Today's question: Fixing a broken water pipe looks like a dirty job. How is the inside of the pipe cleaned afterward?
Plain Talk answer: After work is done, the pipe is flushed to clear it of loose debris that may have collected during the repair. Then the pipe is filled with water containing a disinfectant -- usually chlorine or a derivative. Holding this water in the pipe for a time kills bacteria and other organic contaminants in the system. This is followed by testing to be sure the water is safe.
This is not the end of the story, however. The utility must take care in disposing of all this water that contains so much chlorine. State, provincial and federal regulations control its disposal. A chemical must be added to inactivate the chlorine before the water can be flushed out of the pipe and discharged, unless the highly chlorinated water is discharged to a wastewater treatment plant or another location where it will not have an adverse impact on the environment.
For more information on this and many other water-related topics, check out "Plain Talk About Drinking Water" by Dr. James M. Symons.