Environ. Sci. Technol. 2001, 35, 3780-3785
Dense Medium Plasma Environments: A New Approach for the Disinfection of Water S. MANOLACHE,† E. B. SOMERS,‡ A. C. L. WONG,‡ V. SHAMAMIAN,§ AND F . D E N E S * ,†,# Center for Plasma-Aided Manufacturing, Food Research Institute, and Department of Biological Systems Engineering, University of WisconsinsMadison, Madison, Wisconsin 53706, and Naval Research Laboratory, Washington, DC, 20375
The levels to which microbial colony forming units are permitted in various waters fit for human contact are carefully regulated. Conventional chemical and physical approaches usually are complex processes with significant limitations due to the generation of toxic side-products. In this contribution a novel plasma reactorsdense medium plasma reactors is described, and its efficiency for the disinfection of contaminated water is discussed. It has been shown that owing to the intense stirring of the reaction medium (e.g. contaminated water), as a result of the specially designed spinning electrode and gas-flow system, a volumecharacter discharge is created, which can efficiently kill bacteria. It has been demonstrated that treatment times as low as 20 s are enough for the total inactivation of microorganisms for 200 mL of 105 bacteria/mL contaminated water.
Introduction Decontamination and disinfection of potable water, water used in food-processing industries, and water frequently in contact with human beings (e.g. water in swimming pools and spa pools), are major health issues currently under intense scrutiny due to heightened public awareness. We define decontamination as attenuation of recognized toxic chemicals such as pesticides, and effluent from industrial and residential locations, to levels permitted by legislated regulations. Attenuation can be in the form of complete removal via oxidation to volatile species such as CO/CO2 for organic compounds or chemical conversion to species which are considered permissible in much higher concentrations. Disinfection is defined as the killing or inactivation of diseasecausing organisms. The levels to which microbial colony forming units are permitted in various waters fit for human contact also is regulated carefully. Conventional approaches employed for inactivation of toxins such as hydrolysis, electrochemical oxidation, solvated electron technology, plasma arcs (1) and approaches for the disinfection of water including chemical (chlorination: chlorine, chlorine dioxide, chloramines, ozonization etc.) and physical (UV radiation including solar radiation, high-energy irradiation, ultrasound * Corresponding author phone: (608)262-2182; fax: (608)262-3632. † Center for Plasma-Aided Manufacturing, University of Wisconsins Madison. ‡ Food Research Institute, University of WisconsinsMadison. § Naval Research Laboratory. # Department of Biological Systems Engineering, University of WisconsinsMadison. 3780
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treatment, ultrafiltration, heating, freezing, reverse osmosis) approaches, or the use of their combinations, are complex processes with significant limitations related to the generation of toxic side-products (e.g. chlorinated hydrocarbons known as trihalomethanes which are suspected carcinogens) or low efficiencies for large scale applications. Technologies based on atmospheric pressure plasma environments can present an attractive alternative for the decontamination and disinfection of water. Plasma-enhanced surface modification (functionalization, deposition, and etching) processes have become major technologies for a number of industries, and, in some of the applications (e.g. microelectronics), they represent the only option. Most of these processes were developed for operating under low pressure environments, which are plagued by requiring the use of complex vacuum systems, batch-type-mode operation, and difficult robotics handling. These characteristics make “conventional” plasma-technologies economically viable only for applications where economies of scale processes are targeted for the creation of high-value added items. The use of partial discharge (PD; nonequilibrium, low temperature atmospheric-pressure plasmas), such as corona, pulsed corona, discharges, streamers, barrier (silent atmospheric pressure plasmas) discharges, etc., for the surface modification of materials and for volume-plasma-processing of liquid-phase compounds (e.g. water-based solutions), has become an increasingly active research area in recent years. Atmospheric-pressure-plasma approaches might eliminate some of the shortcomings of low-pressure discharge technologies and would allow the development of plasmaenhanced batch-type and continuous-flow system technologies. A PD is considered a highly localized (confined) electrical discharge produced across an insulating medium, in most of the cases, between two electrodes (2). These discharges are very complex phenomena and exhibit nonstationary behavior with different time-dependent characteristics. Their high variety of modes of appearance (low field drift region, ionization region) and the unpredictable transitions which accompany them are also related to the large variety of reactor and electrode geometries and electrode and reactor building materials under which these discharges can be initiated and sustained. In addition changes in the behavior of PD as a result of the PD-induced modifications of the discharge exposed surfaces can occur. PDs can also exhibit pulsating or quiescent behavior. The pulsating phenomenon is related to the build-up of surface or space charges usually in the presence of dielectric surfaces. These charged zones decrease the electric-field strength below the level necessary to sustain the ionization. Corona discharges are a special category of PD that occur in the absence of a solid insulating layer in the discharge zone. Depending on the polarity of the voltage applied to the stressed electrode (determined by its geometrical shape) and the driving electric field, negative and positive, bipolar, AC, and high frequency (HF) coronas can be defined (3). These discharges behave differently due to the different mechanisms of initiation and sustaining of the discharge (e.g. secondary electrons generated on different pathways). Unipolar coronas (positive glow coronas, negative Trichel pulse coronas and negative glow coronas) are characterized by a close vicinity of the ionization region to the stressed electrode and by predominant ions of corona polarity. Streamer (bipolar) coronas are associated with point-to-plane electrode configurations and usually have a positive point polarity. The plasma produced under these conditions cannot be “ab10.1021/es010704o CCC: $20.00
2001 American Chemical Society Published on Web 08/09/2001
sorbed” fast enough by the point electrode, and as a result an intense, narrow nonstationary plasma stream (streamer) is generated from the point electrode toward the plane counter electrode. Barrier discharges (BDs) are characterized by the presence of one or more dielectric layers intercalated between the electrodes, in addition to the discharge zone (4). Due to the presence of insulating surfaces between the electrodes, these discharges can only be initiated and sustained under AC low (60 Hz) or high frequency (microwave range) conditions. A common driving frequency for operating BDs are between 500 Hz and 500 kHz, and the typical discharge voltages and gaps (the space between the electrodes excluding the dielectric layers) for atmospheric pressure operations are around 10 kV and a few mm, respectively. The atmospheric pressure plasma is initiated and sustained through a large number of independent microdischarges, which mediate the breakdown of the gas molecules at voltages that are independent of the presence or absence of dielectric layers. The dielectric barrier controls the charge distribution on the plasma-exposed surfaces and spreads, as a result the microdischarge channel, into a surface discharge covering a region significantly larger in comparison to the original channel diameter. It was shown that the transported charge by the microdischarge channels is proportional to the gap spacing and the permittivity of the dielectric but does not depend on the pressure. Typical microdischarge characteristics for a 1 mm gap at atmospheric pressure were evaluated as the following: duration: 1-10 ns; filament radius: 0.1 mm; peak current: 0.1 A; current density: 100-1000 A/cm2; total charge: 0.1-1 nC; electron density: 1014-1015 cm-3; electron energy: 1-10 eV; gas temperature: close to average gap temperature. These gas phase discharges have been evaluated extensively for the deposition of macromolecular structures, surface modification of polymeric substrates (5-9), and sterilization of solid surfaces (10-13). It was demonstrated that the original plasma source, a one atmosphere uniform glow discharge plasma is a very efficient tool for the generation of active species in air environments and room temperature, capable of significantly reducing within 15-90 s >105 of a variety of bacteria, including Staphylococcus aureus, Escherichia coli, and endospores from Bacillus stearothermophilus and Bacillus subtilis on various seeded substrate surfaces (fabrics, polypropylene, filter paper, glass, powdered cultured media, etc. (10, 11, 13)). The engineering aspects including energy input, critical electric field strength, treatment time, temperature increase, etc., of pulsed electric field pasteurization has also been evaluated using static and continuous treatment reactors (12). The feasibility of employing pulsed electric field pasteurization systems has been demonstrated. Less data are available on the PD-plasma-enhanced disinfection of water. Destruction of living cells such as Saccharomyces cerevisiae and Bacillus natto was performed under pulsed high voltage cylindrical discharge reactors (14) provided with four different electrode configurations: plateplate (aluminum electrodes, Plexiglas reactor dimensions: ID ) 8 mm, length: 10 mm, peak pulse voltage: 12-20 kV, pulse width: 90 and 160 µs, pulse frequency: 25 Hz); needleplate (aluminum electrodes, identical reactor as in the case of plate-plate system, separation between the needle tip and plate: 9.5 mm, radius of curvature of the needle tip: 0.1 mm in the case of yeast cells dispersed in 1% and 3% NaCl solutions the pulse width decreased to 330 and 90 ns, respectively); wire-cylinder (inner diameter of the cylinder/ reactor electrode: 19 mm, wire electrode diameter: 0.5 mm, length of electrode system: 30 mm, pulse width: 140 µs, in the case of 1% NaCl solution the pulse width was 320 ns); and rod-rod (PVC chamber, thickness: 6 mm, OD of stainless
steel, cone shaped rode electrodes: 4 mm, gap between electrodes: 3 mm). It has been demonstrated that the yeast populations can be destroyed efficiently using the wirecylinder electrode configuration under 20 kV/cm, 140 µs, 250 pulse application number conditions; however, the energy input to a unit volume of cell containing sample for an efficient (106) cell destruction differs significantly with the electrode-type system. It was shown that the wire-cylinder system requires around 10-30 cal/cm3, while the plate-plated system produces similar results at 70 cal/cm3. In the case of rod-rod electrodes, the required energy input is only 5-10 cal/cm3. The pulsed high voltage discharge-mediated formation of chemical species and their effects on microorganisms has also been studied (15). Using a needle-plate electrode system the formation of OH and H free radicals was monitored by optical emission spectroscopy (OES), and the generation of H2O2 was investigated. Both batch- (5 × 5 × 10 cm rectangular reactor, platinum point to plane electrodes, OD of needle electrode: 0.2 mm, gap between the needle and plate electrodes: 5 cm, rotating spark gap with a 50 Hz repetition pulse frequency, pulsed voltage: +14 kV, conductivity of water samples: 1.33 to 1300 µS/cm) and thermostated, recirculation-mode (cylindrical cell, ID: 3 cm, 40 mL, platinum point to plane electrodes, pulse frequency: 50 Hz, pulsed voltage: +19 kV, OD of needle electrode: 0.2 mm, OD of ground-plate electrode: 3 cm, gap between the electrodes: 5 cm, total volume of recirculated water: 60 mL, flow rate: 100 mL/min, temperature of water: 25-30 °C, treatment time: 5 to 30 min) reactors were employed. The emission intensity of atomic H decreased with the increase of water conductivity, while the OH radical intensity recorded a maximum value at 10-5 S/cm. The authors suggested that the H2O2 was generated by the recombination of OH radicals and not by an electrolytic reaction of dissolved oxygen on the negative electrode. It was found that the H and OH radicals were not effective in killing yeast cells in situ; however, the H2O2 generated by the discharge and added ex situ to the cell samples was lethal. In most PD experiments directed for the decontamination and disinfection of water, high voltage, pulsed discharges were involved, which generated filamentary nonstationary discharge channels rendering a very localized nonvolume character. In this contribution a novel discharge reactor is described (dense medium plasma reactor) that allows the development of a multitude of low voltage microdischarges uniformly dispersed in the reaction medium and the efficiency of the DMP environments for the disinfection of water is analyzed.
Methods The experiment comprises two aspects: the quantitative assessment of changes in bacterial numbers and the plasma disinfection tool and procedures. Water was artificially contaminated with bacteria and bacterial concentrations before and after plasma treatment were determined. Sixteen environmental Gram positive and Gram negative bacterial isolates in our culture collection were used. They included species from the following genera: Bacillus, Corynebacterium, Enterobacter, Klebsiella, Micrococcus, Proteus, Pseudomonas, Shigella, and Staphylococcus. Each isolate was inoculated into Trypticase soy broth (TSB; Becton Dickinson, Sparks, MD) and grown overnight at 30 °C. Each culture was then transferred into fresh TSB and incubated for an additional 16-20 h. The 16 cultures were pooled, diluted, and inoculated into filter-sterilized (Corning Filter System, 0.22 µm cellulose acetate filter, Corning Incorp., Corning, NY) Milli-Q (Millipore Corp., Bedford, MA) water (specific conductance 1.037 × 10-6 Ω1- × cm-1 at 25 °C) to achieve a target level of approximately 5 log colony-forming VOL. 35, NO. 18, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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units (cfu)/mL (specific conductance 1.046 × 10-6 Ω1- × cm-1 at 25 °C). A portion of the inoculated water was reserved as the control with no plasma treatment. The remaining water was exposed to plasma for 20, 60, or 120 s. Control and plasma-treated water were assessed for bacterial survival immediately after plasma treatment and after 1, 3, and 7 days of storage at 4 °C. Bacterial numbers were determined by diluting each sample in 0.01 M phosphate-buffered saline (PBS, pH 7.2) and plating the appropriate dilutions in duplicate on brain heart infusion (BHI, Difco, Detroit, MI) agar plates. Plates were incubated at 30 °C for 72 h, and the number of cfus were counted. Each plasma treatment was repeated, and the bacterial counts presented are the average values from both experiments. The relative surface atomic concentrations and the nonequivalent C1s, O1s, and N1s linkages of deposited layers (DL) as a result of storage and standard iron oxide (IO) were carried out using a Perkin-Elmer Physical Electronics 0 5400 small area ESCA system (Mg source; 15 kV; 300 W; pass energy: 89.45 eV; angle: 15 and 45°). Carbon (C1s), oxygen (O1s), and Fe2p1/2 and Fe2p3/2 atomic compositions were evaluated, and the nonequivalent positions of carbon and iron linkages were analyzed. To correct surface-charge-origin binding energy shifts calibrations were performed based on the well-known C1s peak. Fourier transform infrared spectroscopy (FTIR: ATIMattson Research Series IR Instrument) was used to identify the chemical linkages of organic compounds, which accompany the IO and DL. All FTIR evaluations were performed under nitrogen blanket generated from a flow-controlled liquid nitrogen tank. Data were collected in the KBr pellet mode, 600-4000 cm-1 wavenumber region with 250 scans for each sample. All plasma treatments of water samples containing the bacteria have been performed using an original plasma reactor (dense medium plasma reactor: DMP) designed and developed at the Center for Plasma-Aided Manufacturing, University of WisconsinsMadison. The DMP reactor allows the initiation and sustaining of discharges in coexisting liquid/ vapor medium and which may offer a significantly higher efficiency for the processing of liquid-phase materials in comparison to existent plasma technologies. The reactor (Figure 1) is composed of a cylindrical glass chamber (7), provided with two stainless steel upper and bottom caps (9, 17), and a cooling jacket (4). The rotating, cylindrical, stainless steel, upper electrode (19) is equipped with a quartz jacket to avoid the penetration of the reaction media to the electrode sustaining central shaft and bearings. The upper electrode has a cylindrically shaped, cross section disc-end, which is terminated in an interchangeable ceramic pin-array (8) and holder (23). The lower electrode is hollow and also has an interchangeable conical cross section end-piece, and in addition it is provided with channels (25) for the recirculation of the reaction medium. Both the spirally located pin-array and the interchangeable metallic part of the lower electrode can be made of different metals, required by the specific plasma treatment. In this case, for the disinfection of water both the spirally located pin-array and the interchangeable metallic part of the lower electrode are made of stainless steel. The distance between the pin-array and the lower electrode can be varied by a micrometric (thimble) screw system. The reactor is vacuum tight (copper gaskets sealings), and the rotation of the upper electrode is assured through a magnetic coupling system (15, 18). The reactor can be operated in a batch-type or continuous flow mode, depending on the specific application. The rotation of the upper electrode is digitally controlled in the range of 500-5000 rpm. The plasma (multitude of spark discharges) can be initiated and sustained using adjustable and commercially available DC or AC power supplies. Although the actual 3782
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FIGURE 1. Scheme of dense-medium plasma reactor. 1 - DC power supply; 2 - gases evacuation; 3, 26 - coolant exit and inlet; 4, 7 - glass cylinders; 5 - electrical contact; 6 - coolant; 8 - ceramic pin-array; 9, 17 - caps; 10 - nonrotating electrode; 11 - ground; 12 - gas inlet; 13 - motor; 14 - digital controller; 15, 18 - magnetic coupling system; 16 - liquid inlet; 19 - rotating electrode; 20 sealed volume; 21 - quartz isolator; 22 - recirculating pump; 23 - pins; 24 - electrical discharges; 25 - recirculated flux; 27 valve. mechanism for electron emission and energy transport through the liquid is not well characterized at this time, the rotation of the electrode and the spirally arranged pin-array system (which acts as a high current density field emission arc source) will generate under DC or AC voltage conditions many microdischarges covering the whole area of the electrode surfaces. Rotating the electrode serves several important purposes. The action spatially homogenizes the multiple microarcs, activating a larger effective volume of fluid. Spinning the electrode also simultaneously pumps fresh liquid and vapors into the discharge zone, and it thins the boundary layer between the emission tips and the bulk liquid. Reactive or inert gases can also be introduced into the reaction media during the plasma process, through the hollow, lower electrode. The simultaneous presence of a gas contributes also to the homogenization of the reaction system and enhances the microarc-formation process owing to the lowered density of the medium. In typical experiments 200 mL of contaminated water is introduced into the reaction vessel, and argon or oxygen (Liquid Carbonic Co., Brookfield, WI) is injected through the hollow electrode at preselected flow rates depending on the specific experiment. The cooling of the reaction medium is initiated in the next step by flowing tap water through the reactor jacket, and the rotation of the upper (stressed) electrode is started at the desired angular speed. The discharge is initiated by applying DC voltage on the electrodes, and the plasma-state is sustained for the preselected treatment time. The following experimental conditions were
TABLE 1. Effect of Increasing Plasma Treatment Times on Bacterial Viability in Inoculated Water injected gas argon treatment time (s) 0
20
60
120
sampling time initial 1 day 3 day 7 day after treatment 1 day 3 day 7 day after treatment 1 day 3 day 7 day after treatment 3 day 7 day
log10 cfu/mL
% changea
4.75 4.82
+17.62
4.70 3.70 ndb
-11.03 -91.10 -99.99
ndb 2.98 ndb
-99.99 -98.30 -99.99
ndb
-99.99
oxygen log10 cfu/mL
% changea
4.89 5.05 4.72 2.96
+44.50 -32.40 -98.83
ndb ndb 2.91
-99.99 -99.99 -98.95
ndb ndb 2.70 ndb ndb
-99.99 -99.99 -99.35 -99.99 -99.99
a Used 0.7 log cfu/mL to calculate percent decrease. detectable at the lowest dilution plated (
