Field Notes - Recommended Shock Chlorination Procedure for Public Water Systems
by Shaun FielderConditions 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.
- 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
|
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 |
|
6
|
1.47
|
|
8
|
2.62
|
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.
