Recommended Procedure for Chlorine Disinfection of Water Treatment Plant Basin, Elevated Storage, Spring Box, Ground Reservoir, or Cistern
Introduction
A ground reservoir, elevated tank, spring box, cistern, or water treatment plant basin should be thoroughly cleaned and then disinfected with a strong chlorine solution after:
- Original construction
- Any repair or maintenance
- Flooding
- A period of non-use
- Two or more "unsafe" bacteriological water samples that are traced to the well
Adequate chlorine requires a certain chlorine dosage for a minimum contact time - 100 parts per million for 2 hours, or 50 parts per million for 8 hours, or 25 parts per million for 24 hours.
Chlorine for disinfection for these water systems can be either 5.25% sodium hypochlorite solution or 65% calcium hypochlorite powder. A 5.25% hypochlorite solution is common house-hold bleach such "Hilex", "Clorox", or "Purex" available at grocery stores and supermarkets. The 65% calcium hypochlorite powder is available from chemical supply houses and is known commercially as "HTH", Perchloron", or "Pittchlor".
Recommended Procedures
- The unit to be disinfected should be full of water.
- Determine recommended chlorine disinfection dosage for the desired contact time from the following table:
| Volume of Box, Basin, Reservoir or Cistern | 5.25% Sodium Hypochorite (Bleach) | 65% Calcium Hypochorite | ||||
|---|---|---|---|---|---|---|
| 100 ppm* for 2 hours | 50 ppm* for 8 hours | 25 ppm* for 24 hours | 100 ppm* for 2 hours | 50 ppm* for 8 hours | 25 ppm* for 24 hours | |
| 50 gal. | 1 1/2 cups | 3/4 cups | 3/8 cups | --- | --- | --- |
| 100 gal. | 3 cups | 1 1/2 cups | 3/4 cups | --- | --- | --- |
| 200 gal. | 6 cups | 3 cups | 1 1/2 cups | --- | --- | --- |
| 500 gal. | 1 gal. | 7 1/2 cups | 3 3/4 cups | 9 1/2 oz. | --- | --- |
| 1,000 gal. | 2 gal. | 1 gal. | 7 1/2 cups | 1 lbs. 3 oz. | 9 1/2 oz. | --- |
| 2,000 gal. | 4 gal. | 2 gal. | 1 gal. | 2 lbs. 6 oz. | 1 lbs. 3 oz. | 9 1/2 oz. |
| 5,000 gal. | --- | 5 gal. | 2 1/2 gal. | 6 lbs. | 3 lbs. | 1 lbs. 8 oz. |
| 10,000 gal | --- | --- | 5 gal. | 12 lbs. | 6 lbs. | 3 lbs. |
| 20,000 gal | --- | --- | --- | 24 lbs. | 12 lbs. | 6 lbs. |
| 50,000 gal. | --- | --- | --- | 60 lbs. | 30 lbs. | 15 lbs. |
| 100,000 gal. | --- | --- | --- | 120 lbs. | 60 lbs. | 30 lbs. |
* ppm = parts per million
- Completely mix the chorine dosage throughout the unit to be disinfected.
- Leave the chorine solution in the unit for the recommended contact time.
- Do not use the heavily chlorinated water.
- At the end of the contact time, remove the water from the unit and discharge to waste.
DO NOT ALLOW THE WATER TO ENTER A RIVER, LAKE, OR STREAM. - Fill the unit with clean water and collect a water sample for bacteriological testing after one or two days of use.