Just because I'm sick of seeing the liquids-outrunning-solids-on-steep-
slopes myth on the net....

http://www.pmmag.com/CDA/Archives/83...00f932a8c0____

"A few years later I took a college course that we nicknamed Sewer
101. You got it; the course was on the design of sewers and sewage
treatment facilities. The professor was explaining the installation of
sewers in steep terrain. He was suggesting that we change piping
materials for sewers that were pitched more than 30 degrees.

I raised my hand wondering about the problems that would be
encountered. My mind was abuzz with whether my father was right, or
the plumbing inspector was right. I asked, "If you pitch the pipe too
much, won't the liquids run away from the solids, resulting in an
increased amount of stoppages?"

The professor started laughing as I tried to disappear under my desk.
He said, "You're a plumber, right?" He already knew I was because we
had talked about it many times. "Yeah," is all I could say.

"I don't know why they always teach plumbers that myth. Maybe it is
because they want them to install perfectly aligned sewers."

Of course, I found out that the professor was setting me up. This lead
to the presentation of a new equation. The equation calculated the
minimum flow rate to keep solids in suspension in a sewer system. The
inverse of the equation calculated the maximum pitch before the
liquids ran away from the solids. Of course, the answer to the maximum
pitch was infinity. In other words, you could never reach that point.

Then the professor asked, "What is the maximum pitch we could have for
a project?" That one's easy -- a vertical stack! "Yeah, and in a
stack, the solids land at the bottom first." Excuse me?!

"You got it, the solids land first and the liquids come along and pick
them up and carry them down the drain."

Most of the students just took this all in. Me, my eyes were bulging.
This was mind-blowing stuff. All I could do was think back to that
sewer with the 350 feet of distance and 150 feet of vertical drop. The
professor just told me (indirectly, of course) that we didn't have to
put in one of those vertical drops.

If I knew then what I know now, that sewer would have been 2 feet
below grade pitch straight down that hill. The sewage would have been
humming when it hit the public sewer connection.

I have long since forgotten that equation. But I learned an important
lesson. More than 25 years later, some plumbing instructors are still
teaching that if you pitch the pipe too much, the liquid will run away
from the solids. But all of you know better.

As for that 3-inch pipe, you can pitch it 1/8 inch per foot without a
problem. Of course, at 1/4 inch per foot the flow will run faster."