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Hygienic safety of recycled handwashing water: disinfectant stability and disinfection efficiency

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Monday, November 14, 2022, 3 pm

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This is a past event.

Environmental Engineering Graduate Seminar

Tianqi Zhang, EPFL Switzerland 

In low- and middle-income countries where healthcare facilities and schools are not connected to functional water supply systems, stand-alone handwashing systems provide a solution to safely recycle and reuse handwashing water. Here, we designed a handwashing system that uses a two-step treatment (ultrafiltration + chlorine disinfection) for handwashing water reuse. This seminar talk is focused on disinfectant stability and disinfection efficiency in handwashing water.

Based on a survey, the compounds with amide structures abundantly present in handwashing waters from the use of soaps could control the decay of chlorine on a long-term scale. In this study, we selected six different amides as surrogates (acetamide, benzamide, N-methylformamide, N-methylacetamide, N-methylbenzamide, and N-propylbenzamide) and investigated the kinetics and mechanisms of the reactions of amides with chlorine. Hypochlorite (‒OCl) dominates the reactions of amides with chlorine (apparent second-order rate constants at pH 8 were in the range of 6.6×10-3–0.23 M-1s-1). The reaction products of chlorinated amides were observed to react with electron-rich moieties (e.g., phenolics) at similar second-order rate constants as chloramines. Furthermore, the activation energies of the reactions of amides with chlorine were obtained by temperature-dependent reaction kinetics. Based on these results, the chlorine stability in the presence of amides can be modeled as a function of pH and temperature. Currently, chlorine disinfection efficiency using Bacillus Subtilis spores as surrogate microorganisms in the presence of surfactant (from hand soap) is under assessment. The knowledge obtained in this study will provide guidance to apply handwashing systems for safe water reuse.

Dr. Tianqi Zhang held two bachelor’s degrees in Chemical Engineering and Applied Mathematics from Dalian University of Technology in China. Then, he moved to Arizona and obtained his master’s and PhD degrees in Chemical Engineering from the University of Arizona. Right after, he did his post-doc at Georgia Institute of Technology where he was selected as an ORISE Fellow. Currently, he is a post-doc at EPFL in Switzerland. Dr. Zhang’s research interest is focused on environmental and oxidation chemistry, especially in potable and non-potable water reuse.

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