Water Softening

SOFTENING

• The reduction or removal of hardness from water is called water softening.

• Hard water causes the following problems :-

(i) It causes more consumption of soap in laundry work.

(ii) It causes modification of colours and affects the dyeing industries.

(iii) It causes serious difficulties in the manufacturing process such as paper making, al grind ice manufacture, Rayon industry etc.

(iv) It causes choking and clogging of plumbing fixtures.

(v) It causes scale formation in boilers and hot water heating system.

(vi) It makes the food tasteless, tough or rubbery.

 

Objectives of water softening are:

(i) To reduce the soap consumption of water

(ii) Improve the food taste

(iii) To reduce the maintenance of plumbing fixtures

(iv) Prevent scaling of boilers

(v) Improves the efficiency of manufacturing and dying processes.

(vi) Improves the efficiency of filtration etc.

• Temporary/Carbonate Hardness - is caused by the carbonates and bicarbonates of calcium and magnessium. Temporary hardness can be easily removed by boiling or adding lime.

• Permanent / Non-carbonate Hardness is caused by the sulphates, chlorides, nitrates of Calcium and Magnesium. This cannot be easily removed and requires special methods of water softening such as lime-soda process, Zeolite process or Demineralisation.

Methods of removing Temporary Hardness :-

1. Boiling:

• Calcium carbonate is not readily soluble in water.

• It may exist as Calcium bicarbonate in water, which easily dissolves in water  containing Carbon dioxide.

• When the water is boiled, CO2 is released, leading to precipitation of CaCO3 which can be removed by sedimentation process in a settling tank.

• This method cannot be used for Magnesium carbonate and Magnesium bicarbonate, since MgCO₃ is soluble in water.

Limitation:

• Boiling does not remove temporary hardness caused by magnesium.

• Boiling is unfeasible and uneconomical for public water supplies.

 

2. Addition of Lime

Lime (CaO), generally hydrated lime [Ca(OH)2] is added to the water. The following reactions take place :

The calcium carbonate and magnesium hydroxide are precipitated which can be removed in the sedimentation tank.

 

Methods of removing Permanent Hardness:

The different methods of removing permanent hardness are :

(1) Lime - soda process

(2) Base - Exchange process/ Zeolite process

(3) Demineralisation process

 

1. Lime - Soda process:

• In this process, lime [Ca(OH)₂] and soda ash [Na₂ CO₃] are added to hard water; which react with calcium and magnesium salts, to form insoluble precipitates of calcium carbonate and magnesium hydroxide.

• These precipiates can be sedimented out in a sedimentation tank.

• The chemical reactions which takes place are :

This process removes the following:

• Carbonate hardness (both Ca and Mg)

• Non-carbonate hardness of Mg is converted into non carbonate hardness Ca, which is removed by soda.

• Removes free dissolved CO2

• Equipments required for this process are :(Similar to coagulation process)

 (i) Miving Tank - Lime and soda ash are added to raw water and mixe

 (ii) Flocculation

(iii) Sedimentation - The precipitates formed are made to settle in the tank. The detention time varies between 2 to 4 hours.

• The dosage of lime and soda required for softening, depends upon the chemical quality of water and the extent of hardness removal desired.

Advantages of Lime -soda process:

• Economical

• Easily combined with other water treatment methods.

• Lime and soda used in combination with coagulants, reduces the dosage of coagulants.

• Increases pH of water and thus reduces corrosion of pipes.

• Increases pH which kills the pathogens.

• Reduces mineral content of water.

• Removes iron and manganese to some extent.

Disadvantages of Lime-soda process:

• Large quantity of sludge, precipitates of CaCO, and Mg(OH), is formed, which requires proper disposal.

• Careful operation and skilled supervision is required.

• Recarbonation of water is required; otherwise it will cause incrustation of pipe walls. (siiloss beau)

• Zero hardness cannot be achieved. Hardness removal is only upto 50 mg/l.

 

Recarbonation of softened water:

The very fine precipitates of calcium carbonate and magnesium hydroxide, may sometimes not settle in the sedimentation tank. These particles may deposit on filters and cause enlargement of sand grains i.e. incrustation of filter media and distribution pipes.

To prevent this, the softened water leaving the sedimentation tank should be recarbonated by passing carbon dioxide gas.

In Recarbonation process, the insoluable carbonates combine with carbon dioxide to form soluable bicarbonates.

The CO₂ gas to be blown in water can be produced by burning coke, gas or oil.

 

2. Zeolite or Base-exchange or Cation-exchange Process:

• Zeolite are naturai salts or clays, hydrated silicates of sodium and aluminium, Issimodo or synthetic resins.

• General formula is :

• Zeolites have the property of exchanging their cations.

• During water softening, Sodium ions of zeolite are exchanged with Calcium and Magnesium ions in hard waters.

• The chemical reactions involved are:

• The Ca and Mg zeolite is regenerated by treating with 5 to 10% solution of sodium chloride (Brine solution)

• A zeolite softener resembles a sand filter in which the filtering media is zeolite. They may be either gravity filters or pressure filters. The pressure filter type zeolite softener commonly used is shown in figure 2.23. It consists of a closed steel cylinder containing a bed of zeolite (0.75m to 2m thick). The hard water 300 litres per sq.m per minute (0.1 to 0.3m/min) When the sodium salts of zeolite are exhausted, it is regenerated by backwashing with 10% brine solution with 10% bri (Nacl).

Advantages

(i) Water of zero hardness can be achieved; useful for textile industries, boilers

(ii) Plant is compact, automatic, easy to operate.

(iii) No sludge is formed. No problem of sludge disposal.

(iv) RMO (Running, Maintenance and operation) cost is less.

(v) Also removes iron and manganese from water.

(vi) No problem of incrustation of pipes.

Disadvantages:

(i) Not suitable for treating turbid waters.

(ii) Process leaves sodium bicarbonate in water, which causes foaming in industrial or boiler feed waters.

(iii) Costly and not suitable for treating water containing iron and manganese iron or manganese zeolite cannot be regenerated back into sodium zeolit zeolite gets wasted.

3. Demineralisation Process:

• Demineralisation means removal of minerals from the water.

• This can be used for producing water of any desired hardness or mineral

• The demineralised or deionized water is as pure as distilled water and suitable for industrial purposes especially high pressure boilers.

• Demineralisation involves passing water first through a bed of cation exchange resins and then through a bed of anion exchange resins.

(a) Cation-exchange process - is similar to zeolite method, and hydrogen ion exchanged. The cation exchange resins are phenol aldehyde condensation pro having base exchange properties. Their chemical formula is H,R, where H repre hydrogen ions and R represents organic part of the substance.

The chemical reactions involved are :-

In the cation exchange process, acids [carbonic acid, hydrochloric acid, sulp acid etc.] are formed, which are removed by the subsequent anion exchange pro

(b) Anion - exchange Process:

The anion-exchange resins are condensation products of amines with formalde with anion exchange properties i.e., hydroxyl ions (OH) is exchanged. The cher formula of resin is ROH, where OH represents hydroxyl ions and R represents org part of the substance.

The chemical reactions involved are:

ROH + HC1 →RCI + HOH

2ROH + H2SO4  → R2SO4+2HOH

The water coming out from the anion-exchanger will be free from minerals. ziwiadio

When the resins get exhausted, regeneration is done as follows:

(i) Regeneration of cation exchange resin - By treating with dilute hydrochloric acid or sulphuric acid.

(ii) Regeneration of anion-exchange resin - By treating with sodium carbonate solution. 2RCI+ Na₂CO₃ + 2H₂O → 2ROH + 2NaCl + CO₂↑ + H₂O

 

Problem 2.6:

Design a zeolite-softener for an industry, using the following data:

(i) Quantity of soft water required per hour = 25,000 litres.

(ii) Hardness present in raw water as CaCO, = 400 ppm

(iii) Hardness to be obtained in the treated supplies = 50 ppm

(iv) Ion exchange capacity of zeolite = 10 kg of hardness per cu.m of zeolite

(v) Salt required for regeneration of exhausted zeolite

= 50 kg per cum of zeolite.

Also assume that the industry works for 2 shifts of 8 hours each, per day. M suitable assumption wherever needed.

 

Solution :

 • Quantity of soft water required per shift of 8 hrs.

= 25,000 1/hr × 8 hr = 2,00,000 litres.

• Hardness removal is upto 50 ppm out of total hardness of 400 ppm.

• Percentage removal desired= 350/400 x100=87.5%

• As the zeolite process reduces the hardness to zero; a part (87.5%) of raw water is treated to obtain zero hardness and the balance (12.5%) is not treated and added as raw water, to achieve hardness of 50 ppm.

The quantity of water to be treated per shift

= 2 x 10⁵ x 0.875 1.75 x 10⁵ litres

The amount of hardness to be removed per shift

= [Quantity of water treated per shift in litres] x [hardness in mg/l]

= (1.75 x 10⁵) × 400 mg/l

= 70 x 10⁶ mg = 70 kg.

The quantity of zeolite resin required

Hardness to be removed in Kg / Ion-exchange capacity of resin in Kg/cu.m

= 70 Kg / 10Kg/cu.m = 7 cu.m

Assume number of units as 6 with one unit as standby.

Volumt of one unit = 7 cu.m / 5 =1.4 cum

Provide 6 (5+1 standby) units with volume 1.4 cum, i.e. of area = 1m2 and depth =

Regeneration:

In 8 hours of shift time, assume regeneration process will take one hour and the

The quantity of salt required for regeneration

= 50 kg/cum of zeolite

= 50 kg/cum x 7 cum

= 350 kg.

Using 10% brine solution (10 kg salt dissolved in water to make 100 kg solution

350 kg of salt will produce

350×100/10 = 3500 kg of water solution

= 3500 kg /1000kg/m3 =3.5 cum

Provide two tanks of 1.75 cu.m capacity each. Assume the diameter of tank as m, then the depth required

Using free board = 0.15, Overall depth = 1.55 + 0.15 = 1.7m

Overall tank size will be = 1.2 m dia x 1.7 m depth

 

Check for contact period

Flow rate over zeolite bed

= volume of water treated per shift /operation hours of zeolite

= 1.75×10⁵ /7 litres/hr =25,000 litres/hr

Rate of filtration = Flow rate of water over zeolite bed / surface area of zeolite

= 5,000-83.31/m²/min

= 0.083 m/minute > 0.3 m/min

It is less than 0.3 m/min, Hence OK.

Contact period (i.e. Average time of travel through the bed)

= Depth / rate of filtration = 1.4 m/ 0.083 m/min

= 16.9 minutes > 7.5 minutes

Hence design is OK