Catalytic destruction of monochloramine using granular activated carbon for point of use applications
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Chloramines are used for disinfection in many water treatment facilities because of their ability to provide residual protection of water supplies while minimizing the formation of disinfection-by-products. However, chloramines can impart taste and odor to the water, which can lead to customer complaints. Furthermore, the removal of monochloramine from water is essential for certain industries. Previous research at the University of Texas at Austin has demonstrated the potential of several granular activated carbons (GAC) for removal of monochloramine under conditions typical of water treatment plants. The goal of this research project is to further quantify steady-state monochloramine reduction in fixed bed reactors (FBR) with three commercially available GACs, and improve the understanding of the physical and chemical properties that influence removal. The research was divided into 3 phases: 1. A laboratory scale fixed bed reactor experiment was used to quantify steady state monochloramine removal over time. City of Austin tap water viii was used for three GAC types (Jacobi CAT, Norit CAT, Nority CNS) at pH 8 and 9. 2. Physical characterization of each GAC was performed using analysis of nitrogen adsorption isotherms. Specific surface area, pore volume, and pore distribution were determined. Chemical characterization was performed quantitatively using Boehm titrations. Qualitative analysis was performed by analyzing FTIR spectra of untreated activated carbon samples. 3. The Monochloramine Catalysis (MCAT) model was calibrated using results from the Phase 1 and 2 experiments. Simulations of full scale point of use drinking water filters were run for various empty bed contact times and influent monochloramine concentrations. These results were compared against National Sanitation Foundation monochloramine reduction certification criteria. Results show that steady state removal was achieved for all of the activated carbons tested and this removal efficiency can reach nearly 90% using a 0.75-minute empty bed contact time. This steady state performance indicated that catalysis of the monochloramine was occurring, and removal could theoretically occur for very long periods of time. The second stage of the research shows correlation between chemical characteristics (acidity and basicity) and removal efficiency. Furthermore, physical characteristics, mainly micro-porosity, were shown to largely impact performance. Finally, the MCAT model provides a reasonable estimate of steady state removal, and is used to predict full scale point of use performance.