Using the Sharpest Tools for Tertiary Filtration Rehabilitation: CFD Modeling and Media Selection at DC Water's Blue Plains AWTP

The Filtration and Disinfection Facility (FADF) at DC Water's Blue Plains Advanced Wastewater Treatment Plant (AWTP) is the last process before discharging into the Potomac River. Averaging 384 million gallons per day (mgd) and peaking at 555 mgd, the FADF is one of the largest tertiary filtration facilities in the world. The FADF removes total suspended solids and total phosphorous to levels satisfying the AWTP's NPDES permit limits.
The FADF was built in the 1970s, expanded in 1990, and upgraded through a series of projects between 2002 and 2014. In 2013, DC Water witnessed their first filter underdrain failure. Since that time, DC Water has rebuilt some of the filter underdrains in-kind. In 2021, DC Water initiated the Filter Underdrain and Backwash System (FUBS) Upgrades Project. The project includes replacing the block type underdrains and dual-media system with nozzle type underdrains and mono-media. The upgrades will permit DC Water reduce their backwash and air scour rates, thereby reducing overall energy costs and potentially reducing maintenance needs. All while meeting the stringent NPDES parameters.
This presentation will cover DC Water's approach and lessons learned from the rehabilitation of the filters, including:
Selection of filter media and underdrain system
Modifications of the backwash system, modeled using computational fluid dynamics (CFD) and physical modeling
Rehabilitation of select filters for full scale media testing
Operation and performance of existing dual and select mono media filters
To evaluate filter media options, DC Water conducted a pilot study comparing mono-media against the existing dual-media (as control) in 6-in diameter PVC columns. Three media sizes were assessed with two media bed depths. This pilot study helped DC Water narrow down media sizing and depth preferences, the results of which were then carried into the FUBS project for further refinement. The FADF currently has forty filters, each containing two cells (eighty total filter cells). At the start of the FUBS project, over ten cells were out of service due to underdrain failure. Therefore, in advance of the main project, eight filter cells were designated for early upgrade through the DC Water High Priority Rehabilitation Program (HPRP). This early work restores capacity critical to facilitate the FUBS construction phase. Additionally, four of these cells will be utilized as full-scale demonstration filters for testing the short-listed filter media. Two filters will hold 5-ft bed depth of the smaller media (effective size 1.4 +/- 0.1mm), with the other two holding 5-ft bed depth of the larger media (effective size 1.7 +/- 0.1 mm). The filter performance is measured at the individual filter effluent using new online turbidimeters. The media performance will be evaluated against each other and an existing dual-media control filter. In addition to demonstrating filter effluent quality, filter runtimes and backwash process control will be compared amongst the three cells to assess volumetric efficiency. Ultimately, this full-scale demonstration testing will permit final media selection and filter control strategy optimization.
There are 8 washwater wells each containing a 40 mgd vertical turbine pump to supply backwash water and will be replaced with 12 mgd pumps. The wet well geometry does not meet current Hydraulic Institute (HI) design recommendations outlined in HI 9.8-2018. Poor approach conditions to pumps can lead to pump performance and/or maintenance problems. Although the current pumping system is functional, DC Water has witnessed vane tip cavitation at the pumps, reducing overall life of the equipment. A combination of both numerical methods and scale physical modeling was used to verify and optimize the intake approach conditions to ensure the new pumps would mitigate the existing challenges. The mono media also has lower air scour requirements which was met by reducing the existing blower speeds. The existing blower drives were modified to provide the specified air scour rates.
The initial pilot testing performed by DC Water demonstrates that the mono media can provide equal or better suspended solids and phosphorus removal as shown by Figure 1. However, to optimize the media selection the two media sizes that demonstrated the best performance are being further evaluated in full scale testing in four filter cells.
This presentation will illustrate how both computational fluid dynamics (CFD) and physical modeling tools led to robust and verified hydraulic design that will contribute to optimal pump and filter performance. The CFD model was run for the existing wet well conditions with and without a vaned flow conditioner at the pump inlet. Model results showed that adding a vaned flow conditioner to the inlet eliminated rotation and vortices entering the pump. Similarly, a physical hydraulic model constructed at a 1:4 scale also demonstrated that a flow conditioning basket was effective in dissipating all vortex activity and stabilizing flow entering the pump, as seen in Figure 3.
The filter upgrades provide:
Robust and reliable filter underdrain and backwash systems
Operational flexibility in future
Energy savings
The CFD modeling allowed for a broad exploration of approaches to optimize the pump intake system. The use of both CFD and physical modeling tools led to robust and verified hydraulic design for optimal pump performance.