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.

I. Primary Treatment

1. Screening

It is a process of removing the floating materials like, leaves, wood pieces, etc. from water. The raw water is allowed to pass 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 impurities causing bad taste and odour to water.

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

3. Sedimenation

It is a process of removing suspended impurities by allowing the water to stand undisturbed for 2-6 hours in a biy tank. Most of the suspended particles settle down at the bottom, 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 1.1.

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 party 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.

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.

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.

(b) By adding chloramine

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

Cl₂ + NH₃ → CINH₂(Chloramine) +  HCl

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

(c) By adding bleaching powder

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

CaoCl₂ (Bleaching powder ) + H₂O → Ca(OH)₂ + Cl₂

Cl₂ + H₂ O → HCl + HOCI(Hypochlorous acid)

HOCl + Bacterias → Bacterias are killed

Break point Chlorination

Water contains the following impurities

(i) Bacterias.

(ii) Organic impurities.

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

(iv) Free ammonia.

Chlorine may be added to water directly as a gas or in the form of bleaching powder. When chlorine is applied to water, the results obtained can be depicted graphically in the following Fig. 1.2. The graph shows the relationship between the amount of chlorine added to water and the residual chlorine.

It is observed from the graph that initially the applied chlorine is used to kill the bacterias and oxidises all the reducing substances present in the water and there is no free residual chlorine.

As the amount of applied chlorine increases, the amount of combined residual chlorine also increases. This is due to the formation of chloramine and other chloro compounds.

At one point, on further chlorination, the oxidation of chloramines and other impurities starts and there is a fall in the combined chlorine content.

Break point chlorination is the point at which the combined residual chlorine decreases to a minimum point where oxidation of chloramines and other impurities complete and free residual chlorine begins to appear.

Thus, the break point chlorination eliminates bacterias, reducing substances, organic substances responsible for the bad taste and odour, from the water.