The chlorine residual in the clearwell 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 customer, the chlorine residual should be about 0.2 mg/L at the extreme ends of the distribution system.  This residual in the distribution system will also act to control microorganisms in the distribution system which produce 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:

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 two hours at the plant and at various points in the distribution system. 

It is also important to understand the breakpoint curve when changing chlorine dosages.  If the water smells strongly of chlorine, it may not mean that too much chlorine is being added.  More likely, chloramines are being produced, and more chlorine needs to be added to pass the breakpoint. 

Contact Time

Contact time is just as important as the chlorine residual in determining the efficiency of 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 customer.  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:

The CT is the Concentration multiplied by the Time.  As the formula suggests, a reduced chlorine residual can still provide adequate kill of microorganisms if a long contact time is provided.  Conversely, a smaller chlorine residual can be used as long as the chlorine has a longer contact time to kill the pathogens. 

Other Influencing Factors

Within the disinfection process, efficiency is influenced by the chlorine residual, the type of chemical used for chlorination, the contact time, the initial mixing of chlorine into the water, and the location of chlorination within the treatment process.  The most efficient process will have a high chlorine residual, a long contact time, and thorough mixing. 

Characteristics of the water will also affect efficiency of chlorination.  At a high pH, the hypochlorous acid becomes dissociated into the ineffective hypochlorite ion.  So lower pH values result in more efficient disinfection. 

Temperature influences chlorination just as it does any other chemical reaction.  Warmer water can be treated more efficiently since the reactions occur more quickly.  At a lower water temperature, longer contact times or higher concentrations of chemicals must be used to ensure adequate disinfection. 

Turbidity of the water influences disinfection primarily through influencing the chlorine demand.  Turbid water tends to contain particles which react with chlorine, reducing the concentration of chlorine residual which is formed.  Since the turbidity of the water depends to a large extent on upstream processes (coagulation, flocculation, sedimentation, and filtration), changes in these upstream processes will influence the efficiency of chlorination.  Turbidity is also influenced by the source water - groundwater turbidity tends to change slowly or not at all while the chlorine demand of surface water can change continuously, especially during storms and the snow melt season. 

Finally, and 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.