Site test of phytoremediation of an open pond contaminated with domestic sewage using water hyacinth and water lettuce

• A self-designed device was successfully used for in situ pilot experiments.
• Rhizofiltration plays an important role in intercepting and filtering of algae.
• Water hyacinth is suitable for the deep purification of water with high N.
• Compared to water hyacinth, water lettuce has higher P removal efficiency.

This study was undertaken in situ to explore the potential of the alien plants water hyacinth (Eichornia crassipes) and water lettuce (Pistia stratiotes L.) as phytoremediation aquatic macrophytes for nutrients (nitrogen and phosphorus) removal and algal interception from domestic sewage contaminated pond (approximately 10500 m2 in area, average 2.5 m in depth) by using self-designed experimental devices from July 7 to August 8 in 2015. The physicochemical properties of water and plant samples as well as N and P mass balance in the phytoremediation system were investigated. The range of physicochemical parameters of influent were shown as follows: water temperature (WT: 24.5 °C −31.0 °C), pH (6.94–8.25), DO (4.58 mg L−1–15.73 mg L−1), CODMn (5.00 mg L−1–13.15 mg L−1), TN (1.60 mg L−1–5.60 mg L−1) and TP (0.16 mg L−1–0.73 mg L−1). Water hyacinth, which exhibited hyperactive accumulating capacity for nitrogen (58.64% of total reductions), was more suitable than water lettuce for the intensive purification of domestic sewage with high nitrogen concentrations. This result may be attributed to the larger total root surface area (0.97 m2 g−1–1.10 m2 g−1 fresh weight), active absorption area (0.31 m2 g−1–0.36 m2 g−1 fresh weight), and leaf area and higher root activity (71.79 μg g−1 h−1–98.34 μg g−1 h−1), root biomass (kg m−2), and net photosynthetic rate (20.28 μmol CO2 m−2 S−1) of water hyacinth than those of water lettuce regardless of cultivation in oligotrophic water with total nitrogen contents lower than 1.0 mg L−1. Water lettuce exhibited a higher total phosphorus removal efficiency, which benefitted higher P accumulation, adsorption, and precipitation because of its longer roots (approximately 49.0 cm) with higher rhizofiltration capacity. As such, water lettuce achieved higher algal (96.36%) and chlorophyll a (96.65%) removal efficiencies. A combined pattern using both macrophytes was recommended for the phytoremediation of most domestic sewages containing dual contaminants (N and P) in the future.