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Emergency Disinfection of Small Systems

Emergency Disinfection of Small Systems:


Has your system lost pressure lately due to a power outage, failed pumping system, main break, or other unusual events? Or have you just learned you have an Acute or Non-Acute Total Coliform Rule MCL violation? If so, and if you normally provide water that is not disinfected, please read on.


When to Disinfect


A water system should be disinfected any time it experiences any of the following:

  • The system loses pressure for any reason.

  • Any part of the system is "opened up" for maintenance or repairs.

  • Backflow or back-siphonage creates a cross-connection event.

  • Total Coliform, Fecal Coliform, or E.Coli is found present in both routine and repeat Coliform samples. 


A system should also be batch chlorinated if multiple routine Coliform samples in one month show the presence of Total Coliform, Fecal Coliform, or E.Coli. This disinfection should not occur until after the required repeat samples have been collected for each of the unsatisfactory routine samples.


Notify your customers first


Before disinfecting any system that is normally not disinfected, you must notify all users. Of special concern are people with unique medical needs, such as kidney dialysis patients. All water systems should maintain a list of such customers. People with aquariums or ponds that contain fish will also want to know that the water is to be chlorinated.


Adding chlorine to a groundwater source


1. Calculate the volume of water in the well or spring box. To do this, multiply the number of cubic feet by 7.5 to determine the number of gallons. (Or use the table below). 


Well Casing Diameter (inches)

Volume (gallons per vertical foot of water)
















2. Calculate how much chlorine to add to the well or spring box, using this table (use 5.25 % Bleach approved for use on drinking water):


Well Volume (gallons)

5 ppm Desired Chlorine Dosage

20 ppm Desired Chlorine Dosage

50 ppm Desired Chlorine Dosage


1.2 Tbsp 

0.3 Cups

0.8 Cups


2.4 Tbsp

0.6 Cups

1.5 Cups


3.7 Tbsp

0.9 Cups

2.3 Cups


4.9 Tbsp

1.2 Cups

3.0 Cups


6.1 Tbsp

1.5 Cups

3.8 Cups


12 Tbsp

3.0 Cups

7.6 Cups


18 Tbsp

4.6 Cups

11 Cups


24 Tbsp

6.1 Cups

15 Cups


3. Pour the required quantity of bleach (approved for use on drinking water) into the well or spring box.


4. Connect a brand new garden hose to the nearest outside faucet and circulate the water through the hose and back into the source. This will mix the chlorine with the water and the pump will draw the chlorine to the bottom of the well. After you start smelling the chlorine in the water coming out of the hose, use the hose to rinse the upper portion of the well with the disinfectant. Note: If you cannot reach the well with the hose, mix one cup chlorine bleach per bucket of water and pour chlorinated water down the inside of the casing. The bucket method will also work when you are disinfecting a gravity-flow spring box.


Now proceed to Step 5




Adding chlorine to a storage reservoir


If you must chlorinate both your source and your storage reservoir, disinfect the reservoir and distribution system first, then do the source and the pipe leading to the reservoir. This will ensure adequate disinfection of the source. 


1. If the contamination does not appear to be originating at the water source, the system may be disinfected by adding a disinfectant to the storage reservoir rather than the water source. 


2. Determine the amount of chlorine that will need to be added to the storage tank, using the table below:


Reservoir Volume (gallons)

1 ppm Desired Chlorine Dosage (5.25 % Bleach*)

20 ppm Desired Chlorine Dosage (5.25 % Bleach*)

50 ppm Desired Chlorine Dosage (5.25 % Bleach*)



1.5 Cups

1.9 Gal

4.8 Gal


3.0 Cups

3.8 Gal

9.5 Gal


4.6 Cups

5.7 Gal

14 Gal


6.1 Cups

7.6 Gal

19 Gal


7.6 Cups

9.5 Gal

24 Gal


15 Cups

19 Gal

48 Gal


23 Cups

29 Gal

71 Gal


30 Cups

38 Gal

95 Gal


*Approved for use on drinking water


Also if your distribution system is extensive, the volume of water in the distribution piping should be considered when determining how much chlorine to use.


3. Drawdown the level of water in the storage tank, but keep sufficient quantity for fire flow, if required. 


4. Pour the chlorine into the tank as the tank is refilling, in order to get some mixing.


5. (Steps 5-8 are identical for both types of disinfection operations.) Beginning with the outlet closest to the point of chlorine addition (that is, either the source or the reservoir) draw water at every outlet until you can smell chlorine. To be more accurate, use a chlorine residual test kit or have Accurate Testing Labs test it for you. Turn off each outlet once chlorine is detected. 


6. Allow the chlorine to remain in the system overnight (24 hours is preferable.) Chlorine needs time to do an effective job of disinfecting.


7. Use one or more outside faucets, blow-offs, hydrants, etc. to draw water out of the system to remove the chlorine. The system should be thoroughly and repeatedly flushed to remove the chlorine. During this process, make sure you don't damage a pump by drawing water down below the pump intake. Chlorinated water is extremely toxic to fish. It should never be discharged to any water body, wetland, or drainage ditch. High chlorine residual must be de-chlorinated before discharge. 


8. After following this procedure and rendering the water completely free of disinfectant, you should wait a minimum of seven days following disinfection before collecting a bacteriological sample. (Note: If you are disinfecting in follow-up to an Acute Total Coliform Rule MCL violation, you should be working with the DEQ/ DOH Regional Office staff to determine when Coliform sampling should occur relative to chlorination and flushing.) The chlorine residual should be measured and noted on the Coliform lab slip whenever Coliform samples are collected. In a follow-up to an emergency disinfection event, the measurement of a zero residual is worthy of note too. The bacteriological analysis will indicate whether or not the system disinfection was effective. 


If you have any questions about disinfecting your system, please call your DEQ/ DOH Regional Office staff.


North Idaho: Suzanne Scheidt at 208-769-1422 and Jamie Barton at 208 415-5208


Eastern Washington: Pat McCaffery at 509-456-2788


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