A free-water surface constructed wetland (also called free water surface, FWS) is a series of flooded planted channels or basins. It aims to replicate the naturally occurring processes of a natural wetland, marsh or swamp. As water slowly flows through the wetland, particles settle, pathogens are destroyed, and organisms and plants utilize the nutrients. This type of constructed wetland is commonly used as an advanced treatment after secondary or tertiary treatment processes. Pre-treating of wastewater in e.g. a septic tank or biogas settler is necessary to avoid excess accumulation of solids and garbage. Because of the open water surface, there is a risk of mosquito breeding if not properly designed. Plants grown on the wetland may be used for composting or energy production and the effluent can be used for aquaculture and irrigation. This system is appropriate for small sections of urban areas (e.g. decentralised treatment for a community or several housings or small industries) or even more appropriate for peri-urban and rural communities because of the land surface required.