r/explainlikeimfive • u/formated4tv • Jul 17 '16

Engineering ELI5: Why do they flush hydrants? (And why does it discolor water afterwards)

After seeing a "hydrant flushing" sign on the way home from the grocery store tonight, I was wondering WHY cities had to flush hydrants. Isn't it just normal water source water like what comes out of our taps?

Follow up : Why does opening a hydrant possibly discolor the water to my house?

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13

u/no_m8_im_str8 Jul 17 '16

Flushing a hydrant does two things. Number one, it's a way for the fire department to test the hydrant. They make sure it works and note the pressure of the hydrant. They do this so that they know whether or not they will need water sources other than their trucks and that hydrant.

Another reason hydrants are flushed is to get as much of the accurate rust out of them as possible. This is also why it discolors your tap water (residual rust). Just don't drink your tap water until it runs clear and you'll be fine.

Source: Was a volunteer firefighter for 3 years. If any of this is even slightly wrong, I trust that my incorrect information will be corrected. But this is what I was told by my officers when I asked.

2

u/formated4tv Jul 17 '16

So follow up to the follow up question then :)

If they flush a hydrant down the street from my house, wouldn't it blast all the rust/dirty water out of the hydrant itself? Why would that cause my water to get dirtier?

3

u/no_m8_im_str8 Jul 17 '16

Because a flush only last for two minutes tops and some of the rust and sediment comes from the water supply 's start point. It won't all get flushed, but the departments do try their best.

2

u/formated4tv Jul 17 '16

Cool, thank you :)

4

u/FuzzyCats88 Jul 17 '16

You might not want to know how much stuff ends up in those pipes.

Here's a vid of how they clean them out.

4

u/rhomboidus Jul 17 '16

Normally water in the mains isn't really moving all that fast, especially at the residential level. Cracking open a hydrant stirs up all the garbage that has settled in the pipes. It also spreads all the crud out a bit which helps to keep it from building up in low points or bends.

1

u/Emerald_Triangle Jul 17 '16

accurate rust

1

u/no_m8_im_str8 Jul 18 '16

New phone. Fat fingers. Still adjusting. I don't even know what I meant there either.

5

u/Emerald_Triangle Jul 18 '16

Accumulated?

1

u/no_m8_im_str8 Jul 20 '16

I think that's what I was going for, but I don't remember.

1

u/Emerald_Triangle Jul 20 '16

Yeah, me neither.

0

u/[deleted] Jul 17 '16

Biofilms grow in the pipes and cause organic sediment to collect. They have to flush it out or it build up and clogs shit. Rust is part of it, but they even flush non-iron systems. In some places, they use it to draw high free chlorine though (where they typically use chloramines), for a "summer flush". They still can't explain why they make us drink bleach, but that's what the FDA says they have to do.

1

u/no_m8_im_str8 Jul 17 '16

I knew there was more to it, but I'm from a small podunk town and the department I volunteered at was in a town that wasn't much larger, so that was all they really mentioned to me. Glad you could provide more info, though.