Clean water is one of the basic conditions for the persistence of human life. Purified water from sewage plants may still contain large amounts of pathogenic microorganisms and poisonous chemicals. Underwater-pulsed electric discharges have the potential to remedy some of these problems in an ecologically attractive way. In an underwater corona discharge high electric fields, shock waves, UV-light and strong oxidants act simultaneously upon bacteria. To investigate the efficiency of underwater corona discharges for bacterial decontamination we have adopted a scheme initially proposed by Sunka [3]. In this scheme the discharges are created with the help of a thin porous ceramic layer that covers the central anode in a coaxial reaction chamber.
This set-up produces a large number of streamers filling the space between anode and cathode. Several units can be operated in parallel and in series. Therefore, it seems scalable to the large throughputs for municipal sewage plants. An important aspect of this development is to determine the limitations for the lifetime of the anodes.
Recycling and Decontamination of building materials
We have continued our efforts to use electrodynamic fragmentation for the recycling and decontamination of building materials. These investigations are conducted in close cooperation with an industrial partner.
Electrodynamic fragmentation is based on the initiation of pulsed electric discharges in dielectric materials. The pressure pulses created in these discharges do not only lead to a destruction of the material but also to a separation of components in case of composite materials.
The general goals of these activities are:
· Separation of waste concrete into sand grovels and cement powder to save natural resources.
· Concentration of pollutants of contaminated concrete in the fine fraction to reduce the amount of disposal material.
· Increase the rate of removal of concrete and hard rock with near-surface discharges.
Because of its importance for sustainable development and because of the high deposition costs decontamination of polluted concrete has gained the highest priority. Decontamination of concrete polluted with mineral oil is economically feasible if a large decontamination factor can be achieved for a sufficiently large fraction of the material. Then an economic profit can result from a reduction of the material mass to be dumped. To optimize the decontamination process a gradual fragmentation of concrete lumps is required. This shall be achieved by several arcs arranged in a series configuration whose power and energy input is tailored according to the degree of fragmentation already achieved. The material is transported past the arcs in a vibrating canal.
A yet unresolved problem is the long-term increase in water conductivity, which results in a decrease of the fragmentation efficiency. It is supposed that this conductivity increase is due to alkali ions accumulating in the process water. Unlike the ions of the alkaline earth group, which are responsible for the short-term rise of the conductivity, they can not be precipitated with CO2. Other components that can influence the discharge efficiency are suspended fine particles. More detailed investigations are required before an effective concept for a closed water cycle can be established.
References
1. Bluhm H. et al., High power particle beams, pulsed power for industrial applications // Proc. Of the 14th Intern. Conf. On High-Power Particle Beams, Albuquerque USA, 23–28 June 2002, AIP Conf. Proc. Vol. 650, 9–16.
2. Humphries S. Jr. Charged Particle Beams. New York, John Wiley & Sons, Inc., 1990.
3. Sunka P., Babicky V., Clupek M. et al. Generation of chemically active species by electrical discharges in water // Plasma Sources Sci. Technology, 8, 258–265, 1999.
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