University of Illinois at Urbana-Champaign Water Quality Management in Distribution Systems Vernon L. Snoeyink University of Illinois Alabama-Mississippi

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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University of Illinois at Urbana-ChampaignUniversity of Illinois at Urbana-Champaign2

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


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.


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)


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


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


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


Al Post-Precipitation and Dirty Water Complaints: Lake Erie Supply

Al Al + Fe Fe


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


Post-Filter Al Depends on TemperatureChicago Example

0

25

50

75

100

125

150

175

200

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


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


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)


San Luis Obispo, CA, 2000 Aluminum Silicate scale

30” line 8” line

Solution: Change to ferric coagulant and pig lines


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


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


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


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

2

3

4

5

6

7

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


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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University of Illinois at Urbana-ChampaignUniversity of Illinois at Urbana-ChampaignMWRA Rack 1

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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



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.


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


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


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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University of Illinois at Urbana-Champaign Water Quality Management in Distribution Systems Vernon L. Snoeyink University of Illinois Alabama-Mississippi