Treatment of Water for Domestic Supply (or) Municipal Water Treatment

TREATMENT OF WATER FOR DOMESTIC SUPPLY (OR) MUNICIPAL WATER TREATMENT

Rivers and lakes are the most common sources of water used by municipalities. These water should be free from colloidal impurities, domestic sewages, industrial effluents and disease producing bacterias. Hence domestic supply of water involves the following stages in the purification processes.

1. Primary Treatment

1. Screening

It is a process of removing the floating materials like, leaves, wood pieces, etc. from water. The raw water is through a screen, having large number of holes, which retains the floating materials and allows the water to pass.

2. Aeration

The process of mixing water with air is known as aeration. The main purpose of aeration is

(i) to remove gases like CO₂, H₂S and other volatile stat impurities causing bad taste and odour to water.

(ii) to remove ferrous and manganous salts as insoluble ferric and manganic salts.

(iii) Promotes taste and odour of water.

(iv) Increases the oxygen concentration in water.

3. Sedimentation

It is a process of removing suspended impurities by allowing stand undisturbed for 2‒6 hours the water to in a big tank. Most of the suspended particles settle down due to forces of gravity, and they are removed. Sedimentation removes only 75% of the suspended impurities.

4. Coagulation

Finely divided clay, silica, etc. do not settle down easily and hence cannot be removed by sedimentation. Such impurities are removed by coagulation method.

In this method certain chemicals, called coagulants, like alum, Al₂(SO₄)₃ etc., are added to water. When the Al₂(SO₄)₃ is added to water, it gets hydrolysed to form a gelatinous precipitate of Al(OH)₃. The gelatinous precipitate of Al(OH)₃ entraps the finely divided and colloidal impurities, settles to the bottom and can be removed easily.

5. Filtration

It is the process of removing bacteria, colour, taste, odour and suspended particles, etc., by passing the water through filter beds containing fine sand, coarse sand and gravel. A typical sand filter is shown in the figure 1.5.

The sand filter consists of a tank containing a thick top layer of fine sand followed by coarse sand, fine gravel and coarse gravel. When the water passes through the filtering medium, it flows through the various beds slowly. The rate of filtration decreases slowly due to the clogging of impurities in the pores of the sand bed. When the rate of filtration becomes very slow, the filtration is stopped and the thick top layer of fine sand is scrapped off and replaced with clean sand. Bacterias are also partly removed by this process.

II. Sterilisation (or) Disinfection

The process of destroying the harmful bacterias is known as sterilisation or disinfection. The chemicals used for this purpose are called disinfectants. This process can be carried out by the following methods.

1. By using ozone

Ozone is a powerful disinfectant and is readily absorbed by water. Ozone is highly unstable and breaks down to give nascent oxygen.

O₃ → O₂ + [O]

The nascent oxygen is a powerful oxidising agent and kills the bacterias.

Advantages

(a) O₃ is a multi functional agent. It acts as a bleaching, sterlizing, deodourising agent.

(b) It is harmless in drinking water.

Disadvantages

(a) This process is costly and cannot be used in large scale.

(b) Ozone is unstable and cannot be stored for long time.

2. By using ultraviolet (UV) radiations

UV rays are produced by passing electric current through mercury vapour lamp. This is particularly useful for sterilizing water in swimming pool.

Advantages

(i) No requirements of any chemical.

(ii) No changes in colour and odour.

(iii) It is a very good cleaning agent.

Disadvantages

(i) It is costly.

(ii) Turbid water cannot be treated.

3. By chlorination

The process of adding chlorine to water is called chlorination. Chlorination can be done by the following methods.

(a) By adding chlorine gas

Chlorine gas can be bubbled in the water as a very good disinfectant.

Advantages

(i) Chlorine gas is harmless.

(ii) It is highly effective and economical.

Disadvantages

(i) Handling is difficult.

(ii) Excess chlorination causes unpleasant odour.

(b) By adding chloramine

When chlorine and ammonia are mixed in the ratio 2:1, a compound chloramine is formed.

Cl₂ + NH₃ → CINH₂ (Chloramine) + HCI

CINH₂ + H₂O → NH₃ + HOCI (Hypochlorous acid)

Chloramine compounds decompose slowly to give chlorine. It is a better disinfectant than chlorine.

Advantages

(i) It reduces the formation of cancer causing disinfection by products.

(ii) It is more stable and thus provides longer lasting residual disinfectant action.

Disadvantages

(i) Formation of dichloramine and trichloramine affect the taste and odour of water.

(ii) It can cause respiratory problems.

(c) By adding bleaching powder

When bleaching powder is added to water, it produces hypochlorous acid (HOCl). HOCl is a powerful germicide.

CaOCl₂ (Bleaching powder) + H₂O → Ca(OH)₂ + Cl₂

Cl₂ + H₂O → HCI + HOCI (Hypochlorous acid)

HOCI + Bacterias → Bacterias are killed

Advantages

 (i) It is eco‒friendly and effective disinfectant.

 (ii) It has long shelf life.

Disadvantages

(i) Irritation to skin and respiratory system.

(ii) Harmful to aquatic life.

(iii) Decomposes overtime.

Break point chlorination

Chlorine may be added to water directly as a gas (or) in the form of chloramine (or) bleaching powder.

During this process the following impurities, present in the water, are destroyed.

(i) Bacteria

(ii) Organic impurities

(iii) Reducing substances (Fe²⁺, H₂S etc.,)

(iv) Free ammonia

When chlorine is applied to water, the results obtained can be depicted graphically in the following figure 1.6. The graph is drawn between the amount of chlorine, added to water, and the residual chlorine. It is explained as follows.

Explanation of the graph

The graph is explained based on the curves and points.

(i) Curve OA

The curve "OA" shows, the applied chlorine gas is utilized for the oxidation of reducing substances present in the water. So upto point "A" no Cl₂ gas is available for disinfection.

(ii) Curve AB

The curve "AB" shows the formation of chloramine and chloro compound. As the amount of applied chlorine increases the amount of combined residual chlorine also increases.

(iii) Curve BC

The curve "BC" shows the destruction of chloramine and chloro compounds, where the combined residual chlorine content is going on decreasing upto the point 'C'.

(iv) Point 'C'

Point 'C' is known as "Break point chlorination". At this point all the reducing substances, organic impurities, bacteria and ammonia are oxidized and no combined residual chlorine content is available, free residual chlorine begins to appear and acts as a very good disinfectant.

(v) Curve CD

The curve "CD" shows the free residual chlorine gas is going on increasing as the applied chlorine gas increases. It shows that the sterilization effect is very good.

Break point chlorination definition

It is the point where all the bacterias, reducing substances, organic compounds and ammonia are completely removed and the applied chlorine begins to act as a powerful disinfectant.

Advantages of break point chlorination

1. It completely removes colour, odour and bad taste from water.

2. It also destroys the bacterias.

3. It oxidizes all the organic matters and other reducing substances like Fe²⁺, Mn²⁺, H₂S etc.,