Modeling N transformation and removal in a duckweed pond: Model application

Based on the simulated results of a reconstructed N transformation model for duckweed ponds, the roles of different transformation pathways in N removal were elucidated, and the seasonal effects of water conditions on N transformations were analyzed. It showed that the nitrification–denitrification process was the most important pathway for N removal in the duckweed pond. In low temperature periods, nitrification–denitrification accounted for 10.2% of TN removal, but the total amounts of organic nitrogen precipitation and ammonia volatilization only accounted for 0.7%; in high temperature periods, the above two values were 38.8% and 2.1%, respectively, more clearly showing the importance of nitrification–denitrification. In the duckweed pond, different N forms possessed differently dominant transformation pathways of their own. Nitrification, denitrification, ammonification accounted for 113%, 91%, 74% of ammonia, NOx, organic nitrogen removals, respectively; remineralization reduced sedimentary nitrogen by 234% and organic nitrogen precipitation increased sedimentary nitrogen by 297%; ammonia amounted to 79% of the N uptake of the duckweed, and the algae mortality reduced 133% of the N contained in algae. Moreover, due to the light limitation by duckweed, the mortality rate of algae was greatly accelerated, thus lowering algae content in the final effluent of duckweed pond. The most important function of duckweed for TN removal was not their direct uptake, but their indirect acceleration to the nitrification and denitrification rate.