IN-SITU ALGAE CONTROL USING WATER-LIFTING AERATORS IN A EUTROPHICATED SOURCE WATER RESERVOIR

摘要

Taking the eutrophicated Shibianyu Reservoir in Northwest China as a study case, the effect of in-situ algae control using water-lifting aerators was numerically investigated with Fluent. Two submerged water-lifting aerators with different circulation flow rates were installed with different local water depths respectively. Accurate geometry data required for the mesh generation were obtained using a global position system based on real time kinematic technique and a depth meter. The three-dimensional flow velocities were measured with an Acoustic Doppler Profiler. The temporal distribution of velocity at the domain inlet, the vertical distributions of initial water temperature and algae concentration, the density and viscosity with water temperature were all imposed with user defined functions written in C programming language. The algae transport was also simulated with Euler-Euler multiphase model and the turbulence was modeled with RNG model. Before running the aerators, the algae concentrations increased from 4.00 million cells.L~(-1) at the surface to 9.20 million cells.L~(-1) at the depth of 5m, decreased to 0.16 million cells.L~(-1) at the depth of 15m and then remained constant in the deeper water. At day 10 after operating the aerators, the water temperature became nearly uniform in the reservoir, the algae content in the surface area was greatly decreased, but the algae content in the lower water was increased. The dominant microcystis aeruginosa with a floating velocity of 0.000275m/s was transported to and deposited at zones near the bottom and side wall. The distributions of simulated flow velocity, water temperature and algae content agreed well with the measured ones on the field. Based on the simulation results of algae concentration under five water levels, five temperature gradients and three air flowrates, the times required for complete mixing of algae within the water depth of 5m were comprehensively evaluated and the optimized operational conditions were suggested for the reservoir management.