Environmental Engineering Reference
In-Depth Information
because water can be seen on the surface, and plants that are rooted in the soil layer at the bot-
tom emerge through the water surface. The VSB systems contain a bed of crushed stone,
gravel, sand, and soil with aquatic plants planted on the surface. Water level is controlled so it
reaches plants' root systems but not the surface. VSB systems are not able to support aquatic
life (EPA, 2000). The hybrid system combines elements of both the FWS and the VSB in dif-
ferent cells. The use of FWS or VSB systems depends on the type of water to treat. For
instance, if the wastewater contains high levels of nitrogen, then cells working under the FWS
system would be predominant.
The function of plants in constructed wetlands is mainly to provide support for microor-
ganisms. Plants also add an aesthetic value. Most frequently used plants are persistent emer-
gent plants with tolerance of variable concentration of pollutants and constant flooding. Also,
the climatic conditions of the region dictate the type of plants that can be used. In the United
States, some common plants used in constructed wetlands are bulrushes (Scirpus), spikerush
(Efeocharis), sedges (Cyperus), rushes (Juncus), common reed (Phragrnites), and cattails
(Typha) (EPA, 1995).
Constructed wetlands can serve several purposes, such as:
Secondary treatment in conventional wastewater plants (EPA, 2000).
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After anaerobic treatment of wastewater in lagoons or reactors (Alvarez et al., 2008).
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As a polishing step in advanced ecologically engineered systems (Barista, 2001).
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As tertiary treatment for conventional wastewater treatment (Interdonato and McCarthy,
2001).
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In stormwater management (Campbell et al., 1999).
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The advantages of the integration of constructed wetlands in wastewater treatment are mul-
tiple, such as less expensive to build than other treatment options, minimal need of energy
input, low operation costs, and toleration of flow fluctuation. On the downside, they require
more land surface than traditional systems; do not remove significant amounts of nitrogen or
phosphorus (EPA, 2000); can emit methane and nitrous oxide at levels comparable to natural
wetlands (Johansson et al., 2004); and their performance is less consistent than conventional
treatment (EPA, 1995).
STORMWATER MANAGEMENT
Food-processing plants, administrative buildings, warehouses, and distribution centers have
extensive impervious areas in the form of rooftops, access roads, and parking lots. As a result,
precipitation, in the form of rain and snow, instead of infiltrating into the soil collects on these
impermeable surfaces, and the excess flows to the surroundings as a runoff. When natural
ground cover is present, about 10 percent of the precipitation runs off the surface into streams,
rivers, or lakes. From the remaining, 40 percent evaporates and 50 percent infiltrates. If the
ground is covered by 10 to 20 percent with impervious surfaces then the runoff climbs to
20 percent at expenses of less infiltration. As the percentage of impermeable surface increases
to 35 to 50 percent, runoff goes up to 30 percent, and so forth (Fig. 9.9).
The conventional approach to stormwater management has been draining the runoff as
quickly as possible and routing through storm sewers to rivers, lakes, and oceans. This prac-
tice promotes erosion and downstream flooding, reduces groundwater recharge, and loads
surface water with pollutants (DOE, 2001). As water runs over impervious surfaces, it
washes contaminants (e.g., dirt, dust, detergents, rubber, metals, antifreeze, engine oil, diesel
