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Effective Well Chlorination
Chlorine has been used as a disinfectant in wells for many years. Generally, it is used following well construction and remediation to kill any bacteria that may be present due to operations or naturally occurring sources.
The bacteria and viruses introduced into the well by drilling tools, pipes and other materials during operations are usually those living in the soil at the well site. Among these may be the possible disease-producing bacteria known as coliform.
Use of a chlorine solution is the simplest and most effective way to disinfect wells, pumps and distribution systems. The effectiveness of chlorination depends on:
- Chlorine concentration
- Free-chlorine residuals
- pH of water
- Retention time
The chlorine concentration must be high enough so that the residual available chlorine remains several hours after treatment, usually 4 to 12 hours. This ensures that the chlorine demand has been satisfied. High pH waters require higher chlorine dosages than low
pH waters to obtain the same level of dis-infection. Enough retention time must be allowed so that the chlorine can kill the bacteria. High turbidity may tend to reduce the effectiveness of chlorination.
Chlorination essentially utilizes three forms of chlorine: chlorine gas, sodium hypochlorite (liquid), and calcium hypochlorite (solid). The three most commonly used chemicals for chlorination are:
- Liquid sodium hypochlorite (NaOCl) in a 12.5 percent or 5.25 percent solution
- Dry calcium hypochlorite (Ca(OCl)2) 65% activity, and
- Hydrochlorous acid (HOCl).
Hydrochlorous acid by far is the most effective disinfectant form of chlorine. However, since it is in an acidic form (not sodium or calcium salt), the amount of available chlorine produced by the acid is bly dependent on solution pH. The available chlorine decreases as the solution pH increases. For example, the acid maintains over 90 percent activity at pH of 6.0, the activity reduces drastically to less than 30 percent if the pH reaches 8.0. The optimum pH range, where hypochlorous acid maintains 70 to 90 percent activity, is between 6.0 and 7.0. The available chlorine strength commonly used for sterilizing wells is about 50 to 200 ppm or mg/liter.
The following table shows the amount of sodium hypochlorite and calcium hypochlorite required to produce four available chlorine concentrations in 1,000 gallons of water.
Quantities of Chlorine Compounds Required To Produce Chlorine Concentrations of 50, 100, 500, and 1,000 mg/l in 1,000 Gal (3.8 m3) of Water
|Strength (mg/l)||Sodium Hypochlorite
Additions in Gallons/Liters
|Calcium Hypochlorite 65%|
Dry calcium hypochlorite is a fairly stable material when properly stored in a cool, dry place. It will retain 90% of its chlorine content for 12 months after manufactured. Sodium hypochlorite, often referred to as bleach, is only available in solution form due to its instability. Ordinary household bleach is about 5% sodium hypochlorite. Commercially available formulations are usually 10 to 12% by weight. It loses about one half of its strength after 6 months under the best conditions. Sodium hypochlorite solution is alkaline with a pH of 9 to 11 depending on the concentration.
Chlorine can destroy only the bacteria with which it comes into contact. To simply pour chlorine into a well is not enough to completely disinfect it.
The water in the well must be agitated thoroughly for complete mixing and forced into the gravel pack and formation interface for thorough disinfection. The conflict in achieving the proper pH is that additions of hypochlorite actually raise the pH above the natural pH of the well water. Try to lower the pH below that of the well water. This can easily be done with a treatment of SAPP at 15 lb./1000 gal during initial development or after rehabilitation. The pH of this mixture will be around 5.0. However, this is not true for BARAFOS® thinner, the pH of a 15 lb./1000 gal treatment in water with a pH of 7.0 will be around 8.0.
One word of caution, at a pH of below 5.0, chlorine gas begins to form.
Fortunately the reaction is very slow until the pH is at 4.0 or less and the mere addition of hypochlorite begins to neutralize the acid and raise the pH to a safe level. Maintaining the proper pH during chlorination will provide much more active disinfectant and removal of contaminating bacteria.