Disinfection by pulsed power discharges

We study the disinfection of Escherichia coli in a pulsed power discharge reactor. The pulsed power discharge process is an electrohydraulic phenomenon characterized by a rapid release of electrical energy across a submerged electrode pair.; The survival kinetics of pure strains of E. coli suspensions exposed to 5.5 kV, 90 kA electrohydraulic discharges (EHD) is investigated. The probability of survival P_n of a 2 × 107 E. coli CFU mL−1 population after 50 consecutive EHD's follows a logit distribution that corresponds to lethal doses of LD₅₀ = 2.2 and LD₉₀ = 10.8 EHDs. Variation of the initial cell concentration produced nearly constant values of LD₅₀ = 0.9 ± 0.1 in the range 2 × 10 3 ≤ E. coli/CFU mL−1 ≤ 3 × 106. Beyond 106 CFU mL−1 , the LD₅₀ values increase exponentially due to nonlinear light absorbance with increasing E. coli concentrations. Qualitatively similar initial cell concentration dependence is observed for survival under low intensity 254 nm irradiation, in contrast with lower values of LD₅₀ obtained in denser colonies to 20 kHz power ultrasound exposure.; The high intensity (3.3 × 1010 W m−2 ) ultraviolet radiation emitted by the electrohydraulic discharge is completely suppressed in the presence of less than 100 mg L−1 2,2′-dihydroxy-4,4′-dimethoxybenzophenone-5,5 ′-disulfonic acid (BP9), a well-known sunscreen agent. Concentrations of BP9, in the range (0–100) mg L−1, are varied to measure the sterilization kinetics of ∼3 × 107 CFU mL−1 suspensions to varying degrees of high intensity UV exposure. The slope of the logit plots of E. coli as function of BP9 concentration is consistent with the screening of radiation as the sole lethal agent. Computed values of biologically available light fluences are 5.0 × 1011 photons CFU−1 for high intensity, high power, pulsed EHD experiments and 6.6 × 108 photons CFU−1 for separate low power continuous UV experiments. The net availability of 3 orders of magnitude more photons during high intensity UV exposure suggests the possibility of a multiphotonic disinfection mechanism at play in the EHD process relative to low intensity case.; The overall resistance to long term exposure to EHD is also investigated. Selective pressure experiments with E. coli exposed to 11 cycles of 50 consecutive EHDs each show a weak kinetic change in the dose-response curves reflected in the nearly constant values of LD₅₀ = 0.24 ± 0.03. A greater than 98% metabolic similarity in carbon source consumption between initial and final E. coli populations are enzymatically related. In addition, the results indicate that no bacterial contaminants are propagated throughout the experiment.