks to doped catalysts, has achieved degradations of >90% in dyes and >85% in pharmaceuticals with 30% less energy consumption, optimizing the use of energy and resources [83]. Finally, solar systems for photo-Fenton and photocatalysis have achieved removals of >85% in emerging contaminants in 2–4 h of irradiation, highlighting their potential to operate with renewable energy and reduce dependence on fossil sources, key elements to close the water cycle in a circular scheme [88]. From a circular economy perspective, AOPs facilitate safe water reuse and can be coupled with renewable energy systems to reduce operational costs and emissions. Additionally, catalysts—particularly biochar-based or doped materials—are increasingly designed for recyclability, enabling multiple use cycles and minimizing secondary waste.

2.6. Recent Advances in Membrane Separation Processes for Circular Water Management

Nowadays, membrane separation has played an important role in water treatment. This is a selective method that allows the separation of different components, depending on their nature or size [89]. In this process, the membrane acts as a selective barrier, allowing the treatment of effluents and partially or totally restricting the passage of some unwanted components [90]. This technology is classified based on the driving force (pressure, concentration, or electrochemistry) and depending on the pore size, thus determining the type of contaminant that can be eliminated. The membranes have a high permeability to water, which facilitates its transport, while retaining various solutes, such as salt ions and organic molecules [91]. These properties make them effective tools for the removal of unwanted contaminants in drinking water treatment, wastewater treatment, and seawater desalination processes [90,91,92].

According to the driving force, membranes are classified into three main categories: concentration filtration, electrochemical filtration, and pressure filtration. Concentration filtration, also known as concentration gradient, is based on the difference in solute concentration on both sides of the membrane. In this process, the concentration gradient acts as a driving force, inducing the movement of molecules across the membrane. The second is electrochemical filtration, which uses an electric field as a driving force to separate ions and charged molecules in a solution. This method is based on the migration of particles through the membrane under the influence of the electric field. On the other hand, pressure filtration is the most common method in membrane separation processes. This approach consists of applying external pressure to force fluid through the membrane, allowing the separation of components based on their size and charge [93]. At the industrial level, membrane operations whose driving force is the pressure gradient generated between both sides of the membrane, cover a wide spectrum of applications, consolidating themselves as advanced technologies, being a key component in water purification [30]. Depending on the pore size, these membrane technologies can carry out a separation process including:

-

Microfiltration (UF): UF membranes are effective in removing large particles between 0.1 and 10 µm. The pores allow the retention of sediments, microorganisms, and some colloids. This process is characterized by its low energy consumption because it operates at relatively low pressures, which makes it an attractive option for various applications in wastewater treatment and for the food industry [94]. However, since it retains particles by size exclusion, it is not selective and only rejects larger suspended particles [90].

-

Ultrafiltration (UF): is a process that allows the size exclusion separation of macromolecules. The pores of these membranes are in a range of 2 to 100 nm, which allows the removal of colloidal dispersions of clays, sugars, proteins, and microorganisms present in contaminated waters [94]. Generally, these membranes are used in the pharmaceutical industry for the purification of biological products and concentration of solutions. Also, they are used in water treatment to eliminate organic contaminants and some microorganisms [95]. Among its advantages are low-cost, low-pressure requirements and high-water yield.

-

Nanofiltration (NF): NF membranes are capable of retaining small organic molecules and multivalent ions, thereby enabling the removal of specific contaminants in effluents, such as hardness, salts, and organic compounds. These membranes typically exhibit pore diameters in the range of 0.5–2 nm and operate under moderate pressures, offering higher water permeability compared to reverse osmosis (RO). The separation process is primarily governed by size exclusion, complemented by electrostatic interactions, the Donnan exclusion principle, and specific solute–membrane interactions. Beyond contaminant removal, NF membranes have demonstrated significant potential for the separation of industrially relevant compounds, as recently highlighted in the literature. Importantly, NF membranes exhibit higher selectivity towards divalent and polyvalent ions, while allowing partial passage of monovalent ions and small organic molecules. Their high solute–solute selectivity allows the fractionation of industrial wastewater. For instance, treating textile effluents recovers salts, organic compounds, and dyes, thereby promoting the valorization of by-products and reducing pollutant discharge. Furthermore, NF has been applied to solute–solute separations from brines, with particular emphasis on the selective extraction of lithium over magnesium, a strategic process for the sustainable exploitation of critical minerals [33,90].

-

Reverse osmosis (RO): RO membranes are semipermeable and, due to their very small effective pore size (~0.25 nm), they are capable of rejecting more than 98–99% of ions and molecules, allowing only water to permeate [37,96]. This technology has become a fundamental process for water desalination [97], and is considered the most efficient for converting saline water into high-quality drinking water, ranging from 1 to 10 g/L [22,98]. RO is also widely applied in the pharmaceutical and food industries for the purification of ingredients and final products, due to its high efficiency, low cost, and ease of operation.

Conventional RO membranes are typically composed of aromatic polyamide (PA) thin-film composite (TFC) polymers, which ensure high selectivity [37,96]. Its general composition consists of three layers. The first is known as the active layer, and it is a thin layer formed with aromatic polyamide with a thickness of 200 nm. Next is the support layer, which has an irregular and porous structure with reduced dimensions ranging between 20 and 50 μm. The morphology of the first two layers is shown in Figure 5. Because the polyester support layer, in isolation, provides a suitable surface for the active polyamide layer, an intermediate layer of microporous polysulfone is introduced between the selective layer and the support layer with the function of protecting the ultrathin selective layer from the compression generated by high temperatures and pressures. Finally, there is the layer of nonwoven fabric with dimensions between 120 and 150 μm [92].

Overall, the different membrane types not only exhibit specific separation mechanisms and operational ranges but also offer distinct opportunities for integrating circular economy principles.

Table 3. Permeate reuse and concentrate valorization in membrane technologies.

Membrane Type	Permeate	Concentrate	Reference
Microfiltration (MF)	Rinse water or simple industrial processes.	Solids and microorganisms; composting or anaerobic digestion.	[37,95]
Ultrafiltration (UF)	Pretreated water for RO; use in food and pharmaceutical industries.	Proteins and polysaccharides; industrial or energetic valorization.	[95]
Nanofiltration (NF)	Agricultural irrigation or textile industry.	Salts, nutrients, and critical minerals (lithium, magnesium).	[22,33,92]
Reverse osmosis (RO)	Drinking, pharmaceutical, or food-grade water.	High salinity; crystallization and recovery of salts/minerals.	[22,37]

synthesizes these aspects by presenting the main applications of permeate and concentrate in (MF), (UF), (NF), and (RO), emphasizing their role in waste minimization and resource recovery.

To provide a concise overview of the physicochemical technologies discussed,

Table 4. Comparative summary of physicochemical wastewater treatment technologies within a circular economy framework.

Technology	Advantages	Disadvantages	Economic Cost	Circular Economy Opportunities
Coagulation– Flocculation	Simple operation; effective for turbidity, colloids, and phosphorus removal	Generates large volumes of sludge; requires chemical inputs	Low	Sludge reuse (soil, construction)
Adsorption	High efficiency for organics and metals; regenerable; can use waste-based adsorbents	Adsorbent exhaustion and regeneration challenges; variable selectivity	Medium-High (depending on adsorbent)	Spent adsorbents → biochar/catalysts
Chemical Precipitation	High removal of metals and nutrients; enables recovery of struvite and salts	Produces chemical sludge; requires reagent addition	Low-Medium	Nutrient (struvite) and metal recovery
Electrocoagulation	No external coagulants; low-toxicity sludge; effective for diverse contaminants	Electrode consumption and passivation; energy demand	Medium	Metal recovery; H2 valorization
Advanced Oxidation Processes (AOPs)	Degrade refractory organics; non-selective; compatible with renewable energy	High energy and chemical use; potential formation of by-products (e.g., bromates)	High	Catalyst reuse; safe water reuse
Membrane Separation (MF/UF/NF/RO)	High selectivity; produces reusable permeate; concentrate valorization possible	Membrane fouling; concentrate management required	Medium-High	Permeate reuse; salt/mineral recovery

summarizes their main advantages, disadvantages, approximate economic costs, and typical application ranges. This comparative analysis highlights not only the technical effectiveness of each process but also their potential integration within circular economy frameworks. By presenting these aspects side by side, the table complements the detailed discussion and facilitates the identification of opportunities for resource recovery and sustainable implementation.

As summarized in Table 4, each physicochemical technology presents specific strengths, limitations, and application ranges that determine its suitability for different wastewater streams. While coagulation–flocculation and chemical precipitation remain cost-effective options for conventional applications, adsorption and electrocoagulation offer greater versatility in treating complex effluents [25,29,35]. Advanced oxidation processes and membrane separation stand out for their high efficiency and potential for integration with renewable energy and resource recovery, although their costs and operational requirements can be higher [27,33,88,99,100]. Beyond their individual performance, these technologies also provide opportunities for the valorization of residual streams—such as sludge reuse, adsorbent regeneration, concentrate recovery, or hydrogen valorization—that align directly with circular economy strategies. This comparative overview not only supports the selection of appropriate technologies but also underlines the importance of integrating waste valorization pathways to maximize environmental and economic benefits. In addition to comparing technical aspects, Table 3 also highlights opportunities for integrating circular economy strategies. For each technology, examples are provided of how residual streams—such as sludges, spent adsorbents, concentrates, or catalytic materials—can be recovered and valorized [10,17,101]. This perspective underscores that beyond pollutant removal, physicochemical processes have the potential to transform waste into useful products, thereby enhancing both environmental sustainability and economic feasibility. Building on this general analysis, the following section presents the microelectronics industry as a case study to illustrate how these technologies can be implemented in a high-water-demand sector under a circular economy.

3. Circular Economy in Physicochemical Treatments of Wastewater of Microelectronics

Building on the general role of physicochemical technologies in circular water management, this section illustrates their application in a specific high-water-demand industry: microelectronics. This sector is particularly relevant due to its reliance on ultrapure water (UPW) and the generation of complex effluents rich in inorganic, organic, and metallic contaminants. Examining microelectronics wastewater treatment as a case study highlights how physicochemical processes can be integrated into circular economy strategies to achieve both regulatory compliance and resource recovery. Wastewater treatment has become a fundamental pillar across industries [102], but it is especially critical in microelectronics, where chip and semiconductor manufacturing can require up to 10 million gallons of UPW per day, according to the World Economic Forum (WEC). During processes such as cleaning, etching, photolithography, backgrinding, stripping, and dicing, large volumes of wastewater containing organic compounds, inorganics, and heavy metals are generated—among which tetramethylammonium hydroxide (TMAH) and ammonium hydroxide are particularly significant (see Table 1) [26,81,103].

To cope with the cleaning of all the pollutants, the industry must build a Wastewater Treatment Plant (WWTP) before discharging to the residual water municipal system. The cost of this system is proportional to the size of the manufacturing facilities. But no matter the sizes, there are costs and operations that have to be considered (

Table 5. Minimal costs that should be considered to implement and operate WWTP.

Pollutants
Organic	Inorganic	Heavy Metals
1-methyl-2-pyrrolidone	Ammonium	Aluminum	Silver
Acetic acid	Calcium fluoride	Arsenic	Tin
Acetone	Fluoride	Cadmium	Titanium
Ethyl lactate	Hydrogen peroxide	Chromium	Vanadium
Glycerol	Nitrates	Cobalt	Zinc
Organic solvents	Potassium hydroxide	Copper
Perfluorooctanoic acid (PFOA)	Phosphate	Iron
Phenol	Sulfates	Lead
Phosphoric acid		Manganese
Propylene glycol methyl ether acetate		Mercury
Pyrazole		Nickel
Tetramethylammonium hydroxide (TMAH)		Platinum

). In recent years, due to water scarcity, strategies as the Zero Liquid Discharge (ZLD) and the Minimal Liquid Discharge (MLD) have been developed. These three actions, WWTP, ZLD, and MLD, allow the industry to comply with demanding environmental regulations, obtain certifications of environmental social responsibility, improve their public image, and recover valuable and revalued metal salts [81,102].

The conventional treatment technologies in microelectronics wastewater involve physical, chemical, and biological processes. Inorganic, metal, and metalloids can be removed using traditional chemical processes like chemical precipitation. The inorganic pollutants have partially degraded with the traditional process, and other methods like adsorption and membranes have to be coupled (

Table 6. Principal physicochemical processes in a WWPT [106].

Costs	Activity
Inversion	Soil Studies
	Design and engineering
	Construction
	Land
	Administrative, Legal and Financial Expenses
	Replacement
	Management
Operation	Operation and maintenance	Replacement
		Reparations
		Energy
		Chemicals
		Water quality monitoring
		Operation and maintenance manpower
		Sludge disposal
	Administration	Equipment maintenance
		Administrative personal
		Overheads
		Environmental rates

) [26,104,105]. Usually, a WWTP integrates physical, chemical, and biological processes in one system so the wastewater can undergo an ultrafiltration process with membranes. The main drawback of biological methods is that the highly toxic metal could interfere with cellular growth and enzymatic activity. In microelectronics production facilities with WWTP, ZLD, or MLD systems, membrane filtration (F), microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), or reverse osmosis (RO) are the most efficient methods for removing the inorganic pollutants (more than 90%) and producing UPW [81,102]. Of particular interest is the capacitive deionization (CDI), a method that has performed better than NF in the removal of TMAH under basic conditions (Table 3) [103].

One point of concern and opportunity area is the microelectronic sludge (MES). MES is one of the main challenges of wastewater recycling in the microelectronics industry. MES contains heavy metals such as Fe³⁺ and ions as Ca²⁺, Mg²⁺, and Al²⁺ cations [81]. To cope with this issue, many investigations for MES use have been developed. Activated MES could be used for the removal of organic compounds and is able to improve the adsorption efficiency via anion exchange. Carbon-activated, metal hydroxide composite and mesoporous materials could be generated from MES. Even pulverized MES has been tested to replace Portland cement in cement mortar, showing an improvement in strength, and the examples go on. Until now, it has been difficult to define an ideal WWPT since the sizes of each facility, the type of microelectronic compound produced, and the geopolitical location where the plant operates. The Microelectronics Industry has to establish the parameters according to the pollutants generated with the goal of reaching ZLD productions, and parallel finance investigation projects to public and private universities and institutions to improve the systems and take the opportunity areas, like with MES. This implies cooperation that coexists with market competition, since in the end, this would be a win-win for everyone.

4. Conclusions and Future Perspectives

The reviewed literature demonstrates that physicochemical treatments play an essential role in the transition to circular water management, not only because of their ability to achieve high levels of purification, but also for their potential to recover and valorize resources. Established technologies such as coagulation-flocculation, chemical precipitation, and membrane separation have evolved toward hybrid and adaptive configurations that maximize efficiency and minimize waste generation. In this context, advanced oxidation processes have emerged as key tools for the removal of refractory and emerging contaminants, incorporating innovations such as the use of sustainable catalysts, integration with renewable energy, and operation in modular schemes. The reported results consistently show removal efficiencies exceeding 90% for a wide range of contaminants, accompanied by significant reductions in energy consumption and extended equipment lifespans, features that align with the resource efficiency principles of the circular economy.

In water-intensive sectors such as microelectronics, the integration of physicochemical processes—particularly chemical precipitation, advanced membrane filtration, and capacitive deionization—has proven highly effective for removing inorganic pollutants, recovering ultrapure water, and enabling Zero Liquid Discharge and Minimal Liquid Discharge operations. These approaches not only ensure compliance with stringent environmental regulations but also enhance the recovery of valuable resources, including high-value metal salts. An emerging opportunity lies in the valorization of microelectronic sludge (MES), which can be transformed into functional materials such as activated adsorbents, composite hydroxides, or construction additives, thereby aligning waste management with circular economy principles. Current trends also point toward modular and integrated systems capable of treating and valorizing multiple streams simultaneously, reducing operating costs and maximizing resource recovery. The use of recycled materials in equipment manufacturing further exemplifies the synergy between treatment technologies and circular economy objectives. In rural areas, decentralized rainwater harvesting and treatment systems, integrated with filtration and storage, have demonstrated technical and economic viability, supplying water for irrigation and domestic uses. At the technological level, further progress is anticipated in processes for the removal of emerging contaminants and in the safe reuse of nutrient-rich sludge for agricultural fertilization.

Despite these advances, several challenges remain. Technical barriers include the control of toxic by-products in advanced oxidation processes, membrane fouling, and the variability of influent quality, which affects system stability. Economic and operational limitations, such as high energy demand and the cost of reagents, continue to restrict large-scale adoption. Equally pressing are regulatory gaps that hinder the safe reuse of water, sludge, and concentrates, limiting opportunities for by-product valorization. Future progress will require the integration of advanced physicochemical technologies with renewable energy sources, nutrient recovery systems, and cost-effective strategies for valorizing sludge, exhausted adsorbents, and concentrates. Promising research directions include the development of antifouling and longer-life membranes, hybrid and modular reactors capable of treating variable effluents, and sustainable catalysts that can be regenerated over multiple cycles.

Equally important is the establishment of standardized regulatory frameworks and incentive mechanisms that facilitate the safe and large-scale adoption of circular practices, particularly in high-water-demand industries such as microelectronics. Strengthening collaboration between academia, industry, and policymakers will be essential to accelerate innovation, optimize operational costs, and expand the reuse of by-products. The convergence of technological innovation, environmental responsibility, and economic feasibility will be key to closing the water cycle in a manner that is safe, efficient, and profitable.