Section 15

Well Disinfection

After constructing or repairing a well or pump, the entire well and pumping system must be disinfected in order to kill harmful microorganisms (germs and bacteria) that may be on the well casing, gravel, soil, rising main, pumping rod or in the water from the digging operation. If you think you know how to do this, try this chlorination exercise (Appendix E)!

Chlorine is normally used as the disinfecting agent since it destroys bacteria by neutralising the enzymes that are essential for their survival (Richard, 1987). Chlorine is usually sold in 2 forms: sodium hypochlorite (liquid bleach) and calcium hypochlorite (powder or pellets).

Sodium hypochlorite is the main ingredient in liquid bleaches which initially contain about 5% available chlorine; it gradually loses its strength over time, especially in hot climates. While substantial quantities are required to effectively disinfect large diameter wells, it can be easily mixed with the water and it is relatively safe to handle and use.

Calcium hypochlorite comes in strengths ranging from 30-75 percent available chlorine (70 percent is most common). Like sodium hypochlorite, it slowly loses its strength with exposure to air and should be stored in sealed containers in a cool dark place to retain its strength. Much less quantity of this agent is required to effectively chlorinate wells and special, slow-dissolving pellets can be purchased to provide longer-lasting chlorine residuals. However, calcium hypochlorite becomes unstable and may spontaneously combust if it becomes warm and moist and it can even explode & burn if dropped.

Care must be taken when mixing and adding chlorine to a well since exposure to it can result in severe skin/eye irritations and blisters. It is also poisonous; inhaling concentrations of 30 ppm can lead to harsh coughing and concentrations of 1,000 ppm can be fatal in few breaths!

Chlorine is a very reactive substance. When added to a well, it first combines with inorganic compounds (hydrogen sulphide, ferrous iron, manganese); there is no disinfection at this stage. After these compounds have been reduced, remaining chlorine then reacts with organic matter (algae, phenols & slime growth). While some bad tastes and odours may be eliminated, there is only a slight disinfection action and trihalomethanes (carcinogenic, chlorinated organics) may be formed.

After the demand exerted by inorganic and organic compounds has been met, chlorine will combine with nitrogen compounds (primarily ammonia) to form chloramines. This combined chlorine form results in long lasting disinfection, produces minimal chlorine taste/odour and controls organic growths. However, it is slow acting and long contact times are required.

Finally, once even more chlorine is added to the water, the chloramines are destroyed and excessive chlorine, known as the free residual, forms Hypochlorous Acid (HOCL). HOCL is a potent, fast reacting disinfectant which is desirable when contact times are kept as short as possible and/or if there are high concentrations of iron, manganese, colour or bacteria. The amount of chlorine (dose) required to create sufficient quantities of HOCL depends on:

1. Bacterial numbers: If there are large numbers of aerobic or anaerobic bacteria in the water, a high chlorine dosage is required to ensure that all disease causing organisms have been destroyed;
2. pH: Hypochlorous acid (HOCL) will form in waters ranging from pH 6.5-7.5. As the pH increases above pH 7.5, HOCL increasingly dissociates to the hypochlorite ion, which is up to 250 times less effective as a disinfectant that HOCl (in terms of concentration). Under pH 6.5, the concentration of HOCL is reduced and hydrochloric acid (HCl), a very weak disinfectant, is formed. Generally HCl doesn't become a problem in the desired pH range of drinking water.
3. TEMPERATURE: Affects disinfection speed (high temperature = fast disinfection);
4. TURBIDITY: Effective microorganism destruction will only begin after the chlorine demand exerted by turbidity (inorganic and organic compounds) is met. In addition, chlorine is a surface-active agent and, since it can not effectively penetrate solids to kill concealed bacteria, disinfection of turbid water will be incomplete.

In established municipal water treatment/distribution systems, water is filtered to remove solids and excessive concentrations of chemicals prior to chlorination. In these systems, where chlorine and water are brought into active contact by pumping into mixers, holding tanks and/or through long distribution lines, initial chlorine dosages are often 1 ppm or less.

However, new wells often have turbid water, elevated concentrations of iron and/or organic slimes and few many existing wells were not thoroughly disinfected following construction and pump installation. Finally, it is difficult to achieve even mixing of chlorine and water in large diameter wells, chlorine tends to settle to the bottom of wells and high chlorine concentrations must reach the outside of the well tiles and the surrounding gravel pack.

As a result, much higher chlorine doses are required for shock chlorination of wells than are used in operating treatment systems. All newly constructed wells should be chlorinated so that a minimal chlorine dosage of 250 ppm is maintained for at least 12 hours (MOEE, 1987). Once wells have been effectively developed and chlorinated, they can be treated by maintaining a chlorine dosage of at least 50 ppm over a contact time of 12 hours. The higher the amount of organics and inorganics in the water, the higher the initial dose must be to ensure that at least 50 ppm chlorine is present in the well 12 hours after it was added.

Wells must be effectively developed prior to disinfection since the presence of organics (including residual drilling fluid) and fine particulate matter can make disinfection incomplete and can result in the formation of compounds that have unacceptable health and/or aesthetic characteristics.

Table 5: Well Shock Chlorination Procedure

Calculate the volume of water to be treated. To do this, measure the number of metres of water in the well. For wells completed with 10.16 cm (4 in) diameter schedule 40 PVC pipe, the volume of water (l) to be treated is the depth of water in the well in metres times 10.5 l/m (or depth in feet times 0.85 US gallons/foot)(1). If a 7.62 cm (3 in) diameter schedule 40 PVC pipe is used, multiply the depth of water (m) x 8.7 l/m. Determine how much chlorine needs to be added to effectively disinfect the calculated volume of water. Newly constructed wells should be chlorinated with 250 milligrams per litre available chlorine. For wells completed with 10.16 cm (4 in) diameter schedule 40 PVC pipe, for every m of water in a LS-100 well add: 3.8 grams of 70% strength calcium hypochlorite (TIP: there is about 10 grams in a level tablespoon); OR 8.8 grams of 25-35% strength bleaching powder or chlorinated lime; OR 53 millilitres of 5% strength sodium hypochlorite (liquid bleach). If a 7.62 cm (3 in) diameter schedule 40 PVC pipe is installed in a 15.24 cm (6 in) diameter borehole, for every m of water add: 3.1 grams of 70% strength calcium hypochlorite; OR 7.2 grams of 25-35% strength bleaching powder or chlorinated lime; OR 44 millilitres of 5% strength sodium hypochlorite (liquid bleach). If sodium hypochlorite is used, just pour the chlorine into the well. If calcium hypochlorite is used, dissolve the chlorine powder or tablets in a 20 litre (5 gallon) bucket of water before adding it to the well. Add no more than 100 g of calcium hypochlorite to each bucket. When mixed with water, an insoluble residue will likely be formed. This residue should be allowed to settle and the clear supernatant containing the chlorine should be decanted (Richard, 1987). Pour the clear solution into the well. If possible, agitate the water to evenly mix the chlorine. If the pump is already in-place, it should be operated until a distinctive chlorine smell is detectable in the treated water. If it isn't, place the required quantity of chlorine powder or tablets into a weighted porous container and surge it up and down in the well until the contents are dissolved. Leave the chlorine solution in the well for at least 12 hours and preferably for 24. After 12-24 hours, the strongly chlorinated water should be pumped from the well. If the pumping equipment has not yet been installed for some reason, do it now: it will be disinfected by using it to remove the excess chlorine. Choose a disposal place for the chlorine solution where it will have minimal contact with plant and animal life. If a chlorine smell is not present in the discharge water after this contact time, the chlorination procedure should be repeated. Discharge the water in the system to waste until the smell of chlorine disappears. The amount of chlorine remaining in the water will not be harmful. In about a week, collect a water sample for bacteriological examination (see Section 16: "Water Quality Testing"). To be totally safe, boil or chlorinate all drinking water until the bacteriological results are returned (see Appendix T). Two consecutive "safe" tests will probably indicate that the treatment has been effective(2).

Footnotes & References

¹ This assumes a 4 inch dia. ID and a 4.5 inch OD well casing, a 6 inch dia. borehole and a 30% porosity of the gravel pack and/or formation stabilizer in the annular space. The volume of water, therefore, = volume in casing + 0.3 x annulus volume (see Appendix A for volumes). Note that the amount of water within the well casing alone is 8.11 litres per metre of casing below the water table since volume (litres) = depth of water in the well (metres) x (casing diameter in metres/2) x (casing diameter in metres/2) x 1,000 l/m³ x 3.14.

² If tests show continuing bacteriological contamination, a second chlorination is needed. Following the second treatment additional tests should be conducted. After repeated positive bacteriological tests, a well contractor should be contacted to surge the well and the surrounding formation with a strong chlorine solution. Chlorination will sterilize a well and water system; however, unless the source of the bacterial contamination is found and corrected the problem will continue to reoccur and chlorination will not solve the problem. In some cases, a new well may have to be constructed to correct the problem.

Ministry of Environment (1987) Water Wells & Ground Water Supplies in Ontario, ISBN 0-7729-1010-3 WRB

Richard, Y. (1987) "Disinfection in the Treatment of Drinking Water", pp.215-217 in Developing World Water, Hong Kong: Grosvenor Press International.