Atmospheric Pressure Plasma Device With Secondary Flow for Gas-Phase Modification and Selectivity Control in Point-of-Use Water Disinfection Applications

The World Health Organization estimates that 25% of the human population—over 2 billion people—are relying on contaminated water sources and that 3.4 million individuals succumb annually to illnesses attributed to waterborne pathogens that result from insufficient water treatment infrastructure. To address this need, we are developing a plasma disinfection applicator that can be used on demand at the point of use. In this device, plasma chemistry can be controlled by varying pulse frequency, voltage, and two separate gas flows—both of which are air. Here, the destruction of the Escherichia coli (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E. coli</i>) pathogen and bacteriophage MS2 is targeted. Using the plasma applicator, a 6 log reduction in <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E. coli</i> was realized in a 200-mL treatment volume after 4 min of treatment at relatively low power (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$16.8~\pm ~1.3$</tex-math> </inline-formula> W delivered to plasma), and a 6 log reduction of MS2 was realized in 100-mL treatment volume. An important consideration for water purification using plasma devices is that the final product is safe to drink. To that end, an attempt was made to understand the plasma parameter space that maximizes the long-lived disinfection products while minimizing harmful nitrogen byproducts with the aspirational goal of keeping them below EPA maximum contaminant levels (MCLs). A wide sweep of voltages, frequencies, and gas flows was performed in order to determine the optimal operating conditions to maximize the production of ozone (O<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>) and hydrogen peroxide (H<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>O<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>) and to minimize the production of undesirable products nitrite (NO<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${}_{2}^{-}$</tex-math> </inline-formula>) and nitrate (NO<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${}_{3}^{-}$</tex-math> </inline-formula>). Plasma was generated utilizing two different voltage sources: a laboratory dc nanosecond pulser and a low cost commercially available ac power source. In order to compare the performance of the two power sources, emission spectroscopy was performed to analyze the gas-phase chemistry, and colorimetric absorption spectroscopy was performed to analyze the liquid-phase chemistry.