The bacterium Escherichia coli (E. coli) is commonly found in the intestines of warm-blooded animals. Its presence in water, however, indicates fecal contamination, which can transmit serious waterborne illnesses. Chlorine is the most widely used and cost-effective chemical disinfectant globally for treating municipal water supplies. The primary question for public health remains whether this common disinfectant is truly effective at neutralizing E. coli and safeguarding the water supply.
The Chemical Action of Chlorine
When chlorine is introduced into water, it immediately reacts to form two powerful disinfecting agents: hypochlorous acid (\(\text{HOCl}\)) and the hypochlorite ion (\(\text{OCl}^-\)). This combination is known as “free available chlorine” and is responsible for the germ-killing action.
Hypochlorous acid is significantly more potent than the hypochlorite ion, often cited as being 80 to 120 times more effective. This superior efficiency is due to its neutral electrical charge, which allows the \(\text{HOCl}\) molecule to easily penetrate the negatively charged outer wall of the bacterial cell. Once inside the E. coli cell, hypochlorous acid acts as a powerful oxidizer, disrupting the bacterium’s internal machinery, including enzymes and DNA, which leads to rapid cell death.
E. coli Susceptibility and Required Levels
E. coli is highly susceptible to free chlorine compared to more resistant pathogens like cysts (e.g., Cryptosporidium). This means it requires a relatively low dose of chlorine and a short contact time for inactivation.
The effectiveness of disinfection is measured using the “CT value,” which is the product of the disinfectant concentration (C, in \(\text{mg/L}\)) and the contact time (T, in minutes). For example, greater than \(99.9\%\) inactivation of E. coli can be achieved with a free chlorine concentration of \(0.2 \text{ mg/L}\) applied for only \(0.50\) minutes in clean water.
A recommended \(\text{CT}\) value for effective chlorination to eliminate E. coli is around \(15 \text{ mg/L} \cdot \text{min}\) at temperatures above \(10^\circ \text{C}\) and a \(\text{pH}\) below \(7.5\). While chlorine is highly effective at destroying the bacterium’s ability to form colonies, it can sometimes induce a “viable but non-culturable” (VBNC) state in a small fraction of cells, making them difficult to detect with standard laboratory tests.
Variables Impacting Disinfection Success
The effectiveness of chlorine is conditional due to real-world environmental factors that interfere with the chemical process.
One significant variable is the water’s \(\text{pH}\) level, which controls the ratio of the potent hypochlorous acid (\(\text{HOCl}\)) to the weaker hypochlorite ion (\(\text{OCl}^-\)). As the \(\text{pH}\) increases above neutral (\(\text{pH } 7\)), the equilibrium shifts, and the less effective \(\text{OCl}^-\) becomes dominant, drastically reducing disinfecting power.
The presence of organic matter, such as decaying leaves or sediment, also impacts success by consuming the chlorine before it can reach the target bacteria. This consumption is known as the “chlorine demand.” The initial chlorine dose must be high enough to satisfy this demand and still leave a sufficient “free chlorine residual” for disinfection. Lower water temperatures slow down the chemical reaction rate, requiring either a higher chlorine dose or a longer contact time for inactivation.
Chlorine and E. coli in Public Water Systems
In public water treatment, the goal is to eliminate E. coli at the treatment plant and maintain a protective barrier as the water travels through the distribution network. This is achieved by ensuring a minimum level of free chlorine, known as the disinfectant residual, is present throughout the pipes. This residual prevents the regrowth of pathogens or recontamination before the water reaches the consumer’s tap.
Regulatory bodies often require a minimum free chlorine residual of \(0.2 \text{ mg/L}\) to be maintained, though many utilities target \(0.3 \text{ mg/L}\) or higher for microbial control, especially in warmer conditions. The detection of E. coli in a public water system is considered a serious breach, indicating a strong likelihood of recent fecal contamination and requiring immediate public health alerts. E. coli is monitored continuously, and the maximum acceptable concentration in drinking water is zero detectable bacteria per \(100 \text{ mL}\) of water.