Effect of Pre-Chlorination on GAC and Emerging and Regulated DBP Formation and Reduction
- Allison Reinert, Ben Stanford, Scott Alpert - Hazen and Sawyer
- Chad Seidel - Corona Environmental Consulting, LLC
With the USA EPA considering future disinfection byproduct (DBP) rules, there is a driver to investigate the implications of current strategies, such as granular activated carbon (GAC), treatment as well as disinfectant choice and application on emerging and regulated DBPs. Over the past 30 years, significant efforts have been directed toward increasing the understanding of DBP formation, occurrence, and health implications. In the suite of DBP rule compliance strategies, raw or settled water chlorination typically is shunned due to the potential to form high concentrations of THMs and HAAs in the plant. However, settled water chlorination was shown at two Arizona water utilities to extend GAC service life while also producing fewer THMs in the distribution system. However, little is known about how pre-chlorination strategies would apply in other water sources, how it effects the adsorptive removal of pre-formed unregulated DBPs, and whether or not it reduces the formation of unregulated DBPs in the distribution system. As such, the principal objective of this presentation will be to provide a case study example where we evaluated the impact of raw water chlorination followed by GAC treatment on the formation of regulated and unregulated DBPs under various source water quality conditions. The focus will be on the results from a pilot-scale study in North Carolina, with background information on the Arizona utilities and their use of a pre-chlorination strategy in combination with GAC.
The use of GAC for the removal of regulated DBP precursors has been extensively studied in previous work. However, this research focuses on the use of raw water chlorination and pre-filter chlorination to pre-form DBPs prior to GAC treatment. The pilot study at the North Carolina surface water plant consist of the evaluation of four different GACs and two different empty bed contact times, with and without pre-chlorination ahead of GAC. When pre-chlorination was used, influent water to the pilot-scale columns contains approximately 0.5 ppm free chlorine. The full-scale pre-chlorination study at the Arizona utilities, however had no chlorine residual in the influent water to the GAC columns. As such, the impact of chlorine concentration on the integrity of the GAC as well as the efficiency of removing preformed DBPs will be compared and contrasted. Both the pilot-scale study and the full-scale study will be used to test the hypothesis that raw water or settled water chlorination prior to GAC will favor a lower total DBP formation potential in the GAC during the early stages of operation, but the effects of carbon exhaustion and breakthrough may limit the GAC’s ability to remove preformed DBPs along with dissolved organic carbon over time.
The results found within this study will help to provide utilities, regulators, and consultants with a better understanding of the implications of GAC use for the control of regulated and emerging DBPs. Also with a firm understanding of how pre-oxidation affects GAC service life and efficiency, additional evaluations of design and cost implications for using GAC to maintain DBP compliance can be obtained.
For more information, please contact the author at email@example.com.
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