Influence Of Hydrodynamics On Particle Sorting-sedimentation In Taihu Lake And Phosphorus Distribution With Particle Size

Taihu Lake is the third largest freshwater lake in China.Influenced by the urbanization and industrialization of the watershed,the water body of Taihu Lake has been seriously polluted,and the eutrophication problem is particularly prominent.The average water depth of Taihu Lake is 1.9 m,and its sediments are easily disturbed by wind and waves,resulting in the release of phosphorus（P）,which forms endogenous load and plays an important supporting role in the growth of cyanobacteria in bloom.In the process of sediment resuspension disturbed by wind and waves,particle size sorting and settling usually occur,which leads to P redistribution.In addition to hydrodynamic effects,the high biomass of algae in Taihu Lake may also have a potential impact on sediment sorting and settling performance.Under this background,it is of great scientific significance to find out the influence of hydrodynamic forces and algae on particle sorting-settling performance and the corresponding P sorting effect,so as to deeply understand the endogenous load process of sediment P in Taihu Lake and seek hydraulic control methods to control the endogenous load.The main contents,results and related conclusions of this study are as follows:（1）Firstly,the methods and influencing factors of sediment particle separation are studied,mainly considering the influence of several main factors such as particle concentration,separation times,salt solution and temperature on the separation effect.The results show that the lower the particle concentration,the better the separation effect.The more times of separation,the better the separation effect;When the negatively charged colloid in salt energy is added,the settling speed of fine particles becomes very fast.（2）Three representative sediment samples in Taihu Lake were selected to study the influence of hydrodynamic force on particle settling and sorting performance.The results show that the settlement law under the small hydrodynamic disturbance is similar to the static settlement law,breaking through the critical condition（The value of G）,and the stronger hydrodynamic disturbance leads to the weakening of particle settlement performance.The sediment grain size of Zhushan Bay and Dapu is small,which is consistent with the indoor simulation results.The critical G values of these three stations are also different,with Zhushan Bay and Dapu being 11.2 s^-1,Shedu being19.9 s^-1.（3）Two kinds of bloom cyanobacteria,Microcystis aeruginosa and Macrosporium（filamentous）,were added to study the influence of hydrodynamic force on the settling and sorting performance of particles in the presence of algae.The results showed that algae would flocculate with particles under weak hydrodynamic force,but the influence on the settling performance of algae was limited.In addition,flocculation is also related to the species of algae.Microcystis aeruginosa with single cell has a more obvious effect on the sedimentation of particulate matter than Macrosporium sp.（4）Finally,six kinds of particles with particle sizes<2.5μm,2.5-5μm,5-10μm,10-30μm,30-50μm and>50μm separated from Zhushan Bay,Dapu and Shedu were analyzed to determine the particle classification effect of P.The results show that the concentration of pollutants in fine particles<2.5μm and 2.5-5μm are more than those in other four groups of coarse particles,and the concentration of P in particles<50μm（sand）decreases with the particle size.