EFFICIENCY OF DISINFECTION

The efficiency of the disinfection depends upon the following factors:

• Nature of the disinfectant,
• Dose of the disinfectant,
• Length of contact time with the disinfectant,
• Temperature,
• Type and concentration of organisms in water to be disinfected, and
• pH of water

Dose of Disinfectant

A variety of factors can influence disinfection efficiency when using breakpoint chlorination or chloramines. One of the most important of these is the concentration of chlorine residual in the water. The chlorine residual depends upon the chlorine dose and chlorine demand. The chlorine residual in the clear water storage should be at least 0.5 mg/L. This residual, consisting of hypochlorous acid and/ or chloramines, must kill microorganisms already present in the water and must also kill any pathogens which may enter the distribution system through cross-connections or leakage. In order to ensure that the water is free of microorganisms when it reaches the consumer, the chlorine residual should be about O.2 mg/L at the extreme ends of the distribution system. This residual in the distribution system will act to control microorganisms in the distribution system, which produces slimes, tastes, or odors.

Determining the correct dosage of chlorine to add to water will depend on the quantity and type of substances in the water creating a chlorine demand. The chlorine dose is calculated as follows:

Chlorine Dose = Chlorine Demand + Chlorine Residual

So, if the required chlorine residual is 0.5 mg/L and the chlorine demand is known to be 2 mg/L, then 2.5 mg/L of chlorine will have to be added to treat the water. The chlorine demand will typically vary over time as the characteristics of the water change. By testing the chlorine residual, the operator can determine whether a sufficient dose of chlorine is being added to treat the water. In a large system, chlorine must be sampled every day at the plant and at various points in the distribution system.

Contact Time

Contact time is as important as the chlorine dose in determining the efficiency of water treatment chlorination. Contact time is the amount of time which the chlorine has to react with the microorganisms in the water, which will equal the time between the moment when chlorine is added to the water and the moment when that water is used by the consumer. The longer the contact time, the more efficient the disinfection process is. When using chlorine for disinfection a minimum contact time of 30 minutes is required for adequate disinfection. The CT value is used as a measurement of the degree of pathogen inactivation due to chlorination. The CT value is calculated as follows:

CT = (Chlorine dose, C in mg/L) x (Contact time, Tin minutes)

As the formula suggests, even a small chlorine dose, can provide adequate kill of microorganisms if a longer contact time is provided. Conversely, a large chlorine dose, needs only chlorine has a shorter contact time to kill the pathogens.

Temperature of Water

At lower temperatures, bacterial kill tends to be slower and higher doses are needed. The effect of low temperatures is greater with combined chlorine than with free available chlorine.

• The concentration of chemical substances is exerting demand on the chlorine. Many of these compounds are not effectively removed in conventional water treatment processes.
• The chlorine must be well dispersed and homogeneously mixed to assure that the contact time for disinfection is applied throughout the water supply.

Types and Concentration of organisms

Most intuitively, the number and type of microorganisms in the water will influence chlorination efficiency. Since cyst-forming microorganisms and viruses are very difficult to kill using chlorination, the disinfection process will be less efficient if these pathogens are found in the water.

pH

pH is an important factor that influences the efficiency of the disinfection. The concentration of hypochlorous acid and hypochlorite ions in chlorinated water will depend on the pH of water.

A higher pH facilitates the formation of more hypochlorite ions and results in less hypochlorous acid in the water. Lower pH facilitates the formation of less hypochlorite ions and more of hypochlorous acid. Hypochlorous acid is the most effective form of free chlorine residual, i.e., chlorine available to kill microorganisms in the water, while hypochlorite ions are much less efficient disinfectants. So disinfection is more efficient at a low pH (with large quantities of hypochlorous acid in the water) than at a high pH (with large quantities of hypochlorite ions in the water.) At a high pH, the hypochlorous acid becomes dissociated into the ineffective hypochlorite ion. So in lower pH water disinfection will be more efficient.

The Below Table shows the pH and chlorine residual relation for 100% bacterial kill in 60 minutes at temperature of 22°C.

pH	Combined Chlorine Dosage, mg/L
6.5	0.3
7	0.6
7.7	0.9
8	1
8.5	1.2