Corbalis Water Treatment Plant Stage III Expansion, Herndon, Virginia

Over the last decade, Fairfax Water has been undertaking a significant capital investment to increase its water treatment capacity to meet growing customer demands. Fairfax Water provides drinking water on a retail and wholesale basis to over 1.5 million Northern Virginians; one out of every five Virginians receiving public water. By expanding the Corbalis Water Treatment Plant (WTP), sourced by the Potomac River, from 150 million gallons per day (mgd) to 225 mgd, Fairfax Water will be able to meet future customer demands.

The Corbalis WTP produces outstanding water quality with treatment that includes coagulation, flocculation, sedimentation, settled water ozonation, dual media filtration (including granular activated carbon), free chlorine disinfection and ammonia addition to provide a combined chlorine residual in the distribution system. The expansion of the Corbalis WTP is the second expansion in capacity since the plant’s original construction in the early 1980’s. The Stage III expansion increased the plant’s capacity by 75 mgd and included several major process improvements, such as improvements to the ozone generation and diffusion system, conversion from gaseous chlorine to liquid sodium hypochlorite disinfection, installation of plate settlers for treatment of recycled plant process water, installation of an underground dedicated power feed to increase electrical reliability, and a new plant-wide computer based process control system. Prior to the development of the final design, Fairfax Water, with its consulting engineer, Camp Dresser & McKee (CDM) conducted detailed water quality and treatability evaluations. An ozone system upgrade analysis was performed to evaluate the use of ozone for primary disinfection (to meet LT2 regulations). This analysis included bench-scale studies under different water quality conditions to determine ozone demand and decay characteristics for settled water ozonation. This information was used to develop a new approach for optimizing the design of ozone contactors to meet EPA CT-product values for Giardia and Cryptosporidium inactivation. Computation Fluid Dynamic analysis was also performed on the existing ozone contactors, which resulted in cost-effective baffle modifications to increase ozone transfer within the contactors. The design of the Stage III facilities was developed to seamlessly integrate into the existing plant while maintaining an uninterrupted drinking water supply to Fairfax Water’s customers. A competitive construction bid in the amount of $160,079,000 resulted in a contract award to Pizzagalli Construction Company headquartered in Burlington Vermont.

Major systems included in the Stage III project included:

1. Expansion of the raw water control chamber, including automated raw water flow control, chemical addition and mixing
2. A new plant-wide chemical delivery systems which included using the existing centralized bulk storage with new loop piping system to feed pumps at each application point.
3. A complete conversion from gaseous chlorine to bulk delivery liquid sodium hypochlorite.
4. Flocculation-sedimentation basins with 75-mgd capacity, including a new ozone contractor
5. Expansion of the ozone system with two additional 1,500-ppd generators and an additional 26,000 gallons of LOX storage
6. Two 60-mgd raw water pumps
7. Two 33-mgd finished water pumps
8. A new new 75-mgd filtration building housing eight dual-media (GAC/sand) filters
9. A new 14 million-gallon underground clearwell and modification to two existing clearwells to provide 28 million gallons of on-site finished water storage.
10. A new recycle treatment system, which included a new plate settler system, two decant basins and detention basin.
11. Two new solids dewatering belt filter presses with a new polymer feed system
12. Miscellaneous renovations to existing facilities to accommodate the expanded conveyance, storage, and treatment systems at the Corbalis WTP.
13. Complete replacement of the existing UNIX-based distributed architecture, process control system components.
14. Expansion and upgrade of electrical power and mechanical systems.

A partnered effort between Fairfax Water, the design engineer (CDM) and the contractor (Pizzagalli Construction Company), has led to the successful operation of the largest water treatment plant in the Commonwealth of Virginia. Major facilities and capacity were delivered to Fairfax Water on schedule with a total construction cost less than 1% above the original construction contract value.