The H.L. Mooney Water Reclamation Facility (Woodbridge, Va.) has a history of award-winning treatment. The plant won the 2003 George W. Burke Jr. Facility Safety Award from the Water Environment Federation (Alexandria, Va.), the 2006 Safety Award from the Virginia Water Environment Association (Winchester), and the 2006 Peak Performance Gold Award from the National Association of Clean Water Agencies (Washington, D.C.).
It’s no wonder the plant wins accolades when it specializes in novel solutions. For example, when an air header at the bottom of its activated sludge aeration basin broke and was leaking badly, operators knew they couldn’t risk taking the basin out of service for the extended times the repairs would require. So they found another answer: They hired a diver.
Although the diver couldn’t see while in the aeration basin, armed with a good set of instructions and a good sense of touch, he was able to make the repair in several hours, saving the day.
The plant treats an average of 12.5 mgd (47,300 m³/d) through its activated sludge system, with chemical treatment for phosphorus and biological nutrient removal and denitrification filters for nitrogen removal. By design, the plant can remove nitrogen down to a concentration of 8 mg/L, but at times, the denitrification filters can remove nitrogen to near refractory levels.
When the H.L. Mooney Water Reclamation Facility in Woodbridge, Va., completes its current expansion project, the plant will be able to treat flows as great as 24 mgd (91,000 m3/d) and meet nitrogen discharge limits of 3 mg/L. (Photo credit: Prince William County Service Authority)
Even though the plant completed an expansion in 2006 — that project increased the full plant capacity to 18 mgd (68,100 m³/d) and added the denitrification filters, odor control, and ultraviolet (UV) light disinfection — another project is already underway. On Oct. 4, 2007, the plant began its current expansion, which will increase capacity to 24 mgd (91,000 m³/d) and enable nitrogen removal to 3 mg/L by Dec. 31, 2010.
The Prince William County Service Authority (PWCSA), which operates the plant, has been successful in negotiating grant agreements with Virginia’s Department of Environmental Quality for nitrogen removal. In the 2003 expansion, the grant totaled $8.6 million, and in the current expansion, it is expected to total $37.1 million.
Moreover, PWCSA is a member of the Virginia Nutrient Credit Exchange Association (Richmond), a voluntary association of owners of more than 100 regulated municipal wastewater treatment plants and industrial facilities discharging nitrogen and phosphorus within the Chesapeake Bay watershed. One major benefit of membership in the exchange is that it helps ensure compliance with the plant’s nitrogen and phosphorus allocations.
Following the Flow
As influent enters the plant, it is air-stripped for odor control. It then passes through fine screens and grit removal before entering equalization basins. The equalized flow passes to primary clarifiers, where ferric chloride is added for phosphorus removal. Primary effluent enters the biological nutrient removal activated sludge process, where it is nitrified and nitrogen is partially removed before secondary clarification. Lime, methanol, additional ferric chloride, and polymers are added for process control. The secondary effluent continues to the denitrification filters for additional nitrogen and suspended solids removal. The filtered effluent is disinfected by UV radiation and reaerated as it passes down the effluent cascade.
All solids are gravity-thickened, dewatered by centrifuges, and stabilized by fluid-bed incineration. The high-solids centrifuges condition solids so well for incineration that the plant’s fluid-bed incinerator runs without additional fuel. PWCSA hauls the resulting ash, along with the grit and screenings from the headworks, to a landfill.
One of the plant’s biggest challenges is filling the shoes of its Class I veteran operators who are retiring after 25 to 35 years of service. These operators, who served as the backbone of the operation for so many years, have slowly disappeared, and finding and growing quality operators from scratch is a daunting challenge.
To attract and retain qualified staff, the plant offers many opportunities for professional development. For example, PWCSA offers cash incentives to its water reclamation operators based upon license class and ability to operate the plant. The authority also offers a 4-year electrician apprenticeship program that incorporates both informal on-the-job training and classroom instruction, and is developing similar programs for its utility mechanic trainee and laboratory environmental technician positions.