THE USE OF CONSTRUCTED WETLANDS TO UPGRADE TREATED SEWAGE EFFLUENTS BEFORE DISCHARGE TO NATURAL SURFACE-WATER IN TEXEL ISLAND, THE NETHERLANDS - PILOT-STUDY

Since 1988 experiments have been carried out on a pilot scale on polis hing 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. c oli bacteria could be obtained. The natural alternation of low and hig h oxygen levels resulted in an extra nitrogen removal from the low lev els of nitrogen in the effluent of the STP. Phosphorus removal in the wetland system under these conditions was low, therefore chemical prec ipitation 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 syst em for the treatment of 3500 m(3) effluent day(-1) in 1994. The HRT is 2.1 days at dry weather flow. The purpose of this wetland is to furth er polish the STP effluent in order to match the water quality of the natural local surface waters. (C) 1997 IAWQ. Published by Elsevier Sci ence Ltd.