Shock Disinfection of Water Supply Systems
When a water supply has been exposed to bacterial contamination,
it is advisable to disinfect the system using a commercial chlorine
compound such as chlorine bleach. Disinfection should occur
after: construction of a new water supply, repairs are made
to an existing water supply, a positive coliform or E. coli
test, or any time the well cap or lid has been removed.
The disinfection process outlined below is intended to eliminate
the effects of previous contamination, but will not continue
to disinfect or render safe a water supply which is continuously
or intermittently contaminated. Therefore, before disinfecting
the water supply system, all sources of pollution should be
eliminated and proper repairs should be made. Contact a water
system specialist or the Tompkins County Health Department (TCHD)
for advice.
The most convenient source of chlorine is ordinary household
bleach. Chlorine bleach contains about 5.25% chlorine (sodium
hypochlorite) and is available at most grocery stores. Note
that "Ultra" Chlorine Bleach products contain 6% chlorine.
If possible, use NSF approved chlorine, as other types have
additives in them. DO NOT use scented bleach.
Shock Disinfection Procedure:
- Ensure that you have potable drinking water (bottled water
or disinfected well water) available. Your well water will
be heavily chlorinated for 1-2 days after this procedure,
so you may consider doing laundry and showering before you
begin.
- Disconnect carbon or charcoal filters. Chlorinated water
should be allowed to pass through water treatment devices
such as softeners, iron filters and water sand filters to
disinfect them. Water softeners can be shocked separately
by adding 1/2C household bleach into the salt (brine) solution.
Check with the manufacturer to ensure chlorine will not damage
the water treatment equipment. To save energy during this
procedure, turn your hot water heater to pilot if it is gas,
or off if it is electric.
- Clean the well and any storage tanks. Using a hose, rinse
down the inside of the casing of the well. Then run the hose
away from the well until the water is clear.
- Refer to the charts on back of this sheet to determine the
amount of chlorine to use. Dilute the chlorine in a 5 or 10
gallon bucket with water, then pour this dilution into the
well casing, dug well, reservoir, or other structure to be
disinfected.
- Run the hose into the well again for at least 10 minutes
to mix and recirculate the chlorine solution. Cover the well.
- Turn on each indoor and outdoor tap, one at a time. When
the water coming out has a strong chlorine odor, turn that
tap off and proceed to the next one. Allow the chlorine solution
to sit in the distribution piping for the amount of time specified
below. This step sanitizes the distribution piping.
- Flush the chlorinated water from all the lines after the
contact time has elapsed. You may want to run water through
a hose to the road ditch or a location away from the well
or septic system before turning on the indoor faucets. This
is not required, but prevents the exposure of your septic
system to high levels of chlorine. Repeated exposure to elevated
chlorine levels can negatively impact the functioning of your
septic system. Also, be aware that heavily chlorinated water
can be harmful to your lawn, trees, or garden.
- Reconnect any water conditioning equipment you may have
disconnected. Continue to use the water for all household
purposes except those intended for consumption (drinking,
ice-making, washing of fruits and vegetables to be eaten raw).
After one to two weeks, collect a water sample for bacteriological
testing. It is critical when resampling that no chlorine be
present in the water. If the bacteriological problem has not
been eliminated, contact the TCHD for further advice and assistance.
- Ensure that the water continues to be safe to drink by
testing the water for bacteria again, 2 to 3 months after
the shock chlorination procedure.
Quantities of liquid household bleach, 5.25% sodium hypochlorite, required for water well disinfection (note that the stronger solution requires less contact time):
Table 1: 10 hour minimum contact time (100 ppm chlorine
concentration)
Feet of Water in Well |
Well Diameter (inches) |
3 |
4 |
5 |
6 |
8 |
30 |
½C |
¾C |
1C |
1¼C |
2½C |
40 |
½C |
¾C |
1¼C |
2C |
3¼C |
60 |
¾C |
1¼C |
2C |
2¾C |
1¼Q |
80 |
1C |
1¾C |
2½C |
3¾C |
1½Q |
100 |
1¼C |
2C |
3C |
1Q |
2Q |
150 |
1¾C |
3C |
1¼Q |
1¾ Q |
3Q |
200 |
2¼C |
1Q |
1½Q |
2¼Q |
1G |
250 |
2¾C |
1¼Q |
2Q |
2¾Q |
1¼G |
300 |
3½C |
1½Q |
2¼Q |
3½Q |
1½G |
Table 2: 4 hour minimum contact time (250 ppm chlorine
concentration)
Feet of Water in Well |
Well Diameter (inches) |
3 |
4 |
5 |
6 |
8 |
30 |
1C |
1¾C |
2½C |
3¾C |
1½Q |
40 |
1¼C |
2C |
3¾C |
1¼Q |
2Q |
60 |
2C |
3¼C |
1¼Q |
2Q |
3Q |
80 |
2½C |
1Q |
1½Q |
2½Q |
1G |
100 |
3C |
1¼Q |
2Q |
3Q |
1¼G |
150 |
3½C |
2Q |
3Q |
1¼G |
1¾G |
200 |
1½Q |
2½Q |
1G |
1½G |
2½G |
250 |
1¾Q |
3¼Q |
1¼G |
1¾G |
3¼G |
300 |
2¼Q |
1G |
1½G |
2¼G |
4G |
Table 3: Quantities of liquid household bleach, 5.25%
sodium hypochlorite, to disinfect volumes of water in storage
tanks
Chlorine concentration |
Water in Gallons |
100 |
200 |
250 |
500 |
750 |
1,000 |
2,000 |
5,000 |
10,000 |
100 PPM
(10 hr contact) |
3C |
1½Q |
2Q |
1G |
1½G |
2G |
4G |
9½G |
19G |
250 PPM
(4 hr contact) |
2Q |
1G |
1¼G |
2½G |
3¾G |
5G |
10G |
25G |
50G |
KEY: C=cups; Q=quarts; G=gallons
Note: Caution should be exercised when handling bleach solutions.
If chlorine accidentally gets on your skin, immediately flush
the area with clean water.
jm 9/01