Granular Activated Carbon for RWSA - Charting New Territory through Hybrid Treatment
- Mark Bishop, Erik Rosenfeldt - Hazen and Sawyer
With the implementation of the Stage 2 disinfectants/disinfection byproducts (D/DBP) rule, utilities are evaluating advanced treatment techniques to control DBPs and their precursors, such as organic matter. After a detailed evaluation process and public input period, the Rivanna Water and Sewer Authority landed on the use of Granular activated carbon (GAC) adsorption, a treatment option that has been proven effective for removal of organic matter and reduction of disinfection byproduct (DBP) precursors. In order to ensure GAC was implemented in a manner that ensured high quality water production (80% of the MCL) in the most fiscally responsible manner, RWSA and Hazen and Sawyer embarked on an unprecedented evaluation of hybrid implementation of GAC to achieve these goals.
Pilot testing with six inch diameter columns, and rapid small scale column testing (RSSCTs) were undertaken to determining GAC life and effectiveness, and an innovative design tool was developed in order to convert this information along with historical water quality records to predictions of future DBP levels. The approach allowed for a complete understanding of water quality implications associated with full GAC installation as well as several hybrid options, including :
• partial treatment with blending of GAC treated and filtered water,
• seasonal treatment with winter shutdowns to save building costs,
• GAC treatment initiated by a filtered water “TOC trigger” threshold.
The comprehensive evaluation was used by RWSA staff as a tool to assess the benefits of hybrid options in terms of water quality and costs, and evaluate the merits of various alternatives. By exploring an innovative technical solution, encouraging public involvement, and implementing a transparent decision making process, the RWSA was able to build consensus among public and stakeholders. The study, concluded that a hybrid approach, with less than full plant capacity, could save capital cost and still meet water quality objectives for disinfection byproducts under a range of source water quality conditions.
The paper and presentation will provide attendees with new and unique information on the application of GAC contactors for DBP control in drinking water. GAC hybrid design alternatives and concepts will be discussed as well as capital and operating cost comparisons. Practical operational considerations will also be reviewed for various options.
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Horizons showcases significant water, wastewater, reuse, and stormwater projects and innovations that help our clients to achieve their goals, and can help you achieve yours. Articles are written by top engineers and process group leaders, demonstrating and explaining the beneficial application of a variety of technologies and tools.