Utilizing a Two‐Stage Constructed Wetland With Floating Plant Mats for Removing Phosphorus From Agricultural Drainage in Michigan

ABSTRACT A two‐stage constructed floating wetland with a 0.7% ratio of wetland to drainage area was studied for removing phosphorus from subsurface agricultural drainage. This system consisted of two stages, the first being a floating wetland containing Scirpus atrovirens , Carex vulpinoidea , Juncus effusus , Ranunculus hispidus , and Acorus americanus , and the second containing a filter media of pea stone. It was found that total phosphorus (TP) in plant tissues was not statistically different across macrophyte species; however, total biomass did differ. Therefore, to obtain maximum TP removal, plant shoots should be removed when they reach their maximum biomass. Scirpus had the highest average dry mass (17.1 g) and average TP shoot mass and was significantly greater than the other species examined. Total P removal by plant shoots ranged between 0.89% and 19.3% of the soluble reactive phosphorus (SRP) entering the wetland, depending on the year. The average annual SRP and total phosphorus reductions from discharge entering and exiting the system equated to 65 and 36%, respectively. Additionally, the average SRP concentration across all 3 years during the growing season was reduced from 0.087 mg/L to 0.029 mg/L, and TP was reduced from 0.168 mg/L to 0.068 mg/L, keeping both SRP and TP effluent concentrations below maximum concentrations recommended for flowing waters to minimize eutrophication and harmful algal blooms.