Optimization of Secondary Clarifiers in BNR/ENR Applications: Four Cases Studies Comparing Different Clarifier Geometries

The performance of secondary clarifiers is critical to ensure that stringent nitrogen and phosphorus limits are met in Biological Nutrient Removal (BNR) and Enhanced Nutrient Removal (ENR) treatment plants. Additionally, these plants rely in slow growth microorganism populations that are difficult to restore in the event of solids washout that could happen in wetweather storm flows. Secondary clarifiers are a key unit to ensure retention of the solids inventory. This paper illustrates the application of computational fluid dynamics (CFD) modeling to four different BNR/ENR case studies covering a wide variety of clarifier geometries, internal configurations and loading conditions.In Case Study 1, the model was used for the validation of step feed as a wet weather strategy to prevent solids washout during storm events. For Case Study 2, the model was use to evaluate the impact that the tank depth can have on clarifier performance and capacity. In Case 3, the model was used to compare the performances of peripheral-feed and center-feed circular clarifiers; and Case 4 presents the evaluation of rectangular clarifiers. In all cases, the model was used to identify improvements and operating strategies that result in improved reliability, increased capacity and reduced capital cost.