Making the Right Decision: How One County is Meeting Aquifer Requirements with Membrane Treatment
Prepared for the 2013 NC AWWA-WEA Annual Conference by Stephanie H. Kellogg, EI, Engineering Intern; Charles D. Riley, Jr., PE
In the Central Coastal Plain of North Carolina, groundwater from the Cretaceous aquifer system is being withdrawn at a rate that exceeds the available recharge. In 2002, the Central Coastal Plain Capacity Use Area (CCPCUA) regulations were established to reduce existing groundwater withdrawals from the Cretaceous aquifer system. The regulations call for a 25 percent reduction every five years over a 15- year period, equivalent to an overall withdrawal cutback of 75 percent. This applies to wells located in the dewatering and saltwater encroachment zones of the Central Coastal Plain.
Before the CCPCUA rules were established, Craven County’s water supply system relied heavily on groundwater wells that drew from the cretaceous Black Creek Aquifer and Pee Dee Aquifer. Craven County was initially permitted to withdraw an average of 2.69 MGD from these aquifers. However, under the CCPCUA rules, the County’s 2018 total permitted raw water supply capacity would be reduced to 1.39 MGD. Population projections and corresponding water demand projections indicated an average day demand of 2.62 MGD in 2018. If the County took no action, it would experience a raw water supply shortfall of approximately 1.23 MGD.
McKim & Creed and Craven County teamed to evaluate the County’s raw water deficit and to develop several options for comprehensive new potable water infrastructure. Craven County decided to pursue a new raw water supply from the Castle Hayne Aquifer and to utilize membrane treatment. Engineers conducted a pilot membrane system and used the results to predict the expected permeate water quality, fine-tune the system, complete toxicity testing and mixing models for the concentrate, and perform membrane autopsies to analyze scale and fouling. This data is now being used in the design of the new potable water supply and treatment facilities.
In this presentation, we will discuss the process that led to Craven County’s decision to pursue the new raw water supply and membrane treatment. We will address the specific water-quality issues that Craven County faced, including the presence of organic carbon and high iron, as well as hardness (calcium and silica) and alkalinity concentrations. We will review the results from the pilot plant operation and discuss how the obstacles typically associated with membrane treatment will be overcome in the design for the new Craven County Potable Water Supply and Treatment Facilities.