FLOATING TREATMENT WETLANDS ISLAND   |   VETIVER PLANT

Do something on the unsightly mud flats during low tides !! These unwelcome views from the waterfront promenade can be replaced with floating wetland islands that floats or just laid over the mud flats during low tides, thus only the greenery view is visually exposed to the on lookers !!

Constructed treatment wetlands have traditionally involved the use of free floating aquatic plants or sediment-rooted emergent wetland plants, either with water flowing through the root zone (subsurface flow) or amongst the stems (surface flow).

Floating treatment wetlands (FTWs) are an innovative variant on these systems that employ rooted, emergent plants such as the Vetiver grass etc. growing as a floating mat on the surface of the water rather than rooted in the sediments.

Because of this feature, floating treatment wetlands offer great promise for rainfall driven storm water treatment applications as they are little affected by fluctuations in water levels that may submerge and adversely stress bottom-rooted plants.

Over the past two decades, artificially created floating wetlands have been studied in various parts of the world for a range of applications, such as water quality improvement, habitat creation, and aesthetic enhancement in the following treatment applications.
• Combined stormwater-sewer overflow
• Sewage
• Acid mine drainage
• Piggery effluent
• Poultry processing wastewater
• Water supply Reservoirs

Compared to conventional pond and wetland systems, FTWs are considered to possess a number of advantages that may enhance certain contaminant removal processes. The cover and shelter provided by the floating mat promotes conditions conducive to settling by reducing turbulence and mixing induced by wind, waves and thermal mixing.

Compared to conventional sediment-rooted wetlands that are predominantly restricted to water depths of less than 0.5m, FTWs can be constructed deeper to provide extra water volume, reduce flow velocities and enhance settling.

Plant roots are believed to play a key role in the treatment processes within FTWs by virtue of the contact that is afforded as the water passes directly through the network of hanging roots that develops beneath the floating mat. Plant roots provide a living surface area for development of biofilms containing communities of attached-growth micro-organisms responsible for a number of important treatment processes. The thick network of roots and associated biofilms are effective at physically trapping particulates within the water column, which subsequently slough off the roots as heavy particles that are more amenable to settling.

There is some potential for incorporation of materials with a high metal sorption capacity within the floating mat of FTW in order to enhance removal of dissolved copper and zinc. Potentially suitable media includes zeolites, vermiculites, bauxsol, activated carbon, and bio-sorbents such as peat, plant, algal and shell materials. The effectiveness of this approach will be somewhat limited by the amount of interaction between the stormwater and the sorbent material contained within the floating mat. The pumping and circulation of water vertically through the floating mat may be required to optimize metal removal through such a process.

A number of questions exist surrounding the design and practical implementation of FTWs as a stormwater quality improvement device. At this stage, FTWs show great promise for applications where they are retro-fitted onto existing detention ponds in order to enhance their treatment performance, In two stage pond systems, it is suggested that the most suitable location for FTW elements will be within the second pond following the sediment removal forebay. FTWs may also be beneficial for upgrading surface flow wetlands that have suffered vegetative decline due to inappropriate water depths or for incorporation into deep water zones.

A conceptual stormwater treatment train has been suggested for newly constructed systems that consist of a forebay upfront of course sediments and flow attenuation, a surface flow wetland for some removal of organics, suspended solids and nutrients (this component may not be necessary), followed by a FTW for fine particulate and metal removal, The final stage of the treatment system would consist of an open water pond to facilitate re-aeration of the water prior to discharge.

Fundamental experimental work is required in order to define the relationship between loading rate per unit surface area of FTW (either hydraulic or contaminant loading rate) and the typical removal rates or effluent concentrations that can be expected from such systems. This would then enable the surface area of FTW required to achieve a given effluent concentration to be estimated for design purposes.

In short, FTW is still a work in progress and empirical approach is the current design basis to achieve a desired pollutant removal objective.