## 3 Results
### Inactivation Kinetics of Bacteria by Chlorine at 30 min Contact Time  
In order to evaluate the kinetics of \(E\). _coli_ inactivation at higher chlorine concentrations (0.75-1.5 mg/L) after 30 min exposure, inactivation data were fitted to the Chick-Watson empirical model (Table 2) to determine disinfection kinetic parameters, \(R^{2}\) (coefficient of determination), inactivation rate constant, \(k\), and coefficient of dilution, \(n\). From the linear regression, \(R^{2}\) values ranged between 0.64 and 0.77. Significant differences (\(p<0.5\)) in inactivation rate constant \(k\) were observed for each tested \(E\). _coli_ strain at \(-1.99\)\(\pm\) 1.43, \(-1.33\)\(\pm\) 1.36 and \(-1.21\)\(\pm\) 0.08 mg/L for \(E\). _coli_ SAMRC-1, \(E\). _coli_ SAMRC-2 and \(E\). _coli_ SAMRC-3, respectively. The \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Bacterial Strains**} & \multicolumn{4}{c}{**Free Chlorine (mg/L)**} \\ \cline{2-5}  & \(R^{2}\) & \(k\) & \(n\) & \(p\) \\ \hline \(E\). _coli_ SAMRC-1 & 0.64 & \(-1.99\)\(\pm\) 1.43 & \(-4.66\)\(\pm\) 1.47 & 0.03 \\ \(E\). _coli_ SAMRC-2 & 0.71 & \(-1.33\)\(\pm\) 1.36 & \(-5.07\)\(\pm\) 1.40 & 0.02 \\ \(E\). _coli_ SAMRC-3 & 0.77 & \(-1.21\)\(\pm\) 1.08 & \(-4.67\)\(\pm\) 1.12 & 0.01 \\ \hline \hline \end{tabular} \(R^{2}\): coefficient of determination; \(k\): inactivation rate constant; \(n\): coefficient of dilution; \(P\): probability level of \(k\) values (considered significant at \(p<0.05\)), \(n=3\).  
\end{table} 
Table 2: Inactivation kinetics of \(E\). _coli_ at chlorine concentrations of 0.75–1.5 mg/L for 30 min ([PERSON]–[PERSON] model: \(\log N/N_{0}=-kC^{n}T\)). Figure 5: Chlorine inactivation of \(E\). _coli_ at free chlorine concentration of 1.5 mg/L at 30 min exposure (pH = 7, temperature = 22 \({}^{\circ}\)C), \(n=3\). inactivation rate constant \(k\), describes the sensitivity of bacteria to inactivation by disinfectants [57]. The highest inactivation rate constant \(k\) (\(-1.99\)\(\pm\) 1.43 mg/L) observed for _E. coli_ SAMRC-1 suggests higher sensitivity of _E. coli_ SAMRC-1 to free chlorine while _E. coli_ SAMRC-3 with the lowest value of \(k\), reflects a more resistant bacterial population to free chlorine. The coefficient of dilution, \(n\) values were 4.66, 5.07 and 4.67 for _E. coli_ SAMRC-1, _E. coli_ SAMRC-2 and _E. coli_ SAMRC-3, respectively. Differences in \(n\) represent the average number of disinfectant molecules in contact with an organism available to cause inactivation [58] and this was reflected in the increased inactivation for _E. coli_ SAMRC-2 (higher value of \(n\) = 5.07) compared to the other _E. coli_ strains. Variations in \(n\) indicate physiological differences between organisms, inactivation conditions and availability of nutrients in the cultivation media [58]. Difference in the values of \(k\) and \(n\) for two strains of _E. coli_, K12 and 0157:H7 exposed to chlorine concentrations of 0.1-8.0 mg/L for 10 min have been reported [59]. Inactivation kinetics of _E. coli_ at optimal chlorine dose of 1.5 mg/L at intervals of 10 min over 30 min exposure were evaluated using the Chick-Watson (Table 3) and Hom model (Table 4) model. Comparatively, the Hom model gave a good fit for inactivation of _E. coli_ by chlorine disinfectant with \(R^{2}\) values ranging from 0.9 to 0.98 while Chick-Watson model showed with \(R^{2}\) values of 0.91-0.95. Overall, these results indicate a strong correlation between bacterial inactivation and chlorine dose over time and bacterial isolates were adequately exposed to reacting chlorine species during the disinfection process. Values obtained for disinfection exponent \(n\) ranged between 1.57 and 22.43 while time exponent m had values of \(-\)0.23-0.17. For inactivation reactions with coefficient \(n\) greater than 1, the chlorine dose has a greater influence on bacterial inactivation than contact time [60]. These results suggest a greater impact of chlorine dose of 1.5 mg/L on bacterial inactivation than the effect of exposure time.