The Sustainability Review:
Dual Flush Toilets

Replacing a toilet in the near future? Want to reduce your water consumption at home in a dramatic way? Well, the dual flush toilet (DFT) is something that is becoming more common in this country and can significantly reduce your water consumption. The basic concept is that there are 2 modes of flushing waste down the toilet, 1 for solid wastes that uses the standard 1.6 gallons per flush and 1 for liquid wastes that typically uses half of the full flush.

The DFT was originally popularized in Australia with it now being required in new homes by code there. With a more universal understanding of water problems in this country it is becoming more common to see this fixture at a plumbing store and for most manufacturers to offer 1 or more models in their product line. Companies such as Caroma, Kohler, Mansfield, Sterling, Toto and others make dual flush toilets that are easy to obtain. For excellent comments and reviews on DF toilets visit www.terrylove.com, I’ve found this site to be very helpful with real world experiences and a discussion forum.

Let’s do a little math to illustrate the water saving potential of these toilets. If you look at statistics the average person in this country flushes the toilet just a little more than 5 times per day (American Water Works Association, 1999). Most houses these days have a toilet that flushes 1.6 gal/flush; that equals 8 gallons per day or 2920 gal/year per person just for toilet flushing. We have had a dual flush toilet in our home for several years now and I am a believer that almost all waste can be flushed with the liquid waste mode (our model and many are .8 gal/flush). Even if we figure on using the full flush once a day and 4 other low flushes that brings the total water use to 4.8 gal/day or 1752 gal/year; a pretty significant savings in water (1168 gal/person). Just imagine the savings if you still had an older toilet that used 3.5 gal or more to flush.

Do these things work and what are the drawbacks? Lets start with a little design discussion. Most toilets these days flush using either a gravity flow method or pressure assist. You’ll notice the pressure assist varieties because they sound like a jet taking off when you flush them; I’m not a huge fan of these although they do work well. The gravity flow method utilizes a tank that washes down the bowl through a larger than normal opening in the bottom of the toilet. There is a test method called the Maximum Performance testing (MaP) that uses soybean paste to simulate real world conditions of toilet performance (visit www.cuwcc.org for a detailed description and test results). You’ll see there are many DF toilets that perform well in these tests and do not live up to the negative reputation that many “low flow” toilets have.

Drawbacks: Many of the toilets have a small water surface area since they use less water to flush. Consequently there is less water in the bowl creating a larger potential for “skid marks” to appear in the toilet. I personally haven’t felt this to be a problem but something that can occur. We have a Caroma Caravelle 270 and two things come to mind. The seat is a little cheap (it hasn’t broke or shown signs of that and for that matter a replacement could be had) but looking at other DF toilets many have conventional seats. Secondly, with the force of the washdown process we occasionally see some water splashing on the underside of the toilet seat lid.

Where do you buy them? Locally they are available at The Natural Abode and McCoy’s Plumbing and the prices vary from approximately $200-400 depending on the model. This price is very competitive with most quality toilets out there. There are some communities in the country (Seattle and many California cities) that are offering rebates up to $200 if you install a DFT. If your local plumbing shop doesn’t stock the toilets I’m sure they can special order them because the local area distributors have them available.

Mike is looking forward to leading the Tour of Solar Homes later this fall.

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