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Water Treatment Part 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:

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groundwater gw are usually
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
hardness removal precipitation softening
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 caco 3
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
chemical reactions in precipitation softening
Chemical reactions in precipitation softening

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

  • 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 eq to one eq

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

3- Then Lime reacts with magnesium bicarbonate

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

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 eq

1 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)

re carbonation
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
excess lime softening
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

selective calcium carbonate removal
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.
split treatment softening
Split-Treatment Softening

By dividing the raw water into two portions for softening in a two stage system

QP MgR

QR

MgR

QE

MgE

  • 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
slide13
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.

slide14

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

slide15

3.21

0.0

2.00

3.81

(A)

0.0

2.70

3.30

3.81

0.0

0.6

0.8

1.91

Ca++

OH-

(B)

1.25 of excess lime

0.0

0.2

0.8

1.40

1.91

0.0

0.6

0.8

1.91

(C)

0.0

0.4

0.8

1.40

1.91

iron and manganese removal
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

iron and manganese removal1
Iron and manganese removal
  • Fe (HCO3)2 + KMn O4 Fe (OH) 3 + Mn O2
  • Mn(HCO3)2 + KMnO4 MnO2

Potassium permanganate

Potassium permanganate

iron and manganese removal2
Iron and manganese removal
  • Manganese zeolite process
  • Figure 7-20
slide19

Figure 7-20

Aeration optional

Well water

Anthracite medium

Dry KMnO4

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

Manganese treated greensand

Dissolving tank and solution feeder

Under drain

Pressure filter

Finished water

water stabilization
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
ion exchange softening and nitrate removal
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
slide23

Cation exchange softening

Ca ++

Mg ++

CaR

MgR

+ Na+

In Process of Removal

+Na2R

CaR

MgR

Ca ++

Mg ++

excess

Regeneration

+ NaCl

Na2R +

NaCl

slide24

Anion exchange for Nitrate Removal

Nitrate removal

So =4

NO-3

RSO4

RNO3

RCl +

+ Cl -

Regeneration

with NaCl

Disadvantages : high operating costs and problem

of brine disposal

removal of dissolved salts
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
removal of dissolved salts1
Removal of dissolved salts
  • Reserves osmosis
    • Forced passage of the natural osmotic pressure to accomplish separation of water and ions
slide27

Semi permeable

Membrane

P> P0

Po

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

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

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

Saline

water

Fresh

water

Osmosis

Reverse

osmosis

Osmosis

equilibrium

(b)

(c)

(a)

reverse osmosis system
Reverse osmosis system
  • Pretreatment unit
  • Pump to provide high pressure
  • Post-treatment
  • Brine disposal
slide30

Reverse osmosis models

Alkali

Saline water

Permeate (product water)

Scale inhibitor

Waste brine

Pump

Granular-media filter

Acid

10-30% of saline feed

Cartridge filter

source of wastes in water treatment
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)]

example 7 16
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.
slide33
Applying Eq. 7-39

Total 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 gal

Solids in backwash water = 0.30 X 166

= 50 lb/ mil gal

Volume =

Sludge solids (lb)

Solids fraction X 8.34 (lb/ gal)

116

Sludge volume =

= 1390 gal/mil gal

1.0

100

X 8.34

Wash- water volume = 50

500

1,000,000

X 8.34

= 12,000 gal/mil gal

slide34

PRECIPITATE PRODUCED

COMPONENT IN WATER

dewatering and waste disposal of wastes from water treatment plants
Dewatering and waste disposal of wastes from water treatment plants
  • Lagoons
  • Drying beds
  • Gravity thickening
  • Centrifugation
  • Pressure filter
slide36

Gravity thickening

Sludge in flow

Inlet baffle

Supernatant

overflow

Weir

Pickets

Scraper blades

Sludge discharge

example
Example
  • Case 1: Groundwater source with small infrequent possible contamination used for domestic use
  • Solution : chlorination or ozonation or filtration
example 2
Example 2
  • Surface water: floating matter, high suspended matter, high turbidity, considerable, biological contamination, clay
  • Solution: screening, sedimentation, coagulation, flocculation, filtration, chlorination