IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 40, NO. 5, MAY 2012 1311 Perspectives on the Interaction of Plasmas With Liquid Water for Water Purification John Foster, Bradley S. Sommers, Sarah Nowak Gucker, Member, IEEE, Isaiah M. Blankson, and Grigory Adamovsky Abstract—Plasma production or plasma injection in liquid wa- ter affords one the opportunity to nonthermally inject advanced oxidation processes into water for the purpose of purification or chemical processing. Such technology could potentially revolution- ize the treatment of drinking water, as well as current methods of chemical processing through the elimination of physical cata- lysts. Presented here is an overview of current water treatment technology, its limitations, and the future, which may feature plasma-based advanced oxidation techniques. As such, this field represents an emerging and active area of research. The role that plasma-driven water chemistry can play in addressing emerging threats to the water supply is discussed using case study exam- ples. Limitations of conventional plasma injection approaches in- clude limited throughput capacity, electrode erosion, and reduced process volume. At the University of Michigan, we are inves- tigating two potential approaches designed to circumvent such issues. These include direct plasma injection using an underwater DBD plasma jet and the direct production of plasmas in isolated underwater bubbles via a pulsed electric field. These approaches are presented here, along with the results. Index Terms—Atmospheric pressure plasmas, high voltage techniques, organic compounds, plasma applications, plasma chemistry, waste water, water pollution, water pollution control. I. I NTRODUCTION O N July 28, 2010, the United Nations passed a resolution declaring access to clean drinking water a basic human right [1]. The National Academies have listed access to clean drinking water as an Engineering Grand Challenge, advocat- ing the need for the infusion of new technologies to address this worldwide problem [2]. These gestures are timely in that the projected world population is expected to grow to nearly 10 billion by the year 2050. With this increased population growth, an associated growth in industry and agriculture is anticipated—with all segments over time requiring an increas- ing allotment of freshwater. Such increasing demand on fresh water reserves is problematic as the reserves themselves repre- sent a fixed quantity. In this regard, a delicate balance must be achieved between maintaining existing water reserves in order to address industrial and agricultural requirements—which are Manuscript received August 31, 2011; revised October 28, 2011; accepted December 4, 2011. Date of publication April 16, 2012; date of current version May 9, 2012. This work was supported by the NSF CBET #0939879 and the NASA Glenn Faculty Fellowship Program. J. Foster, B. S. Sommers and S. N. Gucker are with the Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109-1005 USA. I. M. Blankson and G. Adamovsky are with NASA Glenn Research Center, Cleveland, OH 4413 USA. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPS.2011.2180028 driven in turn by development and population growth—while at the same time addressing domestic consumption needs. One method of achieving these ends is the infusion of technologies that support water recycling. Recycling, in this context, refers to the reuse of treated industrial and agricultural wastewater for the purpose of supplying water needs as well as poten- tially recharging aquifers. For such a recycling approach to be successful, new technologies will be required to remove harmful contaminants and to monitor the overall health of water streams. The purpose of this paper is to describe the current state of water purification technology by defining the problem, listing the shortcomings of current water treatment technologies, explaining the role of advanced oxidation meth- ods in the future of water treatment, discussing plasmas as a source of advanced oxidation processes including example cases and ongoing related work at the University of Michigan, and suggesting prospects for the future. II. CONVENTIONAL WATER TREATMENT TECHNOLOGIES In order to realize large-scale wastewater reuse and thus relieve stress on freshwater reserves, wastewater must be ren- dered usable after processing. Existing water treatment tech- nology for a typical city water treatment plant focuses on filtration and disinfection. Particulates are filtered from input water streams via a multistep process in which chemical coagu- lation agents are added to the water to encourage the formation of larger particulates, which are then removed via sedimenta- tion and subsequent filtration. This water is then disinfected, typically with chlorine precursors, ozone or more recently UV light [3]. Fig. 1 schematically shows the inner workings of a water treatment plant. In this regard, conventional water treatment addresses only particulates and bacteria. Industrial and agriculturally derived wastewater contains a host of toxins that are not directly addressed by conventional water treatment. Of particular concern are volatile organic compounds (VOC). These organic compounds can concentrate in air as well as in water. Because of this property, VOCs can migrate into drinking water sources and persist there. These toxins are of particular concern as they can contaminate surface and aquifer water sources. These toxins have been linked to a host of human health impacts ranging from damage to the circulatory system to the digestive and nervous systems. VOCs have been detected in over 51% of all US aquifers from which drinking water is derived. The US EPA has set maximum contami- nant levels for a number of these toxins [4]. These compo- nents cannot be removed from water using conventional water 0093-3813/$31.00 © 2012 IEEE