Disinfecting a potable water storage tank--that is, a tank used for storing drinking water--in accordance with standards set by the American Water Works Association (AWWA) can be a difficult task. Unless you're looking to get into the tank cleaning business, you might consider leaving this project up to a professional. The chemicals involved are extremely hazardous and following strict guidelines is necessary to make sure the disinfection is done safely and properly.
Any items that were used during the construction or repair of the water tank should be removed and the tank should be thoroughly cleaned. The inside of the tank should be swept, scrubbed and, ideally, pressure washed. Any and all screened openings should also be washed and inspected to make sure they are in good condition to prevent any debris from entering the newly cleaned tank.
Types of Chlorine
There are three forms of chlorine acceptable for use in tank disinfection: liquid chlorine, sodium hypochlorite, which is also a liquid and calcium hypochlorite, which is available in tablets or small granules. It is important to remember that chlorine is a highly dangerous substance and in most cases should be handled by an experienced professional.
There are three methods for using chlorine to disinfect a water storage tank.
Method 1 The water storage tank is filled to the point of overflow with potable water to which chlorine is added. The amount of chlorine in the water should reach a minimum of 10 milligrams per liter (mg/L) and left for a period of 6 to 24 hours.
Method 2 A solution containing 200 mg/L of chlorine is applied to all surfaces of the tank that will come into contact with water when the tank is filled to the point of overflowing. The solution must stay on for a minimum of 30 minutes. All drainpipes must be filled with a 10-mg/L chlorine solution for the same amount of time. The tank should then be cleaned with potable water and the drainpipes should be cleared.
Method 3 A 50 mg/L chlorine solution should be used to fill 5 percent of the tank. After waiting a minimum of 6 hours, the tank should then be filled to its overflow point with potable water. After another 24 hours, the tank is then drained.
After cleaning, potable water stored in the tank must be tested for the presence of coliform bacteria. Acceptable testing methods include the multiple-tube fermentation technique, membrane-filter technique or enzyme substrate coliform test. It is important to test the water that is feeding the tank as well, to make sure that the water source is not introducing the bacteria to the storage tank. The water drawn from the tank is also tested for odors left over from the chlorine treatment.
Dangers of Chlorine Exposure
It is important to highlight that all chlorine handling and water testing should be done by experienced and in some cases licensed professionals. Mishandling chlorine can result in serious injury. When chlorine comes in contact with the moist tissue of the human body found in the eyes, throat and lungs, an acid is produced that damages the tissue. If you or someone you know has been exposed to a high concentration of chlorine, seek medical assistance immediately.