Floating treatment wetland aided nutrient removal from agricultural runoff using two wetland species

Floating treatment wetlands (FTWs) are an emerging surface water treatment practice for reducing nutrients such as nitrogen (N) and phosphorus (P) in a variety of waste streams. FTWs consist of floating rafts which support wetland plants allowing the plant roots to be submerged below the water surface similar to hydroponic systems. N and P removal is achieved through direct plant uptake and flocculation or sedimentation facilitated by biofilms adhering to plant roots. While research has documented FTW performance for a variety of waste streams, little information is available on runoff from commercial nurseries. Nursery runoff occurs frequently, from storm and irrigation events, and can potentially contain elevated concentrations of total N (TN) and total P (TP) due to crop fertilization and frequent irrigation. Nursery runoff is often captured in large ponds known as tailwater recovery basins (TRBs) for reuse. We assessed the effectiveness of FTWs for treating nursery runoff in TRBs over a 7-day hydraulic retention time across a 19-week growing season using two supplied nutrient concentrations and four treatments with four replicates in a randomized complete block mesocosm experiment. The high nutrient concentration averaged 17.1 mg.L-1 TN and 2.61 mg.L-1 TP and low nutrient concentration averaged 5.22 mg.L-1 TN and 0.52 mg.L-1 TP. Treatments included: 1) Pontederia cordata, 2) Juncus effusus, 3) a raft with no plants, and 4) no raft (open-water). Pontederia cordata removed 90% and 92% of TP and 84% and 89% TN, depending on initial loading; performing better than the other planted and non-planted treatments. Juncus effusus performed better than the mat-only control treatments for TP and TN removal across nutrient concentrations tested, but performed similarly to the open water control when nutrient load was evaluated. Nutrient uptake by Pontederia cordata averaged 0.026 g.m(-2).d(-1) N and 0.0075 g.m(-2).d(-1) P. The study demonstrated that FTWs can be very effective, but nutrient removal depends greatly upon plant species selection.