Since the start in 1997 in Wageningen Kilian Water has been engineering constructed wetlands which have a minimum energy requirement and a maximum reliability. Constructed wetlands are natural wastewater treatment systems which are low in energy consumption and which require minimal maintenance. The reason for the low energy consumption lies in the role of the plants which live in symbiosis (relationship between two species from which both benefit) with numerous micro-organisms which break down the contaminants in the wastewater. This makes the constructed wetland to some extend a solar powered water treatment system which explains the low energy need. It also explains the limited maintenance requirement because the more we allow nature to take care the less remains there for ouselves.
Raw domestic sewage requires anearobic pre treatment in a septic tank or biodigester to allow solid organic matter time to dissolve. We implement vasious types of septic tanks and/or biodigesters depending on the project size and scope.
When using constructed wetlands we do not have to handle excess sludge like we have to in conventional sewage treatment plants, as organic matter effectively disappears from the bottom of the food pyramid upwards.
In a constructed wetland bacteria are consumed by for instance flaggelates or rotifers which can be consumed by for instance nemathodes or worms.
With more than fifteen years of experience Kilian Water can come up with the best possible constructed wetland and anaerobic pre treatment solution for any given waste water stream, provided it is treatable within the limits that nature has set for us.
Because of the relative low load and long hydraulic retention time constructed wetlands turn out to be the most reliable wastewater treatment systems. Hydraulic overloading tends to be no problem, this effectively happens every time whenever it rains heavily and of course any well designed structure can cope with heavy rainfall. Organic overloading is also possible (when temporary many more individuals visit the locality or location) and more pollutants will be simply buffered in the constructed wetland itself. After the peak the bacteria in the wetland will simply take some time to process all the pollutants before going back to the steady state of before.
Only in case a constructed wetland is overloaded for a prolonged period of time, the quality of the effluent (treated wastewater) will worsen. In that case one can argue that the constructed wetland was designed too small.
Treated wastewater from constructed wetlands can be reused for flushing toilets, irrigation or any ather non potable water appication. The type of constructed wetland which is best suitable to deliver the required effluent quality and which is optimal under local conditions differs however.
In arid regions one generally tends to minimize evaporation which disqualifies surface flow constructed wetlands.
In some places land scarce so we have to make the footprint as small as possible which directs us to choose vertical flow constructed wetlands wity or without forced aeration or itermitted effluent discharge options.
On slopes gravity systems can possibly be used.
To make water available for reuse an external storage basin of tank for treated wastewater can be required or in some cases it is even possible to store a significant amount of water in a lower part of the constructed wetland itself.
These and many other desig considerations will be made by Kilian Water when establishing the best suitable design of a biodigester and constructed wetland combination.
In general one can distinguish three types op constructed wetlands:
- Surface flow constructed wetlands
- Horizontal subsurface flow constructed wetlands
- Vertical flow constructed wetlands
Surface flow constructed wetlands appear similar to natural swamp area's in which plants are rooted in a submerged layer of sand or gravel. Aeration of the sediment takes place by the unique property of helophyte plants which act as oxygen pumps providing dissolved oxygen with their roots to a wide variety of micro organisms. We apply surface flow constructed wetlands generally when flow rates are highly unpredictable (run-off from roads) and when anaerobic pre treatment in a septic tank or biodigester is not required, this because of the odor nuisance it would cause. The design is mainly dependent on spatial limitations, ambient temperatures, matrix characteristics, and organic and hydraulic load.
This type of constructed wetland is most commonly used for aerobic post treatment of domestic wastewater and can take a higher hydraulic load than a surface flow constructed wetland. In order to dissolve solid organic matter anaerobic pre treatment in a septic tank or biodigester is required. A thick layer of gravel above the aquifer holds a layer of stagnant air and prevents odor nuisance in the vicinity. Aeration takes place as in surface flow constructed wetlands. The wastewater is however forced to pass thorough the matrix ensuring intensive contact between wastewater and the bacteria in the rhizosphere (root zone of the plants). In this manner all wastewater is treated as no short circuit flow is possible. Horizontal subsurface flow constructed wetlands, when accurately designed, provide an extremely reliable low cost aerobic post treatment solution which is applicable all over the world.
The desire to further reduce the size of constructed wetlands led to the development of vertical flow constructed wetlands. Anaerobic pre treated wastewater coming from a septic tank or biodigester is intermittedly pumped on top of the constructed wetland. By trickling down the wastewater effectively sucks air in the constructed wetland whenever the pump stops, forcing aeration of the rhizosphere. This increases the aeration capacity up to approximately twenty times compared to horizontal subsurface flow constructed wetlands. Apart from that no short circuit flows are possible and due to lower levels of oxygen deeper in the matrix nitrate is removed under anoxic conditions. We can, for instance, adjust the level of the aquifer and the depth of the matrix as design parameters.