Utilization of Dynamic Modeling to Evaluate and Design Secondary Clarifier Improvements
- Joe Rohrbacher, Mike Parker, Alonso Griborio - Hazen and Sawyer
Wastewater treatment plant simulators are powerful tools for the evaluation, sizing and optimization of WWTPs. These simulators incorporate the latest advances in technical knowledge and can reliably predict performance if properly calibrated. Existing treatment plant models include elements to simulate secondary clarifier performance. These models are idealized, one-dimensional models and do not account for hydrodynamics, flocculation or environmental impacts such as temperature; therefore they are limited in their ability to accurately represent clarifiers with various design elements such as inlets, flocculating wells and baffles.
Clarifier modeling packages incorporating computational fluid dynamics (CFD) address the limitations of idealized solids flux based models. CFD models can incorporate hydrodynamics, flocculation, turbulence and temperature in a clarifier settling model and allow for optimization of clarifier design. Although the more advanced clarifier models have yet to be incorporated into a commercially available whole plant simulator model, the two can be used together in order to more accurately predict final effluent quality.
This approach was recently used to evaluate and design secondary clarifier improvements at two WWTPs. In the first case study, a 15 mgd nitrifying activated sludge plant in Virginia was converted to an enhanced nutrient removal facility (ENR). The simulations predicted clarifier failure during the design storm event. Implementation of step feed during wet weather flows was evaluated using the model. The models also were used to predict the impact of polymer addition during poor settling conditions and modifications to the existing clarifier influent wells and baffles. Step feed operation and clarifier modifications were predicted to decrease the effluent suspended solids concentration from over 150 mg/L to less than 15 mg/L during peak wet weather flows. The ENR improvements were designed to incorporate step feed for wet weather flows, polymer feed facilities and modifications to the existing clarifiers. The WWTP was able to defer construction of additional clarifier capacity by implementing these changes that were recommended as a result of the modeling.
The second case study involved a 21-mgd nitrifying WWTP in North Carolina. The existing secondary clarifiers had limited capacity to meet the current effluent limits, and the WWTP has experienced variable settling performance and high effluent solids concentrations during peak flow events. Dynamic simulations indicated that modifying the existing clarifiers would have very limited benefit. The integrated modeling approach was further used to optimally size a new secondary clarifier with provisions for installation of an additional clarifier for future nutrient removal.
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