Q. Do low-flow toilets generate enough water to carry solids through the soil pipe to the city sewage line or the septic system? With so little water being flushed, my concern is that in older homes with cast iron pipes, corrosion in the pipe may catch the solids and cause blockage.
A. Master plumber Rex Cauldwell responds: The small volume of flushing water that’s available to carry the waste down the pipes is a potential problem, and you are wise to be concerned. With a smooth pipe, such as PVC, there isn't a significant problem until you start using very long runs with large-diameter lines (4 inches or larger). With cast iron, you may have a problem from the onset, because the interior surface is rough and the water does not flow smoothly. Short runs usually present no problem, but long runs can.
You mention older pipes. Actually, if the cast iron has been in for a few years, it has built up a scum on the inside to help smooth out the flow. But there is still no doubt that 1.6 gallons may not be enough water to carry the waste for the entire length of the sewer line by itself.
As you’ve guessed, there are some tricks we plumbers use to keep ourselves out of trouble. I still do what my Uncle Bud taught me in the 1950s: I always design the system so the waste line of the 1.6 is scoured by another fixture upstream. This way the additional water from the other fixture will always clean the line — it's just common sense. If this is your situation, you should be okay.
If you have a toilet by itself at the end of a long run of cast iron, you may need a pressure-assist toilet to increase the velocity of the flush. In some cases, the pressurized tank will still not be enough to solve the problem. For example, in my own house, I have a run of more than 250 feet of PVC to the septic tank — 1.6 gallons, pressure-assist or not, simply won't work. For such "big problem" cases, plumbers have been known to modify the toilet to increase the water flow. I never buy a 1.6 that can't be modified. Talk to your plumber about this if you think you’ve got a major problem situation.