Calcite Contactors for Corrosion Control

Lee Odell, P.E.Vice President

CH2M HILLlee.odell@ch2m.com

Overview

• 1 – Corrosion Background• 2 – Treatment Systems Design,

Operation & Maintenance

Corrosion Control

• Purpose– protect public health– improve water quality– extend plumbing equipment– meet regulations

Battery Analogy• Anode• Cathode• Electrical Circuit• Metal lost at anode

Corrosion

e -

Electrolyte

Ano

de

Cat

hode

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-

Major Factors Influencing Corrosion

• 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...

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

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

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

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

ratio

n (m

g/L

)

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)

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

What information is Needed to Design a Calcite Contactor?

• 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• 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.

Contactors

Contactor Arrangement

Contactors

Limestone

• CaCO3 H+ + HCO3

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

Frac

tion

as d

esig

nate

d sp

ecie

s

CO2 HCO3- CO32-

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

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 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

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

02468

1012141618

Alk 10 Alk 20 Alk 50

pH 6pH 6.5pH 7

Questions?