All public service utility watermains must be disinfected/sterilized (commonly with chlorine) and tested for bacteria presence prior to opening to the public. Organic materials getting inside the pipe in a dark, wet environment are sources for bacteria growth. The disinfection, ‘shocking’, or ‘cooking’ of the water main is most commonly done prior to ‘tying-in’ a waterline to an adjacent system. However, sometimes a local authority will allow the contractor to tie one end of the new main directly into an existing system, with the nearest existing gate valve remaining in the off position. Disinfection is typically done with chlorine tablets which have adhesive on one side and are stuck to the inside of each pipe as they are laid or just prior to. Disinfection may also be done (especially when chlorination will be done after the pressure test) by way of a chlorination tap which is often installed near the ‘tie-in points’. Chlorination taps may also be used as flushing points if the water main has yet to pass bacteria testing.
After the initial shocking (usually a minimum of 24 hours), a water main is flushed, refilled, (sometimes flushed again and refilled) to rid the pipe of as much ‘bacteria-susceptible’ water as physically possible (though completely flushing it of all water is next to impossible). The pipe is often flushed at hydrants in between closed valves but can also be flushed at the tapped cap (aka blowoff) end.. If flushing at hydrants, they will typically need to be opened up wide for flushing to allow for significant thrust to drive out all water, dirt, rocks, and potential contaminants. This can pose as a challenge for public safety and also highly chlorinated water can kill grass, vegetation, and aquatic life. Because of this reason, the public authority may require that chlorinated water be pumped into a nearby storm sewer, sanitary sewer, or in extreme cases a tank truck for offsite disposal.
After the final flush/refill it is common for the watermain to be cleared of excess air or ‘burped’ as there should be as little air as possible in the line prior to pressure testing. Once ready, a several samples from different areas of the pipeline are commonly taken for bacteria testing. The amount of samples taken will depend on how stringent the owners requirements and how long of a run of pipeline was installed. This testing is typically done in a laboratory and will usually take anywhere from 48 to 72 hours for results to come back. It is common for 2 passing tests to be required at 24 hour intervals for passing results. If the tests come back and one or both do not pass, the watermain must be flushed, refilled, re-sterilized, and retested until passing tests are achieved.
Order of Operations
The efforts of pressure testing and chlorinating a main for bacteria testing, are often intertwined and will influence eachother. It is common for a public authority to require that bacteria testing is done prior to pressure testing assuring that if bacteria is present in the testing water and there is a leak, it won’t backflow into the existing system or leak into the groundwater. Some authorities however will allow the contractor to pressure test prior to bacteria testing, particularly if they trust that the contractor was cautious about not allowing foreign objects into the watermain that might lead to a failing bacteria test. Bacteria testing as a requirement prior to pressure testing, can be problematic for the contractor because if there are leaks/infiltrations to be repaired, it may require the bacteria testing to be done again after re-flushing, re-filling, and pressure testing passes prolonging the overall process, adding cost to the project and time to the schedule.