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Calcite Contactors for Corrosion Control

Understand the importance of corrosion control, types of corrosion, and effects of corrosion, and learn about Calcite Contactors using limestone for water treatment. Explore the benefits, process, and design considerations for Calcite Contactors.

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Calcite Contactors for Corrosion Control

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  1. Calcite Contactors for Corrosion Control Lee Odell, P.E. Vice President CH2M HILL lee.odell@ch2m.com

  2. Overview • 1 – Corrosion Background • 2 – Treatment Systems Design, Operation & Maintenance

  3. Corrosion Control • Purpose • protect public health • improve water quality • extend plumbing equipment • meet regulations

  4. - e Corrosion Battery Analogy • Anode • Cathode • Electrical Circuit • Metal lost at anode Cathode Anode Electrolyte

  5. - - - - e e e e Simplified Corrosion Cell OH- STEP 4 O2 O2 STEP 1 Water with Dissolved Minerals STEP 3 Fe 2+ CATHODE Base Metal ANODE STEP 2

  6. Major Factors Influencing Corrosion • pH • Temperature • Dissolved Solids • System Deposits • Water Velocity • Microbiological Growth

  7. Types of Corrosion All water systems experiences some degree of corrosion. The objective is to control the corrosion well enough to maximize the life expectancy of the system...

  8. Water Original Thickness Base Metal General Etch Uniform Attack General Corrosion • Preferred situation • Take a small amount of metal evenly throughout the system • Anode very large

  9. Water OriginalThickness Base Metal Localized Pitting Attack Pitting Corrosion • Metal removed at same rate but from a much smaller area • Anode very small • Often occurs under deposits or weak points • Leads to rapid metal failure

  10. Affects of Corrosion • Potential regulatory non-compliance • Shortened pipeline life • Water usage increases • Corrosion product deposits in hot water tanks • Heat transfer efficiency is reduced by deposits • Leaks in equipment develop • Process side and water side contamination occurs • Maintenance and cleaning frequency increases • Equipment must be repaired and/or repaired • Unscheduled shutdown of plant

  11. Effect of pH on the Release of Copper into Solution

  12. Effect of pH and Alkalinity on Lead Solubility Alkalinity (mg/L CaCo3)

  13. Calcite Contactors • Calcite Contactors Use Limestone to Add Calcium Carbonate to Water, Raise pH and add Alkalinity to water. • Benefits: • Easy to Operate • Easy to Maintain • No Risk of Overdosing Chemical • Operate in Upflow Mode • No Need for Controllers/Motor Actuated Valves or backwashing

  14. What information is Needed to Design a Calcite Contactor? • pH • Alkalinity • Calcium • TDS or Conductivity • Flow Rate

  15. Calcite Contactors 1200 College St

  16. Calcite Contactors • Limestone contactors may offer advantages: • easier and safer to operate, • reduces operating cost, • self adjusts the water pH without risk of alkali overdose, • requires minimal maintenance and operator skills, • and does not require continuous feed of chemicals

  17. Process Description • In a calcite contactor, water flows through a bed of crushed sieved limestone in a similar way as it would flow through a sand filter. • The pH of water that flows through the limestone bed will be adjusted until it nears equilibrium with calcium carbonate (CaCO3(s)). • The components of a contactor include: • a contact tank, • limestone bed, • inlet line, • outlet line, • overflow line, • access lid, • backwash line. • There are two types of contactors: (i) open and (ii) closed system contactor. The former is exposed to the atmosphere and the latter is covered from the atmosphere. There are also contactors that are built in pressurized vessels. • Limestone contactors are typically located at the end of the treatment train – after filtration, primary disinfection and chlorine contact.

  18. Contactors

  19. Contactor Arrangement

  20. Contactors

  21. Limestone • CaCO3 H+ + HCO3

  22. Species Distribution DiagramCaCO3 -> H+ and HCO3

  23. Design of Contactors • Candidate Systems: • pH<7.2 • Calcium<60 mg/L • Alkalinity<100 mg/L • Iron <0.2 mg/L • Manganese <0.05 mg/L

  24. Limestone Contactor

  25. Contactor Feasibility Decision Tree • Design contactor length using EPA DESCON program Parameters needed: • pH • Alkalinity (DIC) • Calcium • Iron • Manganese • Temperature • Velocity • % Calcium Carbonate • Particle Size Available at Raymond Letterman’s website http://web.syr.edu/~rdletter/

  26. Descon Design Tool

  27. Design Considerations • Vessel Type – • Open, Pressure • Up-flow, Down-flow • Need Backwash Disposal? • Site Glass • Pressure gauges • Y-Strainer • pH Monitoring?

  28. Operations & Maintenance

  29. Calcite Dissolution Estimate • Ray Letterman/EPA Model • Excel Spreadsheet Model • Converted RTW to limestone dose to achieve pH 7.5 • Used duty cycle of 12 hrs/day • Estimate as cm/month of bed depth

  30. Questions?

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