Since 1988 experiments have been carried out on a pilot scale on polishing of sewage treatment plant (STP) effluent in a constructed wetland system, a combination of a macrophyte bed and a lagoon. The hydraulic retention time (HRT) has been between one and ten days. At HRTs of 2-3 days a favourable oxygen regime and a high degree of removal of E. coli bacteria could be obtained. The natural alternation of low and high oxygen levels resulted in an extra nitrogen removal from the low levels of nitrogen in the effluent of the STP. Phosphorus removal in the wetland system under these conditions was low, therefore chemical precipitation has to take place in the STP.
The favourable results of the study have led to the construction of a full scale 3.5 ha wetland system for the treatment of 3500 m3 effluent day−1 in 1994. The HRT is 2.1 days at dry weather flow. The purpose of this wetland is to further polish the STP effluent in order to match the water quality of the natural local surface waters.