Constructed Wetlands: A promising system
It is easy to leave the tap running without caring about where the water is coming from. How often do we do this when brushing our teeth or washing dishes? However, the real cost of this wastage may be considerable, if we think about the loss of this valuable natural resource and the expense of water treatment. Thinking of wastewater, have you also considered what happens when you flush the toilet or dispose of laundry water containing soap or oils down the sink? Well, our urban wastewater receives treatment before it is discharged to the environment.
However, our situation is not the reality of developing countries, where water is often scarce and water treatment technologies must be reliable and sustainable. Efforts to mitigate negative environmental impacts and reduce public health risks in these countries require the development of low-cost wastewater treatment technologies that effectively eliminate wastewater contaminants; facilitating the breakdown and removal of pollutants using biological processes that avoid the hazards associated with chemical-based systems and their additional cost. A promising wastewater treatment system for this context is constructed wetlands (CW), which can provide sustained access to improved water and sanitation services.
Oksana Voloschenko , a Marie Curie fellow within the ADVOCATE project, is researching the development of this treatment concept, within the topic "microbial nitrogen transformation in horizontal subsurface flow constructed wetlands for the treatment of contaminated groundwater". The focus is on the removal by CW of ammonium (NH4+) a major pollutant in groundwater from agricultural sources. Firstly, we need an overview of how CW serve as natural wastewater treatment systems for projects carried out in Africa, Latin America or India.
In general terms, constructed wetlands consist of beds of aquatic macrophytes (wetland plants). Their root systems provide surfaces for the attachment of microorganisms, enhancement of filtration effects and stabilization of the bed surface. Moreover, the roots contribute to the development of microorganisms by the release of oxygen and nutrients within the host material. Depending on flow conditions we can distinguish surface flow or subsurface flow CW (horizontal and vertical flow), as shown in the diagrams below.
- Surface flow CW consists of large, shallow lagoons that contain submerged, emergent, or floating plant species. They are most commonly used to remove nutrients to prevent eutrophication (algae growth) in the receiving water body.
- Subsurface flow CW consist of shallow basins filled with coarse sand or gravel as filter material. Wetland plants are grown on the surface of the filter bed, and pre-treated wastewater flows through the bed horizontally below the surface. Subsurface flow CW can treat both nitrogen and phosphorus.
How do they work?
If we look at subsurface flow CW the main components are: a waterproof basin, filter material, wetland plants and inlet and outlet structures. This is shown in the diagram below.
The waterproof basin is used to prevent soil and groundwater contamination through wastewater infiltration. Filter material has several functions. It retains solids from the pre-treated wastewater, provides surfaces for the adhesion and development of the microorganisms that play a crucial role in the degradation of organic pollutants and transformation of nitrogen compounds, and supports the development of root systems in the filter material for the wetland plants. Inlet and outlet structures are required for wastewater distribution and collection, respectively.
Our research efforts are focused on understanding the removal mechanism for pollutants, due to the complexity of the wetland systems, and the role that aerobic and anaerobic zones play within the root zone of the plants. This is the Oksana's research. Obviously, as with all water treatment technologies, CW designs have limitations. These are currently are being studied by researchers such as Oksana.
Coban, O., Kuschk, P., Wells, N., Strauch, G., & Knoeller, K. (2014). Microbial nitrogen transformation in constructed wetlands treating contaminated groundwater Environmental Science and