The Bicarbonate Method Of Corrosion Control


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THE BICARBONATE
METHOD OF CORROSION
CONTROL

THEBICARBONATEMETHOD OFCORROSIONCONTROL

THE BICARBONATE METHOD OF CORROSION CONTROL
TABLE OF CONTENTS
PAGE
I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
II. D esigning and Operating Sodium Bicarbonate Addition Systems for Potable Water Corrosion Control . . . . . . . . . . . . . . . . . . 5
III. R esidual Benefits of the Bicarbonate Method . . . . . . . . . . . . . . . . . . . . . . . . 13

I. INTRODUCTION

Drinking water suppliers in many parts of the United States and Canada will have a hard time meeting the new drinking water standards for lead and copper — not because their source water is contaminated with these metals, but almost paradoxically, because of this high purity and lack of dissolved minerals. Such water is considered corrosive or aggressive, and it will eventually satisfy this aggression by attacking whatever it comes in contact with. Unfortunately for the water supplier and consumer, this hunger is often satisfied by metals in the distribution system and in the residential plumbing and fixtures themselves.
Rain water is essentially devoid of any minerals, but it is naturally acidic because of its tendency to dissolve carbon dioxide from the atmosphere as it falls to the earth. The dissolved carbon dioxide reacts with the water to form carbonic acid, which dissociates to form a bicarbonate ion (-HCO3 ) and an acidic ion (H+). Other acid constituents in the atmosphere such as nitrogen or sulfur trioxides will also be absorbed by the falling rainwater, increasing its acidity and lowering its pH.
In areas of the country where geological substrates contain limestone, a well-understood buffering process occurs as this acidic rain water percolates through the soil to its respective reservoir or aquifer. As the water permeates the limestone-containing subsoils, some of the active hydrogen ions react with the calcium carbonate substrates to form more bicarbonate ions. These bicarbonate ions provide a natural buffering effect that stabilizes the pH in a favorable neutral region and prevents dramatic shifts in pH which could result from the absorption of other acidic compounds.

Furthermore, as this naturally treated water enters the distribution system, the prevalent bicarbonate ions will react with exposed metals such as lead and copper. The result is the formation of a tenacious carbonate or hydroxy carbonate film on the metal surface, creating nature’s own corrosion inhibiting system and protecting the metal from further corrosion.
Unfortunately, many geologic drainages have soils which are devoid of neutralizing minerals and the water in these reservoirs will be acidic with little, if any, bicarbonate alkalinity.
Various studies have shown that the corrosive nature of these waters can be greatly reduced by adjusting pH and alkalinity to values similar to those found in watersheds with alkaline mineral deposits. In fact, minimum corrosion occurs when the pH is in the range of 7.5 to 8.5, which corresponds to maximum bicarbonate (-HCO3 ) ion species as shown in the adjacent graph.

The Bicarbonate Method of Corrosion Control – 3

Relationship of the species distribution diagram to the titration curve for the carbonate system at 25°C

The bicarbonate method of corrosion control as developed by Church & Dwight Co., Inc., was specifically designed to treat corrosive source waters which are acidic and scarce in bicarbonate alkalinity. By utilizing what nature has taught us and optimizing with good science, the bicarbonate method will help many water suppliers to successfully meet EPA’s lead and copper criteria.
Waters that are only mildly acidic but devoid of bicarbonate alkalinity can have corrosion successfully controlled simply by increasing the alkalinity to approximately 30 ppm as CaCO3 using pure sodium bicarbonate. The required dosage is 150 pounds of sodium bicarbonate to each million gallons of water for a 10 ppm increase in alkalinity as CaCO3.

Waters that are more acidic (pH < 6.5) may be treated first for pH adjustment using an inexpensive but high-purity alkali such as sodium hydroxide or calcium hydroxide, followed by alkalinity adjustment using pure sodium bicarbonate.

The Bicarbonate Method of Corrosion Control – 4

II. D ESIGNING AND OPERATING SODIUM BICARBONATE ADDITION SYSTEMS FOR POTABLE WATER CORROSION CONTROL

The purpose of this section is to describe in general terms how to design and operate a bicarbonate-based corrosion control system. For the administrator or supervisors, it will provide a feel for the simplicity and economy of the process. For the consulting and plant engineers, it will provide a basic understanding from which a detailed design and cost estimate will flow.
PHILOSOPHY The design concept is to minimize labor and material handling concerns by providing a process which can be easily automated and which has a very high degree of reliability, using equipment and processes familiar to water treatment plant operators.

This is accomplished by using a “day tank” concept in which the solution required to provide a full 24 hours of alkalinity adjustment is made up once per day, requiring one hour less of operator labor. After this, the bicarbonate solution is metered into the treated water flow at a proportional rate. The relatively high solubility and handling ease of sodium bicarbonate makes this concept viable. Once the bicarbonate is in solution form, it can be easily metered into the distribution network by pumps and metering systems familiar to water treatment operators. The same design concept can be utilized for systems of less than 100,000 gpd to those greater than 10 MGD.

Solubility of sodium bicarbonate in water

The Bicarbonate Method of Corrosion Control – 5

BASIS For simplification we have assumed a “typical” water supply into the corrosion treatment plan containing 10 ppm of alkalinity as CaCO3. The design of the corrosion control process is to continuously add 20 ppm of bicarbonate alkalinity as CaCO3 to approach the optimum value of 30 ppm. Since alkalinity is a linear function, any of the quantities given can be adjusted to a specific alkalinity requirement by using a factor calculated from rationing the required alkalinity to 20 ppm. For example, if your system requires only 15 ppm of CaCO3 adjustment, multiply the base case quantities by 15/20 or 0.75. The base case, however, is truly typical and will apply directly to many water supplies.
The base case assumes a bicarbonate feed solution of 7.5%. This represents a saturated solution at 40°F or approximately 95% saturated at 50°F. For solution temperatures below 50°F, the concentration should be reduced to 7%. Conversely at 70°F, a 9% solution can be utilized.

HANDY CONVERSIONS A 7.5% NaHCO3 solution: • Contains 45,000 ppm of bicarbonate alkalinity as CaCO3 • Contains 0.625 pounds of NaHCO3 per gallon of solution
Forty-eight gallons of 7.5% NaHCO3 solution are required to increase 100,000 gallons of water by 20 ppm as CaCO3, or 29 gph for each 1,000 gpm of water flow.
Three hundred pounds of sodium bicarbonate are required to raise the alkalinity of 1 million gallons of water by 20 ppm as CaCO3.

GALLONS PER FEET OF HEIGHT TANK CAPACITY

Tank diameter (ft.)

Gallons/ft.

2

24

3

53

4

94

6

212

8

377

The Bicarbonate Method of Corrosion Control – 6

II. DESIGNING AND OPERATING SODIUM BICARBONATE ADDITION SYSTEMS FOR POTABLE WATER CORROSION CONTROL (CON’T)

SYSTEMS UP TO 5 MGD Sodium bicarbonate is received on pallets of nominal 2,000-pound weight in 50-pound paper bags.
A day tank which will contain 24 hours’ worth of sodium bicarbonate solution is filled with treated water to the operating level and a top-mounted mixer is activated. Sodium bicarbonate bags are manually broken and contents dumped into the day tank. A dust control hood is recommended. For each MGD of system capacity, six bags (300 pounds) of sodium bicarbonate addition is required along with 480 gallons of solution capacity.
The sodium bicarbonate will completely solubilize within minutes; however, it is acceptable to begin metering the solution during bicarbonate addition as long as the day tank is properly agitated.
The metering pump which delivers the sodium bicarbonate solution to the distribution system is started. The rate of solution flow is controlled by the rate of treated water flow in the ratio of 480 gpm or 29 gph for 1,000 gpm.

SIZING RECOMMENDATIONS Minimum size — The minimum practical system size is determined by the smallest sodium bicarbonate package size of 50 pounds. (Of course, very small systems can be designed around consumer packaged “baking soda” in one-pound to four-pound containers). Eight gallons of water are required to dissolve 50 pounds of sodium bicarbonate. Considering room for a mixer and some freeboard, the minimum tank size becomes nominally 100 gallons or roughly 2 feet in diameter by 4 feet tall. This tank would then contain the necessary solution to treat up to 167,000 gallons of water per day.
Storage — A minimum five-day storage space is needed for the sodium bicarbonate. Allow 20 sq. ft. per pallet space. Pallets can be double stacked if lift equipment is available.

System size MGD
1 2 5

Bags (50#) per day
6 12 30

No. of pallet spaces for 5 days storage
1 2 4

Sq. ft. storage space
20 40 80

The Bicarbonate Method of Corrosion Control – 7

Tankage — Tanks should be properly baffled to assure adequate mixing.

System size MGD
0-0.15 1 2 5

Gallons capacity
100 600 1,200 3,000

Diameter 200 4 6 8

Height (ft.) 4 6.5 8 8

Mixing — A mixer, preferably top-mounted to minimize leak potential and to facilitate maintenance, is needed.
Metering — The metering pump should have a turn-down ratio of at least 3-to-1 to handle minimal winter and maximum summertime recommended flows.
The rate of bicarbonate solution flow into the treated water is maintained at a constant rate. If the treated water flow is controlled manually, such as estimated from a calendarized demand chart, and

flow is into an intermediate storage reservoir, then the solution rate can be controlled manually at a proportion of 29 gph of solution per 1,000 gpm of water flow.
If water flow is strictly by demand and cannot be controlled, then it must be metered. A flow sensor on the meter output is used to control the flow of the bicarbonate solution at the ratio of 29 gph per 1,000 gpm of water flow.

5 MGD or less

The Bicarbonate Method of Corrosion Control – 8

II. DESIGNING AND OPERATING SODIUM BICARBONATE ADDITION SYSTEMS FOR POTABLE WATER CORROSION CONTROL (CON’T)

SYSTEMS 6 MGD TO 10 MGD

Sodium bicarbonate is received in 1-ton bulk sacks. The tank is

designed to hold one sack of sodium bicarbonate. The hours of

capacity in the tank will vary but, in general, the day tank

MGD

concept still holds.

6

7.5

The 1-ton sacks are hoisted by a winch on a monorail and

10

positioned over the day tank. The contents are dumped during

a period of several minutes into the well-mixed day tank.

SIZING RECOMMENDATIONS Tank size — 8 ft. diameter by 12 ft. height, capacity = 4,500 gallons

Hours of capacity per tank load 27 21 16

6–10 MGD

The Bicarbonate Method of Corrosion Control – 9

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The Bicarbonate Method Of Corrosion Control