Nitrogen removal from Lake Caohai, a typical ultra-eutrophic lake in China with large scale confined growth of Eichhornia crassipes

•We evaluated N removal from Lake Caohai by large-scale application of Eichhornia crassipes.•The TN concentration in the water obviously decreased after E. crassipes was planted.•N assimilation by E. crassipes was the main pathway of N removal from the lake.•Large scale utilization of E. crassipes for N removal from the lake is practicable.

An ecological engineering project, with large-scale utilization of Eichhornia crassipes (coverage area ∼4.3 km2) for pollution control in an open ultra-eutrophic lake, Lake Caohai, was first implemented in 2011. In this study, the efficiency of N removal using E. crassipes in the lake was evaluated. After E. crassipes was planted in May, the concentrations of TN and NH4+ in Waicaohai, the main part of Lake Caohai, were significantly decreased within a month, and then, remained stable from June to November, 2011, although the lake had received waste water continuously from river inlets. The average concentrations of TN, NH4+-N and NO3--N in water of Xi Yuan Channel (outlet) were reduced to 3.3, 0.02 and 0.8 mg L−1 from 13.8, 4.7 and 5.8 mg L−1 in river inlets, respectively. The DO levels in 2011 were not decreased, but concentrations of TN and NH4+ were significantly reduced when compared with the historical data from 2007 in the lake. Assimilation by E. crassipes was the main pathway to remove N in Lake Caohai, accounted for 52% of the total N influent (936 t), or 64% of the removed N (761 t). These results indicated that large scale utilization of E. crassipes for removal of N in the eutrophic lake is practicable.