Continuous State of Improvement
Name: Alexandria Sanitation Authority’s Advanced Wastewater Treatment Facility
Location: Alexandria, Va.
Plant size: The waste treatment facility has a design capacity to treat 54 mgd for 325,000 people in a 51-sq-mile area. Recently, two new digesters, a pre-pasteurization unit and the rehabilitation of the existing digesters were implemented.
Infrastructure: The Alexandria Sanitation Authority’s Advanced Wastewater Treatment Facility currently provides preliminary, primary and secondary treatment processes flowed by chemical addition for phosphorous removal, sedimentation, filtration, UV disinfection and post aeration. Additionally, solids processing includes gravity thickening of primary and tertiary sludge, mechanical thickening of waste activated sludge, centrifuge dewatering, lime addition and landfill disposal. Incineration of dewatered raw sludge is another option for disposal.
The plant is located on a 30-acre site on the north bank of Huntington Creek, close to the junction of the Potomac River. It provides sewage treatment for 325,000 people in a service area that extends 51 sq miles and includes the city of Alexandria as well as portions of Fairfax County.
The Alexandria Sanitation Authority originally completed construction of their Advanced Wastewater Treatment Facility in 1954 as a 18-mgd trickling facility. In 1984, a $101 million expansion and upgrade project was completed, allowing the plant to handle an average flow of 54 mgd, including advanced treatment capabilities.
Most recently, an upgrade project valued at $275 million and spearheaded by CH2M Hill was commissioned primarily to focus on nitrogen removal and to meet strict standards adopted by the Commonwealth of Virginia. CH2M Hill’s expansion and upgrade project for the multiple treatment and processing facilities was divided into six contracts, involving more than 20 structures on the site. The project began in 1999 and was completed in late 2005.
“The project was a success because it was deemed important from the beginning of the project to keep everyone within the organization informed of the changes and progress being made,” said Cheryl St. Amant, director of technical services for the Alexandria Sanitation Authority. “A good communication program helped to alleviate fears that construction would impact existing plant processes. The carefully worked out communication plan paved the way to acceptance of the ongoing construction activity and eased the implementation of start up of newly constructed facilities. Keep in mind that the plant sits on 30 acres, and there were six different contracts. Communication between the engineer, contractors and plant personnel was essential.”
Meeting water quality standards
In 1999, the Alexandria Sanitation Authority and CH2M Hill began construction to upgrade the 54-mgd design flow facilities in order not only to meet water quality requirements of the Potomac Embayment Standards, but also meet requirements of the Chesapeake Bay Agreement.
In December 2002, initial operation of the Biological Nutrient Removal system was achieved, thereby reducing nitrogen discharges from the plant by approximately 70%.
Upgrades also included the construction of a 119-ft-tall solids processing building. The building included thickening and dewatering centrifuges, a lime-addition system, biosolids storage hoppers, polymer feed systems, a primary effluent pump station, as well as plant-wide odor control and chemical feed systems.
The flow train for the biosolids handling process is connected into a central control point using a SCADA system that was customized for the Alexandria Sanitation Authority’s Advanced Water Treatment Facility. Operations of the new solids handling facility began in July 2003.
Finally, the construction of two new digesters, a pre-pasteurization unit, and the rehabilitation of the existing digesters came online in late 2005. The facility is producing Class A Exception Quality biosolids, which are currently being reused on farms around the state.
Despite the recently completed upgrades, the Alexandria Sanitation Authority is already planning for future upgrades to its Advanced Wastewater Treatment Facility.
“We currently have a design that has been sent out for bid to upgrade the plant to meet the Chesapeake 2000 Agreement,” said St. Amant. “The current contract will bring the plant to a TN of 3 mg/L for current flows, and another contract will follow to upgrade the plant to meet a TN of 3 mg/L at design flows by 2010.”