Secrets of the toilet

Way back on New Year's Eve of 2005, when we were still hosted over at Blogger, I did one of my more popular posts about how a toilet works. Most people don't know. I'm guessing they have some kind of vague mental image that when you push the toilet handle a trapdoor opens up somewhere and the contents of the toilet bowl fall through to the abyss. But that's not what happens and I felt compelled to explain it. The pretext was a long article from the Wall Street Journal about how toilets are tested. The official testing material is . . . miso (a Japanese fermented soybean paste). I thought of this again when I saw (hat tip boingboing) a video of a toilet that swallows just about anything (reasonable) you could put down it, including four full sets of chess pieces, three and a half pounds of dog food, bunches of hotdogs and much more. The video is at the end. To get to it you'll have to swim through my pool of past post on how a toilet works:

Most household toilets work using a bowl siphon. Yes, a siphon, just like you will be doing during the next gas crisis when you steal gas from your neighbor's car. You remember how you did that the last time, right? You put a tube into the gas tank and sucked some gas into it until the gas "went over the top of the bend" and, assuming the other end of the tube outside the car was lower than the level of the end in the gas tank, the siphon would empty out the gas in your neighbor's car into your gas can and you were on the road again (and also assuming you didn't suck the gas all the way into your mouth and wind up aspirating it and get an ensuing hydrocarbon pneumonitis).

So now we have three questions: (1) What makes a siphon work, anyway? (2) What does that have to do with flushing a toilet? (3) What does this have to do with miso?

(1) A siphon works by exploiting the difference in the weight of the water in the two columns. Here's a good way to visualize it. Suppose you have a length of chain, with an excess in a beaker and the rest running smoothly up over a pulley and back down to the table top. Now take the beaker in your hand and raise it. You'll find that the chain "runs out" of the beaker, over the pulley and down to the table on the other side. That's because the length of chain on the other side of the pulley is longer when you raise the beaker on the near side up, and the longer chain is heavier and pulls chain from the "lighter" (higher) side. Here is a good set of pictures of the set up.

(2) So where is the siphon in a toilet?


You can see from this picture there is a siphon in back of the bowl. When you push the toilet handle down, water from the tank in back of the toilet (not shown in this picture) starts to fill the toilet bowl more quickly than the water can flow over the siphon bend, and water, seeking its own level both in the siphon tube and the bowl, completes the siphon and the bowl empties. In fact it empties more quickly than the water runs into it from the tank, which is sized just right so that it refills the bowl again after it empties. The amount of water running into the bowl is sized so that when it is finished running the level is below that needed to complete the siphon. The valve from the tank now closes and water from your plumbing connection refills the tank and you are ready to "go" again. This explains how you can flush a toilet just by filling it from a bucket of water instead of the tank (you didn't know that? try it at home). There's a nice animation here.

(3) Back to miso.

For decades, the toilet industry had a standard way of testing a toilet's flushing capabilities: tossing 3/4-inch plastic balls into the bowl and pulling the handle. But there was one problem: Toilets that are fantastic at flushing down 3/4-inch plastic balls sometimes falter under real-world conditions.

A few years ago, researchers began pondering a better test. After scouring grocery aisles for alternatives, they settled on using miso, which is made primarily of cooked soybeans.

Now, a group of water utilities and plumbing companies is pushing to make the miso test the new standard. This month, the group, which includes Kohler Co. and American Standard Cos., is rolling out a set of rules called UNAR -- that is Uniform North American Requirements for toilet fixtures -- which lay out a flushing standard that toilets have to meet. A key element of the suggested rules, which also include standards for toilet parts, is the use of a miso paste in testing.


Since 1978, toilet makers had been using the plastic-ball test, which involved dropping 100 balls into the toilet; the toilet had to dispose of at least 75 in one flush to pass. (Cheryl Lu-Lien Tam, The Wall Street Journal)

The article is long, and for some, "too much information," so suffice it to say the test involves fashioning miso paste into a cylinder to test flushing efficiency. The UNAR standard is meant to be like the "Energy Compliant STAR standard" for electrical appliances. In the 1950s we used 5 - 7 gallons to flush a toilet (toilet flushing is the largest use of water in the home--by far). That was lowered to 3.5 gallons subsequently, and in 1992 the Energy Policy Act required all new toilets to use only 1.6 gallons per flush. This ushered in the era of the skidmark and the dreaded double-flush. Now, with better designs, low volume toilets are finally getting it together. But manufacturers, and likely regulators, will want a good test for flushing efficiency. UNAR's miso test stands (floats?) poised to become that standard.

One more thing. Miso is the efficiency tester that "dares not speak its name":

Most manufacturers are careful to call the substance "soybean media" at the request of the miso merchants who sell the product. (The miso companies also insist that toilet makers not mention their names in connection with their testing.)

I guess they don't want to be identified with siphons, used to steal other people's gasoline.

And now the Olympic Champion of toilet flushing. Enjoy:

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Nice explanation - but your some of your links don't appear.

By Dave Gill (not verified) on 10 Mar 2009 #permalink

Means every time, we have to watch the flush away- hot dogs, Golf balls, grapes to tell our health condition? Miso is sticky. Got it; one tube siphon down, one is up and invisible. :-)

Thanks for sharing the Olympic Champion video clip. Just one question: what happens to the golf balls when they get to the sewage treatment plant? Wouldn't it be better not to flush them in the first place? Also, what about composting toilets - they don't need any water to flush, plus the material (not the golf balls, but possibly Rummy) can be used as fertilizer.

By Sam Dawes (not verified) on 10 Mar 2009 #permalink

Sam: I suspect you would have preferred if those golf balls had been left on your doorstep, but don't worry about the treatment plant. They screen out the big objects (like guns and baseball bats) that regularly wind up in the sewer. You make a good point about the composting toilet. I should do a post (or a compost) about it one of these days.

I would just like to distance myself from Japanese fermented soybean paste, and it's uses. My user name is derived from a Greek verb.

First was the miso government funded? Second, was the toilet a left or right leaning toilet?

Finally was the toilet bought and paid for with taxpayer bailout money....? If so was the bailout enough.

By M. Randolph Kruger (not verified) on 10 Mar 2009 #permalink


Hopefully, the toilets do their job well enough that no bailing out is necessary.

In the 1950s we used 5 - 7 gallons to flush a toilet

i need to get me one of those old things. surely there's got to be a few of them still available at garage sales or whatever...

By Nomen Nescio (not verified) on 10 Mar 2009 #permalink

Well Karen... Its a shitty business but someone has to doo it.

Fascinating technology toilets...

But it too is now being engineered into obscurity. Sanderson who makes toilets and seats is based down in Tupelo Ms. The nice lady that owns it is a Republican said that in fifty years we will go and then a heater element will burn and break it down to its basic chemicals as a dust...Sanitary at that. No more septic tanks or sewers required. You'll be able to sell the chemicals that are left and drop them off and sell them by the pound. They will be harvested for the mercury and all of the nifty stuff we need. Full electric process apparently. I asked how big it would be and she said that a version in use today (somewhere) is about the size of a bathtub. I think they'll need to work on that part.

By M. Randolph Kruger (not verified) on 10 Mar 2009 #permalink

Randy, thank you for being politically incorrect. You are a funny man.

suzyf: LOL. But it's the cat, not the toilet that's the champ.

Oh dear LOL suzyf, that cat video is incredible. Neurotic cat but incredible. I kept waiting for the cat to "spray" the toilet as his/her property.

There is another reason for the double-flush, and better toilets aren't going to solve this.

The sewer pipes leading from the house to the street used to be sized to carry the heavy water volumes from old 5- 7 gallon toilet tanks and other inefficient appliances and fixtures.

Problem is, when you flush with less water, the smaller amount of water is insufficient to float the waste (poo and toilet paper) all the way down to the sewer main. Thus the waste remains in the pipe until subsequent quantities of water carry it to the street. In a household that's good about water conservation, this can lead to a clog in the sewer pipe and an expensive visit from a sewer technician with an electric snake machine.

One solution is to flush twice right away. The second flush moves the waste down the line before the waste has had an opportunity to solidify and adhere to the pipe.

Another solution is to take your shower immediately after flushing your poop with a single flush. The water from the shower will do the trick nicely.

In the long run, the solution is to replace the larger-diameter sewer pipe with a smaller diameter one, so that the smaller quantity of water from the efficient toilet will be enough to carry waste all the way to the sewer main in the street.

g336: there is a logical order to the 3 S's-- "sh**, shower, and shave"

Toilets account for approx. 30% of water used indoors. By installing a Dual Flush toilet you can save between 40% and 70% of drinking water being flushed down the toilet, depending how old the toilet is you are going to replace.
If you are serious about saving water, want a toilet that really works and is affordable, I would highly recommend a Caroma Dual Flush toilet. Caroma toilets offer a patented dual flush technology consisting of a 0.8 Gal flush for liquid waste and a 1.6 Gal flush for solids. On an average of 5 uses a day (4 liquid/ 1 solid) a Caroma Dual Flush toilet uses an average of 0.96 gallons per flush. The new Sydney Smart uses only 1.28 and 0.8 gpf, that is an average of 0.89 gallons per flush. This is the lowest water consumption of any toilet available in the US. Caroma, an Australian company set the standard by giving the world its first successful two button dual flush system in the nineteen eighties and has since perfected the technology. Also, with a full 3.5â³ trapway, these toilets virtually never clog. All of Caromaâs toilets are on the list of WaterSense labeled HETâs and also qualify for several toilet rebate programs available in the US. Please visit my blog… to learn more or go to to learn where you can find Caroma toilets locally. Visit to see how we flush potatoes with 0.8 gallons of water, meant for liquids only. Best regards, Andrea Paulinelli

After read that I have a dream to be the best toilet flusher! It's ambitious plan which will be hard to obtain but I will do all I have to do to become the hero of toilet!