A constructed vertical macrophyte system for the retention of nitrogen in agricultural runoff

Recent evidence for the importance of luxury rhizome accumulation of N by the common reed Phragmites australis opens the possibility for N retention in constructed vertical wetlands. The removal of nutrients (N and P) from agricultural runoff was investigated in columns planted with P. australis in a sand bed. Nitrate demonstrated a linear removal with detention time (60-300 min) and was accounted for by membrane-limited root uptake. Ammonia was effectively removed from agricultural runoff, with nitrate removed at longer detention times. Detention time based on a targeted nitrate removal therefore represents a suitable design parameter for a vertical macrophyte system. On the other hand, ammonia was more effectively removed at low concentrations, but ineffectively removed at higher concentrations. Nitrogen is effectively accumulated in rhizomes which could be mechanically ground, composted and returned to agriculture.