In vertical flow constructed wetlands the wastewater is loaded onto the surface of a planted filter bed (Fig. 2). The necessary surface area of the filter bed is 3.2 m2/PE. The pollutants are removed or transformed by the microorganisms that are attached to the filtersand and the root system of the plants. It is important that the filter is not saturated or covered with water in order to secure a high oxygen level in the filter (Brix and Schierup, 1990). The bed is planted with common reed (Phragmites australis). The main function of the plants is to counteract clogging of the filter. The standing aboveground biomass also insulates the filter against freezing during winter ( Brix, 1994 and Brix, 1997). The treated wastewater is collected in a system of passively aerated drainage pipes placed in the bottom of the filter. Half of the effluent is recirculated to the sedimentation tank or the pumping well in order to enhance denitrification and to stabilise performance of the system.