Constructed wetlands are an appropriate technology for small communities in rural and suburban areas. Many rural projects with activated sludge plants failed because it was not properly operated, often no skilled stuff is available or the energy costs is no longer affordable. Constructed wetlands are principally using the same natural degradation processes and nutrient uptake but they are acting as extensive systems. There is wide acceptance and interest because of the following advantages (SWAMP 2002):
Simple in construction, operation and maintenance
Low operation and maintenance costs (low energy demand)
High ability to tolerate fluctuations in flow
High process stability
Aesthetic appearance
Constructed wetlands are used in various fields and at various treatment levels. Nevertheless, this lesson deals mainly with the conventional use of Constructed wetlands, which are to treat the pre-treated municipal wastewater, or so-called primary effluent. The typical treatment cycle is shown in Figure 3.
In general, primary effluent constitutes of these characteristics; data shown in mg/l (adapted from Metcalf & Eddy, 2003)
Constructed wetlands may also be applied for primary or tertiary treatment but these cases will only be mentioned in the last chapter of this lesson.
Constructed wetlands may need a post treatment particularly to completely remove nitrogen (nitrification and denitrification) and phosphorus, if the removal of both parameters is required in this region. Its capability to remove N and P has often been overestimated. Both aerobic and anoxic zones are necessary to perform complete nitrification and the subsequent denitrification. To remove significantly phosphorus the Constructed wetlands must be enhanced by an accompanying P removal step, e.g. pre-precipitation in the pre-treatment unit. Recently, there are several researches running concerning the use of the constructed wetland with special P-absorbing capacity materials instead of normal gravel and sand as a substrate.
Terminology
Terminology