can someone explain how tap water is safe to drink?


Surely the many miles of underground pipes are a breeding ground for microbes, right?

In: 122

Water utilities add a little chlorine to tap water which kills microbes. The pipes are airtight, so the chlorine doesn’t evaporate and disappear until after the water leaves the tap.

This is why public utilities use chlorine. The chlorine breaks down when it disinfects, but otherwise has a healthy half life in clean water. The residual chlorine helps keep the water safe while in the system, although it will eventually break down if the water isn’t used. This is one reason you should let a faucet that hasn’t been used in a long time (days/weeks) run a little bit before drinking it.

Ozone can be used to disinfect water, but it has a very short half life, and provides little residual protection. Likewise deep UV light is effective in disinfection, but it has zero residual protection.

It depends on where you live.

While in the USA you’ll get the typical “chlorine” answer, this isn’t the case for every other country.

In the USA up to 4 milligrams of Chlorine per liter of water are permitted. However in Germany for example only a maximum of 0.3 mg/liter are allowed.
Instead, to ensure water quality, Germany mainly uses copper pipes, which are naturally harmful to any kind of germs, and water in those pipes is constantly in motion. Also, water quality is very strictly monitored.

Because of that the water quality is considered to be overall better than in the USA, since it’s at least equally safe and on top of that has far less chemicals put inside.

It’s a combination of chlorine in the water, which kills any contaminants in the water, and highly pressurized water mains, which prevents new contaminants from entering the water supply. That’s why any time there’s a loss in water pressure like a water main breaking, you’re usually told to boil your water before using it.

“The half-life determined in static tests for free chlorine, chlorine dioxide, and combined chlorine was 140, 93, and 1680 min.”


Large water districts that have the miles and miles of pipe you are referencing, typically use monochloramine (produced by the combination of free chlorine and ammonia) – far more persistent in water than free chlorine. To the point that it becomes a problem (and has to be removed before use) for particular end users like aquariums and brewers.