Back to the Future: Evaluation of NF Membranes for a Return to Free Chlorine
Last Modified: Jan 28, 2019
- Kristen E. Cope EIT, Robert Boysen PE, Marc Santos PE - Hazen and Sawyer
- Rafael Martinez, Gabriel Ramirez, Crystal Ybanez - City of Corpus Christi
The City of Corpus Christi (Corpus Christi) operates the O.N. Steven’s Water Treatment Plant (ONSWTP), which is a surface water treatment plant providing drinking water to nearly 500,000 residents through 1,700 miles of pipe with a maximum capacity of 161.5 million gallons per day (MGD). In 2016, Corpus Christi, experienced low chloramine residuals in their drinking water distribution system due to nitrification, but more recent operational changes and upgrades have markedly improved water quality and chloramine residuals. The previous challenges with nitrification have led the City to investigate what treatment technique changes would be required to return to free chlorine as the final disinfectant rather than chloramine to help address potential future regulations on chloramine disinfection byproducts.
The ONSWTP has three raw water supplies, the Nueces River, Lake Texana, and the Colorado River, that are blended at different levels before being introduced into the ONSWTP treatment process. The existing process train is as follows:
1. Surface water sources are blended at the raw water junction box located at the front end of the treatment plant.
2. Blended raw water is then conveyed to a presedimentation basin where it is treated with sodium permanganate for pre-oxidation. A ClO2 system was added in 2017 after the presedimentation basin for the benefits related to lowering disinfection byproduct (DBP) formation and the chlorite residual’s benefits in nitrification prevention.
3. The pre-oxidized water is then pre-chloraminated at the receiving unit.
4. The pre-chloraminated water is split and processed through two parallel treatment plants (referred to as Plant 1 and Plant 2). Both plants utilize conventional treatment, including coagulation (with alum as the primary coagulant), flocculation, 2-stage sedimentation, and dual-media filtration.
5. Additional chlorine and liquid ammonium sulfate (LAS) (as needed) are applied after filtration for secondary chloramination prior to the clearwells.
Like other systems with high total organic carbon (TOC), high summer temperatures, and long distribution system residence times, elevated levels of regulated Disinfection Byproducts (DBPs), including trihalomethanes (THMs) and haloacetic acids (HAA), are formed when free chlorine reacts with the organics present in Corpus Christi’s surface water sources. Therefore, an additional treatment technology was needed if the City were to make the switch back to free chlorine.
A Desktop Evaluation in Fall 2016 was conducted to assess feasible options that would allow the ONSWTP to permanently switch from chloramine disinfection to free chlorine disinfection while simultaneously meeting the Stage 2 Disinfectant/Disinfection Byproduct (D/DBP) Rule. The evaluation included a review of historical raw water quality, a review of DBP levels during temporary, system-wide conversions to free chlorine disinfectant, and bench-scale testing and performance projection modeling of TOC removal for each alternative.
Based on the results of the evaluation, which considered several factors, including effective DBP precursor removal, finished water quality, general site footprint, operational implications, and cost, post-dual-media filter granular activated carbon and nanofiltration and reverse osmosis (NF/RO) membrane processes were identified as potentially viable alternatives for the ONSWTP. Pilot testing was requested for these alternatives to evaluate the efficacy of GAC and membrane treatment to reduce TOC concentration in the treated water with the goal of sufficiently reducing the DBP formation potential, such that free chlorine could be used in the distribution system.
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