Constructed wetlands with subsurface flow for nitrogen removal from tile drainage

Abstract

Diffuse pollution from agricultural drainage is a severe problem for water quality and it is a major reason for the failure to accomplish “good chemical status of surface waters” according to the Water Framework Directive and for the 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. Until now, the vast majority of constructed wetlands designed to treat tile drainage were free-surface constructed wetlands. In 2018, three experimental constructed wetlands with horizontal subsurface flow were built to treat tile drainage from 15.73 ha watershed. The wetlands have a surface area of 79, 90 and 98 m2 and were planted with Phalaris arundinacea and Glyceria maxima in parallel bands. The substrate in the first two wetlands is gravel (4–8 mm) mixed with birch woodchips (10: 1 volume ratio). In one of those wetlands, the water level is kept 10 cm above the surface, in the second one the water is kept below the surface. The third wetland has 20 cm layer of birch woodchips on top of gravel. The mean total nitrogen concentration eliminations during one-year period were 61.2%, 62.6% and 70.9% for wetlands 1, 2 and 3, respectively. The average load removals amounted to 0.516, 0.323 and 0.399 g N m−2 d−1 or 1885, 1180 and 1457 kg ha−1 yr−1 in wetlands 1, 2 and 3 respectively. The plant uptake and nitrogen sequestration in aboveground biomass contributed only marginally to the overall nitrogen removal. The results revealed that the selection of the best option depends on the fact whether concentration or load is taken as the most important parameter.