Section 950. Stabilization

Water that is unstable due either to natural causes or to the treatment given to the water should be stabilized. Water treated with excess lime for softening or manganese removal shall be treated by carbon dioxide or acid.

A. Carbon dioxide addition.

1. The recarbonation chamber design should provide:

a. A detention time of three to 10 minutes;

b. A depth of about eight feet; and

c. A reaction tank with a detention time of 20 minutes.

2. Adequate precautions shall be taken to prevent the possibility of carbon monoxide entering the plant from the recarbonation and reaction chamber.

B. Sulfuric acid.

1. Feed equipment for sulfuric acid shall conform to 12VAC5-590-860.

2. Adequate precautions shall be taken for safety.

C. Removal of free carbon dioxide. Carbon dioxide may be removed by an alkali, following aeration. The addition of an alkali following aeration may not be necessary when the alkalinity of the aerated water is greater than 80 milligrams per liter.

D. Deposition of calcium carbonate film. The desired calcium carbonate film may be obtained by using either soda ash or caustic soda when the alkalinity of the water exceeds about 35 milligrams per liter. Soft waters should be treated with lime to provide the required calcium. Soft waters which also have a low carbon dioxide content may need a mixture of lime and soda ash to provide both calcium and carbonate for the calcium carbonate film.

E. Polyphosphates. Polyphosphates are applicable for sequestering dissolved minerals.

1. Feed equipment shall conform to 12VAC5-590-860.

2. Phosphate chemicals shall be food grade.

3. Stock phosphate solution shall be kept covered and disinfected by carrying approximately 10 milligrams per liter chlorine residual.

4. Satisfactory chlorine residuals should be maintained in the distribution system when phosphates are used.

F. Under some conditions, softening plants can be designed using split treatment in which raw water is blended with softened water to partially stabilize the water. Treatment plants designed to utilize split treatment should, in most cases, also contain facilities for further stabilization by other means.

G. Water unstable due to biochemical action in the distribution system. Residual chlorine throughout the distribution systems may be used to prevent corrosion due to decomposition of organic matter (especially in dead-ended mains), the biochemical action within tubercles and the reduction of sulfates to sulfides.

H. Cathodic protection may be used to prevent or minimize corrosion of the inner surfaces of water tanks and standpipes and the outer surfaces of metal conduits.

I. Laboratory equipment shall be provided for determining the effectiveness of stabilization treatment and concentration of chemicals in the treated water.

Historical Notes

Derived from VR355-18-009.11 § 3.32, eff. August 1, 1991.

Statutory Authority

§ 32.1-12 and 32.1-170 of the Code of Virginia.