Dear All,

I have been asked to model a final settling tank, or clarifier, at a wastewater treatment plant. For those of you who don't know about wastewater treatment, settling tanks are usually the last process before effluent is discharged back to the environment. The velocities in the tanks are relatively low, allowing remaining solids to settle to the bottom of the tank to form a sludge layer. A moving scraper slowly moves the settled sludge to a hopper where it is removed. The clear liquid, the treated sewage, flows over an elevated weir and is passed to the nearest watercourse. The solids concentration in the flow entering the tank will be in the region of 2-8 g/l.

I have spent a few weeks trying to model the process with the eularian model, but I am now getting into trouble. The problem I have is how to represent the changing concentrations. When the flow enters the tank the solids concentration will be say 3 g/l. The solids concentration within the sludge layer will vary. At the top of the layer the concentration will be low, but will increase with depth to perhaps 6-8 g/l at the base of the tank. This means I cannot model the sludge as one single phase with a constant density as this will not reproduce the sludge 'layering'. I have also tried to model the process with multiple sludge phases of different densities, but the lighter phase gets trapped eventually filling up the tank.

Does anybody know how to model this scenario within Fluent 6?

Thanks for your help, John

Hello,the solid concentration in the inlet ranges from 2 g/lto 8 g/l.In this case,it seemed that you should use model of different densities.

[A generalized settling approach in the numerical modeling of sedimentation tanks]

please see this paper,may be helpful for you!