## 3 Results
### Inactivation of E. coli at Free Chlorine Dose of 1.5 mg/L after 30 min Exposure  
The inactivation of the three _E. coli_ strains initial bacterial density of approximately 8.2 log by chlorine disinfectant was assessed at the initial chlorine concentration of 1.5 mg/L. Bacterial survival was progressively monitored at 10 min interval over 30 min exposure (Figures 2-4).  
Figure 3: Chlorine decay and inactivation of _E. coli_ SAMRC-2 at 1.5 mg/L during 30 min exposure. Bacterial survival and chlorine decay were quantified at 10 min interval. The error bars represent standard deviation from the mean, \(n=3\).  
Figure 2: Chlorine decay and inactivation of _E. coli_ SAMRC-1 at 1.5 mg/L during 30 min exposure. Bacterial survival and chlorine decay were quantified at 10 min interval. The error bars represent standard deviation from the mean, \(n=3\).  
At 1.5 mg/L chlorine concentration for 30 min, viable counts of _E. coli_ SAMRC-1 were reduced to 3.6 log (4.67 \(\times\) 10\({}^{4}\) CFU/mL) in 10 min, 4.7 log (3.67 \(\times\) 10\({}^{3}\) CFU/mL) in 20 min and complete inactivation of _E. coli_ in 30 min (Figure 2). A reduction in viable counts of _E. coli_ SAMRC-2 was 4.51 log (5.0 \(\times\) 10\({}^{3}\) CFU/mL) in 10 min, 5.38 log (6.67 \(\times\) 10\({}^{2}\) CFU/mL) in 20 min and complete inactivation was achieved in 30 min (Figure 3) while _E. coli_ SAMRC-3 was reduced to 4.04 log (1.53 \(\times\) 10\({}^{4}\) CFU/mL) in 10 min, 5.19 log (1.07 \(\times\) 10\({}^{3}\) CFU/mL) in 20 min and 7.3 log (8.3 CFU/mL) in 30 min (Figure 4).  
Bacterial inactivation was rapid within the first 10 min while inactivation rate slowly declined afterwards with increase in contact time. Previous studies have shown that the highest reduction of bacterial population occurred within 10-15 min of exposure [50; 51].  
A tailing-off effect observed for _E. coli_ strains is shown in Figure 2. The tailing-off implies a shielding phenomenon in which a certain portion of the bacterial population, after inactivation by chlorine, returns the bacteria suspension into its initial state, but with a lower number of viable cells. Further reaction between the chlorine species and bacteria then results in inactivation of smaller portions of viable bacteria due to shielding by previously inactivated cells [52]. Similar observations of tailing-off effects have been reported in some inactivation experiments. For example, [PERSON] et. al. [53] reported a tailing phase for total coliforms in raw grey water treated with 1-5 mg/L free chlorine for 120 min. Similarly, [PERSON] et. al. [54] reported a tailing-off for _E. coli_ O157 in the chlorine-demand free buffer and [55] observed a tailing-off in the inactivation curve of _Enterococcus_ sp. treated with 90 mg O\({}_{3}\)/L of ozone.  
An estimation of Ct values for 7-log inactivation of _E. coli_ extrapolated from the linear regression lines showed 41.1, 42.4 and 49.9 mg/L-min free chlorine for _E. coli_ SAMRC-1, _E. coli_ SAMRC-2 and _E. coli_ SAMRC-3, respectively (Figure 5).  
Figure 4: Chlorine decay and inactivation of _E. coli_ SAMRC-3 at 1.5 mg/L during 30 min exposure. Bacterial survival and chlorine decay were quantified at 10 min interval. The error bars represent standard deviation from the mean, \(n=3\).  
These \(Ct\) values were higher than compared to the \(Ct\) values for 7 log inactivation of \(E\). _coli_ by free chlorine obtained in previous studies. For example, [PERSON] et al. [56] observed greater than 7-log inactivation of \(E\). _coli_ O157:H7 at a \(Ct\) value of 0.25 mg/L-min. The differences in inactivation may be due to higher initial bacterial density (10\({}^{8}\) CFU/mL) of \(E\). _coli_ used in this study compared to initial bacterial density (10\({}^{6}\) CFU/mL) used in the previous study.