Module 6 — Shock Chlorination
49
Shock Chlorination Method
Shock chlorination is used to control iron-related and sulfate-reducing bacteria and to
eliminate faecal coliform or E. coli bacteria in a water system. To be effective, shock chlorination
should be done on a regular basis at least once or twice per year as part of a routine well
maintenance program. Start the treatments early in the life of your well. Shock chlorination will
disinfect the following:
The entire well depth
The formation around the bottom of the well
The pressure system
Some water treatment equipment
The distribution system.
To accomplish this, a large volume of chlorinated water is siphoned down the well to displace
all the water in the well and some of the water in the formation surrounding the well.
Effectiveness of Shock Chlorination
With shock chlorination, the entire system (from the water-bearing formation, through the
well-bore and the distribution system) is exposed to water which has a concentration of chlorine
strong enough to kill iron-related and sulfate reducing bacteria (see Figure 1, Water System).
Bacteria collect in the pore spaces of the formation and on the casing or screened surface of the
well. To be effective, you need to calculate the correct volumes of chlorine and water required to
reach and disinfect the entire cased section of the well and adjacent water-bearing formation.
The procedure described on the following pages does not completely eliminate nuisance
bacteria from the water system, but it will hold it in check. To control the iron-related bacteria,
you will have to repeat the treatment on a regular basis, likely each spring and fall as a regular
maintenance procedure. If your well has never been shock chlorinated or has not been done for
some time, it may be necessary to contact a licensed water well contractor about other products
and methods for control of bacteria.
Adding chlorine increases the pH of the water. An increase in pH reduces the ability of
chlorine to disinfect or kill bacteria. Your licensed water well contractor may have specialized
chemicals designed to reduce pH and optimize disinfection.
Shock chlorination will not be effective on wells that have been seldom or never been treated.
These poorly maintained wells likely require the services of an experienced licensed water well
contractor who has the necessary equipment and products to effectively and safely clean and
restore the well water quality and production. Sometimes these wells can be restored to near their
original water quality and production capacity after the well driller uses scrubbing equipment and
applies an acid treatment to remove the heavy layers of bacterial slime on the well casing prior
to disinfecting with a chlorine solution. After a thorough cleaning it may be possible to return to
regular shock chlorination treatments to control bacteria buildup and its related problems.
Before you shock chlorinate, consult
your water treatment equipment
supplier to ensure the appropriate
steps are taken to protect your
treatment equipment.
Be aware that chlorinating old,
unmaintained wells may uncover
existing holes in steel casing and
speed up inevitable well failure.