I have seen work crews cleaning watermains and the water they flush out looks terrible. How can the water be safe if the pipes are so dirty?
Almost all water pipes have a thin film of rust and harmless microbes on the inside. Experience has shown that this thin film causes no health problems. Build-up of this material may, however, cause problems such as clogging of fixtures, causing the tap water to look bad, or using up the disinfectant in the water as it passes through the pipes. The City of Cambridge has a regular program of flushing and cleaning the distribution pipes. When all of this material is removed from the walls of long lengths of pipe and it comes out through a fire hydrant all at once, it looks worse than it really is. If you watch the workers do this, you will notice that the water cleans up rather quickly.
