The most commonly applied natural/biological purification system is the Constructed Wetland Filter (CWF). The idea is simple: the waste water is slowly percolating through several layers of sand, gravel and sea shells. On top of this filter, reed plants are growing, the root zone of these plants are instrumental in the purification process.
There are three main varieties of wetland filters:
- Vertical flow wetland filter for extremely polluted waste water
- Horizontal flow wetland filter for light to medium polluted waste water,
- Flow field especially for large quantities lightly polluted waste water.
The Wetlantec CWF was certified in 2012, by the Royal Institute of Water Treatment in the Netherlands. The certificate was issued for the highest possible purification levels, according to Dutch standards. During testing trials, the CWF underscored all existing standards by an impressive margin.
Vertical flow constructed wetland filter
The vertical flow CWF has a longer percolation time compared with other systems, this increases the processing time of the waste water, making this application the better choice for heavily polluted waste water. This longer processing time decreases the processing capacity. In order to increase the capacity, for the removal of higher concentrations of nutrients, there are several options like forced aeration or addition of active carbon to the filter. If budget and space allow, a larger size filter is also an option.
The general principle of this kind of treatment is percolation of the waste water through a layered filter package, that acts as a biochemical reactor. On its way down, several complex biological, chemical and physical processes take place. The main ingredient of the filter is sand; and on top of the filter, some varieties of reeds are planted in order to stimulate evaporation.
The water from the sedimentation tank (often a septic tank) is being pumped into a distribution network in the top layer of the CWF. From here the water percolates through the filter package, and the root zone of the reed plants, into the drains on the bottom of the package. In the root zone, biochemical transformations, supported by oxygen from the reed plants, break down harmful substances in the waste water, creating a purification level of approximately 98%.
The vertical flow CWF’s are currently being used for treatment of concentrated domestic waste water, industrial waste water, process water, even oil contaminated ground water can be (partly) treated with a CWF of Wetlantec.
During the last couple of years, Wetlantec has been designing and installing special CWF’s for the purification of the water in swimming pools. Special attention was given to prevent the development of blue algae and other dangerous pathogens, the filter needs to get modified for this role.
Example Large scale application
The town of Culemborg (Netherlands) was extended with a ‘super eco-subdivision’ called ‘EVA-Lanxmeer”, about twenty years ago. About 150 households provide input for one of the biggest CWF’s in Europe: a 4,500 m2, vertical flow filter. The green reed plants amalgamate automatically in the natural environment of the subdivision and the filters add to the general experience of a self supporting, energy efficient and eco-friendly subdivision.
Horizontal flow constructed wetland filter
A horizontal flow constructed wetland filter is an ideal solution for treatment of lightly / moderately polluted waste water. The processing capacity of such a system is lower then with a vertical flow CWF, and the space demand is higher but construction costs can be lower. In order to increase the processing capacity of a horizontal flow CWF, it is possible to add components to the filter, like artificial aeration.
As the name already predicted, in a horizontal flow CWF, the waste water passes the filters horizontally, usually under gravity, so no pumps are needed. In order to reduce hydraulic resistance, the filter material is more coarse. Due to continuous high water levels in the filter, the decomposition processes are mainly anaerobic. There is also more variation in available planting: not only reeds but various species of domestic wetland and marsh vegetation are possible. From an ecological point of view, the horizontal CWF offers more variation and a colourful exposition of the filters.
When the waste water is flowing through the filters, a combination of biochemical, physical and biological processes take place , that decompose the harmful components of the water and reduce them to nutrients for the plants.
Horizontal flow CWF’s are mainly used for the less critical treatment of lightly polluted waste water. Examples are: Highway and parking lot runoff, (natural) swimming pools, polluted ground water, stagnant surface water bodies and lightly polluted industrial waste water.
The runoff of an eight-lane highway, a main artery around Amsterdam, is being drained into a horizontal flow CWF, and the effluent is of such a high quality that it is being dumped into a local environmental protected area, without any harm done to the ecosystem.
Flow fields are a great way for treating large quantities of lightly polluted waste water, for that reason a flow field is often built behind a “normal” waste water treatment plant, in order to upgrade the effluent to the highest standards so it can be dumped in eco-sensitive areas like swamps and recreational areas.
In a flow field, the water flows through a shallow trench or swale, with depths varying between 30 and 80 cm and being planted with swamp vegetation and reeds, like Bullrushes and Sedge.
The flow field revitalizes the water and brings it back to “natural standards” using a combination of physical and biochemical processes, mainly diffusion and sedimentation, combined with an increase in the oxygen content. Microbiological processes remove the last remaining nutrients from the water. Flow fields are commonly used for “fine tuning” the effluent of another treatment process, when water is being discharged close to municipal wellheads, or in recreational projects with very high quality standards.
Near the village of Erica (Netherlands) water from a nearby canal was needed to supply water levels in a recently restored environmental protected area (partly wetlend). The canal water was of insufficient quality, so Wetlantec was asked to design a purification structure, to reach the water quality standards required. Wetlantec came up with a 1.5 Ha. Flow field, for treatment of the canal water and also built the structure. It has been in operation for several years now and the results are very encouraging.