Investigation of a Physical Disinfection Process Based on Pulsed Underwater Corona Discharges (2007)

To overcome the side effects of commonly used water disinfection methods based on chemicals, advanced oxidation processes (AOP's) are presently considered as an alternative. These processes are based on the effect of highly reactive oxidants (like ozone, OH radicals, H 2 O 2 , et.) on microorganisms and toxic organic compounds. In this work the effects and products from pulsed underwater corona discharges have been investigated for their potential to remove contaminants from the water. Pulsed underwater corona discharges create oxidative and reductive species, UV radiation, shock waves and strong electric field at the tip of propagating streamers. It has been supposed that the combined appearance of these effects leads to an effective destruction of contaminants in the water. It was the aim of this work to quantify the production rates of oxidants and the intensity of other effects from underwater corona discharges, to demonstrate their effectiveness for the inactivation of microorganisms and to propose a suitable corona reactor that is scalable to large throughputs. For that purpose a coaxial corona reactor was designed with a central anode covered by a thin (200-300 μm) porous ceramic layer. The conductive porous ceramic and the anode cathode water gap form a parallel resistive-capacitive voltage divider. For times greater than the dielectric relaxation time of the water (τ=ee 0 /σ) resistive voltage division dominates and the electric field at the ceramic layer can be more enhanced than in the case of capacitive voltage division. Applying a pulsed voltage of 30-50 kV for a duration of 200-400 ns to the anode a large number of streamers is launched homogeneously from the ceramic surface extending up to 10 mm into the anode cathode water gap. An L-C chain Blumlein configuration has been selected to generate the pulses. By changing the number and the values of the LC elements the impedance of the generator and its pulse width could easily be adapted to the requirements of the experiment. Different ceramic layers were tried, but the best results were obtained with almandine which has a very low pH value at its isoelectric point. The number and the length of streamers increased linearly with the anode voltage. The reactor impedance was mainly determined by the water conductivity. Little influence from the appearance of streamers was observed. A large number of chemical probes have been selected to measure the oxidant production quantitatively. OH and H radicals created in core of a streamer channel are the most important primary products. As the channel cools down they recombine or form secondary products like H 2 O 2 and H 2 . H 2 O 2 accumulates in the bulk water and becomes a source of additional OH radicals due to the interaction with UV radiation and the liberation of Fenton reagents from the anode. Therefore, H 2 O 2 plays an important role in the disinfection process. Using DNTA as a chemical probe for OH radicals combined with an OH scavenger and measuring the H 2 O 2 yield simultaneously the initial OH radical concentration at the position of the cold streamer was found to be 30 mM. Different gases were percolated through the reactor to enhance the oxidant production. Saturating the water with N 2 O it was also possible to prove the existence of solvated electrons in the water after the formation of streamer discharges.