Constructed wetlands are a series of connected reed fields that, by means of microbial interactions, purify effluents after biological treatment into dischargeable water. This system has a low energy consumption, is also easy to construct and helps in the development of biodiversity. However, a large surface area is required to install this reed field purification.

In Flanders there are constructed wetlands working as polishing step after a biological treatment (nitrification-denitrification) for processing the liquid fraction of manure or digestate from anaerobic digestion.