Gần đây, việc sử dụng đất ngập nước (ĐNN) để xử lý nước ô nhiễm đã nhận được nhiều quan tâm trên thế giới do biện pháp này tương đối rẻ tiền và có khả năng cải thiện tình trạng của hệ sinh thái khu vực. Hiện nay, trên thế giới có nhiều định nghĩa khác nhau về đất ngập nước tùy theo mỗi quốc gia và mục đích quản lý, sử dụng chúng. Ở Việt Nam, định nghĩa về ĐNN được lấy chính thức theo Công ước Ramsar quy định: ĐNN là những vùng đầm lầy, than bùn hoặc vùng nước bất kể là tự nhiên hay nhân tạo, thường xuyên hay tạm thời, có nước chảy hay nước tù, là nước ngọt, nước lợ hay nước biển, kể cả những vùng nước biển có độ sâu không quá 6 m khi triều thấp. Có nhiều loại đất ngập nước tự nhiên và nhân tạo đã được sử dụng để xử lý nước mỏ ô nhiễm kim loại ở nhiều mức độ khác nhau.
Introduction
Scope
Constructed wetlands are engineered systems designed for wastewater treatment, featuring shallow ponds or channels, typically less than 1 meter deep, that are populated with aquatic plants These systems utilize a combination of natural microbial, biological, physical, and chemical processes to effectively treat wastewater They are usually lined with impervious clay or synthetic materials and include structures to manage flow direction, liquid detention time, and water levels Depending on the design, constructed wetlands may also incorporate inert porous media, such as rock, gravel, or sand, to enhance treatment efficiency.
Constructed wetlands are effective in treating various types of wastewater, such as urban runoff, municipal, industrial, agricultural, and acid mine drainage This manual specifically focuses on constructed wetlands as the primary treatment method for municipal wastewater Although some pre- or post-treatment may be necessary to achieve compliance with discharge or reuse standards, the wetland remains the key component of the treatment system.
This manual explores the capabilities and design approach of constructed wetlands, emphasizing their management requirements for optimal performance Constructed wetlands are a cost-effective and low-maintenance solution that offers aesthetic and ecological benefits, making them ideal for certain applications However, their need for substantial land—ranging from 4 to 25 acres per million gallons of flow per day—limits their suitability for some scenarios They are particularly advantageous for wastewater treatment in small communities with affordable land and a shortage of skilled operators.
Terminology
Understanding the terminology is essential for distinguishing constructed wetlands from other wetland types According to Federal regulations, wetlands are defined as areas that are inundated or saturated by surface or groundwater for a duration and frequency adequate to support specific ecological functions.
Chapter 1 Introduction to the Manual normal circumstances do support, a prevalence of veg- etation typically adapted for life in saturated soil condi- tions Wetlands generally include swamps, marshes, bogs and similar areas.” (40 CFR 230.3(t)) Artificial wetlands are wetlands that have been built or extensively modified by humans, as opposed to natural wetlands which are existing wetlands that have had little or no modification by humans, such as filling, draining, or altering the flow pat- terns or physical properties of the wetland The modifica- tion or direct use of natural wetlands for wastewater treat- ment is discouraged and natural wetlands are not dis- cussed in this manual (see discussion of policy issues in Section 1.7.2).
Constructed wetlands are artificial ecosystems designed specifically for wastewater treatment, typically featuring uniform depths and regular shapes near wastewater sources, often in previously non-wetland upland areas These systems are regulated as wastewater treatment facilities and are not permitted for compensatory mitigation purposes To clarify their primary function, some EPA documents refer to them as constructed treatment wetlands.
Constructed wetlands, often referred to as enhancement wetlands, offer advanced treatment for wastewater that has undergone secondary pretreatment In addition to their primary function, these wetlands provide valuable benefits such as wildlife habitats, research opportunities, and recreational spaces.
Constructed wetlands are categorized into two main types: Free Water Surface (FWS) wetlands, which resemble natural wetlands with aquatic plants rooted in a soil layer and water flowing through their leaves and stems, and Vegetated Submerged Bed (VSB) systems, which differ significantly as they lack standing water VSB systems consist of a media bed, such as crushed rock or gravel, planted with aquatic vegetation, allowing wastewater to flow beneath the surface and interact with the plant roots without being visible to wildlife Proper design and operation of these systems are essential for effective wastewater management.
The term "vegetated submerged bed" is preferred over "subsurface flow wetland" in this manual, as it provides a more precise and descriptive definition of these systems This terminology has been utilized in prior literature (WPCF, 1990; USEPA, 1994).
Some VSBs may meet the strict definition of a wetland, but a
VSBs do not promote aquatic wildlife due to their water level remaining below the surface of the media, which hinders essential biological and chemical interactions found in wetlands with open water columns Although VSBs are often referred to as constructed wetlands in literature, they are included in this manual for reference.
Constructed wetlands differ from created or restored wetlands, which primarily serve as wildlife habitats While created wetlands aim to replicate natural wetlands, they typically incorporate diverse features, including varying water depths, open water areas, dense vegetation zones, and a range of plant types from submerged aquatic species to shrubs and trees, along with nesting islands and irregular shorelines.
Wetlands are often constructed or restored in areas that historically featured these ecosystems, serving primarily as compensatory mitigation This manual does not cover the creation or restoration of wetlands intended for habitat or compensatory purposes.
A vertical flow wetland refers to a sand or gravel filter that is integrated with aquatic plants The effectiveness of this system relies on its function as a filter, which requires regular dosing and draining cycles Therefore, this manual will not cover this type of system.
Relationship to Previous EPA Documents
Several Offices or Programs within USEPA have published documents in recent years on the subject of constructed wetlands Some examples of publications and their USEPA sponsors are:
• Design Manual: Constructed Wetlands and Aquatic Plant
Systems for Municipal Wastewater Treatment (1988)
(Office of Research and Development, Cincinnati, OH,
• Subsurface Flow Constructed Wetlands for Wastewa- ter Treatment: A Technology Assessment (1993) (Office of Wastewater Management, Washington, DC, EPA 832-
• Habitat Quality Assessment of Wetland Treatment Sys- tems (3 studies in 1992 and 1993) (Environmental Re- search Lab, Corvallis, OR, EPA 600-R-92-229, EPA 600-
• Constructed Wetlands for Wastewater Treatment and
Wildlife Habitat: 17 Case Studies (1993) (Office of Waste- water Management, Washington, DC, EPA 832-R-93-
• Guidance for Design and Construction of a Subsur- face Flow Constructed Wetland (August 1993) (USEPA
Region VI, Municipal Facilities Branch)
• A Handbook of Constructed Wetlands (5 volumes,
1995) (USEPA Region III with USDA, NRCS, ISBN 0- 16-052999-9)
• Constructed Wetlands for Animal Waste Treatment: A Manual on Performance, Design, and Operation With Cases Histories (1997) (USEPA Gulf of Mexico Pro- gram)
• Free Water Surface Wetlands for Wastewater Treat- ment: A Technology Assessment (1999) (Office of Wastewater Management, Washington, DC, EPA /832/ R-99/002)
This manual may present information that contradicts other documents due to new insights, previous misconceptions about constructed wetlands, or differing expert opinions in the absence of clear answers It aims to provide an environmental engineering perspective on the design, use, and performance of constructed wetlands, relying on the highest quality data currently available In cases of expert disagreement, the manual adopts a conservative approach based on established treatment mechanisms supported by valid data.
Wetlands Treatment Database
Through a series of efforts funded by the USEPA, a Wetlands Treatment Database, “North American Wetlands for Water Quality Treatment Database or NADB” (USEPA,
The North American Database (NADB), first released in 1994, compiles information on natural and constructed wetlands utilized for wastewater treatment across over 30 U.S states and Canadian provinces, featuring 174 treatment wetland locations The database provides essential site and system-specific details, including flow rates, dimensions, plant species, and contact information, along with literature references and permit data While it includes some water quality metrics such as BOD, TSS, nitrogen series, phosphorus, dissolved oxygen, and fecal coliforms, the inconsistency in data quality and quantity limits its suitability for design or modeling applications.
Version 2 of the NADB is currently undergoing Agency review and contains information on treatment wetlands at
Version 2 identifies over 800 individual wetland cells across 245 locations in the US and Canada, reflecting a significant expansion in the number of wetland sites compared to previous versions.
1, Version 2 also contains information regarding vegeta- tion, wildlife, human use, biomonitoring and additional water quality data As with Version 1, the data is not adequate for design or modeling.
The National Aquatic Database (NADB) primarily compiles existing data on constructed wetlands, but many entries are incomplete or lack sufficient information While the NADB serves as a valuable resource for understanding the general status and locations of operational wetland systems and contact information, it falls short in providing reliable water quality data necessary for wetland design and predicting treatment performance.
Tables 1.1 to 1.4 provide a comprehensive overview of Version 2 of the NADB, highlighting the size range and median size of various wetland types The inclusion of median size is particularly important, as it offers a more accurate representation of the group's characteristics by mitigating the influence of a few exceptionally large wetlands.
Tables 1.1 and 1.2 categorize wetlands based on their type and the wastewater they treat Generally, free water surface wetlands (FWSs) are larger than vertical subsurface flow wetlands (VSBs), with FWSs having a median size that is double that of VSBs Additionally, the summary statistics for "other water" wetlands in Table 1.2 may be skewed due to the significant impact of the Everglades Nutrient Removal project in Florida.
Table 1-1 Types of Wetlands in the NADB
Type of Wetland Qty Min Median Max.
Vegetated Submerged Bed (all Marsh) 49 0.004 0.5 498
Combined FWS & VSB (all Marsh) 8 0.1 0.4 17
Natural Wetlands (all Free Water Surface) 38 0.2 40 1093
*Marshes are characterized by soft-stemmed herbaceous plants, including emergent species, such as cattails, floating species, such as water lilies, and submerged species, such as pondweeds (Niering, 1985)
Table 1-2 Types of Wastewater Treated and Level of Pretreatment for NADB Wetlands
Wastewater Type Pretreatment Qty Min Median Max.
Table 1.3 categorizes wetlands by size, highlighting that most are under 10 hectares (25 acres), with nearly 90% being less than 100 hectares (250 acres) Additionally, regarding design flow rates, the majority of wetlands have a capacity of less than 1000 m³/d (approximately 0.25 mgd), while 82% operate below 4060 m³/d (1 mgd).
Table 1.4 categorizes wetlands by location, encompassing all types of wetlands and wastewater treatment Treatment wetlands are found across 34 states in the U.S and 6 provinces in Canada The prevalence of wetlands in each state is likely influenced more by the presence of advocates for treatment wetlands than by climate or other favorable environmental conditions.
Table 1-3 Size Distribution of Wetlands in the NADB
Range Percentage Range Percentage less than 1 46 less than 10 19 less than 10 75 less than 100 31 less than 100 93 less than 1000 62 less than 1000 99 less than 10,000 93
Table 1-4 Distribution of Wetlands in the NADB by State/Province
Number of Size (hectares) State or Province* Wetlands Min Median Max.
CT, IL, MA, NJ, NWT,
*Two-letter abbreviations are states; three-letter abbreviations are provinces.
History
Kadlec and Knight (1996) provide a comprehensive historical overview of natural and constructed wetlands for wastewater treatment, noting that natural wetlands have likely been utilized for this purpose since the early 20th century, with documented instances of wastewater discharges as far back as 1912 Early researchers of constructed wetlands were inspired by the effective treatment capabilities of natural wetlands, while others recognized the potential of wastewater as a resource for wetland restoration Research on constructed wetlands began in Europe during the 1950s and expanded to the United States in the late 1960s, gaining momentum throughout the 1970s and 1980s, particularly with significant support from the Tennessee Valley Authority and the U.S Department of Agriculture However, the limited involvement of the USEPA in constructed wetlands research has resulted in a scarcity of reliable, quality-assured data.
Table 1.5 presents the start dates for constructed wetlands in the North American Database (NADB), alongside those of natural wetlands for comparison The data indicates that the implementation of Free Water Surface (FWS) wetlands and Vegetated Swale Basins (VSBs) in North America commenced in the early and late 1980s, respectively, with a continuous rise in their numbers In contrast, no new natural wetland treatment systems have been initiated since 1990, and over one-third of the natural wetland treatment systems listed in the NADB are currently non-operational.
Since the release of the USEPA's design manual in 1988, numerous texts and design guidelines for constructed wetlands have been published by various organizations, including the EC/EWPCA, WPCF, Tennessee Valley Authority, and USDA Additionally, international conferences have been held to share research findings on constructed wetlands from around the globe, showcasing the ongoing development and understanding of this innovative approach to water treatment.
Table 1-5 Start Date of Treatment Wetlands in the NADB before 1950's & 1990's
*Year of last wetland included in database for this type of wetland - other wetlands may have started after this date, but are not in the database.
Despite significant resources allocated to constructed wetlands, there are still prevalent questions and misconceptions regarding their application, design, and performance This section outlines four common misconceptions, with further details available in other chapters.
Misconception #1: The design of constructed wetlands is often misunderstood as being well-defined by established equations In reality, these systems are intricate due to their biological, hydraulic, and chemical complexities A significant challenge is the insufficient quality and detail of data on full-scale constructed wetlands, which forces designers to rely on aggregated performance data from various sources, leading to uncertainties in design parameters Often, data from rigorously studied wetlands is mixed with that from less reliable sources, such as small wetlands with minimal pretreatment or poorly monitored larger systems Additional issues include a lack of paired influent-effluent samples, reliance on grab samples, insufficient flow or detention time data, and missing critical information like temperature and precipitation Consequently, the resulting data sets can lead to the development of regression equations that are not reliable for design purposes Furthermore, using data from wetlands treating higher quality wastewater to inform parameters for more concentrated municipal applications raises concerns about the validity of these design guidelines.
A common misconception about constructed wetlands is that they contain both aerobic and anaerobic treatment zones Emergent wetland plants are specifically adapted to thrive in anaerobic conditions, as they can transport oxygen from the atmosphere to their roots Research indicates that these plants can also release some oxygen into the surrounding soil, enhancing the overall treatment process.
Early studies on natural and constructed wetlands have suggested that significant aerobic micro-sites exist within all wetland systems, leading to the belief that densely vegetated wetlands function as aerobic systems due to the presence of "leaking" oxygen However, this assumption is challenged by the fact that findings from tertiary or polishing wetlands do not adequately apply to those treating higher strength wastewater The characteristics of wetlands change significantly when exposed to the high oxygen demand of more contaminated municipal wastewater, impairing treatment mechanisms that work effectively under lighter loads Research and field experience indicate that the minimal oxygen released from plant roots is negligible compared to the oxygen demand posed by municipal wastewater at practical loading rates.
Misconception #3 suggests that constructed wetlands can effectively remove significant amounts of nitrogen; however, studies indicate that harvesting only removes less than 20% of influent nitrogen The primary mechanisms for nitrogen removal are nitrification and denitrification, which rely on the presence of aerobic and anaerobic zones While nitrification of ammonia to nitrate can occur in aerobic zones, it is often limited in constructed wetlands where anaerobic processes dominate This misunderstanding has led to the failure of several constructed wetlands designed for nitrogen removal To effectively remove nitrogen, constructed wetlands must be designed with adequate aerobic (open water) and anaerobic (vegetated) zones, or they should be integrated with other aerobic treatment processes that can facilitate nitrification.
Misconception #4 suggests that constructed wetlands can effectively remove significant amounts of phosphorus; however, their phosphorus removal is primarily limited to seasonal plant uptake, which is minor compared to the phosphorus load in municipal wastewater This uptake diminishes during plant senescence and is also dependent on the sorption capacity of influent solids, soils, or plant detritus, all of which have limited capacities Furthermore, phosphorus removal data in the literature can be misleading, as early studies often reported percent removal based on low influent phosphorus concentrations, making small reductions appear substantial Additionally, phosphorus removal in newly constructed wetlands is not indicative of long-term performance; newly planted systems initially absorb more phosphorus, while mature wetlands experience leaching from dying plants and have saturated sorption sites.
1.7 When to Use Constructed Wetlands
Appropriate technology is defined as a treatment sys- tem which meets the following key criteria:
Affordable - Total annual costs, including capital, op- eration, maintenance and depreciation are within the user’s ability to pay.
Operable - Operation of the system is possible with locally available labor and support.
Reliable - Effluent quality requirements can be con- sistently meet.
Many rural areas in the U.S with small treatment plants, defined as those treating less than 3,800 m³/d (1 mgd), have often overlooked appropriate technology definitions and adopted unsuitable systems like activated sludge In 1980, 39% of small publicly owned treatment facilities utilized activated sludge, and recent data indicates that 73% of treatment plants with capacities under 3,800 m³/d employ this process However, experts agree that the activated sludge process is among the most challenging wastewater treatment methods to operate and maintain.
Presently, small treatment plants constitute more than 90% of the violations of U.S discharge standards At least one
U.S state, Tennessee, has required justification for the use of activated sludge package plants for very small treat- ment plant applications (Tennessee Department of Public
Small community budgets are heavily impacted by the high costs associated with wastewater collection and treatment facilities Limited financial resources and insufficient access to necessary equipment hinder effective operation and maintenance (O&M) The burden of unaffordable capital costs, coupled with challenges in consistently meeting effluent quality standards, exemplifies a failure to adhere to appropriate technology criteria Additionally, there has been a lack of consideration for alternatives such as water reuse and groundwater recharge, with these practices only being adopted in a few states facing water shortages.
Small communities have a limited selection of effective technologies to consider for wastewater treatment, including stabilization ponds, slow sand filters, land treatment systems, and constructed wetlands These options are operable, relatively affordable, and reliable in performance Each technology has specific characteristics and requirements for pre- and post-treatment to achieve desired effluent quality, allowing them to be used individually or in combination based on treatment objectives.
Designers often enhance stabilization ponds with tertiary systems to consistently meet reuse or discharge standards One effective method for upgrading these ponds is the use of Free Water Surface (FWS) wetlands, which improve settling and significantly reduce fecal coliform levels while also removing excess algal growth common in pond effluents FWS wetlands are typically implemented after stabilization ponds due to their capacity to manage the algal solids produced While Vegetated Swale Basins (VSBs) have been used post-pond, they can struggle with excess algal solids, which may hinder their effectiveness VSBs are better suited for use following processes that reduce suspended and settleable solids, such as septic tanks, Imhoff tanks, or anaerobic lagoons.
Constructed wetlands may necessitate additional post-treatment processes to meet specific effluent requirements, particularly for nitrogen and phosphorus removal, which is often overestimated in their capabilities To effectively achieve ammonium oxidation, technologies such as intermittent and recirculating sand filters are recommended A successful case study demonstrated the use of a recirculating gravel filter alongside a vertical flow system Furthermore, free water surface (FWS) systems can efficiently nitrify and denitrify wastewater by alternating between vegetated and open water zones However, if significant phosphorus reduction is mandated, constructed wetlands must be supplemented with additional processes to achieve the desired levels, such as reducing phosphorus from 6-7 mg/L to 1 mg/L.
Constructed wetlands are an effective solution for regions with affordable land and limited skilled labor Their suitability for standalone use or in conjunction with other technologies hinges on specific treatment objectives Furthermore, they serve as viable onsite systems when local regulations permit alternatives to traditional septic tank and soil absorption systems.
An interagency workgroup, including representatives from several Federal agencies, is presently developing Guiding Principles for Constructed Treatment Wetlands: Providing Water Quality and Wildlife Habitat (USEPA,
1999) The essence of the current draft of the guidelines is that constructed treatment wetlands will:
— receive no credit as mitigation wetlands;
— be subject to the same rules as treatment lagoons regarding liner requirements;
— be subject to the same monitoring requirements as treatment lagoons;
— should not be constructed in the waters of the United States, including existing natural wetlands; and
— will not be considered Waters of the United States upon abandonment if the first and the fourth condi- tions are met.
The guidance encourages use of local plant species and expresses concern about permit compliance during lengthy startup periods and vector attraction and control issues.
When to Use Constructed Wetlands
Appropriate technology is defined as a treatment sys- tem which meets the following key criteria:
Affordable - Total annual costs, including capital, op- eration, maintenance and depreciation are within the user’s ability to pay.
Operable - Operation of the system is possible with locally available labor and support.
Reliable - Effluent quality requirements can be con- sistently meet.
Many rural areas in the U.S with small treatment plants, defined as those treating less than 3,800 m³/d (1 mgd), have often overlooked appropriate technology definitions and instead adopted unsuitable methods like activated sludge In 1980, activated sludge systems made up 39% of small publicly owned treatment facilities, and recent data indicates that 73% of treatment plants with capacities under 3,800 m³/d utilize some form of this process However, the activated sludge process is widely regarded by U.S and international experts as one of the most challenging wastewater treatment methods to operate and maintain.
Presently, small treatment plants constitute more than 90% of the violations of U.S discharge standards At least one
U.S state, Tennessee, has required justification for the use of activated sludge package plants for very small treat- ment plant applications (Tennessee Department of Public
Small community budgets are significantly impacted by the high costs associated with wastewater collection and treatment facilities Limited financial resources, coupled with inadequate access to necessary equipment and repair services, hinder effective operation and maintenance (O&M) The combination of unaffordable capital expenditures and challenges in consistently meeting effluent quality standards exemplifies a failure to adhere to appropriate technology criteria Additionally, there has been a lack of consideration for water reuse, groundwater recharge, or alternative discharge methods, with such practices only being adopted in a few states facing water shortages.
Small communities should consider a limited number of effective technologies for wastewater treatment, including stabilization ponds, slow sand filters, land treatment systems, and constructed wetlands These options are operable, affordable, and reliable in performance Each technology has specific characteristics and requirements for pre- and post-treatment to achieve desired effluent quality, allowing them to be used individually or in combination based on treatment objectives.
Designers may enhance stabilization ponds with a tertiary system to consistently meet reuse or discharge criteria Upgrading methods such as Free Water Surface (FWS) wetlands can improve effluent standards by promoting enhanced settling, which reduces fecal coliforms and removes excess algal growth typical of pond effluents FWS wetlands are particularly effective following ponds, as they manage the surplus algal solids produced While Vegetated Swale Basins (VSBs) have been used after ponds, they can encounter challenges due to excess algal solids, compromising their effectiveness VSBs are best suited for systems that minimize suspended and settleable solids, such as septic tanks, Imhoff tanks, or anaerobic lagoons.
Constructed wetlands may necessitate additional post-treatment processes to meet stringent effluent requirements, as their ability to remove nitrogen and phosphorus is often overestimated Effective technologies for ammonium oxidation include intermittent and recirculating sand filters, with successful case studies demonstrating the use of recirculating gravel filters alongside vertical flow systems Furthermore, free water surface (FWS) systems can achieve both nitrification and denitrification, significantly reducing nitrogen levels in wastewater by utilizing a balanced mix of vegetated and open water zones However, if a municipal facility aims for substantial phosphorus removal—such as reducing levels from 6 to 7 mg/L to 1 mg/L—constructed wetlands must be paired with additional processes capable of effectively removing phosphorus.
In summary, constructed wetlands are a suitable solution for regions with affordable land and limited skilled labor Their effectiveness, whether used independently or in conjunction with other technologies, hinges on specific treatment objectives Furthermore, they are viable for onsite systems, especially where local regulations permit alternatives to traditional septic tank and soil absorption systems.
An interagency workgroup, including representatives from several Federal agencies, is presently developing Guiding Principles for Constructed Treatment Wetlands: Providing Water Quality and Wildlife Habitat (USEPA,
1999) The essence of the current draft of the guidelines is that constructed treatment wetlands will:
— receive no credit as mitigation wetlands;
— be subject to the same rules as treatment lagoons regarding liner requirements;
— be subject to the same monitoring requirements as treatment lagoons;
— should not be constructed in the waters of the United States, including existing natural wetlands; and
— will not be considered Waters of the United States upon abandonment if the first and the fourth condi- tions are met.
The guidance encourages use of local plant species and expresses concern about permit compliance during lengthy startup periods and vector attraction and control issues.
To prevent extra permitting and regulatory obligations, constructed wetlands should be designed as a treatment process and located in upland areas rather than in wetlands or floodplains, ensuring they are situated outside of U.S waters.
If your constructed treatment wetland is located within a water of the U.S., it will retain its status unless you obtain a specific CWA Section 404 permit that designates it as an excluded waste treatment system compliant with the CWA After construction, if your wetland qualifies as a water of the U.S., you must secure an NPDES permit for any pollutant discharges into it Furthermore, using a degraded wetland for wastewater treatment and building water control structures like berms or levees will necessitate a Section 404 permit, and ongoing maintenance may also require permitting.
As stated in the guidelines:
An abandoned constructed wetland may regain its status as a water of the U.S if it exhibits wetland characteristics, such as specific hydrology, soils, and vegetation This reversion occurs under two conditions: if the wetland is classified as an interstate wetland or if it is adjacent to another water of the U.S.
(other than waters which are themselves wetlands), or (3) if it is an isolated intrastate water which has a nexus to interstate commerce (e.g., it provides habitat for migratory birds).
None of preceding discussion precludes designing and building a wetland which provides water reuse, habitat or public use benefits in addition to wastewater treatment.
Constructed wetlands designed mainly for treatment are typically not recognized as compensatory mitigation for wetland losses However, in certain instances, portions of a constructed wetland system may qualify for credit, particularly if they include additional wetland areas beyond treatment needs Current policies also promote the use of adequately treated wastewater to aid in the restoration of degraded wetlands, making restoration feasible under specific conditions.
The source water complies with all relevant quality standards and criteria, ensuring its safety and suitability for use Additionally, utilizing this water would provide a net environmental benefit, enhancing the natural functions and values of the aquatic ecosystem If applicable, it would also contribute to restoring the aquatic system to its historical condition.
Restoration efforts should focus on degraded wetlands, particularly in the arid western regions where historic wetlands lack reliable water sources due to upstream water allocations and declining groundwater levels In these areas, pre-treated effluent may serve as the sole water supply for both the wetlands and the ecosystems that rely on them To support these initiatives, the EPA has established regional guidance to help dischargers and regulators demonstrate a net ecological benefit from maintaining wastewater discharges into water bodies.
Policy and permitting issues related to constructed wetlands are typically addressed on a case-by-case basis It is essential for planners and designers to consult with State and Regional regulators to understand site-specific criteria, including location, discharge requirements, and potential long-term monitoring obligations.
Constructed wetlands are highly appealing to the public, serving as attractive landscape enhancements that treat wastewater and contribute positively to the community This aesthetic value plays a crucial role in discussions about upgrading wastewater treatment systems However, the engineering community often overlooks this appeal, while environmentalists may not fully understand the treatment mechanisms and limitations of the technology In some cases, the natural beauty and additional aesthetic benefits of constructed wetlands can outweigh the treatment efficiency or cost-effectiveness of alternative options, leading public opinion to favor them Conversely, there are instances where constructed wetlands may be too expensive or fail to meet required effluent quality standards, necessitating efforts to inform the public about their limitations despite their visual allure.
Constructed wetlands as a wastewater treatment technology present certain risks due to inconsistent acceptance among state regulators and EPA regions While some authorities recognize constructed wetlands as a proven solution, others view them as an emerging technology, influenced by misconceptions Achieving uniform acceptance for constructed wetlands, similar to the gradual acceptance experienced by other natural treatment processes like slow rate or overland flow systems, will require time and further validation.
Use of This Manual
Chapters 1, 2, 7, and 8 offer essential insights for non-technical readers, including decision-makers and stakeholders, to grasp the capabilities and limitations of constructed wetlands This information equips them to engage effectively with designers and regulators when exploring the potential for constructed wetlands to enhance, upgrade, or develop wastewater treatment infrastructure.
Chapters 3 through 6 provide information for technical readers, such as design engineers, regulators and plan- ners, to plan, design, build and manage constructed wet- lands as part of a comprehensive plan for local and re- gional management of municipal wastewater collection, treatment, and reuse.
Chapter 2 describes constructed wetland treatment sys- tems and their identifiable features It answers the most frequently asked questions about these systems and in- cludes a glossary of terms which are used in this manual and generally in discussion of constructed wetland sys- tems There are brief discussions of other aquatic treat- ment systems that are in use or are commercially avail- able and an annotated introduction to specific uses for constructed wetlands outside the purview of this manual.
Chapter 3 discusses the treatment mechanisms occur- ring in a constructed wetland to help the reader under- stand the most important processes and what climatic con- ditions and other physical phenomena most affect these processes A basic understanding of the mechanisms in- volved will allow the reader to more intelligently interpret information from other literature sources as well as infor- mation in chapters 4 and 5 of this manual.
Chapters 4, 5, and 6 provide an in-depth exploration of the design, construction, startup, and operational challenges of constructed wetlands The analysis highlights a significant lack of reliable data for developing confident treatment models, as much of the existing literature suffers from inadequate quality assurance and control Additionally, many studies fail to document critical variables that could clarify performance characteristics Utilizing high-quality existing data, Chapters 4 and 5 propose a practical framework for the applicability and design of both Free Water Surface (FWS) and Vertical Subsurface Flow (VSB) systems, while also establishing realistic performance limits Chapter 6 addresses the practical construction and startup issues encountered with these systems to date.
Chapter 7 contains cost information for constructed wet- lands Subsequent to standardizing the costs to a specific time, it becomes clear that local conditions and require- ments can dominate the costs However, the chapter does provide a reasonable range of expected costs which can be used to evaluate constructed wetlands against other alternatives in the facility planning stage Also, there is sufficient information presented to provide the user with a range of unit costs for certain components and to indicate those components that dominate system costs and those that are relatively inconsequential.
Chapter 8 presents eight case studies to allow readers to become familiar with sites that have used constructed wetlands and their experiences The systems in this chap- ter are not ones which are superior to other existing facili- ties, but they are those which have been observed and from which lessons can be learned by the reader about either successful or unsuccessful design practices.
References
Brix, H 1997 Do Macrophytes Play a Role in Constructed Treatment Wetlands? Water Science & Technology, Vol 35, No 5, pp.11-17.
Campbell, C.S and M.H Ogden 1999 Constructed Wet- lands in the Sustainable Landscape John Wiley and Sons, New York, New York.
Cole, Stephen 1998 The Emergence of Treatment Wet- lands Environmental Science & Technology, Vol 3, No.5, pp 218A-223A.
Cooper, P.F., and B.C Findlater, eds 1990 Constructed Wetlands in Water Pollution Control Pergamon Press, New York, New York.
The 1990 guidelines by the European Community/European Water Pollution Control Association (EC/EWPCA) outline the design and operational standards for reed bed treatment systems Compiled for the EC/EWPCA Expert Contact Group on Emerging Hydrophyte Treatment Systems and edited by P.F Cooper, these guidelines serve as a crucial resource for effective water pollution control in Europe.
Government Accounting Office 1980 Costly wastewater treatment plants fail to perform as expected CED-81-
Hammer, D.A., ed 1989 Constructed Wetlands for Waste- water Treatment Lewis Publishers, Inc Chelsea,Michigan.
IAWQ 1992 Proceedings of international conference on treatment wetlands, Sydney, Australia Water Science
IAWQ 1995 Proceedings of international conference on treatment wetlands, Guangzhou, China Water Science
IAWQ 1997 Proceedings of international conference on treatment wetlands, Vienna, Austria Water Science &
Kadlec, R.H and R.L Knight 1996 Treatment Wetlands.
CRC Press LLC Boca Raton, FL.
Moshiri, L., ed 1993 Constructed Wetlands for Water Qual- ity Improvement Lewis Publishers, Inc., Chelsea, MI.
Niering, W.A 1985 Wetlands Alfred A Knopf, Inc., New
Natural Systems for Waste Management and Treat- ment Second edition McGraw-Hill, Inc., New York,
Tennessee Department of Public Health 1977 Regula- tions for plans, submittal, and approval; Control of con- struction; Control of operation Chapter 1200-4-2, State of Tennessee Administrative Rules Knoxville, TN.
Tennessee Valley Authority 1993 General Design, Con- struction, and Operation Guidelines: Constructed Wet- lands Wastewater Treatment Systems for Small Us- ers Including Individual Residences G.R Steiner and
J.T Watson, eds 2nd edition TVA Water Management
Resources Group TVA/WM 93/10 Chattanooga, TN.
U.S Department of Agriculture 1995 Handbook of Con- structed Wetlands 5volumes USDA-Natural Re- sources Conservation Service/US EPA-Region III/ Pennsylvania Department of Natural Resources. Washington, D.C.
U.S Environmental Protection Agency 1988 Design Manual: Constructed Wetlands and Aquatic Plant Sys- tems for Municipal Wastewater Treatment EPA/625/ 1-88/022 US EPA Office of Research and Develop- ment, Cincinnati, OH.
U.S Environmental Protection Agency 1993 Subsurface Flow Constructed Wetlands for Wastewater Treatment:
A Technology Assessment S.C Reed, ed., EPA/ 832/ R-93/008 US EPA Office of Water, Washington, D.C.
U.S Environmental Protection Agency 1994 Wetlands Treatment Database (North American Wetlands for Water Quality Treatment Database) R.H Kadlec, R.L. Knight, S.C Reed, and R.W Ruble eds., EPA/ 600/C- 94/002 US EPA Office of Research and Development, Cincinnati, OH.
U.S Environmental Protection Agency 1999 Final Draft Guiding Principles for Constructed Treatment Wetlands: Providing Water Quality and Wildlife Habitat Developed by the Interagency Workgroup on Constructed Wetlands (U.S Environmental Protection Agency, Army Corps of Engineers, Fish and Wildlife Service, Natural Resources Conservation Services, National Marine Fisheries Ser- vice, and Bureau of Reclamation) Final Draft 6/8/1999. http://www.epa.gov/owow/wetlands/constructed/ guide.html
Water Pollution Control Federation 1990 Natural Systems for Wastewater Treatment Manual of Practice FD-16,S.C Reed, ed., Water Pollution Control Federation,Alexandria, VA.
Constructed wetlands are engineered wastewater treatment systems consisting of multiple treatment cells designed to treat municipal wastewater These systems specifically provide secondary treatment, receiving primary effluent and improving it to meet or exceed secondary effluent standards Unlike enhancement systems or polishing wetlands, which further treat already treated secondary effluent before environmental discharge, constructed wetlands focus on the initial secondary treatment process.
This distinction highlights the intensity of treatment rather than the methods employed, as the constructed wetlands outlined in this manual are designed to handle higher-strength wastewater compared to the polishing wetlands commonly utilized in wastewater treatment systems.
While constructed wetlands discussed in this manual provide secondary treatment in a community’s wastewa- ter treatment system, this technology also can be used in combination with other secondary treatment technologies.
Constructed wetlands can be strategically positioned upstream from infiltration systems to reduce secondary treatment costs They can also be used to enhance wetlands by discharging secondary effluent for further polishing However, it is important to note that constructed wetlands are not suitable for treating raw wastewater A hypothetical wastewater treatment train utilizing constructed wetlands in series is illustrated in Figure 2-1.
Understanding the difference between constructed wetlands for secondary treatment and enhancement systems for tertiary treatment is essential for recognizing the limitations of existing wetland-based treatment system data Most available information on constructed wetland treatment systems comes from larger polishing wetlands and a limited number of smaller secondary treatment wetlands Historically, unverified data from these varied sources has been aggregated and statistically analyzed, leading to guidance that may not accurately reflect the performance of constructed wetland systems.
Introduction to Constructed Wetlands
Understanding Constructed Wetlands
Constructed wetlands are engineered wastewater treatment systems designed to treat municipal wastewater by providing secondary treatment Comprising one or more treatment cells, these systems effectively process primary effluent, achieving secondary effluent standards or better Unlike enhancement systems or polishing wetlands, which further treat already treated secondary effluent before environmental discharge, constructed wetlands focus on improving the quality of primary wastewater.
This distinction highlights the intensity of treatment rather than the methods employed, as the constructed wetlands outlined in this manual process higher-strength wastewater compared to the polishing wetlands, which have been commonly utilized in wastewater treatment systems for many years.
While constructed wetlands discussed in this manual provide secondary treatment in a community’s wastewa- ter treatment system, this technology also can be used in combination with other secondary treatment technologies.
Constructed wetlands can be strategically positioned upstream of an infiltration system to reduce secondary treatment costs Additionally, they can discharge secondary effluent to enhancement wetlands for further polishing However, it is important to note that constructed wetlands are not suitable for treating raw wastewater A hypothetical wastewater treatment train utilizing constructed wetlands in series is illustrated in Figure 2-1.
Understanding the distinction between constructed wetlands for secondary treatment and enhancement systems for tertiary treatment is crucial for recognizing the limitations of previous accounts and performance databases Most available data on constructed wetland treatment systems comes from larger polishing wetlands and a limited number of smaller secondary treatment wetlands Historically, unverified data from these varied sources has been aggregated and statistically analyzed, leading to guidance for constructed wetland systems that may not be reliable.
Chapter 2 Introduction to Constructed Wetlands inconsistent results In contrast, guidance offered in this manual is drawn from reliable research data and practical application in constructed wetlands for secondary treat- ment of higher-strength municipal wastewater.
Constructed wetlands consist of two main system types that, while sharing similar features, differ in the positioning of the hydraulic grade line Design variations for these systems primarily influence their shapes and sizes, allowing them to be tailored to specific site characteristics for enhanced construction, operation, and performance Additionally, both types can be equipped with liners to prevent infiltration, depending on local soil conditions and regulatory guidelines.
Free water surface (FWS) constructed wetlands mimic the appearance and function of natural wetlands, featuring open-water areas, emergent vegetation, and varying water depths These systems typically consist of several key components, including berms that enclose treatment cells, inlet structures for even distribution of influent wastewater, a mix of open-water and vegetated areas, and outlet structures that help maintain balanced water levels The design's shape, size, and complexity are often influenced by the specific site characteristics rather than predetermined design criteria.
Vegetated submerged bed (VSB) wetlands are designed with gravel beds that can be planted with wetland vegetation, enhancing both functionality and aesthetics As illustrated in Figure 2-3, a typical VSB system includes berms along with inlet and outlet structures to effectively manage and distribute wastewater flow Key variables influencing the design of VSB systems include the shape and size of the beds, the choice of treatment media such as gravel type and size, and the selection of vegetation, which primarily impacts the visual appeal rather than the system's performance.
The apparent simplicity and natural function of con- structed wetlands may obscure the complexity of interac-
Figure 2-1 Constructed wetlands in wastewater treatment train
Figure 2-2 Elements of a free water surface (FWS) constructed wetland
Figure 2-3 Elements of a vegetated submerged bed (VSB) system
Top Slope ( 0% - flat) Inlet Zone
Va riab le-Le ve l Outlet
Effective wastewater treatment relies on specific influent conditions Unlike natural wetlands, constructed wetlands are intentionally designed to meet defined performance standards Once operational, these systems require regular monitoring to ensure they function correctly Based on monitoring outcomes, minor modifications and routine management may be necessary to sustain optimal performance.
This chapter provides essential insights into the ecology of constructed wetlands, aimed at planners, policymakers, local government officials, and stakeholders engaged in utilizing constructed wetlands for effective wastewater treatment.
Wetlands play a crucial role in ecology, and while this article provides a brief overview of their basic components and functions, it intentionally omits detailed descriptions to maintain focus on their treatment performance For those looking to deepen their understanding of wetland ecology, numerous resources are available It is assumed that designers and operators possess a general knowledge of this field, with more comprehensive information on constructed wetlands presented in the following chapters.
This manual focuses on municipal wastewater treatment systems that use constructed wetlands, designed to mimic the functions of natural wetlands It also briefly discusses related systems that incorporate elements of natural wetland ecosystems Furthermore, it introduces constructed wetlands for on-site domestic wastewater treatment and non-municipal wastewater management solutions.
VSB wetlands operate independently of wetland vegetation for effective treatment, eliminating the need for open-water areas This chapter outlines specific design and management considerations relevant solely to these systems.
FWS wetlands For reference purposes, important terms are highlighted in bold type and are explained in a glos- sary at the end of the chapter.
Ecology of Constructed Wetlands
Constructed wetlands are engineered ecological systems that utilize physical, chemical, and biological processes for effective wastewater treatment To successfully design and operate these systems, it is essential to have a solid understanding of their components and the interrelationships within the system.
Constructed wetlands utilize ecological principles derived from natural wetlands to treat wastewater Unlike their natural counterparts, constructed wetlands offer a higher level of control over the treatment processes, allowing for more effective management of water quality.
Constructed wetlands maintain a stable water flow, unlike natural wetlands that experience significant fluctuations due to changing precipitation This consistent flooding in constructed wetlands leads to elevated levels of total suspended solids (TSS), biochemical oxygen demand (BOD), and other wastewater constituents, impacting their ecological dynamics compared to natural systems.
Constructed wetlands primarily receive a consistent volume of wastewater from sewers, while precipitation and surface runoff can vary seasonally and annually To assess water loss in these systems, it is essential to measure outflow, estimate evapotranspiration, and consider seepage in unlined areas Despite the predictable inflow, accurately modeling the water balance of constructed wetlands requires an understanding of weekly and monthly fluctuations in precipitation and runoff, as well as their impact on wetland hydraulics and the detention time necessary for effective treatment For a detailed exploration of these modeling challenges, refer to Chapter 3.
Temperature fluctuations influence the treatment efficiency of constructed wetlands, but their impact varies among different wastewater constituents While colder temperatures can reduce the treatment performance for certain elements, the removal of Biochemical Oxygen Demand (BOD) and Total Suspended Solids (TSS) through processes like flocculation and sedimentation remains relatively stable.
During colder months, the lack of plant cover enhances atmospheric reaeration and solar insolation, as plants typically provide shading and surface coverage in the growing season Additionally, ice cover significantly influences constructed wetlands by modifying wetland hydraulics and limiting solar insolation, atmospheric reaeration, and biological activity While the insulating properties of ice slow the cooling rate of the water column, they do not hinder physical processes like settling, filtration, and flocculation Furthermore, the senescence and decay of plants under ice cover lead to a reduction in effluent biochemical oxygen demand (BOD).
Botany of Constructed Wetlands
The effective performance of constructed wetlands relies on ecological functions akin to those of natural wetlands, primarily driven by interactions within plant communities Research indicates that free water surface (FWS) constructed wetlands achieve superior treatment of typical wastewater pollutants, such as total suspended solids (TSS) and biochemical oxygen demand (BOD), in vegetated cells compared to those without plants Despite these findings, the exact mechanisms through which plant populations enhance treatment efficiency remain largely unexplored Some studies suggest a potential link between plant surface area and the density and functionality of attached microbial populations, yet this relationship has not been conclusively demonstrated.
Constructed wetlands experience notable shifts in plant communities after initial planting, often deviating from the intended species composition and density While many changes are predictable and do not significantly impact treatment performance, some alterations can lead to suboptimal results, necessitating enhanced management efforts Understanding fundamental principles of plant ecology can provide valuable insights into the dynamics of constructed wetlands.
In constructed wetlands, microscopic bacteria play a crucial role in the ecological food web, facilitating complex energy transformations fueled by influent wastewater These microbes are essential for the nitrogen cycle, converting nitrogen into various biologically useful forms that support plant metabolism while influencing oxygen levels Additionally, microbial activity aids in the uptake of phosphorus by transforming insoluble forms into soluble ones, making them accessible to plants Moreover, microbes process organic carbon compounds, releasing carbon dioxide in aerobic zones and various gases, including methane, in anaerobic areas The presence of plants, plant litter, and sediments further enhances microbial activity by providing solid surfaces for their concentration.
Microbial activity in cold regions exhibits seasonal variations, with reduced activity during colder months However, the difference in performance between warm and cold climates is less pronounced in full-scale constructed wetlands compared to small-scale experiments, likely due to the complex interplay of physical, chemical, and biological processes occurring across a larger area.
Algae are commonly found in wet environments and play a significant role in Free Water Surface (FWS) systems While they are essential in specific treatment systems like lagoons, their presence can notably influence the treatment efficiency of constructed wetlands Therefore, it is crucial to consider algae during the design phase of these systems.
Algae in open areas, particularly where submergent vegetation exists, can create a living canopy that obstructs sunlight, leading to decreased dissolved oxygen levels Additionally, open water near constructed wetland outlets often fosters seasonal blooms of phytoplanktonic algae, resulting in increased suspended solids and particulate nutrients in the effluent.
Floating aquatic plants like duckweed exhibit high primary production rates, leading to significant biomass accumulation in fully vegetated FWS wetland and pond systems Additionally, water hyacinth thrives in tropical pond environments, improving total suspended solids (TSS) and algal removal However, both species can obstruct sunlight and reduce dissolved oxygen (DO) levels by hindering atmospheric reaeration at the water-air interface.
The rapid growth of certain aquatic plants has led to the development of specialized wastewater treatment systems that utilize these plants to extract nutrients from wastewater However, challenges arise due to their low solid content, typically under 5% wet weight, necessitating drying before disposal, which can cause odor and water quality issues Harvested duckweed, rich in protein, is often used as green manure in agricultural soils, while water hyacinths are either partially dried for landfilling or decomposed in controlled environments to generate methane Despite various attempts to recover beneficial by-products, these efforts have largely been unsuccessful in North America due to prevailing social and economic factors.
Emergent herbaceous wetland plants play a crucial role in wetland ecosystems due to their unique adaptations that enable them to thrive in saturated or flooded soils Key traits include lenticels for air exchange, aerenchymous tissues for gas transport, and specialized growth structures like buttresses and pneumatophores that enhance root aeration Additionally, these plants possess adventitious roots that absorb gases and nutrients directly from the water, along with a heightened physiological tolerance to chemical by-products from anaerobic conditions.
Emergent vegetation in FWS systems is crucial for improving flocculation, sedimentation, and filtration of suspended solids by creating optimal hydrodynamic conditions Additionally, these wetland plant species enhance the winter performance of constructed wetlands by insulating the water surface from cold temperatures, capturing snow, and minimizing heat loss due to wind (Wittgren and Maehlum, 1996).
Limited evidence shows that the selection of plant species significantly impacts the performance of constructed wetlands A study at the Iron Bridge Wetland in Florida revealed that two similar free water surface (FWS) treatment cells, differing mainly in their dominant plant species, exhibited comparable effectiveness in treating biochemical oxygen demand (BOD), total suspended solids (TSS), total nitrogen (TN), and total phosphorus (TP) As the research and implementation of constructed wetlands have grown, the documentation of performance differences among emergent marsh plant species has become less beneficial for wetland designers.
When selecting emergent herbaceous species, wetland designers should consult experienced local practitioners to ensure the choice of successful native species For guidance on the initial selection and establishment of plant species suited to wetland environments, refer to Table 2-2.
Table 2.1 Characteristics of plants for constructed wetlands
General Types General Characteristics Function or Importance Function or Importance Design & Operational of Plants and Common Examples to Treatment Process for Habitat Considerations
Aquatic roots or root-like structures serve essential functions in aquatic ecosystems, primarily by creating dense floating mats that limit the growth of invasive species like duckweed These mats restrict nutrient uptake and shading, which hinders oxygen diffusion in the water, particularly in stagnant environments without strong currents By blocking sunlight from submerged plants, these mats also provide shelter for aquatic life, ensuring a balanced ecosystem.
Common duckweed (Lemna), present as an invasive species and food for animals.
Rooted floating plants, such as water lilies (Nymphea) and pennywort (Hydrocotyle), typically feature floating leaves and serve essential functions in aquatic ecosystems They create dense floating mats that limit water depth, restrict sunlight penetration to submerged plants, and enhance oxygen diffusion from the atmosphere These plants are anchored to the bottom but do not rise above the water column during daylight hours Additionally, they provide shelter and food for various animals while inhibiting the growth of other aquatic plant species.
Aquatic plants are typically fully submerged and serve essential functions such as providing shelter and food for animals, particularly in water zones They also facilitate microbial attachment and enhance retention time in open water, contributing to a balanced ecosystem.
Fauna of Constructed Wetlands
Animal species play a crucial role in the management of constructed wetlands, particularly in free water surface (FWS) wetlands Although animals generally have less biomass than wetland plants, their impact on energy and mass flows can be significant For instance, insect pest outbreaks in constructed wetlands can lead to the defoliation of entire marshes and floating aquatic plants, disrupting mineral cycles and negatively affecting water quality treatment performance.
Bottom-feeding fish, particularly carp, disrupt sediment in constructed wetlands, hindering their ability to filter out suspended solids and pollutants Similarly, large seasonal populations of waterfowl have been observed to impact the effectiveness of these wetlands, as seen in Columbia, Missouri, and other locations In VSB wetlands, the influence of avian species is notably significant.
Wildlife species in constructed wetlands can have both positive and negative impacts While birds common to these environments attract birdwatchers and foster public support for municipalities and industries using this treatment technology, their presence also requires careful management to mitigate human health risks associated with exposure to primary effluent Additionally, regulatory concerns for vulnerable wildlife species can hinder the development of constructed wetlands at specific sites, particularly when dealing with wastewater that contains toxic substances.
Free water surface wetlands mimic the ecology of natural wetlands and aquatic habitats, attracting various animal species that depend on wet environments throughout their life cycles Constructed wetlands support a diverse array of animal groups, including protozoans, insects, mollusks, fish, amphibians, reptiles, birds, and mammals, as summarized in Table 2-3.
Ecological Concerns for Constructed Wetland Designers
Wetland ecology plays a crucial role in the effectiveness of constructed wetlands, highlighting their intricate ecosystems and wildlife accessibility Although the ecology of Vegetated Subsurface Bioreactor (VSB) systems is primarily linked to their subsurface environments, it is essential to also consider the wetland plants and surface features that define VSB wetlands.
Table 2.3 Characteristics of Animals Found in Constructed Wetlands
Members of Group Commonly Found in Function or Importance to Treatment
Animal Group Treatment Wetlands Process Design & Operational Considerations
Invertebrates play a crucial role in mosquito control, with their diversity significantly impacting populations that fluctuate seasonally The interactions between chemical and biological cycles, as well as the presence of plant mono-cultures, can lead to increased susceptibility to insect infestations Additionally, the functions of protozoa, insects, spiders, and crustaceans in supporting higher organisms remain poorly understood, highlighting the need for further research in this area.
Certain fish species, such as mosquitofish, mudminnow, bowfin, catfish, killifish, and carp, have adapted to thrive in anaerobic conditions often found in polluted waters These fish play a crucial role in controlling insect populations, particularly mosquito larvae, while also impacting their habitats by uprooting plants and resuspending sediments Their nesting areas are vital for maintaining ecological balance in such environments.
Amphibians and Frogs, alligators, snakes, turtles Consumers of lower organisms Turtles have an uncanny ability to fall
Reptiles into water control structures and to get caught in pipes, so turtle exclusion devices are needed; monitoring of control structures and levees for damage or obstruction is needed.
A diverse range of 35 to 63 bird species, including forest and prairie waterfowl, plays a crucial role as consumers of lower organisms However, heavy usage by migratory birds can lead to a seasonal increase in pollutant loads, impacting both biodiversity and bird populations, which fluctuate throughout the year and across different habitats.
Mammals in the ecosystem include small rodents like shrews, mice, and voles, as well as larger species such as rabbits, nutria, muskrats, and beavers These animals are primarily consumers of plants and lower organisms However, populations of nutria and muskrats can grow to nuisance levels, leading to significant vegetation removal and damage to levees In such cases, structural controls and animal removal may be necessary to maintain ecological balance and protect infrastructure.
Constructed wetlands play a crucial role in attracting wildlife, which must be carefully considered during their design and management Animals contribute significantly to the ecological functions of these systems, although their specific roles are not extensively studied While many species can be beneficial to constructed wetlands, others may become nuisances Therefore, focusing on both desirable and undesirable wildlife, along with understanding the primary and ancillary functions of constructed wetlands, is essential for their successful operation and management.
2.5.1 Primary and Ancillary Functions of
Constructed wetlands primarily serve to store water and enhance water quality, with some specifically designed for groundwater recharge Additionally, they provide various benefits similar to those of natural wetlands, which will be explored in the following chapters.
Ancillary functions of wetlands encompass the primary production of organic carbon by plants, oxygen generation through photosynthesis, and the support of wetland herbivores and predator species that extend beyond wetland boundaries Additionally, these functions help reduce the export of organic matter and nutrients to downstream ecosystems while fostering cultural values related to education and recreation In many constructed wetland projects, achieving one or more of these ancillary functions is a key objective For further details on these functions, please refer to the works of Feierabend (1989) and Sather (1989).
Successful wildlife management in FWS wetlands re- quires maintaining a balance between attracting benefi- cial species and controlling pest species (EPA, 1993a).
Constructed wetlands, while home to many attractive wildlife species, often face challenges from nuisance species that hinder their effectiveness Burrowing rodents like beavers, nutria, and muskrats damage berms and levees and consume beneficial vegetation Additionally, mosquitoes pose health risks and annoyance, while bottom-feeding fish, such as carp, disrupt aquatic plants and increase total suspended solids (TSS) and pollutants by disturbing sediments Large populations of waterfowl can also be problematic, as their nutrient-rich droppings further strain the water quality in these ecosystems.
Effective control of wildlife access in constructed wetlands is highly site-specific, necessitating measures tailored to geographic location, nuisance species, wetland design, and management preferences Throughout the planning, construction, and operation phases of wetland projects, various control methods are implemented For instance, carp management can be addressed during the design phase by utilizing winter water level drawdowns and freezing in northern climates, while physical removal of stranded individuals, although labor-intensive, may also be employed To limit access by large rodents and prevent damming, culverts can be screened, but trapping may be required to mitigate burrowing and damage Additionally, restricting open-water areas can deter certain waterfowl species, although netting over these areas may conflict with the goal of promoting natural wildlife control methods.
Wetland wildlife often inhabit areas beyond the confines of constructed wetland cells, making them a public resource that requires protection for reasons beyond their value to these ecosystems While the benefits of constructed wetlands for wildlife habitat may be debated, it is essential to address this issue throughout all project phases to identify the best design and management strategies that either encourage or deter wildlife presence.
Mosquitoes play a crucial role in the ecological food web, yet they are often viewed as pests While constructed wetlands can enhance wildlife appeal, their potential to breed mosquitoes poses challenges for permitting, funding, and the overall establishment of these wetlands.
Effective mosquito control in constructed wetlands can be achieved through various methods, including the use of mosquito fish, which significantly reduce populations when habitat conditions are optimized Maintaining appropriate water levels and ensuring channels are clear of dead vegetation enhance the effectiveness of mosquito fish Additionally, lowering water levels in spring supports mosquito fish spawning and improves their access to mosquito larvae during peak breeding season (Dill, 1989).
In warm climates, it is crucial to monitor mosquito fish habitats for high water temperatures and variations in effluent strength and composition Additionally, bats and various bird species serve as effective predators; however, the establishment of standardized planning and management practices for optimal conditions remains a challenge.
Effective mosquito habitat management during planning and construction can be achieved by implementing steep slopes on water channels, which minimize standing water in shallow areas While natural, undulating banks enhance the aesthetic appeal of wetlands, they are not effective for mosquito control A proven channel design for controlling mosquito populations features steep sides with flat aprons leading to steep banks, allowing operators to lower water levels during breeding seasons This design helps eliminate standing water in emergent vegetation, creating a deeper channel that supports fish species that prey on mosquitoes Therefore, flexible drainage capabilities are crucial for effective mosquito management.
Human Health Concerns
Constructed wetlands have been extensively studied for their biological effectiveness and aesthetic appeal, particularly those designed for polishing secondary effluent These successful tertiary treatment wetlands often feature interpretive centers, informative signage, and boardwalks that enhance visitor experiences In contrast, constructed wetlands that treat primary effluent for secondary treatment may not be as accessible or inviting to visitors, making it preferable for people to observe them from a distance.
Partially treated wastewater in constructed wetlands for secondary treatment poses similar health risks to humans as untreated wastewater in primary treatment and lagoons The potential for dermal contact and disease transmission remains a significant concern in free water surface (FWS) wetlands, just as it does in open lagoons.
Variable- Level Outlet Floating Plants
Outlet Collector Pipe Submerged Plants
Figure 2-4 Profile of a three-zone FWS constructed wetland cell polishing systems, where influent wastewater has already met effluent quality requirements which are set by regula- tory authorities.
Constructed wetlands that receive primary effluent pose a higher risk of human exposure to wastewater at the inlet, where only primary treatment has occurred In contrast, the outlet end presents a lower concern, as the wastewater has undergone secondary treatment or better, ensuring a safer quality for environmental education and awareness activities in polishing wetlands.
Humans are often viewed as an undesirable presence in many FWS wetlands that treat municipal wastewater to meet secondary treatment standards (30 mg/L of BOD and TSS) However, constructed wetlands can function as recreational spaces and outdoor laboratories, particularly at the outlet where treated wastewater meets secondary effluent criteria Effective management must take into account public access, perceptions, and potential health risks, as highlighted by Knight (1997) Therefore, it is essential to incorporate fencing, signage, and other safety measures during the proposal, design, and operational phases of the system.
Mosquito populations can be a nuisance, but certain species pose significant health risks that need to be managed This concern is particularly relevant in warmer climates, such as the southern regions.
In the United States, the encephalitis mosquito (Culex tarsalis) benefits from the extended breeding season created by constructed wetlands However, effective control of these mosquito populations has been achieved through water-level manipulation and the introduction of mosquito fish in a two-tiered pond design This design enables water levels to be lowered, concentrating mosquito populations in smaller areas, which enhances the efficiency of mosquito fish predation.
Most of the health concerns described above do not apply to VSB systems, in which wastewater typically is not ex- posed at the land surface.
Onsite System Applications
On-site constructed wetland systems offer an effective solution for wastewater treatment and disposal at individual properties These systems employ similar technologies to municipal VSB systems, providing cost-effectiveness and low maintenance A key distinction is that on-site constructed wetlands discharge treated effluent into the soil rather than surface water Specifically, these systems are designed to treat septic tank effluent or primary effluent through small-scale VSB systems for subsurface disposal.
On-site constructed wetlands are smaller in scale compared to municipal systems, usually covering only a few hundred square feet While municipal vertical flow systems (VSB) cater to hundreds of residential, commercial, and industrial properties, on-site systems are designed to serve individual homes or clusters of a few residences.
An on-site Vertical Subsurface Bed (VSB) system usually features a lined VSB that processes primary effluent from a septic tank, often accompanied by a second unlined VSB for additional treatment This second VSB allows treated wastewater to infiltrate into the soil for disposal Variations of this system may include supplemental absorption trenches to enhance soil absorption and options for direct surface discharge, with or without further disinfection Each VSB is typically equipped with wetland vegetation to support the treatment process.
Research on constructed wetland systems has demonstrated effective treatment capabilities for various wastewater components, such as BOD, TSS, and fecal coliforms However, the efficiency in removing ammonia nitrogen varies across different studies (Burgan and Sievers, 1994; Huang et al., 1994; Johns et al., 1998; Mankin and Powell, 1998; Neralla et al., 1998; White and Shirk, 1998).
Related Aquatic Treatment Systems
Various aquatic treatment systems have been developed to address municipal and other wastewater treatments, many of which do not fit the definition of constructed wetlands outlined in this manual This overview aims to give readers supplementary background information and references for further study.
Polishing wetlands effectively remove trace metals such as cadmium, chromium, iron, lead, manganese, selenium, and zinc through sedimentation Plant uptake facilitates the deposition of these metals into the soil via roots, necessitating the harvesting of plants to reduce metal concentrations in the system However, in certain instances, effluent metal concentrations have surpassed influent levels, likely due to the evaporation of wastewater.
The Advanced Ecologically Engineered System (AEES) is a proprietary treatment system that incorporates various treatment units, including both FWS-like and VSB-like components, as part of its comprehensive treatment process.
The "Living Machine" system integrates traditional wastewater treatment components such as sedimentation, anaerobic bioreactors, and clarifiers within a greenhouse environment Funded by federal grants, four demonstration projects were evaluated for their treatment performance across various wastewater types and conditions, including raw wastewater in different climates and the improvement of pond water quality An independent evaluation by a firm contracted by the U.S EPA confirmed that while some treatment performance goals were met, others were not Although marketed as a natural system, the impact of wetland plants seems to enhance aesthetics more than actual treatment efficacy For more detailed information, readers are encouraged to consult additional sources.
Living Technologies, 1996; Reed et al., 1995; Todd and
Floating macrophyte systems, while benefiting from natural wetland processes, also necessitate mechanical components to meet desired treatment outcomes Larger systems utilizing duckweed and water hyacinth effectively treat wastewater by targeting key constituents such as BOD and TSS However, the removal of these floating macrophytes often demands additional mechanical systems for drying, disposal, and managing residuals.
Frequently Asked Questions
Constructed wetlands are an innovative technology that harnesses ecological processes similar to those found in natural wetland ecosystems to effectively remove contaminants from wastewater Utilizing wetland plants, soils, and microorganisms, these systems not only treat wastewater but also offer additional benefits, including reliability without the need for energy sources or chemicals, minimal operational demands, and significant land use Furthermore, employing constructed wetland technology can facilitate the creation or restoration of wetlands, enhancing the environment by providing wildlife habitats, greenbelts, and opportunities for passive recreation and other ecological amenities.
2 What are wetland treatment systems?
A wetland treatment system typically refers to two main types of passive treatment systems, one of which is the free water surface (FWS) constructed wetland This system features a shallow wetland environment that incorporates a variety of plant types, including emergent aquatic plants like cattails, bulrushes, and reeds, as well as floating plants such as duckweed and water hyacinth, alongside submergent aquatic plants like sago pondweed and widgeon grass.
A FWS wetland features open-water zones filled with submergent and floating vegetation, along with islands that provide essential habitats These wetlands can be either lined or unlined, based on specific regulatory or performance standards They support intricate aquatic ecosystems, serving as vital habitats for both aquatic and wetland bird species.
A vegetated submerged bed (VSB), often referred to as a subsurface flow wetland, is distinct from traditional wetlands as it lacks hydric soils In a VSB, emergent wetland plants are anchored in gravel, allowing wastewater to flow through the gravel rather than over the surface This shallow system utilizes large gravel particles to ensure long-term subsurface flow without clogging, with plant roots and rhizomes extending into the gravel's pore spaces Current research suggests that VSB systems can function effectively without the presence of plants, indicating that wetland ecology is not a crucial element for their performance.
3 Are constructed wetlands reliable? What do they treat?
Constructed wetlands are a highly effective water reclamation technology when designed, built, operated, and maintained correctly They excel at removing pollutants from municipal and industrial wastewater as well as stormwater, targeting contaminants such as biochemical oxygen demand (BOD) and suspended solids Additionally, constructed wetlands can effectively eliminate heavy metals like cadmium, chromium, iron, lead, manganese, selenium, and zinc, along with toxic organic compounds from wastewater.
4 How does a constructed wetland treat wastewa- ter?
Natural wetlands serve as effective watershed filters, sediment sinks, and biogeochemical engines that recycle and transform nutrients Similarly, constructed wetlands replicate these functions for wastewater treatment, performing essential tasks such as sedimentation, filtration, digestion, oxidation, reduction, adsorption, and precipitation As wastewater flows through a constructed wetland, these processes occur sequentially, allowing wastewater constituents to mix with the detritus of marsh plants.
5 What is the difference between treatment and en- hancement wetlands?
Constructed wetlands are engineered to effectively treat municipal and industrial wastewater, as well as manage stormwater runoff Enhancement marshes, also known as polishing wetlands, provide community benefits such as water reclamation, wildlife habitats, water storage, mitigation banks, and opportunities for passive recreation and environmental education These wetland systems can be developed as standalone entities or combined into a single design that integrates various treatment and enhancement objectives.
6 Can a constructed wetland be used to meet a sec- ondary effluent standard?
Both FWS and VSB constructed wetlands can be used to meet a 30/30 mg/L BOD and TSS discharge standard It is not advisable to put raw wastewater into a constructed wetland.
7 Can a constructed wetland be used to meet an advanced secondary/tertiary discharge standard?
FWS constructed wetlands, when provided with adequate pretreatment and sufficient area, can achieve monthly average discharge standards of less than 10 mg/L for BOD, TSS, and TN Numerous instances of FWS wetland systems consistently meeting these stringent standards are documented across the United States.
VSB systems have been widely utilized in England for enhancing secondary effluents and managing effluent from combined sanitary and storm sewers However, in the United States, these systems have struggled to consistently achieve advanced treatment objectives when dealing with primary treatment influent.
8 How much area is required for constructed wet- lands?
As a general rule, a constructed wetland receiving wastewater with greater degrees of pretreatment
Constructed wetlands typically require less space than traditional methods like oxidation ponds and trickling filters when treating higher-strength wastewater Historically, the area needed for constructed wetlands has varied widely, ranging from less than 2 to over 200 acres per million gallons per day (MGD) The specific area required depends on the effluent quality standards to be achieved and the necessary buffer zones.
9 Do these systems have to be lined?
The necessity for liners in constructed wetlands varies based on state regulations and the properties of surface and subsurface soils Generally, when soils are porous, the requirement for liners may be reduced.
Constructed wetlands typically require well-drained soils, such as sands with minimal loams, clays, and silts, to function effectively Conversely, if the soils are poorly drained and predominantly clay-based, the use of a liner may not be necessary.
Over time, these systems can create a peat layer at the bottom that hinders infiltration However, the idea of a "leaky wetland" could harness natural processes to treat wastewater as it percolates through the soil, ultimately replenishing groundwater, which may offer significant advantages in specific regions.
10 What is the role of the plants in constructed wet- lands?
In FWS constructed wetlands, emergent and floating aquatic plants are crucial as they create a canopy over the water, limiting phytoplankton growth and promoting the accumulation of free-floating species like duckweed, which hinders atmospheric reaeration This environment also aids in reducing suspended solids within the wetland While emergent plants contribute minimally to the uptake of nitrogen and phosphorus, the decomposition of litter from previous seasons significantly influences effluent quality by transforming into humic soil and lignin particles.
The role of plants in vertical flow constructed wetlands (VSB systems) has been debated, as initial assumptions suggested that they significantly contributed to oxygen supply for microbes However, studies comparing planted and unplanted systems have not substantiated this claim Despite this, planted VSB systems are often preferred for their aesthetic appeal and do not seem to negatively impact the overall performance of these systems.
11 How much time is needed for a constructed wet- land to become fully operational and meet discharge requirements?
Glossary
Abiotic Nonbiological processes or treatment mechanisms in a constructed wetland.
Adsorption Adherence by chemical or physical bonding of a pollutant to a solid surface.
Adventitious roots enhance a plant's competitive edge by emerging from stems into the air for terrestrial species or into water for aquatic varieties, allowing them to access additional nutrients and resources directly from their environment before penetrating the soil substrate.
Aerenchymous tissues in aquatic plants facilitate gas transfer, playing a crucial role in wastewater treatment systems These tissues enable emergent aquatic plants to deliver oxygen to their roots, supporting their growth and function in aquatic environments.
Aerobic processes in wastewater treatment systems take place in the presence of dissolved oxygen.
Algae are single-celled to multicelled organisms that rely on photosynthesis for growth Most algae are classified as plants.
Anaerobic processes in wastewater treatment systems take place in the absence of dissolved oxygen and instead rely on molecular oxygen available in decomposing com- pounds.
Aspect ratio The length of a constructed wetland divided by its width (L/W).
Atmospheric reaeration introduces atmospheric oxygen into the water at the water’s surface, which provides dis- solved oxygen to the aquatic environment.
Autotrophic Types of reactions that generally require only inorganic reactants; for example, nitrification.
Biochemical oxygen demand (BOD) refers to the amount of dissolved oxygen required for the decomposition of organic matter in wastewater treatment Expressed in milligrams per liter (mg/L), BOD serves as an indicator of the organic strength of wastewater and the effectiveness of treatment processes Throughout this manual, "BOD" specifically denotes the results from the standard 5-day BOD test used in the U.S.
Biomass is the total amount of living material, including plants and animals, in a unit volume.
Biotic is a term which implies microbiological or biological mechanisms of treatment.
BOD removal refers to the reduction of dissolved oxygen demand necessary for biological decomposition in water This process can be achieved through biological decomposition in open-water areas, as well as through flocculation and sedimentation in fully vegetated zones and vegetated shallow basins (VSBs).
Bulrush, a common name for various plants in the genus Scirpus, thrives in wetlands and is essential for constructed wetlands Key species like S validus, S californicus, and S acutus are known for their ability to adapt to diverse environmental conditions, including fluctuations in water depth and quality These large, upright bulrush species develop dense stands with numerous round-sectioned stems that can last for several years, while other varieties, such as the three-square bulrush, contribute to the ecological diversity of these habitats.
S americanus (olynei), S fluviatilis, and S robustus, which offer tolerance to salinity, a variety of color shades, and attractiveness to various animal species.
Canopy Uppermost or tallest vegetation in a plant com- munity.
Cattail is the common name for a number of plants of the genus Typha that are common in constructed wetlands in the United States, with at least three species predominant:
T latifolia, T domingensis, and T angustifolia Along with their hybridized forms, these species occupy numerous water-depth and water-quality niches within constructed wetlands The wetland designer is advised to consult local botanists and geographic references to determine which local cattail species or hybrid is best adapted to the spe- cific water quality, water depth, and substrate planned for a constructed wetland.
Common reed (Phragmites) is one of the most commonly utilized plants in constructed wetlands globally, yet it is rarely employed in the United States While it thrives in shallow constructed wetlands, its invasive nature in certain natural wetlands leads to discouragement of its transport and intentional introduction in various regions Additionally, common reed is regarded as providing minimal food or habitat value for wetland wildlife species (Thunhorst, 1993).
Constructed wetlands are wastewater treatment systems that rely on physical, chemical, and biological processes typically found in natural wetlands to treat a relatively con- stant flow of pretreated wastewater.
Deciduous Woody plants that shed their leaves in cold seasons.
Dentrification Biotic conversion of nitrate-nitrogen to ni- trogen gases.
Detritus Loose, dead leaves and stems from dead veg- etation.
A dike is a soil wall that serves to contain or separate constructed wetlands from their surrounding environment In wastewater treatment systems, maintaining dissolved oxygen (DO) levels in the water column is essential for the aerobic biochemical processes that occur within these constructed wetlands.
Dominant plant species The plant species that exerts a controlling influence on the function of the entire plant com- munity.
Duckweed naturally disperses across large water surfaces due to wind, unless it is anchored by dense stands of emergent plants or artificial barriers In constructed wetlands, this leads to dense growths of duckweed in fully vegetated areas, effectively sealing the water surface and inhibiting atmospheric reaeration Consequently, the combination of this sealing effect and the oxygen demand from incompletely treated municipal wastewater creates anaerobic conditions in these zones.
Emergent herbaceous wetland plants are rooted in soil, with their leafy structures extending above water surfaces These plants possess a decomposable nature and enough internal support to maintain upright growth, regardless of surrounding water Typically, they thrive in shallow water or near the banks of water bodies, adapting to varying water levels.
Evapotranspiration Loss of water to the atmosphere through water surface and vegetation.
Exotic species A plant not indigenous to the region.
Fecal coliform A common measure for pathogenicity of wastewater This analytical test reveals the number of these types of organisms in counts/100 milliliters (#/100mL)
Filtration is the process of filtering influent solids from the wastewater and typically is provided by plant stems and leaves and other vegetation in the water column.
Floating aquatic plants are prevalent in FWS systems and include species such as water hyacinth, duckweed, and water fern Additionally, rooted plants that grow in a floating manner, such as pennywort, water lily, frog's bit, spatterdock, and pondweed, are also commonly found in these environments.
Floating aquatic systems consist of shallow basins that are topped with floating plants Water hyacinth, a common species in these systems, is sensitive to temperatures outside the tropics and is recognized as an invasive species Alternatively, duckweed can be stabilized using artificial barriers within these environments.
Flocculation is a crucial process in wastewater treatment where tiny particles aggregate to form larger, settleable solids This process effectively combines colloidal particulates, allowing for their removal through sedimentation.
Free water surface (FWS) wetlands are engineered ecosystems designed for wastewater treatment, utilizing flocculation and sedimentation processes As wastewater flows through these wetlands, it interacts with stands of aquatic plants in shallow water, effectively purifying the water.
FWS wetlands feature open areas where aerobic bio-oxidation enhances physical removal processes These systems function and appear similar to natural wetlands and are commonly referred to as "surface flow systems."
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