Virginia Plant Tackles Tightening Nutrient Permit Limits With Gusto

Operators at the H.L. Mooney Advanced Water Reclamation Facility meet the challenges of being neighborly and cutting down nutrient discharges.
Virginia Plant Tackles Tightening Nutrient Permit Limits With Gusto
Rachel Carlson, water reclamation operations manager, stands with the operating staff at the H.L. Mooney facility. From left, Timothy Krisner, Patrick Hills, Mike Failor, Charles Egharevba, Carlson, Michael Lawson, Robert Peterson, Vincent Taylor and James Nicely.

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Thirty-five years ago, the wastewater treatment plant serving the east half of Virginia’s Prince William County lay mostly isolated, surrounded by woods.

Today, development has encroached on three sides; high-end homes in subdivisions stand as close as 200 yards away. The remaining side abuts Neabsco Creek in an area with wildlife preserves and ecologically sensitive wetlands.

Meanwhile, the plant faces tightening permit limits on nitrogen and phosphorus under the Chesapeake Bay Program. It all means the plant, now known as the H.L. Mooney Advanced Water Reclamation Facility, needs to bring its “A game” every day, limiting odors, controlling treatment processes and minimizing emissions from its fluidized bed biosolids incinerator.

It’s a challenge taken with gusto by water reclamation operations manager Rachel Carlson, process engineer Maureen O’Shaughnessy, and teams of plant operators, maintenance specialists and laboratory technicians who work for the Prince William County Service Authority (PWCSA).

Their efforts have earned the H.L. Mooney plant three consecutive Peak Performance Platinum Awards from the National Association of Clean Water Agencies (NACWA), representing seven years without an effluent permit violation.

Exploding growth

For years, the PWCSA has been challenged to keep up with growth. “The county population is close to 420,000 people,” notes Kipp Hanley, a communications staff member. “From 2000 to 2010, the county grew by almost 40 percent. Prince William is in the top 10 or 20 counties nationally in household income.”

Based in Woodbridge, about 25 miles southwest of Washington, D.C., the PWCSA serves an area that is largely suburban; the western reaches are more rural and include some homes on wells and septic systems. The authority supplies Prince William County with drinking water, purchased from neighboring Fairfax County. The 24 mgd (design) Mooney plant serves about 88,000 residential and commercial connections. There are no major industrial users.

“Before the PWCSA was formed in 1983, this area had a number of small wastewater treatment plants,” says O’Shaughnessy. “There was a big effort in the 1970s to clean up the Potomac River, and this plant’s construction was part of that. After it was built, five smaller plants were closed, pumping stations were built and the wastewater came here.”

The plant was upgraded to 18 mgd in 1997; that project replaced an older multiple-hearth incinerator with the fluidized bed unit. A $131 million upgrade in 2012 boosted capacity to the current 24 mgd and enhanced nitrogen and phosphorus removal. “Our effluent has to meet a total nitrogen limit of 3.0 mg/L,” says O’Shaughnessy. “For phosphorus, it’s 0.18 mg/L. Nitrogen is the limiting nutrient in Chesapeake Bay, because it’s saline. The Potomac is freshwater, so phosphorus is the limiting factor. That’s why our limits are pretty low on both.”

Targeting nutrients

Two pump stations deliver influent to the plant’s pre-aeration step. Pretreatment consists of two band screens (Hydro-Dyne), a manual bar screen (Peabody Wells) and a grit classifier (WEMCO). “Right now, we’re bypassing the grit classifiers and the flow goes into three equalization basins,” says Carlson. “The grit settles out in those basins. We try to keep one online at a time. Between the three, we have about 8 million gallons capacity.”

Preliminary treatment is followed by five primary clarifiers, where ferric chloride is added for phosphorus removal. More phosphorus is removed biologically in five aeration basins, which use a four-stage Bardenpho process. “We add methanol to the second anoxic zone, so we are able to remove enough nitrate to meet the 3.0 mg/L limit,” says Carlson. Six blowers (Hoffman & Lamson) supply the air via fine-bubble diffusers (Sanitaire - a Xylem Brand).

The flow then passes through nine secondary clarifiers and into deep-bed sand filters. “Right now, we’re using them as conventional filters, although we have the capability to add methanol there if need be to meet our nitrogen limit,” Carlson says. After disinfection (TrojanUV3000Plus), final effluent flows over an aeration cascade to the outfall on Neabsco Creek, a Potomac tributary.

Odor controls are integral to the process. Packed tower scrubbers (Indusco) treat odorous air from pre-aeration, pretreatment and from the sludge thickeners. An odor counteractant is sprayed regularly around the primary clarifiers.

Solids side

Commingled sludges from the primary and secondary clarifiers are fed to four gravity thickeners (Ovivo) and then to a holding tank from which the material goes to three centrifuges (Alfa Laval) that yield cake at 26 to 27 percent solids. Each centrifuge has a cake pump (Schwing Bioset) that feeds material to the fluidized bed incinerator (Hankin Environmental Systems).

“We chose the fluidized bed technology because it puts out much cleaner flue gas and is much more energy efficient to operate than our old multiple-hearth incinerator,” Carlson says. The air pollution control equipment is being upgraded to comply with the U.S. EPA’s latest sewage sludge incinerator (SSI) maximum achievable control technology (MACT) emission limits.

“We’re adding a wet electrostatic precipitator and a new technology, called a solvent polymer composite,” says Carlson. “It’s a scrubber with media that removes mercury from the flue gas. It’s an emerging technology that our authority pilot tested and decided to install.”

The fluidized bed consists of a 5-foot-deep layer of special heat-resistant sand above the combustor. Heated air blows upward through the sand, creating turbulence that breaks up the injected sludge cake. Combustion temperatures of 1,200 to 1,600 degrees F instantly ignite the material. Combustion is self-sustaining.

Sterile, zero-odor incinerator ash exits the system and goes to on-site settling ponds. The ponds are periodically drained, and the ash is scooped out with a front-end loader and spread on drying pads before being hauled to a landfill. As a backup in case the incinerator should go down for an extended time, the facility includes a mobile Schwing Bioset lime stabilization unit, which can create Class A or Class B biosolids for land application.

Keys to excellence

For the plant’s continuous excellence, O’Shaughnessy points to a highly qualified staff and the success of the 2012 plant upgrade. “Almost every process at the plant was affected in some way,” she says. “We added new screens, the new grit system, more aeration basins, more final clarifiers, new filters.

“One of our biggest problems before the upgrade was ammonia breakthrough. Once we got the extra aeration basins, the ammonia breakthrough was under control, and our long string of compliance really started coming together. The extra clarifiers helped us reduce the solids loading to the filters and improve the plant’s function, keeping more of the solids in the secondary system.”

A Wonderware SCADA system (Invensys) monitors all processes, including aeration. Six Hach LDO probes are strategically placed in each basin; valves automatically open and close to maintain a DO setpoint that tapers through the tank from 2.0 mg/L to about 0.75 mg/L.

Staff diversity

Responsibilities for the plant fall to four closely coordinated departments: maintenance, operations, laboratory and administration. Operations staff members man the plant around the clock on four rotating 12-hour shifts, each with four operators.

Maintenance is divided into mechanical and instrumentation/electrical specialties, each with six people. “We have a well-rounded maintenance department,” O’Shaughnessy says. “We have on-call staff so that if there is an issue during off hours, someone is always available to come in and make repairs.”

A computerized maintenance management system (MP2) keeps preventive tasks on track, automatically issuing work orders for scheduled activities. The team also deploys predictive maintenance tools such as vibration analysis, fluid analysis and infrared thermography. Emergency power capability is supplied by two 2.5 MW diesel engine-generators (Caterpillar).

The plant’s commercial laboratory is accredited by the State of Virginia for water and wastewater testing. Its 12 technicians handle some 40,000 samples a year, including all process control and compliance samples from the H.L. Mooney plant as well as samples from some neighboring plants.

The administrative team oversees all activity and keeps the other departments apprised of higher-level developments, such as impending changes in regulations.

The daily routine

Workdays at the H.L. Mooney plant start with a morning meeting that involves staff members from all the areas. “We go over the plant priorities and coordinate on any major work that needs to be done, especially if we need to take any equipment out of service so that maintenance can make repairs or a replacement,” says Carlson. “As for the lab, we have a routine of the samples we grab each day for analysis.”

The plant has a safety committee with all departments represented. Weekly safety meetings and safety training sessions cover all work shifts. There are periodic safety inspections and walk-throughs of plant processes to check for hazards and verify compliance with safety policies. Joseph Del Priore, safety manager, has his office in the plant, which has won recognition for its safety program, most recently the 2014 Water Environment Federation George W. Burke Facility Safety Award.

Ever improving

The accomplishments and accolades to date aren’t enough for the H.L. Mooney team. More plant upgrades are in the future. They include an upgrade to the programmable logic controller in the UV disinfection facility and the addition of a fourth disinfection channel. Another project includes replacing the mechanisms in the primary clarifiers and adding aluminum covers to reduce odors.

Besides that, a unique feature of the plant ensures that innovation and improvement will continue: The secondary process includes a testing train. “We have one set of aeration basins and two clarifiers that can be completely isolated from the rest of the secondary system,” Carlson says. “That enables us to try different setups, such as different DO controls and different carbon sources for nitrogen removal, and it doesn’t affect the other systems.

“It’s like a separate treatment train after the primaries. It shares the same primary influent but, once the water goes into the aeration basin, it has its own isolated clarifiers. The clarifier effluent joins the rest of the flow before it goes into the filters. It’s nice to have that flexibility for testing process improvements. In the future, we were looking to try different technologies for methanol reduction.”

It’s just one more reason to think this plant has more NACWA Platinum in its future.

Recruitment Pipeline

The H.L. Mooney Advanced Water Reclamation Facility isn’t unique in facing a wave of operator retirements. It is unique in the way its team deals with the issue.

The plant has a recruitment program in which trainees are hired for plant operator and mechanical maintenance positions and placed on a path that can lead directly to careers. “It’s attractive to people who maybe didn’t see themselves getting into wastewater,” says Rachel Carlson, water reclamation operations manager. “They come in and we provide them with training and anything they need to become successful future operators and mechanics.”

For the operations department, it’s a four-step process. Trainees receive plant-specific training materials and take outside study classes, such as short schools in preparation for Virginia wastewater operator licenses.

Each time a trainee completes the criteria for a given step in the process, he or she receives a noncompetitive promotion to a higher pay grade. “It makes them valuable to us because they have the training they need, and it gives them the opportunity to make more money,” Carlson says. After all four steps, they can become Class 1 licensed operators at the plant.

“It’s a seven-year program,” says Carlson. “Trainees have the potential, if they come in and really excel, to almost double their salary within seven years. We’re trying to lure people into a career that they might not have considered as a possibility.”

So far, the program is paying off handsomely. “We’ve acquired several operators, and we just recently welcomed three new trainees,” says Carlson. “Those who have come in so far have all been successful and are at different steps of the process. It’s a form of succession plan for us. It ensures that we’ll have people with the knowledge and skills required to operate this plant.”


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