CE 121 chlorination

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Chlorination

Chlorine Breakpoint


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y Chlorination of wastewater is the application of chlorine toa wastewater to accomplish some definite purpose. Thepurpose of chlorination may not always be disinfection andmay, in fact, involve odor control or some other objectivewhich will be noted. Chlorine may be applied in twogeneral ways, gaseous and liquid. In general, the effectivechemical form of chlorine that either destroys the microbeor acts against odor, etc., is the same. Gaseous forms of

chlorine are generally first dissolved in water prior toaddition to the wastewater stream, while liquid forms ofchlorine (called hypochlorites) are sold in the form ofwater soluble salts.


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y In order to determine at what points in the treatment process,and how much chlorine should be applied to accomplish thepurpose desired, it is necessary to know what reactions occurwhen chlorine is mixed with a wastewater. When chlorine ismixed with pure water, it immediately dissolves, forming first

hypochlorous acid and then hypochlorites:


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y The forms of chlorine (hypochlorous acid andhypochlorite ion) are called "free" residual chlorine, asopposed to the reaction products of chlorine with othercompounds that can also be detected using analyticaltechniques that are called "combined" residual chlorine.

y Free residual chlorine is a more effective disinfecting agent

than combined residual chlorine, and generallyhypochlorous acid is a much more effective disinfectantthan hypochlorite ion. In wastewaters, free residualchlorine is seldom detected and chlorine is usually foundin the "combined" residual form.

y Chlorine is a extremely active chemical it will react to manysubstances in waste waters even in a small amount is addedto wastewater it will react rapidly.


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y The quantity of reducing substances, both organic and inorganic, inwastewaters, varies, so that the amount of chlorine that has to be addedto wastewater for different purposes will also vary. The chlorine usedby these organic and inorganic reducing substances is defined as the

chlorine demand. Chlorine demand is equal to the amount of chlorineadded minus that remaining as combined chlorine after a period oftime, which is generally 15 minutes. This relationship can be writtenas:

Chlorine Demand = (Applied Chlorine Dose) - (Chlorine Residual)

It is important to note that disinfection is carried out by that amount of

chlorine remaining after the chlorine demand has been satisfied. Thisquantity of chlorine in excess of the chlorine demand is defined as residualchlorine and expressed as milligrams per liter.


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y Theoretically, while microorganisms are killed as thechlorine demand is being satisfied, disinfection is

generally the result of chlorine residual or the amountof chlorine remaining after the chlorine demand hasbeen satisfied.

y Chlorine is seldom applied to wastewaters to reach

"breakpoint" levels. This is because the amount ofchlorine required prior to observing free availablechlorine would be very high (approximately 150mg/L). Generally chlorine is applied only to give a

combined residual.


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y Disinfection

y Chlorine is a very effective disinfecting agent and has beenthe agent of choice in reducing bacterial numbers in

wastewater effluents. As noted, neither primary norsecondary methods of wastewater treatment cancompletely eliminate pathogenic bacteria which are alwayspotentially present. When wastewaters or treated effluentsare discharged to bodies of water which are, or may be used

as a source of public water supply, or for recreationalpurposes, treatment or disinfection for the destruction ofpathogenic organisms is required to minimize the healthhazards of pollution to these receiving waters.


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y At times it is necessary to dechlorinate, or removechlorine from a wastewater by the addition ofdechlorinating agents. Generally this must be done to

counteract the reactive effects of chlorine in effluentsamples, for example, in determining coliforms orBOD's. At present only a small number of treatmentplants in New York State dechlorinate, however, in thefuture it may become a more common practice todechlorinate treated wastewater.


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y The point at which the chlorine dosage in a watertreatment process has satisfied the Chlorine Demand. Toeliminate the taste and odor associated with processedwater, sufficient chlorine must be added to reach thebreakpoint. Increasing the chlorine dose beyond thebreakpoint produces a free chlorine residual, which is freeto kill microorganisms. When chlorine is added to water, itfirst combines with constituents in the water such as iron,manganese, and nitrites. It is important to add enoughchlorine to the water initially to ensure that these

constituents are oxidized and to ensure that a residual isformed to react with the ammonia and organic matter inthe water. Taste and odor problems result when chlorinedosages are either below the breakpoint, or well beyond thebreakpoint.


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y The graph below shows what happens when chlorine (eitherchlorine gas or a hypochlorite) is added to water. First (betweenpoints 1 and 2), the water reacts with reducing compounds in the

water, such as hydrogen sulfide. These compounds use up thechlorine, producing no chlorine residual.


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yNext is Chemical Equations.

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CE 121 chlorination