Utilization of Computational Fluid Dynamics Modeling to Optimize Design

The Nansemond Treatment Plant (NTP) Advanced Nutrient Reduction Improvements (ANRI) Phase II improvements project is a vital component of Hampton Roads Sanitation District's (HRSD) overall Sustainable Water Initiative for Tomorrow (SWIFT) Program to improve nutrient waste load reductions to the Chesapeake Bay watershed. This project consists of expanding the existing NTP facility to 50 MGD to accommodate the transfer of screened and equalized raw wastewater from the Boat Harbor (BH) service area. It also includes process improvements to prepare for the next phase of SWIFT improvements at the NTP. As part of the design, of these improvements Computational Fluid Dynamics (CFD) Modeling was utilized for several unit processes to optimize design and construction. These included:

*Influent Distribution Box (IDB) Mixing and Flow Distribution: A new IDB is being constructed to combine influent flows from the Nansemond service area and the BH service area and distribute them equally to the downstream primary clarifiers. CFD modeling was used to evaluate the effectiveness of mixing these flows completely prior to flow splitting. The results indicated the need for a mixing chamber followed by a perforated baffle wall to evenly distribute the flow and reduce the approach velocity of the combined flow prior to splitting flows to the primary clarifiers using proportional length weir gates. CFD modeling results for the IDB are shown in Figure 1.

*Backflow modeling for partial denitrification annamox (PdNA): The density difference between wastewater in anoxic and aerobic zones can cause buoyancy-driven counterflow (BDCF), potentially impacting biological nutrient removal and increasing methanol consumption for denitrification. BDCF is a hydraulic phenomenon that occurs due to a density difference between interfacing fluids, causing a pressure gradient and resultant counterflow between the liquid zones. CFD modeling was performed to understand the impact of BDCF at the first anoxic/aerobic zone and aerobic/second anoxic zone interfaces for the aeration tanks. Modeling indicated significant BDCF was likely occurring between the first anoxic and aerobic zones reducing process efficiency. A hood-shaped baffle system was designed to reduce BDCF while minimizing head loss across the system. Figures 2 and 3 show BDCF before and after implementation of the baffle system.

*AAA Basin Flow Distribution: Hydraulic modifications are being made to accommodate higher flow rates through the existing AAA basins. The AAA basins consist of 7 trains, each subdivided into six cells using baffle walls. The flow split to the basins is dictated by the hydraulics through the baffle walls. Maintaining similar hydraulic losses through each basin is critical to maintain even flow split. To accommodate higher flows through each basin due to the plant expansion, openings in the baffle walls needed to be added and/or widened. As each basin's modifications are completed sequentially, the hydraulic losses through the basins will differ causing a variation in the hydraulic flow split to occur. To maintain desired flow distribution to each basin throughout construction, CFD Modeling was performed to evaluate the expected flow split under various flows and operating conditions. Based on the results of the modeling, temporary modifications to the inlet of each basin using temporary orifice plates for each phase of construction were developed. This allows a more even distribution of flows to each of the basins will be maintained throughout construction. An example of the flow distribution is shown in Figure 4.

*Secondary Clarifier Influent Channel Flow Distribution: Two additional 160-ft secondary clarifiers are being added to the plant with this project, for a total of four 160-ft diameter clarifiers and three 85-ft diameter clarifiers. The flow from nine process trains discharge to a common flow distribution channel prior to overflowing flow splitting weirs to the clarifiers. The clarifier influent distribution channel extends over 500 ft with proportionally sized flow splitting weirs spread out along the distribution channel. However, proportional flow split at the weirs was not occurring due to the hydraulic conditions along the channel. CFD Modeling was performed to evaluate the expected flow split under various flows and operating conditions. Initial modeling indicated a significant deviation in flow split to the clarifiers. Modifications to inlet weir plates, installation of adjustable weir gates, and an operational strategy for operating the inlets to the clarifiers was developed based on the results of the modeling to more evenly distribute flows to the secondary clarifiers. The presentation will provide details of the various CFD Modeling performed, options that were developed to optimize the design, the results of those changes in the CFD models, and how these improvements were implemented in the design.