Membrane Treatment - The Answer for Smaller Communities

Authors:

  • David S. Briley, Dwayne Amos - Hazen and Sawyer
  • Pat Irwin - Currituck County

How do you fix a small public utility’s water supply issues that include potential health concerns related to septic fields affecting the surficial aquifer, limited water supply yield, limited treatment capacity, saltwater intrusion, lack of fire protection in some areas, and poor-quality potable water? Currituck County, located in the northeastern portion of North Carolina, elected to implement the use of reverse osmosis (RO) or hyper filtration at both of its water treatment plants (WTPs) to provide a reliable barrier of protection against contaminants to provide sustainable potable water for its customers. The concept has proven effective and has resolved many of the concerns facing the County’s customers.

This paper will explore the advancements in the membrane process including chemical pretreatment, element selection and cleaning experience of the membrane system. Finished water quality comparisons are made which demonstrate how the County was able to improve finished water quality with the addition of RO. Furthermore, this paper will address the changing permitting associated with the concentrate disposal outfalls. Selection of the discharge site and permitting for the RO concentrate discharge was a long, multi-step process and was the critical component for both WTP projects.

Currituck County owns and operates two water treatment facilities, the Southern Outer Banks WTP and the Mainland WTP. The Southern Outer Banks WTP was completed in Spring of 2005 with an initial capacity of 2 mgd. A current project will increase the treatment capacity to 3 mgd including provisions for future expansion to 5 mgd. The Southern Outer Banks WTP includes two separate process trains: a conventional process train and a reverse osmosis (RO) train. The County manages two wellfields for its raw water supply. The lower total dissolved solids (TDS) surficial aquifer is processed by the conventional treatment train, while the higher TDS brackish Yorktown aquifer is treated by the RO process train. The County blends the filtered water with RO permeate prior to post-treatment to improve the overall water quality.

A relatively new concept in North Carolina is the use turbochargers. By taking advantage of advancements in membrane technology and the reduced energy requirements of first stage elements, use of turbochargers has proven to be a cost-effective solution for interstage pressure boost to the second stage. The turbocharger simply uses the excess residual concentrate pressure of the first stage elements to increase the driving pressure in the second stage. A unique feature of the Southern Outer Banks WTP is its disposal of RO concentrate. This facility uses an ocean outfall (i.e., subterranean outfall) that discharges through two HDPE screens located approximately 250 feet offshore. It is the only subterranean ocean outfall permitted in North Carolina for RO concentrate discharge.

Increased water demands and low-yielding surficial aquifers led Currituck County to invest in a new 1.5-mgd RO water treatment facility at the Mainland WTP. To meet the water supply needs of its customers, the County expanded its existing water supply into the Yorktown aquifer, which is a high-yielding aquifer with high chloride concentrations. The RO system will have an initial capacity of 1.5 mgd, with plans for a 5.0-mgd buildout. At buildout, the plant will produce 1.67 mgd of RO concentrate. The RO concentrate discharge location is at the mouth of the North River near the confluence with the Albemarle Sound, approximately 21 miles from the WTP. The RO concentrate line includes a 2,000-foot HPDE pipeline installed beneath the North River via horizontal directional drilling, and a 12-nozzle diffuser header anchored on the river bottom.

For more information, please contact the author at dbriley@hazenandsawyer.com.

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