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Calcite Contactors for Corrosion Control Lee Odell, P.E. Vice President CH2M HILL

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Presentation on theme: "Calcite Contactors for Corrosion Control Lee Odell, P.E. Vice President CH2M HILL"— Presentation transcript:

1 Calcite Contactors for Corrosion Control Lee Odell, P.E. Vice President CH2M HILL

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

5 Simplified Corrosion Cell Fe 2+ CATHODE ANODE O2O2 OH - e - STEP 1 STEP 2 STEP 3 STEP 4 Water with Dissolved Minerals Base Metal O2O2 e - e - e -

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

9 Base Metal Localized Pitting Attack Water Original Thickness 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 CaCO 3 H + + HCO 3

22 Species Distribution Diagram CaCO3 -> 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

26 Descon Design Tool Filter Tank Sizes and Velocity (gpm/sq ft) 24"36"48"60" 25 gpm8. 50 gpm15. 100 gpm31.814.28.05.1 150 gpm47.821.211.97.6 200 gpm63.728.315.910.2

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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