Technological Innovations in the Application of Constructed Wetlands: A Review

@uv.mx; Tel.: +55-442-226-6337 Abstract: Constructed wetlands (CWs) are highly effective in wastewater treatment and have generated lines of research with a focus on technological development and implemented innovations. This work concentrates on the most recent technical and scientiﬁc advances that have obtained optimal results in the construction of CWs using sustainable materials and the use of ornamental plants and other aquatic plants. Efﬁciency is also documented through models and simulation with neural networks, the use of the random forest method, and the use of software such as MODFLOW, MODPATH, and COMSOL Multiphysics. The information shown is structured by geographical area and addresses regions of Africa, Asia, Europe, North America, South America, and Oceania. It is important to consider that the optimization and innovation of CW for pollutant removal may beneﬁt developing countries that do not have sufﬁcient infrastructure to meet the demand for municipal and industrial wastewater.


Introduction
Constructed wetlands (CWs) are a type of sustainable and innovative technology that emerged in Germany in the 1950s; however, it was not until the 1990s and early 2000s that these systems were adopted by more countries around the world [1].They have the function of treating wastewater effectively as secondary and tertiary treatment through chemical, biological, and physical processes [2,3], and are an alternative to conventional treatment plants with activated sludge; the most common method currently used for wastewater treatment [4].In addition, they can be used as tourist attractions [5].Currently, there are horizontal, vertical, surface, and subsurface flow CWs, or a combination of these types [6].Horizontal flow CWs are the most common and their operation depends on the horizontal transfer of wastewater through the porous medium.In turn, vertical flow CWs are usually used as a central unit in multi-stage systems [7,8].However, all of these types provide an economical solution for the insufficient wastewater treatment that exists in developing countries, due to their low labor requirements and zero energy requirements, representing approximately 1/3 of the cost of a conventional treatment plant [9][10][11].Examples schematizing horizontal and vertical flow CWs are shown in Figures 1-3.The typical configuration of a CW system should include a pre-sedimentation medium, usually a septic tank [15], although there are processes with xenon and ultraviolet lamps that work as a pretreatment.
Being relatively new alternatives, there is limited information and research in certain regions.However, it is considered an active line of research and the number of publications on the subject is increasing [16], considering the above, innovation and technological growth on this matter should be recognized, such as the use of sustainable materials for the design of CWs, the application of plants and techniques associated with vegetation, and the optimization of treatment processes that is obtained thanks to the development of mathematical models and software simulations.
The investigation of sustainable materials used in CW systems is mainly based on the study of alternatives for the filter substrate, especially for two important reasons: the sustainability of natural and economic resources.Although the construction of a CW generally does not imply very high costs, the substrate is usually the most expensive component [17] and, in rural communities with few resources, it could represent an economic impact [18].Considering the above, there are studies which show that plastic can be reused and function as a substrate in CWs without losing efficiency in pollutant removal [19], while addressing the current problems in relation to pollution caused by plastics that compromises the environment and public health [20].
Design factors to consider in a CW include a variety of biological, chemical, and physical parameters interacting under various atmospheric conditions and environmental circumstances.This causes a complex analysis of the sizing processes, as well as uncertainty in the behavior that the CW system will exhibit over time.The arguments presented above encourage the study of numerical modeling and simulation techniques to assess the impact of the system, and thus optimize processes [21].
This review aims to search for innovative studies applied to CW systems, mainly with a focus on the use of sustainable materials and examples of modeling and numerical simulation in order to document the technological evolution that has been developed in recent years and, based on the information obtained, make a comparison of the different applications and latest developments in different parts of the world.

Literature Review Methodology
The exhaustive search was carried out in databases such as ScienceDirect, Springer-Link, and Cambridge University Press, selecting the articles related to the use of sustainable materials applied in CWs, manuscripts that emphasize innovations in the use of plants and types of treated water, as well as investigations of numerical models evaluating the quality of CW systems, to later organize them into a regional format.
The search was carried out within the first 100 results obtained with the following keywords: Constructed wetlands, modeling and simulation, sustainable materials, and innovation in CW.In addition, the innovations proposed in the last 10 years were taken into account.It is worth mentioning that studies were not found for all cases in all regions.

Operation of a Constructed Wetland
The substrate occupies most of the volume in a CW, being key for all physical, chemical, and biological processes to take place.Substrates are usually made up of filter material such as sand or gravel whose main function is to remove solids.The substrate also influences bacteriological development by creating an aerobic condition thanks to the oxygen released by the roots of plants, facilitating the proliferation of microbiological biofilms, see [22][23][24] and [25].The function of plants is to create an oxidation-reduction environment in the rhizosphere, directly influencing the removal of pollutants by transporting oxygen and regulating bacteria [26].The main strains of bacteria found in CWs are Proteobacteria, Bacteroidetes, and Actinobacteria.These are responsible for processing organic matter and removing nitrogen or other contaminants [27].

Sustainable Materials
The high demand for conventional construction materials represents the exploitation of natural resources used to produce these inputs, for which the use of alternatives that do not affect the efficiency of work is essential to settle the debt of environmental damage generated by the construction industry [28].CWs are systems that generally use gravel or sand substrates as filter media [29], these materials are being highly exploited in the construction industry and their high demand causes significant disturbances in rivers, which are the main natural suppliers of gravel and sand [30].
Other residues that can be beneficial for the functions of a CW are some forms of agricultural waste and, in cases with low-carbon wastewater, this waste functions as biomass that contributes solid carbon to the system, helping in the denitrification of the water [31], as has been demonstrated with coconut fiber [32].

Asia
A study carried out in India mentions the adsorption capacity of coconut fiber added to CWs, and highlights its efficiency in chlorine treatment, Chemical Oxygen Demand (COD) and Biological Oxygen Demand (BOD) (65.7%, 93.7%, and 95.6%), respectively, which makes this material an alternative for the millions of tons of coconut waste produced in this country by commercial industries [33].
The carbon-enriched CW substrate is an alternative that accelerates the removal of nitrate (N) and total nitrogen (TN).In Hong Kong, the behavior of a vertical flow CW series was analyzed when sucrose, kale remains, and rush leaves were added.The CW system has the function of treating wastewater from hydroponic crops, which have a high content of N and total phosphorus (P).The addition of carbon into the substrate demonstrated an increase in the removal efficiency of TN, N, and P up to 93.5%, 99.8%, and 55.2%, respectively [34].

Europe
The use of concrete produces carbon dioxide emissions [35], causing an increase in global warming [36].Through the recycling of by concrete using it as a substrate in CWs, Hube et al. propose an alternative route for the treatment of water in cold climates in Iceland [37], obtaining organic waste disposal efficiencies and TN of 51% and 34% at a temperature of 5 • C.

Latin America
In Mexico, in 2019, Zamora et al. [17] carried out an investigation testing the performance of a CW over 180 days, comparing the use of different ornamental plants such as Canna indica, Cyperus papyrus, and Hedychium Coronarium.The system is configured as seen in Figure 4.In the study, it was shown that the use of plants improves the efficiency of contaminant removal between 20 and 40%.In addition, the substrate used as a filter medium was compared, evaluating the addition of Polyethylene Terephthalate (PET) to tepezil, which resulted in findings that demonstrated little or no difference in water decontamination when plastic is added to the substrate.For this reason, it is considered an economical and environmentally friendly alternative since this petroleum-derived material can be reused in CW systems.

North America
Biodegradable plastics are those that degrade naturally over time [38] and, like PET, have yielded promising results when added to CW systems in the Everglades Wetlands Research Park in the state of Florida.The performance of a floating CW and a vertical flow CW used for the treatment of marine aquaculture effluents using polycaprolactone as an addition to the original substrate was evaluated.By this means a better retention of TN was obtained; however, there was a disruption of the microbial community in the system, which encourages further research to understand the behavior of microbial degradation of plastic [39].In addition to the environmental benefits, Aranibar et al. [19] mention the structural benefit of using plastics as an alternative filter substrate, due to a significant reduction in weight applied in the CW, considering a volumetric weight of gravel of around 1475 kg/m 3 , in contrast to the 172 kg/m 3 that plastic contributes.The study carried out in Chile reaffirms that there is no significant difference in the efficiency of a CW system that uses gravel compared to a system that uses plastic, verifying their studies in a pilot-scale CW.

North America
Biodegradable plastics are those that degrade naturally over time [38] and, like PET, have yielded promising results when added to CW systems in the Everglades Wetlands Research Park in the state of Florida.The performance of a floating CW and a vertical flow CW used for the treatment of marine aquaculture effluents using polycaprolactone as an addition to the original substrate was evaluated.By this means a better retention of TN was obtained; however, there was a disruption of the microbial community in the system, which encourages further research to understand the behavior of microbial degradation of plastic [39].
Table 1 shows the investigations that used sustainable materials as an alternative to a conventional design and the type of innovation proposed in each investigation.In addition to the environmental benefits, Aranibar et al. [19] mention the structural benefit of using plastics as an alternative filter substrate, due to a significant reduction in weight applied in the CW, considering a volumetric weight of gravel of around 1475 kg/m 3 , in contrast to the 172 kg/m 3 that plastic contributes.The study carried out in Chile reaffirms that there is no significant difference in the efficiency of a CW system that uses gravel compared to a system that uses plastic, verifying their studies in a pilot-scale CW.
Table 1 shows the investigations that used sustainable materials as an alternative to a conventional design and the type of innovation proposed in each investigation.

Plants
Plants play an important role within a CW system and the most used are the typical plants of natural wetlands.However, the use of ornamental plants shows a very similar efficiency, in addition to adding a landscape function to wetlands.Depending on the geographical area, Canna, Iris, Heliconia, and Zantedeschia are usually used [40].
A recent study points to Mexico, Brazil, the United States, China, and India as the countries with the highest number of publications on the use of ornamental plants in CWs [41].Some recent research regarding the analysis of plants used in CWs is presented below.

Asia
A study conducted in China by Li et al. [42] demonstrates the importance of plants in the process of anaerobic oxidation of ammonia, arguing with a test where a TN removal of up to 85% was achieved in systems with plants against 72-82% achieved in control systems.Furthermore, in China, Gu et al. [43] point out that the age of the plant plays an important role in the microbiological community, stating that an old plant significantly increases the elimination of nitrogen in summer, as observed in Figure 5, six-year-old plants produce more bacteria for nitrification processes, while those for iron reduction and denitrification processes decrease.In addition, TN removal is increased to 400 mg/m −2 /d −1 compared to one-year-old plants.

Europe
In Italy, the application of Phragmites Australis was proposed to improve the efficiency of a CW used for post-treatment of an activated sludge plant responsible for wastewater derived from a textile plant and the organic material demonstrated an important metal absorption capacity greater than 40%, 70%, and 45% in the analysis of copper, iron, and zinc, respectively [44].

Latin America
In order to evaluate the performance of CWs in the treatment of wastewater used in the manufacture of synthetic leathers, using Polygonum hydropiperoides to check if it was capable of removing chromium was experimented with in Brazil.The plant used was enriched with phytohormones and bacteria that promote growth, obtaining a very similar result, greater than 99% in all cases; however, it was observed that phytohormones and bacteria minimize the phytotoxic effects of wastewater, which encourages further research with different bacteria and hormones to improve the performance of Polygonum hydropiperoides in this type of water [45].
In the municipality of Actopan in Mexico, Marín-Muñiz et al. [46] studied the behavior of 8 CW systems against a wide variety of domestic pollutants.The microcosms were conditioned with monocultures and polycultures of ornamental plants such as Canna hybrid, Alpinia purple, and Hedychium coronarium, highlighting the landscape advantages and the highly effective removal of pollutants from the systems .

Europe
In Italy, the application of Phragmites Australis was proposed to improve the efficiency of a CW used for post-treatment of an activated sludge plant responsible for wastewater derived from a textile plant and the organic material demonstrated an important metal absorption capacity greater than 40%, 70%, and 45% in the analysis of copper, iron, and zinc, respectively [44].

Latin America
In order to evaluate the performance of CWs in the treatment of wastewater used in the manufacture of synthetic leathers, using Polygonum hydropiperoides to check if it was capable of removing chromium was experimented with in Brazil.The plant used was enriched with phytohormones and bacteria that promote growth, obtaining a very similar result, greater than 99% in all cases; however, it was observed that phytohormones and bacteria minimize the phytotoxic effects of wastewater, which encourages further research with different bacteria and hormones to improve the performance of Polygonum hydropiperoides in this type of water [45].
In the municipality of Actopan in Mexico, Marín-Muñiz et al. [46] studied the behavior of 8 CW systems against a wide variety of domestic pollutants.The microcosms were conditioned with monocultures and polycultures of ornamental plants such as Canna hybrid, Alpinia purple, and Hedychium coronarium, highlighting the landscape advantages and the highly effective removal of pollutants from the systems .
Table 2 shows the investigations that used plants for CW design and the efficiency demonstrated in each investigation.Table 2 shows the investigations that used plants for CW design and the efficiency demonstrated in each investigation.Below, Table 3 lists the reviewed species and the context of their application.

Modeling and Simulation
Laboratory experiments are important to gain basic insights into CW behavior [47]; however, for the technological development of innovative scientific applications, it is necessary to apply mathematical models and simulations in specialized software to observe the possible behavior of the different parameters involved in the execution processes of little-studied technologies.At the hydraulic level, it is important to evaluate the behavior of factors such as influent and effluent flow, as well as the characteristics of the porous medium, such as size and position [48].
Analysis using models and simulation can help to obtain an adequate CW design, anticipating problems such as flow obstruction, which represents an excellent area of opportunity for future research, since there is currently an incorrect approach in the search for a solution to this problem [49].
In the literature we can also find mathematical models applied to the evaluation of the remediation behavior of the different types of contaminants [50].The modeling can also confirm the importance of the use of plants in CW systems [51].In this section, applications of mathematical models and simulations with different methods and software are mentioned, which seek to solve different CW design problems.

Africa
CWs are a treatment alternative that can be very useful in developing countries [52].In Egypt, Salem et al. [53] applied the random forest method to mathematically model the behavior of contaminant removal parameters such as COD, BOD, total suspended solids (TSS), TP, and TN in a HSSF-CW, to verify reliability.From the model, 112 water samples were monitored across a period from 2018 to 2020, obtaining purification efficiencies of 72, 77, 78, 49, and 31% for BOD, COD, TSS, TP, and TN respectively.The model showed a coefficient of determination (R 2 ) of 0.66, 0.68 and 0.79 for the prediction of BOD, COD and TSS respectively, which represents relative reliability in the model obtained.

Asia
The non-linearity of the relationship between the hydraulic performance and the design parameters of some CW types makes it difficult to find models to evaluate the behavior of a CW.In these cases, a neural network model seems viable [54].This modeling technique can be used for the prediction of pollutant removal, as proposed by Mohammed and Ismail in 2021 [55], where a prediction model was made using neural networks to anticipate the behavior of a CW system for wastewater treatment in a cheese factory in Iraq, obtaining an R 2 quality of up to 0.9929 in the COD analysis and a mean square error of up to 0.0028 in the ammonium analysis.
Recently, in China, Li et al. [56] confirmed the precision of neural networks to simulate the results of a CW obtaining R 2 correlation coefficients of 0.99, 0.91, 0.92, and 0.82 for COD, ammonium, TN, and TP respectively.
Increasingly complex engineering requires rigorous analysis and the use of software has become more common [57].In China, for example, Nawaz et al. used HYDRUS to predict the removal of Nitrobenzene, a highly toxic product that is used to produce pesticides, dyes, explosives, etc.The model was calibrated with four experimental specimens with different substrate configurations, obtaining variable efficiencies of 50.26%, 55.86%, 65.97%, and 76.22%, having an R 2 correlation of 0.8869, 0.9729, 0.9247, and 0.9621, respectively, compared to the simulations [58,59].
In Iran, hydraulic conductivity, inflow and outflow, and pressure head were evaluated in a horizontal flow CW, whose simulations were carried out using a 3D model on the Comsol Multiphysics ® platform and considering Darcy's law as the basis of the model to determine the load and pressure losses through the porous medium.The results of the simulations were validated with real data and the hydraulic retention time was optimized, being 4.65 days for this example [60].

Europe
In Italy, Rizzo et al. obtained a simulation that replicated the behavior of a CW system analyzed by Galvão and Matos [61] through the use of HYDRUS-CWM1 software.The efficiency evaluation for COD removal according to the model was 68%, compared to 67% in the experimental measurements [62].However, in France, Morvannou et al. found deficiencies in the representation of the oxygen re-aeration rate and the measurement of ammonium adsorption in organic matter of domestic wastewater using the same software [63].

Latin America
A study carried out in Brazil in 2019 by Fioreze and Mancuso [64], demonstrates the importance of three-dimensional modeling of the flow of a horizontal subsurface flow CW (HSSF-CW).In the study, a HSSF-CW was modeled for wastewater treatment by 36 people (150 L/day per person).The simulations executed in ModFlow and ModPath optimized the design parameters such as the selection of the filtering substrate, obtaining the best results with coarse sand in the central area and crushed gravel in the flow inlet and outlet areas.It is worth mentioning that, with the simulations, it was possible to obtain an almost constant speed that was measured within the range of 0.07207 ± 0.08655 m/d and a hydraulic height between 0.5203 and 0.5837 m, which is quite close to the optimal height defined for the system (0.6 m).
On the other hand, in a context of chemical and biological parameters in 2022, and based on two HSSF-CW systems already installed in Costa Rica and El Salvador, a rigorous mathematical model to predict processes and life cycles of microorganisms that are transported by the mass of the substrate was proposed.Aguado et al. obtained purification efficiencies above 90% as results, a value very close to that shown in reality, and in other studies, which is why it is considered an adequate model that is capable of anticipating the response of the system [65].

North America
In Canada, Xiao et al. [66] analyzed a CW for stormwater treatment in the city of Edmonton.Hydraulic modeling software such as QUAL2K and HEC-RAS were used to review hydraulic design and water quality parameters, in which pollutant removal percentages were obtained from the study, they were low compared to other geographical areas, especially in the analysis of TN, obtaining 13.59% and 25.64% with the simulations in the mentioned software.The low levels of efficiency obtained in the models may be due to the fact that the type of climate in the area causes low bacterial activity, reducing the efficiency of the system [67].
Evapotranspiration in arid areas is a significant phenomenon for water loss in CWs [68], contributing to a decrease in treatment efficiency that was verified by the first-order serial tank model developed by Beebe et al. in South Carolina, obtaining a correlation coefficient of 0.96 compared to real data [69].
In Canada, the Comsol Multiphysic ® software was also used for the analysis of a vertical flow CW, managing to predict obstructions in the substrate thanks to an R2 correlation of 0.89 in the simulation of the porosity of the filter medium [70].

Oceania
Pollution in rainwater is a matter of concern for governments around the world as it can affect the water quality where the runoff flows [71].In Australia, Dharmasena et al. [72] published an article in 2021 on a CW that, in addition to treating residential wastewater, has a rainwater harvesting system.The study proposes modeling and simulation of water runoff flow, in addition to a model to verify water quality, which was validated by taking samples during eight storms in 2018.The flow model was validated with a correlation coefficient of 0.78 and a p-value < 0.05, however, the modeling simulations showed that only 62% of the expected water flow enters the CW, while the other 38% ends up diverting before reaching the treatment system.Regarding water quality, the model was validated with coefficients of determination and p-values of 0.749 (p < 0.05), 0.491 (p < 0.1), and 0.856 (p < 0.05) for the evaluation of TSS, TP, and TN respectively.
Table 4 shows the cited articles that use models and simulation as a fundamental part of the research and the type of proposal used.

General Innovations 7.1. Green Walls
Green walls (GWs) provide attractive landscaping in modern buildings and, in addition, they have the ability to adapt as CWs and treat gray water, coupled with their ability to improve air quality.There are few, but important, studies that analyze the efficiency of GWs used as CWs [73].
At the University of Turin, a GW used as a CW was analyzed with two types of filter substrate (coconut fiber and perlite), with the aim of comparing four different biomass additives.The system gave contaminant removal results greater than 95% for BOD, and greater than 98% for Escherichia coli [74].
The addition of coconut fiber showed good adsorption behavior, and in combination with crushed tile residues it is a highly efficient substrate in the treatment of COD and TSS, reaching an effectiveness of up to 70% and 75%, respectively [75].
GWs are also manufactured commercially.In Australia, Prodanovica et al. [76] analyzed the performance of a GW of the Gro-Wall ® brand, to compare the performance of different types of plants in the removal of contaminants in artificial gray water.The results showed a superior effectiveness of up to 10% when plants were occupied in the configuration of the system, demonstrating a high effectiveness of treatment for TN; however, a significant variation was found in the elimination of TP (27-53%).This deficiency is attributed to the short hydraulic retention time.

Hospital Waters
Pharmaceutical waste from hospitals in the water represents a significant toxicity hazard for the aquatic environment.Nevertheless, for some authors this situation has not received the attention it deserves.CWs have generally been used [77] to treat domestic wastewater [78]; however, in Northern India, a CW was built that was configured with a tube decanter to capture the effluent of wastewater from hospitals, focusing on antibiotics, anti-inflammatories, and anticonvulsants, from which highly variable pollutant removal efficiencies were obtained as the study was carried out in three different seasons.Thus, more research should be conducted in order to evaluate the effectiveness of a CW for hospital water treatment [79]. Figure 6 schematically shows the configuration of the CW under study.

Hospital Waters
Pharmaceutical waste from hospitals in the water represents a significant toxicity hazard for the aquatic environment.Nevertheless, for some authors this situation has not received the attention it deserves.CWs have generally been used [77] to treat domestic wastewater [78]; however, in Northern India, a CW was built that was configured with a tube decanter to capture the effluent of wastewater from hospitals, focusing on antibiotics, anti-inflammatories, and anticonvulsants, from which highly variable pollutant removal efficiencies were obtained as the study was carried out in three different seasons.Thus, more research should be conducted in order to evaluate the effectiveness of a CW for hospital water treatment [79]. Figure 6 schematically shows the configuration of the CW under study.

Substrate Additions
The substrate in a CW plays a very important role in the retention and adsorption processes of contaminants [23].In the city of Sanjiang in China, a substrate enriched with pyrite, sponge iron, and iron scrap was analyzed, increasing the removal of N and TP by 60 and 70%, respectively-this meant an improvement of 40% in the efficiency of the CW

Substrate Additions
The substrate in a CW plays a very important role in the retention and adsorption processes of contaminants [23].In the city of Sanjiang in China, a substrate enriched with pyrite, sponge iron, and iron scrap was analyzed, increasing the removal of N and TP by 60 and 70%, respectively-this meant an improvement of 40% in the efficiency of the CW [80].
Table 5 organizes the relevant innovations of CWs that do not fall into the category of sustainable materials, plants, or modeling and simulation.However, due to their relevance they were taken into account.

Political Positions
The poor application of regulations on environmental preservation leads to policies that do not have the expected impact on the world's ecosystems [81,82].CWs provide ecosystem services to humans and other organisms [83], yet these ecosystems have little economic support from governments [84].CWs help in water preservation, pollution degradation, enhance biodiversity, and regulate climate [85].During the twentieth and twenty-first centuries, the loss of natural wetlands ranged from 64% to 71% [86].Thus, the governments of the world should support and encourage the creation of CWs, especially for the treatment of municipal waters in developing countries [87].

Discussion
Since its conception in the 1950s, CW studies have increased and recently there has been innovation in fields such as numerical modeling and simulation, whose research has shown correlations close to 100%.This suggests the birth of new lines of research that can be supported without the need to build a system but putting the most encouraging research into practice.
There are complex models for CW design such as the PkC* approach, in addition to water flow simulation software such as ModFlow and others.The ability to model processes that occur in CWs helps optimize their design, considering that the geometry and materials used may vary depending on the context in which they are used.
The variety of waters that CWs can treat is wide including, for example, domestic, industrial, hospital, municipal, rain, textile, and agricultural waters.The CWs reviewed in this research demonstrate the removal effectiveness of highly variable contaminants that depend on the pollutant measured, type of flow, climate, and type of water treated.The circumstances favor an individualized study for each case, but in general terms, they attained effectiveness levels greater than 50%.Additionally, the use of plants generally improves pollutant removal efficiency, it is therefore important to explore new plants and test them in different contexts.
CWs also have disadvantages and limitations, such as the need for excessive space, which may be unfeasible in cities with high urban expansion.In addition, zero maintenance leads to clogging and performance depends on many factors, causing efficiency variation.
Cost-benefit analyses help to efficiently manage resources and suggest feasibility of CW application in developing countries due to the performance shown and the low construction and operating expense compared to conventional treatment plants.
The sustainable materials reviewed present design alternatives whose scope depends on the material used and may present disadvantages such as transport costs, insufficient material, or additional labor, particularly for the use of recycled concrete, requiring a cost-benefit and environmental impact assessment when its use is applicable.
If we compare the advantages of applying CWs with the disadvantages, the technical and economic feasibility is evident-as are the social and environmental benefits obtained from these systems in the current environmental context.

Conclusions
From the search for innovations applied to CWs around the world, the following conclusions can be reached:

•
The study of sustainable materials for CWs is mainly based on adding or substituting recycled material to the filter substrate layer.In general, plastic is widely investigated as a substitute or complement to the filtering substrate in a CW.The results of these investigations are promising, as this addition does not affect the purifying capacity of the system.In view of this, more research is necessary to optimize the bacteriological processes in the plastic, in addition to proposing new materials such as coconut fiber that can replace or complement typical materials.

•
The Asian continent is positioned as the geographical area with the greatest diffusion of research and application of CWs with state-of-the-art technology.

•
Likewise, the situations in Latin America and North America are contrasted.This is due to the differences in weather conditions, since it is known that the temperature of a region directly influences the performance of a CW, with hot places being those where better contaminant removal efficiencies are obtained.Thus, it is difficult to find CW application research in countries like Canada or some cold areas of the United States.

•
Plants are very important to improving the efficiency of a CW.There are some very versatile species that work with various types of polluted water and in various regions, such as Phragmites australis, but there are also other species under study such as ornamental flowers.It is important to analyze different species under different demands and types of water to verify their efficiency and versatility.

•
This wastewater treatment technique is more commonly used in developing countries like Latin America, some African countries, and some Asian countries like India.

•
Models and simulations of CW processes using neural networks show high degrees of correlation with what is observed in reality.The use of numerical models decreases the risk of a failed design and encourages the construction of CWs since their operation can be optimized according to specific needs.

•
Finally, based on the studies reviewed, we can conclude that the lines of research on CWs should focus on innovating and optimizing the elements that integrate a CW to make it more feasible and promote its use in all communities that may need it.

Figure 1 .
Figure 1.Example of a horizontal flow CW [12] adapted from Bakhshoodeh et al. (Reproduced with permission from Bakhshoodeh et al., Ecol Eng, published by Elsevier 2017).

Figure 2 .
Figure 2. Example of a vertical flow CW [13] adapted from Munir et al. (Reproduced with permission from Munir et al., Contam Hydrol, published by Elsevier 2023).

Figure 1 .
Figure 1.Example of a horizontal flow CW [12] adapted from Bakhshoodeh et al. (Reproduced with permission from Bakhshoodeh et al., Ecol Eng, published by Elsevier 2017).

Figure 1 .
Figure 1.Example of a horizontal flow CW [12] adapted from Bakhshoodeh et al. (Reproduced with permission from Bakhshoodeh et al., Ecol Eng, published by Elsevier 2017).

Figure 2 .
Figure 2. Example of a vertical flow CW [13] adapted from Munir et al. (Reproduced with permission from Munir et al., Contam Hydrol, published by Elsevier 2023).

Figure 2 .
Figure 2. Example of a vertical flow CW [13] adapted from Munir et al. (Reproduced with permission from Munir et al., Contam Hydrol, published by Elsevier 2023).

Figure 1 .
Figure 1.Example of a horizontal flow CW [12] adapted from Bakhshoodeh et al. (Reproduced with permission from Bakhshoodeh et al., Ecol Eng, published by Elsevier 2017).

Figure 2 .
Figure 2. Example of a vertical flow CW [13] adapted from Munir et al. (Reproduced with permission from Munir et al., Contam Hydrol, published by Elsevier 2023).

Figure 5 .
Figure 5.Comparison of nitrogen removal according to age by Gu et al. [43] (Reproduced with permission from Gu et al.Science of the Total Environment, published by Elsevier 2023).

Figure 5 .
Figure 5.Comparison of nitrogen removal according to age by Gu et al. [43] (Reproduced with permission from Gu et al.Science of the Total Environment, published by Elsevier 2023).

Table 1 .
Sustainable materials in CW systems.

Table 1 .
Sustainable materials in CW systems.

Table 2 .
Innovations with plant application.

Table 3 .
Use of plants in different contexts.

Table 4 .
Modeling and simulation applied in CW systems.

Table 5 .
General innovations in CW systems.