Chlorine kills bacteria, including disease-causing organisms and the nuisance organism, iron bacteria. However, low levels of chlorine, normally used to disinfect water, are not an effective treatment for giardia cysts. A chlorine level of over 10 mg/1 must be maintained for at least 30 minutes to kill giardia cysts.
Chlorine has been used since 1908 to disinfect water supplies in the United States to protect public health. The effectiveness of chlorination depends on the chlorine demand of the water, the concentration of the chlorine solution added, the time that chlorine is in contact with the organism, and water quality. These effects can be summatized in the following manner:
The safest way to be sure that the amount of chlorine added is sufficient is to add a little more than is required. This will result in a free chlorine residual that can be measured easily. This chlorine residual must be maintained for several minutes depending on chlorine level and water quality. Table 4 lists the free chlorine residual level needed for different contact times, water temperatures and pH levels.
Kits are available for measuring the chlorine residual by looking for a color change after the test chemical is added. The test is simple and easy for a homeowner to perform. If chlorination is required for the water supply, the chlorine residual should be tested regularly to make sure the system is working properly.
The kit should specify that it measures the free chlorine residual and not the total chlorine. Once chlorine has combined with other chemicals it is not effective as a disinfectant. If a test kit does not distinguish between free chlorine and chlorine combined with other chemicals, the test may result in an overestimation of the chlorine residual.
Chlorine will kill bacteria in water, but it takes some time (Table 4) . The time needed depends on the concentration of chlorine. Two methods of chlorination are used to disinfect water: simple chlorination and superchlorination.
| Table 4. Necessary chlorine residual to disinfect water for various contact times, water temperatures and pH | |||
|---|---|---|---|
| Water Temp. 50 degrees F | |||
| Contact time (minutes) | Necessary chlorine residual (mg/l) | ||
| pH 7 | pH 7.5 | pH 8 | |
| 40 | 0.2 | 0.3 | 0.4 |
| 30 | 0.3 | 0.4 | 0.5 |
| 20 | 0.4 | 0.6 | 0.8 |
| 10 | 0.8 | 1.2 | 1.6 |
| 5 | 1.6 | 2.4 | 3.2 |
| 2 | 4.0 | 6.0 | 8.0 |
| 1 | 8.0 | 12.0 | 16.0 |
| Water Temp. 32 - 40 degrees F | |||
| Contact time (minutes) | Necessary chlorine residual (mg/l) | ||
| pH 7 | pH 7.5 | pH 8 | |
| 40 | 0.3 | 0.5 | 0.6 |
| 30 | 0.4 | 0.6 | 0.8 |
| 20 | 0.6 | 0.9 | 1.2 |
| 10 | 1.2 | 1.8 | 2.4 |
| 5 | 2.4 | 3.6 | 4.8 |
| 2 | 6.0 | 9.0 | 12.0 |
| 1 | 12.0 | 18.0 | 24.0 |
Example: What is the necessary chlorine residual for well water with pH 7.5?
The well water is 38 degrees F when it enters the house. The pump delivers 7 gallons per minute and after the chlorine is added it is held in a 100 gallon holding tank.
Simple chlorination involves maintaining a low level of free residual chlorine at a concentration of 0.30.5 mg/1 for at least 30 minutes. The residual is measured at the faucet most distant from the where chlorine is added to the water supply.
To ensure the proper contact time of at least 30 minutes, a holding tank can be installed (Table 5). Pressure tanks, while often thought to be sufficient, are usually too small to always provide 30 minutes of contact time.
| Table 5. Available contact time from a 50-gallon holding tank | |
|---|---|
| Water flow rate (gallons per minute) | Holding time (minutes) |
| 5 | 7 |
| 7 | 5 |
| 10 | 3.5 |
Another way to maintain necessary contact time is to run the chlorinated water through a coil of pipe (Table 6).
| Table 6. Available contact time from 1000 feet of 1-1/4 inch pipe | |
|---|---|
| Water flow rate (gallons per minute) | Holding time (minutes) |
| 5 | 9.2 |
| 7 | 6.6 |
| 10 | 4.6 |
When the water cannot be held for at least 30 minutes before it is used, supe chlorination is an alternative. For superchlorination, a chlorine solution is added to the water to produce a chlorine residual of between 3.0 and 5.0 mg/1, which is about ten times stronger than for simple chlorination. The necessary contact time for this concentration is reduced to less than five minutes (Table 4). The water will have a very strong chlorine smell. If this is not desirable, the chlorine can be removed just before it is used with a carbon filter (Note: not currently allowed under Ohio Department of Health for private water supplies).