Introduction to Treatment Wetlands

Treatment wetlands are constructed ecosystems dominated by aquatic plants that naturally cleanse water. Throughout Florida, the U.S., and the world, treatment wetlands are providing a cost effective alternative for water and wastewater management.

Wetland Solutions, Inc. (WSI) is a leader in applying this technology to a variety of water quality challenges, including treatment of urban stormwaters, municipal wastewaters, lake and reservoir eutrophication, and runoff from confined animal feeding operations, agricultural lands, and industrial sites. We believe that our experience with developing cost-effective and environmentally sound treatment wetland alternatives at numerous sites provides our clients with the most appropriate studies and designs available.

Wetlands, both natural and constructed, have demonstrated effective treatment of contaminated waters. Wetland systems offer numerous benefits, especially in this age of concern about wetland losses and the focus on preserving open space. However, for a given application and project site, treatment wetlands may or may not be the best alternative and should be carefully evaluated in terms of reliability, cost, and other benefits or disadvantages that may be less tangible. WSI has the practical experience to help clients decide when treatment wetlands are the best alternative, and if so, how to build a system that provides the greatest benefit:cost ratio.

In the following pages, we present an overview of the strengths of wetland water treatment systems, the technology involved, and our experience and important considerations when implementing effective natural and constructed wetland water treatment systems.

Background of the Technology

The feasibility of using wetlands for water treatment has been investigated since the 1950s when researchers first observed removal of trace organics and biochemical oxygen demand (BOD) in laboratory constructed wetlands. Since then, many scientists and engineers in North America and abroad have conducted further studies of treatment potential in both constructed and natural systems. Designs range from large open-water areas fringed with cattails and bulrushes, to shallow water ponds completely covered with sedges and rushes, to natural forested wetlands. However, because of the variety of wetland types, each with differing hydrology, vegetation, and substrate, the observed treatment efficiency varies between systems.

In general, wetland water treatment systems have been found to lower concentrations and mass loads of biochemical oxygen demand (BOD), total suspended solids (TSS), and total nitrogen concentrations to 10 to 30 percent of the concentrations entering the systems (70 to 90 percent reduction). For total phosphorus, metals, and organic compounds, removal efficiencies vary more widely, typically from about 20 to 90 percent. Removal of pollutants in treatment wetlands is limited by the form and concentration of the constituents, water flow rates and residence time, the presence of oxygen, substrate type, and the entire chemical makeup of the water to be treated. WSI has been instrumental in collecting and analyzing treatment wetland data, and in developing models to accurately predict wetland performance.

What Types of Treatment Wetlands are Used?

Three types of treatment wetlands are typically used for water quality improvement: natural treatment wetlands, constructed surface flow treatment wetlands, and subsurface flow treatment wetlands. Each of these alternatives is briefly described below.

Natural Treatment Wetlands
Natural treatment wetlands have been used for the disposal and treatment of secondary and tertiary wastewater effluents for many years. Hundreds of wastewater treatment facilities discharge to natural wetlands nationwide. While many of those systems were not originally designed for wastewater treatment, studies have led to an understanding of the natural potential of wetland ecosystems for pollutant assimilation and to the design of new natural treatment wetlands.

Properly using a natural wetland to treat secondary wastewater or stormwater involves a number of considerations. Hydraulic loads must be matched to the hydroperiod requirements and tolerances of the dominant wetland vegetation. The pretreated water should be well distributed throughout the wetland for optimal treatment and to reduce localized impacts. Ideally, alternative discharge areas should be used so that portions of the natural wetland can be taken offline periodically and allowed to undergo a natural hydroperiod.

WSI has completed a number of the definitive studies of the performance of natural wetlands for water quality enhancement. These studies demonstrate that through careful design, some natural wetlands can consistently and cost-effectively provide advanced treatment of wastewater and stormwater constituents while retaining or enhancing their important ecological functions.

Constructed Surface Flow Treatment Wetlands
Constructed surface flow treatment wetlands are typically shallow, man-made impoundments planted with emergent, rooted vegetation. Water flows overland through the wetland and primarily above the sediment surface. These wetlands may be planted manually or naturally colonized by volunteer plant communities. Some constructed surface flow treatment wetlands contain monocultures of cattails (Typha spp.) or bulrushes (Scirpus spp.), while others are planted with diverse plant communities that are more adaptable under changing seasonal and water quality conditions.

Unlike a natural treatment wetland in which hydrology is largely fixed by the tolerance limits of the existing plant community, a constructed treatment wetland can be designed to regulate water depth and residence time, two important factors in treatment wetland design. Also, the design of constructed treatment wetlands can feature parallel cells or cells in series. Such a system can be operated to rotate discharge points or to use slightly different treatment capabilities of the various available plant species groups. Constructed treatment wetlands have relatively low construction, operation, and maintenance costs compared with conventional advanced treatment technologies.

The plants in constructed treatment wetlands are not typically harvested to remove nutrients. Rather, the microbial flora (bacteria and fungi) that attach to the plants have the natural assimilative capacity to remove biodegradable organics and nitrogen (that is, organic carbon, ammonia, and nitrate) efficiently and reliably. Metals and phosphorus can be sequestered in plant materials and wetland sediments. Molecular diffusion and subsequent microbial activity transform or permanently sequester dissolved pollutants in the incoming wastewater and those released from sedimented solids.

Subsurface Flow Treatment Wetlands
Subsurface flow treatment wetlands are similar to constructed surface flow wetlands in many respects and often use some of the same emergent plant species. However, subsurface flow systems are designed to achieve effluent flow either horizontally or vertically through a porous substrate such as gravel or sand supporting the emergent vegetation. The large surface area resulting from the porous medium and the plant roots provide ample sites for microbial activity. When treating an equivalent volume of flow, subsurface flow treatment wetlands may use somewhat less acreage than constructed surface flow treatment wetlands. Subsurface flow treatment wetlands are less likely than lagoons to have odor and/or mosquito problems.

Subsurface flow wetlands typically are more expensive than surface flow wetland systems and are used in a limited set of circumstances. Appropriate applications include onsite systems for homes and schools, industrial pre-treatment systems, applications near airports where birds might pose a nuisance, use in cold climates, and for secondary wastewater treatment for relatively small communities.

When Should a Treatment Wetland be Considered?

Treatment wetlands are not recommended for all applications. Such systems are most appropriate under the following conditions:

• Large tracts of suitable land are available
• They can be developed with wastewater reuse options
• The influent does not contain high levels of toxic pollutants
• There is a shortage of local groundwater or surface water supplies and augmentation with highly treated effluent is considered beneficial
• The region has a history of wetland loss and associated decline in wetland-dependent wildlife habitat
• Regulatory agencies are interested in the potential benefits of the technology

Design of Treatment Wetlands

In most cases, designing treatment wetlands involves several phases of activity. First, a feasibility study is conducted to confirm the expected benefits and appropriateness of the system for a given situation or for a set of alternative scenarios. Conceptual system designs are developed to estimate capital and operational costs. Choosing a treatment wetland option is based on technical feasibility of the design, operational simplicity, ease of implementation, environmental compatibility, and expected cost.

After the wetland system has been deemed the preferred treatment option from technical and economic standpoints, site evaluations are conducted for selected sites. Evaluations include reviewing available information regarding soil properties, land topography, hydrogeology, vegetation, wildlife and fisheries resources, hydrology, and archaeology and determining land ownership and water quality discharge requirements.

After conceptual design and site selection have been completed, preliminary design begins (this may entail more detailed study of site characteristics such as detailed topography and geotechnical conditions). Final design depends on the alternative selected and includes development of construction plans and specifications, as well as operation, management, and monitoring plans. Necessary permits are also typically obtained in this phase.

Benefits of Treatment Wetlands

Constructed and natural treatment wetlands provide several major benefits compared to more conventional treatment alternatives:

• In some cases, they are less expensive to construct than traditional secondary and tertiary wastewater treatment systems.
• In nearly every case they require less maintenance and are less expensive to operate than traditional treatment systems.
• They introduce a reuse option (environmental enhancement) for the wastewater facility.
• Discharge from treatment wetlands can augment surface water sources for existing water bodies year-round, thus ensuring flow and enhancing local natural aquatic resources.
• With proper design, portions of the treatment wetland may provide important wetland wildlife habitat, as well as human recreational opportunities such as birdwatching, hiking, and picnicking.
• Treatment wetlands are viewed as an asset by regulatory agencies in many regions and as a potentially effective method for replacing natural wetlands lost through agricultural practices, industrial and municipal development, and groundwater withdrawal. These systems may provide self-mitigation for any unavoidable impacts to on-site natural wetlands.

Both constructed and natural treatment wetlands can be utilized for water quality purification in Florida. Use of natural wetlands is dependent upon meeting stringent standards to protect the “type, nature, and function” of the existing wetland. Constructed wetlands can be built and utilized in any part of the state and country, making them advantageous for many water quality management applications.

Ecosystem studies of numerous large-scale constructed treatment wetlands throughout the U.S. have helped to inform regulatory agencies of the potential benefits of wetland creation for successful water treatment. Permitting of discharges from constructed treatment systems is based on well-thought-out plans for design, operation, and monitoring, as well as discharge chemistry and receiving water body quality.