WETLANDS - Water Quality Engineering in Natural Systems - page 292

water conservation and wildlife habitat goals. A disad-
vantage of constructed treatment wetlands is their rela-
tively slow rate of operation in comparison with
conventional wastewater treatment technology. Con-
structed wetlands have proven effective in removing
metals from waste streams; however, if a large volume
of waste is involved, a sizable expanse of wetlands and
rigorous rotation of areas in the treatment process may
be required (Stone, 1999). Also, once the plants take up
and concentrate the metals, they themselves become a
hazardous waste that must be disposed of properly.
Although constructed wetlands mimic natural wetlands,
they represent created ecosystems that are not part of
the original wetland resources of a region, and therefore
continuous flooding with polluted water

appearance to natural marshes with areas of open water
and predominant overland flow. In a horizontal subsur-
face flow (HSSF) wetland, shown in Figure 8.8b, and
sometimes called a vegetated submerged bed , the water
flows horizontally through a gravel bed, the water
surface is kept below the surface of the bed, and the bed
is planted with wetland vegetation. A vertical flow (VF)
wetland, shown in Figure 8.8c, distributes water across
the surface of a sand or gravel bed planted with wetland
vegetation, and the water is treated as it percolates
through the plant root zone.
FWS, HSSF, and VF wetlands can be used to treat
municipal wastewaters. However, FWS wetlands that
receive less than secondary-treated wastewater are
potential health hazards and odor sources and are not
well suited for use near parks, playgrounds, and similar
public facilities; gravel bed HSSF systems often domi-
nate these applications. In using constructed treatment
wetlands to treat urban runoff, agricultural runoff, and
livestock and poultry wastes, FWS wetlands are most
commonly used, primarily because of their ability to
effectively handle pulse flows and changing water levels.
Wetland treatment of landfill wastes use both FWS,
HSSF, and VF systems. Overall, HSSF systems are more

is more

acceptable.

8.3.1 Classification

There are three types of constructed wetlands: free
water surface wetlands , horizontal subsurface flow wet-
lands , and vertical flow wetlands . These types of wet-
lands are illustrated in Figure 8.8. A free water surface
(FWS) wetland, shown in Figure 8.8a, is similar in

emergent vegetation

deep zone

deep zone

water-level control

inflow

outflow

impermeable
liner

rooting
media

(a)

water level

top of gravel bed

water-level control

inflow

outflow

main bed
media

coarse
media

coarse
media

(b)

inflow

sand

outflow

gravel

perforated pipe

(c)

Figure 8.8. Wetland treatment systems: (a) free water surface wetland; (b) horizontal subsurface flow wetland; (c) vertical flow
wetland.