PHOSPHATE AND AMMONIUM REMOVAL BY CONSTRUCTED WETLANDS WITH HORIZONTAL SUBSURFACE FLOW, USING SHALE AS A SUBSTRATE

The objective was to investigate the performance of constructed wetlan ds with horizontal subsurface flow, using shale as a substrate, in rem oval of phosphate (P) and ammonium (N) from sewage. Shale was selected on the basis of its physico-chemical properties and its potential for P removal, investigated in an earlier study. A laboratory-scale const ructed wetland system (CWS) employing horizontal subsurface flow was s et up in a greenhouse, with and without Phragmites australis (reeds), and its capacity for simultaneous phosphate and ammonium removal from a synthetic sewage was monitored over a period of ten months. Both the planted and unplanted systems showed an extremely high P removal of 9 8-100% over the whole period of investigation. Ammonium N was also com pletely removed in the planted tanks, whereas in the unplanted ones th e rates of removal varied between 40 and 75%; removal of nitrate N var ied between 85 and 95% in planted and between 45 and 75% in unplanted tanks. pH, E-h and temperature did not differ significantly among plan ted and unplanted tanks, but the inlet E-h was correlated with P remov al (r(2) = 0.73; p < 0.05). The presence of Phragmites australis contr ibuted significantly (p < 0.05) to P and N removal. In addition the pl ants showed excellent growth (up to 2 m in the first year), with good root and rhizome development, and showed potential for heavy metal rem oval. It was concluded that the shale-based system (which uses a readi ly available material) shows promise as a substrate for constructed we tland systems. (C) 1997 IAWQ.