Hardness of Water: Causes of hardness, expression of hardness – units – types of hardness
Hardness is defined as the concentrations of calcium and magnesium ions which is expressed in terms of calcium carbonate. These minerals in water can cause a few day to day problems. They react with soap to produce a deposit called soap curd that remains on the skin and clothes as it is insoluble and sticky, it cannot be removed by rinsing.
Hard water may also shorten the lifespan of plumbing and water heaters. When water containing calcium carbonate is heated, a hard scale is formed that can plug pipes and coat heating elements. Scale is a poor conductor of heat. With increased deposits on the unit, heat does not get transmitted to the water fast enough and overheating of the metal results in its failure. Build-up of deposits will inturn reduce the efficiency of the heating unit, thereby increasing the cost of the fuel.
Units for measuring hardness:
mg/litre of CaCO3.
2.parts per million of CaCO3 .
Usually ,the hardness of water is expressed in terms of calcium carbonate equivalents.
The formula used to convert the mass of hardness producing salt to mass of CaCO3 equivalents is given below:
Note: Molecular masses of hardness producing salts are given below:
There are two types of water hardness, temporary and permanent.
Permanent hardness in water is the hardness due to the presence of the chlorides, nitrates and sulphates of calcium and magnesium, which will not be precipitated by boiling. The lime scale can build up on the inside of the pipe restricting the flow of water or causing a blockage. This can happen in the industries where hot water is used.
Temporary Hardness is due to the bicarbonate ion, HCO3, being present in the water. This type of hardness can be removed by boiling the water to expel the CO2, as indicated in the equation below:
Ca(HCO3)2 ↔ CaCO3 + CO2 + H2O
Permanent hardness is due to presence of calcium and magnesium nitrates, sulphates and chlorides etc. This type of hardness cannot be eliminated by the method of boiling.
Hardness Concentration of Calcium carbonate (mg/L)
Soft Water 0 to 75
Medium Hard Water 75 to 150
Hard Water 150 to 300
Very Hard Water over 300
temporary hardness → prolazna tvrdoća
Temporary Hardness is due to the bicarbonate ion, HCO3-, being present in the water. It can be removed by water reboiling, whereby white solid emerges calcium carbonate that is limescale.
Ca(HCO3)2↔ CaCO3 + CO2 + H2O
Health Effects Of Hardness
The presence or absence of the hardness minerals in drinking water is not known to cause a health risk to the consumers. Hardness is generally considered an aesthetic water quality factor. The presence of some dissolved mineral material in drinking water is the reason for the waters characteristic and pleasant taste. At higher concentrations however, hardness can create the following consumer issues :
• Produces a white mineral deposit on the dishes additional noticeable on clear glassware.
• Reduces the efficiency of the devices that helps in heating of water. As hardness deposits integrate thickness, they act like insulation, reducing the efficiency of the heat transfer.
• Produces soap scum which is most noticeable on tubs and showers.
It has also been determined that the areas of higher hardness in drinking water maybe related to lower incidents of heart disease. This potential relationship is now being investigated.
Disadvantages of hard water:
Scales have a thermal conductivity value which is less than that of the bare steel. Even a very thin layer of scales serves as an insulator and resist in the transfer of heat.
Internal diameter of the pipes of the boilers progressively decreases due to the formation of scales .
Evaporative capacity of the boiler surface reduces due to the scale formation as they block the passage of heat transfer.
Surface of tubes becomes rough and resist the proper flow of the water.
5. Different parts of the boiler become weak and distorted due to overheating. As a result, operation of boilers becomes dangerous especially in the high pressure boilers.
6. Boiler efficiency decreases as the valves and condensers of the boilers are choked.
7. Due to scale formation boiler tubes are clogged down.
8. Water does not come in direct contact with tubes and plates of boilers due to the formation of scales. This results in overheating and sometimes burning of these plates and tubes.
(A) Domestic Uses:
C17 H35 COO Na + H2O→ C17 H35 COOH + NAOH
C17 H35 COOH + C17 H35 COO Na → Lather
C17 H35 COO Na + CaSO4 → (C17 H35 COO) 2 Ca ↓ + Na2SO2
(in water) (White Scum)
(B) Industrial Uses:
1. Boiler Feed: Should not contain nitrates- scale and sludges.
2. Paper Mill: Should not contain iron and lime- destroy resin of soap.
3. Sugar industries: Sulphates and Alkaline carbonates- Deliquescent.
4. Dyeing Industries: Should not contain iron and hardness.
5. Laundries: Should be soft.
3.1.1 Estimation of temporary & permanent hardness of water by EDTA method
Ethylene diamine tetra acetic acid (EDTA) is a reagent that helps in the formation of EDTA-metal complexes with many metal ions. In alkaline conditions (pH>9) it forms stable complexes with the alkaline earth metal ions Ca2+ and Mg2+. The EDTA reagent can be used to measure the total quantity of the dissolved Ca2+and Mg2+ ions present in a water sample. Thus the total hardness of the water sample can be estimated by the method of titration with a standard solution of EDTA.
Suitable conditions for the titration are achieved by the addition of a buffer solution having a pH value of 10. The buffer solution stabilizes the pH at 10. There are H+ ions produced as the reaction proceeds, and without the buffer solution the pH would decrease.
The EDTA reagent cannot under these conditions distinguish between the hardness caused by Ca2+ and Mg2+ or (directly) between temporary and permanent hardness. Therefore the results of this experiment are generally expressed in terms of the quantity of insoluble CaCO3 that would have to be converted into soluble salts to give the same total number of moles of dissolved Ca2+ and Mg2+ ions. This enables the total hardness of water from various sources to be compared easily.
Because it is a primary standard, and is also more soluble in water, the di-sodium salt of the EDTA is more commonly used as a reagent rather than the EDTA itself. If Na2H2Y represents the above salt, it ionises in the aqueous solution to form H2Y2-, which complexes in a 1:1 ratio with either Ca2+ or Mg2+ ions (which are represented as M2+). The reaction can be represented as follows:
H2Y2- + M2- → MY2- + 2H+
The indicator Eriochrome Black T is used to detect the end point. This is an indicator that has a different colour when complexed to the metal ions than when it is a free indicator. The reaction between the red indicator-metal complex and the EDTA reagent at the end point can be represented as follows:
50 ml Burette, 20 ml Pipette, 250 ml Conical flask, 100 ml Beaker, 250 ml beaker, Glass funnel.
1. Wash the pipette, burette and conical flask with the de-ionised water. Rinse the burette with the EDTA solution and the pipette with the hard water.
2. Using the funnel, fill the burette with EDTA solution. Open the tap to fill the part below the tap. Then remove the funnel. Adjust the level of the solution to zero mark. Make sure that the burette is vertical.
3. Use the pipette to transfer 50 cm3 of the hard water sample to the conical flask. Add 2-3 cm3 of the buffer (pH 10) solution .
4. Add 0.03 g of the solid indicator to the contents of the flask in the following manner: Add gradually to the flask, swirling after each addition. A deep wine red colour is obtained.
5. Carry out one \’rough\’ titration to find an approximate end point, followed by a number of accurate titrations until two titres agree to within 0.1 cm3. At the end point, the colour should be dark blue, with no tinge of wine-red colour.
6. From the data, calculate the total hardness of the water sample.
Experimental verification of EDTA solution
1. A sample of 100 ml of hard water consumes 25 ml of 0.01 M EDTA solution. Calculate the hardness of the sample of water.
100 ml of hard water
25 ml of 0.01 M EDTA solution
A sample of 100 ml of water consumed 12.5 ml of 0.01 M EDTA solution. In another titration 100 ml of the same sample, after boiling for half an hour consumed 8.2 ml of the same EDTA solution. Calculate the carbonate and non-carbonate hardness of the sample of water.
100 ml of water
12.5 ml of 0.01 M EDTA solution
3.2 Boiler troubles – Scale & sludges, Priming and foaming, caustic embrittlement and boiler corrosion
Scale: It is a hard and firm deposit, which is difficult to remove and the responsible salts are CaCO3, Mg(OH)2.
Scale is hard, thick, strong adherent precipitate, formed due to salts like CaSO4, Ca(HCO3)2. Cause reduced fuel economy, important boiling, boiler explosion etc. It can be prevented by special methods like:
(i) External treatment of ion exchange.
(ii) Internal carbonate, phosphate, Calgon conditioning.
(iii) Mechanical hard scrubbing methods.
Disadvantages of scale formation:1. Scales have a thermal conductivity value less than the bare steel. Even a very thin layer of scales serves as an insulator and resist in heat transfer.2. Internal diameter of the pipes of boilers progressively decreases due to scale formation.3. Evaporative capacity of the boiler surface was reduced due to scale formation as they blocked the passage of heat transfer.4. Surface of tubes becomes rough and resist to proper flow of the water.5. Different parts of the boiler become weak and distorted due to overheating. As a result operation of boilers becomes dangerous especially in high pressure boilers.6. Boiler efficiency decreases because the valves and condensers of boilers are choked.7. Due to scale formation boiler tubes clog down.8. Water does not come in direct contact with tubes and plates of boilers due to scale formation. This results in overheating and sometimes burning out of these plates and tubes.
Sludge : It is a loose and slimy precipitate, which is easy to remove and the responsible salts are MgCO3, Ca(HCO3)2 and CaCl2.
Formation of scales and sludges:
When hard water is used in boilers to get the steam, the impurities that are present in the hard water settles down on the sides of the boiler. This residue in due course adhere to the boiler vessel surface in the form of a sludge and scale. This is called as boiler scale. The calcium salts responsible for the formation of boiler scale.are Ca(HCO3)2 and CaSO4.
Causes for sludge and scale:
1. The salt deposit formed is a poor conductor of heat. Therefore fuel is wasted in raising the temperature of the boiler.
2. Due to the increase in the temperature the plates may melt. This may lead to explosion of boiler.
3. At higher temperature more oxygen may be absorbed by the boiler metal which causes corrosion of boiler metal.
4. The sudden spalling of the boiler scale exposes the hot metal to super-heated steam which causes corrosion of boiler.
Prevention of the sludge and scale formation can be done by two methods, namely, Internal conditioning method and External conditioning methods.
Lime soda process
Ion exchange process
Treatment with sodium aluminate
Priming and foaming:
During the production of steam in the boiler, due to rapid boiling, some droplets of liquid water are carried along with steam. Steam containing droplets of liquid water is called wet steam. These droplets of liquid water carry with them some dissolved salts and the suspended impurities. This phenomenon is called as carry over. It occurs due to priming and foaming.
Causes for priming and the prevention methods for priming:
Priming caused by:
1. Sudden boiling.
2. Very high water level in boiler.
3. Presence of excessive foam on the surface of the water.
4. Poor boiler design.
Good boiler design.
Providing proper evaporation of water.
Maintaining uniform heat distribution.
Adequate heating surface.
Maintaining low water levels.
The formation of stable bubbles above the surface of water is called foaming. These bubbles are carried over by steam leading to excessive priming.
Presence of oil and grease.
Presence of finely divided particles.
Prevention of foaming:
1. Adding anti foaming agents like castor oil and small amount of polyamide.
2. Clay, suspended solids, droplets of oil and grease can be removed by treated water and with clarifying agents such as hydrous silicic acid and aluminium hydroxide.
3. The concentration of salts and sludge in the boiler can be controlled by internal treatment and blow down operation.
It is a phenomenon that occurs in boilers where caustic substances (NaOH) accumulate in boiler materials. Residual sodium carbonate, which is used for the softening process undergoes hydrolysis forming sodium hydroxide at high pressure and temperatures.
The alkaline water enters the minute holes and cracks by capillary action on the interior of the boiler. The water then diffuses out of the cracks, leaving behind hydroxide salts that accumulate when more water evaporates. The hydroxide attacks the surrounding material of the boiler and dissolves iron as sodium ferrite.
Na2CO3 + H2O ↔ 2NaOH + CO2-
3Fe + 3NaOH ↔ 3Na2FeO2(sodium ferroate)
3Na2FeO2 + 4H2O ↔ 6NaOH+Fe3O4 + H2
Further dissolution of iron depends on regeneration of sodium hydroxide.
Prevention of Caustic Embrittlement:
1. As softening agent we can use sodium phosphate instead of sodium carbonate.
2. The hairline cracks can be sealed by waxy materials like Tannin and Lignin.
Boiler corrosion occurs due to dissolved gases such as oxygen, CO2, SO2 and H2S and due to hydrolysis of dissolved salts such as MgCl2 in the boiler water. As a results deep holes are formed in the boiler, which can be minimized by following methods:
1. Prevention of oxygen by chemical method:
i) Adding Sodium Sulphite:
Na2SO3 + O2 → 2Na2S0
This method results in other precipitates which can have some side effects. So this method is less preferred.
ii) Adding Hydrazine:
N2H4 + O2 → N2 ↑ + 2H2O;
This method results in inert gas and pure water and has no side effects. So it is preferred.
2. Prevention from CO2:
1. Chemical method: By adding calculated amount of ammonium hydroxide.
2NH4OH + CO2 → (NH4)2CO3 + H2O
2. Mechanical deaeration method.
3. Corrosion due to dissolved salts like MgCl2 by Neutralization: Excess acidic nature is neutralized by adding alkali\’s and vice versa.
HCl + NaOH → NaCl + H2O.
3.3 Treatment of boiler feed water
The removal of scale forming substance by adding chemicals directly into the boiler is called Internal treatment. The chemicals used for this purpose is called boiler compounds.
The boiler compounds kerosene, tannin, gelatin, agar agar, etc. get coated over the scale forming particles and convert them into non sticky, non-adherent and loose precipitate in boilers.
3.3.1 Internal treatment ( Phosphate, Colloidal and Calgon conditioning)
The residual salts that are not removed by external methods can be removed by directly adding some chemicals into the boiler water. These chemicals are called as Boiler compounds and the method is known as Internal treatment.
The various examples are:
Carbonate conditioning, Phosphate conditioning, Calgon conditioning, etc.,
(a)Carbonate conditioning: Used for the low pressure boilers. Here the salts like CaSO4 are converted to easily removable CaCO3. But sometimes it produces NaOH, CO2 and hence carbonic acid is produced. So it is less preferred.
CaSO4 + Na2CO3 → CaCO3 + Na2SO4.
(b)Phosphate conditioning: Used for high pressure boiler. No such risk of CO2 liberation.
3CaSO4 + 2Na3PO4 → Ca3(PO4)2 + 3Na2SO4.
(c)Calgon conditioning: Calgon is the trade name of sodium hexa meta phosphate- Na2 [Na4 (PO3)6].
With calcium ions it forms a soluble complex and prevents scale and sludge formation. It is used for high and low pressure boilers.
2CaSO4 + Na2[ Na4 (PO3)6] → Na2 [Ca2(PO3)6] + 2Na2SO4.
(d) Colloidal conditioning: In case of low pressure boilers, scale formation can be avoided by adding organic substances like Kerosene, tannin ,agar-agar etc., which get coated over by the scale firming precipitates which yields a coated non sticky and loose deposits.
(e) Treatment with sodium aluminate (NaAlo2). Sodium aluminate gets hydrolyzed yielding NaoH and a gelatinous precipitate of aluminium hydroxide.
NaAlO2+2H2O → NaOH+Al[OH] 2
The NaoH so formed precipitate some of the magnesium as Mg(OH)2 I.e.;
MgCl2+2 NaOH → Mg(OH) 2+2NaCl
The precipitate of Mg (OH)2 and Al (OH)3 produced inside the boiler entraps finely suspended and Colloidal impurities including oil drops and silica.
Chemicals used in internal treatment
Phosphates were used as the main conditioning chemical, however nowadays chelate and polymer type chemicals are mostly used. These new chemicals have the advantage over phosphates of maintaining scale-free metal surfaces. All internal treatment chemicals, whether phosphate, chelate, or polymer, condition the calcium and magnesium within the feedwater. Chelates and polymers form soluble complexes with the hardness, whereas phosphates precipitate the hardness. Sludge conditioners are also used to aid within the conditioning of precipitated hardness. These conditioners are selected in order that they are both effective and stable at boiler operating pressures. Synthetic organic materials are used as antifoam agents. For feedwater oxygen scavenging, the various chemicals used are sodium sulfite and hydrazine. Condensate system protection may be accomplished by the use of volatile amines or volatile filming inhibitors. A reputable company supplying treatment chemicals should be consulted. These companies supply the chemical formulations under their brand names and they give details on the dosage and methods.
Internal treatment for sulfates
The boiler temperature makes the calcium and magnesium sulfates within the feedwater insoluble. With the phosphates used as internal treatment, calcium reacts with the phosphate producing hydroxyapatite, which is much easier to condition than calcium sulfate. With chelates or polymer used as internal treatment, calcium and magnesium react with these materials producing soluble complexes that are simply removed by blowdown.
Internal treatment for silica
If silica is present in the feedwater, it tends to precipitate directly as scale at hot spots on the boiler metal and or combines with calcium forming a hard calcium silicate scale. In the internal treatment for silica, the boiler water alkalinity has to be kept high enough to carry the silica in solution. Magnesium, present in most waters, precipitates some of the silica as sludge. Special organic materials or synthetic polymers are used to condition magnesium silicate from adhering to the boiler metal.
Internal treatment for sludge conditioning
Internal treatment for hardness leads to insoluble precipitates in the boiler that form sludge. Additionally, insoluble corrosion particulate (metal oxides) are transported to the boiler by condensate returns and from pre-boiler feedwater corrosion leading to suspended solids. Suspended solids, carried to the boiler by feedwater or subsequently formed inside the boiler, adversely affect both boiler cleanliness and steam purity. These solids has a varying tendency to deposit on the boiler metal. Conditioners prevent these solids from depositing and forming corrosive or insulating boiler scale. Some of the principal types of the sludge conditioners are:
Lignins are effective on the phosphate type sludge.
Tannins fairly effective on high hardness feedwater.
Starches effective on high silica feedwater and where oil contamination is a problem.
Synthetic polymers Highly effective sludge conditioners for all types of sludges.
Internal treatment advantages
Internal treatment is basically simple and with the help of a qualified consultant an effective program is simply established. Scales or deposits, corrosion and carryover are minimized thereby improving efficiency and reducing energy consumption, preventing tube failures and unscheduled costly repairs, and reducing deposits, corrosion and contamination in the downstream equipments or processes.
3.3.2 External treatment – Lime Soda process, Zeolite process and ion exchange process
Cold Lime-Soda Process
In this method, calculated quantity of chemicals and water, along with accelerators and coagulators are added to a tank fitted with a stirrer. On vigorous stirring, thorough mixing takes place. After softening the soft water rises upwards and the heavy sludges settle down.
The softened water passes through a filtering media ensuring complete removal of the sludge and finally the filtered water flows out through the top. Cold lime soda process is used for partial softening of municipal water, for softening of cooling water etc. In actual purpose, magnesium hardness is brought down to almost zero but calcium hardness remains about 40 ppm.
Continuous cold lime-soda softener
Hot Lime-Soda Process
This process is similar to the cold lime-soda process. Here the chemicals along with the water are heated near about the boiling point of water by exhaust steam. As the reaction takes place at high temperature, there are the following advantages:
(i) The precipitation reaction becomes almost complete.
(ii) The reaction takes place faster.
(iii) The sludge settles rapidly.
(iv) No coagulant is needed.
(v) Dissolved gases (which may cause corrosion) are removed.
(vi) Viscosity of soft water is lower, hence filtered easily.
(vii) Residual hardness is low compared to the cold process.
Hot lime-soda process consists of three parts:
(a) Reaction tank in which complete mixing of the ingredients takes place.
(b) Ionical sedimentation vessel where the sludge settles down.
(c) Sand filter where sludge is completely removed.
The soft water from this process is used for feeding the boilers.
Continuous hot lime-soda softener
(i) Lime soda process is economical.
(ii) The process improves the corrosion resistance of the water.
(iii) Mineral content of the water is reduced.
(iv) pH of the water rises, which reduces the content of pathogenic bacteria.
(i) Huge amount of sludge is formed and disposal is difficult.
(ii) Due to residual hardness, water is not suitable for high pressure boiler
Zeolite or Permutit Process:
The name Zeolite is derived from Greek words (Zien + Lithos) which mean “boiling stone”. The chemical structure of sodium Zeolite may be represented as Na2O, Al2O3, xSiO2, yH2O where (x = 2 – 10 and y = 2 – 6). Zeolite or permutit is hardness producing Ca+2 and Mg2+ ions in water Zeolites are of two types:
(a) Natural Zeolites: Natural Zeolites are more durable.
e.g, Non-porous green sands, natrolite, Na2O, Al2O3, SiO2, 2H20.
(b) Synthetic Zeolites: They are porous and possess a gel structure. These Zeolites are prepared from solution of sodium silicate and aluminium hydroxide. Synthetic Zeolites have higher exchange capacity per unit weight.
Sodium Zeolites are used in water softening and are represented as Na2Z (Z stands for the insoluble zeolite radical framework). These are also known as base exchangers.
Zeolites process for treating water:
Zeolites water softeners are made in both pressure type and gravity type. A Zeolite softener consists of steel tank packed with a thick layer of permutit. The water enters at the top and passes through the bed of Zeolite. The water is softened by passing it through the Zeolite bed, where Ca+2 and Mg2+ ions are removed from the water by Zeolite and simultaneously releasing equivalent amount of Na+ ion exchange.
Process: In this process hard water passes at a specified rate through a bed of active granular sodium zeolite present in a zeolite as CaZ and MgZ respectively while the outgoing water contains equivalent amount of sodium salts. The chemical reactions taking place in zeolite softener are:
Ca(HCO3)2 + Na2Z → CaZ + 2NaHCO3
Mg(HCO3)2 + Na2Z → MgZ + 2NaHCO3
CaSO4 + Na2Z → CaZ + Na2S04
CaCl2 + Na2Z → CaZ + 2NaCl.
Small quantities of iron and manganese present as the divalent bicarbonates may also get removed simultaneously.
Fe (HCO3)2 + Na2Z → Fe2 + 2NaHCO3
Mn (HCO3)2 + Na2Z → Mn2 + 2NaHCO3.
Regeneration: After some time when the zeolite is completely changed into calcium and magnesium zeolites then it gets exhausted , saturated with Ca2+ and Mg2+ ions and it ceases to soften water. It can be regenerated and reused by treating it with a 10% brine (sodium chloride) solution.
CaZ + 2NaCl → Na2Z + CaCl2
MgZ + 2NaCl → Na2Z + MgCl2.
Advantages of Zeolite Process:
(1) It removes the hardness almost completely.
(2) The equipment used is compact occupying a small space.
(3) The process automatically adjusts itself for variation in hardness of incoming water.
(4) The process does not involve any type of precipitation thus no problem of sludge formation occurs.
(5) The plant can be installed in the water supply line itself decreasing the cost of pumping.
(6) It requires less time for softening.
Disadvantages of Zeolites Process:
(1) The outgoing water contains more sodium salts.
(2) The method only replaces Ca2+ and Mg2+ ions by Na+ ions.
(3) Zeolite process leaves all the acidic ions (like HCO3– and CO32–) as such in the softened water.
(4) High turbidity water cannot be softened efficiently by Zeolite process.
Limitations of Zeolite Process:
(1) The water must be free from turbidity and suspended matter. Otherwise Zeolite bed (Permutits) will be clogged and the rate of flow will be decrease.
(2) Hot water should not be used as the Zeolite tend to dissolve in it.
(3) Colored ions such as Mn2+ and Fe2+ must be removed first because these ions produce manganese and ion Zeolites which cannot be easily regenerated.
Ion exchange or demineralization process removes almost all ions present in the hard water.
The soft water produced by lime-soda and zeolite processes does not contain hardness producing Ca2+ and Mg2+ ions but it will contain other ions like Na+, K+, SO42-, cl- etc., On the other hand demineralized (DM) water does not contain both anions and cations. Thus a soft water is not demineralized water whereas demineralized water is soft water.
This process is carried out by using ion exchange resins, which are long chain, cross linked, insoluble organic polymers with a micro process structure. The functional groups attached to the chains are responsible for the ion exchanging properties.
Removal of non-metal inorganic
Removal or recovery of metal
(i) Cation exchanger:
Resins containing acidic functional groups (- COOH, – SO3H) are capable of exchanging their H+ ions with other cations of hard water. Cation exchange resin is represented as RH2.
Examples: Sulphonated coals.
R—SO3H; R—COOH = RH2
(ii) Anion exchanger :
Resins containing basic functional groups (—NH2, —OH) are capable of exchanging their anions with other anions of hard water. Anion exchange resin is represented as R (OH)2. Examples:
Cross-linked quaternary ammonium salt.
R—NR3OH; R—OH; R—NH2 = R(OH)2
The hard water is first passed through a cation exchange which absorbs all the cations like Ca2+,Mg2+,Na+, K+,etc. present in the hard water.
RH2 + CaCl2 → RCa + 2HCl
RH2 + MgSO4 → RMg + H2SO4
RH + NaCl → RNa + HCl
The cation free water is then passed through a anion exchange column which absorbs all the anions like Cl-, SO42, HCO3- etc., present in the water.
R\’ (OH) 2+ 2HCl→ R\’Cl2 + 2H20
R\'(OH) 2 +H2SO4→\’ R’SO4 +2H20
The water coming out of the anion exchanger is completely free from cations and anions.This water is known as demineralized water or deionized water.
The ion exchange resin is contained in a vessel with a volume of several cubic feet. Retention components at the top and bottom consist of screens, slotted cylinders, or alternative suitable devices with openings smaller than the resin beads to prevent the resin from escaping from the vessel. When the resin bed may be a uniform mixture of cation and anion resins in a volume. This arrangement is known as a mixed-bed resin, as opposed to an arrangement of cation and anion resins in discrete layers or separate vessels.
The use of various volumes of the two kinds of resins is due to the difference in exchange capacity between cation and anion resins. Exchange capacity is the amount of impurity that a given amount of resin is capable of removing and it has units of moles, equivalents or moles. The anion resin is less dense than the cation resin. So, it has a smaller exchange capacity and a larger volume is required for anion resins than for the cation resins to obtain equal total exchange capabilities.
1. All the cations and anions are completely removed by two column of cation exchange column and anion exchange column filled with resins.
2. Resins are long chain, insoluble, cross linked, organic polymers. There are two types:
i. Cation exchange resins – RH+.
(e.g) Sulphonated coals, RSO3H.
ii. Anion exchange resin – R’OH–.
(e.g) Urea formaldehyde, Amines R–NH2.
3. The water is fed into cylinder-I where all the cations are replaced by RH2 resins.
2RH+ + Ca2+ → R2Ca2+ + 2H+.
4. The cation free water is fed to cylinder II where all the anions are replaced.
2ROH– + SO42– → R2SO42– + 2 OH–.
5. Therefore the resultant water is free from all types of ions.
Ion exchange process
Advantages of Ion exchange method:
i) It can be used for high pressure boilers .
ii) It can treat highly acidic or alkaline water.
iii) We can get pure water with hardness as low as 2 ppm.
Drawbacks of Ion exchange method:
ii) Fe, Mn cannot be removed as they form complexes with resins.
iii) It cannot be used for turbid water as they clog the resins.
When the cation exchange resin in exhausted, it can be regenerated by passing a solution of dil.HCl or dil.H2SO4.
RCa + 2HC1 → RH2 + CaCl2
RNa + HCl → RH + NaCl
Similarly, when the anion exchange resin is exhausted it can be regenerated by passing a solution of dil.NaOH.
R\’Cl2+ 2 NaOH → R\'(OH)2 + 2 NaCI
The water obtained by this process will have very low impurities.
Highly acidic or alkaline water can be treated by this method.
The equipment is costly.
More explosive chemicals are needed for this process.
Water containing turbidity Fe and Mn cannot be treated because turbidity reduces the output and Fe,Mn form stable compound with the resins.
1. A pond water sample contains 100 mg of Ca(HCO) per litre, calculate the hardness of water in terms of CaCo equivalent.
100 mg of Ca(HCO) per litre
In the present case,
Mass of Ca(HCO) (w) = 100 mg/litre
2. A sample of water contains 200 mg of Ca+ per litre. What is the hardness of the sample in terms of CaCO equivalent?
In the present case,
Mass of Ca + (w) = 200 mg/litre
Equivalent mass (E) of Ca + = 20
Equivalent of CaCO = w x 50/E = 200 x 50/20 = 500 mg/litre or ppm.
3.4 Potable Water- Its Specifications – Steps involved in treatment of potable water – Disinfection of water by chlorination and ozonization
Water for Drinking purpose ( Potable water )
Municipal water is mainly used for drinking purposes and for cleaning, washing and other domestic purposes. The water that is fit for drinking purposes is called potable water.
(1) Characteristics of Potable water
1. It should not have turbidity and other suspended Impurities.
2. It should be colorless, odorless and tasteless.
3. It should not contain toxic dissolved impurities.
4. It should be free from the germs and bacteria.
5. It should not be corrosive to the pipe lines.
6. It should not stain the clothes.
7. It should be moderately soft.
(2) Standards of drinking water as recommended by WHO
Water quality standards in India
The domestic water treatments involves treatment for the potable or drinking purpose. Taste, Odor, Hardness, Contamination. These are the most common four reasons why people install water treatment systems in our home.
An activated carbon filter removes many volatile organic chemicals, some pesticides, radon gas, hydrogen sulfide and mercury. It also reduces odor, color and taste problems. The water is filtered through carbon granules that trap contaminants. But infrequently the maintained filters can result in the higher concentrations of contaminants and can serve as a breeding ground for bacteria.
The distillation removes radium, odor, off-tastes, heavy metals, some pesticides, nitrate, fluoride and salt. The units with volatile gas vents can remove some volatile organic chemicals as well. In distillation, the water is evaporated, leaving the impurities behind. The steam is then cooled and it becomes distilled water. But the distillation process is slow and consumes a lot of energy, making it expensive. It also consumes large amounts of water if the coolant used in the distillation process is water. The distilled water can corrode materials such as iron and copper in plumbing systems.
The reverse osmosis removes inorganic minerals such as radium, sulfate, calcium, magnesium, potassium, nitrate, fluoride, boron and phosphorous. It also helps to remove salts, certain detergents, volatile organic contaminants, some pesticides and taste and odor producing chemicals. The water is then filtered through a membrane that has passages smaller than the contaminant molecules. Under the sink the installations are costly and take up a lot of space. In addition, reverse osmosis is slow and wasteful of water and filter replacement is costly. Some microorganisms may be small enough to pass through the reverse osmosis membrane and colonize the holding tank.
The cation or anion exchange removes barium, radium and taste, color and odor-producing chemicals. It will then remove dissolved iron and manganese when they are present in low concentrations. Also, anion exchange units will remove nitrate and fluoride, but cation exchange units will not. The water softening process works by passing the hard water through resin beads.
The magnesium and calcium ions in the water exchange places with sodium ions on the beads, softening the water. People with hypertension or high blood pressure should consult their doctor about personal health risks associated with drinking softened water because of the added sodium Mechanical filtration removes dirt, sediment, loose scale and insoluble iron and manganese. The water is cleared by sand, filter paper, compressed glass wool or other straining material. The mechanical filtration does not do much to remove harmful, dissolved chemicals.
Chlorination is the most common disinfection method in drinking water treatment plants and can be done at any stage throughout the water treatment process. Each point of chlorine application will control a different water contaminant concern, thus providing a complete spectrum of treatment from the time the water enters the treatment facility to the time it leaves. The chlorination process is integrated into water treatment plants as a primary or secondary disinfection method.
Primary disinfection is the application of a disinfectant in the drinking water treatment plant. The amount of chlorine needed and time needed to react and disinfect is called the Contact Time (CT) and is a product of the concentration of residual chlorine (mg/l) and the disinfectant contact time. The CT values required to achieve the necessary disinfection depends on the microorganism targeted, pH and temperature. The other design factors influencing the amount of chlorine required are contact chamber design, adequate mixing and the presence of sunlight.
The following are possibilities for chlorination as a primary disinfection method:
In pre-chlorination, chlorine is applied to the water almost immediately after it enters the treatment facility to eliminate algae and the other organisms from water so they won’t cause a problem in later treatment stages. Pre-chlorination is found to remove tastes and odors and control biological growth throughout the water treatment system, thus preventing growth in the sedimentation tanks and the filtration media. The addition of chlorine also oxidizes any iron, manganese and/or hydrogen sulphide molecule that are present in it, so that they too can be removed in the sedimentation and filtration steps.
2. After sedimentation and before filtration:
This controls the biological growth, removes iron and manganese, removes taste and odors, controls the algae growth and removes the color from the water.
3. Final treatment step:
The most common stage for chlorination is as a final treatment step to disinfect the water and maintain chlorine residuals that will remain in the water as it travels through the distribution system. Chlorinating as a final step is more economical because a lower CT value is required, as by the time the water has been through the sedimentation and filtration, a lot of the unwanted organisms have been removed, so a less chlorine and a shorter contact time is required to achieve the same effectiveness.
Secondary disinfection may be applied to the treated water as it leaves the treatment plant or at re-chlorination points throughout the distribution system, to introduce and maintain a chlorine residual in the drinking water distribution system. Overall, a chlorine residual provides two main benefits:
1. It can limit the growth of bio-film within the distribution system and its associated taste and odor problems.
2. A rapid drop in the disinfectant residual may provide a immediate indication of treatment process malfunction or a break in the integrity of the distribution system.
A chlorine residual may also reduce the risk of recontamination in the event of an intrusion into the distribution system.
2. Effective in purifying water from pathogens and some inorganic compounds (iron, manganese, hydrogen sulphide).
3. Non-toxic (in free chlorine form).
4. Reduces taste and odor problems caused by algae and some chemical compounds.
1. CDBP’s may be toxic.
2. There are taste and odor problems with chlorine and CDBP.
Ozonation is an efficient treatment to reduce the amounts of micro pollutants released in the aquatic systems by wastewater treatment plants. Although no residual by-products are generated by ozone itself, some concerns are raised regarding oxidation by-products when water containing both organics and ions, such as bromide, iodide and chlorine ions, are treated with ozonation. A typical ozonation system consists of an ozone generator and a reactor where ozone is bubbled into the water to be treated.