Treatment performances of floating wetlands: A decade of studies in North Italy

Floating treatment wetlands (FTWs) are one of the most recent eco-technologies to treat low-quality polluted wastewaters in already existing retention ponds, without subtracting other soil surface for the treatment of wastewaters. This study assessed the depuration performances of eight different FTWs (1 pilot-scale and 7 full-scale plants) treating municipal wastewaters, agricultural runoff and digestate liquid fraction, in North Italy. The concentration of different pollutants was monitored before and after the FTWs barriers: total nitrogen (TN), nitrate nitrogen (NO3-N), ammonia nitrogen (NH4-N), total phosphorus (TP), orthophosphate (PO4-P), chemical oxygen demand (COD) and turbidity. Considering all the case studies, pollutants inflowing concentrations were highly variable, due to the different composition of wastewaters used in the trials (TN: 0.3–520.0 mg L−1, median value 19.1 mg L−1; NO3-N: <0.001–61.5 mg L−1, median value 2.6 mg L−1; NH4-N: <0.001–310.0 mg L−1, median value 3.6 mg L−1; TP: 0.02–44.9 mg L−1, median value 5.7 mg L−1; PO4-P: <0.001–32.9 mg L−1, median value 4.1 mg L−1; COD: 2.1–6152.0 mg L−1, median value 67.5 mg L−1). Wastewater nitrogen forms and COD concentrations were reduced after passing the FTW with median abatements of 44.1% for TN, 34.6% for NO3-N, 16.7% for NH4-N, and 25.2% for COD. Only Phosphorus concentrations were not affected by the FTWs. Wastewater turbidity ranged from 3.4 to 3075.0 NTU and the median value of 77.7 NTU was significantly reduced by 22.1% after the FTWs, thus suggesting that plant root systems were active in physical wastewater filtration. In three case studies E. coli was also monitored and abatements ranged from 19.5 and 95.1%. The results obtained after ten years of study indicated that FTWs represented an efficient and economic wastewater management system, which provided a positive ecosystem service and therefore reduced the human footprint on the environment.