Structural Materials in Constructed Wetlands: Perspectives on Reinforced Concrete, Masonry, and Emerging Options

Abstract

Constructed wetlands (CWs), increasingly adopted as nature-based solutions (NBS) for wastewater treatment, require a rigorous assessment of the durability and structural performance of the materials used in their supporting systems. In contrast to the extensive literature addressing hydraulic efficiency and contaminant removal, the structural behavior of CWs has been scarcely examined, with existing studies offering only general references to reinforced concrete and masonry and lacking explicit design criteria or deterioration analyses. This study integrates evidence from real-world CW installations with a systematic review of 31 studies on the degradation of cementitious materials in analogous environmental conditions, following PRISMA 2020 guidelines, with inclusion criteria based on quantified wastewater-related exposure conditions (e.g., chemical aggressiveness, persistent saturation, and biogenic activity). Results indicate that reinforced concrete, despite its structural capacity, is susceptible to biogenic corrosion, accelerated carbonation, and sulfate–chloride attack under conditions of persistent moisture, with reported degradation rates in analogous wastewater infrastructures on the order of millimeters per year for concrete loss and tens of micrometers per year for reinforcement corrosion. Masonry structures, similarly, exhibit performance constraints when exposed to mechanical overloads and repeated wetting–drying cycles. In contrast, emerging alternatives—such as nanomodified matrices and concretes incorporating supplementary cementitious additives—demonstrate potential to enhance durability while contributing to a reduced carbon footprint, without compromising mechanical strength. These findings reinforce the need for explicit structural design criteria tailored to CW applications to improve sustainability, durability, and long-term performance.

1. Introduction

CWs have gained increasing prominence as NBS for wastewater treatment, offering a sustainable alternative to conventional technologies [1]. Beyond their well-established capacity for pollutant removal, these systems require structural components capable of withstanding hydraulic loads, substrate and vegetation weight, and the circulation of operation and maintenance personnel [2]. In tropical and rural settings, however, high humidity, saturation cycles, aggressive chemical compounds, and corrosion or carbonation processes can accelerate the deterioration of structural materials [3]. Despite these challenges, no specific design guidelines or regulatory frameworks exist for the structural elements of CWs, which limits their large-scale technical standardization and creates uncertainty regarding service life and safety.

Research on CWs has focused primarily on their hydraulic and biological performance, including flow configuration [4,5,6], macrophyte selection [7,8,9], and pollutant removal efficiency [10,11,12]. Although concrete can achieve service lives exceeding 30 years in humid environments [13,14] and masonry remains an attractive low-cost alternative for rural communities, existing CW literature has focused predominantly on hydraulic performance and pollutant removal efficiency, rather than on structural behavior. As a result, neither material has been systematically evaluated in terms of its structural performance, load-bearing capacity, or durability-driven design criteria within CWs. The literature rarely reports fundamental parameters such as design strength, reinforcement detailing, wall thickness, foundation criteria, or deterioration mechanisms under the combined hydraulic, biological, and environmental loads characteristic of these systems. This contrasts with other hydraulic infrastructures—such as sewer networks, reservoirs, and pipelines—where the degradation of concrete and masonry in aggressive environments has been widely studied, although with limited translation to the context of CWs.

In response to this gap, the present study introduces a structural and materials-focused perspective specifically applied to CWs. The objectives are to: (i) systematize available records of CWs constructed with reinforced concrete or masonry, highlighting both documented information and common omissions related to geometry, configuration, and material specifications; (ii) compile and analyze indirect evidence on the behavior of cementitious materials in CW-analogous environments through a systematic review conducted under PRISMA 2020 guidelines, with emphasis on chemical, physicochemical, and biogenic deterioration; and (iii) discuss protective strategies, material selection criteria, and emerging alternatives capable of enhancing durability and contributing to a reduced carbon footprint without compromising structural performance. The principal contribution of this work is the development of an integrated conceptual framework that bridges structural engineering, materials science, and environmental engineering, providing guidance for future experimental research, numerical validation, and the formulation of technical criteria applicable to the design and construction of CWs across diverse socio-environmental contexts.

2. Current State of Structural Knowledge in Constructed Wetlands

The structural design of CWs has received considerably less attention in the scientific literature compared with the vast body of work addressing hydraulics, pollutant removal, or vegetation performance. Most academic studies examine CWs primarily as treatment systems, focusing on flow behavior, removal efficiencies, or plant–substrate interactions. Consequently, the materials that form their supporting structures are rarely described in detail. As a result, technical knowledge regarding the mechanical performance, degradation mechanisms, and long-term durability of structural components in CWs remains limited, fragmented, and often anecdotal.

To clarify the current state of knowledge, a systematic search was conducted in Scopus, Web of Science, ScienceDirect, and Google Scholar. The search employed keywords and Boolean combinations such as “structural concrete,” “concrete,” “constructed wetlands,” “masonry,” “brick,” “partition,” and “structural system,” linked through AND/OR operators. Searches in Scopus, ScienceDirect, and Web of Science revealed studies that examined the use of concrete and masonry derived from construction and demolition waste, but only as substrate materials. Publications that addressed these materials as structural systems—considering aspects such as load-bearing capacity, reinforcement configuration, wall thickness, foundation criteria, or durability—were virtually absent.

Google Scholar provided a small number of manuscripts that briefly mentioned structural systems used in CW construction. However, even in these cases, the descriptions lacked engineering detail and did not document design considerations, degradation mechanisms, or long-term structural behavior under the combined hydraulic, biological, and environmental loads typical of CWs. This gap underscores the need for systematic engineering research to support the development of reliable structural criteria for CW implementation.

2.1. Reports on the Use of Reinforced Concrete

Reinforced concrete is the most commonly used structural material for CWs due to its high compressive strength, durability, and broad commercial availability—characteristics that allow its adaptation to vertical-flow, horizontal-flow, and surface-flow configurations [15,16]. As summarized in

Table 1. Records of wetlands built with a structural system made of concrete or masonry.

Country/Author	Wastewater Type	Scale	CW Structural System	Material-Specific Structural Characteristics	Flow Type	Substrate/Layers	HRT	Surface
Mexico/Lomeli et al. [20]	Domestic municipal wastewater	Large scale (430.49 m2)	Reinforced concrete and wetland settler with masonry	Concrete and masonry covered with cement-sand mortar; waterproof cells; walls without detailed structural specification	Mixed: VSSF, HSSF, and surface	Layer 1: boleum 5–7″ (40 cm, porosity 8%); Layer 2: 1/2″–11/2″ gravel (35% porosity); total porosity 43%	3 days	430.49 m2
Mexico/Monzón-Reyes et al. [21]	Domestic + agro-industrial (coffee)	Large Scale/Complete Treatment Plant	Masonry coated with cementitious waterproofing	It does not specify resistance; it describes construction in masonry covered with waterproof cement-based material	SF (Surface Flow) → HF (Horizontal Subsurface Flow) in series	Red volcanic gravel (3–5 cm); 60 cm base with 5 cm gravel + 3 cm top layer	4 days	C1: 15 m2, C2: 13 m2, C3: 11 m2, C4: 4 m2 (Total = 43 m2)
China/Wang et al. [22]	Slightly polluted lake water (Lake Xijiu)	Pilot	Masonry (brick)	Structure built with bricks, waterproofed with ABS plastic; total depth 1.5 m; Bottom slope 0.5%.	Subsurface Horizontal Flow (HFCW)	Gravel 8–16 mm; porosity 0.40; surface layer 20 cm	2 days	16 m3 volume—estimated area ≈ 15.2 m2
Thailand/Koottatep et al. [23]	Septic Sludge (Septage)	Pilot	Ferrocement (walls) + reinforced concrete slab (bottom)	Square unit 5 × 5 m, ferrocement walls, reinforced concrete slab, drainage system with hollow blocks and perforated PVC pipe, ventilation with vertical pipes	Vertical-flow (VF)	10 cm fine sand (1 mm) + 15 cm gravel 25 mm + 40 cm gravel 50 mm (total 65 cm)	Variable according to phase (impounding 0–12 days; weekly load)	25 m2 per unit (5 × 5 m)
India/Anand et al. [17]	Domestic wastewater (MBBR primary reactor effluent)	Pilot (two stages)	Reinforced concrete	Four rectangular chambers built with reinforced concrete walls and slabs. Dimensions of each module: 4 m × 3 m × 1.2 m. The walls have a cementitious finish and house embedded inlet and outlet pipes. Rigid and watertight system, typical of concrete tanks for French CWs.	French VF CW vertical flow with intermittent feed	First stage: gravel filter medium. Second stage: fine sand filter medium. Macrophytes: Typha latifolia and Canna indica in both stages.	24 h per stage (48 h total)	48 m2 total (24 m2 per stage)
China/Yi et al. [24]	Synthetic industrial runoff with PAHs (phenanthrene)	Pilot	Concrete-coated masonry (concrete-plastered brick walls)	Rigid rectangular structure (4 m × 2 m × 1 m) built with brick and covered with concrete; reinforced edges; confined and watertight bed	Horizontal Subsurface Flow (HSSF)	30 cm coarse gravel (20–30 mm); 30 cm fine sand (2–3 mm); 20 cm upper floor; Gravel inlet area 40–60 mm	Not explicitly reported (hydraulic load: 0.128 m3/m2/day)	8 m2
China/Deng et al. [25]	Mixed domestic-industrial	Full-scale	Combined concrete + masonry structure (rigid hydraulic cells with concrete walls and sections with confined masonry)	CW cells built with reinforced concrete perimeter walls, secondary areas with coated masonry, compacted soil base, and PVC pipe connections. Structural gravel beds of 0.8–1 m.	Hybrid System: VFCW + SFCW + HFCW	VFCW: fine and coarse gravel. SFCW: flooded soil. HFCW: 0.8 m gravel with pipes.	12–28 h	5000 m2
China/He et al. [18]	Domestic Wastewater from College Dorms	Pilot	Reinforced concrete (two independent structural tanks: IVF-CW and HSF-CW built on concrete ponds)	Rectangular concrete structures with bottom slopes (0.5% in VF and 1% in HSF); connection between downstream and upstream VF via lower holes; PVC pipe for distribution; confined and watertight beds	IVF-CW: vertical flow down + up; HSF-CW: Intermittent Horizontal Subsurface Flow	Downflow VF: 0.8 m grava (30–50 mm + 10–40 mm). Upflow VF: 0.75 m grava. HSF: 0.6 m grava (30–50 mm + 10–40 mm).	IVF-CW: 1 día; HSF-CW: 2.5 días	IVF-CW: 24 m2 (3 × 8 m); HSF-CW: 24 m2 (3 × 8 m)
United States/Bedessem et al. [19]	Groundwater contaminated with petroleum derivatives (BTEX, TPH-DRO, MTBE)	Pilot (4 cells)	Reinforced concrete (cells built within an existing concrete structure, with new concrete partition walls)	Structure: concrete floor 7.9 × 7.9 m; 4 cells of 7.0 × 1.7 × 1.1 m; new concrete walls; insulation with solid foam; perforated PVC pipes for flow distribution and subsurface aeration.	Vertical upflow (main); possibility of horizontal operation; some cells with subsurface aeration	Layers per cell: 0–15 cm wet soil (A and B only); 15–71 cm washed sand; 61–71 cm pea gravel (C and D); 71–86 cm coarse gravel 12.5 mm; geotextile; sand around pipes (86–92 cm)	0.61–0.76 days (depending on cell)	Usable area per cell: 11.9 m2; Total structure: 7.9 × 7.9 m

, its use is predominant in large-scale facilities and pilot systems designed to withstand substantial hydraulic pressures and long-term operational conditions, as documented by Anand et al. [17], He et al. [18], and Bedessem et al. [19].

However, the environmental conditions characteristic of CWs—permanent moisture, the presence of organic matter, and exposure to chemically aggressive compounds—pose significant durability challenges for reinforced concrete structures. The literature consistently reports deterioration processes such as steel reinforcement corrosion, microcracking, and accelerated carbonation, all of which may compromise structural integrity in the absence of waterproofing systems or protective admixtures [26,27]. Despite this, none of the studies summarized in Table 1 provide quantitative assessments of these deterioration mechanisms under real operating conditions.

Consequently, although reinforced concrete is the best-documented structural alternative for CWs (Figure 1), its specific mechanical and durability performance under hydrobio logical environments has not yet been systematically evaluated. This gap limits the development of evidence-based design criteria that ensure long-term structural safety and sustainability.

2.2. Reports on the Use of Masonry

Masonry is frequently used in CWs due to its low cost, local availability, and ease of construction—attributes that make it a practical option for rural and community-based projects [28,29]. As shown in Table 1, brick and concrete block systems are commonly implemented in small- to medium-scale CWs and, in some cases, combined with reinforced concrete elements, as reported in the hybrid configurations described by Deng et al. [25], Lomelí et al. [20], and Monzón-Reyes et al. [21].

Despite its widespread application (Figure 2), the structural information available in the literature is scarce. Most studies do not specify key parameters such as wall compressive strength, mortar type, degree of confinement, or the waterproofing systems employed. Existing reports suggest that masonry performs adequately in structures less than approximately 1.5 m in height and under moderate hydraulic pressures; however, it is more vulnerable to concentrated loads, bending stresses, and differential settlements—particularly when foundations are insufficiently designed. Its inherent porosity and the frequent absence of surface sealing increase the risk of mortar degradation and leakage under conditions of continuous saturation.

None of the reviewed studies provides a systematic evaluation of masonry deterioration under wetting–drying cycles or its stability during long-term exposure to wastewater. This gap highlights the need for targeted research on the mechanical behavior and durability of masonry in CW environments, particularly in settings where it continues to be a preferred construction alternative.

2.3. Limitations of the Available Literature

The literature review indicates significant gaps in the structural understanding of CWs. Although CWs share certain functional characteristics with other hydraulic infrastructures, their mechanical behavior is considerably more complex due to the combined action of static, hydraulic, biological, and time-dependent loads, all of which evolve as vegetation grows and the system matures. This inherent complexity is not adequately reflected in the existing scientific evidence.

As shown in Table 1, there is an almost complete absence of studies that examine the structural performance of CWs from an engineering standpoint. Most publications simply describe the construction materials used—typically reinforced concrete or masonry—without evaluating their mechanical behavior under operational conditions, such as lateral hydraulic pressures, saturated media loads, vegetation-induced forces, or concentrated loads associated with routine maintenance activities [18,25]. The literature also seldom reports internal stresses, deformations, differential settlements, or long-term degradation mechanisms.

A major limitation is the lack of analytical, numerical, or experimental models that characterize the response of walls, slabs, and foundations under the combined effects of saturated granular media, wastewater flow, vegetation growth, and environmental exposure. For example, hydraulic loads generated by saturated substrates—commonly composed of gravel, sand, or lightweight aggregates such as pumice—can induce significant lateral pressures and permanent loads, especially in horizontal subsurface-flow systems [30,31,32]. Although these loads have been studied in other hydraulic structures, the methodologies have not been formally transferred to CW design, resulting in a clear methodological gap.

In reinforced concrete systems, such loads may produce excessive bending, buckling, or deformation when reinforcement detailing or wall-to-foundation continuity is insufficient [33,34]. Masonry structures are even more vulnerable to lateral pressures that can cause cracking along mortar joints, differential settlements, or loss of confinement [35,36]. Despite these well-documented mechanisms, the magnitude, spatial distribution, and temporal evolution of hydraulic loads in full-scale CWs remain unquantified, preventing the definition of reliable safety factors or optimized structural cross-sections.

Similarly, loads associated with vegetation and maintenance activities have received minimal attention. Macrophyte species commonly used in CWs—such as Canna indica, Cyperus papyrus, or Colocasia esculenta—often exceed 2 m in height [37] and develop dense root systems capable of exerting tensile forces on the substrate [38]. Under strong winds, high saturation, or excessive biomass accumulation, vegetation-related forces may increase stresses on unrestrained edges, walkways, and thin slabs. In addition, maintenance personnel generate concentrated loads that can exceed 150 kg/m² on access platforms and pedestrian paths (Figure 3); however, these loads are rarely considered in existing CW design practices. These omissions raise concerns about the structural adequacy of walkways, inspection platforms, and thin concrete elements frequently used in operational systems.

Environmental exposure is an additional source of concern. CWs operate in conditions of persistent humidity, partial or total saturation, and prolonged contact with chemically aggressive compounds present in wastewater [39,40,41]. In reinforced concrete, these environments promote carbonation, leaching, steel corrosion, and loss of watertightness [42,43]. Masonry systems face risks associated with efflorescence, moisture absorption, mortar deterioration, and loss of bond between units, especially in the absence of adequate waterproofing [44,45]. Despite the clear relevance of these degradation mechanisms, the literature does not offer a systematic quantification of deterioration rates, reduction in useful life, or structural implications under real operating conditions.

Taken together, these limitations highlight a widespread lack of structural analysis in the investigation of concrete water systems (CWs). Hydraulic forces generated by saturated media, biological loads imposed by macrophyte growth, concentrated loads associated with maintenance activities, and harsh environmental conditions that accelerate material degradation remain largely undocumented, both individually and in their combined effects. This lack of quantitative evidence hinders the development of predictive models, reliable reinforcement strategies, and durability-oriented design criteria.

Therefore, there is an urgent need to generate coordinated, interdisciplinary research lines that formally integrate civil engineering, materials science, and environmental engineering. Such an agenda should include the experimental characterization of actual degradation mechanisms in CW environments; the development of structural models that capture coupled hydromechanical behavior; the optimization of wall, slab, and foundation systems exposed to saturated, anoxic, or chemically aggressive conditions; and the evaluation of sustainable building materials capable of maintaining their structural performance in these demanding environments.

Adapting well-established tank structures, anaerobic digesters, and hydraulic containment structures can serve as a solid starting point for developing a comprehensive structural understanding of CWs—an understanding that is entirely absent from the current scientific literature.

3. Indirect Evidence of the Behavior of Concrete in Environments Similar to Built Wetlands

Although CWs are hydraulic systems with unique dynamics of transport, biogeochemistry, and contaminant retention, their interaction with cementitious materials exhibits chemical, physical, and microbiological similarities to other environments where concrete has been extensively studied. Wastewater conditions (both real and simulated), prolonged anaerobic exposure, microbially induced sulfuric acid generation, and the presence of aggressive ions, such as sulfates, chlorides, ammonium, and nitrogenous compounds, provide analogous scenarios useful for understanding the behavior of concrete in CWs.

To gather this evidence, a systematic search was conducted in major scientific databases, primarily Scopus and Web of Science, following the PRISMA 2020 guidelines for systematic reviews. The search strategy was structured using Boolean combinations of keywords and operators: (concrete OR masonry) AND (“wastewater” OR “sewage” OR “anaerobic conditions”) AND (deterioration OR corrosion OR degradation).

The initial database search yielded 78 records across Scopus and Web of Science. For the purposes of this review, studies were included if they met at least one of the following exposure-relevance conditions: (i) continuous or intermittent contact with wastewater, sewage effluents, or wastewater-derived media (including sewer liquids, vapors, brines, or process waters); (ii) documented chemically aggressive conditions, such as pH values below 6.5 or above 9.0, or exposure to acidic/alkaline solutions representative of wastewater-related deterioration; (iii) documented presence or explicit consideration of aggressive species and by-products commonly associated with wastewater environments, such as sulfates, chlorides, ammonium, sulfides/H₂S, dissolved CO₂, sulfuric acid generated by microbial activity, or organic acids; (iv) prolonged saturated or near-saturated exposure conditions (field or laboratory) lasting several months, including long-term monitoring campaigns; and/or (v) technical/industrial or field-assessment studies reporting deterioration mechanisms, failure modes, protection system performance, or maintenance-related factors in wastewater infrastructures (e.g., sewers, tanks, pumping stations, tunnels, or pipes) that are structurally and environmentally analogous to CW supporting systems.

Studies focused exclusively on dry environments, potable water systems without aggressive exposure, or short-term laboratory tests without chemically or microbiologically aggressive conditions were excluded.

This approach enabled the identification of literature addressing the degradation of concrete and masonry in contexts dominated by wastewater, microbial corrosion, prolonged exposure to sulfides and sulfates, and persistent anaerobic conditions—scenarios that analogously reflect the hydrochemical functioning of CWs.

In this context, “environments similar to constructed wetlands” are defined as those characterized by persistent moisture or saturation, limited oxygen availability (anaerobic or sub-oxic conditions), and the presence of chemically aggressive compounds commonly reported in CWs, such as sulfates, chlorides, dissolved CO₂, ammonium, and organic matter. While hydraulic residence times and biological dynamics may differ, the chemical and physicochemical exposure conditions governing concrete and masonry deterioration are considered comparable and transferable as a first-order approximation.

The process of identification, screening, eligibility assessment, and final inclusion of studies was structured according to the PRISMA 2020 flowchart, shown in Figure 4, generated using the PRISMA2020 R package and the Shiny application developed by Haddaway et al. [46]. In general, exclusion criteria included the removal of duplicates, studies lacking a direct focus on the behavior of concrete or masonry in contaminated environments, works focused exclusively on theoretical modeling without material validation, and articles whose results were too nonspecific or inconclusive regarding deterioration mechanisms.

After this process of purification and critical evaluation, a total of 31 studies were ultimately included. These works empirically and analytically document the behavior of cementitious materials under exposure conditions highly comparable to those that characterize constructed wetlands, particularly in terms of chemical aggressiveness, microbial activity, and persistent moisture.

After this process of purification and critical evaluation, a total of 31 studies were ultimately included. These works empirically and analytically document the behavior of cementitious materials under conditions highly comparable to those that characterize constructed wetlands, particularly in terms of chemical, microbiological, and physicochemical aggressiveness.

Table 2. Studies on the deterioration of concrete or masonry in environments similar to built wetlands.

Author	Polluting Environment Evaluated	Material Studied	Mechanisms of Deterioration Analyzed	Methods/Tests/Models Used	Main Findings	Relevance to CWs
Rahane et al. [47]	Spent wash: pH < 4, high BOD, organic compounds, nutrients, and sulfates	Conventional Concrete and Geopolymer Concrete	Risk of corrosion, sulfate attack, alteration of setting, and mechanical properties	Tests for consistency, setting, slump, compressive strength (IS 10262–2009; IS 1489–2015)	Replacing up to 15% mixing water with spent wash increases resistance by 20%. At >20%, workability and resistance decrease (−6%). Direct effects on plastic and hardened properties	Demonstrate how acidic wastewater alters the mechanical properties of concrete; relevant for CWs exposed to industrial effluents, high organic load, or sulfates.
Bhutto et al. [48]	Drinking water and sewer systems, prolonged exposure to sewage, constant humidity, corrosion in pipes and structures	Reinforced concrete, steel, ductile iron, asbestos cement	Degradation of infrastructure due to aging, humidity, corrosion; Accelerated deterioration due to operating conditions	Field study and survey of 12 industry stakeholders; Economic and constructive analysis	The rising costs of materials (cement, steel) and their deterioration cause delays, cost overruns, and reduced structural performance. Emphasizes the need for proper material management and selection	Indirect evidence of the behavior of structures exposed to wastewater and CWs; identifies vulnerability of concrete and metals to corrosion; relevant to CWs with infrastructure exposed to conditions
Teppan [49]	Wastewater infrastructure, chronic exposure to wet media, and corrosive chemicals	Reinforced concrete with different protective coating systems (liners)	Premature deterioration due to lack of protection: chemical attack, moisture, internal corrosion, coating failures	Technical-industrial review; chemical and mechanical analysis of protective products; Evaluation of installation and design protocols	Most premature deterioration does not come from the concrete itself, but from poorly installed coatings. Implementing comprehensive protocols and advanced coatings allows for extended service life up to 100 years	Relevant for confirming that CWs require adequate protective coatings to prevent chemical degradation; demonstrates effective strategies for corrosive environments similar to CWs
Alzgool et al. [50]	Exposure to brine wastewater with varying concentrations (2.5–15%)	Reinforced concrete (beams with steel in tension and compression)	Initial corrosion of steel, salinity effects on flexural and compressive strength	Experimental tests on 72 beams; immersion in brine; tests at 7, 14, 21, and 28 days; Mechanical evaluation (compression, bending, shear)	Substituting 10% drinking water with brackish water improves strength (22% compression, 2.6% bending). No significant corrosion damage observed in 65 days	It indicates that waters with moderate salinity do not deteriorate the concrete quickly; it provides evidence on initial material performance under chemically aggressive conditions, comparable to CWs with salt loads
Kashaija et al. [51]	Exposure to sewage vapors and sewage liquids in pumping stations and sand traps	Cementitious materials (mortar/cement paste)	Formation of secondary minerals (gypsum, ettringite), surface degradation, morphological and mineralogical changes	On-site exposure for 1–7 months; stereoscopic microscopy; SEM; XRD	Wastewater vapors generate greater deterioration than liquid wastewater; intense formation of mineral deposits; accelerated damage in aggressive wet and gaseous environments	High relevance: CWs have areas with humid atmospheres rich in gases. Thus, this study helps to understand deterioration by vapors, not only by immersion.
Pramanik et al. [52]	Sewerage environments, presence of H2S, acidification by H2SO4, vapors and residual liquids	Concrete in sanitary infrastructure	H2S → H2SO4 conversion, microbial biocorrosion, chemical degradation, resistance reduction, cementitious matrix dissolution	Systematic review; chemical tests; on-site testing; microbial simulations; Comparative analysis of mitigation techniques	Biocorrosion is a highly destructive mechanism that drastically reduces the useful life; H2S is the critical precursor; special coatings and optimized mixtures are effective in mitigating damage. There is a lack of standardization of methods.	Highly relevant: CWs also produce H2S, SO42−, CO2, and biofilm. This article provides the basis for anti-corrosion strategies and material improvement in CWs.
Ibrahim et al. [53]	Aggressive exposure to 5% H2SO4, full immersion, wet-dry cycles in acidic medium	Surface-treated concrete with vinyl ester, vinyl ester + nanocomposites, silane, and colloidal silica coatings	Sulfuric acid attack, mass loss, surface degradation, deterioration of the cementitious matrix, and initial permeability	Acid immersion tests, wet-dry cycles, mass loss measurement, microstructural, mineralogical, and thermal analysis	Vinyl ester coatings and their nanocomposites improved durability by ~64% compared to silane; reduce acid penetration and deterioration; colloidal silica improves performance; Concretes with W/B = 0.60 require more protection.	Very relevant: CWs operate in environments with sulfates, organic acids, CO2, and H2S. This study provides surface protection strategies applicable to walls and slabs in contact with acid effluents.
Xu et al. [54]	Prolonged exposure (36 months) to H2SO4 simulating sewer environments; Acid Biogenic Attack	Steel Fiber Reinforced Concrete (SFRC)	Acid corrosion, loss of alkalinity, gypsum formation, progressive deterioration of surface layers, and changes in dynamic modulus	Mechanical testing (cubic compression, axial loading, and bending), morphological monitoring, mineralogical analysis, pH, and damage depth measurements	Corrosion begins after 12 months; at 36 months, the damaged and transition layers reach ~2 mm and ~8 mm; gypsum formation; slight decrease in compressive strength, but improvement in axial loads and flexural strength; Noticeable increase in toughness.	SFRC shows improved resilience under acidic environments similar to those generated in CWs; it demonstrates potential for walls and slabs exposed to anaerobic wastewater and biogenic conditions
Deldar et al. [55]	Concrete mixed and/or cured with treated wastewater (TWW) with high chloride content; Accelerated corrosion environment	Conventional concrete subjected to TWW in mixing and curing	Chloride-induced corrosion, decreased mechanical strength, early cracking, loss of steel mass	Mechanical Testing, Porosity, Absorption, UPV, pH, Electrochemical Potentials, Voltage-Accelerated Corrosion	Mechanical strength decreased <10%; after 6 months, he recovered almost to the level of control. However, corrosion was greater (23–25%), with early cracking and greater mass loss in the steel. The use of TWW in curing had minimal additional impact	Demonstrates that treated wastewater, although acceptable for mixing or curing, increases significantly
Tan and Zhang [56]	Prolonged exposure (180 days) to sewage, oxalic acid, and seawater environments	Nano-TiO2 Modified Coral Concrete	Chemical erosion, dissolution of Ca(OH)2, formation of expansive compounds (Mg(OH)2), loss of mass, reduction in resistance	Tests for compressive and bending strength, mass loss, corrosion coefficient, XRD, and SEM	Nano-TiO2 (4%) improved compression (+22%), flexural (+33%), and erosion resistance in a wastewater environment. Aggressive environments (oxalic acid, MgCl2) reduce resistance and increase mass loss. Nano-TiO2 densifies the matrix, but certain media generate expansive products.	Relevant: demonstrates that nano-modified admixtures can improve the durability of concrete in contact with wastewater, and evidences the adverse effects of acidic and saline environments similar to conditions in CWs of industrial effluents.
Fang and Achal [57]	Aerobic and anaerobic environments, high salinity (0–5%), pH variation (7.5–11.5), nitrate waters	Q235 Carbon Steel + Calcium Carbonate Microbial Bioprecipitation	Non-ureolytic MICP (via denitrification), corrosion inhibition, protective CaCO3 precipitation	Tapel electrochemical assays, pH variation, Ca2+ concentrations, precipitation tests, and microbial analysis	The bacterium Stutzerimonas stutzeri CF3 generates CaCO3 even in aggressive environments, improves the corrosion resistance of steel, and has high adaptation to extreme conditions (variation in pH, salinity, and nitrates). High effectiveness in anaerobic and saline environments.	It demonstrates that microbial processes can protect materials in humid, anoxic, and high-salinity environments, very similar to the internal conditions of CWs. It supports the development of self-protective and self-healing concretes.
Rebai et al. [58]	Domestic wastewater, seawater (high salinity), acid solutions (sulfates and chlorides)	Conventional concrete and nano-modified concrete with additives (PCE superplasticizers, Vinsol resin, gluconate)	Deterioration by sulfation and chloruriation, microcracking, dissolution of portlandite, formation of ettringite; Pore refinement protection mechanisms, electronic barriers, controlled aeration, and optimized hydration kinetics	ASTM/EN Durability Testing, SEM-EDS, XRD, Mercury Porosimetry	Conventional concrete loses 25% strength in 56 days in seawater; The nano-modified concrete reduces loss by 15% and improves durability by 40%. Reduces pore size (<50 nm), inhibits ettringite, and has low absorption. Nano-modifier additives block ion pathways and produce protective micro-bubbles	Mentions how cementitious nanotechnology can improve durability in sulfated, chlorinated, and acidic environments, similar to those that occur in wet and anaerobic CW zones. Useful for proposing reinforced concretes for distribution chambers, canals, and wetland structures exposed to aggressive wastewater.
Alazzawi et al. [59]	Real and simulated wastewater (sulfates, chlorides, phosphates, H2S, ammonium)	Portland concrete with and without epoxy resins (2–6% of cement)	Deterioration by chemical attack, ionic penetration, gasification (H2S), susceptibility to corrosion; Mechanisms of protection by epoxy coatings	Immersions in real and simulated wastewater; chemical analysis of water; Permeability, Penetration, and Compressive Strength Tests	The 4% epoxy resin reduced the loss of compressive strength (~19.55% in real wastewater), representing the most stable mixture. In simulated wastewater, resistance increased by 11% with 2% resin. Water absorption decreased by 0.96% with 6% resin. H2S and salts increase chemical damage in concrete without additives	The anaerobic environments of CWs can generate H2S and similar salts. Hence, epoxy resins appear as an option for distribution chambers, lids, walls, and areas of prolonged contact with wastewater.
Chu et al. [60]	Microbial Induced Corrosion (MICC) in Wastewater Transport Systems	Hardened cement paste coated with copper by chemical electrodeposition	Protection against MICC by means of metal coatings (Cu/CuO); effect of pH and CuSO4 concentration on compaction, hardness, and bonded mass	Chemical plating without electricity; variation in CuSO4 concentration (0–10 g/L) and pH (7–11); Vickers hardness tests, SEM microscopy, antibacterial analysis	CuSO4 = 10 g/L produced the densest coating (0.3% mass gain) and hardness of 212 HV. pH = 9 generated the highest coating quality. The coated surfaces showed great density, compaction, and improved antibacterial properties, reducing the risk of MICC	CWs can generate environments conducive to MICC in anaerobic or biofilm zones. Cu coatings could be an alternative for elements submerged or exposed to aggressive biofilm.
Mendizabal et al. [61]	Environments with high sulfur (H2S) loads; WWTP entrance chambers; Pressurized sewer system	Sulfide-exposed concrete infrastructure in WWTP inlet chambers	Accelerated corrosion due to generation and accumulation of H2S; identification of sources; Predictive Modeling for Corrosion Management	Sulfide sensors (data every 5 min); ANN–LSTM for H2S prediction; analysis of sulfur loads (kg/day); H2S–flow correlation	RM1 had higher sulfide loads (3.6–4.2 kg/day). H2S showed an inverse pattern with the flow. The LSTM model predicted H2S concentrations with RMSE = 0.34 and NSE = 0.57. The approach enables early warning and early control systems to reduce corrosion	CWs have anaerobic zones where H2S can be generated. Sulfide prediction allows anticipating periods of greater aggressiveness and planning maintenance and selection of materials in CWs
Silva Martínez et al. [62]	Treated domestic wastewater; increasing concentrations (0–100%); presence of phosphates, nitrates, nitrites, sulfates, and chlorides	Recycled steel fiber reinforced mortars (tire wire) and commercial 4D fibers	Fiber corrosion; loss of adhesion; chemical changes in surface sediments; degradation by aggressive anions	Pull-out tests, chemical sediment analysis, two-way ANOVA, repeated ANOVA, t-tests	The best mechanical performance was with 50% treated wastewater + recycled fibers (A50-1). The greatest degradation was with 100% wastewater + 4D fiber (B100-2). Corrosion was 4.13% in A50-1 and 1.53% in B100-2. Aggressiveness is associated with phosphates, nitrates, nitrites, sulfates, and chlorides.	It shows that the chemical composition of the treated influent can significantly modify the durability, especially due to the content of aggressive anions also present in CWs. It provides evidence on fiber-wastewater interaction in cementitious matrices.
Słomka-Słupik [63]	Industrial coking wastewater; solution of NH4Cl (ammonium + chlorides), highly aggressive compounds	Hardened Cementitious Paste/Concrete in Wastewater Tanks	Dissolution of calcium phases (Ca(OH)2, C-S-H), formation of corrosion products: gypsum, secondary calcite, secondary ethringite, thaumasasite; Corrosion Front Advance	Accelerated immersion in NH4Cl; mineralogical analysis (identification of phases); Temporary observation of the advance of the corrosive front	NH4+ and Cl− generate strong chemical degradation. The corrosion front advances over time and completely transforms the microstructure. Expansive minerals are formed that disintegrate the surface area. A model is proposed to estimate the destruction of tanks exposed to solutions with ammonium and chlorides	Ammonium and chloride ions are also present in sewage and anaerobic effluents, characteristic of CWs. Provides evidence on accelerated dissolution of concrete under environments rich in ammonium salts, common conditions in septic areas and wetland loading chambers
Capuozzo et al. [64]	Wastewater with H2S, SOB microorganisms, and sulfuric acid generated by biological oxidation	Concrete in sewage pipes; Concrete–Biofilm Interaction	Microbial Influenced Corrosion (MIC), SOB Biofilm Growth, Gypsum Formation, Sulfuric Acid Penetration, Corrosion-Free Double Front	1D mathematical model of diffusion-reaction; two free borders (biofilm and gypsum layer); Stefan-type equations; Numerical simulations	SOB growth oxidizes H2S and generates H2SO4 that moves into the concrete. The plaster layer progresses over time. The model identifies key factors: CaCO3 concentration, acid diffusion, biofilm dynamics, and H2S levels. Under specific conditions, biofilms can act as a protective partial barrier, limiting acid diffusion	CWs can generate anaerobic environments where H2S, a precursor of biogenic corrosion, is formed. This article provides tools to model MIC in infrastructure exposed to H2S effluents and is applicable to chambers, pipes, tanks, and septic areas associated with CWs
Kong et al. [65]	Typical sequential environments of wastewater pipes: abiotic neutralization, neutral conditions, and acidophilic biological attack	Mortars: OPC (Portland), CAC (calcium aluminate), and AAM (alkaline-activated fly ash)	Acid-base neutralization, biogenic attack (acidophilic bacteria), mass loss, loss of strength, biofilm formation, staged corrosion progression	Simulated Stage Test (3 Phases), Mass Loss/Strength Measurement, Visual Evaluation, Neutralization Depth Analysis, 2-Stage Linear Model (Start + Progression)	Alkaline-activated mortars (AAMs) showed better performance: less loss of mass and strength, less adhesion of biofilms, and a more integral appearance. However, there is a greater neutralization depth compared to OPC. Estimated shelf life: AAM ≈ 2.5 times greater than OPC, and higher than CAC	It suggests that alternatives to Portland concrete (such as AAM) can increase durability in wet, anaerobic, or biogenically attacked environments, which are very common in CWs; Applicable for internal structures, pre-treatments, chambers, and areas with biomass accumulation.
Kashaija et al. [66]	Wastewater treatment plant; real exposure in sand-trap (liquids + biofilms); Exposure times 10–240 days	Concretos: Portland (PC) y calcium sulfoaluminate cement (CSAC)	Microbial biodeterioration, biofilm-cement interaction, ettringite and calcite formation, mass loss, mineralogical changes	In situ; prolonged exposure; microbial analysis (16S rRNA of bacteria and archaea); mineralogical and geochemical analysis; Statistical correlation	CP showed progressive deterioration, correlated with the abundance of acidogenic and sulfate-reducing bacteria (>50% of the community at 75 days). CSAC showed remarkable resistance, with very low biodeterioration. Dominance of Gammaproteobacteria, Bacilli, Clostridia, Bacteroidia, Campylobacteria, Actinobacteria, and Desulfovibrionia	Evidence that certain alternative cements (CSACs) can resist wet, anaerobic, and acidic environments similar to those of CWs. It suggests that PC is vulnerable to biodegradation and that alternative compositions could improve wetland structures.
Abbas et al. [67]	Wastewater transport systems; biogenic attacks on buried pipes; environments with H2S and biogenic sulfuric acid	Precast concrete pipes	Biogenic attack of H2S → H2SO4; internal erosion; degradation by soil-pipe interaction; structural failures by design and manufacture	Systematic review of scientific literature, technical reports, standards, documented failures; tabular analysis and graph of critical factors; Comparison of design methods	Manufacturing is a critical factor in performance. Common failures include cracking, internal erosion, and attack by biogenic sulfuric acid. Direct design outperforms traditional indirect design. More robust quality assessments and long-term monitoring are required to understand deterioration in buried pipes.	Biogenic attack on wastewater pipes is a global problem, and the mechanisms identified (H2S, sulfuric acid, internal erosion) are analogous to the risks in CW structures, especially inlet chambers, sedimentators, and channels.
Tarnowski et al. [68]	Sanitary sewer systems with high hydraulic retention; anaerobic environments producing H2S; Manholes and Expansion Chambers	Concrete Pits in Sanitary Sewer Systems	H2S formation → oxidation by SOB → sulfuric acid → sulfated corrosion of concrete	Continuous monitoring of H2S with dual sensors (base and top of the well); analysis of the register of pumping stations logs; Structural Impairment Assessment	Severe degradation observed in just 5 years: from surface film to concrete losses of several centimeters. Within 11 years, some wells were at risk of collapse. The condition of the well did not depend on the maximum peak of H2S, but on the total cumulative exposure time. Measured values exceeded the sensor range (>500 ppm). The system’s hydraulic retention raised H2S emissions.	The presence of anaerobic zones + prolonged retention periods also occurs in deep or poorly ventilated CWs. Cumulative corrosion by H2S can compromise feed chambers, settlers, and maintenance records in CWs. This indicates the need for gas control and monitoring.
Aliyev et al. [69]	Untreated urban networks; uncontrolled industrial wastewater; environments with corrosive gases (H2S, CO2, NH3), humidity and flow blockage	Reinforced concrete collectors and pipes in urban systems	Chemical and biogenic corrosion by gases; surface degradation of concrete; loss of coating due to acid attack; effect of ventilation on gas concentration; Impact of untreated industrial dumping	Field analysis in deteriorated networks; identification of failure modes; review of coating and ventilation technologies; Crash Evaluation	The absence of treatment and the discharge of industrial effluents generate severe corrosion and blockage of collectors. Surface degradation is reported due to the action of corrosive gases and environmental contamination. Durability can be improved through coatings (mastics, paints, liners), material enhancement, and natural or forced ventilation to dilute gases such as H2S.	CWs that receive industrial or non-pretreated effluents may exhibit accelerated degradation in concrete chambers. It reinforces the need for ventilation and protective coatings for inlet structures and pipes connected to wetlands.
de Andrade et al. [70]	Sewerage tunnel with a sulfate-rich environment; permanent humidity; corrosive gases typical of the sanitary system	Shotcrete used as a lining in tunnels	Secondary ettringite formation in air vacuums; sulfate penetration; increased water absorption; differential degradation between primary and secondary layers; weak areas from trapped pores	Video inspection; core extraction; physical-mechanical analysis; chemical, mineralogical, and microstructural characterization; Degraded Depth Analysis	A sulfate-rich environment generated ettringite formation up to 8 mm in the shotcrete, especially in trapped pores. High water absorption and reduced resistance were detected at the interface between the two layers of shotcrete, identified as a critical point of failure. The mechanical tests showed high dispersion, but the microstructural characterization allowed for measuring the degradation depth with high precision. The study underlines the difficulty of evaluating concrete thrown when there is no complete information on the construction process.	Relevant when CWs use shotcrete structures (chambers, shafts, tunnels). He points out that shotcrete is vulnerable to sulfates, especially if there is trapped air. It reinforces the importance of porosity and layer control during construction to avoid weak points in wet and sulfated areas of wetlands.
Baas and Escalante [71]	Drinking water and wastewater conduction systems; aggressive environments (sulfates, chlorides, CO2, H2S)	Pressurized Concrete Pipe (CPP)	Chemical corrosion (acid attack, sulfates, chlorides); biogenic corrosion; inherent mechanisms for the protection of concrete; supplementary methods (coatings, liners, inhibitors, cathodic protection, internal barriers)	Technical review; compilation of industrial practices; Comparative analysis between protection methods	Pressurized concrete pipe possesses inherent corrosion resistance properties due to its alkalinity and mortar coating. However, in extremely aggressive environments, additional systems are required, such as internal coatings, polymeric liners, cathodic protection, and reinforced structural design. More than 80 years of use show its durability when proper controls are implemented. The study summarizes criteria for the selection of protection systems depending on the level of aggressiveness of the wastewater or drinking water.	It provides relevant criteria for selecting coatings and protection methods for hydraulic structures in CWs exposed to sulfates or H2S. Useful for inlet chambers, distribution tanks, and pipes connected to wetlands at risk of corrosion. It reinforces the importance of design and coatings in wet/aggressive environments typical of CWs.
Lin et al. [72]	Hydrogen sulfide (H2S); typical sewer environment; Mechanical loads (compression + bending moment)	Sewer concrete pipes	Biogenic corrosion by H2S; mechanical degradation due to loads; load-corrosion coupling; Loss of stamina	Development of an experimental apparatus to simultaneously apply loads and H2S; 180 samples tested; compressive and flexural strength tests; H2S vs. H2S + loading Comparison	Deterioration due to the coupling of loads + H2S increases the loss of strength by more than 10% compared to corrosion alone. The effect increases with longer exposure time, higher load, and higher concentration of H2S. If the charge is doubled, the mating effect increases by 1.6 times. Structural loads accelerate chemical degradation. It offers criteria to predict the useful life of pipes.	Fundamental for CWs because it demonstrates that structures subjected to loads (soil weight, water columns, pulsating flow, settlements) suffer accelerated deterioration when exposed to H2S from the pretreatment or anaerobic chambers. Relevant to the design of distribution boxes, wells, settlers, and concrete structures in CWs.
Zhang et al. [73]	Complex acidic, alkaline, and ionic environments; prolonged immersion in industrial process wastewater; presence of aggressive ions (Cl−, SO42−, etc.)	Concrete and steel in reaction tanks; GO/SiO2 coatings	Ionic corrosion; detachment of concrete; corrosion of steel by aggressive ions; protection by graphene oxide nano-coatings + SiO2	Preparation of GO/SiO2 coatings; accelerated immersion tests; permeability measurement; ion leaching analysis; Tests under “Extreme Corrosion Environments”	Uncoated samples showed high ionic leaching, decreased permeability, and significant degradation of concrete. GO/SiO2 coatings showed high stability, reduced leaching, increased resistance to acidic/alkaline environments, and improved performance as GO content increased. GO increases resistance to ionic penetration and chemical stability.	Very relevant for CWs: it shows that nanocoatings based on oxygen–silicon and graphene can protect structures exposed to aggressive wastewater. It provides innovative alternatives to protect sedimentators, distribution chambers, inlet boards, pretreatments, and contact tanks where there is high chemical corrosion.
Godinho et al. [74]	Environments with biogenic sulfuric acid (H2SO4 generated by sulfur-oxidizing bacteria); controlled chemical conditions + real environment in treatment plant; Continuous exposure 950 days	Reinforced Concrete with Supplemental Cementitious Additions (SCM): Fly Ash (FA), Commercial Crystallizing Additives, Sodium Silicate Solution	Biogenic acid attack; dissolution of portlandite; plaster formation; progression of the neutralization front; mechanisms of “chemical buffering”; Phase stabilization (hemicarbonate, hydrotalcite) that increases resistance	Compressive strength tests; loss of mass; neutralization depth; mapping of surface degradation by images; SEM–EDX; XRD; thermodynamic modeling GEMS/Cemdata18; Simultaneous laboratory-field exposure for 950 days	Fly ash significantly reduced degradation and mass loss under chemical and biogenic attack. The crystallizing additives showed little improvement under chemical conditions, but they did increase resistance in real biogenic environments. Thermodynamic modeling explained that durability is improved by (1) depleting portlandite, (2) saturating gypsum, and (3) stabilizing hemicarbonate/hydrotalcite. A unique connection was generated between laboratory degradation and the real field.	CWs generate biogenic environments with sulfates, sulfides, low pH, and S-oxidizing microorganisms. The findings demonstrate that: FA and SCM reduce acid damage, crystallizing additives work better under real biological conditions, and stable phase formation (hydrotalcite) increases shelf life. Provides guidelines for selecting strong cements in CWs.
Ahmed et al. [75]	Domestic, treated municipal, industrial wastewater and its chemical components: TDS, suspended solids, BOD, COD, chlorides, sulfates	Concrete and mortars made from wastewater (WW) as a partial or total replacement for drinking water	Variation in workability and strength; chemical effects (TDS, chlorides, sulfates); increased porosity; risk of corrosion of the reinforcement; reduction in C–S–H formation; Increased micro-cracks	Bibliometric review (Bibliometrix/Biblioshiny); systematic analysis of 91 studies; SEM for microstructure; Collection of mechanical results (strength, porosity, absorption, chloride diffusion)	45% of studies report improvements in endurance when using WW; 35% moderate increases; 20% unchanged. However, greater replacement with WW increases porosity, absorption, and diffusion of chlorides, affecting durability. The microstructure showed lower C–S–H and more voids. The effects are strongly dependent on the chemical composition of WW. Sulfate and acid resistances are poorly studied. Specific analyses are required according to the type of WW.	CWs generate treated wastewater with high chemical and biological loads, similar to those used in WW. The study shows that wastewater can affect porosity, chloride/sulfate diffusion, and microstructure, which impacts the durability of concrete in wetlands. Indirect evidence for selecting mixtures resistant to WW and aggressive environments.
Rajiv et al. [76]	Secondarily treated wastewater (STW) from municipal plants; Typical chemical variations of post-treatment effluents	Concrete made of: STW as mixing water, 10% fly ash (FA), and 1–3% sodium nitrite (corrosion inhibitor)	Decrease in workability due to chemical composition of the STW; changes in mechanical strength; possible protective effect of sodium nitrite against corrosion	Slump test, compressive strength, indirect stress, bending; ML models (Random Forest, etc.) for resistance prediction	Workability 25% with STW. Strengths only decrease marginally: compression −2.91%, tension −4.95%, bending −1.75%. The changes are attributed more to FA and NaNO2 than to wastewater. The Random Forest model showed high prediction (R2 = 0.87). It is demonstrated that STW can replace drinking water without significantly compromising performance.	CWs generate treated water that is secondarily similar to STW. The study shows that these waters have a low negative impact on resistance, but they do affect the workability and initial microstructure. It confirms the feasibility of using concrete exposed to treated wastewater in CW structures. The use of inhibitors (nitrite) can be key for environments with sulfide and chlorides.
Wang et al. [77]	Biogenic sulfuric acid (MICC), sulfuric mineral acid, and actual sewage liquid; Areas above and below the sewage level	Sewer Pipe Concrete	Microbial Induced Corrosion (MICC), Mineral Sulfuric Acid Attack, Thickness Loss, Microstructural Degradation, Neutralization Depth	Advanced microscopy, kinetic degradation analysis, immersion tests (90 and 525 days), corrosion face characterization, zone comparison (over/low level)	MICC produced 7.8 mm loss in 525 days; H2SO4 ore produced 4.46 mm in 90 days; immersion in real wastewater generated almost zero corrosion; the neutralization depth was 0.68 mm (MICC), 0.98 mm (ore), and 1.22 mm (sewage).	It provides a comparison of relevant corrosion mechanisms in environments with sulfates and H2S; useful for providing concrete durability in hydraulic components and structures exposed to acid effluents associated with CWs.

synthesizes the key information from these studies and provides a robust foundation for the analysis developed in the following sections, which focus on the interaction between complex wastewater environments and the structural and microstructural response of concrete and masonry.

3.1. Chemical and Physicochemical Attack on Aggressive Wastewater

The collected studies indicate that even simple contact between concrete and wastewater can significantly modify its properties in both fresh and hardened states. Partial substitution of mixing water with distiller’s vinasse increases compressive strength by up to 20% at a 15% replacement level; however, workability and strength decrease when this threshold is exceeded, underscoring the sensitivity of the cementitious matrix to highly acidic effluents rich in sulfates and organic matter [47]. Comparable findings are reported when drinking water is replaced with treated or brackish wastewater: mixtures containing 10% brine show initial strength gains and no early evidence of reinforcement corrosion, although the exposure period evaluated is short relative to the expected service life of a hydraulic structure [50]. In mixtures prepared with treated wastewater with elevated chloride content, overall mechanical strength is only moderately reduced (<10%), but steel corrosion increases by approximately 25%, accompanied by early cracking associated with ion ingress [55].

The impact of highly aggressive environments becomes more pronounced when concrete is exposed to acidic solutions or media containing multiple salts. Immersion in 5% sulfuric acid produces significant mass loss and rapid surface deterioration, effects that can be partially mitigated with vinyl ester coatings reinforced with nanocomposites or colloidal silica [53]. In marine or mixed environments (seawater, domestic wastewater, and sulfate-/acid-rich solutions), conventional concretes exhibit strength losses of up to 25% after 56 days, whereas nanomodified matrices containing superplasticizers and targeted additives limit this reduction to around 15% by refining pore structures and inhibiting expansive ettringite formation [58]. Similar improvements have been documented in nano-TiO₂-modified coral concretes, where an optimal dosage of 4% enhances compressive and flexural strength and reduces erosion under wastewater exposure, although certain acidic and saline media (e.g., oxalic acid, MgCl₂) still generate expansive byproducts and substantial mass loss [56].

More severe scenarios, representative of industrial effluents reaching pretreatment stages of constructed wetlands, have also been investigated. In industrial reaction tanks, prolonged immersion in acidic and alkaline wastewaters containing high concentrations of Cl⁻ and SO₄²⁻ leads to intense leaching, reduced permeability, and coating detachment. The application of GO/SiO₂ protective films significantly enhances chemical stability and reduces ion transport, with performance improving as graphene oxide content increases [73]. Cement pastes exposed to NH₄Cl solutions—analogous to coking wastewater—show accelerated dissolution of calcium-bearing phases and formation of gypsum, secondary calcite, ettringite, and thaumasite. The corrosion front advances progressively, causing surface disintegration, and a deterioration evolution model has even been proposed to describe this process [63].

The importance of wastewater composition is further confirmed in studies where domestic, municipal, or industrial effluents are used as mixing water. A bibliometric and systematic synthesis of 91 studies reports that 45–80% of the publications document equal or higher strengths when wastewater is used; however, higher concentrations of total dissolved solids (TDS), chlorides, and sulfates generally increase porosity, water absorption, and chloride diffusion, while reducing C–S–H formation and promoting additional microcracking [75]. More specific evaluations show that using secondarily treated water combined with fly ash and sodium nitrite results in strength reductions of less than 5%, although workability decreases by approximately 25%, changes primarily attributable to the mineral additives and inhibitor rather than the water itself [76]. In mortars reinforced with recycled steel fibers, aggressive ions in treated wastewater (phosphates, nitrates, nitrites, sulfates, chlorides) modify both the bond strength and corrosion rate of the fibers, with responses varying depending on the specific combination of water and fiber type [62].

Evidence obtained from real infrastructures complements these experimental findings. In drinking and wastewater distribution networks, as well as in urban systems lacking adequate treatment, surface degradation of concrete has been reported due to the combined effects of corrosive gases (H₂S, CO₂, NH₃), persistent humidity, and uncontrolled industrial discharges—conditions exacerbated by poor ventilation [48,69]. In tunnels lined with shotcrete, sulfate-rich environments and entrapped air voids promote localized ettringite formation at depths of up to 8 mm, particularly at interfaces between shotcrete layers [70]. Pressurized concrete pipes for drinking and wastewater systems generally maintain good intrinsic strength when alkalinity and mortar coatings are preserved; however, in environments containing elevated sulfates, chlorides, CO₂, and H₂S, additional protective measures—such as polymeric linings, internal coatings, inhibitors, or cathodic protection—are recommended [70].

Together, these results provide a solid framework for predicting the behavior of concrete in CWs that receive industrial, brackish, or domestic effluents with high concentrations of salts, sulfates, nutrients, and organic matter. They further emphasize the need for thorough characterization of influent water chemistry before defining mixture proportions or selecting protection systems.

3.2. Biogenic Corrosion, Aggressive Gases, and Combined Deterioration Mechanisms

In addition to purely chemical deterioration, the literature highlights the significant role of microbiologically induced corrosion and corrosive gases such as H₂S in sewer infrastructure. Numerous peer-reviewed and experimental studies concur that the microbial oxidation of H₂S to H₂SO₄ on concrete surfaces is among the most aggressive deterioration mechanisms in wastewater systems, substantially shortening service life when protective coatings or optimized mixes are not used [52]. Field investigations at pumping stations and grit chambers further show that wastewater vapors often cause damage more rapidly than direct liquid contact, with abundant gypsum and ettringite deposits observed on surfaces exposed to humid, gas-rich atmospheres [51].

The influence of H₂S on the degradation of manholes and expansion chambers has been well documented through continuous gas monitoring and structural inspections. In some systems, concrete losses of several centimeters have been reported within five years, progressing to near-failure conditions after only 11 years. Damage intensity is associated not with instantaneous gas peaks but with cumulative exposure, which in certain cases surpasses the measurable range of sensors (>500 ppm) due to extended hydraulic retention times [68]. In treatment plant inlet chambers, where sulfide loads may accumulate to several kilograms per day, artificial neural network models (RNA-LSTM) have successfully predicted H₂S concentrations with moderate error, enabling the development of early-warning tools for corrosion management [61].

The mechanisms of biogenic attack have also been explored across different cementitious systems. Sequential tests that simulate abiotic neutralization, neutral exposure, and subsequent acidophilic bacterial activity indicate that alkali-activated mortars experience lower mass loss and strength reduction than ordinary Portland cement (OPC) and calcium aluminate cement (CAC), although they present deeper neutralization fronts. The estimated service life of alkali-activated mortars is approximately 2.5 times greater than that of Portland cement mortars under biogenic attack [65]. Field studies in wastewater treatment plants further confirm that concretes formulated with calcium sulfoaluminate binders exhibit minimal biodeterioration after several months of operation, in contrast with Portland cement systems whose degradation correlates with biofilm formation dominated by acidogenic and sulfate-reducing bacteria [66].

Microbiologically induced corrosion (MIC) has also been analyzed through numerical modeling. A one-dimensional diffusion–reaction model with two moving boundaries has been used to describe the development of sulfur-oxidizing bacterial biofilms and the progression of the gypsum layer generated by the resultant sulfuric acid. The model identifies CaCO₃ concentration, acid diffusion, biofilm dynamics, and H₂S levels as key drivers of deterioration, and suggests that, under certain conditions, the biofilm may partially restrict acid penetration [64]. A related comparative study reported thickness losses of approximately 7.8 mm in 525 days due to MIC, whereas mineral sulfuric acid produced 4.46 mm of loss in only 90 days, and immersion in wastewater without significant acid generation resulted in negligible deterioration [77].

From a structural perspective, the interaction between mechanical loading and biogenic corrosion is highly relevant for constructed wetlands. Experimental work on concrete pipes exposed simultaneously to H₂S and compressive or flexural loads shows that strength loss exceeds that caused by corrosion alone by more than 10%, with synergistic effects increasing with exposure time, load magnitude, and H₂S concentration; doubling the mechanical load can increase the coupling effect by up to 1.6 times [72]. Long-term exposure tests (36 months) on steel fiber-reinforced concrete under simulated sewer conditions indicate that, despite moderate reductions in compressive strength, toughness, and flexural capacity improve, suggesting that fiber reinforcement may offer enhanced resistance to extended acid attack [54].

Biocorrosion is also influenced by microbial processes that, paradoxically, can contribute to material protection. Experiments on calcium carbonate bioprecipitation using denitrifying bacteria demonstrate that CaCO₃ layers can form on Q235 carbon steel even under high-salinity and variable-pH environments, significantly reducing corrosion currents [57]. Additionally, copper electrodeposition onto cement pastes produces compact, hard, antibacterial coatings that mitigate susceptibility to microbiologically induced corrosion, provided that bath pH and CuSO₄ concentrations are adequately controlled [60].

4. Protection Strategies, Material Selection, and Criteria for CWs and Future Prospects for New Materials

The evidence gathered indicates that, in environments analogous to CWs, the durability of structures depends not only on the intrinsic characteristics of the concrete but also on the proper selection of coatings (including mortars applied to both concrete and masonry), construction quality, and operational management. Analyses of sewerage and drinking water systems show that a considerable share of premature infrastructure deterioration is linked to inadequate material selection, aging without maintenance, and unfavorable operating conditions, which ultimately result in significant cost overruns and reduced structural performance [48]. In systems protected with polymeric liners, failures are frequently associated not with the underlying concrete but with poor installation practices; therefore, comprehensive design and construction protocols can extend service life to nearly 100 years under wastewater exposure [49].

Several studies highlight the potential of organic, polymeric, and nanostructured coatings to mitigate the ingress of aggressive agents. Under severe acid attack, vinyl-ester barriers reinforced with nanocomposites outperform silane-based coatings by approximately 60% in terms of durability [53]. In Portland cement concretes exposed to real and simulated wastewater containing sulfates, chlorides, phosphates, H₂S, and ammonium, the addition of 2–6% epoxy resin (by cement weight) reduces compressive strength loss, lowers water absorption, and improves resistance to chemical attack when compared with unmodified mixtures [59]. In industrial tanks storing highly aggressive process water, GO/SiO₂ coatings have demonstrated promising performance by minimizing ionic leaching and preserving the integrity of both the concrete matrix and the embedded reinforcing steel [73].

The use of alternative binders and mineral additions constitutes another robust mitigation strategy. Calcium sulfoaluminate cements and alkali-activated mortars have exhibited markedly superior resistance to biodeterioration in wastewater treatment facilities and sewer networks compared with conventional Portland cement systems [65,66]. The incorporation of fly ash as a supplementary cementitious material reduces mass loss and neutralization depth under biogenic sulfuric acid attack by lowering portlandite availability and stabilizing phases such as hemicarbonate and hydrotalcite. Commercial crystallizing additives also appear more effective under real biological conditions than under idealized chemical scenarios [74]. Additionally, cementitious nanotechnology—based on PCE additives, resins, and gluconates—contributes to pore refinement, reduced water absorption, and limited ettringite formation, thereby enhancing durability in matrices exposed to sulfate- and chloride-rich environments [58]. The use of nano-TiO₂ has also been shown to densify the matrix of coral concrete subjected to wastewater exposure [56].

Regarding the direct use of wastewater in concrete production, available evidence suggests that partial substitution of potable water with industrial, municipal, or secondarily treated effluents is feasible when the chemical composition is properly controlled and when suitable additives or corrosion inhibitors are incorporated. Studies involving wash water, brackish water, and secondary effluents report both improvements and moderate reductions in strength, while highlighting the need to evaluate workability, porosity, and risks associated with chloride and sulfate content [47,50,75,76]. In mortars incorporating recycled steel fibers, the compatibility between fiber type and wastewater chemistry is critical to prevent adhesion loss and localized corrosion [62].

From a system design and operation standpoint, reviews of precast and prestressed concrete pipes indicate that manufacturing quality and process control are key determinants of long-term performance. Failures such as cracking, internal erosion, and biogenic sulfuric-acid degradation are closely associated with deficiencies in fabrication, inspection, and unfavorable hydraulic and ventilation conditions [67,71]. Evidence from urban networks without pretreatment shows that natural or forced ventilation—combined with limiting uncontrolled industrial discharges—can significantly reduce corrosion and clogging in sewer systems [69].

Looking ahead, research trends point toward the development of more resilient, multifunctional, and adaptable materials for concrete, mortar, and masonry systems exposed to CW-like environments. Notably, self-healing concretes employing encapsulated bacteria or polymeric microcapsules capable of controlled CaCO₃ precipitation [78,79] have demonstrated the capacity to autonomously seal microcracks and extend service life. Geopolymeric and alkali-activated binders derived from industrial by-products (fly ash, granulated slag, activated sludge) [80,81,82] are increasingly reported in the literature as alternatives with reduced clinker demand and enhanced chemical resistance, particularly against sulfate, chloride, and organic acid attack. Hybrid nanocomposites incorporating graphene, functionalized nanosilica, and doped metal oxides [83,84,85,86,87,88,89] also show promise in enhancing barrier performance, reducing interconnected porosity, and limiting microbial colonization. In masonry, the development of smart mortars with antimicrobial, self-healing, and hygrothermal-regulating properties is emerging as a strategy to improve performance under conditions of high humidity and chemical variability [90,91,92,93]. These next-generation materials—currently transitioning from research to large-scale application—offer a pathway toward more durable, sustainable, and adaptable infrastructure aligned with resilient engineering principles and nature-based solutions.

Key Degradation Factors and Practical Recommendations for CW Structures

Figure 5 schematically summarizes how characteristic environmental conditions in constructed wetlands lead to specific degradation mechanisms in concrete and masonry structures, and how these mechanisms can be addressed through targeted engineering responses, design strategies, and durability-oriented criteria.

Although deterioration mechanisms affecting constructed wetland (CW) structures are often discussed in isolation across different types of hydraulic infrastructures, the present review enables the identification of a coherent set of key degradation factors that systematically govern structural performance in CW environments. These factors arise from the combined action of chemical aggressiveness, persistent saturation, biological activity, and mechanical loading, which distinguishes CWs from conventional dry or intermittently wetted structures.

As illustrated in Figure 5, the synthesis of the 31 studies reviewed and evidence from real CW installations allows the dominant degradation drivers to be grouped into five interrelated categories: (i) chemical exposure, including sulfates, chlorides, ammonium, organic acids, and dissolved CO₂; (ii) biogenic processes, such as H₂S generation, sulfur-oxidizing bacterial activity, and biofilm development; (iii) hydromechanical actions, including lateral pressures induced by saturated substrates, long hydraulic retention times, and pore-pressure fluctuations; (iv) material-related vulnerabilities, such as high porosity, inadequate concrete cover thickness, insufficient confinement in masonry systems, and poor interface detailing; and (v) operational factors, including limited ventilation, uncontrolled industrial discharges, and insufficient inspection and maintenance practices.

Importantly, this synthesis moves beyond descriptive observation by explicitly linking each degradation factor to actionable engineering responses, as summarized in the right-hand column of Figure 5. Practical recommendations emerging from the reviewed evidence include: (a) the use of sulfate- and biogenic-resistant binders, such as calcium sulfoaluminate or alkali-activated systems, in permanently saturated and anaerobic zones; (b) the targeted application of polymeric or nanostructured coatings (e.g., epoxy, vinyl ester, GO/SiO₂-based systems) in areas exposed to wastewater vapors, acidic effluents, or aggressive ions; (c) the incorporation of durability-oriented performance indicators—including corrosion rate, crack width, chloride penetration depth, and loss of reinforcement cross-section—into routine inspection and maintenance protocols; (d) the explicit consideration of lateral pressures and coupled hydromechanical effects in structural design models, particularly for walls and slabs confining saturated substrates; and (e) the implementation of ventilation, monitoring, and preventive maintenance strategies to control H₂S accumulation and mitigate biogenic corrosion in confined zones.

By structuring degradation factors, mechanisms, and engineering responses within a unified conceptual framework, the novelty of the present review lies not in identifying new damage phenomena but in translating dispersed evidence from analogous hydraulic infrastructures into an integrated, engineering-oriented framework specifically applicable to constructed wetlands. This approach provides a practical basis for future experimental research, numerical modeling efforts, and the development of durability-oriented design and maintenance guidelines for CW structures.

5. Perspectives on Modeling, Analysis, and Structural Design of Constructed Wetlands

The scarcity of studies explicitly focused on the structural behavior of constructed wetlands (CWs) has limited the development of design methodologies grounded in mechanical analysis, numerical modeling, and durability-oriented criteria. Most existing CW research prioritizes hydraulic performance, pollutant removal efficiency, or ecological aspects, while the structural response of walls, slabs, and foundations under real operating conditions remains largely unexplored. Given this lack of dedicated studies, a viable and scientifically consistent alternative is the adaptation of conceptual and computational tools originally developed for analogous hydraulic infrastructures, such as storage tanks, treatment lagoons, anaerobic digesters, lined canals, tunnels, and water containment structures.

These infrastructures share fundamental characteristics with CWs, including lateral confinement, interaction with saturated granular media, prolonged exposure to chemically aggressive environments, and degradation induced by persistent humidity and wetting–drying cycles. Consequently, their analytical and numerical approaches provide a solid starting point for advancing the structural engineering of CW systems.

5.1. Engineering Performance Indicators for Constructed Wetland Structures

Despite the recognition that CWs operate under mechanically and chemically aggressive conditions, their structural performance is rarely assessed using explicit engineering indicators. To move beyond qualitative descriptions, this study proposes a set of quantitative performance indicators that can be adopted as a first approximation for evaluating the structural behavior and durability of CW components. These indicators are widely used in analogous hydraulic infrastructures—such as wastewater tanks, anaerobic digesters, tunnels, buried retaining structures, and sewer systems—and are therefore transferable to CWs under comparable exposure conditions.

Relevant performance indicators include:

• Steel corrosion rate (μm/year or mm/year), commonly reported for reinforced concrete structures exposed to wastewater or saturated environments and directly linked to reinforcement section loss and service-life reduction.
• Chloride penetration depth (mm), defined as the depth at which the critical chloride concentration reaches the reinforcement, typically evaluated through diffusion-based models.
• Loss of steel cross-section (%), associated with corrosion progression and reduced load-bearing capacity.
• Variation in concrete porosity (%), reflecting chemical degradation processes such as calcium leaching, sulfate attack, or carbonation under prolonged saturation.
• Crack width evolution (mm), particularly relevant for loss of watertightness and accelerated ingress of aggressive agents.
• Maximum lateral deformation of walls (mm), induced by saturated substrate pressure, hydraulic loading, or coupled hydromechanical effects.
• Global factor of safety (FS) against sliding, overturning, or flexural failure, commonly used as an overall indicator of structural stability in containment and retaining systems.

Although specific threshold values for CWs remain largely undocumented, the ranges reported for analogous hydraulic infrastructures provide a robust basis for preliminary assessment, comparative analysis, and model calibration. The adoption of these indicators enables the formulation of performance-based design and maintenance strategies and facilitates the development of predictive numerical models tailored to CW operating conditions.

5.2. Numerical Modeling Approaches Applicable to CW Structures

A synthesis of the scientific literature reveals that sufficiently mature numerical and conceptual tools already exist for assessing structural behavior in systems comparable to CWs.

Table 3. Scientific articles relevant to modeling, analysis, and structural design applicable to CWs.

Author	Type of Modeled Structure	Method/Software	Analyzed Variables	Applicability to CWs
Hosseinzadeh et al. [94]	Embedded basement walls (buried structures)	Dynamic finite element analysis (FEM)—soil–structure interaction	Dynamic lateral earth pressures, excavation depth, topography, geotechnical parameters, and number of levels	Useful for estimating lateral pressures on concrete walls of buried or rigidly confined CWs, considering seismic effects and soil–structure interaction
Muni et al. [95]	Rigid retaining walls subjected to lateral pressure	FEM using ABAQUS	Internal friction angle (ϕ), dilatancy angle (ψ), active and passive pressure distribution, AFR and NAFR effects	Provides criteria to model lateral pressures on concrete walls of CWs (HSSF/VSSF) and to optimize analyses considering the influence of the dilatancy angle of saturated substrates
Joseph et al. [96]	Cantilever retaining walls with relief shelves	PLAXIS 3D (geotechnical FEM)	Displacements, maximum shear force, maximum bending moment, wall height, shelf length, multi-criteria analysis	Helps assess stress reduction in CW structural walls through optimized geometries; useful for designing concrete walls subjected to lateral pressure from saturated substrates
Keshavarz and Khani [97]	Soil–structure retaining wall under lateral pressure with anisotropic seepage	UBM (Upper Bound Method), FELA (Finite Element Limit Analysis), SCM (Stress Characteristic Method)	Active and passive earth pressures, anisotropic hydraulic conductivity, soil friction angle, soil–wall friction, stress distribution, and failure zones	Enables estimation of lateral pressures on CW walls under saturation and preferential flow conditions; useful for modeling water–substrate–wall interaction and evaluating stability in wetlands with hydraulic gradients
Nam et al. [98]	Stress state in confined soil (K0) assessed by cone penetration testing (CPT)	Vertical and inclined CPT tests + CEL-FEM (Coupled Eulerian–Lagrangian)	Cone resistance (qc), cone inclination (θ), relative density (DR), estimation of at-rest lateral earth pressure (K0)	Allows estimation of K0 for CW substrates, useful for modeling soil–wall interaction, evaluating lateral loads on concrete or masonry structures, and improving the design of confined cells
Kamiloğlu [99]	Inverted T-shaped cantilever retaining walls with narrow granular backfill	FEM + RSM (Finite Element Method + Response Surface Method) + physical tests using particle image velocimetry	Soil friction angle (φ), soil–wall friction (δ), heel length (β), foundation thickness (α), backfill inclination (ψ), backfill width (θ), failure patterns, and active thrust	Enables modeling of lateral earth pressures on rigid walls similar to concrete/masonry CW walls; useful for estimating lateral loads from saturated substrates and evaluating structural safety of hydraulic cells
Liang et al. [100]	Saturated porous media (two-phase 3D elements: water–soil)	ABAQUS (custom 3D element via u–U subroutine) + implicit integration algorithm	Fluid–solid dynamics, soil nonlinearity, pore-pressure–deformation coupling, transient response, validation against analytical solutions	Enables hydro-mechanical modeling of saturated substrates in CWs, particularly for estimating deformations, dynamic consolidation, and fluid–solid coupling in walls and structural beds
Guo et al. [101]	Saturated structural concrete (CAC: coral aggregate concrete) subjected to chloride ingress	COMSOL Multiphysics + PHREEQC coupled via MATLAB	Reactive chloride transport, pH variation, salt precipitation (Friedel’s salt, Kuzel’s salt), penetration profile evolution, chemical changes under saturation	Enables modeling of chloride ingress and chemical degradation of concrete in CWs exposed to saline wastewater or aggressive environments; useful for assessing structural durability and service life of walls and slabs
Wang et al. [102]	Metallic tools and components subjected to CO2 corrosion in humid environments	CO2 corrosion model validated by SEM, EDS, XRD, and polarization curves	Lattice structure and crystallinity of corrosion products, electrochemical mechanisms, corrosion rate, and humid environment effects	Provides insight into accelerated corrosion mechanisms in CO2-saturated environments, analogous to processes affecting concrete walls or slabs in CWs exposed to wastewater with high CO2 loads and acidic compounds
Dhir et al. [103]	Unreinforced masonry (URM) walls retrofitted with “splints and bandages” systems	Continuum FEM (Abaqus) + DMEM (HiStrA) + comparison with mesoscale FE model	In-plane behavior, load–displacement curves, fracture energy, tensile failure, material calibration, damage mechanisms	Useful for modeling masonry structural walls in CWs, particularly to evaluate band-reinforced systems, lateral loads from saturated substrates, and calibration of mechanical parameters in heterogeneous materials
Ram et al. [104]	Masonry walls with interlocking hollow blocks and adhesive or mortar joints	Micro-scale FEM + explicit mesoscale model (software not specified, experimentally validated)	Block geometry, adhesive/mortar mechanical properties, out-of-plane response, block and joint failure, plastic hinge formation, bending and damage	Useful for analyzing masonry walls in CWs using non-conventional materials or adhesives, evaluating stability under saturated substrate thrust, and modeling joint and block failure
Fernández et al. [105]	Deep circular excavation (shaft-type cavity) under dry and saturated conditions	FEM, NLA (Numerical Limit Analysis), MPM (Material Point Method) with Strength Reduction Factor (SRF)	Failure mechanisms, failure surfaces, bearing capacity, large deformations, basal stability, behavior under saturation	Useful for modeling failures due to basal and wall instability in CWs constructed in saturated soils, evaluating large ground deformations around CWs, and predicting failure mechanisms under hydraulic loading
Hafna et al. [106]	Homogeneous soil slopes (3D slope stability) under dry and partially saturated conditions	PLAXIS 3D (three-dimensional FEM)	Soil cohesion, slope height, safety factor, saturation conditions	Enables modeling of slope stability and lateral slopes of excavated CWs, analyzing sliding failures at edges, and evaluating stability under moisture variations
Vickneswaran and Ravichandran [107]	Geotechnical systems subjected to extreme hydroclimatic events (intense rainfall, droughts, coupled flows)	PLAXIS with modified constitutive model (MMC—Modified Mohr–Coulomb) and coupled flow–deformation modeling	Slope safety factors, temporal stress variation, settlements of shallow and deep foundations, and hydro-mechanical response under extreme rainfall	Useful for modeling hydro-mechanical responses of CWs under full saturation, extreme rainfall, or droughts, predicting deformations in retaining walls, differential settlements, and stability under variable hydraulic loads
Liu et al. [108]	Rock slopes subjected to prolonged infiltration and saturated environments	Coupled hydro-mechanical model with nonlinear creep law (proprietary numerical implementation)	Strength degradation due to saturation, elastic modulus reduction, cohesion loss with pore pressure, and accelerated creep deformation	Relevant for long-term analysis of CW walls and foundations exposed to permanent saturation; useful for predicting time-dependent deformations, stiffness loss, and combined effects of moisture and hydraulic loads
Chen et al. [109]	Reinforced concrete (RC) circular tunnel subjected to chloride attack in a marine environment	Multi-stage analytical model validated with COMSOL Multiphysics (diffusion–corrosion–structural deterioration)	Chloride transport, steel corrosion rate, corrosion product volume, corrosion-induced deformations, concrete mechanical parameters (E, ν, porosity, w/c)	Highly useful for concrete CWs exposed to wastewater, enabling prediction of service life, corrosion initiation and propagation, deterioration due to moisture and contaminants, and oxide-induced deformations in walls and slabs
Shao et al. [110]	Reinforced concrete (cement-based materials) subjected to chloride diffusion, calcium leaching, and sulfate attack	Coupled numerical model implemented and verified in COMSOL Multiphysics	Porosity, calcium leaching, chloride diffusion, sulfate attack, disruption of bound chlorides, evolution of expansive products	Highly applicable to concrete CWs exposed to chemically loaded wastewater; enables evaluation of sulfate degradation, leaching, porosity increase, and reduction in structural service life in walls and slabs under constant effluent contact
Tanhatan Naseri and Gucunski [111]	Reinforced concrete slabs subjected to chloride and oxygen diffusion	Comparative numerical modeling (software not specified, diffusion- and damage-based simulations)	Oxygen vs. chloride diffusion, cracking patterns, corrosion rate, time to reach critical state	Useful for understanding damage propagation by aggressive ions (e.g., chlorides and oxygen) in concrete CW walls and slabs exposed to wastewater; allows evaluation of accelerated cracking, degradation, and service life reduction
Rahman et al. [112]	Concrete with compression-induced damage affected by chloride migration	Multiphysics FEM (phenomenological model + Nernst–Planck equations)	Chloride diffusion coefficient, compressive stress-induced damage, chloride–matrix binding, non-steady-state migration	Enables understanding of how mechanical damage in CW concrete walls (due to lateral pressure from saturated substrates) accelerates chloride penetration, compromising durability and promoting corrosion and leakage

compiles representative studies addressing retaining walls, buried structures, saturated media, concrete tunnels, masonry systems, and chemically aggressive environments, highlighting their potential applicability to CW structural analysis.

The studies summarized in Table 3 demonstrate that a wide range of numerical techniques—particularly finite element methods (FEM)—are capable of capturing lateral pressure distributions, internal stress development, deformation patterns, and failure mechanisms under saturated and confined conditions.

5.3. Structural, Hydromechanical, and Durability-Oriented Perspectives

First, it is necessary to move towards a systematic integration of relevant actions into the structural models of constructed wetlands. This implies that the analysis schemes include, at a minimum: (i) permanent loads (self-weight of walls and slabs, weight of the saturated substrate, operating water level, weight of attached elements); (ii) variable loads (maintenance personnel traffic, water level fluctuations, wind on aboveground biomass, seismic actions where applicable); (iii) the mechanical and hydraulic properties of the materials (strength parameters of concrete and masonry, friction angle and substrate cohesion, hydraulic conductivity, porosity, degree of saturation); and (iv) environmental conditions (presence of chlorides, sulfates, and CO₂, wetting-drying cycles, temperature, and exposure time). Formalizing this minimum set of variables allows future constructed wetland models to be comparable and reproducible in different contexts.

Secondly, evidence from studies on rigid walls, buried structures, and saturated media suggests that finite element models (FEMs) are the most promising tool for capturing the distribution of lateral pressures, internal stresses, and deformations in CWs. The use of ABAQUS, PLAXIS, or other FEM codes has proven effective for analyzing walls subjected to lateral pressure, buried structures, and complex soil-structure configurations (Hosseinzadeh et al. [94]; Muni et al. [95]; Joseph et al. [96]; Kamiloğlu [99]; Ram et al. [104]). From the perspective of constructed wetlands, a clear line of research involves adapting these approaches to represent hydraulic cells of concrete or masonry, evaluating moments, shears, displacements, and safety factors against sliding, overturning, or flexural failure under the specific operating conditions of the CWs.

A third relevant perspective is the incorporation of coupled hydromechanical models, especially in concrete water structures (CWs) built on deformable soils or exposed to significant fluctuations in water level. Studies integrating water flow and porous medium deformation in saturated or partially saturated systems show that it is possible to simulate variations in pore pressure, differential settlements, and loss of confinement using tools such as PLAXIS 2D/3D, coupled formulations in ABAQUS, and advanced methodologies such as the Material Point Method (MPM) or Finite Element Limit Analysis (FELA) (Keshavarz and Khani [97]; Liang et al. [100]; Fernández et al. [105]); Transferring these approaches to constructed wetlands would allow, in future studies, for more accurate estimation of lateral wall deformations, the influence of substrate compaction, and the failure mechanisms associated with wetting-drying cycles or extreme hydrological events.

Fourth, the durability of concrete and masonry in aggressive environments suggests a broad research perspective in modeling coupled mechanical-chemical deterioration. Carbonation, calcium leaching, sulfate attack, and steel corrosion processes can be described using transport and reaction models based on Fick’s diffusion laws, as well as multiscale approaches that integrate chemical reactions with stiffness loss, cracking, and reinforcement section reduction. Recent studies have shown that tools such as COMSOL Multiphysics and coupled analytical or numerical models accurately reproduce chloride diffusion, calcium leaching, damage propagation, and service life evolution in saturated concrete and reinforced concrete structures subjected to marine or polluted environments (Guo et al. [101]; Shao et al. [110]; Chen et al. [109]; Tanhatan Naseri and Gucunski [111]; Rahman et al. [112]). Projected onto CWs, this opens the possibility of designing structures that not only meet immediate resistance criteria but also meet explicit service-life requirements in environments laden with chlorides, sulfates, CO₂, and organic compounds.

Finally, a fifth perspective relates to the consolidation of guidelines and future structural design standards specific to concrete walls. The integration of AEF, coupled hydromechanical models, and chemical deterioration simulations provides the basis for establishing quantitative dimensioning criteria (wall and slab thicknesses, slenderness ratios, reinforcement details, cover requirements), as well as for objectively comparing different material alternatives (conventional concrete, modified mixes, confined masonry, hybrid systems). In the medium term, these advances could translate into technical recommendations or specialized codes that reduce dependence on empirical criteria and promote the use of safer, more durable, and more sustainable structural solutions in CWs.

Taken together, these perspectives show that the transfer and adaptation of methodologies established in other hydraulic infrastructures (such as those mentioned in Table 3) constitutes a promising way to build, in the coming years, a robust body of knowledge on the structural behavior of CWs and on the role that structural materials play in their long-term performance.

6. Conclusions

The present study reveals a clear gap in the literature on CWs: although hydraulics, treatment performance, and vegetation ecology have been widely documented, the structural dimension remains largely unexplored. The reviewed cases show that reinforced concrete and masonry constitute the backbone of most pilot- and full-scale CW support systems; however, fundamental design parameters—including material strength, reinforcement detailing, wall thickness, and foundation criteria—are seldom reported. This omission limits the development of reliable calculation methodologies and hampers the transfer of best practices from other hydraulic infrastructures to CWs.

The indirect evidence obtained through a PRISMA 2020 systematic review of 31 studies involving concrete and masonry in CW-like environments enabled the identification of a consistent set of deterioration mechanisms: chemical and physicochemical attack by sulfates, chlorides, ammonium, and organic acids; biogenic corrosion driven by H₂S and H₂SO₄ generation; leaching, carbonation, and microcracking; and the interaction between mechanical loading and aggressive exposure conditions. Collectively, these findings indicate that CW structures operate under highly demanding durability scenarios. Persistent moisture, biofilm development, and long hydraulic retention times intensify the degradation of concrete and masonry mortars when adequate protection strategies are absent.

Building on this gap, the evidence synthesized in this study supports a transition from predominantly qualitative descriptions toward a performance-oriented structural assessment of CWs. A set of transferable engineering indicators—such as steel corrosion rate, chloride penetration depth, loss of reinforcement cross-section, variation in concrete porosity, crack-width evolution, maximum lateral wall deformation, and global safety factors—emerges as a practical basis for evaluating structural integrity and durability under CW operating conditions. In parallel, the maturity of numerical approaches originally developed for analogous hydraulic infrastructures indicates that finite element–based formulations, as well as coupled hydro-mechanical and chemo-mechanical models, can be realistically adapted to CW systems as a first-order approximation. At this stage, these approaches should be understood as part of an integrated conceptual framework, intended to guide future numerical validation and experimental calibration, rather than as fully validated design tools for CW structures. Together, these elements provide a concrete pathway for defining representative loading scenarios, supporting quantitative design checks, and developing reproducible, durability-oriented structural analyses tailored to CW applications.

Based on these insights, the study identifies three complementary pathways to enhance the structural performance of CWs: (i) the strategic use of organic, polymeric, and nanostructured coatings—such as vinyl ester, epoxy, GO/SiO₂, and metallic layers—to limit ingress of aggressive agents and reduce biocorrosion; (ii) the incorporation of alternative binders and mineral additions, including calcium sulfoaluminate cements, alkali-activated mortars, fly ash, crystalline additives, nano-TiO₂, and PCE-based nanomodifiers, which improve resistance to sulfate, chloride, and acid attack; and (iii) the critical assessment of wastewater as mixing water, which, although mechanically feasible, demands strict control of chemical composition and careful evaluation of porosity and reinforcement corrosion risks. These pathways, summarized within the conceptual framework presented in Figure 5, provide a structured basis for translating degradation mechanisms into engineering-oriented material selection, design decisions, and maintenance strategies for CW structures. Together, these strategies provide an initial framework for adapting durability criteria established for sewers, tanks, and pipelines to the specific exposure conditions encountered in CWs.

Finally, the assessment of emerging technologies suggests that future structural design in CWs will rely on the adoption of more resilient, multifunctional, and environmentally responsible materials. These include self-healing concretes incorporating bacteria and microcapsules, geopolymers and alkali-activated systems synthesized from industrial by-products, hybrid nanocomposites with enhanced barrier performance, and smart mortars for masonry with antimicrobial and self-repairing capabilities. However, their implementation in CWs requires interdisciplinary research programs that integrate laboratory experimentation, field monitoring, hydromechanical numerical modeling, and the development of regulatory guidelines specific to structures exposed to humid and biogenically aggressive environments. Advancing CWs as truly durable nature-based solutions, therefore, demands the explicit integration of structural engineering and materials science into their conceptualization and design.