Water Supply Engg. AE Coaching 1

Break Point Chlorination

1 Introduction

• We know that when chlorine is added to water, it first reacts with inorganic and organicimpurities along with formation of chloramines. The amount of chlorine that reacts withthese impurities constitute the chlorine demand of water.

2 Applied-Residual Chlorine Curve

• Let us now consider a plot between residual chlorine and applied chlorine. If water hasno chlorine demand, any applied chlorine will appear as residual chlorine and thus therelation between residual and applied chlorine will be a straight line having 45◦ slope.

• Stage I: Water generally has some chlorine demand. On addition of chlorine to water, itimmediately reacts with inorganic substances and oxidises them. There will not be anyresidual chlorine left as all chlorine is consumed.

• Stage II: After meeting the immediate demand, chlorine reacts with ammonia to formchloroamines. Thus, there will be some residual chlorine present in water in the form ofcombined chlorine. With increase in applied chlorine, the residual chlorine also increasestill the peak (point C) is reached.

• Stage III: Beyond point C, any increase in applied chlorine results in sharp decrease inthe residual because of two reasons:

1. A lot of applied chlorine is utilized in oxidisation of organic matter.

2. The increased concentration of applied chlorine breaks down chloroamines formed inStage II by changing them back into nitrogen compounds, thus reducing the residualchlorine.

The oxidisation of organic matter is accompanied by bad smell and taste.

• Stage IV: At point D, the oxidisation of organic matter is completed and the bad smelland taste also vanishes. The residual chlorine at point D represents the true residualfree chlorine since the chlorine demand has now been completely satisfied. On furtherapplication of chlorine, the applied chlorine will directly appear as residual chlorine aschlorine demand is completely satisfied. The slope of line E will thus be 45◦.

Point D on the curve is called as break point because any chlorine that is added to water beyondthis point simply ‘breaks’ through the water and appears as residual chlorine. Therefore, atbreak point, nearly all residual chlorine will be free chlorine.

3 Significance of Break Point Chlorination

The chlorine is applied to water at a dose equal to or slightly greater than that at which thebreak point occurs. It has the advantages of:

• It will leave the desired chlorine residual.

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Water Supply Engg. AE Coaching 2

• It will remove taste and odour.

• It will have adequate bacterial killing effect.

• It will ensure that oxidation of ammonia and other compounds is complete.

Generally, the chlorine dose at which break point occurs varies from 3—7 ppm.

Figure 1:

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