Environmental Health & Safety

1. Minimize the entry of contaminants into the repaired main. If possible, repair the break without depressurizing the main by use of clamps, sleeves or other devices. If the main must be shut down and depressurized during repair, excavated areas should be dewatered to the extent practical to prevent dirty water from contacting the pipe. If a pipe is cut and a section removed, examine the inside of the remaining pipe ends and remove pieces of pipe, scale, or other debris. Provide temporary plugs to open ends of pipes.
   
   
2. If the main has been depressurized and opened, disinfect the pipe by swabbing with a concentrated chlorine solution or by maintaining a high chlorine residual in the repaired section of main for a brief contact period. The swabbing method is quick and is generally effective under repair conditions that do not pose a threat of significant contamination.

1. Swabbing Method. All new pieces of pipe, couplings, clamps, sleeves, and other materials used in repair are thoroughly swabbed with a concentrated (1‰ available) chlorine solution to disinfect all surfaces that will come in contact with potable water. The 1‰ chlorine solution may be prepared by adding 2 ounces of calcium hypochlorite (65‰-available chlorine) or 26 fluid ounces of household bleach (5‰-available chlorine) to 1 gal of water. The chlorine solution is typically applied using clean rags or a sprayer. Longer pieces of pipe may be disinfected using a clean mop. When working with hypochlorite compounds, proper personal protection should be worn such as rubber gloves, goggles, and respiratory protection under conditions of inadequate ventilation.
   
   
2. Hypochlorite Injection or Addition of Premixed Solution. In these methods of disinfection, a high chlorine residual is maintained in the repaired section of main for a brief contact period.

Preliminary Steps — Both methods require the repaired section of main be isolated from the distribution system. This will require the shut off of all service connections along the section of main to be disinfected. Temporary connections for filling the main with water as well as a method of flushing the main through a hydrant or other temporary outlet must be provided. The isolated section of main must be initially flushed to remove dirty water, debris, and air.

Hypochlorite Injection — In the hypochlorite injection method, liquid sodium hypochlorite is injected into the flowing main by means of a chemical-feed pump to achieve a high chlorine residual (see Figure 1). The initial required chlorine dose is 300 mg/L, verified by measuring chlorine residuals in the flushed water through an outlet in the end of the section. The minimum amount of hypochlorite solution, which is required to treat one pipe volume with an initial chlorine dose of 300 mg/L, can be determined using the following table:

Table 7 includes the minimum volumes of sodium hypochlorite solution (5 and 12.5‰-available chlorine) necessary to achieve an initial chlorine dosage of 300 mg/L in 100 feet of main. Volumes in excess of the table values will be necessary because pumping must continue until the minimum chlorine dose is verified at the flushing outlet.

Addition of Premixed Solution — An alternate method is the preparation of a premixed chlorine-water solution in sufficient volume to completely fill the repaired section of main. A hypochlorite compound is added to potable water in a tanker truck or other large container to form a solution with at least 300 mg/L free chlorine (see Table 7). The solution from the tanker truck or large container is pumped into the repaired section of main until the main is full, as indicated by a discharge through a hydrant or other outlet at the other end of the section.

Minimum Contact Period — The minimum contact period for an initial chlorine dose of 300 mg/L is 15 minutes. After the minimum 15-minute contact period, a chlorine residual of at least 100 mg/L should be verified. Lower initial chlorine doses may be used for longer contact periods (e.g., 100 mg/L initial chlorine dose with a 3-hour contact time).

Final Steps — The heavily chlorinated water is flushed from the main until the chlorine residual is reduced to the level normally present in water supplied to the area. Consideration should be given to the collection of bacteriological samples after disinfection is completed to provide a record of the effectiveness of the procedures where repairs were conducted under conditions that posed a threat of significant contamination.