Contact Time pH And Temperature Effects

Hypochlorous acid and hypochlorite ion are known as free available chlorine. The chloramines are known as combined available chlorine and are slower than free chlorine in killing microorganisms. For identical conditions of contact time, temperature, and pH in the range of 6 to 8, it takes at least 25 times more combined available chlorine to produce the same germicidal efficiency. The difference in potency between chloramines and HOC1 can be explained by the difference in their oxidation potentials, assuming the action of chloramine is of an electrochemical nature rather than one of diffusion, as seems to be the case for HOC1. The effect of pH alone on chlorine efficiency is shown in Figure 3. Chlorine exists predominantly as HOCI at low PH levels. Between pH of 6.0 and 8.5, a dramatic change from undissociated to completely dissociated hypochlorous acid occurs. Above pH7.5, hypochlorite ions prevail; while above 9.5, chlorine exists almost entirely as OC1. Increased pH also diminishes the disinfecting efficiency of monochloramine.

Figure 3. Relative amounts of HOCI and Ocl formed as a function of pH.

Figure 3. Relative amounts of HOCI and Ocl formed as a function of pH.

It has also been demonstrated that the germicidal effectiveness of free and combined chlorine is markedly diminished with decreasing water temperature. In any situation in which the effects of lowered temperature and high pH value are combined, reduced efficiency of free chlorine and chloramines is marked. These factors directly affect the exposure time needed to achieve satisfactory disinfection. Under the most ideal conditions, the contact time needed with free available chlorine may only be on the order of a few minutes; combined available chlorine under the same conditions might require hours.

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