

Penyisihan unsur pencemar dengan cara penambahan chemical agent/bahan kimia sehingga terjadi reaksi kimia, contoh : koagulasi dan presipitasi
Prinsip dasar: perubahan bentuk terlarut/tersuspensi menjadi bentuk yang terendapkan (kecuali desinfeksi) sehingga lumpur yang terendapkan termasuk kategori
B3 (perlu treatment khusus)



Kelebihan pengolahan secara kimia:


Efisiensi tinggi (dapat mencapai angka yang diinginkan)
Waktu dentensi relatif singkat sehingga volume reaktor/unit pengolahan relatif lebih kecil
Kekurangan


Ada penambahan zat aditif sehingga meningkatkan konsentrasi Total Dissolved Solid
(TDS). Penyisihan TDS relatif sulit dan mahal: membran atau destilasi
Meningkatkan beban pengolahan

Biaya bahan kimia cukup mahal = biaya untuk energi

Benno Rahardyan
Faculty of Civil and Environmental Engineering - ITB




Perhaps you have on occassion noticed mineral deposits on your cooking dishes, or rings of insoluble soap scum in your bathtub.
These are not signs of poor housekeeping, but are rather signs of hard water from the municipal water supply.
Hard water is water that contains cations with a charge of +2, especially Ca2+ and Mg2+.
These ions do not pose any health threat, but they can engage in reactions that leave insoluble mineral deposits. These deposits can make hard water unsuitable for many uses, and so a variety of means have been developed to "soften" hard water; i.e., remove the calcium and magnesium ions.

Hard water is water contaminated with compounds of calcium and magnesium. Dissolved iron, manganese, and strontium compounds can also contribute to the
"total hardness" of the water, which is usually expressed as ppm CaCO3.
Water with a hardness over 80 ppm CaCO3 is often treated with water softeners , since hard water produces scale in hot water pipes and boilers and lowers the effectiveness of detergents.

Table 1. Hardness Classification
----------------------------------------------------------
Concentration of hardness minerals in grains per
milligrams per
liter (mg/l) or gallon (gpg) Level of Hardness parts per million (ppm)
---------------------------------------------------------- below 1.0
1.0 to 3.5
3.5 to 7.0
soft
slightly hard moderately hard less than 17
17 to 60
61 to 120
7.0 to 10.5 hard above 10.5 very hard
121 to 180 more than 180
----------------------------------------------------------


Mineral deposits are formed by ionic reactions resulting in the formation of an insoluble precipitate. For example, when hard water is heated, Ca2+ ions react with bicarbonate (HCO3-) ions to form insoluble calcium carbonate (CaCO3), as shown in Equation 1.

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


This precipitate, known as water is heated, scale, coats the vessels in which the reduce the efficiency of heat transfer serious effect for industrial-sized water boilers scale can accumulate on the inside of appliances, such as dishwashers, and pipes.

Neutralization
CO
2
+Ca(OH)
2
 CaCO
3
(s) + H
2
O
Ca +2 Precipitation at pH 10
Ca +2 +2HCO
3
+Ca(OH)
2
 2CaCO
3
(s) + 2H
2
O

Mg +2 Precipitation at pH 11
Mg +2 +2HCO
3
+Ca(OH)
2
 2MgCO
3
+ CaCO
3
(s) + 2H
2
O
Mg +2 + CO
3
= +Ca(OH)
2
 Mg(OH)
2(
(s) + CaCO
3
(s)
Ionic Balance: addnon non hardness ionic (Na + ) :
Mg +2 +NaOH  Mg(OH)
2(
(s) + Na +
Ca +2 +Na
2
CO
3
 CaCO
3
(s) + 2Na +

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


For large-scale municipal operations, a process known as the
"lime-soda process" is used to remove Ca2+ and Mg2+ from the water supply.
The water is treated with a combination of slaked lime,
Ca(OH)2, and soda ash, Na2CO3. Calcium precipitates as
CaCO3, and magnesium precipitates as Mg(OH)2. These solids can be collected, thus removing the scale-forming cations from the water supply.
To see this process in more detail, let us consider the reaction for the precipitation of Mg(OH)2.
Consultation of the solubility guidelines in the experiment reveals that the Ca(OH)2 of slaked lime is moderately soluble in water. Hence, it can dissociate in water to give one Ca2+ ion and two OH- ions for each unit of Ca(OH)2 that dissolves.


The OH- ions react with Mg2+ ions in the water to form the insoluble precipitate. The Ca2+ ions are unaffected by this reaction, and so we do not include them in the net ionic reaction. They are removed by the separate reaction with CO32- ions from the soda ash.

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

Ion-exchange devices consist of a bed of plastic
(polymer) beads covalently bound to anion groups, such as -COO-.
The negative charge of these anions is balanced by
Na+ cations attached to them. When water containing Ca2+ and Mg2+ is passed through the ion exchanger, the Ca2+ and Mg2+ ions are more attracted to the anion groups than the Na+ ions.
Hence, they replace the Na+ ions on the beads, and so the Na+ ions (which do not form scale) go into the water in their place.

The ion exchange process

Calcium (Ca2+) and magnesium (Mg2+) ions that cause water hardness can be removed fairly easily by using an ion exchange procedure.

Water softeners are cation exchange devices. Cations refer to positively charged ions. Cation exchange involves the replacement of the hardness ions with a nonhardness ion.


Water softeners usually use sodium (Na+) as the exchange ion. Sodium ions are supplied from dissolved sodium chloride salt, also called brine. In the ion exchange process, sodium ions are used to coat an exchange medium in the softener.
The exchange medium can be natural "zeolites" or synthetic resin beads that resemble wet sand.

The exchange medium can be natural "zeolites" or synthetic resin beads that resemble wet sand.


NaZeolite + Ca2+ --> CaZeolite + Na+ and
NaZeolite + Mg2+ --> MgZeolite + Na+

NaCl + CaZeolite --> NaZeolite + CaCl and
NaCl + MgZeolite --> NaZeolite + MgCl

Ion exchange softeners replace Ca ++ and Mg ++ with Na + ions.
Zeolite medium is recharged with Na + by NaCl brine when depleted.




Exchange sodium ions for calcium and magnesium ions in water
May be dietary hazard - hypertension (adds  140 mg/l of sodium in “Hard” water)
Use potassium salt (KCl) for health reasons


 many people with high blood pressure or other health problems must restrict their intake of sodium.
Because water softened by this type of ion exchange contains many sodium ions, people with limited sodium intakes should avoid drinking water that has been softened this way. Several new techniques for softening water without introducing sodium ions are beginning to appear on the market.

Water softeners are classified in five different categories:

Manual: There are several types of manual softeners. The
 operator opens and closes valves to control the frequency, rate and time length of backflushing or recharging.
Semi-automatic: The operator initiates only the recharging cycle. A button is pushed when the softener needs recharging
 and the unit will control and complete the recharging process.
Automatic: The automatic softener usually is equipped with a timer that automatically initiates the recharging cycle and every step in the process. The operator needs only to set the timer and add salt when needed. It is the most popular type of softener used.


Demand initiated regeneration (DIR): All operations are initiated and performed automatically in response to the water use demand for softened water. DIR systems generally have two softening tanks and a brine tank. While one tank is softening the other tank is recharging.

Off-site regeneration (generally rental units): A used softening tank is physically replaced with a recharged tank.
Spent softening tanks are then recharged at a central location.

Ion Exchange Water
Softener with
Sensor- Controlled
Recharge





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
Daily water use (household population)
Water hardness
Desired regeneration schedule


Rated by grains of hardness treated between regenerations
Example:
Water hardness = 200 mg/l
Softener Capacity = 2000 gr
Household Population = 4 persons
Calculate:
Water Use = 4 persons x 200 l/person-day = 800 l/day
Daily Hardness Treated = 800 l/day x 200 mg/l = 160 gr/day
Regeneration Interval = 2000 gr/ 160 gr/day = 12.5 days

Pump Capacity
(l/det)
50
80
120
140
200
Softener Capacity
(gr)
750
1000
1500
2000
3000
Water Hardness
(mg/l)
350
500
850
1200
1500

Water
Use
+
-
-
-
-
-
Initial Cost
-
+





Scale will not buildup on pipes, water heater elements, bathtubs etc.
Sudsing action of soaps is not improved






CO
2
+Ca(OH)
2
 CaCO
3
+H
2
0
Ca(HCO
3
)
2
+Ca(OH)
2
=2CaCO
3
+2H
2
0
Mg(HCO
3
)
2
+Ca(OH)
2
=MgCO
3
+CaCO
3
+2H
2
0
MgCO
3
+Ca(OH)
2
=Mg(OH)
2
+2CaCO
3
MgSO
4
+Ca(OH)
2
=CaSO
4
+ Mg(OH)
2





CaSO
4
+Na
2
CO
3
=CaCO
3
+ Na
2
SO
4
CaCl
2
+Na
2
CO
3
=CaCO
3
+ 2NaCl
MgSO
4
+Ca(OH)
2
=CaSO
4
+ Mg(OH)
2
MgCl
2
+Ca(OH)
2
= Mg(OH)
2
+ CaCl
2






CO
2
+Ca(OH)
2
 Ca +2 +2OH -
CO
2
+2OH  HCO
3
-
HCO
3
+OH  CO
3
-2 +H
2
0
Ca +2 +CO
3
-2  CaCO
3
Mg +2 + 2OH  Mg(OH)
2

Prior to softening some preliminary treatment may be advisable if
1.
2.
Raw water turbidities exceed 3,000 NTU at times
Raw water has high concentration of free carbon dioxide (more than 10 mg/l)
3.
4.
5.
The raw water is high in organic colloids of a type that impedes crystallization of calcium carbonate
Raw water quality is highly variable over short periods of time
Recalcining of sludge is to be practiced

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






Single or two stage recarbonation ater conventional lime-soda treatment
Sludge recirculation
Excess lime treatment with split treatment or recarbonation
Post-treatment with polyphophates
Coagulation with alum, activated silica, or polymers
The use of three-stage treatment
The substitution of cation exchangers for soda ash to remove non carbonate hardness
The use of caustic soda instead of soda ash





CO
2
+2NaOH  Na
2
CO
3
+H
2
0
Ca(HCO
3
)
2
+2NaOH=CaCO
3
+Na
2
CO
3
+2H
2
0
Mg(HCO
3
)
2
+4NaOH=Mg(OH)
2
+2Na
2
CO
3
+2H
2
0
MgSO
4
+2NaOH=Mg(OH)
2
+ Na
2
SO
4

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


German degree = Ca and Mg equivalent with 10 mg CaO/liter
French degree = Ca and Mg equivalent with 10 mg CaCO3/liter
English degree = one grain (0.06480 g) of CaCO3 per gallon (3.785 L)
USA = ppm (mg/L) CaCO3

1 milli-equivalent per liter =
-
-
-
-
2.8 German degree
5.0 French degree
3.5 English degree
50 mg CaCO
3
/liter
< 2 meq/L  soft water
> 5 meq/L  hard water

Total hardness = amount of Ca and Mg non carbonate hardness + carbonate hardness.
Carbonate hardness = Ca and Mg equivalent to bicarbonate content
The difference between total hardness and bicarbonate (also called carbonate) hardness is the non carbonate hardness, which corresponds with ions like Cl , and SO4 --

HCO
3
-
I
II
Ca +2
HCO
3
-
Ca +2 Mg +2
Mg +2
Total hardness
Total hardness

Substance mg/l meq/l mg CaCO3/l
Ca(OH)
2
CaCO
3
Ca
3
(PO
4
)
2
Mg(OH)
2
MgCO
3
Mg
3
(PO
4
)
2
1280 34.9
15 0.3
8.4
110
Nearly insoluble
0.29
14.5
2.62
1730
15
131
Nearly insoluble

-
-
-
Disadvantage of phosphate method
Rather expensive (cost of sodium orthophosphate)
Treated water will contain some rest of
PO 3-
4
For drinking water it is not necessary and even not reccomendable to remove all hardness

I CO
2
+ Ca(OH)
2
II Ca(HCO
3
)
2
 CaCO
3
+H
2
0
+ Ca(OH)
2
=2CaCO
3
+2H
2
0
IIIa Mg(HCO
3
IIIb MgCO
)
2
+ Ca(OH)
3
+ Ca(OH)
III Mg(HCO
3
)
2+
2
2Ca(OH)
2
=MgCO
3
+CaCO
3
+2H
2
0
2
=Mg(OH)
2
+2CaCO
=Mg(OH)
2
3
+2CaCO
3
+2H
2
0 2
1
1
IV CaCl
2
+ Na
2
CO
3
=CaCO
3
+ 2NaCl 1
Va MgCl
2
+ Ca(OH)
2
=CaCl
2
+ Mg(OH)
2
Vb MgCl
2
V MgCl
2
+ Na
2
+
CO
3
Ca(OH)
=CaCO
2
+ Na
2
3
CO
II, III carbonate hardness reactions
+ 2NaCl
3
=CaCO
3
IV, V non carbonate hardness reaction
+ Mg(OH)
2
+ 2NaCl 1 needed Ca(OH)2 in meq needed Na2CO3 in meq
1

I CO
2
+ Ca(OH)
2
II Ca(HCO
3
)
2
 CaCO
3
+H
2
0
+ Ca(OH)
2
=2CaCO
3
+2H
2
0
IIIa Mg(HCO
3
IIIb MgCO
)
2
+ Ca(OH)
3
+ Ca(OH)
III Mg(HCO
3
)
2+
2
2Ca(OH)
2
=MgCO
3
+CaCO
3
+2H
2
0
2
=Mg(OH)
2
+2CaCO
=Mg(OH)
2
3
+2CaCO
3
+2H
2
0 2
1
1
IV CaCl
2
+ Na
2
CO
3
=CaCO
3
+ 2NaCl 1
Va MgCl
2
+ Na
2
CO
3
=MgCO
3
+ 2NaCl
Vb
V
MgCO
MgCl
2
3
+
+ Ca(OH)
2
Ca(OH)
2
+
=Mg(OH)
Na
2
CO
II, III carbonate hardness reactions
3
IV, V non carbonate hardness reaction
2
+2CaCO
=CaCO
3
3
+ Mg(OH)
2
+ 2NaCl 1 needed Ca(OH)2 in meq needed Na2CO3 in meq
1

I
CO
2
HCO
3
-
I II V
Lime needed : [CO2] + [HCO3-] + [Mg+2]
Soda needed : [Ca+2] [HCO3-] + [Mg+2]
IV
V
II
CO
2
Ca +2
HCO
3
-
Mg +2
Ca +2
Mg +2
Lime needed : [CO2] + [HCO3-] + 2[Mg+2]
I II III

-
-
Advantages
Dosage of NaOH solutions is very simple
By using NaOH the amount of sludge is much less than with Ca(OH)2 as the precipitating agent)

I CO
2
+2 NaOH  Na
2
II Ca(HCO
3
)
2
CO
3
+H
2
0
+ 2NaOH =CaCO
3
+ Na
2
CO
3
+2H
2
0
1
1
IIIa Mg(HCO
3
IIIb MgCO
)
2
+ 2NaOH =MgCO
3
+ 2NaOH =Mg(OH)
III Mg(HCO
3
)
2+
3
+ Na
2
2
+ Na
2
CO
4NaOH =Mg(OH)
2
CO
3
+2 Na
2
3
+2H
2
CO
3
0
+2H
2
0 2
IV CaCl
2
+ Na
2
CO
3
=CaCO
3
+ 2NaCl 1
V MgCl
2
+ 2NaOH = NaCl + Mg(OH)
2
II, III carbonate hardness reactions
IV, V non carbonate hardness reaction
1 needed Ca(OH)2 in meq needed Na2CO3 in meq

I
II
CO
2
I II V
NaOH needed : [CO2] + [HCO3-] + [Mg+2]
Soda needed : ( [Ca+2] HCO3- ) – ( [CO2] + [HCO3-])
[Ca+2] [CO2] 2[HCO3-]
HCO
3
-
I, II
IV
Ca +2 Mg +2
CO
2
HCO
3
-
Ca +2
Mg +2
NaOH needed : [CO2] + [HCO3-] + 2[Mg+2]
I II III





CaSO
4
+Na
2
CO
3
=CaCO
3
+ Na
2
SO
4
CaCl
2
+Na
2
CO
3
=CaCO
3
+ 2NaCl
MgSO
4
+Ca(OH)
2
=CaSO
4
+ Mg(OH)
2
MgCl
2
+Ca(OH)
2
= Mg(OH)
2
+ CaCl
2

Diketahui air mengandung ion


Cl=142 mg/l,
HCO3-=183 mg/l



Ca++ = 120 mg/l
Mg++=36 mg/l

CO2 terlarut = 66 mg/l
Harga bahan kimia
Na2CO3 = Rp. 4500/kg


NaOH = 4000/kg
Ca(OH) = 2500/kg

1.
2.
3.
Hitung tingkat kesadahan yang dapat dicapai dengan metode pengendapan yang paling murah.
Jawaban didasarkan atas perhitungan dan reaksireaksi
Hitunglah tingkat kesadahan yang dapat dicapai dalam proses pelunakan menggunakan Ca(OH)2 jika diketahui bahan proses ini memerlukan kelebihan dosis Ca(OH)2 sebanyak 18,5 mg/l
Idem soal 2 menggunakan NaOH jika diperlukan kelebihan dosis NaOH sebesar 16 mg/l






Cl-=142 mg/l =142/35.5 = 4 meq/l
HCO3-=183 mg/l = 183/61 = 3 meq/l
Ca2+ = 120 mg/l = 120x2/40=6 meq/l
Mg2+ = 36 mg/l = 36x2/24 = 3 meq/l
CO2 = 66 mg/l = 66x2/44 = 3 meq/l
Ca2+ Mg2+ > HCO3 bukan hanya kesadahan sementara
Kesadahan total = 9 meq/l
Kesadahan sementara = 3 meq/l
Kesadahan tetap = 6 meq/l

Total Ca(OH)2 yang dibutuhkan
333+18,5 = 350 mg/l
Naik 350/333 = 1,05
Ksp CaCO3 <  Ca2+ + CO3= pada suhu tertentu adalah 0,3 meq/l
=[Ca2+][CO3=]/[CaCO3] dengan memperhatikan
Ca(OH)2 = Ca2+ + OH-
350 350
---- mmol/l ----- mmol/l
74 74
Dengan kenaikan 1,05 pada ion Ca maka Ksp  0,3 x 1,05 = 0,315 meq/l
Mg(OH)2 = Mg2+ + OH-
Ksp = 0,24 meq/l
Total 0,555 meq/l = 1,64 D CaCO3

1 D = 10 mg/L CaCO3 = 10/(40+12+48)*2
= 0,2 meq/L CaCO3
Ca(OH)2 = Ca2+ + 2OH-
18,5 mg
18,5 x 2
--------- = 0,5 meq/L
40+34
Tingkat kesadahan 0,5/0,2 = 2,5 D
NaOH = Na+ + OH-
16 mg
16/40 =
0,4 meq
Kesadahan 0,4/0,2 = 2 D