Download - Calcite Contactors for Corrosion Control Lee Odell, P.E. Vice President CH2M HILL [email protected]
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Calcite Contactors for Corrosion Control
Lee Odell, P.E.
Vice President
CH2M HILL
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Overview
• 1 – Corrosion Background
• 2 – Treatment Systems Design, Operation & Maintenance
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Corrosion Control
• Purpose– protect public health– improve water quality– extend plumbing equipment– meet regulations
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Battery Analogy• Anode• Cathode• Electrical Circuit• Metal lost at anode
Corrosion
e -
Electrolyte
An
od
e
Cat
ho
de
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Simplified Corrosion Cell
Fe 2+
CATHODE
ANODE
O2
OH-
e-
STEP 1
STEP 2
STEP 3
STEP 4
Water withDissolved Minerals
Base Metal
O2
e-e- e-
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Major Factors Influencing Corrosion
• pH
• Temperature
• Dissolved Solids
• System Deposits
• Water Velocity
• Microbiological Growth
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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...
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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
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Base Metal
Localized Pitting Attack
WaterOriginal
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
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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
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Effect of pH on the Release of Copper into Solution
0
1
2
3
4
5
6
2 3 4 5 6 7 8 9
pH
Cop
per
Con
cent
rati
on (
mg/
L)
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Effect of pH and Alkalinity on Lead Solubility
0
0.2
0.4
0.6
0.8
1
1.2
6.5 7 7.5 8 8.5 9 9.5 10
pH
The
oret
ical
Lea
d So
lubi
lity
(mg/
L)
10 15 20 25 30Alkalinity (mg/L CaCo3)
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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
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What information is Needed to Design a Calcite Contactor?
• pH• Alkalinity• Calcium• TDS or Conductivity• Flow Rate
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Calcite Contactors
1200 College St
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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
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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.
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Contactors
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Contactor Arrangement
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Contactors
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Limestone
• CaCO3 H+ + HCO3
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Species Distribution DiagramCaCO3 -> H+ and HCO3
0
0.2
0.4
0.6
0.8
1
4 5 6 7 8 9 10 11 12
pH
Fra
ctio
n a
s d
esig
nat
ed s
pec
ies
CO2 HCO3- CO32-
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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
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Limestone Contactor
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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/
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Descon Design ToolFilter Tank Sizes and Velocity (gpm/sq
ft)
24" 36" 48" 60"
25 gpm 8.0 3.5 2.0 1.3
50 gpm 15.9 7.1 4.0 2.5
100 gpm 31.8 14.2 8.0 5.1
150 gpm 47.8 21.2 11.9 7.6
200 gpm 63.7 28.3 15.9 10.2
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Design Considerations
• Vessel Type – – Open, Pressure– Up-flow, Down-flow– Need Backwash Disposal?– Site Glass– Pressure gauges
• Y-Strainer• pH Monitoring?
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Operations & Maintenance
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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
0
2
4
6
8
10
12
14
16
18
Alk 10 Alk 20 Alk 50
pH 6
pH 6.5
pH 7
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Questions?