Aquatic weeds and their utilization 369Based on data used in this example, duckweed culture will give more
financial return annually.
7.7.5 Emergent aquatic weeds and constructed wetlands
Wetland is an area that is inundated or saturated by surface or groundwater at a
frequency or duration sufficient to maintain saturated conditions and growth of
related vegetation. Emerged aquatic weeds, such as cattails (Typhas), bulrushes
(Scripus) and reeds (Phragmaites) are the major and typical component of the
wetland systems (Figure 7.3). Wetland is a natural system where complex
physical, biological and chemical reactions essential for wastewater treatment,
listed in Table 7.7, exist. To avoid interference with the natural ecosystems,
constructed wetland in which hydraulic regime is controlled have been used for
treatment of a variety of wastewaters (Hammer 1998; WPCF 1990). Constructed
wetlands can range from creation of a marsh land to intensive construction
involving earth moving, grading, impermeable barriers, or erection of tanks or
trenches (U.S. EPA 1988).
Two types of constructed wetlands have been developed for wastewater
treatment, namely, free water surface (FWS) and subsurface flow (SF). An FWS
system consists of parallel basins of channels with relatively impermeable bottom
and soil and rock layers to support the emergent vegetation, and the water depth is
maintained at 0.1 – 0.6 m above the soil surface. An SF system, also called ‘root
zone’, or reed bed, consists of channels or trenches with impermeable bottom and
soil and rock layers to support the emergent vegetation, but the water depth is
maintained at or below the soil surface (Figure 7.18). To reduce the short-
circuiting, the length to width ratios of constructed wetlands units should be more
than 2:1 and bed slopes of 1 – 5 %. Although it might appear that SF constructed
wetlands could be subjected to frequent clogging problems, performance data
reported so far have shown them to function satisfactorily with a high degree of
removal efficiencies (Reed and Brown 1992 and 1995). An SF constructed wetland
can be operated in a vertical flow mode in which wastewater is applied uniformly
over the wetland surface and the effluent is collected at the bed bottom. High
percentages of BOD and nitrogen removal have been reported from this type of
operation (WEF 2001).
Table 7.11 shows removal efficiencies of some constructed wetlands located in
the USA, which, in general, are comparable with other constructed wetlands in
operation worldwide. Although more than 80 percentage of N and P removal can
be expected in constructed wetlands, not much information about the removal of
faecal micro organisms is available (see section 7.1.3). Based on the performance
data, a summary of wetland design consideration is given in Table 7.12, which
should serve as guidelines in the design and operation of natural and constructed
wetlands for wastewater treatment.