Transcript 6 cal-contactors lee odell final. - PNWS-AWWA

Calcite Contactors for Corrosion Control
Lee Odell, P.E.
Vice President
CH2M HILL
[email protected]
Overview
• 1 – Corrosion Background
• 2 – Treatment Systems Design,
Operation & Maintenance
Corrosion Control
• Purpose
– protect public health
– improve water quality
– extend plumbing equipment
– meet regulations
Corrosion
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Anode
Cathode
e
Electrolyte
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Battery Analogy
Anode
Cathode
Electrical Circuit
Metal lost at anode
Simplified Corrosion Cell
OH-
STEP 4
O2
O2
STEP 1
Water with
Dissolved
Minerals
Fe 2+
STEP 3
CATHODE
Base Metal
ANODE
ee-
e-
eSTEP 2
Major Factors Influencing Corrosion
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pH
Temperature
Dissolved Solids
System Deposits
Water Velocity
Microbiological Growth
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...
General Corrosion
Water
Original
Thickness
Base Metal
General Etch Uniform Attack
• Preferred
situation
• Take a small
amount of metal
evenly
throughout the
system
• Anode very
large
Pitting Corrosion
Original
Thickness
Water
Base Metal
Localized Pitting Attack
• 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
Affects of Corrosion
• Potential regulatory non-compliance
• Shortened pipeline life
• Water usage increases
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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
Copper Concentration (mg/L)
Effect of pH on the Release of Copper
into Solution
6
5
4
3
2
1
0
2
3
4
5
6
pH
7
8
9
Theoretical Lead Solubility
(mg/L)
Effect of pH and Alkalinity on Lead
Solubility
1.2
10
1
15
20
25
30
Alkalinity (mg/L CaCo3)
0.8
0.6
0.4
0.2
0
6.5
7
7.5
8
8.5
pH
9
9.5
10
Calcite Contactors
• Calcite Contactors Use Limestone to Add Calcium
Carbonate to Water, Raise pH and add Alkalinity to
water.
• Benefits:
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Easy to Operate
Easy to Maintain
No Risk of Overdosing Chemical
Operate in Upflow Mode
No Need for Controllers/Motor Actuated Valves or
backwashing
What information is Needed to Design
a Calcite Contactor?
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pH
Alkalinity
Calcium
TDS or Conductivity
Flow Rate
Calcite Contactors
1200 College St
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
Process Description
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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:
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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.
Contactors
Contactor Arrangement
Contactors
Limestone
• CaCO3
H+ + HCO3
Species Distribution Diagram
CaCO3 -> H+ and HCO3
Fraction as designated species
CO2
HCO3-
CO32-
1
0.8
0.6
0.4
0.2
0
4
5
6
7
8
pH
9
10
11
12
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
Limestone Contactor
Limestone
Contactor
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/
Descon Design Tool
Filter 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
Design Considerations
• Vessel Type –
– Open, Pressure
– Up-flow, Down-flow
– Need Backwash Disposal?
– Site Glass
– Pressure gauges
• Y-Strainer
• pH Monitoring?
Operations & Maintenance
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
18
16
14
12
10
pH 6
pH 6.5
pH 7
8
6
4
2
0
Alk 10 Alk 20 Alk 50
Questions?