Waste Water Treatment Gets an Oxygen Boost
By Alban Poirier, Vincent Perrin, and Jérôme Cluzeau, AIRLIQUIDE, Gaz Industriels Services, DAP, Les Loges en Josas, France
Waste water treatment basin |
Oxygen plays an important role in waste water treatment. Most of the bacteria that are responsible for the decay of organic material are aerobic, so the dissolved oxygen in the waste water must be replenished by an outside source. The aeration process allows bacteria and sludge to be put into contact. Efficient oxygenation is, in fact, essential to the success of aerobic biological treatment. In the most difficult cases, particularly with industrial effluents, pure oxygen boosting (rather than air boosting) is a very efficient solution; it can be applied to most basins, even those that were not originally designed for oxygen. CFD can be used to validate the technical choices for oxygenation without industrial risks and to predict the performance of existing and future basins.
Pathlines illustrate the flow field for high (top), nominal (middle), and low (bottom) oxygen flow rates for VENTOXAL |
In a recent project, FLUENT was used to characterize the hydrodynamic behavior of an industrial rectangular waste water tank in the presence of four floating turbine aerators and two types of oxygen transfer device: a TURBOXALT (floating at the basin free surface) and a VENTOXALT (immersed in the waste water basin). The goal of the project was to simulate the initial performance of the basin, and then to improve its performance before the installation of the equipment by optimizing two parameters: the location of the new equipment and the flow rate distribution. Using a Lagrangian (DPM)calculation, a discrete phase of oxygen bubbles was coupled to the continuous phase of water, taking into account the hydrodynamic effect of the oxygen plume. Of particular interest were the oxygenation homogeneity, the mixing efficiency, and the interaction of the plumes with the different equipment.The presence of low velocity zones, which represent a significant risk for sludge deposits (and the development of filamentous bacteria) and short-circuiting (hydrodynamic flows with low residence time) were sought as well.
Validation of a simplified VENTOXAL CFD model of an industrial basin |
Cross-section of the velocity field 0.57 m above the bottom of the basin, showing the low velocity zone along the right edge |
Validation of these two AIR LIQUIDE oxygen boosting devices was difficult because of the high reciprocal impact of the gaseous phase on the flow. In particular, one of the fitting parameters is the length of the bubble streams, measured experimentally and compared with the modeling results using a typical bubble size (measured and correlated). The VENTOXAL device was first simulated on an instrumented biologic water treatment plant, and provided computed velocity and concentration fields for comparison with data. The validation allowed simplifying modeling assumptions to be identified. The TURBOXAL was also the object of preliminary 3D CFD simulation, used for the development of this new device, that took into account the impeller rotation. These results also allowed simplified 3D model to be developed for the wastewater basin simulation that used values of axial,radial and tangential velocities. A simplified model of the turbine aerator was developed as well, based on technical data supplied by the manufacturer.
The CFD simulation allowed the influence of various parameters to be studied. For example,varying the location of the oxygen boosting equipment or modifying the flow rate distributions were considered in order to improve the mixing performance of the basin. The oxygen residence time could also be optimized by displaying the upward velocity of the plumes.
Iso-surface of 0.1 m/s velocity, showing the low flow zones in the basin |
Once the basin began operating, the experimental data showed very good agreement with the CFD modeling results. The plumes were located exactly where the CFD model predicted they would be, Wanda very good quality of floc was observed at the exitstation as a result of a good velocity distribution .Overall, the CFD modeling effort saved time in determining the best implementation of the equipment, and it will contribute to new oxygen boosting projects in the future.
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