Monday, August 20, 2012

CONSTRUCTED WETLANDS AS A METHOD OF WASTEWATER TREATMENT


          Since most chemical reactions take place in water based medium, water plays a major role in industrial world. With the polar nature of water it easily gets polluted from those chemical and biological ingredients use in manufacturing process. This polluted water is called industrial effluents or simply the wastewater. Since industrial wastewater contain toxic chemical compounds like Cyanides, biological waste like plant materials and pathogens, toxic heavy metals like Cadmium, Lead, exceeding the accepted levels for human use. When this wastewater released in to natural water bodies it may result social and health problems to the people living around the water resource and also to the people who use it for day today work. 
Minamata Disease (methylmercury poisoning) on Public Health & Social Justice 
socialmedicine.org
In early days people tends to release this waste water to natural water bodies without a proper treatment. Therefore in early days outbreaks due to hazards waste were common. Minamata outbreak in Japan is such a disaster coursed by not having a proper treatment for wastewater. Wastewater treatment is an expensive process, if we are using conventional methods like chemical precipitation, Ion exchange; Reverse Osmosis and Electro dialysis. Not only they are expensive but also they produce toxic sludge therefore small industries won’t consider about having a wastewater treatment plant within their manufacturing plants due to the high production cost. Instead they release wastewater to the nearby water stream without treating it.

Constructed wetland (vertical flow) in Shenyang (China)
blumberg-engineers.com
With the development of the science, scientist comes up with an eco friendly and economical friendly two methods to treat waste water by combining knowledge of botany and the knowledge of Chemistry. Those are phytoremidation which use living plants and Biosorption which use dead plant materials. Even though phytoremidation consume much time than Biosorption it is easier then biosorption. Therefore phytoremidation technique is used in initial process of wastewater treatment. And constructed wetlands are use in phytoremidation technique where plants are growing in the artificial wetland.  
According to the US-EPA and Ramsar Convention Wetland is an area where area is saturated by surface water, ground water, flowing fresh or brackish water, permanently or temporary and characterized by a prevalence of vegetation that is adapted to the environmental conditions. Therefore wetlands contain three significant characters such that presence of water, unique soils that differ from upland soil and the presence of vegetation adapted to saturated conditions. Natural wetlands has a great capability of improving the water quality, this feature of wetlands is use in constructed wetland to treat wastewater. With the high demand for water quality, neediness of constructed wetland for wastewater treatment becomes high.
Subsurface Flow Wetland design
Maintaining a constructed wetland require less technology, less capital cost but to contract a wetland for wastewater treatment require some engineering and hydrological knowledge. Since fundamental biological processes like nutrient content, soil pH, water pH, species density, productivity, nutrient cycling which are important to pollution control are depend on the hydrology of the wetland. Furthermore another important factor is the Hydraulic Retention Time and the Water Budget, where treatment performance directly related to the time which wastewater is in contact with the plant species since plants need some time to absorb the pollutants. Taking all this to account engineers can construct two type of Constructed wetlands
Free Water Surface flow wetland design 
First one is Subsurface Flow Wetland design (SSF). In this method gravel substrate is used as the filling medium of the wetland and plants are growing on it. Second one is the Free Water Surface flow wetland design (FWS). In this method waste water is retain as shallow water. Other than these two designs constructed wetlands can be categorized according to the flow direction of the wastewater. They are,
  •      Horizontal- flow Wetlands.
  •      Vertical – flow wetlands.

When constructing a wetland for wastewater treatment, scientists use two type of model to achieve the maximum usage of the wetland. One is the UK model of constructed wetlands. And the other one is The First order Plug-Flow Biokinetic model. In brief UK model is the simplest model used in domestic wastewater treatment in UK. In this model horizontal flow system for SSF system is designed. On the other hand, in first order plug-flow biokinetic model, both SSF and FWS systems can use and this model is more complex than UK model since it involves Organic denegation, nitrification, adsorption, disinfection and other biologically driven processes.    


Spreading of gravel in the horizontal flow constructed wetland
athene.geo.univie.ac.at
References:-
1.Subsurface Flow Constructed Wetlands For WasteWater Treatment A Technology Assessment; United States Environmental Protection Agency Office Of Water EPA 832-R-93-008 (4204) July 1993.
2.Seswoya. R , Zainal. M,Y. ;Subsurface - flow constructed wetland: Proposed design area for high strength effluent domestic wastewater;