Water Science & Technology—WST Vol 57 No 12 pp 1881–1887 © IWA Publishing 2008 doi:10.2166/wst.2008.319

A two-stage subsurface vertical flow constructed wetland for high-rate nitrogen removal

Guenter Langergraber, Alexander Pressl, Raimund Haberl, Klaus Leroch and Roland Rohrhofer

By using a two-stage constructed wetland (CW) system operated with an organic load of 40 g COD·m^-2·d^-1 (2 m² per person equivalent) average nitrogen removal efficiencies of about 50% and average nitrogen elimination rates of 980 g N·m^-2·yr^-1 could be achieved. Two vertical flow beds with intermittent loading have been operated in series. The first stage uses sand with a grain size of 2–3.2 mm for the main layer and has a drainage layer that is impounded; the second stage sand with a grain size of 0.06–4 mm and a drainage layer with free drainage. The high nitrogen removal can be achieved without recirculation thus it is possible to operate the two-stage CW system without energy input. The paper shows performance data for the two-stage CW system regarding removal of organic matter and nitrogen for the two year operating period of the system. Additionally, its efficiency is compared with the efficiency of a single-stage vertical flow CW system designed and operated according to the Austrian design standards with 4 m² per person equivalent. The comparison shows that a higher effluent quality could be reached with the two-stage system although the two-stage CW system is operated with the double organic load or half the specific surface area requirement, respectively. Another advantage is that the specific investment costs of the two-stage CW system amount to 1,200 EUR per person (without mechanical pre-treatment) and are only about 60% of the specific investment costs of the singe-stage CW system.

Keywords: constructed wetland; high-rate nitrogen removal; subsurface vertical flow; two-stage system

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