## 3 Results
### Effect of Lethal Dose of Chlorine on E. coli Survival  
To investigate the efficacy of increasing chlorine doses on the inactivation of the test _E. coli_ isolates, three _E. coli_ strains in suspensions (initial bacterial concentration of 8.2-8.23 log) were exposed to chlorine concentrations of 0.75-1.5 mg/L for 30 min. The survival of the three _E. coli_ isolates at higher free chlorine concentrations were compared with survival at 0.5 mg/L (Figure 1).  
At 0.75-1.25 mg/L free chlorine, the removal efficiency of _E. coli_ SAMRC-1 was 6.41-6.45 log and virtually 100% inactivation was achieved at free chlorine concentration of 1.5 mg/L within 30 min at residual chlorine concentrations of (0.29 \(\pm\) 0.02)-(0.9 \(\pm\) 0.05) mg/L. In comparison, chlorine concentration of 0.5 mg/L achieved the lowest log removal of 4.1 log. For _E. coli_ SAMRC-2, a removal efficiency of 6.2-7.9 log was achieved at 0.75-1.25 mg/L free chlorine and complete inactivation of the _E. coli_ population at chlorine concentration of 1.5 mg/L at residual chlorine concentrations of (0.55 \(\pm\) 0.05)-(1.23 \(\pm\) 0.04) mg/L. In contrast, chlorine concentration of 0.5 mg/L showed a lower removal efficiency of 4.2 log. The removal efficiency for _E. coli_ SAMRC-3 ranged between 6.2 and 6.9 log at chlorine concentrations of 0.75-1.25 mg/L while the highest removal of 7.3 log units was achieved at chlorine concentration of 1.5 mg/L. Residual chlorine concentrations after 30 min were (0.42 \(\pm\) 0.08)-(1.22 \(\pm\) 0.01) mg/L. In contrast, 0.5 mg/L achieved disinfection efficiency of 3.7 log.  
Water guidelines recommend a maximum contaminant level (MCL) of zero for total coliforms including _E. coli_ for effluent discharge into water bodies [26] and a greater removal efficiency of _E. coli_ isolates was achieved at higher chlorine doses of 0.75-1.5 mg/L compared to log inactivation at 0.5 mg/L in 30 min. This might be due to the presence of higher chlorine species reacting with bacteria cells which causes greater inactivation of bacteria. It has been hypothesized that solutions with higher oxidizing species have higher oxidation potential to inactivate bacteria [28].  
A study also reported greater than 5 log reductions of Shiga-toxicine _E. coli_ serotypes 026 and 0103 after treatment with a chlorine dose of 1.0 mg/L in 5 min [47]. High sensitivity of _E. coli_ O157:H7 and _Listeria monocytogenes_ to free chlorine concentrations equal to or higher than 1.0 mg/L was recorded with no detection of bacteria after chlorine treatment [48]. The Minimum Inhibitory Concentration (MIC) of planktonic bacterial isolates was also achieved at free chlorine concentrations ranging between 1.0 and 2.0 mg/L [16]. Low percentages of intact cells of coliforms and _E. coli_ were found  
Figure 1: Reduction of _E. coli_ at free chlorine concentrations at 0.5–1.5 mg/L for 30 min. Error bars represent standard deviation from the mean, \(n\) = 5.  
in drinking water samples after treatment with free chlorine concentrations above 0.5 mg/L, while higher percentages of intact cells were observed at free chlorine concentrations below 0.5 mg/L [49].  
The difference between the control and chlorine treated _E. coli_ strains was not statistically significant (\(p<0.05\)) at all the treatment levels and there were no statistically significant differences (\(p<0.05\) in inactivation among the chlorine treated _E. coli_ isolates at 0.75-1.5 mg/L. However, a chlorine concentration of 1.5 mg/L was found to be optimal for _E. coli_ inactivation.