Ion Exchange and Disposal Issues Associated with the Brine Waste Stream
- Julie Karleskint, Daniel Schmidt, Robert Anderson, and Jayson Page - Hazen and Sawyer
The City of Arcadia, Florida has a 3 MGD Lime Softening Water Treatment Plant that is over 40 years old. The plant treats a groundwater source that exceeds water quality standards for radionuclides. The groundwater also has higher concentrations of hardness, sulfides, organic carbon and fluoride than desirable. Currently, treatment is primarily focused on meeting the radionuclides standards by raising the pH to 8.5 with lime, followed by settling and filtration. Since the plant has reached the end of its serviceable life, treatment alternatives were evaluated to determine the most cost effective treatment option for replacing the existing treatment plant. A Facility Use Plan was prepared that evaluated different options including lime softening, ion exchange, nanofiltration and purchasing water from the local water supply authority. In evaluating these options, ion exchange was determined to be the most cost effective option and could enhance water quality in the process.
A bench scale pilot study was performed on the source groundwater using both cation and anion exchange columns. The pilot test was set up to determine the effectiveness of the anion and cation exchange resins, efficiency of brine regeneration with time, vessel headloss as a function of time in service and the approximate run time for both ion exchange resins. The pilot test verified that the ion exchange system will produce a quality effluent that meets the design criteria at reasonable ion exchange vessel loading rates, run times and regenerant waste stream volumes. It was also determined that water quality goals could also be achieved by blending a portion of raw water with water treated via ion exchange to reduce the amount of ion exchange regenerant produced for disposal in the wastewater collection system. Reducing the volume of regenerant waste was a pivotal factor in the selection of the ion exchange process.
Since the wastewater plant discharges to a reclaimed water system, there was concern that the ion exchange regenerant waste stream salt ion concentrations could adversely impact local groundwater quality. An evaluation determined that local groundwater quality standards should not be exceeded provided the regenerate waste stream to the wastewater collection system was kept under key limits. The evaluation showed that the total volume limit could be maintained by blending raw and ion exchange treated water streams which extended ion exchange column run times and reduced the total volume of ion exchange regenerant waste generated per day. In addition, by implementing blending of raw and ion exchange treated water streams and controlling the discharge rate of ion exchange regenerant waste to the wastewater collection system, the City can produce a drinking water that meets water quality standards and improve the current water quality while also meeting local groundwater water quality standards.
Based on the pilot test results and preliminary engineering study, it was determined that a 1.5 mgd cation and anion ion exchange facility would be constructed with the capability of blending raw and ion exchange treated water streams followed by disinfection, storage and high service pumping. Additional treatment for pH adjustment to stabilize the water and control corrosion will also be provided. The ion exchange regenerant waste will be captured and stored in tanks on-site and slowly pumped under control to the wastewater collection system to meet local groundwater quality standards. This was determined to be the most cost effective treatment option for the City and they are proceeding with the construction. This paper will examine the treatment options evaluated and blending ratios identified for meeting both drinking water and groundwater quality standards for the regenerant waste stream.
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