Pengolahan Kimia

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Pengolahan Kimia
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• 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)

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• 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

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Softening

• Benno Rahardyan
• Faculty of Civil and Environmental Engineering -
  ITB

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Private Water System Resources
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• Hard water

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What is "Hard Water"?

• 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.

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water hardness

• 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.

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Problems with Hard Water

• 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 scale, coats the
  vessels in which the water is heated,
• 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.

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• Softening

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Precipitation
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• Neutralization
• CO2Ca(OH)2 ?? CaCO3(s) H2O
• Ca2 Precipitation at pH 10
• Ca2 2HCO3-Ca(OH)2 ?? 2CaCO3(s) 2H2O

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• Mg2 Precipitation at pH 11
• Mg2 2HCO3-Ca(OH)2 ?? 2MgCO3 CaCO3(s)
  2H2O
• Mg2 CO3 Ca(OH)2 ?? Mg(OH)2((s)
  CaCO3(s)
• Ionic Balance addnon non hardness ionic (Na)
• Mg2 NaOH ?? Mg(OH)2((s) Na
• Ca2 Na2CO3 ?? CaCO3(s) 2Na

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Precipitation

• 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.

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• 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

• 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.

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• 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.

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The exchange medium can be natural "zeolites" or
synthetic resin beads that resemble wet sand.
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• Softening Process
• NaZeolite Ca2 --gt CaZeolite Na and
• NaZeolite Mg2 --gt MgZeolite Na
• Recharging Process
• NaCl CaZeolite --gt NaZeolite CaCl
• and
• NaCl MgZeolite --gt NaZeolite MgCl

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

• 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

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• 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.

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Types of water softening equipment available

• 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.

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Types of water softening equipment available

• 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.

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Ion Exchange Water Softener with Sensor-
Controlled Recharge
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Softener Selection Considerations

• Required grain capacity
• Daily water use (household population)
• Water hardness
• Desired regeneration schedule
• Initial cost
• Water conservation
• Other (Iron removal, etc.)

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Ion Exchange Water Softener Capacity

• 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

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Recommended Softener Sizes
Pump Capacity (l/det) Softener Capacity (gr) Water Hardness (mg/l)
50 750 350
80 1000 500
120 1500 850
140 2000 1200
200 3000 1500
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Ion Exchange Water Softener Recharge Control
Method
Water Use
Initial Cost -

• -Time Clock
• -Flow Meter
• -Hardness
• - Sensor

-
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Water Softening

• Permanent magnet water softeners dont work
• Electrostatic and catalytic descalers may
  descale water, but dont soften it
• Scale will not buildup on pipes, water heater
  elements, bathtubs etc.
• Sudsing action of soaps is not improved

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Typical Programmable Water Softener Controller
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Reactions

• CO2Ca(OH)2?CaCO3H20
• Ca(HCO3)2Ca(OH)22CaCO32H20
• Mg(HCO3)2Ca(OH)2MgCO3CaCO32H20
• MgCO3Ca(OH) 2Mg(OH)22CaCO3
• MgSO4Ca(OH) 2CaSO4 Mg(OH)2

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• CaSO4Na2CO3CaCO3 Na2SO4
• CaCl2Na2CO3CaCO3 2NaCl
• MgSO4Ca(OH)2CaSO4 Mg(OH)2
• MgCl2Ca(OH)2 Mg(OH)2 CaCl2

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• CO2Ca(OH)2?Ca22OH-
• CO22OH- ?HCO3-
• HCO3-OH- ?CO3-2H20
• Ca2 CO3-2 ? CaCO3
• Mg2 2OH- ?Mg(OH)2

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Pretreatment and other variations

• Prior to softening some preliminary treatment may
  be advisable if
• Raw water turbidities exceed 3,000 NTU at times
• Raw water has high concentration of free carbon
  dioxide (more than 10 mg/l)
• 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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Variation of process

• 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

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• CO22NaOH?Na2CO3H20
• Ca(HCO3)22NaOHCaCO3Na2CO32H20
• Mg(HCO3)24NaOHMg(OH)22Na2CO32H20
• MgSO42NaOHMg(OH)2 Na2SO4

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Systems expressing hardness of water

• 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

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Expressing hardness in milliequivalent/liter

• 1 milli-equivalent per liter
• 2.8 German degree
• 5.0 French degree
• 3.5 English degree
• 50 mg CaCO3/liter
• lt 2 meq/L ? soft water
• gt 5 meq/L ? hard water

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• 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--

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HCO3-
I
Total hardness
Ca2
Mg2
HCO3-
II
Total hardness
Ca2
Mg2
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Solubility in water
Substance mg/l meq/l mg CaCO3/l
Ca(OH)2 1280 34.9 1730
CaCO3 15 0.3 15
Ca3(PO4)2 Nearly insoluble Nearly insoluble
Mg(OH)2 8.4 0.29 14.5
MgCO3 110 2.62 131
Mg3(PO4)2 Nearly insoluble Nearly insoluble
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• Disadvantage of phosphate method
• Rather expensive (cost of sodium orthophosphate)
• Treated water will contain some rest of PO43-
• For drinking water it is not necessary and even
  not reccomendable to remove all hardness

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Lime soda process

• I CO2Ca(OH)2?CaCO3H20 1
• II Ca(HCO3)2Ca(OH)22CaCO32H20
  1
• IIIa Mg(HCO3)2Ca(OH)2MgCO3CaCO32H20
• IIIb MgCO3Ca(OH)2Mg(OH)22CaCO3
• III Mg(HCO3)22Ca(OH)2Mg(OH)22CaCO32H20 2
• IV CaCl2Na2CO3CaCO3 2NaCl 1
• Va MgCl2Ca(OH)2CaCl2 Mg(OH)2
• Vb MgCl2Na2CO3CaCO3 2NaCl
• V MgCl2Ca(OH)2Na2CO3CaCO3 Mg(OH)2 2NaCl
  1 1

needed Ca(OH)2 in meq needed Na2CO3 in meq
II, III carbonate hardness reactions IV, V non
carbonate hardness reaction
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Lime soda process

• I CO2Ca(OH)2?CaCO3H20 1
• II Ca(HCO3)2Ca(OH)22CaCO32H20
  1
• IIIa Mg(HCO3)2Ca(OH)2MgCO3CaCO32H20
• IIIb MgCO3Ca(OH)2Mg(OH)22CaCO3
• III Mg(HCO3)22Ca(OH)2Mg(OH)22CaCO32H20 2
• IV CaCl2Na2CO3CaCO3 2NaCl 1
• Va MgCl2Na2CO3MgCO3 2NaCl
• Vb MgCO3Ca(OH)2Mg(OH)22CaCO3
• V MgCl2Ca(OH)2Na2CO3CaCO3 Mg(OH)2 2NaCl
  1 1

needed Ca(OH)2 in meq needed Na2CO3 in meq
II, III carbonate hardness reactions IV, V non
carbonate hardness reaction
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Lime soda process
II
V
I
Lime needed CO2 HCO3- Mg2 Soda needed
Ca2 - HCO3- Mg2
HCO3-
CO2
I
IV
V
Ca2
Mg2
CO2
HCO3-
II
Mg2
Ca2
Lime needed CO2 HCO3- 2Mg2
I
II
III
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Sodium hydroxide-soda process

• 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)

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NaOH process

• I CO22 NaOH?Na2CO3H20 1
• II Ca(HCO3)2 2NaOH CaCO3Na2CO32H20
  1
• IIIa Mg(HCO3)2 2NaOH MgCO3Na2CO32H20
• IIIb MgCO3 2NaOH Mg(OH)2Na2CO3
• III Mg(HCO3)2 4NaOH Mg(OH)22Na2CO32H20
  2
• IV CaCl2Na2CO3CaCO3 2NaCl 1
• V MgCl2 2NaOH NaCl Mg(OH)2 1

needed Ca(OH)2 in meq needed Na2CO3 in meq
II, III carbonate hardness reactions IV, V non
carbonate hardness reaction
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NaOH process
I
II
V
NaOH needed CO2 HCO3- Mg2 Soda needed
(Ca2 - HCO3- ) (CO2 HCO3-)
Ca2 - CO2 - 2HCO3-
HCO3-
CO2
I
I, II
IV
Ca2
Mg2
CO2
HCO3-
II
Mg2
Ca2
NaOH needed CO2 HCO3- 2Mg2
I
II
III
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• CaSO4Na2CO3CaCO3 Na2SO4
• CaCl2Na2CO3CaCO3 2NaCl
• MgSO4Ca(OH)2CaSO4 Mg(OH)2
• MgCl2Ca(OH)2 Mg(OH)2 CaCl2

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• Diketahui air mengandung ion
• Cl142 mg/l,
• HCO3-183 mg/l
• Ca 120 mg/l
• Mg36 mg/l
• CO2 terlarut 66 mg/l
• Harga bahan kimia
• Na2CO3 Rp. 4500/kg
• NaOH 4000/kg
• Ca(OH) 2500/kg

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1. Hitung tingkat kesadahan yang dapat dicapai
   dengan metode pengendapan yang paling murah.
   Jawaban didasarkan atas perhitungan dan
   reaksi-reaksi
2. 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
3. Idem soal 2 menggunakan NaOH jika diperlukan
   kelebihan dosis NaOH sebesar 16 mg/l

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• Cl-142 mg/l 142/35.5 4 meq/l
• HCO3-183 mg/l 183/61 3 meq/l
• Ca2 120 mg/l 120x2/406 meq/l
• Mg2 36 mg/l 36x2/24 3 meq/l
• CO2 66 mg/l 66x2/44 3 meq/l
• Ca2 Mg2 gt HCO3- ? bukan hanya kesadahan
  sementara
• Kesadahan total 9 meq/l
• Kesadahan sementara 3 meq/l
• Kesadahan tetap 6 meq/l

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• Total Ca(OH)2 yang dibutuhkan
• 33318,5 350 mg/l
• Naik 350/333 1,05
• Ksp CaCO3 lt? Ca2 CO3
• pada suhu tertentu adalah 0,3 meq/l
• Ca2CO3/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

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• 1 D 10 mg/L CaCO3 10/(401248)2
• 0,2 meq/L CaCO3
• Ca(OH)2 Ca2 2OH-
• 18,5 mg
• 18,5 x 2
• --------- 0,5 meq/L
• 4034
• 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

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