Water TreatmentPart 3

Groundwater Treatment

Dr. Abdel Fattah Hasan

Groundwater (GW) are usually:• Cool and uncontaminated• Has uniform quality• Usually used directly for municipal use (just chlorine is added to

avoid post contamination)

Sometimes GW is polluted or contaminated with:• Hardness• Fertilizers • WW• Pesticides• Radionuclides• Toxic metals, such as Arsenic

GW Treatment Options

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Hardness Removal - Precipitation softening

• Hardness of water is caused by divalent cations, such as Ca & Mg ions

• Max. hardness for public supply: 300 -500 mg/l as CaCO3

• Moderate hardness for public supply: 60 -120 mg/l as CaCO3

• Precipitation softening uses lime CaO and soda ash Na2CO3 to remove Ca and Mg

• Lime slurries are usually has the form Ca(OH)2

• Lime treatment has the incidental benefits of bacterial actions, removal of iron and aid in clarification of turbid surface water

• Carbon dioxide can be applied after lime treatment to lower pH by converting the excess hydroxide ion and carbonate ion to bicarbonate ion

Converting Ca and Mg into mg/l CaCO3

• Ca Hardness as mg/l CaCO3 = Ca (meq/l) X 50

• Mg Hardness as mg/l CaCO3 = Mg (meq/l) X 50

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Chemical reactions in precipitation softening

• CO2 + Ca(OH)2 = CaCO3 + H2O

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

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

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

• Mg(HCO3)2 + 2Ca(OH)2 = 2 CaCO3 + Mg(OH)2 + 2H2O

• MgSO4 + Ca(OH)2 = CaSO4 + Mg(OH)2

• CaSO4 + Na2CO3 = CaCO3+ Na2SO4

1- Lime added to water reacts first with any available CO2

2- Then Lime reacts with any calcium bicarbonate present in water

1 eq to one eq

2eq of lime to one eq Mg(HCO3)2

3- Then Lime reacts with magnesium bicarbonate

4- Non-carbonate Ca (sulfate or chloride) require addition of soda ash and non-carbonate Mg (sulfate or chloride) require both lime and soda ash

1 eq to one eq1 eq to one eq

Note: Ca ion can be effectively removed by lime addition (pH = 10.3), but Mg ion demand higher pH, so lime should be added in excess of about (35 mg/l; 1.25 meq/l)

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Re-carbonation

• Used to stabilize excess lime of treated water by adding CO2:

– Ca(OH)2 + CO2 = CaCO3 + H2O

– CaCO3 + CO2 + H2O = Ca(HCO3)2

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Excess lime softening

• Calcium can effectively reduced by lime addition, but magnesium removal need excess lime

• Lime and Soda ash dosages to be estimated by chemical equations PLUS excess lime for Mg removal

• Practical limits (remains after estimation of theoretical dosages from chemical equations) for hardness removal are:

- CaCO3: 30 mg/l as CaCO3 (= 0.6 meq/l Ca++)

- Mg(OH)2 :10 mg/l as CaCO3 (= 0.2 meq/l Mg++)

Note: Sodium (Na) concentration is usually increased by the amount added in the Soda ash

Excess Lime Softening

Selective Calcium Carbonate Removal

• Used to soften water with low Mg hardness (less than 40 mg/l as CaCO3)

• Enough lime is added to remove Ca without Excess.

• Soda ash mayor may not be required, depending on the contents of non-carbonate hardness.

• Recarbonation is usually performed to reduce scale formation.

Selective Calcium Carbonate Process

Split-Treatment SofteningBy dividing the raw water into two portions for softening

in a two stage system

•Split treatment can result in chemical savings •Recarbonation my not be required•Split around 1st stage is determined by the level of Mg desired in treated water•Mg in treated water = (QP X MgR + QE X 10)/QR

QR

MgRQE

MgE

QP MgR

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• Example: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 of Mg(OH)2 is 10 mg/l as CaCO3

CO2= 8.8 mg/l

Ca++ = 40mg/l Mg++ =14.7mg/l Na+ = 13.7mg/l Alk (HCO3

-) =135 mg/l as CaCO3

SO4= 29mg/l

Cl- = 17.8mg/l(a) Sketch a meg/l bar graph and list the hypothetical combination of chemical

compounds in solution(b) Calculate the softening chemicals required, expressing lime dosage as CaO

and soda ash as Na2CO

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

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component mg/l EW Meq/l

CO2 8.8 22 0.4

Ca+ 40 20 2

Mg++ 14.7l 12.2 1.21

Na+ 13.7 23 0.6

Alk (HCO3-) 135 mg/l as CaCO3 50 2.7

SO4= 29 48 0.6

Cl- 17.8 35 0.51

component Meq/l Lime Meq/l

Soda Ash Meq/l

CO2 0.4 0.4 0

Ca(HCO3)2 2 2 0

Mg(HCO3)2 0.7 1.4 0

MgSO4 0.51 0.51 0.51

4.31 0.51

Calcium hardness = 2X 50 = 100

mg/l as CaCO3

Magnesium hardness = 1.2X 50= 60.5

mg/l as CaCO3

Required lime dosage = 4.31 X28 +35 =

156

Dosage of Soda ash = 0.51* 53= 27

mg/l Na2CO3

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Na+ Mg++ Ca++ CO2

0.0 2.00

C1- SO4= HCO3

- 0.4

3.21 3.81

0.0 2.70 3.30 3.81

Na + Mg ++ Ca++

0.0 0.6 0.8 1.91

Ca++

C1- SO4 CO3= OH-OH-

0.01.25 of excess lime 0.2 0.8 1.40 1.91

Na+ Mg++ Ca++

0.4

0.6 0.8 1.91

C1- SO4= CO3

= HCO3-

0.0

0.0

0.8 1.40 1.91

(A)

(B)

(C)

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Iron and manganese removal• Fe++ and Mn++ soluble in groundwater exposed to air these

reduced to insoluble Fe+++ and Mn++++

• Rate of oxidation depend on pH, alkalinity, organic content and present of oxidizing agents

• Filtration – sedimentation and filtration– Fe++ ( ferrous) + oxygen Fe Ox ( ferric oxidizes)– Manganese can not oxidized as easily as iron need to increase pH

• ِ�aeration –chemical oxidation – sedimentation- filtration– Fe++ + Mn++ + oxygen FeOx + MnO2 ( ferric oxidizes)

Free chlorine residual

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Iron and manganese removal

• Fe (HCO3)2 + KMn O4 Fe (OH) 3 + Mn O2

• Mn(HCO3)2 + KMnO4 MnO2

Potassium permanganate

Potassium permanganate

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Iron and manganese removal

• Manganese zeolite process

• Figure 7-20

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Aeration optional

................................................................................................................................................

Well water

Finished water

Under drain

Manganese treated greensand

Anthracite medium

Pressure filter

Dissolving tank and solution feeder

Dry KMnO4

Figure 7-20

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Water Stabilization

• Ferrous metal when placed in contact with water results in an electric current caused by the reaction between the metal surfaces and existing chemicals in water

• Fe Fe++ + 2electron• 2 elec + H2O + ½ O2 OH-

• 2Fe++ + 5H2O + ½ O2 Fe(OH)3 + 4 H+

• To Protect ductile iron pipe against internal corrosion is by lining with thin layer of cement mortar placed during manufacturing

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Ion- exchange softening and nitrate removal

• Ions of a particular species in solution are replaced by ions of a different species attached to an insoluble resin

Ion Exchanger

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Ca ++

Mg ++

+ Na2RCaR

MgR

+ Na+

CaR

MgR

+ NaCl Na2 R + Ca ++

Mg ++

excess

NaCl

Cation exchange softening

In Process of Removal

Regeneration

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RCl + So =

4

NO-3

Nitrate removal

Regeneration with NaCl

RSO4

RNO3

+ Cl -

Anion exchange for Nitrate Removal

Disadvantages : high operating costs and problem

of brine disposal

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Removal of dissolved salts

• Distillation : (desalination of sea water)– heating sea water (35000 mg/l mostly NaCl) to

boiling point and converting it into steam to form water vapor that is condensed yielding salt free water

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Removal of dissolved salts

• Reserves osmosis – Forced passage of the natural osmotic pressure

to accomplish separation of water and ions

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Fresh water

Saline water

..........................................................................................................................................................................................................................................................................

Osmosis

................................................................................................................................................................................................................................

Osmosis equilibrium

Semi permeable Membrane

Po

(a) (b)

......................................................................................................................................................................................

P> P0

Reverse osmosis

(c)

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Reverse osmosis system

• Pretreatment unit• Pump to provide high pressure• Post-treatment • Brine disposal

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Saline water

Granular-media filter

Cartridge filter

Scale inhibitor

Acid

Reverse osmosis models

Waste brinePump

Alkali

Permeate (product water)

10-30% of saline feed

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Source of wastes in water treatment

• Residue from chemical coagulation• Precipitation from softening• Filter back wash• Settled solid from pre-sedimentation

Total Sludge Solids produced in WT (lb/mil gal) =8.34 x [0.44 x alum dosage (mg/l)+ 0.74 x Turbidity (NTU)]

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Example 7-16

• A surface water treatment plant coagulant a raw water having a turbidity of 9 units by applying an alum dosage of 30 mg/l. – Estimate the total sludge solids production in pounds per million

gallons of water processed. – Compute the volume of sludge from the settling basin and filter

backwash water using 1% solid concentration in the sludge and 500 mg/l of solids in the waste water. Assume 30% of total solids are removed in the filter.

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Applying Eq. 7-39Total sludge solids = 8.34 (0.44 X 30 + 0.74 X 9)= 166 lb/ mil gal Solids in sludge = 0.70 X 166 = 116 lb/ mil galSolids in backwash water = 0.30 X 166 = 50 lb/ mil gal

Volume = Sludge solids (lb)

Solids fraction X 8.34 (lb/ gal)

Sludge volume =116

1.0

100 X 8.34

= 1390 gal/mil gal

Wash- water volume = 50

500

1,000,000X 8.34

= 12,000 gal/mil gal

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Formula MEQ/L APPLICABLE EQUATION

CaCO3MEQ/L

Mg (OH)2

MEQ/L

CO2 0.40 7-15 0.40 0

CA(HCO3)2 2.00 7-16 4.00 0

MG(HCO3)2 0.70 7-17 1.40 0.70

MGSO4 0.51 7-18 & 7-19 0.51 0.51

EXCESS LIME

1.25 7-20 1.257.56

01.21

Minus the practical limits (solubility)

- 0.606.96

- 0.201.01

Resident = CaCO3 + Mg(OH)2

= 6.96 X 50 + 1.01 X 29.2 = 378 mg/ l

COMPONENT IN WATER

PRECIPITATE PRODUCED

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Dewatering and waste disposal of wastes from water treatment plants• Lagoons

• Drying beds• Gravity thickening• Centrifugation• Pressure filter

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Sludge discharge Scraper blades

Pickets

Sludge in flow Inlet baffle

Weir

Supernatant

overflow

Gravity thickening

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Filtration pressure

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Centrifugation

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Example

• Case 1: Groundwater source with small infrequent possible contamination used for domestic use

• Solution : chlorination or ozonation or filtration

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Example 2

• Surface water: floating matter, high suspended matter, high turbidity, considerable, biological contamination, clay

• Solution: screening, sedimentation, coagulation, flocculation, filtration, chlorination