water softening (ir)

IR.AHMAD JUSOH/UMT/2009 w0men_iser

WATER SOFTENING HARDNESS Defined as the sum of all polyvalent cations (i.e., major: calcium and

magnesium, and minor: iron, manganese, strontium and aluminium), however they are not present in significant quantities in natural water.

Water hardness is largely the result of geological formation of the

water source. The common units of expression are mg/L as CaCO3 or meq/L.

Many people object to water containing hardness more than 150

mg/L as CaCO3. A common water treatment goal is to provide water with hardness in the range of 75 to 120 mg/L as CaCO3.

Hardness of 200‐500 mg/L as CaCO3 is considered excessive for a

water supply and results in high soap consumption as well as objection scale in heating vessels and pipes.

The classification of water hardness is as follow:

0 –75 = soft, 75‐ 150 = moderately hard, 150‐300 = hard, > 300 = very hard, all unit as mg/L as Ca CO3.


The natural process by which water becomes hard is shown

schematically in Figure 1. As rainwater enter the topsoil, the respiration of microorganisms increases the CO2 content of the water.

The CO2 reacts with the water to form H2CO3. Limestone, which is

made up of solid CaCO3 and MgCO3, reacts with the carbonic acid to form bicarbonates of Calcium and Magnesium [Ca(HCO3)2 and Mg(HCO3)2] respectively.

While CaCO3 and MgCO3 are both insoluble in water, the

bicarbonates are quite soluble. Gypsum CaSO4 and MgSO4 may also go into solution to contribute to

the hardness. Since Calcium and magnesium predominate, it is often convenient in

performing softening calculations to define the total hardness (TH) of a water as the sum of elements

TH = Ca 2+ + Mg 2+

Where the concentration of each elements in units of mg/L as CaCO3 or meq/L.

Total hardness is often broken down into two components:

1. That associated with the HCO3‐ anion (called carbonate hardness

(CH), and

2. That associated with other anions (called non‐carbonate hardness (NCH). Total hardness may also be defined as

TH = CH + NCH


Carbonate hardness is defined as the amount of hardness equal to

the total hardness or the total alkalinity, which ever is less. Carbonate hardness is often called temporary hardness because

heating the water removes it. When pH is less than 8.3, HCO3

– is the dominant form of alkalinity, and the alkalinity is taken to be equal to the concentration of HCO3

– Non‐carbonate hardness is defined as the total hardness in excess

of the alkalinity. If the alkalinity is equal to or greater than the total hardness, then

there is no non‐carbonate hardness. Non‐carbonate hardness is called permanent hardness because it is

not removed when water is heated.


Bar charts of water composition are often useful in understanding the process of softening. The bar is constructed with cation (i.e., Ca, Mg, Na and K) in the upper bar and anions (i.e., HCO3, SO4 and Cl ) in the lower bar. Rain v v v v v v v Topsoil Bacterial Action ‐‐‐> CO2 ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ Subsoil v v v v v CO2 + H2O ‐‐‐> H2CO3 v____v_____v_____v_____v____v____v Limestone CaCO3 (s) + H2CO3 Ca (HCO3)2

Example: Given the following analysis of a ground water, constructed a bar chart of the constituents, expressed as mg/L of Ca CO3 or meq/L. ________________________________________________________________________ Ion mg/L as ion EW Ca CO3 /EW ion mg/L as Ca CO3 meq/L Ca2+ 103 2.50 258 5.15 Mg2+ 5.5 4.12 23 0.45 Na+ 16 2.18 35 0.70 HCO3

‐ 255 0.82 209 4.18 SO4

‐2 49 1.04 51 1.02 Cl‐ 37 1.41 52 1.03


Solution: 0.0 5.15 5.60 6.30 Ca2+ Mg2+ Na+ HCO3

– SO42 – Cl –

0.0 4.18 5.20 6.23 The concentration of the ions has been converted to CaCO3 equivalent and the results are plotted in above figure. The total cat ions 316 mg/L as CaCO3, of which 281 mg/L as CaCO3 is hardness, the total anions 312 mg/L (i.e. less than 316 mg/L due to other ions which were not analyzed.) of which the carbonate hardness is 209 mg/L as CaCO3. Therefore the non carbonate hardness should be 72 mg/L as CaCO3 (NCH=TH – CH or 281 – 209 = 72). The relationship between the total hardness, carbonate hardness and non‐carbonate hardness are illustrated in Figure 3.14. In Figure 3.14a, the total hardness is 250 mg/L as CaCO3, the carbonate hardness equal to the alkalinity (HCO3‐ = 200mg/L as CaCO3), and the non‐carbonate hardness equal to the difference between the total harness and the carbonate hardness (NCH = TH – CH = 250 – 200 = 50 mg/L as CaCO3). In Figure 3.14b, the total harness is again 250 mg/L as CaCO3. However, since the alkalinity (HCO3‐) is greater than the total hardness, and since the carbonate hardness cannot be greater than the total hardness, the carbonate hardness is equal to the total hardness that is 250 mg/L as CaCO3. Therefore there is no non‐carbonate hardness. Note that in both cases it may be assumed that the pH is less than 8.3 because HCO3‐ is the only form of alkalinity present.


Table Equilibrium of solid and dissolved species of common ions

Mineral Formula Solubility mg/L CaCO3

Calcium bicarbonate Ca (HCO3)2 1,620 Calcium Carbonate CaCO3 15 Calcium Chloride CaCl2 336,000 Calcium sulfate CaSO4 1,290 Calcium hydroxide Ca (OH)2 2,390 Magnesium bicarbonate

Mg (HCO3)2 37,100

Magnesium Carbonate MgCO3 101 Magnesium Chloride MgCl2 362,000 Magnesium hydroxide Mg (OH)2 17Magnesium sulfate MgSO4 170,000 Sodium bicarbonate NaHCO3 38,700 Sodium Carbonate Na2CO3 61,400 Sodium Chloride Na Cl 225,000 Sodium hydroxide Na OH 370,000 Sodium sulfate Na2SO4 33,600


Example: Water has an alkalinity of 200mg/L as CaCO3. The Ca

2+ concentration is 160 mg/L as the ion, and the Mg 2+concentration is 40 mg/L as the ion. The pH is 8.1. Find the total, carbonate, and non carbonate hardness. Solution: The MW of Ca and Mg are 40 and 24 respectively, with both have a valency of 2 and thus the EW of Ca and Mg are 20 and 12 mg/meq respectively. EW for CaCO3 is 50 mg/meq. TH=160 mg/L (50mg/meq) + 40 mg/L (50mg/meq) = 567 mg/L as CaCO3 (20mg/meq) (12mg/meq) In this case, the alkalinity is less than the total hardness; the carbonate hardness (CH) is equal to 200 mg/L as CaCO3. The non‐carbonate hardness (NCH) is equal to the difference NCH = TH – CH = 567 – 200 = 367 mg/L as CaCO3 Note that we can add or subtract concentration of Ca 2+or Mg 2+ if there are in equivalent unit, for example, moles/L, mg/L or mill equivalents/L.


LIME – SODA SOFTENING

The lime‐soda water softening process uses lime, Ca (OH)2 and soda ash, Na2CO3, to precipitate hardness from solution.

Carbon dioxide and carbonate hardness (calcium and Magnesium bicarbonate) are complexed by lime.

Non‐carbonate hardness (Calcium and magnesium sulfates, chlorides and nitrates) requires addition of soda ash for precipitation.

In order to precipitate CaCO3, the pH of the water must be raised to about 10.3. To precipitate magnesium, the pH must be raised to about 11.0

Mg is more expensive to remove, so we leave as much Mg 2+ in the water as possible.

Similarly, there is more expensive to remove non‐carbonate hardness because we must add another chemical (soda ash). Therefore, we leave as much non‐carbonate hardness in the water as possible.

The common source of hydroxyl ion is calcium hydroxide [Ca(OH)2]. It is cheaper to use quicklime (CaO), commonly called lime, than hydrated lime [Ca (OH)2].

The quicklime is converted to hydrated lime in the water treatment plant by mixing CaO and water to produce a slurry of [Ca (OH)2]. The conversion process is called slaking.


i. CHEMICAL REACTIONS IN THE LIME‐SODA PROCESS ARE:

1. In order to raise the pH, we must first neutralize any free acids that

may be presented in the water. CO2 is the principal acid present in unpolluted, naturally occurring water.

Noted that no hardness is removed in this step. CO2 + Ca (OH)2 = CaCO3 v + H2O

2. Precipitation of carbonate hardness due to calcium

In order to precipitate CaCO3 we have to raise the pH to about 10.3. To achieve this we must convert all of the bicarbonate to carbonate.

Ca (HCO3)2 + Ca(OH)2 = 2CaCO3 v + 2H2O

3. Precipitation of carbonate hardness due to magnesium

In order to remove carbonate hardness due to magnesium, we must add more lime to increase the pH to about 11.

The reaction may be considered to occur in two stages

Mg(HCO3)2 + Ca(OH)2 = CaCO3 v + MgCO3 + H2O

Note that the hardness of the water did not change because MgCO3 is soluble.


With the addition of more lime the hardness due to magnesium is removed.

MgCO3 + Ca(OH)2 = Mg(OH)2 v + CaCO3 v

4. Removal of non‐carbonate hardness due to calcium

If we need to remove non‐carbonate hardness due to calcium, no further increase in pH is required.

We must provide additional carbonate in the form of soda ash

(Na2CO3). Ca2+(SO4,Cl and NO3) + Na2CO3 = CaCO3v + 2Na

+(SO4,Cl and NO3)

5. Removal of non‐carbonate hardness due to Magnesium

If we need to remove non‐carbonate hardness due to Magnesium, we will have to add both lime and soda ash.

MgSO4 [Cl, (NO3)] + Ca (OH)2 = Mg(OH)2 v + CaSO4 [Cl, (NO3)]

Note that although the magnesium is removed, there is no change in hardness because calcium is still in the solution.

To remove the calcium we must add soda

CaSO4 [Cl, (NO3)] + Na2 CO3 = CaCO3v + Na2SO4 [Cl, (NO3)]

Note that is the same as the one to remove non‐carbonate hardness due to calcium.


ii. PROCESS LIMITATION (EXCESS LIME APPROACH)

Lime soda softening cannot produce a water at completely free of hardness because of the solubility (little) of CaCO3 and Mg(OH)2. Thus the minimum calcium hardness can be achieved is about 30 mg/L as CaCO3, and the magnesium hardness is about 10 mg/L as CaCO3. we normally tolerate a final total hardness on the order of 75 to 120 mg/L as CaCO3, but the magnesium content should not exceed 40 mg/L as CaCO3 ( because a greater hardness of magnesium forms scales on heat exchange elements).

Notes: a) For Mg removal less than 20 mg/L as CaCO3, the basic excess lime

(20 mg/L as CaCO3) is sufficient.

b) For Mg removal of 20 to 40 mg/L as CaCO3, we must add excess lime equal to the Mg to be removed (e.g. 25.8 mg/L of Mg 2+, thus the excess lime should be 25.8 mg/L).

c) For Mg removal of more than 40 mg/L as CaCO3, we need to add excess lime of 40 mg/L as CaCO3).

In order to achieve reasonable removal of hardness in a reasonable time period, an excess of Ca(OH)2 beyond the stoichiometric limit usually implemented. Based on empirical experience, a minimum excess of 20 mg/L of Ca (OH)2 expressed as CaCO3 must be provided.

There are several advantages of lime softening in water treatment: the total dissolved solids are dramatically reduced, hardness is taken out of solution, and the lime added also removed. Lime also precipitates soluble iron (i.e., Fe 2+) and manganese often found in ground water. In processing surface waters, excess lime treatment provides disinfection and aids in coagulation for removal of turbidity.


Example: Excess Lime

Water defined by the following analysis is to be softened by excess lime treatment. Assume that the practical limit of hardness removal for CaCO3 is 30 mg/L, and that of Mg(OH)2 is 10 mg/L as CaCO3 .

Parameter (cations) Parameter (anions)

CO2 = 8.8 mg/L

Ca 2+ = 40.0 mg/L Alk(HCO3–)= 135 mg/L as CaCO3

Mg 2+ = 14.7 mg/L SO42 – = 29.0 mg/L

Na+ = 13.7 mg/L Cl – = 17.8 mg/L

a) Sketch a meq/L bar graph, and list the hypothetical combinations

of chemicals compound in solution. b) Determine the calcium, magnesium and total hardness as CaCO3.

Carbonate and non‐carbonate hardness.

c) Calculate the chemicals softening required, expressing lime dosage as CaO and soda ash as Na2CO3.

d) Draw a bar graph for the softened water before and after carbonation. Assume that half the alkalinity in the softened water is the bicarbonate form.

Solution:

Component Concentrationmg/L

EquivalentWeight

Meq/L

CO2 8.8 mg/L 22.0 0.40

Ca 2+ 40.0 mg/L 20.0 2.00

Mg 2+ 14.7 mg/L 12.2 1.21

Na+ 13.7 mg/L 23.0 0.60

Total cations 3.81

Alk(HCO3–) 135.0 mg/L 50.0 2.70

SO42 – 29.0 mg/L 48.0 0.60

Cl – 17.8 mg/L 35.0 0.51

Total anions 3.81


0.4 0.0 2.0 3.21 3.81

CO2 Ca2+ Mg2+ Na+ HCO3

– SO42 – Cl –

0.4 0.0 2.7 3.3 3.81

a) From the meq/L bar graph the hypotheticals combination are Ca (HCO3)2 2.0 meq/L, Mg (HCO3)2 0.70, Mg SO4 0.51, Na2SO4 0.09, and NaCl, 0.51 meq/L.

b) Calcium hardness 2.0 x 50 = 100 mg/L as CaCO3 and magnesium

hardness = 1.21 x 50 = 60.5 mg/L as CaCO3 or Total hardness = 160.5 mg/L as CaCO3. Carbonate hardness 2.7 x 50 = 135.0 mg/L. Non‐carbonate hardness = 0.51 x 50 = 25.5 mg/L.

c) Chemical required: The required lime dosage equals the amount needed for the softening reactions 1.25 meq/L (35 mg/L) CaO of excess lime to participate the magnesium.

Dosage of lime = 4.31 x 28 + 35 = 156 mg/L CaO. Dosage of soda ash = 0.51 x 53 = 27 mg/L Na2CO3.

Component Meq/L Lime Ca O meq/L Soda ash meq/L

CO2 0.4 0.4 0

Ca (HCO3)2 2.0 2.0 0

Mg (HCO3)2 0.7 2 x 0.7 = 1.4 0

Mg SO4 0.51 0.51 0.51

Total required 4.31 0.51

d) The hypothetical bar graph after excess lime treatment is shown

below: The dashed box to the left of zero is the excess lime (1.25 meq/L CaO) added to increase the pH high enough to precipitate the Mg (OH)2. The practical limit of 0.6 meq/L Ca 2+ (30 mg/L as CaCO3 and 0.2 meq/L Mg 2+ (10 mg/L as CaCO3).


Recarbonation converts the excess lime to calcium carbonate precipitate. Further carbon‐dioxide addition converts CO3

2– to HCO3

–, and finish water with a total hardness of 40 mg/L. The amount CO2 required is (1.25 + 0.2 + 0.4) meq/L x 22 mg/meq = 40.7 mg/L of CO2.

1.25 0.0 0.6 0.8 1.91

Ca2+ Ca2+ Mg2+ Na+ OH – OH

– CO3

2– SO42 – Cl –

0.0 0.2 0.8 1.4 1.91 0.0 0.6 0.8 1.91

Ca2+ Mg2+ Na+

HCO3– CO3

2– SO42 – Cl –

0.0 0.4 0.8 1.4 1.91


SELECTIVE CALCIUM REMOVAL Waters with a magnesium hardness of less than 40 mg/L as CaCO3 can be soften by removing only a portion of the calcium hardness. The processing can be a single‐stage system of mixing, sedimentation, recarbonation and filtration. Enough lime is added to the raw water to precipitate calcium hardness without providing any excess for magnesium removal. Soda ash may be required depending on the non‐carbonate hardness. Recarbonation is usually practised to reduce scaling of the filter sand and produce a stable effluent. (see example problem 10.5 and 10.4 pp 433‐437). Example: Selective Calcium Removal Consider selective calcium carbonate removal softening of a raw water with a bar graph as shown below: Calculate the lime dosage as CaO, and sketch the soften water bar graph after recarbonation and filtration. Solution The only hypothetical combination involving calcium is 2.0 meq/L of Ca (HCO3)2 ; therefore, no soda ash is needed and the lime required is 2.0 meq/L which equal to 28 mg/meq x 2 meq/L = 56 mg/L of CaO. The soften water bar graph has 0.6 meq/L of calcium hardness (the practical limit of 30 mg/L) and the total alkalinity is 0.8 meq/L, which is 0.6 meq/L from the practical limit and 0.2 meq/L associated with magnesium in the raw water bar graph. The degree of recarbonation determines the relative amount of carbonate and bicarbonate anions. The other ions in the soften water are the same as in the raw water. 0.0 2.0 2.6 2.9

Ca2+ Mg2+ Na+ HCO3

– SO42 – Cl –

0.0 2.2 2.7 2.9 0.0 0.6 1.2 1.5

Ca2+ Mg2+ Na+ CO3

2– HCO3– SO4

2 – Cl – 0.0 0.8 1.3 1.5

water softening (ir)

Documents