Constructed Wetlands for Wastewater Treatment
Constructed wetlands are purpose built wetlands which are specially designed for the treatment of wastewater. A carefully chosen selection of plants and a specially designed substrate provide the right biological environment for cleansing and reoxygenating the water. These wetlands are modelled on natural wetland systems, but are designed to achieve optimum treatment efficiencies. Natural wetlands and their plant communities have evolved to thrive on nutrient rich, silted waters. They have even been used inadvertently for sewage treatment since the first towns and villages channelled their waste into them.
Wetlands for wastewater treatment are considerably less costly than mechanical treatment systems and have lower running costs. They can be used for a wide variety of applications, including:
- Domestic sewage treatment
- Agricultural washings, wastewaters and field runoff buffers
- Industrial effluents and yard runoff filtration
- Group housing and municipal sewage schemes and sludge disposal
- Landfill runoff and other long-term low maintenance situations
- Urban and road storm-water runoff, for both flow buffering and water filtration
- Mine tailings and metal reclamation
- Stream, river and lake restoration
- Recreational and amenity applications
- Flood control and habitat enhancement
Advantages of Constructed Wetlands for Wastewater Treatment:
Wetlands are relatively inexpensive to install and can have low to zero running costs and electricity requirements, as long as pumps are not needed. Ideally gravity should bring the effluent down to a constructed wetland and then on to the groundwater or stream.
One of the primary advantages of constructed wetlands over other systems is that they are very adaptable. They can function effectively with a variable waste load (e.g. schools or caravan sites where the usage is seasonal)
They can also fit in around existing or overloaded systems. This means that it is now possible for small towns and villages to treat their wastewater discharges adequately where only a large septic tank may have existed before.
They require no chemicals to operate, thus keeping maintenance costs and inputs to a minimum. However, for situations where low phosphate discharges are required, wetland are excellent at capturing the ferric and alum sludge residues from effluent coming from conventional dosing and settling systems.
They are attractive to look at and can attract a host of birds, dragon-flies and other fauna throughout the year.
Advantages in Summary:
- High treatment efficiency possible.
- Low capital and running costs.
- Minimum maintenance
- Low energy requirements.
- Tolerant of variable loads.
- Beneficial for wildlife.
- Aesthetically pleasing.
- No chemicals necessary.
- Suitable for polishing effluent from existing overloaded systems.
Why Build a Constructed Wetland?
There are several reasons why a constructed wetland may be used for treating wastewater. For a domestic situation with just a single septic tank, the soil percolation may be too high or too low, or other site conditions may be unfavourable for a percolation area alone.
If soil percolation is too low then the septic tank effluent will not flow down through the percolation area but may instead pond on the surface of the lawn. If percolation is too high then the effluent will travel rapidly through the sandy or gravelly soil and pollute the groundwater. This is particularly undesirable if you or your neighbours have a well supplying their drinking water.
Industrial situations may find constructed wetlands suitable for dealing with effluents with high volume and relatively low toxicity or nutrient status. Wetlands have been used for the mining industry, food industry sector, metals industries for reclaiming metals and a variety of other wastes. To check if a wetland would be suitable for your effluent treatment situation just email or call us.
How Constructed Wetlands Work
There are many Physical, Chemical and Biological mechanisms that play a part in the treatment of wastewater within a Constructed Wetland System. The principal mechanisms are outlined as follows:
Sedimentation - Plant stems and leaf litter in the marsh slow the flow of water and allow sediments in the wastewater to be deposited onto the bed of the marsh. Further sedimentation can occur in the still water of the pond where finer sediments settle out.
Bacterial Action - Wetland plants have adapted to grow in saturated conditions. One such adaptation is the ability to draw oxygen from the leaves to the roots. Oxygen is available in sufficient quantities for aerobic bacteria to thrive in the root zone of the marsh. The saturated area of the marsh below the root zone supports anaerobic bacteria. Bacteria also adhere to the dead plant material on the marsh surface. All of these groups of bacteria feed on the wastewater, playing a major part in the water cleansing process
Filtration - Soil, root zone and plant litter all help to filter pollutants from the wastewater.
Adsorption - Attractive forces acting between particles in the wastewater draws them together, allowing them to settle to the base of the wetland. Adsorptive forces also adhere pollutant particles to plant material and soil colloids.
Precipitation - Substances such as heavy metals can become insoluble under certain conditions and settle onto soil and plant material.
Decomposition - Different organic pollutants in the wastewater are oxidised and reduced in the treatment process.
Nutrient Uptake - Plants growing in the marshes and pond use pollutants in the wastewater for growth. The lush growth in constructed wetlands is due to this abundance of nutrient availability.
Volatilisation - Some elements within the wastewater, such as nitrogen and sulphur, also exist in gaseous form. Conditions in constructed wetlands can allow these elements to be released to the air. This can be an important pathway for their removal.
© FH Wetland Systems, 2012