Download Drinking water

CEE 160L – Introduction to
Environmental Engineering and
Science
Lecture 9
Drinking Water
Typical Characteristics of DW Sources:
Groundwater (GW) vs. Surface Water (SW)
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Groundwater
High mineral content
Low turbidity
Low color
Low or no DO
High hardness
High Fe, Mn
Low bacteria
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Surface water
Low mineral content
High turbidity
Colored
DO present
Low hardness
Taste and odor
High Bacteria
Groundwater Treatment
Lime/soda ash softening (removal of hardness)
Sedimentatio
n
basin
Ground water
from wells
Recarbonation
Rapid Flocculation
Mix
Basin
Sludge
Disinfection
Filter
CO2
Storage
To
Distribution
System
Hardness
• Definition: Has high dissolved minerals
– calcium bicarbonate and magnesium bicarbonate
• Effects of hardness
1. Causes soap scum and water spots
Ca2+ + (Soap)-  Ca(Soap)2
(s)
• Increases amount of soap required for cleaning
2. Causes scaling on pipes (especially in boilers)
CaCO3 and Mg(OH)2
Remove scale
and biofilms by
“pigging” small
sections.
Soft vs. Hard Water
• Soft water
– “I can’t rinse the soap off.”
– Actually, rinsing is more complete with soft
water.
• Hard water
– Soap precipitate gets on skin and in hair.
• Therefore, it doesn’t feel slippery.
Hardness Values
Description
Soft
Moderately hard
Hard
Very hard
Hardness range
(mg/L as CaCO3)
0 - 75
75 - 100
100 - 300
> 300
Durham treated water (Brown Water Treatment
Plant): 21 mg/L as CaCO3
Formation of Hardness
Precipitation
Topsoil
Subsoil
Rock
CH3COOH  H+ + CH3COOCO2 + H2O  H2CO3 H+ +HCO3CaSO4 (s)  Ca2+ + SO42CaCO3(s)  Ca2+ + CO32MgCO3(s)  Mg2+ +CO32-
Non-carbonate hardness
Carbonate hardness
http://www.geo.arizona.edu/geos256/azgeology/bcd/cavewebpg.htm
Lime/Soda Softening Process
• Neutralization of carbonic acid
– CO2 + Ca(OH)2  CaCO3(s) + H2O
• Precipitation of Ca carbonate hardness1
– Ca2+ + 2HCO3- + Ca(OH)2  2CaCO3(s) + 2H2O
• Precipitation of Mg carbonate hardness
– Mg2+ +2HCO3- +Ca(OH)2  Mg2+ +CO32-+ CaCO3(s) + 2H2O
– Mg2+ + CO32- + Ca(OH)2  Mg(OH)2(s)+ CaCO3(s)
Ksp (MgCO3)= 10-5
Ksp (Mg(OH)2)=10-10.7
1
the portion of the hardness associated with
carbonate or bicarbonate ions
Lime/Soda Softening Process
• Removal of Ca noncarbonate hardness
Ca2+ + Na2CO3  CaCO3(s) + 2Na+
• Removal of Mg noncarbonate hardness
Mg2+ + Ca(OH)2  Mg(OH)2(s)+ Ca2+
Ca2+ + Na2CO3  CaCO3(s) + 2Na+
(cont’d)
Ion Exchanger
Recarbonation
CO2 + H2O  H2CO3 +CO32-  2HCO3Recarbonation Because of the lime
that was added, the
water has a pH over
11. Compressed
carbon dioxide gas
is bubbled through
the water to lower
pH and stop the
chemical reactions
of softening. The
high pH also creates
a bitter taste in the
water.
Recarbonation
removes this
bitterness. (Many
places use liquid
CO2)
Process Sequence
Ca(OH)2, Na2CO3
Rapid Mix
Recarbonation
Coagulation and Flocculation
• Purpose
– To bring small colloids into larger flocs for settling
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Usually negatively charged
Coagulant
addition
Al3+,
Fe3+
Colloidal particles
(0.001 - 1 m)
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+ ++
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Flocs (1 - 100 m)
Sedimentation tanks
Filtration – remove suspended
solids
• Purpose
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Series of treatments
Disinfection
• Purpose
– Kill microorganisms in the water
• Disinfectants
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Cl2
Hypochlorite salts (NaOCl, Ca(OCl)2)
Most common
Chloramines (NH2Cl, NHCl2, NCl3)
in US
Chlorine dioxide (ClO2)
Ozone (O3)
UV radiation Very effective against protozoan cysts
Chlorine Reactions in Water
• Cl2
(g) + H2O
HOCl + H+ + Cl-
– pH dependent
– Essentially complete within a few
milliseconds
• HOCl
H+ + OCl-
[HOCl] + [OCl-] = free available chlorine
80-100X more effective than OCl- for killing E. coli
Surface Water Treatment
Coagulation and Flocculation
(colloid removal)
Screen
Surface water
from supply
Rapid Flocculation
Rapid
Mix
Basin
Sand Filter
Disinfection
Storage
Sedimentatio
n
basin
Sludge
To
Distribution
System
Colloid Treatment – Low Hardness
Typically for Surface Water
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Small particles
 0.001 to 1 m
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Usually negatively charged
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Particles repel so suspension is stable
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Treat by coagulation- flocculation