A series of high-flow simulations conducted last winter by the Prince William County (Virginia) Service Authority will help the utility identify near- and long-term infrastructure needs at its wastewater treatment plant.

The H.L. Mooney Advanced Water Reclamation Facility in Woodbridge receives wastewater from two remote pumping stations that converge at the plant headworks. There, influent enters two preaeration chambers before going through pretreatment, where flow can be split between two automatic screens and one manual bar screen. 

After screening, the flow is conveyed through the grit channels and then into one of three equalization basins. Flow is then gravity fed from those basins to the primary clarifiers. Under high-flow conditions, hydraulic bottlenecks have been seen at the preaeration influent chambers, upstream of the screening channels, in the grit channels and at the equalization basin influent gates.

Investigating causes

To determine the cause of the bottlenecks and identify possible solutions, the authority hired the Stantec consulting firm to provide a dynamic hydraulic model for the front of the plant all the way through to the primary clarifier effluent.

In earlier stages of the project, the consultant identified areas where additional level sensors and flowmeters were needed to gather data for the model. Once those instruments were installed, the project called for five high-flow event simulations to capture the level and flow data actually being observed in each process area.

To simulate large rain events, staff from the authority’s Water and Wastewater Facilities (WWF) Department stopped pumping during the morning peak to accumulate wastewater in a nearby pumping station wet well and upstream collections system. They then rapidly put the pumps into service to push a peak flow to the plant. The peak flow events lasted about 30 minutes before the pump station ran out of wastewater. 

Operations staff members were deeply involved because they were responsible for taking the tanks in and out of service and cleaning the tanks once taken out of service. Key contributors included Maureen Oshaughnessy, process engineer and project manager; Doug Chapman, acting operations manager; Robert Jenkins, WWF manager; William Hann, WWF mechanic; and Nathan Griffith, modeler.

Prepping for emergencies

For each simulation, plant staff took tanks and channels in and out of service to gain valuable data on how the 24 mgd (design) plant would respond to increased flows under each configuration.

“Testing the capabilities of our plant during a high-flow scenario is critical to understanding what we may need to address before a real emergency occurs,” says Rachel Carlson, plant superintendent. “This project has been very enlightening so far, and we couldn’t have done it without the cooperation of WWF staff.”

The next step of the modeling project is for plant staff to use the collected data to calibrate the model. The final phase will use the model to identify hydraulic issues and provide recommendations to resolve bottlenecks. That work will likely be completed by the end of 2020.

Options to address the bottlenecks may include additional screening channels, adding a pump station to increase flow out of the equalization basins into the primary clarifiers, or constructing a new pretreatment facility. Those improvements will likely cost $5 million to $15 million.  

Saving the day

In the nine months ending in May 2019, the authority saw a large increase in average daily flows, in part due to exceptional amounts of rainfall in the Mid-Atlantic region. The increase was attributed mainly to inflow and infiltration into the more than 600 miles of sewer mains that feed the treatment facility. During that period, the plant saw a 25% increase in its average flows. Ironically, planned high-flow simulations had to be cancelled due to actual large rain events that occurred late last winter.

One of the high-flow simulations prevented a potential sewage spill at the plant, which is situated close to the Potomac River. Wastewater entering the plant goes into one of three equalization basins, which are used to help balance fluctuation in flows. While preparing for a flow simulation in February, plant staff discovered a leak in one of those basins.

During a simulation to test the hydraulic limitations on the flow discharged from equalization basin No. 3, a leak was observed from a joint in the tank wall. The simulation was stopped immediately, and plant staff cleaned up the small amount of liquid that had seeped out.

According to Carlson, the joint likely leaked due to age and the additional head pressure on the full tank. Each of the three equalization basins is put into service every six months, barring high flows that require two basins to be working at the same time.

“The service authority is all about preventive maintenance,” Carlson says. “By running this exercise, we have potentially saved ourselves a lot of headaches.” 

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About the author

Kipp Hanley (jhanley@pwcsa.org) is a copywriter with the Prince William County (Virginia) Service Authority.