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 wetlands with horizontal subsurface flow, using shale as a substrate, in removal 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 constructed wetland system (CWS) employing horizontal subsurface flow was set 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 98-100% over the whole period of investigation. Ammonium N was also completely removed in the planted tanks, whereas in the unplanted ones the rates of removal varied between 40 and 75%; removal of nitrate N varied between 85 and 95% in planted and between 45 and 75% in unplanted tanks. pH, E(h) and temperature did not differ significantly among planted and unplanted tanks, but the inlet E(h) was correlated with P removal (r² = 0.73; p < 0.05). The presence of Phragmites australis contributed significantly (p < 0.05) to P and N removal. In addition the plants showed excellent growth (up to 2 m in the first year), with good root and rhizome development, and showed potential for heavy metal removal. It was concluded that the shale-based system (which uses a readily available material) shows promise as a substrate for constructed wetland systems.