Issue

The 53 mgd Central Arizona Project (CAP) Water Treatment Plant, which is currently being expanded to treat 75 mgd, is the largest water treatment plant in Scottsdale, Arizona.  Water from this plant can travel more than five days to the farthest reaches of the distribution system. Stage 1 and Stage 2 of the Disinfectants/Disinfection Byproducts Rule (D/DBP Rule) reduce acceptable distribution system concentrations for total trihalomethanes (TTHMs) and five haloacetic acids (HAA5) and change the way they are monitored. 

Meeting these requirements means treatment processes must be modified to reduce free chlorine contact time, reduce concentrations of DBP-forming organics, or both.  Complicating the problem further is the concern that converting secondary disinfection from free chlorine addition to chloramination raises the potential for biologicial regrowth and nitrification in a warm-weather distribution system.   

Solution

Scottsdale conducted an extensive integrated water master plan that recommended a post-filtration granular activated carbon (GAC) adsorption process at its CAP WTP to meet the Disinfectants/Disinfection Byproducts Rule Requirements. Malcolm Pirnie was selected to provide engineering services from preliminary design through construction for a GAC facility sized to treat the ultimate plant capacity. 

The project built upon Malcolm Pirnie’s experience from design and construction of the award-winning 175 mgd GAC facility at the Richard G. Miller Water Treatment Plant for the Greater Cincinnati Water Works. Malcolm Pirnie also enlisted Cincinnati’s GAC facility operations staff to help optimize the design. Construction will be completed in early 2008. Key features of the project include:

• Diversion Pumping of Process Flow.  The GAC facility would not fit within the existing plant hydraulic profile, so a diversion structure and pump station were included.  The diversion structure allows the GAC facility to be totally bypassed as water quality dictates, reducing operations and maintenance costs for GAC replacement. The pump station combines diversion, GAC backwash, and service water pumping systems in one location, utilizing water treated by the plant’s filtration facilities. Flows to the GAC post-filter contactor facility can range from 10 to 75 mgd.
  
• Post-filter GAC Contactor Facility. The facility includes 11 GAC post-filter contactors, each containing 11.3 feet of GAC bed depth (390,000 pounds), providing an empty bed contact time (EBCT) of 20 minutes at design flow. A stainless steel underdrain system like Cincinnati’s is utilized. To prevent draining and air entrainment in the media, a seal well provides a failsafe way to maintain the water level in the contactors.
• Contactor-to-Waste Capability. Like a granular media filter designed for turbidity removal, the post-filter GAC contactors were designed with contactor-to-waste capability to avoid the possibility of a turbidity spike occurring after backwash events.
• Backwash and Contactor-to-Waste Equalization and Pumping. Contactor-to-waste and backwash water flows can be equalized and either sent to the residual handling facilities or recycled to the head of the plant.
• GAC Media Replacement Provisions. The facility design includes a network of fixed GAC transport eductors, piping, valves, and loading stations for hydraulic transfer of media, and the site layout accommodates staging of multiple tanker trucks for GAC delivery and spent GAC removal.  Truck drainage and washdown water is fully contained and sent to the equalization facilities. A truck scale was also provided for verifying load weights.

Benefits

The $42 million GAC facility puts Scottsdale ahead of the curve in responding to the requirements of the Disinfectants/ Disinfection Byproducts Rule and substantially eliminates the potential for seasonal taste and odor issues at the city’s largest WTP.