Do low-flush toilets actually save water?

March 7, 2008

Dear Cecil:

Has anyone ever proved that low-flush toilets actually save water? It seems like the multiple flushes needed to clear my bowl mean they increase water use, at least at my house, not decrease it.

This morning in the bathroom I blew my nose using toilet paper and had a difficult time deciding whether to throw it in the trash or flush it down the toilet. Is it better to let it decompose in a garbage heap somewhere, or flush it and let the sewage treatment centers do what they do best?

Cecil replies:

Some may feel the following is overly mired in the messy details of lavatory use. But as more than a quarter of all the water used indoors in the U.S. goes down the toilet — about six billion gallons daily — conservationwise, flushing is serious business.

Is your seat down? Good. Let's sit and think. In the old days residential-use toilets in the U.S. were designed to unleash anywhere from 3.5 to 7 gallons per flush (GPF); the low-flow movement took off in 1994, when a federal statute kicked in requiring that new units employ no more than 1.6 GPF. How often do people use their toilets? Here we face the thrilling diversity of human experience: some pee four times a day, some ten, while anything upward of three bowel movements a week is considered normal. The bottom line is that for practically everyone the daily ratio of (shall we say) liquid-only events (LOEs) to solid events (SEs) is greater than 1:1, and for some may be 6:1 or more. For argument's sake let's say a typical day on the toilet involves five LOEs and one SE. Using a 3.5 GPF unit that consistently gets the job done in a single flush, that's six events, six flushes, for a total of 21 gallons. But the puniest low-flow model should have little problem handling the LOEs, so even if five flushes were required to bring the SE to a satisfactory conclusion, the ten flushes would use only 16 gallons. In this scenario, as long as you're flushing fewer than eight times per SE, the low-flow toilet is saving water.

Obviously, hanging around repeatedly flushing a toilet is practically no one's idea of a good time (cats on YouTube notwithstanding), but there's evidence to indicate that the typical low-flow user's experience hasn't been quite so grueling. It's true that some of the first low-flows were pretty dodgy — often manufacturers just stuck a new valve or dam mechanism in an existing model rather than revamping the bowl and other elements to work with the reduced flush volume. While the resulting units did conserve water, satisfaction with them was mixed, and anecdotally at least they acquired a bad rap. In 2000 University of Arizona researchers conducted a study measuring water use in 170 Tucson households where low-flow toilets had been installed seven years earlier. Among their findings: (1) more than half the homes had no detectable toilet trouble; (2) about 11 percent of the low-flows got double-flushed at least once a day, somewhat but not much more than the rate seen for higher-volume models; (3) low-flows were seemingly more prone to developing flapper leaks; and (4) in over a quarter of the homes at least one of the low-flows was using significantly more than the 1.6 GPF it was meant to, whether because of malfunction or tampering.

Toilet makers eventually improved their low-flow designs, or at least some did. How to sort the weak from the strong? We look to 2003's landmark Maximum Performance project. As part of this U.S.-Canada effort, experts developed "test media" that could accurately mimic human feces under trial conditions — soybean paste, extruded through a 22-millimeter die into 100-millimeter chunks — then piled it into various low-flow models to see how much each could move. What they found was a wide range of effectiveness: 45 percent of the units tested couldn't handle the minimum acceptable payload of 250 grams, while 25 percent were able to flush more than twice that amount. It may take some shopping around, but clearly you can get a low-flow toilet that doesn't entail overtime with the plunger.

If that's not enough conservation, you could try a dual-flush toilet, which lets you select a partial- or full-volume flush as need requires. Or you could get hardcore and flush only when there's something solid to dispose of. True, this means added gaminess in the bathroom, but there's no question that letting the yellow mellow, etc, saves tons of water over the long haul. Oh, and David: toss that TP in the trash — if it's a brand that doesn't break down readily, flushing it just means it'll get screened out at the treatment plant, whereupon someone's got to cart it to the dump anyhow. And if you somehow can't resist dropping it in the bowl, needless to say, at least don't waste a flush on it.

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