Essay on Disinfection of Water | Water Treatment | Water Engineering

In this essay we will discuss about:- 1. Factors Affecting Bacterial Efficiency of Chlorine 2. Chlorine Compounds 3. Addition of Chlorine to Water 4. Free Available Chlorine 5. Chlorine Demand 6. Methods of Chlorination 7. Forms of Chlorine.

Essay # 1. Factors Affecting Bacterial Efficiency of Chlorine:

The bacterial efficiency of chlorine under ideal conditions depends on the following factors:

(i) Time of Contact:

The number of organisms destroyed per unit of time is proportional to the number of organisms remaining. Experiments have shown that the death rate goes on decreasing with time.

(ii) The Concentration of Disinfectants:

The time required to effect a constant percentage of bacteria or other organisms by any disinfectant depends upon the concentration of the disinfectant. The time concentration relation is of great importance because it has been found that two disinfectants that are equally potent at one concentration (of the organisms) may have quite different efficiencies at some other concentration of the disinfectant.

(iii) The Number of Organism:

The higher the number of organism, the greater the percentage destruction.

(iv) Temperature of Water:

The rate of reaction with enzyme increases with temperature. Therefore, the death-rate increases with rise in temperature.

(v) pH Value of Water:

The change of pH value also has a great effect on the disinfecting power of chlorine, whether in the form of hypochlorite or of simple chloramines. It has been seen that at lower pH values, a smaller contact period is required for the same percentage of kill.

(vi) The Presence of Various Chemicals:

Various chemicals present in water will react with the chlorine, which is added for disinfection and will thus reduce the concentration of chlorine available for direct disinfection.

Essay # 2. Chlorine Compounds:

Various chlorine compounds which are used as disinfectants are hypochlorites of calcium and sodium, the chloromines, chlorine dioxide and complex chlorine compounds such as (CH₂, CO)₂ NCI etc. For comparing the activity and utility of these various compounds, it is necessary to know the available chlorine in each compound. Table 14.1 shows the percentage of available chlorine by weight, found in various pure compounds compared with the percentage of chlorine by weight, present in the compounds.

Essay # 3. Addition of Chlorine to Water:

The following observations are seen while adding chlorine in water containing various impurities:

(i) When chlorine is added in water containing ammonia and organic nitrogen compounds, Monochloramine (NH₂CI) dichloramine (NHCI₂) and tri-chloramine (NCI₃) are released, but their distribution depends on the pH-value of water. NCI₃ is not formed in sufficient amount at normal pH-Value unless the break point is approached.

(ii) When water is free from organic impurities, hypochlorous acid (HOCI), hypochlorite ion (OCI) and elemental chlorine are released and their distribution depends on the pH-Value of water. The two prevailing species (HOCI) and (OCI) are called ‘free available chlorine’ in practice and are responsible for the disinfection of water.

(iii) If water contains sewage and waste waters, when chlorine is added, complex organic chloromines are released.

Essay # 4. Free Available Chlorine:

When chlorine is dissolved in water, it hydrolyses immediately as:

CI₂, + H₂O = HOCI + H⁺ + CI

At ordinary temperatures, this reaction is completed within a few seconds. In strong solutions of chlorine, only a portion of chlorine reacts this way but normally the hydrolysis is complete.

But after some time, the hypochlorous acid further ionizes as follows:

HOCI = H⁺ + OCI

Thus, any free chlorine or hypochlorite when added to water will immediately distributed itself into elemental chlorine. HOCl and OCI and their ratio is controlled entirely by temperature and pH-Value. When pH-Value is less then 5 only elemental chlorine exist; when pH value is between 5 and 10, Both HOCI and OCI exists, but after pH-value of 10 only hypochlorite exists.

From all the above three forms of chlorine only HOCl is the most destructive. HOCl is about 80 times more destructive of bacteria than OCI ions, therefore water should have such pH-Value at which this form of chlorine can remain in maximum concentration.

Essay # 5. Chlorine Demand:

Chlorine demand is defined as the difference between the amount of chlorine added to water and the amount of chlorine (free available and combined available) remaining at the end of a specified contact period.

The chlorine demand for a sample of water depends on:

(a) Nature and concentration of chlorine consuming substances present in water.

(b) Time of contact.

(c) pH-value of water.

(d) Temperature of water.

(e) Variable conditions in the process of chlorination.

(f) And so many other factors.

The chlorine dose for a given water can be determined by two methods. In the first method tests are made and the residual chlorine is determined for various chlorine doses. The results are then plotted on a graph paper and a smooth curve is drawn.

From the curve, the dose corresponding to the desired residual is determined directly. Second method is mathematical method in which the required dose is extrapolated or interpolated from the observed values.

Essay # 6. Methods of Chlorination:

i. Break Point Chlorination:

When chlorine is added to water it reacts with organic and inorganic matter and forms common compounds. Some portion of chlorine remains as residual chlorine, which is not the true but combined residual and is much weak in disinfecting action. Now, if the chlorine dose is increased, the combined available residual chlorine also increase.

When the chlorine dose is so increased, the compounds get oxidized and the substances which are newly formed, do not react with Orthotolodine to show any residual. If the addition of chlorine is continued and a graph is plotted between chlorine dose and residual chlorine, a curve is obtained as shown in Fig. 14.1.

On studying the curve it will be noticed that residual chlorine in the beginning increases with the applied chlorine dose, but after point C it suddenly drops up to point D and then increases. Portion OC shows formation of chloramines and portion CD shows their oxidation.

Point D at which residual chlorine again starts increasing is known as ‘Break-Point chlorination’. After reaching this point if chlorine is added, it remains as free residual chlorine and curve becomes a straight line.

The shape of the curve depends on the type of ammonia present in the water. A greater amount of ammonia nitrogen tends to produce a high ‘hump’ and sharp ‘breack point’, while a greater amount of albuminoid nitrogen tends to smooth out both the ‘hump and break’ The amount of ammonia in water also governs the extent of magnitude of hump and breack areas.

The following are the advantages of Break-point chlorination:

1. It completely oxidizes the ammonia and other impurities of water.

2. The colour of water which is due to organic matters is also removed.

3. It completely destroys all the disease bacteria.

4. It removes taste and odour from the water.

5. It prevents growth of weeds in water.

ii. Super-Chlorination:

Super-chlorination is defined as the administration of a dose considerably in excess of that necessary for the adequate bacterial purification of water. Under certain circumstances such as during epidemics of water-borne diseases, high dose of chlorine is given to the water, generally 2 to 3 p.p.m. beyond the break point, for the safety of public.

The adding of chlorine in excess is called ‘super-chlorination’ and gives a strong odour and taste of chlorine in the treated water, which can be removed by dechlorination.

iii. Dechlorination:

It is defined as the partial or complete reduction of residual chlorine in water by chemical or physical treatment. In this method, some chemicals are added for the purpose of reducing the chlorine residual to a desired value.

There are several methods of dechlorination, but the most common are:

(a) Sulphur Dioxide (SO₂):

It is mostly used for larger plants.

The chemical reaction is:

SO₂ + Cl₂ + 2H₂O → H₂SO₄ + 2HCl

The acids so formed are neutralized by the natural alkalinity present in the water. Theoretically, 0.905 p.p.m. Sulphur dioxide is required for each p.p.m. of chlorine.

(b) Sodium Bi-Sulphate (NaHSO₃):

It is used for smaller treatment plants. It is cheaper and more suitable than sodium sulphite.

The chemical reaction is:

NaHSO₃ + Cl₂ + H₂O → NaHSO₄ + 2HCI

(c) Sodium Thiosulphate (Na₂S₂O₃):

It is usually used to dechlorinate the samples of water that are collected for bacteriological analysis. It may be kept in the bottle either as a solution or crystalline form.

The chemical reaction is:

2Na₂S₂O₃ + Cl₂ + H₂O → Na₂S₂O_A + 2HCl

(d) Activated Carbon:

It may be used for smaller plants. Granular activated carbon absorbs chlorine into its pores where the chlorine oxidizes the carbon to carbon dioxide.

The chemical reaction is:

C + 2Cl₂ + 2H₂O → CO₂ + 4HCl

(e) Aeration:

Aeration removes only a small portion of free available chlorine from water because calcium or sodium hypochlorites formed by the reaction of the hypochlorous acid with the alkalinity of the water are not volatile. Among the combined available chlorine residual, aeration readily removes very little of monochloraminc. Aeration also removes readily the hypochlorous acid.

(f) Ammonia:

It can be economical to use as dechlorinator because it reacts and forms chloramines.

iv. Plain Chlorination:

In some places where good surface water is available, it is used with no other treatment except chlorination. In hilly areas, only chlorination is done and the water is safeguarded against disease. Such type of chlorination is known as plain chlorination. The water of lakes and springs is pure and can be used after plain chlorination.

If the water contains organic matters, more dose of chlorine should be added and contact period should be increased. The chlorine is added in the water in the pipe leading from an impounding reservoir to the city.

v. Post-Chlorination:

When the chlorine is added in the water after all treatment, it is known as Post- chlorination. Generally this is done after filtration process. The chlorine may be added in the suction pipe, but it is more suitable to add it in the clear water well. The minimum contact period should be 30 minutes, before use of water.

vi. Pre-Chlorination:

When chlorine is added to raw water before any treatment it is known as pre-­chlorination. Thus chlorine is added in small dosage before raw water enters sedimentation tanks. The dosage should be so adjusted that about 0.10 to 0.50 ppm of chlorine comes to the filter plant.

vii. Double Chlorination:

When chlorine is added to raw water at more than one point, it is known as double chlorination. When raw water is highly contaminated and contains large amount of bacterial life, it becomes necessary to adopt pre-chlorination and post-chlorination for such water.

Essay # 7. Forms of Chlorine:

Chlorine is generally available in the following forms:

(i) In the form of liquid chlorine.

(ii) In the form of gaseous chlorine.

(iii) In the form of chlorine dioxide.

(iv) In the form of chloramines i.e., the mixture of chlorine and ammonia

(v) In the form of bleaching powder.

Application of Gaseous Chlorine:

When water is to be treated in large water-works, gaseous chlorine is used. The gaseous chlorine is absolutely pure and has 100% available chlorine. Chlorine is available in amount of 50, 75 and 100 kg cylinders stored under a pressure of 7 to 11 kg/cm² in seamless steel cylinders.

Very large steel cylinders containing one tonne of chlorine are also used in very large plants. The chlorine gas may first be dissolved in a small quantity of water and the solution so prepared is fed to the point of application.

Gaseous chlorine should not be applied directly because:

(i) There is possibility of corrosion in pipes and connected fittings due to the accumulation of un-dissolved gas.

(ii) Chlorine gas may not fully diffuse in water.

(iii) At low temperatures there is possibility of chlorine concentration around the diffuser.

(iv) Chlorine cannot be fed in pipes above 1.75 to 2.1 kg/cm² pressure.

Equipment for Application:

Fig. 14.2 shows a chlorinator designed and manufactured by M/s National Hydraulics. This equipment is injector type, but gravity feed type models are also available. The injector requires steam of water about 550 litres for each kg of chlorine, at a pressure three times that of chlorine gas.

The injector assembly consists of a tray of cast iron carrying the ebonite injector body.

The injector unit consists of an ebonite suction block in which are fitted the injector throat, reflex valve and vent bell.

The gas delivered to injector is solutioned by a high pressure water supply. If a high pressure is not available it may be necessary to install a booster pump. The float operated inlet valve is of special design and is supported in tray by the service water supply pipe.

Adjustment of water in the tray is effected by screwing the ebonite cap of water in the tray is effected by screwing the ring is split and is provided with a taper headed screw which expands the rings at the spited and locks it in the cup.

The valve is in correct adjustment when it maintains the water level with the top of the overflow fitting when the injector is in operation but no gas passing. This being the condition when the maximum amount of make-up water is required by the injector.

Application of Liquid Chlorine:

In filtered water, if liquid chlorine is applied at such point where adequate mixing is done, it is most effective in disinfection. Liquid chlorine can be applied to any pressure. The rate of application can be controlled manually, automatically, mechanically at various points. The plant provision should be made to mix the chlorine in water immediately after application and during the period of reaction. Water should not be drawn from the mains before 15 minutes after application.

Equipment for Application:

Liquid chlorinators are used for the application of chlorine in water. Fig. 14.3 shows liquid chlorinator of M/s Wallace and Tiernan Co.

Fig. 14.3 is self-explanatory. To avoid difficulties in operation and other fluctuations, it is most necessary to keep it clean and free from dirt, oils, etc. For cleaning such equipment’s only derivatives of methane such as wood alcohol, chloroform or carbon tetrachloride should be used.

Use of Bleaching Powder:

Hypochlorites of calcium and sodium may be used for the chlorination of small water works, private industry, colony or estate. When the hypochlorites are added in water following chemical action takes place.

The hypochlorite ions obtained above further combine with the hydrogen ions present in water and form hypochlorous acid as follows:

The hypochlorous acid so formed kills the bacteria present in the water, as it has been described earlier. Theoretically, calcium hypochlorite should have 70% free chlorine, but usually the chlorinating ability of the calcium hypochlorite varies from 25-30%. The chlorinating ability of bleaching powder varies from 65-70% for high chlorine compounds.

The commercial bleaching powder normally contains low values of chlorine which vary from 25-30%. The value of chlorine content continuously decreases if the powder is exposed to the atmosphere. Before using the bleaching powder, its strength should first be checked by actual practicals in the laboratory.

But now-a-days due to availability of gaseous and liquid chlorine, the bleaching powders are not used in the water works for chlorination.

Example:

The water works of a town of population 25,000 has to meet its water demand at the rate of 135 litres/capita/day. If the disinfection is to be done by the bleaching powder having 45% available chlorine, determine the quantity of the bleaching powder required per year. The required dose of chlorine at the water-works is 0.3 ppm for disinfection.

Solution:

Disinfection by Chloramines:

Chloramines is the name given to the compound which contains chlorine and ammonia. Chloramines are stable compounds and are good disinfectants. They can remain in water as residuals for long period; therefore, they can provide greater safety against future contamination.

Chloramines do not cause bad tastes and odours even when their residuals are present in the water. Their usefulness increases when phenol is present in the water, because phenol reacts with chlorine only and causes tastes, but it does not react with chloramines.

Chloramines are produced by adding ammonia to the filtered water before adding chlorine. The quantity of ammonia and chlorine depends on the quantity of water and local characteristics of the distribution system.

Following are the reactions involved:

The monochloramine (NHCl₂) predominates at pH value over 7.50, NHCl₂ at pH of 5.0 to 6.50 and NCI₃ at pH below 4.40 the ammonia is added to water generally in the ratio of 1: 2 to 1:4 as NH₃: Cl₂. The ammonia may be used in the form of gas or as solution.

One precaution is necessary when water is treated with the chloramines. The water, after treatment is completed should be supplied to consumers after an interval of about 20 to 60 minutes.

Higher dose of chloramine or longer contact period is required for disinfection of water with chloramines.

Testing of Chlorine Residuals:

The determination of the residual chlorine after the desired contact period is necessary.

It is tested by the following two methods:

(a) Orthotolidine Test

(b) Orthotolidine-Arsenite Test

(c) Starch Iodide Test.

(a) Orthotolidine Test:

This is most easy and common test for determining the residual chlorine. In this test 100 ml of chlorinated water sample is collected in the test tube. 1 ml of orthotolidine solution is added in the test tube. The colour formed is noted and the value of the residual chlorine is directly determined by comparing the colour so obtained with the standard colours of known chlorine residuals.

But as this test gives the value of total residual (free and combined) chlorine, therefore care should be taken while performing this test. The free residual chlorine forms the yellow colour during the first 5 seconds, while the combined residual chlorine forms yellow colour, after about 5 minutes. Therefore, the value of colour compared after 5 seconds will give the value of free chlorine.

For obtaining the value of combined residual of chlorine, its colour should be compared with the standard colours after adding of orthotolidine in the test tube. Therefore, the value of chlorine residual obtained after 3 minutes will be the combined value of free as well as combined chlorine. To get the value of combined chlorine only the value obtained after 5 seconds should be subtracted from the value obtained after 5 minutes.

The presence of iron, manganese, nitrites greatly affects the .yellow colours, so under their presence the orthotolidine test does not give correct results.

(b) Orthotolidine-Arsenite (OTA) Test:

As the presence of some metals as minerals causes difficulty in performing the orthotolidine test which gives wrong result. Therefore in the water containing soluble minerals, to determine the residual chlorine, Orthotolidline-Arsenite test is performed.

In this test, sample of chlorinated water is collected, and sodium arsenite is added in it, which will dechlorinate the sample water. Now orthotolidine is added as described above. The colour obtained will be due to the interfering agents like nitrites, manganese, iron etc. The colour obtained is compared with the standard colour and let the concentration of chlorine be K₁.

Now second sample of water is taken and orthotolidine solution is added first and just after 5 seconds sodium arsenite is added. The sodium arsenite will not allow the combined chlorine to form colour. Therefore, the colour formed in this case will be due to free chlorine and interfering agents like iron, manganese, nitrites etc. The colour is compared with the standard colours, and let concentration of chlorine be K₂.

Now the third sample of water is taken and only orthotolidine solution is added in it. The colour obtained will be due to free chlorine, combined chlorine and due to interfering agents like iron, manganese, nitrites etc. The colour so obtained is compared with the standard colours. Let the chlorine concentration in this case be K₃.

The determination of residual chlorine is done as under:

(i) Chlorine concentration due to interfering agents

= K₁

(ii) Chlorine concentration due to free chlorine + interfering agents

= K₂

(iii) Chlorine concentration due to free + combined chlorine + interfering agents

= K₃

.^.. Quantity of ‘free’ residual chlorine = (K₂– K₁,)

.^.. Quantity of ‘combined’ residual chlorine

= (K₃– K₂,)

If while performing the test blue tinge is obtained it will indicate the presence of high alkalinity in the water. In such cases the quantity of orthotolidine should be doubled.

(c) Starch Iodide Test:

For performing this test one litre of water sample is collected in heat­proof earthen ware container known as casserole. Now 13 ml of potassium iodide solution is added and the water is stirred thoroughly. Now 5 ml of starch solution is added. The blue colour is removed by titrating this water sample against NI 10. Normality solution of sodium thiosulphate.

The amount of residual chlorine is determined by the following simple equation:

But as starch-iodide test is costly and time consuming this is not used in the water works.