Anna University Long Questions and Answers

Unit - I

Chapter 1

Water and its Treatment

Anna University Long Questions & Answers

Part - B

 

1. Define and explain the significance of the followings

(i) turbidity

(ii) hardness

(iii) pH

(i) Turbidity

Turbidity is the reduction of clarity of natural water due to the presence of finely divided, insoluble impurities suspended in water.

Significance

(i) Turbidity affects the taste and odour of drinking water.

(ii) As turbidity affects the disinfection process, it must be removed.

(iii) Turbidity have many negative effects on aquatic life, it block light to aquatic plants, aquatic organisms.

(iv) Turbidity affects the growth rate of algae.

(v) It increases water temperature because suspended particles absorbs more heat.

(ii) Hardness

Hardness is the property (or) characteristics of water, which does not produce lather with soap.

Types of hardness

Depending upon the types of dissolved salts present in water, hardness of water can be classified into two types

1. Temporary hardness.

2. Permanent hardness.

1. Temporary hardness (or) Carbonate hardness (CH) (or) Alkaline hardness

This is due to the presence of bicarbonates of calcium and magnesium. It can be removed by (i) boiling the water (ii) adding lime to the water.

2. Permanent hardness (or) Non-carbonate hardness (NCH) (or) Non-alkaline hardness

This is due to the presence of chlorides and sulphates of calcium and magnesium. It cannot be removed by boiling the water. But, it can be removed by (i) Lime-soda process (ii) Zeolite process.

Significance of Hardness

1. Hardness affects the amount of soap that is needed to produce foam (or) lather.

2. Hardness is very important in industrial uses, because it forms scale in heat exchange equipment boilers and pipe lines.

3. Hardness mitigates metal toxicity because Ca²⁺ and Mg²⁺ help keep fish from absorbing metals such as lead, arsenic and cadmium into their blood stream.

(iii) pH

The hydrogen ion concentration is represented by the pH value, which is defined as

pH = - log10[H⁺]

Significance of pH

(i) pH determines the solubility (amount that can be dissolved in water).

(ii) It also determines the biological availability (amount that can be utilized by aquatic life).

(iii) A rise (or) fall in pH can indicate chemical pollution (or) acid rain. Many animals cannot live in water at a pH level below 5 (or) above 9.

 

2. What is meant by disinfection? How is it carried out? Explain in details about the break point chlorination.

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.

Fig: Break point chlorination

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. 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.

 

3. Explain the following boiler troubles suggesting the remedical methods:  (i) Sludge and scale formation (ii) Caustic embrittlement. (CBE. A.U. Jan 2009, AU Jan 2013, June 2014, Dec 2015)

(or)

What are sludge and scale in boilers? How are they formed Suggest any two methods to prevent their formation. (AU May 2015)

(i) Scale and Sludge Formation in boilers

When water is continuously converted into steam in boilers, the concentration of dissolved salts in water increases progressively. When the concentration of the salts reaches their saturation point, they are thrown out in the form of precipitates on the inner walls of the boilers. The least soluble one gets precipitated first.

(a) Sludge

• If the precipitate is loose and slimy it is called sludge. Sludges are formed by substances like MgCl₂, MgCO₃,

Fig (a) Sludge in boiler (b) Scale in boiler

MgSO₄ and CaCl₂. They have greater solubilities in hot water than cold water.

(b) Scale

On the other hand, if the precipitate forms hard and adherent coating on the inner walls of the boiler, it is called scale. Scales are formed by substances like Ca(HCO₃)₂, CaSO₄ and Mg(OH)₂

Prevention of scale formation

1. Scales can be removed using scraper, wire brush etc.

2. They can be removed by thermal shocks.

3. By using suitable chemicals like dil. acids.

4. They can be removed by frequent blow down operation...

(ii) Caustic Embrittlement

Caustic embrittlement means intercrystalline cracking of boiler metal.

Boiler water usually contains a small proportion of Na₂CO₃. In high pressure boilers this Na₂CO₃ undergoes decomposition to givé NaOH.

Na₂CO₃ + H₂O → 2NaOH + CO₂

This NaOH flows into the minute hair cracks and crevices, usually present on the boiler material, by capillary action and dissolves the surrounding area of iron as sodium ferroate.

Fe + 2NaOH → Na₂FeO₂ + H₂↑

This causes brittlement of boiler parts, particularly stressed parts like bends, joints, rivets, etc., causing even failure of the boiler.

Prevention

Caustic embrittlement can be prevented by using sodium phosphate, tannin, lignin.

 

4. Write brief note on Calgon Conditioning. (A.U. Dec 2014)

Calgon is sodium hexa meta phosphate Na₂ [Na₄(PO₃)₆]. This substance interacts with calcium ions forming a highly soluble complex and thus prevents the precipitation of scale forming salt.

2CaSO₄ + Na₂[Na₄(PO₃)₆] → Na₂ [Ca₂(PO₃)₆] + 2a₂SO₄.

The complex Na₂ [Ca₂(PO₃)₆] is soluble in water and there is no problem of sludge disposal. So calgon conditioning is better than phosphate conditioning.

 

5. How will you protect boiler from corrosion. (A.U.Dec. 06)

(or)

What are the factors which causes boiler corrosion? How can it be minimised. (A.U. Dec 2014)

(a) Chemical method

Sodium sulphite, hydrazine are some of the chemicals used for removing oxygen.

2Na₂SO₃ + O₂ → 2Na₂SO₄

N₂H₄ + O₂  → N₂ + 2H₂O

(b) Mechanical de-aeration

Dissolved oxygen can also be removed from water by mechanical deaeration (Fig.).

Fig. Mechanical deaeration of water

In this process, water is allowed to fall slowly on the perforated plates fitted inside the tower. The sides of the tower are heated, and a vacuum pump is also attached to it. The high temperature and low pressure produced inside the tower reduce the dissolved oxygen content of the water.

(c) Removal of dissolved Carbon dioxide

(a) Carbon dioxide can be removed from water by adding a calculated amount of NH₄OH into water.

2NH₄OH + CO₂  → (NH₄)₂CO₃ + H₂O

(b) Carbon dioxide along with oxygen can also be removed mechanically by de-aeration method.

(d) Removal of acids

Corrosion by acids can be avoided by the addition of alkali to the boiler water.

HCl + NaOH → NaCl + H₂O

 

6. Explain the demineralization of water by ion-exchange process. How are exhausted cation and anion exchange resins regenerated? (AU June 2017) (TNV A.U. Jan 2010, June 2016)

(or)

Draw a suitable diagram and describe the ion exchange process for the softening of hard water. (A.U Dec 2015)

This process removes almost all the ions (both anions and cations) present in hard water. Demineralisation process is carried out by using ion exchange resins.

Fig. Demineralisation process

(i) Cation exchanger

Resins containing acidic functional groups (-COOH - SO₃H) are capable of exchanging their Ht ions with other cations of hard water. It is represented as RH₂.

(ii) Anion Exchanger

Resins containing basic functional groups (-NH₂, -OH) are capable of exchanging their anions with other anions of hard water. It is represented as R (OH)₂

Process

The hard water first passed through a cation exchange column, (Fig which absorbs all the cations like Ca²⁺, Mg²⁺, Na⁺, K⁺, etc., present in the hard water.

RH₂ + CaCl₂ → RCa + 2HC1

RH + NaCl → RNa + HCl

The cation free water is then passed through a anion exchange column, which absorbs all the anions like Cl^-, SO^2-₄, HCO^-₃, etc., present in the water.

R'(OH)₂ + 2HCl → R' Cl₂ + 2H₂O

The water coming out of the anion exchanger is completely free from cations and anions. This water is known as demineralised water or deionised water.

Regeneration

When the cation exchange resin is exhausted, it can be regenerated by passing a solution of dil HCl or dil H₂SO₄.

RCa + 2HCl → RH₂ + CaCl₂

Similarly, when the anion exchange resin is exhausted, it can be regenerated by passing a solution of dil NaOH.

R'Cl₂ + 2NaOH → R'(OH)2 + 2NaCl.

 

(or)

7. How is the softening of water carried out using the zeolite process? (AU June 2014, TCY A.U. Jan 2010, July 2016)

(or)

What are zeolites? How are they used in softening of water? Use a diagram for your explanation. (A.U May 2015, Dec 2014)

(a) Zeolite

Zeolites are naturally occurring hydrated sodium aluminosilicate. Its general formula is Na₂O. Al₂O₃ . xSiO₂ .yH₂O.(x = 2 - 10, y = 2 - 6) Natural zeolites are green sand and non-porous. The synthetic form of zeolite is known as permutit, which is porous and possess gel like structure, hence it is generally used for water softening.

(b) Zeolite process

Zeolites is porous and possess gel like structure, hence it is generally used for water softening. It is represented by Na₂Ze. The sodium ions which are loosely held in Na₂Ze are replaced by Ca²⁺ and Mg²⁺ ions present in the water.

Process

When hard water is passed through a bed of sodium zeolite (Na₂Ze), kept in a cylinder (Fig.), it exchanges its sodium ions with Ca²⁺ and Mg²⁺   ions present in the hard water to form calcium and magnesium zeolites. The various reactions taking place during softening process are

CaSO₄ + Na₂Ze → CaZe + Na₂SO₄

Fig. Zeolite Process

The softened water is enriched with large amount of sodium salts, which do not cause any hardness, but cannot be used in boilers.

Regeneration

After some time zeolite gets exhausted. The exhausted zeolite is again regenerated by treating with 10% solution of NaCl.

CaZe + 2NaCl → Na₂Ze + CaCl₂

Advantages of Zeolite process

(i) Water will have only hardness of 1-2 ppm.

(ii) No sludge is formed during this process.

(iii) Its operation is easy.

Disadvantages of Zeolite process

(i) Turbid water cannot be treated.

(ii) Acidic water cannot be treated.

(iii) Water containing Fe, Mn cannot be treated.

 

8. Explain the internal conditioning of water? Take two examples for your explanation. (A.U May 2015)

(or) What is meant by internal conditioning of water (A.U. June 2017)

It is the process of removal of scale forming substance, which were not completely removed in the external treatment, by adding chemicals directly into the boiler.

(i) Colloidal conditioning

Scale formation can be avoided by adding colloidal conditioning agents like kerosene, agar-agar, gelatin, etc., It is used in low pressure boilers. These colloidal substances get coated over the scale forming particles and convert them into non-adherent, loose precipitate called sludge, which can be removed by blow down operation.

(ii) Phosphate conditioning

Scale formation can be avoided by adding sodium phosphate. It is used in high pressure boilers. The phosphate reacts with Ca²⁺ and Mg²⁺ salts to give soft sludges of calcium and magnesium phosphates.-.

3 CaSO₄ + 2 Na₃PO₄ → Ca₃(PO₄)₂ + 3Na₂SO₄

Generally 3 types of phosphates are employed.

(a) Trisodium phosphate - Na₃PO₄ (Too alkaline) - used for too acidic water.

(b) Disodium hydrogen phosphate - Na₂HPO₄ (weakly alkaline) - used for weakly acidic water.

(c) Sodium dihydrogen phosphate - NaH₂PO₄ (acidic) - used for alkaline water.

 

9. What is desalination? Explain any one method of desalination (Coim A.U. Jan 2010, A.U. Jan 2013)

(or)

What is reverse osmosis? How wil you purify the sea water by reverse osmosis? Mention its advantages. (TNV A.U. Jan 2010, Chen A.U. Dec 2014, May 2015)

(or)

Explain with neat diagram, the desalination of brackish water of reverse osmosis method. (A.U. May 2017)

Desalination

The process of removing common salt (sodium chloride) from the water is known as desalination.

Reverse Osmosis

When two solutions of different concentrations are separated by a semi-permeable membrane, solvent (water) flows from a region of lower concentration to higher concentration.

Fig. Reverse Osmosis

This process is called osmosis. The driving force in this phenomenon is called osmotic pressure.

If a hydrostatic pressure in excess of osmotic pressure is applied on the higher concentration side, the solvent flow is reversed i.e., solvent flows from higher concentration to lower concentration. This process is called reverse osmosis (Fig.). Thus, in the process of reverse osmosis pure water is separated from salt water. This process is also known as super-filtration.

The membranes used are

Examples:

cellulose acetate, cellulose butyrate, etc.

Advantages

(i) The life time of the membrane is high, and it can be replaced within few minutes.

(ii) It removes ionic as well as non-ionic, colloidal impurities.

(iii) Due to low capital cost, simplicity, low operating, this process is used for converting sea water into drinking water.