Removal of nitrogen via harvesting of emergent vegetation in constructed wetlands for wastewater treatment

The amount of nitrogen which could be removed from wastewater via harvesting is given by N standing stock in aboveground biomass and the harvesting schedule. Available data from 38 constructed wetlands around the world indicate maximum standing stock of 47.7, 60.4 and 59.5 g N m(-2) for Phalaris arundinacea, Phragmites australis and Typha spp., i.e. the most commonly used plants in constructed wetlands. Most data are based on the single harvest but some results indicate that in temperate or cold climates the multiple harvest does not increase substantially the amount of removed nitrogen because of low re-growth. The inflow loading values vary widely among systems; for the systems designed for the secondary treatment of sewage inflow N loadings usually range between 500 and 1000 g N M(-2) yr(-1) but values up to > 6000 g N m(-2) yr(-1) have been reported. These figures clearly indicate that the amount of P removed via harvesting is usually low. However, for tertiary treatment systems with inflow < 200 g N m(-2) yr(-1) this removal route could be significant.