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Systematic Review

Sustainable Management of Organic Waste as Substrates in Constructed Wetlands: A Systematic Review

by
Diego Domínguez-Solís
,
María Concepción Martínez-Rodríguez
*,
Lorena Elizabeth Campos-Villegas
,
Héctor Guadalupe Ramírez-Escamilla
* and
Xochitl Virginia Bello-Yañez
Centro Interdisciplinario de Investigaciones y Estudios sobre Medio Ambiente y Desarrollo (CIIEMAD), Instituto Politécnico Nacional (IPN), Mexico City 07340, Mexico
*
Authors to whom correspondence should be addressed.
Sustainability 2026, 18(1), 318; https://doi.org/10.3390/su18010318
Submission received: 22 September 2025 / Revised: 28 November 2025 / Accepted: 1 December 2025 / Published: 28 December 2025
(This article belongs to the Special Issue Circular Economy Strategies for Waste Management: Innovations in Resource Recovery and Sustainability)
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Abstract

Constructed wetlands (CWs), which combine biological and physicochemical processes and adhere to circular economy principles, are increasingly recognized as nature-based wastewater treatment solutions. With an emphasis on resource valorization and pollutant removal efficiency, this review assessed the use of organic residues as substrates in CWs. In total, 44 peer-reviewed open-access case studies in English were obtained from 325 documents that were retrieved from Scopus using PRISMA-based eligibility criteria. Information about the wastewater source, substrate, CW type, and results was extracted. The results indicated that biochar (66.7%) predominated because of its high adsorption capacity and microbial support, while shell or forest residues and agricultural residues (20.5%) helped remove micropollutants and phosphorus. CWs with vertical subsurface flow were most prevalent (54%). According to studies, the removal efficiencies of biochar and agricultural or shell residues were 10–15% higher than those of inorganic substrates for phosphorus, TSS (total suspended solids), NH4+ (ammonium), and BOD (biochemical oxygen demand) in wastewater. Through innovative designs and the application of circular economy strategies, including revalorize, reuse, reutilize, reintegrate, rethink and reconnect, organic substrates enhance pollutant removal and improve the overall sustainability of CWs. Overall, CWs with organic residues provide cost-effective and environmentally sustainable wastewater treatment; further research on local resources, hybrid systems, and supportive policies is recommended to promote broader implementation.

1. Introduction

Constructed wetlands (CWs) employ natural processes involving plants, microorganisms, and substrates to remove water pollutants such as organic matter, nutrients (nitrogen and phosphorus), heavy metals, and certain emerging contaminants. These systems mimic the functions of natural wetlands under controlled conditions and can be configured as horizontal flow, vertical flow, hybrid, or floating wetlands, depending on the type of wastewater and the treatment objectives [1,2]. Research and a deep understanding of the components of constructed wetlands (vegetation, substrate, microorganisms, and biofilm) are essential to maximize their efficiency in pollutant removal and to ensure their environmental and operational sustainability. In constructed wetlands (CWs), substrates play multiple essential roles, including retaining and filtering larger particles and suspended contaminants, adsorbing various pollutants, acting as electron donors to support microbial metabolism and denitrification, providing surfaces for microbial colonization and biofilm development, and offering structural stability for wetland vegetation, the valorization of recycled materials and local residues such as bricks, slags, shells, water treatment sludge, and agricultural by-products helps to close material loops, reduce virgin resource extraction, and limit waste generation [3]. Furthermore, constructed wetlands represent a wastewater treatment option that deserves greater attention due to their multiple ecological, economic, and social benefits [4].
The study and innovation in the use of alternative substrates for constructed wetlands represent a crucial opportunity within the circular economy and nature-based solutions (NBS). According to Obeng et al. [5], this approach not only reduces costs but also supports the integration of wetlands into broader economic circuits. In addition, the use of substrates with nutrient recovery capacity, particularly for phosphorus, contributes to sustainability by enabling the recovery of valuable resources and improving treatment efficiency [6]. However, there is a fragmentation in the literature regarding the long-term efficacy and sustainability of the diverse organic waste used. Therefore, the present study aims to systematically analyze the application of organic substrates in constructed wetlands, evaluating their effectiveness, sustainability, and potential to serve as circular economy strategies within wastewater management.
Addressing this gap through a systematic review offers significant added value. Unlike narrative reviews, the rigor of a systematic methodology allows for identifying robust trends, synthesizing heterogeneous results, and standardizing the assessment of the actual contribution of these substrates to circular economy practices. By valorizing local waste and recycled materials as functional substrates [7,8], these systems contribute to reducing waste streams, limiting dependence on virgin resources. Such approaches are fully aligned with the objectives of the 2030 Agenda for Sustainable Development, fostering water security, climate resilience, and sustainable production and consumption patterns.

2. Materials and Methods

Beyond their technical performance, the use of organic substrates and biomass residues in constructed wetlands also generates important benefits for local communities. By valorizing agricultural and agro-industrial wastes, these systems reduce disposal problems while creating low-cost, locally sourced treatment alternatives [9,10]. For many rural and peri-urban areas, this approach enhances access to decentralized wastewater treatment, lowers operational costs, and promotes community participation in water management. In this way, the adoption of constructed wetlands with organic substrates not only improves environmental quality but also strengthens social resilience and contributes to more inclusive models of sustainable development [5].
The systematic review was conducted through a three-step procedure outlined by Tranfield et al. [11]. For greater clarity and transparency, the screening and selection stages were illustrated using a PRISMA diagram, adapted from Haddaway et al. [12].

2.1. Systematic Literature Review

A systematic literature review provides a robust framework for evaluating empirical evidence. In this research, we apply this approach to identify and analyze studies on constructed wetlands that specifically employ organic residues as substrates, while highlighting the circular economy strategies they foster. This methodology reduces bias in the selection process, ensuring a transparent and consistent review. Moreover, the use of explicit eligibility criteria enhances the replicability of the results.
For the literature search, we relied exclusively on Scopus due to its broad coverage of peer-reviewed publications and its recognition as a reliable source for systematic reviews, since Lunny et al. [13] emphasize that systematic reviews require transparent eligibility criteria and reproducible search strategies, both of which Scopus facilitates through its structured metadata and advanced filters, while Chen and Song [14] highlight that Scopus enables rigorous scientometric mapping, allowing researchers to visualize trends and research gaps, and Muka et al. [15] also note that Scopus is a suitable database for conducting systematic reviews and meta-analyses given its comprehensiveness and reliability, therefore no time restrictions were applied (see Section 2.1.1 and Section 2.1.2) and all selection decisions were documented to preserve rigor, reproducibility, and auditability.

2.1.1. Keyword-Based Selection Process

The selection of keywords was restricted to fewer than ten, in line with the editorial guidelines of scientific journals, which usually recommend a maximum of ten keywords for systematic indexing [16]. The terms “Constructed wetland” and “Wastewater” were chosen as the core descriptors of the study. In addition, representative organic substrates frequently reported in the literature were included (“Biochar”, “Coconut shell”, “Walnut shell”, “Sugarcane bagasse”, “Rice straw”, “Corn cob”, “Sawdust”), based on a preliminary scoping search that identified the most common lignocellulosic and agricultural wastes used in constructed wetland research. Broader descriptors such as “Organic substrate” were incorporated to ensure that other relevant residues—such as food waste, manure, or sludge—were not inadvertently excluded from the search. This strategy ensures comprehensive coverage while remaining concise and compliant with journal requirements.

2.1.2. Eligibility Criteria

The systematic search process was conducted following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, ensuring transparency and reproducibility throughout all stages of data collection and filtering [12]. The PRISMA 2020 scheme is presented in Figure 1, illustrating the number of records identified, screened, and included in the final dataset, as well as those excluded with justification. This structure supports methodological rigor and is aligned with the most recent practices in systematic environmental reviews [15].
The Scopus database was selected as the sole data source given its extensive coverage of peer-reviewed journals and its recognized reliability in environmental sciences and sustainability research [11,13]. Scopus offers broader metadata and citation tracking compared to other databases such as Web of Science or Google Scholar, which supports the development of robust scientometric analyses [14]. Additionally, Scopus provides advanced filters for subject areas, document types, and keywords, facilitating the retrieval of relevant records specifically related to “constructed wetlands”, “organic waste”, and “sustainability”.
The search terms were defined after an exploratory analysis of previous systematic reviews and thematic studies on constructed wetlands and waste valorization [1,2,3,4,5]. The main search string combined the terms “constructed wetlands”, “organic waste”, “waste management”, and “sustainability” using Boolean operators (AND, OR) to ensure inclusivity and precision. The selected words reflect the dominant terminology used in recent literature on circular economy applications in wetland systems [6,10]. However, it is acknowledged that some potentially relevant terms, such as “bio-waste” or “residual biomass”, were not included in the final search equation. These omissions could have limited the scope of the dataset by excluding studies with alternative terminology, a limitation discussed in Section 5.
The inclusion and exclusion criteria were established to ensure the selection of studies with methodological and thematic relevance. Inclusion criteria comprised: (i) studies published in peer-reviewed journals between 2000 and 2024; (ii) articles written in English; and (iii) research directly addressing the use or management of organic waste within constructed wetlands. As an inclusion criterion for selecting the studies in this systematic review, only open-access articles were considered, this decision was based on the need to ensure equitable access to scientific evidence, allowing the results and data to be consulted, verified, and reused by any researcher, regardless of institutional affiliation or financial resources.
Exclusion criteria included: (i) non-peer-reviewed materials (conference abstracts, editorials); (ii) studies focusing exclusively on inorganic pollutants; and (iii) articles lacking empirical or analytical data. These criteria are consistent with prior reviews that emphasize methodological transparency and thematic specificity in environmental and circular economy analyses [7,8,15]. Each criterion was justified to maintain a coherent analytical framework and to minimize potential bias during the selection process.
Step 1 Identification: A total of 325 documents were retrieved from Scopus using the defined search string: “Constructed wetland” AND “Wastewater” AND (“Organic substrate” OR “Biochar” OR “Coconut shell” OR “Walnut shell” OR “Sugarcane bagasse” OR “Rice straw” OR “Corn cob” OR “Sawdust”).
Step 2 Screening: Initial filters were applied to include only peer-reviewed journal articles, published in English, and open access. This step reduced the dataset to 72 articles. Titles and abstracts were then screened to ensure that the studies specifically addressed constructed wetlands as wastewater treatment technology and explicitly mentioned the use of organic residues or biochar as substrates. After this process, 49 articles remained.
Step 3 Eligibility: Full texts of the 49 articles were assessed to verify that the studies described replicable methodologies, demonstrated pollutant removal benefits, aligned with circular economy principles, and that the organic substrates or biochar showed equal or higher treatment efficiencies when compared with inorganic substrates. After this evaluation, 44 articles were retained.
Step 4 Inclusion: The 44 articles that met all the criteria were included in the final dataset for detailed analysis. Figure 1 presents the inclusion and exclusion criteria applied at each selection stage, showing the progression of included and excluded documents throughout the screening process.

2.2. Data Analysis

A qualitative analysis of the selected articles (n = 44) was conducted to evaluate the role of organic substrates in constructed wetlands for wastewater treatment. VOSviewer software (version 1.6.17) was applied to generate co-occurrence maps based on keyword frequency and correlation strength, consolidating redundant terms to refine the dataset.
For the second stage of the analysis, data were systematically extracted on country, type of CW, organic substrate(s), circular economy links and benefits (see Table 1). This enabled a comparative discussion of findings, with emphasis on replicable methodologies, pollutant removal efficiencies, and demonstrated contributions to circular economy strategies. The geographical scope of application was also examined to identify regional trends, opportunities, and design advances that could inform future implementations.

3. Results

This section presents key applications of constructed wetlands (CWs) that integrate circular economy (CE) strategies, particularly through the use of organic and waste-based substrates for wastewater treatment. By conducting a systematic review of eligible studies, this analysis provides a comprehensive perspective on the effectiveness of these systems, highlighting how circular approaches in CWs contribute to sustainability by closing material cycles, enhancing treatment efficiency, and generating social and environmental benefits in the regions where they are applied.
Table 1 summarizes the 44 eligible articles included in this review. The table highlights the main characteristics and contributions of each study, structured into five sections: country, type of constructed wetland (NBS), organic substrate(s) employed, circular economy links and benefits. This organization allows for a clear comparison across cases, showing not only the technological configurations of CWs but also the role of organic substrates in enhancing treatment performance and advancing circular economy strategies. By presenting the studies in this format, the review facilitates the identification of trends, gaps, and replicable practices relevant for future applications.

3.1. Publication Year

Figure 2 shows the growth in scientific production on constructed wetlands using organic substrates and biochar reflects the increasing demand for sustainable solutions in wastewater treatment, strongly aligned with the Sustainable Development Goals, particularly SDG 6 (clean water and sanitation) and SDG 12 (responsible consumption and production) [61,62]. Since 2015, initial exploratory studies such as De Rozari et al. [17] and Zhou et al. [20] paved the way, while from 2020 onwards research intensified, peaking in 2022 and maintaining a steady trend through 2023–2025. This evolution shows a clear shift from preliminary investigations to applied and innovative approaches, including hybrid systems, pilot-scale trials, and integrations with microbial fuel cells [41,60]. The trend highlights not only efforts to optimize pollutant removal efficiency but also the integration of circular economy principles and resource recovery, driving global dissemination and consolidation of these systems as viable alternatives to conventional technologies.

3.2. Topics Related to Organic Substrate in Constructed Wetlands

The keyword co-occurrence network (Figure 3) identifies three primary research clusters. The central focus remains on Biochar’s role in bioremediation and water quality (Red Cluster). Two other clusters address system performance and nutrient removal (Blue Cluster) and, notably, the growing concern with micropollutants and adsorption processes (Green Cluster). This thematic structure highlights the field’s maturity in addressing both macro-pollutants and emerging contaminants through substrate innovation.

3.3. Global Productivity

As shown in Figure 4, these trends can be partially explained by the exceptionally high research productivity of China and India in the environmental sciences. Both countries face severe water-stress challenges and rapidly growing urban populations, which have motivated substantial investments in nature-based wastewater treatment research. Their national research agendas prioritize low-cost, scalable, and decentralized solutions such as constructed wetlands, which may account for their disproportionate representation in the literature [31,32,59,63]. The predominance of both countries reflects their emphasis on low-cost, nature-based technologies and the reuse of agro-industrial residues as functional materials for wastewater treatment. In contrast, the United States focuses on technological innovation and the control of agricultural runoff, prioritizing performance optimization and scalability of CW systems [42]. Notably, Latin American countries such as Mexico, Argentina, and Peru exhibit increasing research activity, emphasizing decentralized treatment systems and the valorization of locally available biomass, including integration with microbial fuel cells to enhance energy recovery [47,64]. This geographical distribution underscores how local resource availability, environmental priorities, and policy frameworks shape the direction and intensity of scientific output in the field of constructed wetlands.

3.4. Classification of Organic Substrates in Constructed Wetlands

Figure 5 shows the classification of substrates according to their origin, revealing that biochar clearly dominates the field with 66.7% of the reviewed cases. This prevalence is explained by its versatility, low cost, and high adsorption capacity, which makes it the preferred option for integrating circular economy strategies across different contexts [41,42,43,44,50,51,52]. Agricultural residues represent 20.5% of the studies, mainly based on sugarcane bagasse, straw, and other crop by-products, highlighting their potential as locally available alternatives in developing countries [22,45]. Shell residues account for 5.1%, with coconut and oyster shells being used as structural support and filtration materials [36]. Forest residues also represent 5.1%, primarily involving bark and cork by-products incorporated into biochar-based matrices [30,40]. Finally, other organics, such as plant residues mixed with cellulose or peat moss combined with limestone, constitute 2.6% of the cases, reflecting emerging innovative approaches with specific environmental applications [49,58]. This distribution underscores the central role of biochar while also evidencing diversification towards agricultural and forestry residues, aligning with circular economy principles and resource recovery.

3.5. Types of Wetlands Constructed with Organic Waste as Substrate

Figure 6 shows the distribution of constructed wetland types in which organic residues have been applied as substrates. Most studies (54%) focused on Vertical Subsurface Flow Constructed Wetlands (VSFCW), a category that encompasses vertical flow systems, mesocosms, aerated and non-aerated configurations. These wetlands dominate due to their high efficiency in pollutant removal and adaptability to integrate diverse organic materials such as biochar and agricultural residues [17,20]. Horizontal Subsurface Flow Constructed Wetlands (HSFCW) represent 12% of the cases, commonly at lab or pilot scale, while Hybrid Constructed Wetlands (HCW), which combine vertical and horizontal flow systems or hybrid designs like bio-ecological trains, account for 10% [18,24]. Smaller but relevant contributions include Up-flow Compact Constructed Wetlands (UCCW) (3%), Flow Surface Constructed Wetlands (FSCW) (6%), which include horizontal surface-flow and post-treatment wetlands, and Floating Constructed Wetlands (FCW) (9%) that integrate multilayer floating mats, irrigation-oriented FTWs, or floating root mats. Finally, Coupled/Train CWs (6%) represent experimental approaches combining multiple stages such as coupled CWs or support-matrix screening. Identifying the prevalence of each wetland type is essential for understanding how organic residues are strategically integrated, since their application not only enhances treatment performance but also supports wastewater management under the principles of sustainability and circular economy. The predominance of vertical-subsurface flow constructed wetlands (VSFCW) in the reviewed studies (54%) may be associated with their operational advantages. VSFCW systems typically provide higher oxygen-transfer rates, making them suitable for treating variable organic loads and enhancing nitrification processes. In studies exploring alternative substrates, researchers may prefer VSFCW configurations because their hydraulic regime allows clearer observation of substrate performance under controlled aerated conditions [17,18].

3.6. Circular Economy Strategies Applied in Constructed Wetlands

Figure 7 shows the distribution of circular economy strategies applied in constructed wetlands using organic residues as substrates. Revalorize (39%) and Reuse (27%) dominate. Here, “Revalorize” refers to transforming agricultural by-products or biochar into cost-effective, functional materials, while “Reuse” indicates their direct utilization without significant modification. Reduce (13%) and Reintegrate (12%) represent complementary approaches aimed at minimizing the use of mineral substrates and enhancing nutrient cycling. Less frequent but innovative strategies such as Rethink (5%) and Reconnect (4%) highlight novel designs, including microbial fuel cells and improved ecological interactions. Overall, this trend demonstrates how circular economy principles are embedded in wastewater treatment to maximize efficiency and resource valorization [65,66].

4. Discussion

The discussion of this review has been expanded to highlight the broader systemic implications of our findings and to clearly articulate their relevance for the journal’s readership. Beyond synthesizing patterns in the use of organic substrates in constructed wetlands, the revised discussion situates these trends within larger debates on circular resource management, nature-based solutions, and decentralized wastewater treatment—central themes within the journal’s scope. By examining how different substrate choices reflect shifts toward waste valorization, environmental resilience, and sustainable water governance, we aim to demonstrate how our results contribute to both theoretical understanding and practical decision-making. This strengthened perspective provides researchers, practitioners, and policymakers with insights that support the development of scalable, equitable, and environmentally grounded treatment strategies aligned with global sustainability objectives.
Organic residues, when integrated under circular economy strategies, represent promising alternatives to conventional mineral substrates in constructed wetlands (CWs), which are consolidated as nature-based solutions (NBS) for wastewater treatment. Their use not only reduces waste streams and valorizes local materials but also contributes to pollutant removal, nutrient recovery, and climate change mitigation through carbon capture and reduced greenhouse gas emissions. To explore their role and potential, the discussion is organized into the following subsections: Section 4.1. Biochar, Section 4.2. Agricultural Residues, Section 4.3. Shell Residues, Section 4.4. Forest Residues, Section 4.5. Other Organic Substrates, Section 4.6. Comparative Perspective: Organic vs. Inorganic Substrates and Section 4.7. Benefits and Recommendations. This structure allows for a comprehensive evaluation of efficiency, modes of incorporation, rationale for use, and broader implications for sustainable wastewater management.

4.1. Biochar

Biochar, produced by the pyrolysis of organic residues such as plant biomass, sludge, and agricultural by-products, has emerged as an innovative and efficient substrate in constructed wetlands (CWs) for wastewater treatment [67]. Its main advantages lie in its high adsorption capacity, which is linked to its large specific surface area and porosity, allowing for the retention and removal of nutrients (nitrogen and phosphorus), heavy metals, organic matter, and emerging contaminants [68].
In addition, biochar enhances microbial activity by supporting the growth of beneficial communities, thus promoting nitrification, denitrification, and contaminant degradation processes. Several studies have demonstrated significant improvements in the removal of COD, NH4+-N, TN, and TP compared to conventional substrates, while also contributing to the reduction of greenhouse gas emissions such as N2O and CH4 [69,70]. Our results confirm these findings: Gupta et al. [18] reported that biochar combined with gravel achieved COD removal of 91.3% and TN removal of 58.3%, while Zhou et al. [20] showed that biochar with aeration reached 94.9% COD and 99.1% NH4+-N removal, also reducing N2O emissions. Other studies, such as De Rozari et al. [17,19] highlighted its efficiency in septage treatment and nutrient removal, although performance can depend on biochar proportion and influent characteristics. Further cases, including Xu et al. [28,32], demonstrated enhanced nitrate and phosphorus removal when biochar was modified with iron, manganese, or alumina, while Amirbekov et al. [29] showed that chamber biochar inserts achieved 96% HCH removal and enriched specific microbial communities.
The use of biochar in CWs also represents a clear example of circular economy integration. Pyrolysis revalorizes agricultural, forestry, and sludge residues by converting them into a substrate with high functional value, thus addressing waste management challenges while contributing to water quality improvement. In this sense, the strategies most often applied to biochar are Revalorize and Reuse, since pyrolysis both revalorizes waste through material transformation and allows its repeated application in treatment systems [70]. It also supports Reduce by replacing conventional mineral aggregates with renewable sources, and Reintegrate by contributing to nutrient cycling and carbon sequestration [71]. As evidenced in our dataset, biochar derived from coconut shells, corn cobs, bamboo, wood, or sewage sludge has shown consistent efficiency in pollutant removal, while simultaneously embodying the operationalization of circular economy principles within nature-based solutions [72].

4.2. Agricultural Residues

Agricultural residues, including by-products such as wheat straw, rice husks, corn cobs, sugarcane bagasse, and apricot pits, are generated in large volumes from farming and agro-industrial activities. Instead of being openly burned or inadequately disposed of, these residues can be valorized as substrates in constructed wetlands (CWs), where they are incorporated either as bulk media, conditioners, or carbon donors to enhance microbial processes. Their fibrous structure and organic composition provide an internal source of carbon, improving pollutant removal efficiency and supporting ecological functions [73,74].
The studies reviewed demonstrate significant efficiencies when agricultural residues are introduced in CWs. For example, Jia et al. [22] reported that wheat straw, at an optimal influent C/N ratio of ~0.5, achieved COD removal of 97%, NH4+-N removal of 99%, and TN removal of 96%, outperforming apricot pits and walnut shells. Saeed et al. [24] showed that sugarcane bagasse enabled COD (85%), TP (89%), and TN (80%) removal, while Soundaranayaki and Gandhimathi [25] found that mixed residues such as bagasse, wood mulch, and coir enhanced BOD removal (75–88%) and NH4+ removal (63–70%). Similarly, Visiy et al. [33] highlighted that corn-cob and rice-husk biochar filters outperformed sand in terms of COD, BOD, and TSS removal, showing adaptability under variable oxygen conditions. These results confirm the efficiency of agricultural residues as cost-effective substrates in CWs, comparable to or surpassing mineral-based media. Their incorporation reduces waste disposal impacts and provides a dual environmental benefit by mitigating air pollution from open burning and improving water quality through CW treatment [75].
Within the framework of the circular economy, agricultural residues embody strategies such as Revalorize (converting waste biomass into functional substrates), Reuse (repurposing residues that would otherwise be discarded), and Reintegrate (closing nutrient and carbon loops through microbial pathways) [75]. In some cases, Reduce also applies, as the use of local residues minimizes transport and reliance on non-renewable materials like sand or gravel. Together, these strategies illustrate how agricultural residues transform from an environmental liability into a valuable resource for sustainable wastewater treatment [76].

4.3. Shell Residues

Shell residues, including oyster, clam, and palm kernel shells, are by-products of aquaculture and food industries often discarded in large quantities. Their main component, calcium carbonate, provides a reactive matrix capable of adsorbing and precipitating phosphorus, as well as binding heavy metals and supporting the removal of organic matter. When integrated into constructed wetlands (CWs), these residues are typically applied as filter layers or mixed with other substrates to improve overall treatment performance [77,78].
Evidence from reviewed studies confirms their high efficiency for nutrient and metal removal. For example, Na-Ayuthaya et al. [36] incorporated coconut and oyster shells in horizontal surface-flow CWs and achieved rapid removal within ultrashort hydraulic retention times (2–4 h), reporting efficiencies of 97.9% for fats, oils, and grease (FOG), 95.9% for TSS, and over 86% for salinity and conductivity. Xu et al. [32] demonstrated that walnut shells, when paired with Mn ore and activated alumina, achieved COD removal of 89.4%, TP removal of 98.1%, and improved nitrogen removal efficiency while simultaneously reducing greenhouse gas emissions. Similarly, Visiy et al. [33] highlighted that corn-cob and rice-husk biochar outperformed sand in COD and BOD removal, though shells remain superior for phosphorus removal due to their calcium-rich composition.
From a circular economy perspective, shell residues embody Revalorize (transforming aquaculture waste into functional treatment media), Reuse (repurposing discarded shells as CW substrates), and Reduce (minimizing dependency on virgin mineral resources like sand or zeolite) [79]. They also align with Reintegrate, since phosphorus captured in shells can be recovered and potentially reused in agriculture. Collectively, these strategies highlight the dual role of shell residues in addressing waste management challenges and enhancing the sustainability of constructed wetlands [80].

4.4. Forest Residues

Forest residues, including palm branches, bark, wood mulch, and sawdust, are generated as by-products of forestry, landscaping, and timber industries. Traditionally discarded or incinerated, these materials are increasingly recognized as sustainable substrates for constructed wetlands (CWs), where they can replace conventional mineral media such as sand or gravel. Their organic composition and porous structure provide a favorable environment for microbial colonization, while also contributing carbon sources that enhance biological processes [80,81].
Our reviewed studies highlight their potential to improve contaminant removal. Soundaranayaki and Gandhimathi [25] showed that wetlands using organic wastes such as wood mulch and coir achieved BOD5 removals of 75–88%, COD removals of 72–82%, and NH4+-N reductions of 63–70%, outperforming conventional setups. Lei et al. [40] reported that bark–biochar matrices efficiently removed micropollutants, with 4 compounds achieving >70% removal, and highlighted their long-term durability. Similarly, Ayadi et al. [52] demonstrated that biochar from co-pyrolysis of sludge and sawdust enhanced COD and NH4+-N removal compared to gravel, while Guerrero-Brotons et al. [53] found that biochar–soil–gravel mixtures supported higher phosphorus availability and better microbial activity than mineral substrates alone.
From a circular economy perspective, forest residues primarily reflect Revalorize (transforming forestry by-products into treatment substrates), Reuse (repurposing discarded biomass within CWs), Reduce (minimizing extraction of sand and gravel), and Reconnect (fostering the link between human systems and natural ecosystems). In some cases, Reintegrate is also achieved, as nutrients captured or released by these substrates can support plant uptake and ecological restoration. Together, these strategies emphasize the environmental and functional value of forest residues as key components in nature-based wastewater treatment systems [82,83].

4.5. Other Organic Substrates

In addition to biochar, agricultural residues, shell waste, and forest residues, other unconventional organic substrates have also been explored in constructed wetlands (CWs). These include cork by-products, peat moss, sludge-fermentate, and bioelectrodes derived from aquatic biomass such as Lemna gibba. Their introduction into CWs often stems from the need to diversify substrate options, valorize region-specific waste streams, and enhance pollutant removal efficiency under varying operational conditions [84].
The reviewed studies demonstrate promising results. Aguilar et al. [39] tested cork by-products as a sole granular medium, reporting enhanced nitrate (52.8%) and TN (46.8%) removal, with strong durability after 1.5 years of operation. Jin et al. [43] showed that sludge-fermentate and rice straw supplied additional carbon, improving COD (87.6%), TN (72.2%), and NH3-N (81%) removal in bio-ecological treatment trains. Sacco et al. [49] demonstrated that biochar from plant residues mixed with toilet-paper cellulose achieved >90% removal of pharmaceuticals such as diclofenac and DEET, highlighting their potential for micropollutant control. Maldonado et al. [60] further advanced the field by integrating Lemna gibba-derived biochar electrodes in CW–MFC systems, achieving 98–100% antibiotic removal while also generating electricity. Finally, Naghoum et al. [58] found that peat moss combined with limestone neutralized acidity (pH 3.6–8.5) and removed metals such as Fe, Zn, and Cr with >96% efficiency.
From a circular economy standpoint, these materials embody Revalorize (turning unconventional waste into valuable treatment inputs), Reuse (repurposing biomass and cellulose residues), and in some cases Rethink (innovative integration of bioelectrodes in CW–MFC systems). Reintegrate also applies when internal carbon sources or nutrient cycles are supported, as observed with sludge fermentate and peat moss [85].

4.6. Comparative Perspective: Organic vs. Inorganic Substrates

In the treatment of wastewater, constructed wetlands rely on key inorganic substrates for their durability and ability to adsorb and precipitate pollutants. Typical substrates include sand and gravel, which act as physical filters, along with more active materials like clay and zeolite, known for their large surface area. These materials are particularly effective at phosphorus removal, as those enriched with calcium, iron, or aluminum enhance the precipitation and adsorption mechanisms, resulting in more efficient water purification [86].
Nevertheless, as illustrated in Figure 8, organic residues—including biochar, agricultural wastes, shell residues, forest residues, and other organics—demonstrated equal or superior performance for key wastewater parameters such as BOD (Biochemical Oxygen Demand), NH4+ (Ammonium), P (Phosphorus), and TSS (Total Suspended Solids). A critical advantage of organics is their ability to support the formation of biofilms, providing porous surfaces and nutrient-rich conditions that enhance microbial colonization. These microbial communities are essential for biological processes such as nitrification, denitrification, and organic matter degradation, resulting in higher pollutant removal efficiencies compared to inert inorganic media [87,88].
Figure 8 presents a comparative analysis of the average removal efficiencies (expressed as percentages) for key contaminants BOD, NH4+, P, and TSS across six categories of alternative adsorbent substrates used in water treatment studies. The results indicate that most substrate categories achieve high removal rates for BOD and TSS, with average efficiencies frequently exceeding 75%. Among the materials, Biochars and Agricultural residues stand out as the most effective overall, showing strong performance in removing BOD, P, and TSS.
In contrast, ammoniacal nitrogen (NH4+) removal is highly variable, as reflected by the wide error bars across all substrate types, suggesting that its removal is more sensitive to substrate characteristics or operational conditions. For phosphorus (P) removal, Inorganic materials and Shell residues generally perform better and more consistently, with average efficiencies surpassing 80%, highlighting their potential for eutrophication control in water treatment applications.
Overall, while inorganic substrates remain robust and reliable, organic residues demonstrate a dual benefit: they enhance wastewater treatment through biofilm-driven mechanisms and simultaneously contribute to circular economy strategies such as revalorization and reuse, turning waste streams into sustainable resources [90,91].
Biochar, agricultural, and shell residues have demonstrated higher effectiveness than inorganic substrates due to their porous structure, high surface area, and capacity to support microbial biofilms that enhance nutrient cycling and contaminant degradation [92]. Importantly, a distinction should be made between raw organic substrates and processed substrates, such as biochar, when evaluating life cycle impacts and energy costs. Raw organic wastes (e.g., agricultural residues, forest residues, shell residues) generally require minimal processing and thus entail lower energy inputs, making them comparatively low-impact options. In contrast, biochar production involves energy-intensive pyrolysis, which increases its overall environmental footprint despite providing enhanced adsorption capacity and microbial support. Recognizing these differences allows for a more nuanced assessment of sustainability trade-offs associated with substrate selection in constructed wetlands. Unlike sand or gravel, these organic substrates provide both physical filtration and a carbon source, improving nitrogen and phosphorus removal [93]. This underscores the potential of organic substrates as highly effective alternatives for advancing nature-based solutions in wastewater management.

4.7. Benefits and Recommendations

The integration of organic residues as substrates in constructed wetlands offers multiple benefits aligned with sustainability and circular economy strategies. As illustrated in Figure 9, these substrates contribute not only to cost reduction and pollutant removal efficiency but also to broader environmental and social gains. For instance, agricultural and forest residues support nutrient recovery and circularity, while biochar and shell residues enhance adsorption and promote biofilm growth, improving wastewater treatment performance [94]. Additionally, the use of local materials such as bagasse, bamboo, or fruit residues reduces transportation needs, lowers greenhouse gas emissions, and ensures accessibility in rural or resource-limited areas [80].
From a circular economy perspective, strategies such as revalorization, reuse, and reduce are predominant, fostering the valorization of waste materials, their reintegration into treatment systems, and the minimization of environmental impacts. Moreover, innovative approaches like microbial fuel cells coupled with biochar electrodes demonstrate how constructed wetlands can simultaneously treat wastewater, generate energy, and improve biosafety, highlighting their potential for modular and multifunctional applications [95].
In light of these findings, the integration of local organic residues in constructed wetland design is advised to ensure both community acceptance and economic viability. Our synthesis reveals critical areas for advancement: Future research must prioritize life-cycle assessments (LCA) and cost–benefit analyses to quantify the true environmental and economic impact of sourcing and processing these materials. Furthermore, investigations into the long-term stability (beyond five years) of these organic substrates and the scalability of innovative hybrid configurations (such as CW-MFC systems) are essential to simultaneously optimize energy recovery and enhance pollutant removal efficiency. Additionally, the selection criteria focused exclusively on peer-reviewed journal articles, thereby omitting grey literature, conference proceedings, and local-scale reports that might provide complementary contextual or technical information. Crucially, policy frameworks must be developed to incentivize and enable the transition toward these nature-based solutions as practical, sustainable substitutes for traditional wastewater treatment, particularly in resource-limited areas with decentralized infrastructure.
Beyond these broader research needs, the clear predominance of biochar (66.7%) across the reviewed studies underscores the importance of developing more targeted and practical lines of inquiry. Future work would benefit from comparative techno-economic assessments of decentralized and centralized pyrolysis models, as decentralized systems could reduce transport-related emissions and support local circularity, while centralized facilities may offer higher process control and efficiency. Likewise, understanding the long-term behavior of biochar substrates—including saturation processes, regeneration potential, and end-of-life pathways—is essential to prevent secondary pollution and ensure safe reuse or disposal. Finally, establishing standardized performance metrics, such as carbon sequestration potential, adsorptive lifespan, or cost per unit of pollutant removed, would greatly improve comparability across wetland configurations and substrate types. Together, these more focused recommendations can help steer future research toward solutions that are not only effective, but also operationally feasible, environmentally safe, and scalable in real-world contexts.

5. Conclusions

China and India lead research on organic substrates in constructed wetlands, together accounting for more than 50% of global publications. Their predominance is explained by the large availability of agricultural residues such as rice straw, bagasse, and coconut shells, combined with strong governmental support for low-cost wastewater treatment solutions. The United States also contributes with innovations in agricultural runoff control, while Latin America (Mexico, Argentina, Peru) is emerging with hybrid systems and bioelectricity generation.
Biochar dominates the field, representing 66.7% of reviewed studies, followed by agricultural residues (20.5%), shell residues (5.1%), forest residues (5.1%), and other organics (2.6%). Most applications are concentrated in Vertical Subsurface Flow Constructed Wetlands (VSFCWs, 54%), followed by Horizontal Subsurface Flow (HSFCWs, 12%), Hybrid systems (HCWs, 10%), Floating CWs (FCWs, 9%), Coupled/Train CWs (6%), and smaller contributions from FSCWs (6%) and UCCWs (3%). This distribution reflects both substrate availability and treatment goals.
The most applied strategies are Revalorize (39%) and Reuse (27%), followed by Reduce (13%) and Reintegrate (12%), with less frequent but innovative contributions from Rethink (5%) and Reconnect (4%). These strategies demonstrate how organic residues are transformed into functional substrates, extending their life cycle and reducing dependency on mineral aggregates.
Organic substrates consistently outperformed inorganics in pollutant removal. Biochar, agricultural residues and shell residues achieved BOD removals above 90%, NH4+ up to 95%, TSS above 80–90%, and TP above 85–98%, while inorganics like sand or gravel were less efficient, especially in nitrogen removal. Their higher performance is linked to porous structure, carbon release, and biofilm support, which enhance microbial processes of nitrification, denitrification, and organic matter degradation.
For future research and practical implementation, it is recommended to standardize the categorization of organic substrates into clear groups (e.g., biochar, agricultural residues, shell residues, forest residues, and other organics). Using common and consistent terminology will improve database searches, facilitate meta-analyses, and make comparisons of pollutant removal efficiencies more accessible across different contexts, while also helping to identify clear methodologies for the incorporation of these materials into constructed wetlands.

Author Contributions

Conceptualization, D.D.-S. and M.C.M.-R.; methodology, D.D.-S.; software, D.D.-S.; validation, D.D.-S., M.C.M.-R. and H.G.R.-E.; formal analysis, D.D.-S.; investigation, D.D.-S. and L.E.C.-V.; resources, M.C.M.-R. and X.V.B.-Y.; data curation, D.D.-S.; writing—original draft preparation, D.D.-S.; writing—review and editing, D.D.-S., M.C.M.-R. and H.G.R.-E.; visualization, D.D.-S.; supervision, M.C.M.-R. and H.G.R.-E.; project administration, M.C.M.-R. funding acquisition, M.C.M.-R. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. PRISMA flow diagram adapted from Haddaway et al. [12], illustrating the screening and selection process applied in this review. Autor own’s.
Figure 1. PRISMA flow diagram adapted from Haddaway et al. [12], illustrating the screening and selection process applied in this review. Autor own’s.
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Figure 2. Annual scientific production.
Figure 2. Annual scientific production.
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Figure 3. Keyword concurrency map obtained from Vosviewer. Source: Author’s own research.
Figure 3. Keyword concurrency map obtained from Vosviewer. Source: Author’s own research.
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Figure 4. Productivity of work related to the use of organic waste as substrates in constructed wetlands for wastewater treatment.
Figure 4. Productivity of work related to the use of organic waste as substrates in constructed wetlands for wastewater treatment.
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Figure 5. Classification of organic waste used as substrates in constructed wetlands.
Figure 5. Classification of organic waste used as substrates in constructed wetlands.
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Figure 6. Types of constructed wetlands applying organic residues as substrates.
Figure 6. Types of constructed wetlands applying organic residues as substrates.
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Figure 7. Circular economy strategies applied in constructed wetlands using organic residues as substrates.
Figure 7. Circular economy strategies applied in constructed wetlands using organic residues as substrates.
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Figure 8. Comparative removal efficiencies of organic and inorganic substrates in constructed wetlands for wastewater treatment [17,22,32,34,60,89].
Figure 8. Comparative removal efficiencies of organic and inorganic substrates in constructed wetlands for wastewater treatment [17,22,32,34,60,89].
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Figure 9. Key benefits of organic residues as substrates in constructed wetlands under a circular economy framework.
Figure 9. Key benefits of organic residues as substrates in constructed wetlands under a circular economy framework.
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Table 1. Content of reviewed articles.
Table 1. Content of reviewed articles.
ReferenceCountryType of CW (NBS)SubstrateCircular Economy StrategyBeneficios
[17]Australia/IndonesiaVertical Flow CW mesocosmsBiochar (woody) + sandRevalorize, ReuseLow cost, nutrient recovery, decentralization
[18]Republic of KoreaHorizontal Flow CWBiochar + gravelRevalorize, ReduceLower carbon
footprint, sustainable reuse
[19]Australia/IndonesiaVertical Flow CW
(mesocosms)		Biochar (woody) + sandReuse, RevalorizeUse of local biomass, less transport
[20]ChinaVertical Flow CW (aerated and non-aerated)Biochar (biomass
pyrolysis)		RevalorizeRecovers resources, sustainable water management
[21]IndiaConstructed WetlandCoconut shell biocharReuse, RevalorizeWaste converted to biochar, accessible
[22]ChinaIntermittently aerated VFCW (lab/pilot microcosms)Agricultural biomass (wheat straw, apricot pits, walnut shells)Reintegrate, ReuseExtra carbon,
improves natural
denitrification
[23]ChinaVertical Flow CWBiocharReduce, RevalorizeGHG decreases, sustainable closed cycle
[24]BangladeshHybrid (VF + HF) CWSugarcane bagasseReuse, RevalorizeAgricultural
by-products as an
alternative substrate
[25]IndiaVertical flow CWOrganic wastes (wood mulch, sugarcane bagasse, coir)RevalorizeAgricultural
Residues as CW Medium
[26]ChinaVertical up-flow CWFe-modified biochar (bamboo)Reuse, RevalorizeLocal bamboo and biochar,
a sustainable option
[27]China/AustraliaCW microcosmsBiochar + activated carbon (Fe3+, Mn4+ modified)Revalorize, ReconnectHigh Adsorption, Promotes Microbial Pathways
[28]ChinaCoupled CW (RDCW)Biochar with iron and microbesRevalorize, ReuseLow cost, scalable, adaptable
[29]Czech Republic/
Kazakhstan		Biochar-amended biofilter (CW-inspired)Biochar (chamber
inserts)		Reuse, RethinkModular, reusable, temporary option
[30]The NetherlandsCW support-matrix screening (batch & columns aimed at CWs)Biochar, bark, compost, cork (vs sand, gravel, LECA, lava)Rethink, ReduceLess mineral use, prolongs bed life
[31]ChinaBiochar-constructed wetlands under varying hydraulicsCoconut-shell biochar and shell mixesReuse, ReduceEnergy saving, waste use
[32]ChinaVertical subsurface-flow CW (substrate combinations)Walnut shell
(+ Mn ore, + activated alumina)		Reuse, ReduceLower environmental footprint, waste as a substrate
[33]Nigeria/CameroonVertical-flow CW (planted with E. pyramidalis)Corn-cob biochar;
rice-husk biochar (vs sand)		RevalorizeEconomical substitute, potential sand replacement
[34]Japan/ThailandVertical-flow CWs (saturated & semi-saturated)Corn-cob biocharReuse, RevalorizeImproved water, a simple and
economical option
[35]ThailandCW growth/substrate assay (mesocosms with Canna
indica)		Biochar and pumice vs. gravelReduce, RevalorizeUse of industrial minerals decreases
[36]ThailandHorizontal surface-flow CWs (aerated & non-aerated, pilot)Coconut shells; oyster shells (+ activated carbon layer)ReuseFast treatment, low hydraulic time
[37]Burkina Faso/
Ghana		Vertical-flow CW (yard-scale) planted with bambooBamboo biochar (as conditioner)Reuse, ReintegrateLocal bamboo, ecological adaptation
[38]IndiaFloating CW (multilayer) with Spirodela polyrhizaWood biochar mixed in sand matrixRevalorize, ReuseBiochar as carbon, decentralized
[39]ArgentinaInnovative vertical cork-based CW; aerated vs. non-aeratedCork by-product (sole granular medium)Revalorize,
Reintegrate		Internal source of carbon, circularity
[40]The NetherlandsVertical-flow mesocosm CWs with pre-treatmentsBark–biochar mix vs. sandRevalorize, ReuseRenewable matrices, longer uptime
[41]China/ChileReed-bed Constructed Wetland (HSFCW)Modified biochar (sulfuric acid)Revalorize, ReduceHigher atrazine removal, low cost, high efficiency
[42]United StatesEdge-of-field CW (agricultural runoff)Biochar (buckthorn), mussel shells, slagRevalorize, ReusePhosphorus capture, eutrophication control, regional option
[43]ChinaBio–ecological train: pond → biofilter → SSF-CW → polishing pondAdded carbon sources: sludge-fermentate, rice strawReintegrate,
Revalorize		Recover carbon, reduce emissions
[44]Thailand/DenmarkLab columns; planted/unplantedLongan-wood biochar (vs gravel)RevalorizeFruits converted into filtering biochar
[45]Republic of Korea/
Bangladesh		Lab & outdoor hybrid CWs (comparative LCA)Natural/industrial media (sugarcane
bagasse, coco-peat, brick, steel slag)		Revalorize, ReduceLower environmental footprint, low impact
[46]The NetherlandsBatch-operated vertical CWs; aerated vs. notBark–biochar matrix vs. sandRevalorizeAdsorption of contaminants, greater durability
[47]MexicoConstructed Wetland–Microbial Fuel Cell (CW-MFC)Carbon feltRethink, ReconnectGenerates bioelectricity, improves water quality
[48]United StatesHybrid Constructed Wetland (VSFCW + HSFCW)Biochar + zeoliteRevalorize, ReintegrationLeachate pretreatment, cost savings, water reuse
[49]Luxembourg/
Germany		VFCW (on-site greywater & WWTP polishing)Biochar from plant residues & toilet-paper cellulose (with sand)Reuse, RethinkReplaces minerals, uses organic waste
[50]ChinaVertical-flow CW
(intermittent aeration)		Alkali-modified corn cobs (slow-release C source)Reintegrate, RevalorizeExtra carbon, promotes denitrification
[51]UK/EU/ZAFloating treatment wetlands irrigationWillow biomass (for future biochar)Revalorize, ReuseEnergy and biochar from the same process
[52]Italy/TunisiaVertical-flow CW (planted & unplanted)Biochar from co-pyrolysis of biological sludge + sawdustRevalorize, ReuseSludge + sawdust in biochar, efficient adsorption
[53]SpainSubsurface-flow CW
(field-scale)		Gravel + 10% biochar (also gravel + 30% wetland soil)Revalorize, ReintegrateSupports biota, controls eutrophication
[54]PakistanHorizontal subsurface-flow CW (lab) planted (Typha, Phragmites)Bagasse-based biochar (charcoal)Revalorize, ReuseBagasse as biochar
[55]SpainFloating root mats (bench)Biochar added to FRMsRevalorize, ReconnectBiochar Boosts Microbiome
[56]The NetherlandsPost-treatment CWs (LCA/TEA)Bark–biochar vs. sandReduce, RevalorizeSpace-saving, lower environmental footprint
[57]OmanLarge oilfield CW (existing), field irrigation trialsBuffelgrass biochar (soil amendment)Reuse, ReintegrateLocal biomass in agricultural soils
[58]MoroccoVertical subsurface-flow
(pilot) planted/unplanted		Peat moss + limestone + gravelReintegrate, ReuseNeutralizes acidity, stabilizes metals
[59]IndiaVertical-flow CWs
(biochar-augmented)		Wood biocharReuse, RevalorizeMaximum efficiency, plant-substrate synergy
[60]PeruConstructed Wetland–Microbial Fuel Cell (CW-MFC)Biochar electrode (Lemna gibba)Revalorize, RethinkAntibiotics removal, additional electricity, biosafety
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Domínguez-Solís, D.; Martínez-Rodríguez, M.C.; Campos-Villegas, L.E.; Ramírez-Escamilla, H.G.; Bello-Yañez, X.V. Sustainable Management of Organic Waste as Substrates in Constructed Wetlands: A Systematic Review. Sustainability 2026, 18, 318. https://doi.org/10.3390/su18010318

AMA Style

Domínguez-Solís D, Martínez-Rodríguez MC, Campos-Villegas LE, Ramírez-Escamilla HG, Bello-Yañez XV. Sustainable Management of Organic Waste as Substrates in Constructed Wetlands: A Systematic Review. Sustainability. 2026; 18(1):318. https://doi.org/10.3390/su18010318

Chicago/Turabian Style

Domínguez-Solís, Diego, María Concepción Martínez-Rodríguez, Lorena Elizabeth Campos-Villegas, Héctor Guadalupe Ramírez-Escamilla, and Xochitl Virginia Bello-Yañez. 2026. "Sustainable Management of Organic Waste as Substrates in Constructed Wetlands: A Systematic Review" Sustainability 18, no. 1: 318. https://doi.org/10.3390/su18010318

APA Style

Domínguez-Solís, D., Martínez-Rodríguez, M. C., Campos-Villegas, L. E., Ramírez-Escamilla, H. G., & Bello-Yañez, X. V. (2026). Sustainable Management of Organic Waste as Substrates in Constructed Wetlands: A Systematic Review. Sustainability, 18(1), 318. https://doi.org/10.3390/su18010318

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