PRIVATE WATER SYSTEMS - Completion - Disinfection

Disinfection

The final step in the completion of a well is to destroy any disease- producing organisms introduced into the well during construction. Only flowing artesian wells need not be disinfected.

Disinfection is most conveniently achieved by the introduction of a strong chlorine solution into the well to burn up the organisms. Highly chlorinated water may be prepared by dissolving calcium hypochlorate, sodium hypochlorate, or gaseous chlorine into water. A solution containing about 100 parts per million of available chlorine should be used for sterilizing wells and piping systems. To provide this concentration in the well, a slightly stronger solution will need to be introduced down the well to account for dilution by the water in the well.

Before final disinfection of the well and piping system, the structure should be thoroughly cleaned. Foreign substances such as sediment, soil, grease, joint dope and scum may harbor and protect bacteria and therefore should be removed.

Disinfecting agents can only kill bacteria that they come in contact with. To just pour a dose of chlorine solution into a well is not enough. The water must be agitated to mix the solution thoroughly. In addition, surfaces of all components above the water level must be flushed or washed down with the sterilizing solution.

Time of contact with the chlorine solution is another important factor. The chemical, after being agitated in the well, should be left for at least 4 hours - preferably longer.


Calcium hypochlorate

Calcium hypochlorate is a white, granular material containing about 70% chlorine by weight. It is now marketed in tablet form under trade names of Pit-Tabs, HTH Tablets, and Chlor-Tabs. It is also commonly referred to as high-test calcium hypochlorate.

Dry calcium hypochlorate is a stable material, so it loses its chlorine very slowly. When properly packed and stored it will retain up to 90% of its available chlorine for 12 months. It becomes quite corrosive when wet or moist and loses its chlorine quickly. The table shown below shows the quantities of calcium hypochlorate, with 70% available chlorine, that are required to make 100 gallons of sterilizing solutions.
Desired Chlorine
Strength ppm
Chlorine, lbDry Calcium
Hypochlorate

50 0.05 0.07
1000.10 0.14
150 0.15 0.20
200 0.20 0.30
300 0.25 0.40
400 .035 0.50


Sodium Hypochlorate

This compound is very unstable, so it is available only in solution form. Almost all chlorine bleaches available in stores are sodium hypochlorate with about 5% available chlorine. The solution loses chlorine at a rate such that a 10% solution will be reduced to about half strength after about 6 months.

In the table below, you will see the quantities of liquid bleach required to make 100 gal of sterilizing solution of various concentrations for use in disinfecting wells and pumps.
Desired
Chlorine Strength
Quarts of Bleach of Various Strengths
per 100 pal water

ppm 5% 7%10%
50 ppm 0.40.3 0.2
100 ppm 0.80.6 0.4
150 ppm1.2 0.9 0.6
200 ppm1.6 1.2 0.8
300 ppm 2.41.7 1.2
400 ppm 3.22.3 1.6


Gaseous Chlorine

Disinfecting solutions can also be made by bubbling chlorine gas through the water. The chlorine dissolves in the water and forms a mixture of hypochlorous and hydrochloric acid.

Extreme caution should be exercised when handling chlorine gas. It is highly toxic. Except when preparing large quantities of sterilizing solution for disinfecting large water mains, the use of chlorine gas is not convenient and the hypochlorites are used much more extensively.

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