Forms of Chlorination: 7 Forms | Water Treatment | Water Engineering

Depending upon the stage of treatment at which chlorine is applied to water and also upon the expected results of application of chlorine; chlorination may be of the following forms:

Form # 1. Plain Chlorination (or Simple Chlorination):

Plain chlorination (or simple chlorination) indicates that only chlorine treatment and no other treatment is given to raw water, and after giving only chlorine treatment to raw water it is supplied to the consumers. The application of chlorine to raw water helps to remove bacteria and colour from water and it also controls the growth of algae.

Plain chlorination can be resorted to in situations where:

(i) Raw water is relatively clear having low turbidity and colour, turbidity not exceeding 5 to 10 NTU;

(ii) Raw water is drawn from relatively unpolluted sources;

(iii) Raw water contains little organic matter and iron and manganese do not exceed 0.3 mg/l; and

(iv) Sufficient contact period between the point of chlorination and the consumer end is available.

The usual dose of chlorine for plain chlorination is between 0.5 and 1 p.p.m.

Form # 2. Pre-Chlorination:

When chlorine is added to raw water before any treatment it is known as pre-chlorination. Under pre-chlorination chlorine is added before raw water enters sedimentation tanks.

Pre-chlorination has the following advantages:

(i) It helps in reducing the quantity of coagulants required because of the oxidization of organic matter.

(ii) It helps to improve coagulation.

(iii) It reduces taste and odour of water.

(iv) It controls the growth of algae in sedimentation tanks as well as in filters.

(v) It reduces the bacterial load on filters.

(vi) It helps in maintaining filter media (i.e., filter sand) clean and thus the interval of cleaning filters may be increased, or longer filter runs may be maintained.

(vii) It prevents putrefaction of sludge in the settling tanks.

For pre-chlorination the dose of chlorine should be such that water has residual chlorine of 0.1 to 0.5 p.p.m. when it enters the filter plant.

Form # 3. Post-Chlorination:

Post chlorination indicates the application of chlorine to water after all the treatments for purification of water are completed. This is the standard form of chlorination in which chlorine is added to water as it leaves filters and before it enters distribution system.

The dose of chlorination should be such that residual chlorine of about 0.10 to 0.20 p.p.m. appears in water at the point of its entry into the distribution system. The residual chlorine present in water is useful for its protection against contamination in the distribution system.

Form # 4. Double or Multiple Chlorination:

Double or multiple chlorination refers to the application of chlorine to water at two or more points in the purification process. When raw water is highly contaminated and contains large amount of bacterial life, it becomes necessary to adopt double chlorination.

Double chlorination consists of pre-chlorination in which chlorine is applied just before raw water enters sedimentation tanks, and post chlorination in which chlorine is added to water as it leaves filters and before it enters distribution system. The advantages of double chlorination are similar to those of pre-chlorination. In addition, the second unit of chlorination plant serves as a stand-by unit.

Form # 5. Break Point Chlorination (or Free Residual Chlorination):

When chlorine is added to water the following two actions take place:

(i) It kills bacteria present in water, thus disinfection is accomplished, and

(ii) It oxidizes the organic matter present in water, i.e., chlorine demand is satisfied.

If water has no chlorine demand, any chlorine added to such water will appear as residual chlorine and hence relation between applied and residual chlorine will be as indicated by line A, having a slope of 45° as shown in Fig. 9.38 which shows relationship between applied and residual chlorine. However, water generally has some chlorine demand, on account of which the relation between applied and residual chlorine will be as indicated in Fig. 9.38.

When chlorine is first added it performs the function of killing bacteria and also reacts with readily oxidizable substances such as iron, manganese, nitrites, sulphides and organic matter that may be present in water, and reduce most of these by oxidation to chloride ions. This is indicated by stage I in Fig. 9.38 during which since entire amount of applied chlorine is utilized for killing bacteria and oxidizing the various substances present in the water there is no residual chlorine available.

After meeting this immediate demand, the chlorine continues to react with compounds such as ammonia, proteins, amino acids and phenols that may be present in the water to form chloramines and chloro-organic compounds which constitute the combined available chlorine.

This represents stage II during which the relation between applied and residual chlorine is represented by curve B shown in Fig. 9.38. The combined available chlorine is also recorded as residual chlorine along with the freely available residual chlorine.

Therefore with the increase in the applied chlorine the residual chlorine also increases and the curve B goes on rising till point C is reached where the amount of residual chlorine is reported to be maximum. At this stage with further increase in the applied chlorine there is sudden decrease in the residual chlorine.

The sudden decrease in the residual chlorine is due to the fact that the increased concentration of the applied chlorine breaks down chloramines by changing them to nitrogen compounds, thus reducing the residual chlorine, and also a lot of applied chlorine is utilized in oxidization of the organic matter present in the water.

This represents stage III which is sometimes accompanied by bad smell and taste. During stage III the relation between applied and residual chlorine is represented by curve CD (Fig. 9.38). At point D the bad smell and taste suddenly disappear and the oxidization of the organic matter is also complete.

Further at point D the residual chlorine has its minimum value which reveals the true residual free chlorine since chlorine demand has been satisfied. A further increase in the applied chlorine beyond point D results in an increase in the residual chlorine as represented by line E the slope of which will be 45° because the entire applied chlorine will appear as residual chlorine. This represents stage IV (Fig. 9.38).

Point D on the curve is known as break point because any chlorine that is added to the water beyond this point breaks through the water and appears as residual chlorine. The break point in the chlorination of water maybe defined as the point on the applied-residual chlorine curve at which all, or nearly all, the residual chlorine is free chlorine.

It may, however, be stated that if water does not contain readily oxidizable substances, then stage I will not exist and only the reactions of stages II, III and IV will take place, in which case the applied-residual chlorine curve will be passing through the origin as indicated in Fig. 9.38 (a).

The application of chlorine to water with chlorine dose equal to or slightly higher than that at which break point occurs is known as break point chlorination or free residual chlorination.

Break point chlorination has the following advantages:

(i) It will remove taste and odour.

(ii) It will have adequate bactericidal effect

(iii) It will leave desired chlorine residual.

(iv) It will complete the oxidation of ammonia and other compounds.

(v) It will remove manganese.

The break point stage should be determined by laboratory tests. However, in some cases a distinguishable break point is not obtained, and in some the changes in the quality of raw water may affect rapid changes in break point. A recognizable break point may be induced by addition of ammonia to water. Generally the chlorine dose, at which break point occurs, lies between 3 and 7 p.p.m, and this is greatly affected by the quantity of free ammonia present in water.

Form # 6. Super-Chlorination:

The application of chlorine to water beyond the stage of break point is known as super-chlorination. The dose of chlorine applied to water for super-chlorination should be such that the residual chlorine content after break point may be 0.5 to 2 p.p.m. Higher residual chlorine content is useful in removing odours and tastes from water, particularly those resulting from chloro-products formed between the decomposition products from vegetable matter and algae.

The excess chlorine may be added to water at any point or points in the treatment process, but it is most commonly added at the end of filtration. Super-chlorination is followed by a retention or contact period of 30 to 60 minutes. However, the presence of excess chlorine in water imparts unpleasant taste and odour to water. As such when super-chlorination is practised, it becomes necessary to remove the excess chlorine by any suitable method of dechlorination before water is supplied to the consumers.

Super-chlorination is usually adopted when there is an epidemic in the locality, or when water is liable to sudden fluctuations in chlorine demand due to high content of organic impurities, or when water contains cysts of E. histolytica, an organism causing amoebic dysentery.

Form # 7. Dechlorination:

The process of removing excess chlorine from water is known as dechlorination. The purpose of dechlorination is to avoid chlorinous taste from water before distribution to the consumers. The dechlorination should be done in such a way that at the end of dechlorination some residual chlorine remains in water.

Dechlorination of water may be accomplished by adding certain chemicals to water. The chemicals used for dechlorination of water are sodium thiosulphate (Na₂S₂O₃), sodium metabisulphate (Na₂S₂O₅), sodium sulphite (Na₂SO₃), sodium bisulphite (NaHSO₃), ammonia (NH₄OH), and sulphur dioxide (SO₂).

The sulphites are granular solids that may be added to water by dry or by solution feed. Although sodium sulphite is cheaper in cost per unit weight, but the required dose of sodium thiosulphate is lower for the same results.

Ammonia may be useful and economical as a dechlorinator because of its reaction with chlorine to form chloramines.

Sulphur dioxide gas is relatively inexpensive as a dechlorinator and it may be applied in the same manner as chlorine with a contact period of not less than 10 to 15 minutes, and a dose of 0.3 to 0.6 p.p.m depending upon the excess chlorine present in water.

The reaction between sulphur dioxide and chlorine is indicated by the following equation:

This equation indicates that about 0.9 part of sulphur dioxide (by weight) is required for 1 part of chlorine to be removed. However, in actual practice for the removal of 1 part of chlorine about 1.12 parts of sulphur dioxide is required which is about 25% in excess of the theoretical requirements indicated by the above equation.

Activated carbon may also be used for dechlorination, which removes chlorine by adsorption. It may be applied as a powder to water before filtration. Alternatively by filtering super-chlorinated water through beds of granular activated carbon excess chlorine oxidizes the carbon to carbon dioxide, due to which odour, taste and colour are removed.

Dechlorination may also be accomplished by treating water with magnesium metal, by prolonged storage of water particularly when exposed to sunlight, and by aeration.