Document Type
Presentation
Publication Date
8-1-2014
Abstract
Algal pollution is a common water quality problem in many source water reservoirs worldwide and directly threatens the safety of drinking water. Water-lifting aeration has been recently developed to improve the source water quality and to control the excessive algae growth by transporting algae from the surface water to the bottom water of the reservoir. Taking the Shibianyu Reservoir supplying 20% source water to Xi’an, 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 (V8 Star) and a depth meter (HD 17). The three-dimensional flow velocities were measured with an Acoustic Doppler Profiler (WH600kHz, LAUREL). The water-lifting aerator was simplified as a cylinder and the periodic velocity at the aerator’s outlet was numerically calculated with Euler-Euler multiphase model. The temporal distribution of velocity at the domain inlet, the vertical distributions of initial water temperature and volume fraction of algae, the density and viscosity with water temperature were all imposed with user defined functions written in C programming language. The volume fraction of algae was calculated with the algae content and algae diameter. The algae transport was also simulated with Euler-Euler multiphase model and the turbulence was modeled with RNG model. Before running the aerators, the water temperature decreased from 18℃ at the surface to 16℃ at the water depth of 46m, and to 10℃ at the depth of 50m; the volume fraction of algae increased from 2.0e-3 at the surface to 3.3e-3 at the depth of 5m, decreased to 9.1e-5 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. Due to its floating velocity of 0.000275m/s, the microcystis aeruginosa, which is the dominant species of algae, 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 transport under other characteristic water levels and temperature gradients, the effect of algae control using water-lifting aeration was comprehensively evaluated and the optimized operational conditions were suggested for the reservoir management.
Comments
Session R28, Eco-Hydraulic Modeling: Lake Processes