Field Notes - Recommended Shock Chlorination Procedure for Public Water Systems

by Shaun Fielder

Conditions that require a Shock Chlorination Procedure

A number of conditions may require a shock chlorination procedure for public water systems. These include:

  • bacterial contamination as indicated by coliform testing.
  • following construction or repair of a component of the system.
  • preventative maintenance procedures.

Preliminary steps for shock procedure

These steps should be performed before initiating a shock procedure:

  • The water system components should be inspected and any deficiencies should be corrected. Compare field inspection notes with the most recent state sanitary survey and make necessary corrections; often deficiencies are sources of coliform problems.
  • Following a coliform bacteria hit, take additional coliform samples as required by the state. Additionally, take samples at the source and storage to determine if they are contaminated.
The field inspection and additional coliform sampling should determine the location of the contamination. The goal is to shock chlorinate only the component that is contaminated. Therefore, if follow-up coliform sampling indicates that the distribution system has coliform present but that the source and storage are not contaminated, the shock procedure is required on the distribution system only. If no clear cause of the coliform can be determined, then all components of the system should be shocked.
  • Inspect the storage component of the water system and clean as required, as any sediment or biofilms can shield coliform during shock procedures. In addition, a flushing procedure should be performed on the distribution system to eliminate sediment, stagnant water and possible biofilms.

Shock Parameters and Disinfectant Needed

Special Note: During the shock procedure the water is not potable. Proper communication with customers is required to ensure that they do not drink the product. The shock chlorine dosage should be at least 50 mg/l with a contact time of 24 hours. Higher doses allow for reduced contact time (Refer to Table 1).

Table 1. Shock Disinfection, dose, contact time and amount of disinfectant required per 100 Gallons to be disinfected.

Chlorine Dose
Required Contact Time
If using Bleach 5.25% Strength add
If using HTH (solid) 65% Strength add
50 mg/l
24 hours
1 pint
1 ounce or 2 teaspoons
100 mg/l
12 hours
1 quart
2 ounces or 4 teaspoons
200 mg/l
2 hours
2 quarts
4 ounces or 8 teaspoons

Adapted from: "A Training Manual for Small Water System Operators", Agency of Natural Resources DEC, Water Supply Division in Conjunction with Northeast Rural Water Association, Reprint 1998.

The operator needs to decide which strength of disinfectant to use. Be advised that HTH (Calcium Hypochlorite) and bleach (Sodium Hypochlorite) are strong oxidants. Safety equipment including goggles, gloves and protective clothing should be worn while working with these chemicals. With any chemical product, operators are required to have the Material Safety Data Sheet (MSDS) on file and need to be familiar with them. It is recommended that 5.25 % bleach be used for the shock procedures.

Certain component conditions may require the use of HTH disinfectant. These include a high volume to disinfect, a drilled well water column deeper then 200 feet, and a well that produces more than the submersible pump can displace. The latter two conditions may prevent the bleach from reaching lower portions of the water column. The bleach is less dense than water and may "float" on the upper portions of the column. A circulation procedure can eliminate this potential problem. A more effective approach is to add a combination of bleach and HTH.

Component Volume Calculations

  • After determining the target dose and strength, the volume of the component to be shocked needs to be determined. Table 2 gives volume information on drilled wells. To determine volumes of springs, storage systems or other components the operator may have to measure them. This information may also be found in the system's operation and maintenance manual or permit to operate.

Table 2. Storage capacity of wells.

Diameter of casing in inches
Storage capacity in gallons per
foot of water column

Source: "A Training Manual for Small Water System Operators", Agency of Natural Resources DEC, Water Supply Division in Conjunction with Northeast Rural Water Association, Reprint 1998.

Here's an example of volume calculation on a deep well with 6-inch diameter casing, total depth of 254 feet, and a static level of 50 feet:
The well's total depth minus static level equals the total water column. In this example, the water column is 204 feet. Since the well has a 6-inch casing, 204 feet times 1.47 gallons per foot gives us 300 gallons to disinfect. If using 50 mg/l (contact time is 24 hours) and bleach as the disinfectant, 3 pints of 5.25 % bleach will be required (refer to table 1).

Addition of Disinfectant and Required Field Actions

  • Add the disinfectant to system component. If the contamination area has not been determined, the entire system may require disinfection.
  • For wells with a water column of more than 200 feet or a production that exceeds the submersible output, add a combination of HTH and Bleach. Use enough bleach to disinfect to first 200 feet of water column and use HTH on the remainder of the column; see table 1 for amounts to add. Crushing the HTH tablets will give better mixing potential with the water column.
  • If field conditions allow it, a circulation procedure for deep water should be performed. If possible run a garden hose back to the wellhead and flush down the well casing. Ensure disinfectant can be smelled and the casing has been adequately washed down. A small section of PVC pipe about 6 feet in length and about 2 inches in diameter can be inserted into the top of the well and used as a funnel for the circulated water. This prevents contact with poor electrical connections.
  • Allow shock water to sit for the required contact time.
  • At end of required contact time, the shock water should be flushed to waste. Do not pump shocked water to surface waters and avoid flushing high volumes into septic systems.
  • If flushing deep wells to waste, monitor static levels to ensure that the aquifer is not drawn down significantly. This will prevent damage to the submersible pump.
  • Perform additional bacteria testing to ensure the system was properly disinfected.
For more detailed information on disinfection procedures, please refer to the following;

"Disinfecting Water Mains", Manual C651-99, published by the American Water Works Association, 1999.

"Disinfection of Storage Facilities", Manual C652-92, published by the American Water Works Association, 1992.

"Disinfection of Water Treatment Plants", Manual C653-97, published by the American Water Works Association, 1997.

"Disinfection of Wells", Manual C654-97, published by the American Water Works Association, 1997.