A model for the effects of water hyacinths on water quality in an experiment of physico-biological engineering in Lake Taihu, China

A model for an experiment of physico-biological engineering purifying lake water in Lake Taihu by using water hyacinths (Eichhornia crassipes (Mart.) Solms) was constructed. The model included 14 state variables. They are NH4+-N, NO3−-N, PO43−-P, nitrogen and phosphorus in detritus, phosphorus in the pore water, exchangeable phosphorus and nitrogen in the sediment layer, density, nitrogen and phosphorus in phytoplankton, and nitrogen and phosphorus in water hyacinths. The external forcing functions were solar radiation, water temperature, concentrations of nutrients, phytoplankton and detritus in inflow water and the retention time of the water in physico-biological engineering. The results of the model simulating the growth of phytoplankton and water hyacinths, and the cycling of nitrogen and phosphorus inside physico-biological engineering were coincident with the results observed. In order to decide which process would affect the water quality in the experiment, to which parameter the water quality indexes such as phytoplankton, NH4+-N, NO3−-N PO43−-P are sensitive has been analyzed by the use of the model. It has been discussed how to culture and harvest the water hyacinths. The filtering effect has also been estimated. The model could be used as a tool to guide physico-biological engineering design and its management.