The deep purification of four aquatic macrophytes for tailrace of rural sewage treatment plants

To provide theoretical basis for optimized application of restoration engineering of aquatic macrophytes in deep purification of tail water of rural sewage treatment plant, a dynamic simulation experiment with running water was conducted, in which the purifying effects of Eichhornia crassipes, Pistia stratiotes, Hydrilla verticillata, and Iris pseudacorus on effluent from sewage treatment works (STW) were compared. Based on the results, plant systems significantly improved water quality of effluent from STW. The purifying effects of E. crassipes and P. stratiotes on nitrogen (N) and phosphorus (P) in STW effluent were better than those of I. pseudacorus and H. verticillata. The average concentration of total nitrogen (TN) in STW effluent was 12.22 mg·L^-1. The average removal efficiency of TN by E. crassipes, P. stratiotes, I. pseudacorus, H. verticillata and control group were 46.25%, 45.74%, 43.41%, 38.39% and 29.22%, respectively. The average concentration of total phosphorus (TP) in STW effluent was 0.38 mg·L^-1. The average removal efficiency of TP by the five experimental treatments were 36.84%, 34.21%, 31.58%, 28.95% and 26.32%, respectively. The average concentration of permanganate index (COD_Mn) in STW effluent was 3.88 mg·L^-1. The average removal rates of COD_Mn in the five experimental treatments were 42.27%, 30.93%, 32.47%, 32.47% and 37.89%, respectively. The net growth rates of E. crassipes, P. stratiotes, H. verticillata, and I. pseudacorus biomass were 550.5%, 418.8%, 210.6% and 80.3%, respectively. The amount of TN absorbed by E. crassipes, P. stratiotes, H. verticillata, I. pseudacorus were 7.36 g, 2.33 g, 5.12 g and 4.46 g, respectively. The amounts of absorption of TP by the four aquatic plants were 0.60 g, 0.19 g, 0.33 g and 0.78 g, respectively. The ability of E. crassipes to assimilate nitrogen was stronger than the other three aquatic plants. Meanwhile, the ability of H. verticillata to assimilate phosphorus was stronger than the other three aquatic macrophytes. The apparent contribution rates of absorption by E. crassipes, P. stratiotes, H. verticillata, I. pseudacorus to nitrogen removal were 15.29%, 4.90% 11.17% and 11.34%, respectively. Meanwhile, the apparent contribution rates of absorption by the four plants to phosphorus removal were 50.34%, 17.17%, 35.24% and 76.34%, respectively. It was feasible and effective to cultivate the floating plant of E. crassipes and submerged plant of H. verticillata in different water layers and combinations in space for deep purification of STW effluent.