Simple Devices Protecting Our Water System

We are all used to turning on the faucet and having clean, drinkable water on demand. But think about what happens afterwards in your home: that water is used to wash dishes or water lawns and many other uses that render it undrinkable. What stops this nasty water from flowing back into your pipes and out of your kitchen faucet? A backflow preventer. This simple, but vital, part of your plumbing turns your water pipes into one-way systems that give out clean, drinkable water. This isn’t just about making your water taste nice: backflow preventers protect your water supply from things like brain-eating amoeba and E Coli that could kill.

The problem is water pressure. Your water flows out of your tap because it is delivered under pressure: the pipes in your home contain water at a pressure of around 50 pounds per square inch (psi). That pressure comes from the higher pressure (often about 200psi) in the water supply system. But if this pressure in the supply system drops because of something like a water main break, this situation can be reversed: the water main will be at a lower pressure, which could suck water back up from your pipes into the water supply line. The same situation applies if you have a garden sprinkler system: if a broken or leaky pipe somewhere in your house reduces the pressure, that system could end up sucking up the filthy water that is sitting in and on your lawn. In the video below, you can see how this happens: the back pressure in the pipes sucks the water back into the pipes.

A backflow preventer stops this happening. Like a diode in an electric circuit, it only allows the water to pass one way, from the water system into your home, or from your home water pipes into the system that waters your lawn. If the normal pressure between the two is reversed, the flow of water is blocked so the foul water can’t get back into your pipes.

dishwasher-airgapIn the USA, a backflow preventer is a legal requirement for any commercial water system where potable (drinkable) and non-potable water system meet. In practical terms, this means that anywhere that water comes in contact with something unfit to drink, there has to be a way to stop that water flowing back. Plumbers call these points cross connections, and there are a lot of federal and state regulations that govern where they should (and should not) be installed. In your home, one important location where a backflow preventer is installed is the connection to a sprinkler system. But there is another backflow prevention device that people are much more familiar with while not actually knowing its purpose.

If you have a dishwasher you also have a backflow preventer in the form of an air gap which is shown in the image above. This is a passive device, it allows the waste water to pass from the dishwasher to the drain pipes under the sink. Water never comes out of this small fixture (unless it is installed wrong… you’ll see questions about this on plumbing forums quite often). It simply introduces a path for air to enter that drain line. Worst case scenario: the sewer backs up and fills the sink, a water main break introduces negative pressure outside the house, and the dishwasher fill valve is open. Without an air gap this situation would siphon sewage into the water supply. The simple air gap is above the rim of the sink, ensuring that air will be sucked into the water supply, preventing contamination of the system. Of course there is another air-gap on your sink, the faucet itself must be above the rim of the sink to prevent backflow.

Our modern lives are surrounded by these engineering breakthroughs that are simple, yet make a huge difference in protecting our infrastructure. Do you have a favorite ubiquitous engineering principle? We’d love to hear about it below. We’re always looking for suggestions for upcoming articles.

39 thoughts on “Simple Devices Protecting Our Water System

  1. E. coli will not kill you, you should have millions of them in your intestine colicolon! and help to digest your .
    BUT!
    Escherichia coli is an indicator for water contaminated with fecal matter.
    So you should not drink water, if you can find E. coli in it, because when there are E. coli there are almost every time other, very dangerous microbes. It’s like not drinking muddy water: when there is mud, ther may be other stuff not supposed to be drunk.

    1. It really depends on what strain of E. Coli you are dealing with, some are somewhat symbiotic, some do nothing and some are harmful to us, but I agree that E.Coli in water means it’s not safe to drink…

  2. The backflow showed in the video is only a very simple one and when a backflow event occurs it just dumps the water. However the one mostly likely installed in a commercial setting is one that does not dump water allowing them to be installed underground. There are models also that are designed for when chemicals are mixed into the water supply, such as injecting fertilizer into a sprinkler system.

    1. Not true. Commercial and industrial backflow preventers almost always dump to atmosphere. Otherwise all you have is a check valve.

      There is no such thing as a direct burial backflow preventer.

  3. “In the USA, a backflow preventer is a legal requirement for any commercial water system where potable (drinkable) and non-potable water system meet”
    Depending on the state (in my case Virginia) https://www2.iccsafe.org/states/Virginia/Plumbing/PDFs/Chapter%208_Indirect%20Special%20Waste.pdf
    you don’t have to have an air gap if you loop the discharge hose and make it high enough, or connect it to a garbage disposal. After talking to my home inspector about it, he informed me that it used to be required but isn’t anymore due to modern dishwashers having a built in check valve.

    1. John, that is why I didn’t get too much into the weeds on the laws about this: they vary so much there is no practical way to say anything except “check your local laws”.

  4. Distribution pressures from municipal water systems are usually somewhere between 50 and 70psi. It makes no sense to distribute at higher pressures and then regulate down. It doesn’t behave like AC power distribution where current is indirectly proportional to potential energy. Pressing up most systems to 200 psi would be disastrous.

    Also if municipal water pressure drops below 20 to 30 psi (each utility has a their own low pressure limit determined by their permit) a boil water alert is issued due to the chance of standing water intrusion. Bottom line, if a main brakes you are going to be boiling your water for a few days until all the nasty stuff gets flushed out system wide.

    1. I’ve seen one place where the municipal pressure was usually between 155 and 160 psi. We had to have a jockey pump on the sprinkler system to keep it above the municipal pressure, and even then we had issues with water hammers occasionally opening sprinkler heads.
      However, this is definitely the exception, not the norm. The location I mentioned was a factory situated in the lowest part of a town fed by a water tower on a nearby hill.

      I guess what I’m saying is that I agree with you, but there are exceptions in some places.

      1. I’ve seen a few on the high side but they are pretty rare. 360′ of head is pretty wild though.
        Typical though, PE makes a design oversight and every one else is left to deal with it :)

      2. The water pressure is entirely dependent on the type of distribution system (Elevated Tanks using gravity to create the pressure plane, PID controlled pumps, Hydro-pneumatic, etc…), and the proximity of the service to the source. I just implemented a system that actually has a combination of all three of those listed above. The pressure plane close to the hydro-pneumatic pump stations is around 100-115 PSI, at the end of the line that those serve the pressure is between 40 and 50 PSI. The gravity pressurized portions of that system have between 60 and 100 PSI depending on the elevation of the house in relation to the source tank. The PID controlled pressure planes are similar to the Hydro-pneumatic portions, only without the buffer should power fail.

        1. Sounds like we are in the same industry. Mainly work in the south east US where there isn’t much elevation change. I certainly can see how 100 psi would be normal due to elevation at certain points. I’ve worked some reuse distribution systems that run close to those pressures as well. Not arguing that systems don’t run at 100+ psi but it isn’t typical. My gripe was more with the following statement:
          “That pressure comes from the higher pressure (often about 200psi) in the water supply system. ”
          200 psi is extremely high and certainly not something I would consider an ‘often’ occurrence. To me it read like service areas were tapped into a 200 psi main using PRVs or something.

    1. agreed. I don’t feel like this really fits the voice of the hackaday community and there are plenty of other places on the interwebs to get this kind of perspective. You want an article idea? Research and talk about all the different types of diodes, not just electric and liquid. There are at least a dozen. Include hacks that people have done that make use of those principals. You have like a whole bunch on hackaday.io to choose from… and then there’s the web in general. That perspective would actually be in the hackday spirit. Yes, it’s a lot of work.

      P.S. you will see questions about this on plumbing forums, not forms*

    2. There isn’t a hack. But this site tends to attract a lot of geeks, and geeks like to know things. Especially things that aren’t particularly useful to us, but explain the world around us.

      I thought this was an interesting article. We don’t have to get all autistic and start complaining when things don’t fit their strict categories. There’s some other hacks you can go read if you like. It isn’t “exclusively hack a day” or they’d only be able to have one hack per 24-hour period. As long as there is at least one actual hack, on each actual day, they’re giving what they promise.

  5. One of these valves should be in every shower that has a convenience shutoff for lathering. It took me years to realize what was going on. When I shut off the warm flow at the shower nozzle and turn it back on again just minutes later, I get hot water instead for 20 or 30 seconds. It is a annoying, dangerous, and wastes hot water. This may not happen in a single floor situation. The cause is the cold water in the cold pipe. It sinks relative to the rest of the water in the loop pulling hot water into the cold pipe. It’s called a thermal siphon. It’s how to have hot water on tap in a multistory building with central heat. There they would use a separate cold line of course.

    1. I had an unusual situation. When I flushed the toilet I could “smell” the heat of the water. The toilet had hot water. What had happened is that the water heater (electric immersion) was wired incorrectly and in the bath position, the thermostat was not in use. This caused the water heater to heat above boiling, and for hot water to go back up to the cold water tank and make the cold water hot, hence the hot water in the toilet. The biggest danger is a plastic cold water tank melting, and flooding the house with boiling water.

      Related to the article in the UK and Ireland water based central heating systems are common and often contain anti-corrosion chemicals. Officially they should only be connected to the water supply with a one way valve AND a disconnected flexible loop, so hopefully no chance of the chemicals entering the drinking water, however in many cases the loop is left connected or not fitted at all.

      I too wonder where the “hack is” or even any suggestion of DIY mods.

      1. A warning to people in UK in Ireland to turn off their water heater immediately if they suspect this is happening. We had the same situation occur, the immersion heater thermostat had failed and our tank was filling with hot water. Aside from the cost of running 4kw 24/7 several people have been killed in recent years when the tank collapsed and floods scalding water onto the unsuspecting occupants below.

  6. There were several instances this summer of water boil orders being given because of E coli in the county I live. There is a irrigation water supply that is separate from the culinary. I believe it is gray water. Because of the drought the municipalities were slow to turn on the irrigation supplies. Impatient people used a hose to connect the culinary supply from the house to drive the sprinkler system and then didn’t disconnect it when the sprinkler supply was turned on. The irrigation supply is quite a bit higher pressure than the culinary supply so it forced its way into the culinary supply, infecting the whole area. That green lawn was totally worth it though :\

    1. Frankly, arrest those people. Not just for wasting water, but for risking the health, and life, of everyone around them. Fuck your lawn. There are priorities, and other people’s human needs come above that of your bloody grass. Which will grow back quick enough anyway.

      If there isn’t enough rainfall to support your garden, you’ve got the wrong plants.

  7. Don’t forget the Water-hammer arrestors.
    Nasty water hammer can and will break pipes on faucets eventually …. especially if they are not the “screw/unscrew” faucets. If they are the lift lever type, they can be opened and closed so fast you’ll hammer your water system badly.

    1. Do an article on water hammer, HAD! Sounds interesting. I think you did one on those ram-pump things ages ago, the ones that increase water pressure just using existing water, and drain off a lot of water to make a small amount go higher. That was interesting, though I’ve still no idea how they work.

  8. LIke several other people that have commented, I am really having a hard time understanding why this is on HAD…it is quite the non-sequitur. Maybe tomorrow we can learn why Federal Pacific load centers are so dangerous or how to light the pilot in a furnace.

  9. Love the reverse-analogy of the check valve being a bit like a diode. Almost certainly when any of us first had a diode explained to us we were told it is a bit like a one-way valve…

  10. Love it, awesome. Thanks for posting this article. If you are looking at every day engineering that makes our lives awesomer, I am very impressed with nikasil coating and its ability to stand years and years of punishing high revs.

    1. I’m really intrigued about this “water hammer”. I suspect a lot of people might know what it is but maybe not how it works? I’m gonna go look myself. I dunno if it’d be appropriate for a full article here, but it’s something I’d like to learn about.

      When weird, unplanned consequences come about from what you thought were simple principles, it’s interesting and educative. It’s also the beginning of chaos theory!

      1. it’s because the hose acts like an inductor, when the valve is opened the water flows and the column of water gains inertia, when the valve is closed the water slams into the end of the tube, sending a pressure wave reflecting back toward the house, which then was blocked by the check valve, thus the hose section remained at a higher pressure than the house, allowing more spectacular squirting behavior the next time the handle-trigger-valve on the hose was opened.

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