Encyclopedia of Environmental Science and Engineering, Volume I and II

740 NATURAL SYSTEMS FOR WASTEWATER TREATMENT

of the emergent vegetations, and water through the system at

a relatively shallow depth. 2,9 The water surface in FWS wet-

lands is exposed to the atmosphere, and the intended flow

path through the system is horizontal. Pretreated wastewater is

applied continuously to such systems, and treatment occurs as

the water flows slowly through the stems and roots of emergent

vegetation. Free water surface systems may also be designed

with the objective of creating new wildlife habitats or enhanc-

ing nearby existing natural wetlands. Figure 2(a) provides a

schematic flow diagram of free water surface wetlands.

Subsurface flow (SF) wetlands The subsurface flow wet-

lands also consist of a basin or channel with a barrier to pre-

vent seepage. The basin or channel is filled to a suitable depth

with a porous media. Rock or gravel are the most commonly

used media types. The media also supports the root systems 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 is usually horizontal.^12 The schematic flow diagram

of submerged flow wetlands is provided in Figure 2(b).

Comparing the two types of constructed wetland sys-

tems, the SF type of wetlands offer several advantages over

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 larger available surface area for attached growth

organisms. As a result, the treatment response may be faster,

and smaller surface area may be needed for the same waste-

water conditions. 2,10 Furthermore, the subsurface position of

the water, and the accumulated plant debris on the surface of

the SF bed, offer great thermal protection in cold climates

as compared to the FWS type.^14 The reported disadvantage

of the SF type system however, is clogging of the media

possible overflow.

Basic Design Considerations of

Constructed Wetlands

Constructed wetlands are relatively recent development,

and are gaining popularity for treatment of wastewater from

small communities, and residential and commercial areas. In

this section the basic design information and economics of

constructed wetlands are compared.

Site selection A constructed wetland can be constructed

almost anywhere. In selecting a site for a free water surface

wetland the underlying soil permeability must be consid-

ered. The most desirable soil permeability is 10E-6 to 10E-7

m/s.^10 Sandy clay and silty clay loams can be suitable when

compacted. Sandy soils are too permeable to support wetland

vegetation unless there is an impermeable restricting layer

in the soil profile that result in a perched high ground water

table. Highly permeable soils can be used for wastewater

flows by forming narrow trenches and lining the trench walls

and bottoms with clay or an artificial liner. In heavy clay

soils, addition of peat moss or top soil will improve soil per-

meability and accelerate initial plant growth.

Plants Although natural wetlands typically contain a wide

diversity of plant life, there is no need to attempt to reproduce

the natural diversity in a constructed wetland. Such attempts

in the past have shown that eventually cattails alone or in

combination with either reeds or bulrushes will dominate

FIGURE 1 An enlarged root hair of wetland plants.

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