The Role of Treatment Wetlands in Water Treatment

from Hydrophyte Volume 27 Issue 1 - January 2023

by South Florida Aquatic Plant Management Sociey

Supplemental Information Can Be Found At:

By: Jay Capasso, Lisa Krimsky, and Jehangir Bhadha

Introduction

Wetlands are often deﬁned as transitional areas between land and water that have shallow water at or near the soil surface for at least part of the year, including during the growing season. These environments support biological, chemical, and physical processes that can improve water quality. As a result, we often refer to wetlands as “nature’s kidneys” and construct man-made wetlands for water treatment purposes.

Constructed (man-made) wetlands are predominately used for wastewater treatment purposes as regulations discourage the use of natural wetlands. These treatment wetlands usually treat or “polish” waste waters that have already undergone primary and secondary treatment at a wastewater treatment plant. Polishing via treatment wetlands is usually the ﬁnal step of treating wastewaters before wastewater is discharged to local surface waters. This step can help reduce the levels of pollutants that have the potential to cause human or environmental harm such as pathogens, viruses, heavy metals, nutrients (nitrogen and phosphorus), pharmaceuticals, suspended sediments, etc. Many of these pollutants are only problematic once they have reached a certain threshold so helping to reduce their levels is imperative.

Types of Treatment Wetlands

There are two common types of treatment wetlands: Surface flow wetlands and subsurface flow wetlands. Surface flow wetlands contain surface waters that flow over a vegetated landscape. These treatment wetlands resemble natural wetlands as they include surface waters and various types of submerged, floating, and emergent aquatic vegetation.

In subsurface flow wetlands, water flows either vertically or horizontally under the soil surface which is generally planted with emergent aquatic vegetation. These wetlands act as a filter by removing suspended solids and nutrients from wastewaters as they flow through the soil medium.

There are advantages and disadvantages to both types of treatment wetlands. Surface flow wetlands are not as expensive to construct or as prone to clogging as subsurface flow wetlands. Subsurface flow wetlands reduce environmental odors, require less land area to establish, and reduce mosquito breeding habitat compared to surface flow wetlands.

Biotic and Abiotic Wetland Water Treatment Processes

Biotic processes occur when plants or microbes uptake, and sometimes transform, nutrients and metals present in surface waters and wetland soils. Aquatic plants also have physical e ects on water treatment processes. Aquatic vegetation can slow the flow of water moving through the wetland increasing the “hydraulic residence time” or the length of time water spends in the wetland. Three general types of aquatic plants include floating aquatic vegetation, emergent aquatic vegetation, and submerged aquatic vegetation. Floating aquatic vegetation float on the top of the water surface and are not attached to wetland soils. Examples of floating aquatic vegetation include duckweed (Lemnoideae) and water hyacinth (Eichhornia crassipes). Submerged aquatic vegetation grow below surface waters and examples include tape grass (Vallisneria americana) and southern naiad (Najas guadalupensis). Emergent aquatic vegetation are plants that are rooted in wetland soils and grow above the water surface and examples include pickerelweed (Pontederia spp.) and blue flag iris (Iris virginica).

Floating aquatic vegetation causes shade in the water and can block sunlight from reaching surface waters and wetland soils, which can reduce the growth of algae and submerged aquatic vegetation. Thick stands of floating aquatic vegetation can also limit the di usion of oxygen in surface waters, resulting in low oxygen conditions that favor microbially mediated reactions such as denitrification. Emergent aquatic vegetation also provides shade to underlying surface waters and wetland soils. It can help to stabilize wetland soils, inhibiting the resuspension of suspended sediments to surface waters. Emergent aquatic vegetation can also di use oxygen from the atmosphere through roots into wetland soils increasing microbial decomposition and nitrification. Submerged aquatic vegetation can influence water treatment processes by stabilizing wetland soils.

These vegetations also photosynthesize within the surface waters, increasing dissolved oxygen levels and increasing pH of surface water through the removal of carbon dioxide. These conditions can promote decomposition and precipitation of certain elements such as when phosphorus binds with calcium carbonate at a higher pH.

There are also abiotic processes that occur within wetlands to facilitate water treatment. Sorption which refers to the binding of chemicals onto soil particles limiting their bioavailability and loss to surface waters is likely the most important water treatment process that occurs in wetland soils. Other abiotic processes that occur in treatment wetlands include photo oxidation, which refers to sunlight energy and its ability to break down and oxidize compounds. Volatilization is another abiotic process, which occurs when compounds are broken down into gaseous forms and released to the atmosphere.

Treatment wetlands in Florida

Aside from Alaska, Florida contains the highest acreage of wetlands compared to any other state in the United States. This is largely due to the state’s flat topography and frequent rainfall. The state of Florida has numerous treatment wetlands being used for wastewater treatment purposes, including 17 natural wetlands and 21 man-made constructed wetlands. Most of these wetlands are surface flow wetlands. Wetlands are an essential component of the Florida landscape and treatment wetlands provide benefits beyond wastewater treatment purposes. These sites also provide recreational opportunities including bird watching, fishing, and sites for environmental education.

For more information see UF/IFAS

Extension Article below: Wetlands https://edis.ifas.ufl.edu/publication/FR419