Water Quality Management in Distribution Systems

1. Water Quality Management in Distribution Systems Alabama-Mississippi AWWA Education Workshop January 2013 Vernon L. Snoeyink University of Illinois

2. 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

3. 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.

4. 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)

5. 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

6. 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

7. 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 (ieheadloss) • Deposits in pipes give bacteria a place to grow. As deposits increase, expect more problems with microbial growth

8. Al Post-Precipitation and Dirty Water Complaints: Lake Erie Supply Al Al + Fe Fe

9. Control pH to Prevent Al Post-Precipitation

10. Post-Filter Al Depends on TemperatureChicago Example

11. 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

12. 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)

13. San Luis Obispo, CA, 2000 Aluminum Silicate scale 30” line 8” line Solution: Change to ferric coagulant and pig lines

14. 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., ProcWQTC,Amer. Wat. Wks. Assoc., Denver, CO, 1997) Cold Hot

15. Control of Magnesium Silicate Deposit Formation Use chemical equilibrium model • Reduce Mg, but not easy to change the process • Reduce Si, but difficult to do • Reduce and control pH: Best choice

16. Iron in Distribution SystemsCorrosion, Tubercles and Iron Release

17. Available cross-section for flow – MWRA (Boston) Unlined Cast Iron Pipes Boston # 2 Boston # 4 Boston # 6 Boston # 5 Boston # 1 Boston # 3

18. Mississippi Unlined Cast Iron

19. A “Good” Tubercle has a Non-Porous Outer Layer From Sontheimer, Ref. 1.

20. A “Poor” Scale has a Porous Outer Layer After Sontheimer, Ref. 1

21. 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 (a-FeOOH) • Porous Interior: Fe(II) and some Fe(III) compounds Shell-like Layer Porous Interior

22. At A: Fe2+ + 5/2 H2O + ¼ O2  Fe(OH)3(s) + 2 H+ Fe(III) ppt Cathode Anode Cathode Formation of a Tubercle At A: Fe  Fe2+ + 2 e At C: ½O2 + 2 H+ + 2 e  H2O N. B.: Must balance charge at A and C Continued Fe (II) flux at A, Oxidized iron crust develops

23. X- e e Shell-like layer e Fe2+ X- X- Fe Electron/Charge Flow in a TubercleDO Present Fe 2+ + 2 H2O  Fe(OH)2(s) + 2 H+ Tubercle growth from mass increase 4 e + O2 + 4 H+ 2 H2O

24. Fe (Total) in mg/L DO in mg/L Stagnation Time (hrs) Iron Release – Effect of DO (NIWC Pipes)

25. Fe2+ Fe2+ DO DO Iron Release from Corrosion Scales Flowing Water with oxidants Stagnant Water with oxidants “Anoxic layer” Prolonged Stagnation Oxidant supply restored

26. “Red Water” formation Iron Release from Corrosion Scales Physical Chemical Abrasion or Erosion As Fe2+ Nucleation Oxidation Particle Red Water

27. 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:

28. MWRA Rack 1

29. 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

30. 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

32. 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.

33. 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

34. Final Thoughts • Water quality changes depend on water quality and the type of pipe material. • Control water quality to reduce energy required to distribute water, control biofilms and minimize metal ion release • 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

35. Iron References • Sontheimer, H., Chapt in Internal Corrosion of Water Distribution Systems, AWWARF, Denver, CO, 1985. • Lytle, D. et al. Effect of Ortho- and Polyphosphates on Iron Particles. J AWWA, 94(10), 87, ‘02. • Lytle, D. et al. The Effect of pH and DIC on the Properties of Iron Colloidal Suspensions. AQUA, 52, 165-180, 2003. • Sarin, S. et al….Iron Release from … Cast-Iron Pipe. J AWWA,95(11),85, 2003. • Sarin, S., et al. Iron Release …: Effect of Dissolved Oxygen. Water Research,38(5), 1259-1269, March 2004. • Sarin, P. et al… Model for .. Iron Release and Colored Water Formation.J Environ Engin,130(4), 364, 2004.