Growth common supporting medium. The organic matter and

Growth biomass attached to a
supporting medium in an attached growth system. This condition is called
biofilm. The attachment is affected by several influencing factors, including
the presence of polymer molecules on the surface, cell-cell interactions and
also the composition of the media used.  Usually
the supporting medium is made of solid natural, artificial polymers or
agglomerates of the biomass itself. For examples, rocks, stones, sand, rubber
and plastic are the common supporting medium. The organic matter and excessive
nutrients from wastewater will be fed off by the biofilms which grow on the
supporting media. Note that it is very important to have difference in the
density gradient of the biofilms and the liquid flowing so that the existence
of velocity gradient is allowed (Shama Sehar & Iffat Naz, 2016).

A.   
Constructed
Wetland System

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Constructed wetlands are engineered
method using phytoremediation to remove the pollution from any water body – can
be municipal wastewater, ponds or lakes or even raw wastewater. It is a
well-designed system consists of growing or fixed biofilms comprising beds
together with its sapped graded medium and corresponding microbial inhabitants
of the plants used. This system can be categorized into two major groups, which
are surface flow and subsurface flow. In surface flow, the water flows over the
surface and the bacteria and substrate contact angle with water is lower than
that in subsurface flow, resulting in the much enhanced treatment efficiency of
subsurface flow systems. On the other hand, subsurface flow has water flowing
beneath the surface, targeting to overcome the issue of unpleasant smells. To
discuss it in further depth, subsurface flow systems have horizontal and
vertical subsurface flow wetlands depending on the flow path.

            In
general, wastewater treatment in constructed wetland systems involves a series
of physical, chemical and biological processes, such as adsorption, filtration,
sedimentation, chemical precipitation, microbial activities and macrophyte
uptake. Phytoremediation is said to be hard to implement since there are various
factors contribute to the removal efficiency, including hydraulic retention
time, temperature, macrophytes, composition of substrate or quantity of biomass.
In this system, the role of selected plants is very significant for the removal
of nutrients from wastewater, and they also speed up the purification process
by increasing the chemical and biological reactions in the rhizosphere (Shama
Sehar & Iffat Naz, 2016).  

There are a lot of advantages for
this method including the requirement of low operational and maintenance costs,
less energy consumption and they are easily accepted by public since it is
environmentally friendly. However, the purification process might take a very
long time with some up to decades. Hence, it is necessary to find an
alternative way to speed up the process. For example, scientists have been
continuously doing the research on the genetic alteration of certain plants
especially in the root parts to increase the rate of nutrients uptake and
enhance its sustainability with heavy metals in the wastewater. However, up to
today, the possible improvement that can be made in order to improve the uptake
rate in the genes of Hydrocharis
morsus-ranae is still unknown.

 

Figure 1: Common Constructed
Wetland System for Wastewater Treatment (Sehar et al., 2015)

 

B.    
Floating
Treatment Wetlands (FTWs)

                               
i.           
What
is Floating Treatment Wetlands?

Floating treatment wetlands (FTWs)
are a comparatively new emerging water treatment practice that consists of
wetland plants as phytoremediators planted on floating mats constructed of
buoyant material. The rafts float on a water surface and can be utilized to
improve water quality by filtering, consuming, or breaking down pollutants from
the water (Lynch et al., 2015). These floating treatment wetlands are proved to
be more efficient than traditional constructed wetlands as they offer great
potential for treatment of urban stormwater.

Plants, in our case, Hydrocharis morsus-ranae secretes sugar
and oxygen for microbes and bacteria to feed on. In return, microbes and
bacteria which usually found abundant around the root and rhizome parts give
the plants food such as nitrogen, phosphorus and ammonia. This is the
application of phytoremediation to reduce the amount of pollution in surface
water.

Figure 2: Floating Treatment
Wetland Receiving Urban Stormwater Runoff (University of Maryland Center for
Environmental Science, 2013)

 

                             
ii.           
Pollutant
Reduction Mechanisms in FTWs

There were few major pollutant
reduction mechanisms that have been identified from floating treatment
wetlands. Firstly, the plants that served as phytoremediators directly uptake
pollutants from the wastewater, most efficiently nutrients using a process known
as biological uptake. Then, the microorganisms growing on the floating rafts
and plant root systems digest and consume these organic matters from the
wastewater through microbial decomposition. Lastly, the root systems filter out
the sediments and associated pollutants.

                           
iii.           
Benefits
and Limitation of FTWs

Floating treatment wetlands provide
more design flexibility. They can be sized to fit into almost any pond or lake.
They also have high tolerance of different water depths. Due to their
floatation on the water surface, the plants are not affected by fluctuations of
water levels which will stress bottom-rooted plants in stormwater systems, as
long as they are anchored to the bottom. Besides, they largely enhance the
pollutant-removal effectiveness of existing stormwater. FTWs can be designed to
operate as an extended detention basin so that large runoff can be captured and
allowed to be flowed over slowly, hence increase the amount of water that
receives treatment and maximize the retention time (Headley & Tanner, 2008).
The fibrous matrix and plant roots also create a large surface area for
pollutant-eating microbes and bacteria to work for the nutrients uptake.

In general, FTWs are actually an
effective way for anaerobic digestion, nitrification processes, de-nitrification,
removal of Total Suspended Solids (TSS), and also reduction in Biochemical
Oxygen Demand (BOD), fecal coliforms, phosphorus and turbidity (Martin
Ecosystems, n.d.).

However, there are several
limitations too. Although the floating treatment wetlands will not be affected
by water level fluctuations as long as they are anchored to the bottom, but
anchoring them will be a great challenge. Moreover, implementation of FTWs
required significant labors. This is because FTWs need to be harvested and
removed seasonally. Not only that, the plants and the microorganisms could be
harmed if some contaminants, especially oil and herbicides are allowed to flow
through the system. 

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