NATURAL SYSTEMS FOR WASTEWATER TREATMENT 741
in a wastewater system owing to the high nutrient levels.^2
The emergent plants most frequently found in the waste-
water wetlands include cattails, reeds, rushes, bulrushes and
sedges. Some of the major environmental requirements of
these plants are given in Table 3.Physical facilities The constructed wetlands behave typ-
ically like a plug flow reactor. Constructed wetlands offer
significant potential for optimizing performance through
selection of proper system configuration, including aspect
ratios, compartmentalization, and location of alternate and
multiple discharge sites.
The aspect ratio, defined as the ratio of length to width,
typically varies from 4:1 to 10:1. However, based on research
data developed on experimental constructed wetlands, the
aspect ratios approaching 1:1 may be acceptable.^13 Several
alternative flow diagrams and configurations of constructed
wetlands are provided in Figure 3.Hydrologic factors The performance of the constructed wet-
lands system is dependent on the system hydrology as well as
many other factors such as precipitation, infiltration, evapotrans-
piration, hydraulic loading rate, and water depth. These factors
affect the removal of organics, nutrients, and trace elements
not only by altering the detention time but also by either con-
centrating or diluting the wastewater. 9,10 For a constructed
wetland, the water balance can be expressed by Eq. (1)[d V /d t ] Q i − Q e P − ET (1)where,
Q i influent wastewater flow, m^3 /d
Q e effluent wastewater flow, m^3 /d
P precipitation, m^3 /d
ET evapotranspiration, m^3 /d
[d V /d t ] change in volume of water per unit time, m^3 /d
t time, dInfluent
Lined Basin Soil Media
Effluent
Water surface
Wetland Plants
Influent
Lined Basin
Soil MediaEffluent
Water surface
Wetland Plants
(a)
(b)
FIGURE 2 Schematic flow diagrams of constructed wetland systems, (a) Free water surface, (b) Subsurface flow.^13C014_001_r03.indd 741C014_001_r03.indd 741 11/18/2005 10:44:18 AM11/18/2005 10:44:18 AM