## 3 Experimental Results and Discussion
### Effect of the Number of Treatment Chambers on E. coli Inactivation  
After the PEF treatment process, the treated _E. coli_ suspension from every condition was incubated by the drop plating method for 18 h on a standard agar culture plate for viable cell counting. The temperature change of the treated _E. coli_ suspension was unnoticeable with the thermometer. The temperature was the same as the room temperature (25 \({}^{\circ}\)C), which is consistent with the simulation results. This would be due to the fact that the treated solution has dissipated accumulated thermal energy through the long outlet metal ground electrode. Figure 8a presents the average _E. coli_ colony count. The viable cells of the control group were around \(1.7\times 10^{10}\) CFU/mL, while the average viable cells of other conditions were around \(6.99\times 10^{7}\)-\(2.57\times 10^{8}\) CFU/mL. From the statistically analysis by ANOVA, it could be confirmed that the _E. coli_ were significantly reduced (\(p<0.05\)) in every condition, as shown on a log scale in Figure 8b. The average _E. coli_ log reduction was 1.82, 2.25, and 2.39 for \(N_{HV1}\), \(N_{HV3}\), and \(N_{HV5}\), respectively. The average _E. coli_ inactivation efficiencies showed the same trend for every repeated experiment. From the experimental results, it could be confirmed that an increase in the number of treatment zones could increase the efficiency of _E. coli_ inactivation. The average _E. coli_ log reduction of \(N_{HV3}\) and \(N_{HV5}\) was higher than that of \(N_{HV1}\) by a factor of 1.24 and 1.31, respectively. This would owe to the fact that increasing the number of treatment chamber units increases the possibility for _E. coli_ to be inactivated by electroporation. Regarding the treatment condition, as _E. coli_ pass the treatment zone, they could possibly be subjected to the high electric field for 75 pulses, for a 7.5 ms duration. Since a treatment unit has two treatment zones, _E. coli_ were hence treated twice. Therefore, for n treatment units, _E. coli_ would be subjected to the high electric field for \(150\times\) n pulses. Figure 8: Results from the different numbers of high voltage (HV) electrodes. (**a**) Amount of _Escherichia coli_ (_E. coli_) colony, and (**b**) _E. coli_ log reduction. However, the increase in the treatment units decreases the energy per pulse since the resistance of the liquid resistors is reduced due to the increase in parallel liquid resistors. This reason results in the voltage drops in contrast to the pulsed current across the treatment chamber, as shown in Figure 7. Since the treated medium is conductive, this fact possibly contributes, to a certain degree, to the joule heating during the treatment process. Moreover, from the simulation results (data not shown here), it is found that a serial connection of the treatment units increases the liquid flow velocity near the surface of the elliptical insulators. This is due to the fact that the diameter of the fluid path is narrower at the treatment zone; therefore, the liquid flow velocity is affected by variation of the tube volume, as described by the equation of continuity [13; 14; 22; 28; 33]. This effect influences the treatment efficiency since the liquid flows faster at the treatment zone and is then subjected to decrease in treatment time. Therefore, the time for which the _E. coli_ are subjected to a high electric field is shorter than expected, causing efficiency reduction. Another issue of concern is that the number of series treatment units should also be optimized, since the temperature rise in the treatment zone increases with an increase in treatment units, as shown in the simulation results in Figure 6. However, regarding the hypothesis of this research and the experimental results, it could be confirmed that a proper increase in the number of treatment chambers can improve the efficiency of microbial inactivation. Only a short treatment time (milliseconds) efficiently reduces the number of viable _E.