Enhanced nitrogen elimination in vertical-flow constructed wetlands in cold climates

The performance of several constructed wetlands in the hilly region of Austria is summarised with respect to nitrogen elimination. Both processes involved (nitrification and denitrification) are discussed separately for one- and two-staged constructed wetland systems. For one-stage vertical flow beds (Vfb) the recirculation of nitrified effluent into the settlement tank of the mechanical pretreatment is the most appropriate way to enhance denitrification. Denitrification rates of 70% could be reached by a combination of recirculation and "in situ" denitrification within the soil profile of the Vfb (amounts to about 20% of the denitrification rate). The areal rate constants for nitrification (kNH(4)-N - values) turned out to be about 0.08 m/d for wastewater temperatures between 5 and 10degreesC and 0.12 m/d for wastewater temperatures above 10degreesC. Beside this temperature dependence there is a slight increase of the rate constant with increasing hydraulic load (up to 60 mm/d) due to a better usage of the whole filter area. Methanol dosage as a carbon source for denitrification into the second stage of a two-stage constructed wetland in respect of theoretical and actual COD/N-ratios is presented as another way for total nitrogen elimination. The combination of a horizontal flow bed as a first stage (COD and TSS reduction) with a Vfb (nitrification) stage as a second stage is recommended for larger constructed wetland systems.