Development of a flocculation sub-model for a 3-D CFD model based on rectangular settling tanks

摘要

To assess performance and evaluate alternatives to improve the efficiency of rectangular Gould IIntype final settling tanks (FSTs), New York City Department of Environmental Protection and CitynCollege of NY developed a 3D computer model depicting the actual structural configuration of thentanks and the current and proposed hydraulic and solids loading rates. Fluent 6.3.26t was thenbase platform for the computational fluid dynamics (CFD) model, for which sub-models of the SSnsettling characteristics, turbulence, flocculation and rheology were incorporated. This wasnsupplemented by field and bench scale experiments to quantify the coefficients integral to thensub-models. The 3D model developed can be used to consider different baffle arrangements,nsludge withdrawal mechanisms and loading alternatives to the FSTs. Flocculation in the front halfnof the rectangular tank especially in the region before and after the inlet baffle is one of the vitalnparameters that influences the capture efficiency of SS. Flocculation could be further improved byncapturing medium and small size particles by creating an additional zone with an in-tank baffle.nThis was one of the methods that was adopted in optimizing the performance of the tank wherenthe CCNY 3D CFD model was used to locate the in-tank baffle position. This paper describes thendevelopment of the flocculation sub-model and the relationship of the flocculation coefficients innthe known Parker equation to the initial mixed liquor suspended solids (MLSS) concentration X0.nA new modified equation is proposed removing the dependency of the breakup coefficient to theninitial value of X0 based on preliminary data using normal and low concentration mixed liquornsuspended solids values in flocculation experiments performed.