University of Illinois at Urbana-ChampaignUniversity of Illinois at Urbana-Champaign
Water Quality Management in Distribution Systems
Vernon L. Snoeyink
University of Illinois
Alabama-Mississippi AWWAEducation Workshop
January 2013
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Distribution System Problems
• 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
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Design and Operating Factors Causing Water Quality Degradation
• 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:Al2(SO4)3 + 6HCO3- 2Al(OH)3 + 6CO2 + 3SO42-
Very important: If not at equilibrium before distribution, or if the pH decreases during distribution,
precipitation of Al(OH)3 can occur
Halton, Ont
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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)D0.63S0.54
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
-8.00
-7.00
-6.00
-5.00
-4.00
-3.00
-2.00
-1.00
0.00
0 2 4 6 8 10pH
Log
CA
l
Al+3
Al(OH)4-
Al(OH)+2
Al(OH)3
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Post-Filter Al Depends on TemperatureChicago Example
0
25
50
75
100
125
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175
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225
250
01/20/01 03/06/01 04/20/01 06/04/01 07/19/01 09/02/01 10/17/01 12/01/01 01/15/02
Date
Tota
l Alu
min
um
- In
dep
end
ent
Lab
(u
g/L
)
0
5
10
15
20
25
Raw
Wat
er T
emp
erat
ure
(°C
)
JWPP Outlet (Post-Phosphate)
JWPP Filtered Water (Pre-Phosphate)
Raw Water Temperature
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Control of Residual Aluminum
• Control pH, but remember the impact on total dissolved solids
• Alternative coagulant, e.g. FeCl3
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)
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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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Post-Precipitation of Magnesium SilicateAustin, TX
Mg2+ + silicate Mg silicate solid
• Add lime to remove calcium• Finished water:
– SiO2 = 7-8 mg/L, Mg = 75 mg/L as CaCO3. 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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Control of Magnesium Silicate Deposit Formation
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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Iron in Distribution SystemsCorrosion, Tubercles and Iron
Release
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Available cross-section for flow – MWRA (Boston) Unlined Cast Iron Pipes
Boston # 1 Boston # 3 Boston # 5
Boston # 2 Boston # 4 Boston # 6
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Mississippi Unlined Cast Iron
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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
AfterSontheimer,Ref. 1
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Scale Structure: Champaign IL Tubercle
• 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 (Fe3O4) and goethite (-FeOOH)
• Porous Interior: Fe(II) and some Fe(III) compounds
Shell-like Layer
Porous Interior
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Formation of a Tubercle
At C: ½O2 + 2 H+ + 2 e H2O
At A: Fe2+ + 5/2 H2O + ¼ O2 Fe(OH)3(s) + 2 H+
Fe(III) ppt
At A: Fe Fe2+ + 2 e
AnodeCathode Cathode
N. B.: Must balance charge at A and C
Continued Fe (II) flux at A, Oxidized iron crust develops22
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4 e + O2 + 4 H+ 2 H2O
Electron/Charge Flow in a TubercleDO Present
Fe2+
Fe
Shell-like layer
Tubercle growth from mass increase
Fe 2+ + 2 H2O Fe(OH)2(s) + 2 H+
e
ee
X-
X-
X-
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Iron Release – Effect of DO (NIWC Pipes)
0.0
0.2
0.4
0.6
0.8
1.0
0
1
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0 20 40 60 80 100 120
Fe (Total) in mg/L
DO in mg/L
Fe
(Tot
al)
in m
g/L
DO
in
mg/
L
Stagnation Time (hrs)24
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Iron Release from Corrosion Scales
Flowing Water with oxidants
Stagnant Water with oxidants “Anoxic layer”
Prolonged Stagnation
Oxidant supply restored
Fe2+Fe2+
DODO
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Iron Release from Corrosion ScalesPhysical Chemical
As Fe2+
OxidationParticle
Abrasion or Erosion
Nucleation
Red Water
“Red Water” formation
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Case History: MWRA
pH and alkalinity are very important– MWRA (Boston) Case History
– Low alkalinity (2x10-4; 10 mg/L as CaCO3) resulted in highly variable pH 7-10
– Result: colored water (yellow) and high lead values
– Pipe loop results:
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Important Considerations
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
• 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 NH2Cl as Cl2; free ammonia in distribution system
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Causes of Biofilms in Distribution Systems
Ammonia and biodegradable organic matter promote the growth of biofilms. For example, the reactions
NH4+ + 2O2 NO3- + 2H+ + H2O
and
Organics + O2 CO2 + H2O + …
provide the energy for the bacteria to grow.
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Effect and Control of Biofilms
Effects
•Increase energy required
•Deplete DO and produce odors (e.g. H2S)
•Produce NO2- and deplete chlorine residual
•Growth of opportunistic pathogens
Control
•Minimize NH3 and biodegradable organics
•Provide good in-plant biological treatment
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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
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Iron References1. 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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