The Use of Constructed Wetlands for Nitrogen Removal from Agricultural Drainage: A Review

Diffuse pollution from agricultural drainage is a severe problem for water quality and it is a major reason for eutrophication of both freshwaters and coastal waters. Constructed wetlands were proposed as a suitable tool for removal of nitrogen from agricultural drainage in the early 1990s. Since then constructed wetlands with free water surface have been successfully used in Europe, North America, Asia, and Australia. The predominant form of nitrogen in agricultural drainage waters is nitrate and therefore denitrification is considered as the major removal process. The literature survey of 41 full-scale constructed wetlands revealed that removed nitrogen amount varied widely between 11 and 13 026 kg N ha^–1 per year with the median removal of 426 kg N ha^–1 per year. A very close relationship was found between inflow and removed nitrogen loads but the overall percentual efficiency decreases with increasing inflow nitrogen load. It has also been found that nitrogen removal depends on the size of constructed wetland (W) as compared to catchment (C) area. The survey indicated that the W/C greater than 1% does not result in any substantial increase of nitrogen removal. There are still some gaps in our knowledge that need to be evaluated, namely the optimum vegetation maintenance.