A water surface. However, peduncle density is

A FTW consists of floating-leaved
macrophytes, in our case, Hydrocharis
morsus-ranae growing on a mat or structure floating on the surface of a
pond-like water body. H. morsus-ranae
has underground organs, leaf peduncles grow through the water column and leaves
float on the water surface. However, peduncle density is much lower as compared
to submerged or emergent species. In FTWs, the plants will grow in a hydroponic
manner, taking their nutrition directly from the water column in the absence of
soil. Beneath the floating mat, a hanging network of roots, rhizomes and
attached biofilms is formed. This hanging root-biofilm network provides a
biologically active surface area for biochemical processes as well as physical
processes such as filtering and entrapment. Therefore, a general FTW design objective
is to maximize the contact between the root-biofilm network and the polluted
water passing through the system.

According to Vymazal and
Kropfelova, constructed wetlands with floating leaved macrophytes have a good potential
for removal of suspended solids as leaves on the surface minimize the effect of
wind causing potential resuspension and water movement. Besides, H. morsus-ranae is also proved to be
beneficial in removing nitrogen and phosphorus from wastewater (Reddy, 1984). Organics
like nitrogen and phosphorus are predominantly removed via sedimentation of
particulate matter and microbial degradation. There are very limited data on
floating-leaved system wetlands; however, these processes are mostly aerobic in
the water column due to oxygen production by algae.

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In the water column, nitrogen removal
is able to realize through volatilization at higher pH values created by
photosynthesis of plankton and periphytic community attached to the leaf
peduncles and blades. There is always possibility of occurrence of algae in
these systems as leaves of floating-leaved species usually do not completely
cover the water surface. Aerobic conditions may favor nitrification while
denitrification is unlikely in the water column but it may appear in the
sediment-water interface in case of anoxic conditions. Phosphorus can be taken
up in the water column by algae but this removal compartment is only short-term
and nutrients are washed out quickly from decaying algal tissue. Besides, both
nitrogen and phosphorus can be taken up by roots and rhizomes.

i.       
Design
and Setup of FTWs

Since maintaining the aerobic
condition in water column is important for aerobic bacteria to work, adding
aeration below the water column is a great way to reach water treatment
goals.  This aeration delivers an
oxygenated environment in which the aerobic bacteria can flourish. The biofilm
that forms on the undersurface of the water column, and that colonize the
biofilter media becomes habitat for large numbers of beneficial bacteria. The
bacteria do the necessary work of consuming excess nutrients in the water
creating in this way a perfect floating treatment wetland.

Besides, in floating treatment
wetlands, buoyancy is provided artificially through the use of a floating
structure or raft which supports the growth of the plants. For example,
Floating Islands International injects expanded polystyrene foam into their polyester
matrix in order to provide the desired level of buoyancy. In other
applications, sealed PVC or PP pipes, polystyrene sheets, bamboo or inflatable
vinyl pillows have been used to provide flotation. In general, there are two
main approaches that have been adopted. Firstly, the construction of a buoyant
frame that supports some sort of mesh on which the plants grow. Secondly, usage
of a buoyant material which itself serves to support the growth of the plants.

Figure
3: Design of FTWs using Buoyant Frame and Suspended Mesh (Smith & Kalin,
2000)

RECOMMENDATION
AND FUTURE IMPROVEMENT

            Although
phytoremediation has far more advantages than its disadvantages, with variety
of techniques, and is economical, ecological and aesthetical, but its
development is rather slower than any other inventory fields due to several
reasons. One of the reasons causing slow development in this technology is
because the time needed for remediation might needs several years or even up to
decades. Most of the people would rather go for easier and faster way that is
conventional treatment. Hence, the commercialization of phytoremediation is
comparably slower and not really common.

            There
is only small amount of successful applications up to date, because majority of
the projects started in recent years. However, the potential of
phytoremediation techniques to be largely applied in industries is huge.  To
improve the rate for phytoremediation to occur faster, attention has to be given
to genetic modification in the plants used in phytoremediation. At the
University of Washington, the human gene that encodes cytochrome P450 IIE1 was
introduced into tobacco. Cytochrome P450 oxidize several halogenated organic
compounds, among them are TCE, ethylene bromide, tetrachlorocarbon, chloroform
and vinyl chloride. Transgenic tobacco degraded TCE 640 times faster than
non-modified plants (Doty et al. 2000). Until now, only a small part of plants
have been screened for their metabolic ability to degrade pollutants, however,
more research is needed to understand how genetic modification can increase
their degradation rate.

            To
understand the long term performance of Floating Treatment Wetlands and gain
improvement from that, several recommendations are made. Firstly,
denitrification measurements within both the FTWs matrix and wet pond sediments
underneath the raft need to be made, so that a better nitrogen mass for the
interaction between FTWs and ponds can be understood. Besides that, it is necessary
to conduct field monitoring studies on the pollutant removal performance of
FTWs applications in a broader range of aquatic settings, especially for
sediment. Furthermore, application of engineering knowledges in FTWs technology
needs to be enhanced. In such, modeling work needs to be done to develop design
tools to optimize nutrient and sediment removal by adjusting FTWs surface area
and configuration within water column (Lane et al., 2016).

           Although FTWs are man-made effort,
but actually it is a production that mimics the real life wetlands. Hence,
there are several projects which already proved that these systems actually
augment the performance of stormwater wet ponds as a mean of improving water
quality of urban runoff, and at the same time serves as a new habitat for
wildlife like dragonflies, damselflies and small fishes that swam around and
underneath the plants. Hence, FTW is a good sustainable option for removing
pollution, without using chemicals and physical efforts, and in turn, do not cause
any harm to nature and surroundings. 
This is exactly what man needs in today’s world, without depriving the
Earth’s resources, creating a sustainable environment for us and for future
generation.