Evaluation of a wetland system designed to meet stringent phosphorus discharge requirements

In this study, a wetland system was created and managed to increase plant removal of influent phosphorus (P). This was accomplished by making P the most limiting nutrient, by enhancing mass transfer of P to the root surface and by harvesting the shoot biomass to regenerate the nutrient removal capacity of the wetland. A mixture of grass species that are tolerant of wet conditions (66% Reed canary grass) was grown in long (3.66‐m) and narrow (0.l‐m) troughs containing silica sand. Four replicates of three depths of sand (1.3, 2.5, and 5.1 cm) were investigated at a hydraulic loading rate between 1 500 and 1 800 m3/ha · d. Removal of P was greater than 90% in all treatments, and P was reduced from 0.480 mg/L to less than 0.001 mg/L in the 5.1‐cm‐deep sand treatment. The treatments removed approximately 40% of the influent nitrate (22 mg/L in; 13 mg/L out). Nutrient removal occurred 24 hours a day with small diurnal fluctuations. Grass was harvested biweekly by cutting to a uniform 7.6‐cm height. Comparison of the actual amount of nutrients removed in the harvested biomass with that calculated from differences between influent and effluent concentrations showed that approximately 50% of the N and approximately 80% of the P were removed from the effluent in the biweekly grass clippings.