EVALUATION OF A WETLAND SYSTEM DESIGNED TO MEET STRINGENT PHOSPHORUS DISCHARGE REQUIREMENTS

In this study, a wetland system was created and managed to increase pl ant removal of influent phosphorus (P). This was accomplished by makin g P the most limiting nutrient, by enhancing mass transfer of P to the root surface and by harvesting the shoot biomass to regenerate the nu trient removal capacity of the wetland. A mixture of grass species tha t are tolerant of wet conditions (66% Reed canary grass) was grown in long (3.66-m) and narrow (0.1-m) troughs containing silica sand. Four replicates of three depths of sand (1.3, 2.5, and 5.1 cm) were investi gated at a hydraulic loading rate between 1 500 and 1 800 m(3)/ha . d. Removal of P was greater than 90% in all treatments, and P was reduce d from 0.480 mg/L to less than 0.001 mg/L in the 5.1-cm-deep sand trea tment. The treatments removed approximately 40% of the influent nitrat e (22 mg/L in; 13 mg/L out). Nutrient removal occurred 24 hours a day with small diurnal fluctuations. Grass was harvested biweekly by cutti ng to a uniform 7.6-cm height. Comparison of the actual amount of nutr ients removed in the harvested biomass with that calculated from diffe rences between influent and effluent concentrations showed that approx imately 50% of the N and approximately 80% of the P were removed from the effluent in the biweekly grass clippings.