S U M M A R YMicro-organisms in aqueous suspensions were killed when voltages above a certain threshold value were discharged through electrodes immersed in the suspensions. The relation of the peak pressure, peak current and the arcing time to the killing effect were studied, but no clear-cut relationship was ascertained. At the discharges shock waves with durations of 20-105 psec. and pressure amplitudes of 40-250 bar were generated. Although bigger and more solid objects were considerably damaged, the micro-organisms were inconspicuously affected by the pressure shock wave alone. I N T R O D U C T I O NWhen micro-organisms suspended in water are subjected to submerged high-voltage electrical discharges, a large proportion of them are killed (Brandt et al. 1962; Allen & Soike, 1966). The spectrum of sensitive organisms is wide and the efficiency high, which might make this method useful for practical disinfection purposes. Several physical and chemical activities are initiated by the electrical discharge. From the plasma channel emanate a pressure shock wave, photon radiation and more or less unstable chemical compounds. Information about the relation of the microbicidal effect to the different non-biological effects of the discharge might elucidate the mechanism of action and lead to a high microbicidal efficiency of the process being obtained. M E T H O D SThe electric circuit used and its physical and electrical quantities The electric circuit used is shown in Fig. I. The step-up transformer (3) is supplied with a variable voltage from a variac ( 2 ) connected to the ax. mains (I). The high voltage capacitor (6) is charged through a resistor (4) and a rectifier (5). The capacitor voltage will reach a ceiling value, U, determined by the spark gap (8) operating in air. The breakdown of gap (8) causes a current to flow between the submerged eletrodes (9). The capacitor will discharge through the circuit elements (7-8-9) of Fig. I . The capacitance, C, is measured in microfarad (pF.) and the inductance, L, in microhenry (pH.). The electrodes (9) were submerged in water.

Submerged electrical discharges between copper-containing electrodes rendered the treated liquid microbicidal. Part of this activity was unstable and decreased rapidly during the first few minutes. It might have been caused by cuprous ions or substances with oxidative activity. The stable microbicidal activity was due to copper released from the electrodes. This copper existed only partly in ionic form. Inorganic salts and most organic substances tested decreased the bactericidal effect of discharge-treated water. Such substances also diminished the protracted killing effect which was observed when bacteria suspended in water of relatively high purity were subjected to transient electric arcs.

The microbicidal effect of electrical discharges on microorganisms suspended in the discharge liquid was reduced when the liquid contained high concentrations of inorganic salts (conductivity k ≥ 5 mmho/cm). A higher discharge voltage and a smaller distance between the submerged electrodes counteracted this reduction. The decrease in the microbicidal effect was accompanied by a change in the electrical current and by a decreased yield of microbicidal photons from the electric discharge.