Water Quality Management in
Distribution Systems
Alabama-Mississippi AWWA
Education Workshop
January 2013
Vernon L. Snoeyink
University of Illinois
University of Illinois at Urbana-Champaign
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• Excessive precipitation of calcium, magnesium, and aluminum
• Corrosion of iron, copper, and lead, and release of corrosion products
• Dissolution of cement mortar lining
• Manganese accumulation and release
• Excessive biological growth
Consider water quality, energy & materials
University of Illinois at Urbana-Champaign
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• Disease outbreaks often caused by faulty distribution systems, e.g. cross connections
• Excessive residence times: distribution system and premises
• Negative pressure transients: Pressure waves owing to rapid valve closure, etc
Ref:
“
Drinking Water Distribution Systems: Assessing and
Reducing Risks ” , The National Academies Press,
Washington, DC 2006.
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Calcium Carbonate Precipitation Decreases
Pipe Diameter and Increases Energy Use
Control:
•
Langelier Index, LI, useful
•
Calcium carbonate precipitation potential, CCPP, best
•
Calculate CCPP with
RTW/Tetra model from
AWWA
•
Requires Ca, alkalinity, pH and temperature as inputs
•
Acceptable CCPP: a few mg/L
(also good for cement mortar)
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Al Post-Precipitation Increases Required
Energy and Decreases Quality
• Alum is added to destabilize particles
• Basic reaction:
Al
2
(SO
4
)
3
+ 6HCO
3
-

2Al(OH)
3
+ 6CO
2
+ 3SO
4
2-
Very important: Halton, Ont
If not at equilibrium before distribution, or if the pH decreases during distribution, precipitation of Al(OH)
3 can occur
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Al Post-Precipitation Increases
Energy Loss
• Increase in roughness increases the energy,
S, required to deliver a quantity Q.
• Hazen-Williams Equation
Q = CA(0.55)D 0.63
S 0.54
Where Q = flow rate, A = pipe x-sectional area, D = pipe diameter, and S = energy slope and C = Hazen-Williams
Coefficient
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Al Post-Precipitation Increases Energy
Loss and Affects Water Quality
• For Halton, a C factor decrease from 135 to 85 yields a Q reduction of 37% for a fixed energy input (ie headloss)
• Deposits in pipes give bacteria a place to grow. As deposits increase, expect more problems with microbial growth
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Al Post-Precipitation and Dirty Water
Complaints: Lake Erie Supply
Al Al + Fe
Fe
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Control pH to Prevent Al Post-Precipitation
4 pH
6
- 5.00
- 6.00
- 7.00
- 8.00
0.00
0
- 1.00
- 2.00
- 3.00
- 4.00
2
A l
+ 3
A l(O H )
4
-
A l(O H )
3
A l(O H )
+ 2
8 10
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Post-Filter Al Depends on Temperature
Chicago Example
250
225
200
175
150
125
100
JWPP Outlet (Post-Phosphate)
JWPP Filtered Water (Pre-Phosphate)
Raw Water Temperature
25
20
15
10
75
50
25
5
0
01/20/01 03/06/01 04/20/01 06/04/01 07/19/01
Date
09/02/01 10/17/01 12/01/01
0
01/15/02
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• Control pH, but remember the impact on total dissolved solids
• Alternative coagulant, e.g. FeCl
3
Remove deposit
• Dissolve by using water undersaturated with
Al(OH)
3
• Pigging
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Aluminum Silicate Case History
San Luis Obispo, CA
• Al from coagulation and silica in the source water precipitate in the distribution system
Al + silicate

Al silicate solid
• Precipitation kinetics are too slow to go to completion in the water treatment plant
• C factor: 80-90 range (Probably lower)
University of Illinois at Urbana-Champaign
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San Luis Obispo, CA, 2000
Aluminum Silicate scale
30” line
8” line
Solution: Change to ferric coagulant and pig lines
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University of Illinois at Urbana-Champaign

Post-Precipitation of Magnesium Silicate
Austin, TX
Mg 2+ + silicate

Mg silicate solid
• Add lime to remove calcium
• Finished water:
– SiO
2
= 7-8 mg/L, Mg = 75 mg/L as CaCO
3. pH 9.7-10
– Magnesium hydroxy silicate, lizardite or chrysotile. (
Ref:
Price et al., Proc WQTC,Amer. Wat. Wks. Assoc., Denver, CO, 1997)
Cold
Hot
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Use chemical equilibrium model
1. Reduce Mg, but not easy to change the process
2. Reduce Si, but difficult to do
3. Reduce and control pH: Best choice
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Available cross-section for flow – MWRA (Boston) Unlined
Cast Iron Pipes
Boston # 2 Boston # 4 Boston # 6
Boston # 1 Boston # 3 Boston # 5
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18

A “Good” Tubercle has a Non-Porous Outer Layer
From
Sontheimer,
Ref. 1.
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A “Poor” Scale has a Porous Outer Layer
After
Sontheimer,
Ref. 1
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Scale Structure: Champaign IL Tubercle
Shell-like Layer
Porous Interior
• Corrosion scales are porous deposits usually with a shell-like layer
• Permeability of shell-like layer is important
• Reservoir of Fe(II) ions exists in the scale interior
• Composition
• Shell-like layer: Magnetite
(Fe
3
O
4
) and goethite ( a
-FeOOH)
• Porous Interior: Fe(II) and some
Fe(III) compounds
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At A: Fe

Fe 2+ + 2 e
At C: ½O
2
+ 2 H + + 2 e

H
2
O
At A: Fe 2+ +
5/2
H
2
O + ¼ O
2

Fe(OH)
3(s)
+ 2 H +
N. B.: Must balance charge at A and C
Fe(III) ppt
Cathode
Anode Cathode
Continued Fe (II) flux at A, Oxidized iron crust develops
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Electron/Charge Flow in a Tubercle
DO Present
Fe 2+ + 2 H
2
O

Fe(OH)
2(s)
+ 2 H +
4 e + O
2
+ 4 H +

2 H
2
O
Tubercle growth from mass increase e X e e
Fe 2+
Shell-like layer
X -
X -
Fe
University of Illinois at Urbana-Champaign
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Iron Release – Effect of DO (NIWC Pipes)
1.0
0.8
0.6
0.4
0.2
0.0
0
DO in mg/L
Fe (Total) in mg/L
7
6
5
4
3
2
1
20
0
40 60 80 100 120
Stagnation Time (hrs)
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Iron Release from Corrosion Scales
Flowing Water with oxidants
Stagnant Water with oxidants
“Anoxic layer”
Fe 2+
Prolonged
Stagnation
Fe 2+
Oxidant supply restored
DO
DO
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Physical
Abrasion or Erosion
“Red Water” formation
Iron Release from
Corrosion Scales
Particle
Chemical
As Fe 2+
Nucleation Oxidation
Red Water
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pH and alkalinity are very important
– MWRA (Boston) Case History
– Low alkalinity (2x10 -4 ; 10 mg/L as CaCO
3
) resulted in highly variable pH 7-10
– Result: colored water (yellow) and high lead values
– Pipe loop results:
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University of Illinois at Urbana-Champaign
MWRA Rack 1
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Some procedures to harden and decrease permeability of soft scales:
– Constant pH (pH and alkalinity control)
– Minimize stagnation
– CCPP control
– Orthophosphates
– Polyphosphates can be used to mask color
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• Biofilms: microorganisms that grow in slimy layers attached to the pipe wall
• Example: Champaign-Urbana, IL
– Ammonia ~1-1.5 mg/L, add chlorine to produce
~3 mg/L of NH
2
Cl as Cl
2
; free ammonia in distribution system
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University of Illinois at Urbana-Champaign
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Ammonia and biodegradable organic matter promote the growth of biofilms. For example, the reactions
NH
4
+ + 2O
2

NO
3
+ 2H + + H
2
O and
Organics + O
2

CO
2
+ H
2
O + … provide the energy for the bacteria to grow.
University of Illinois at Urbana-Champaign
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Effects
• Increase energy required
• Deplete DO and produce odors (e.g. H
2
S)
• Produce NO
2
and deplete chlorine residual
• Growth of opportunistic pathogens
Control
• Minimize NH3 and biodegradable organics
• Provide good in-plant biological treatment
University of Illinois at Urbana-Champaign
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Final Thoughts
1.
Water quality changes depend on water quality and the type of pipe material.
2.
Control water quality to reduce energy required to distribute water, control biofilms and minimize metal ion release
3.
Strategy to solve distribution quality problems
– Compare influent and effluent quality
–
Monitor energy loss
– Characterize scales
–
Bench tests or pipe loop studies may be required
University of Illinois at Urbana-Champaign
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1. Sontheimer, H., Chapt in Internal Corrosion of Water
Distribution Systems , AWWARF, Denver, CO, 1985.
2. Lytle, D. et al. Effect of Ortho- and Polyphosphates on
Iron Particles. J AWWA , 94 (10), 87, ‘02.
3. Lytle, D. et al. The Effect of pH and DIC on the
Properties of Iron Colloidal Suspensions. AQUA , 52,
165-180, 2003.
4. Sarin, S. et al….Iron Release from … Cast-Iron Pipe. J
AWWA, 95 (11),85, 2003.
5. Sarin, S., et al. Iron Release …: Effect of Dissolved
Oxygen. Water Research,38 (5), 1259-1269, March 2004.
6. Sarin, P. et al… Model for .. Iron Release and Colored
Water Formation . J Environ Engin,130 (4), 364, 2004.
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