A comprehensive look at the most commonly used disinfectant.

Although there are several chemical disinfectants that may be used to treat a well, chlorine is the water supply disinfectant most commonly used. Chlorine is a bactericidal chemical that destroys or inactivates coliform bacteria that it comes into contact with. Chlorine attempts to disrupt the normal life processes of an organism. This is done by penetrating the cell wall of the organism and upsetting the natural life cycle processes or altering the organism's enzymes. With the cycle so disrupted, the organism either dies or cannot reproduce and the water is made bacteriologically safe.

Treatment with a chlorine solution is an essential component of efforts to eliminate microorganisms that have invaded an existing well or have been introduced into a new well during its construction. The objective of water supply system chlorination is to expose all parts of the water system to a chlorine solution of sufficient strength for an adequate time period. The “water system” includes the water-bearing formation around well screen or a rock borehole, well casing, pump, pressure tank and piping.

Sodium hypochlorite and calcium hypochlorite are the most common sources of chlorine used for disinfection of onsite water supplies.

Because light and heat accelerate decomposition of sodium hypochlorite solutions, product degradation is less pronounced when containers are stored in a dry, cool and darkened area or in containers protected from light.

Sodium Hypochlorite

Common household bleach is a clear-to-slightly yellow colored liquid with a distinct chlorine odor. Common laundry bleach is 5.25 percent to 6.0 percent available chlorine, when bottled. Use unscented only. Scented bleaches may leave an odor for extended periods of time, even after the chlorine has been flushed out of the water supply. Do not use bleach products that contain additives such as surfactants, thickeners, stabilizers and perfumes. These additives may contain hazardous chemicals and should not be used for treating drinking water. Always check product labels to verify product content and use instructions.

Swimming pool chlorine is 10.0 percent to 12.0 percent available chlorine. Note that there are types of chlorine other than sodium hypochlorite available for swimming pool use, and these should not be used for treatment of water supplies unless certified as meeting American National Standards Institute (ANSI)/National Sanitation Foundation Inc. (NSF) Standard 60. Swimming pool chlorine products may contain UV inhibitors, algaecides or other additives that should not be added to water supplies. Always check product labels to verify product content.

Higher concentrations of chlorine in sodium hypochlorite solutions generally are not available. Above 15 percent, the stability of hypochlorite solutions is poor, and decomposition and the concurrent formation of chlorate is of concern.

Sodium hypochlorite solutions are of an unstable nature due to high rates of available chlorine loss. Over a period of one year or less, the amount of available chlorine in the storage container may be reduced by 50 percent or more. Solutions more than 60 days old should not be counted upon to contain the full amount of available chlorine originally in solution. The stability of hypochlorite solutions is greatly affected by heat, light, pH, initial chlorine concentration, length of storage and the presence of heavy metal cations. These solutions will deteriorate at various rates, depending upon specific factors:

  • The higher the concentration, the more rapid the deterioration.

  • The higher the temperature, the faster the rate of deterioration.

  • The presence of iron, copper, nickel or cobalt catalyzes the deterioration of hypochlorite. Iron is the worst offender.

Because light and heat accelerate decomposition of sodium hypochlorite solutions, product degradation is less pronounced when containers are stored in a dry, cool and darkened area or in a container protected from light. A test kit should be used to check the final chlorine residual in a prepared chlorine solution to assure that you have the intended concentration.

As a general rule, one gallon of 5.25 percent bleach in 100 gallons of water will make a solution of 500 parts per million (ppm).

Calcium Hypochlorite

Calcium hypochlorite is available as a dry white powder, granules or tablets. It is 60 percent to 70 percent available chlorine and has a 12-month shelf life if kept cool and dry. If stored wet, it loses chlorine rapidly and is corrosive. As a general rule, three-quarters of a pound (about 11⁄2 cups) of granular calcium hypochlorite mixed in 100 gallons of water will make a 500-ppm solution. A chlorine test kit should be used to check the final chlorine residual in a prepared chlorine solution to assure that you have the concentration intended.

The use of calcium hypochlorite tablets dropped into the top of a well is not recommended as the sole means of disinfecting a well for the following reasons:

1. Tablets are designed to be slow dissolving. This characteristic is not conducive to getting all the available chlorine into the chlorine solution during the desired chlorination time interval.

2. Conditions in a well are not conducive to dissolving chlorine tablets. The water is cold and there is very little agitation or turbulence in the bottom of a well. Tablets are designed for use in applications where the water is warm and water is flowing past the tablets such as in a basket in the recirculation line of a swimming pool.

3. It is difficult to get uniform distribution of chlorine if the tablets are dumped into a well. There will be a strong concentration of chlorine around the tablets, but not in other portions of the well.

4. Tablets poured into the top of a well may lodge on the interior of the pitless adapter or on top of the submersible pump, causing corrosion. Cases of severe corrosion of submersible pumps leading to premature failure due to chlorine tablets have been reported.

5. The tablets cause high concentrations of chlorine in the bottom of the well, causing chemical interactions with the ground water leading to excessive scaling. If tablets are to be used as a source of chlorine for a chlorine solution, they must first be broken up and dissolved in a 5-gallon pail or bulk tank. Otherwise, they may remain in the bottom of the well for extended time periods and provide poor distribution of chlorine.

Which is Best?

Current experiences of water well drilling contractors and ground water specialists suggest that sodium hypochlorite is more effective of the two. However, this may be associated with the quality of the ground water in the well being treated rather than with the source of the chlorine itself.

In some places, there can be an abundance of calcium-based materials in both bedrock wells and those finished in glacial deposits. Calcium hypochlorite already has a high concentration of calcium (the white cloudy appearance). At 180 ppm (or approximately 10 grains) of hardness, water is saturated with calcium to the point that it precipitates out of the solution, changing from the dissolved state to a solid state. Introducing a calcium hypochlorite solution into a calcium-rich aquifer can cause the formation of a calcium carbonate (hardness) precipitate that may partially plug off the well intake. The plugging can interfere with the distribution of the chlorine solution and possibly reduce the production capabilities of the well. Sodium hypochlorite does not have the tendency to create the precipitate, which may be why it appears to be a more effective disinfectant. If the calcium carbonate concentration in the ground water is above 100 ppm, the use of sodium hypochlorite is recommended instead of calcium hypochlorite.

Sodium hypochlorite or calcium hypochlorite that contain other chemicals or additives, such as stabilizers, perfumes, algaecides or other chemicals that are used for water supply disinfection should be certified that they are in compliance with or surpass the ANSI/NSF Standard 60 for Drinking Water Treatment Chemicals - Health Effects, or an equivalent standard.

Biofilms work to prevent chlorine from penetrating into the material to make contact with the microorganisms that may exist there.

Germicidal Efficiency

The major factors affecting the germicidal efficiency of the free chlorine residual process are chlorine residual concentration, contact time, pH and water temperature. Increasing the chlorine residual, the contact time or the water temperature increases the germicidal efficiency. Increasing the pH above 7.5 drastically decreases the germicidal efficiency of free chlorine. A 99.6 percent to 100 percent kill can be achieved by maintaining a 50-ppm chlorine residual at an approximate pH of 6.4 to 8.6 for 4 to 6 hours at a temperature of 68 degrees F to 84 degrees F.

The form of chlorine that is in a chlorine stock solution is an important factor in how effective the solution is as a disinfectant. Chlorine dissolved in water, regardless of whether sodium hypochlorite or calcium hypochlorite is used as the source of the chlorine, generally exists in two forms, depending on the pH of the water - hypochlorous acid (biocidal) and hypochlorite ion (oxidative). Hypochlorous acid is the most effective of all the chlorine residual fractions. Hypochlorous acid is 100 times more effective as a disinfectant than the hypochlorite ion. It generally is thought that the death of bacterial cells results from hypochlorous acid oxidizing essential bacterial enzymes, thereby disrupting the metabolism of the organism. The germicidal efficiency of hypochlorous acid is due to the relative ease with which it can penetrate cells. This penetration is comparable to that of water, and can be attributed to both its modest size (low molecular weight) and its electrical neutrality (absence of an electrical charge). The hypochlorite ion is not as strong an oxidizing agent as hypochlorous acid and the negative charge and the size of the ion impedes its ability to penetrate an organism's cell wall. Hence, the hypochlorite ion is not as effective a disinfectant agent as hypochlorous acid.

Effect of pH on Chlorine

Chlorine is a more effective disinfectant at pH levels between 6.0 and 7.0, because hypochlorous acid (the most effective form of chlorine) is maximized at these pH levels. Controlling the pH of a chlorine solution increases the effectiveness of the chlorination process. Any attempt to disinfect water with a pH greater than 9 to 10 or more will not be very effective.

The pH determines the biocidal effects of chlorine. By controlling the pH of the solution that the chlorine is in, the form of chlorine (hypochlorous acid or hypochlorite ion) can be controlled. If the amount of hypochlorous acid can be maximized by controlling the pH, the effectiveness of the chlorine is significantly increased. Chlorine will raise the pH when added to water. Raising the pH reduces the amount of hypochlorous acid present. By increasing the concentration of chlorine, and subsequently raising the pH, the chlorine solution actually is less efficient as a biocide. At higher pH levels, hypochlorite ion is formed, which is the least effective of the two forms of chlorine.

Controlling the pH of the water in the aquifer is not practical. However, buffering or pH-altering agents may be used to control pH in the chlorine solution being placed in the well.

Temperature's Effect

As temperatures increase, the metabolism rate of microorganisms increases. With the higher metabolic rate, the chlorine is taken into the microbial cell faster, and its bactericidal effect is increased significantly. Therefore, the higher the temperature, the more likely the disinfection will produce the desired results. Steam injection has been used to elevate temperatures in a well and the area surrounding the well bore as a means of treating for biofouling organisms, and this process may have some application in controlling ground water temperatures. However, it is limited to wells with steel casings, and the expense of the treatment generally renders it impractical for use at residential wells.

Interfering Substances

Dirty surfaces and turbid water cannot effectively be treated with chlorine. There may be substances in the water and on surfaces within the well that bind up or use up available chlorine, resulting in less chlorine (free chlorine residual) being available to serve as a disinfectant - thereby decreasing the effectiveness of the chlorination process. This binding or using up of chlorine is called chlorine demand. Interfering substances may include inorganic matter, such as sand, silt and clay; organic matter (synthetic chemicals or biological material); drilling mud/additives; dissolved iron and other minerals in the ground water or in the water being used for preparing the chlorine solution; or drill cuttings.

Reaction with reducing ions such as iron, manganese, nitrite, sulfide and sulfite form the initial chlorine demand. The chlorine reduced to chlorides in these reactions has no disinfection ability. Additional chlorine will then react with ammonia and certain organic compounds to form chloro-organic compounds and chloramines. Combined residuals reach a peak, then decline as more chlorine is added, offering limited disinfection ability. The point at which combined chlorine residuals reach a minimum and a free chlorine residual begins to appear is known as the breakpoint. Up to this point, chlorine demand (dosage minus residual) varies with dosage. Beyond the breakpoint, the free chlorine residual increases directly with dosage.

The major chlorine demand of concern in well disinfection is not in the water, but on surfaces of the well. Nuisance organisms (organisms able to reproduce in the environment of the well) often produce large quantities of organic matter. The result is a high chlorine demand and sufficient organic energy for the possible propagation of coliform if the water temperature is above 55.4 degrees F.

Many nuisance organisms are filamentous or slime-formers and produce particles that settle to the bottom of the well, along with soil particles, scale and other debris. It is wise to assume that 20 percent of the chlorine demand exists at the bottom of the well. In wells that have been inadequately disinfected several times, more than 90 percent of the chlorine demand may be at the bottom due to settle slimes and other debris loosened in previous disinfection attempts.

Only clean surfaces in a well render themselves to effective disinfection with chlorine. Proper development of a newly constructed water supply, proper preparation of an existing water supply and thorough flushing of a water supply can effectively clean exposed surfaces, remove turbid water and help remove most interfering substances.

Chlorine Concentration

Exposure of an unprotected coliform organism to even very low concentrations of chlorine will kill the microorganism. However, the microorganisms may be protected from exposure to the chlorine by protective slimes, cuttings, drilling fluid, scale, etc. These interfering substances, as discussed earlier, will use up available chlorine, thereby allowing the microbes to survive because of insufficient chlorine concentration.

The initial chlorine concentration in the chlorine solution needs to be high enough to assure that there is sufficient chlorine to make up for this “chlorine demand,” and still have a residual of chlorine left to kill the vulnerable microorganisms. In practice, chlorine concentrations should be kept between 50 ppm and 500 ppm, with the standard recommended concentration being 200 ppm. This allows for enough chlorine to satisfy chlorine demand (interfering substances) and still provide sufficient free available chlorine to disinfect (50 ppm). Exceeding these levels may cause damage to the well or actually reduce the effectiveness of the chlorination process as follows:

  • At higher concentrations of chlorine (in excess of 500 ppm), the corrosivity of the stock solution is significantly increased, creating a potential for damage to metal well components (submersible pumps, check valves, etc.). The use of chlorine solutions with chlorine concentrations in excess of 500 ppm is not recommended.

  • In the presence of higher concentrations of chlorine, the surface of biofilms and mineral scale may be oxidized to form a hard, tight surface. This sealing of the surface layers then reduces the chance that chlorine will penetrate into the material to make contact with the microorganisms that may exist there.

Contact Time

Contact time is the length of time that the chlorine solution is left in the water supply. Sufficient time is needed to allow the residual chlorine to penetrate into biofilms or other materials that may be present, in order to reach any microorganisms that may be present. During the contact time, the chlorine residual should be maintained in the water supply system for 4 hours to 12 hours. For increased contact time to be most effective, the pH of the chlorine solution in the well must be maintained between 6 and 7 to keep the chlorine in a non-oxidative state (hypochlorous acid). If pH control is not going to be used during periods of increased contact time, use concentrations of chlorine that are 50 ppm or less. The longer the contact time, the more likely the chlorination procedure will be successful, especially if proper concentrations of chlorine are used at controlled pH conditions. Caution should be exercised during periods of extended contact times to minimize corrosive damage to pumps and other well components by controlling levels of pH and using lower concentrations of chlorine.