By Jim Maxon, Stephen Fowler and Richard Kehn
Plant owners and operators must be aware of innovative and current methodologies to solve their water and wastewater treatment and operations problems. As such, suppliers with sophisticated software models can anticipate potential limitations and enhance treatment system hydraulics.
What are the incentives? These investigations create large opportunities for productivity and cost savings, such as minimizing capital costs, controlling mechanical spares, maximizing equipment uptime in anoxic basins, and locating basin inlets/outlets to prevent short-circuiting.
Vertical turbine mixers - whether flash/rapid mixers, flocculators or anoxic or digester mixers - contribute to the success of these efforts. Likewise, mechanical mixers can be modeled in water and wastewater operations using computational fluid dynamic (CFD) techniques. CFD is the branch of fluid mechanics where flow fields are modeled and numerically solved using computers. Understanding fundamental fluid dynamics is required to ensure the proper use of CFD as a design tool. This includes applying the correct model to the situation (laminar versus turbulent regimes), employing sound volume meshing techniques, and applying appropriate boundary conditions to the basin geometry under evaluation, as well as experimental validation of CFD results from the mixer's impeller characteristics and performance.
Water and wastewater treatment basins present special atypical geometries to evaluate. Mixers can be modeled using either two dimensional (2-D) or three dimensional (3-D) CFD techniques. 2-D modeling is suited for axisymmetric geometries and requires knowledge of the mixer's impeller outlet velocity profile. Further, it can provide a quick approximation of the flow pattern in the mixing tank and is often useful to make rapid decisions. 3-D modeling, however, uses the actual impeller geometry and considers the rotation of the impeller, thus generating the correct flow pattern (see Fig. 1).