مواد ازالة التاكل المستخدمة لمعالجة المياه والمضافة لمياه الرادياتير (الصودا الكاوية)

CORROSION CONTROL:

Using Caustic Soda to Control Corrosion and Calcium Carbonate Precipitation

technolab el-bahaa group

general.dr

bahaa badr

chemical advisor

The decay of distribution system infrastructure due to corrosion is a major challenge facing water utilities in the 21st century.

In addition to red water problems which commonly result from the corrosion of unlined cast iron distribution pipes, there are numerous other problems associated with pipeline corrosion that affect water quality and the cost of operating and maintaining a distribution system.

Generally, the goal of corrosion control strategies is not only to reduce the corrosion of distribution mains but also to minimize the release of lead and copper from residential plumbing fixtures.

Lead and copper are regulated under the Lead and Copper Rule of the Safe Drinking Water Act and in many cases determine the corrosion control treatment technique employed by utilities.

Two commonly used treatment techniques for controlling the release of lead and copper include lowering the corrosivity of water with pH adjustment and/or with the addition of a corrosion inhibitor.

Increasing the pH decreases the solubility of lead and copper in water and may lead to the formation of an insoluble calcium carbonate precipitate along the pipe wall.

The deposition of calcium carbonate in pipes may help reduce corrosion by providing a barrier on the pipe wall which prevents water from coming into contact with and corroding the pipe material.

Similarly, corrosion inhibitors can reduce the corrosion of distribution system pipes and prevent lead and copper dissolution by forming an insoluble coating of metallic orthophosphate.

Utilities often employ pH adjustment as a corrosion control strategy because treatment plants already have chemical feed systems.

For waters with moderate to high levels of hardness and alkalinity, increasing the pH of the finished water may also promote the precipitation of calcium carbonate within the distribution system.

Excessive deposition, however, will significantly reduce the hydraulic carrying capacity of the pipes.

This problem is particularly true for utilities that use lime for post treatment pH adjustment.

Lime adds calcium ions and increases the pH which shifts the distribution of carbonate alkalinity from bicarbonate (HCO3-1) to carbonate (CO3-2) alkalinity.

When the concentrations of calcium and carbonate ions exceed the solubility product, calcium carbonate precipitate is formed.

Therefore, if lime is added indiscriminately, the result can be excessive amounts of calcium carbonate precipitation in post treatment storage facilities or within the distribution system.

Water quality conditions to achieve finished water pH of 7.5 using lime or caustic Soda without Calcium Carbonate precipitation (with 1mg/L orthophosphate addition).

Experience has shown that it is difficult to develop a uniform deposition of calcium carbonate throughout the distribution system.

Deposition generally occurs close to the treatment plant and less protection is provided as the water travels further from the plant, typically where the lead and copper containing fixtures are located.

To prevent reduction in hydraulic carrying capacity of pipes, corrosion control strategies have generally evolved into producing a low corrosivity water with a low scale forming potential.

As a guideline, water leaving the treatment facility should have a calcium carbonate precipitation potential (CCPP) greater than 0 but less than 10 ppm to provide protection without contributing to the excessive deposition within the distribution system.

One possible means of reducing the corrosivity of water while minimizing the excess deposition of calcium carbonate is to switch post treatment chemicals for pH adjustment from lime to caustic soda.

Caustic soda, or sodium hydroxide (NaOH), will increase the pH of the water without adding calcium ions that are needed for calcium carbonate precipitation to occur.

Consequently, for a given pH adjustment, caustic soda should reduce the calcium carbonate precipitation potential compared to lime.

The use of caustic soda instead of lime may reduce calcium carbonate precipitation while achieving the desired finished water pH.

However, the benefit of using caustic soda over lime can be realized only under certain water quality conditions.

Generally, caustic soda provides some benefit over lime for soft waters. The benefit of caustic over lime decreases as the calcium hardness increases.

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