2    CONSTRUCTED WETLANDS FOR INDUSTRIAL
WASTEWATERS
Sherwood C. Reed, Principal
Environmental Engineering Consultants
Norwich, Vermont 05055
Michael Hines, Manager
Environmental Compliance Department
Tennessee Valley Authority
Knoxville, Tennessee 37922
INTRODUCTION
Natural wetlands, such as bogs, swamps and marshes have probably been used as convenient
receptacles for wastes since human civilization began. In recent decades, as scientific interest in these
wetlands increased, it was repeatedly documented that improvements in water quality occurred as
wastewater passed through the wetland. Concurrently, it was recognized that these natural wetlands
are an important part of our environment; so laws, regulations and public attitudes have emerged for
their protection. As a result of these competing interests, the concept of utilizing constructed wetlands
for wastewater treatment has been developed. This approach takes advantage of the renovative
capabilities of the wetland ecosystem, while protecting the remaining natural wetlands. Interest in
these constructed wetlands emerged in the early 1980s and has grown almost exponentially since. It is
estimated, based on recent surveys and database preparation,1'2 that close to 1000 constructed wetland systems are in operation, design, or planning, in the United States. These systems are treating
domestic, municipal, and industrial wastewaters and range in size from small, on-site units serving a
single family dwelling to large scale (>20 mgd) systems for municipal and industrial wastewaters.
Industrial applications include pulp and paper, oil refining, chemical production and processing,
mining, textile processing and dyeing, and food processing.
CONSTRUCTED WETLAND TYPES
Wetlands are defined as land where the water surface is near the ground surface long enough each
year to maintain saturated soil conditions along with the related vegetation. Marshes, bogs, and
swamps are all examples of naturally occurring wetlands. A "constructed wetland" is defined as a
wetland specifically constructed for the purpose of pollution control and waste management, at a
location other than existing natural wetlands. There are two basic types of constructed wetlands, the
free water surface wetland and the subsurface flow type wetland. Both types utilize emergent aquatic
vegetation and are similar in appearance to a marsh.
The free water surface (FWS) wetland typically consists of a basin or channels with some type of
barrier to prevent seepage, soil to support the roots of the emergent vegetation, and water at a
relatively shallow depth flowing through the system. The water surface in this case is exposed to the
atmosphere, and the intended flow path through the system is horizontal.
The subsurface flow (SF) wetland also consists of a basin or channel with a barrier to prevent
seepage, but the bed then contains a suitable depth of porous media. Rock or gravel are the most
commonly used media types in the U.S. The media also supports the root structure of the emergent
vegetation. The design of these systems assumes that the water level in the bed will remain below the
top of the rock or gravel media. The flow path through the operational systems in the U.S. is
horizontal.
Several advantages are thought to apply to the SF type of wetland as compared to the FWS type. If
the water surface is maintained below the media surface, there is little risk of odors, public exposure,
or insect vectors. In addition, it is believed that the media provides greater available surface area for
treatment than the FWS concept. As a result, the treatment responses may be faster for the SF type
and therefore it can be smaller in area than a FWS system designed for the same wastewater condi-
48th Purdue Industrial Waste Conference Proceedings, 1993 Lewis Publishers, Chelsea, Michigan 48118. Printed in
U.S.A.