Constructed Wetlands / Soil Filters
A soil filter is a natural treatment method, in which biologically degradable material is removed by combined biological-chemical-physical processes. Two basic types have been applied by WECF and partners: constructed wetlands as plants for treatment of greywater or domestic wastewater and simplified filters for treatment of greywater from personal hygiene.
A technical drawing of a constructed wetland from Otterwasser
Constructed wetlands consist of three main parts; sedimentation tank, filter and last storage for infiltration or reuse, where the effluent can also be sampled and monitored. The filter consists of gravel and sand of different sizes. Constructed wetlands can also be made using low-tech equipment; a pump, pipes and foil and clay can be used as a sealant as well. If there is enough slope a pump may not be needed. Soil filter beds are typically planted with reeds.
A constructed wetland being built, with foil as a sealant
Simplified filters provide only a basic treatment, they are filter beds usually without a pre-sedimentation. They are cheaper than the constructed wetlands. They are not sealed away from groundwater; instead the effluent water trickles through it and then filters to the ground. Therefore it is difficult to take samples of the effluent and to check the performance of the filter. Thus these simplified filters are recommended only for wastewaters that are not so dangerous for the groundwater. In general, soil filters are a low-tech treatment method; but the construction cost of constructed wetlands can be quite high, as is the needed area per capita (2-5 m2/capita). Especially sand, gravel prices and area cost play an important role in the total cost.
The advantage of a soil filter in comparison to conventional technologies is the lower energy demand and the lower need for maintenance, which adds up to low running costs. Energy is only needed for pumping (if not enough slope) and the only maintenance required is to empty the sedimentation tank (if applicable, once a year) and harvesting the plants. A well-designed system can have a life span of more than 20 years, before the filter material has to be cleaned or changed. In the design phase, the amount of space and the type of sand needed have to be chosen carefully in order to prevent the filter from clogging problems, which can lead to very high maintenance costs. The cost and area needed depend on the type of wastewater treated.
Constructed wetlands can treat both greywater from the bath and kitchen, and blackwater from toilets – in this case the area demand is high (about 5 m2 per capita). If only greywater from bath and kitchen is to be treated, the needed area for a constructed wetland is already much smaller (about 2 m2 per capita). A simplified filter with a sedimentation tank can also be used. If only bathroom greywater is to be treated, a simplified filter without a sedimentation tank can be used.
A finished constructed wetland from our successful project in Bulgaria
