civil engineers: how does shit flow in sewage pipes?

(Mods, Sorry!
I was going to title this thread “sanitation engineers: flow rates and drop manholes”
But you gotta admit it–the title I used is catchier, and might get more responses. So I decided to be rude. Sorry…I won’t do it again, I promise)

Now my technical questions:
How do the solids move in sewage pipes? Some pipes are filled with liquid, and the solids float along.But some pipes are totally dry (like the connection to my house) When I flush the toilet, why doesn’t the small quantity of water flow faster than the solids, which I would expect to get “left behind” and clog the pipe.

And how do drop manholes work? The flow drops straight down, at a 90 degree angle. I understand how this slows down the liquid flow, and allows for lower gradients in the pipes. But how do the solids re-enter the flow of water, after falling straight down and presumably dumping a splotch of gooey mess on the concrete below?

Also–why do manholes of sanitary sewers have a concave channel in the floor, and storm sewers don’t?

What you are thinking of as being a “solid” is actually very soft and will break up as you flush your toilet. The drain coming out of the toilet is only 3" diameter. The soft solids and tissue paper kind of “melt” into dirty water; they aren’t going to be “left behind.”

Drop manholes are necessary only when the grade change is so great that a regular manhole cannot be used. Again, the solids have dissolved so that is is one homogeneous icky flow.

Sanitary sewers are designed with a minimum grade of 0.4 %. At that low slope, they don’t flow so fast and the smooth concave invert in the manhole helps facilitate flow thru the manhole.

Storm sewers are designed to “flow full”, so that velocities are in the 4 to 8 fps range. Head pressure is one aspect of getting a lot of storm water thru the pipe. A smooth paved invert is not necessary in a storm drain inlet or manhole because of the high velocity head (velocity squared divided by 2 x gravity). Sanitary sewers are not supposed to “flow full”; the flow is not supposed to be turbulent, just kind of smoothly “ease on”.

Hope that helps.

The solids are carried along by the water and move at the same rate. I have observed this, in person, close up. I’m so glad I don’t have that job any more …

Shit flows downhill, gas rises, when in doubt, vent. :slight_smile:

If you ever cut a sewer pipe, even one in your house, you will quickly notice that it’s all just sludge. From house to house, the slude pretty much looks the same. And it’ll run with the water pulled along by gravity.

I help remodel basements, and when we need to access the sewer pipe in the house, everybody’s crap looks like common sludge. Nice ‘n’ runny.

If you forget to flush, or the flush didn’t work, and it has been some time you should notice that much of the human solid waste has somewhat disolved, the TP is the solid that takes a bit longer

They promoted low flush toilets in my area for years. Now they think that not having enough water to push the stuff is causing the pipes to clog up more frequently :slight_smile:

One time someone left the door to the top of the sewage pumping plant off & I looked down it & there were lots of solids swirling around. I guess some solids make it after all.

I’ve had a few that had a definite will to live.

“piss flow downhill, turds tumble”

old CE catchphrase, from a previous life

Could someone elaborate on “drop manholes”, maybe with a diagram link?

I web searched but didn’t find much useful information. Maybe I didn’t search the right terms.

One site suggests that drop manholes are desinged for the liguid to swirl around the walls, alowing the center to vent air and gases. Is the sludge you all speak of able to swirl like water would? Is the bend more commonly two 45° bends as opposed to one 90°, thus making it easier for a gravity assist of whatever solid may yet be there?

I Googled for “drop manhole” and found several pdf drawings illustrating the drop manhole.

A good way to check the “swirl”-y ness of gray water is to pop the top on a manhole. You may find one in a yard in your neighborhood, so you don’t get run over out in the street. :stuck_out_tongue:
It flows pretty well.

The concave inverts are curvilinear for smoothness of transition. The degree of angle depends on the orientation of the in & out pipes.

Curvilinear. Okay, that explains it. The diagrams I did find weren’t too specifc on that, and the photos I found were not done well.

But, having the bend as a curve istead of a straight edged angle makes perfect sense.


At one time I was all excited about this heavy-duty garbage disposal type thing I was going to invent to put on the sewer pipe where it leaves the house. Nobody else seemed very interested. I don’t know why.

There was a show on The Learning Channel a while back about the sewer system in France. It’s pretty ancient as is still quite a marvel of design. Anyway, every so often they drop this huge fricking ball (we’re talking like 10 ft in diameter) into the main lines to flush them out. Looking for something about it on google revealed this:

Oh well, I can’t seem to quote it but it contains lots of sewer cleaning methods whose names get me hot, such as: Power rodding, balling, jetting, pigging, etc…

Drop manholes are for extreme grade changes. The water from the upper (inlet) pipe does not drop directly into the manhole.
Aprox. 1 foot before the inlet pipe reaches the manhole there’s a vertical pipe (behind the wall of the manhole) which the water drops into then it turns 90 degrees (still behind the wall) and comes out horizontally at the bottom of the manhole in the troft.

There’s still a pipe sticking out at the top part also, but only for cleaning access. -There’s the equivelent of a waterfall before it reached the manhole.

As for the lateral pipe being empty of a constant flow, some solids do sit in a sewer pipe until more water is used to flush it out. Only the most perfect pipes stay empty of water and debris.

I’m in the sewer business and have used cameras in both residential lines and municipal.

Ok, Rooves, do low flush toilets cause problems in the sewer pipe because they don’t use enough water? I too have wondered about this.

As far as houses go, yes.

If you have problems with your sewer line like sags, bad (or offset) joints or tree roots obstructing flow it will aggravate the problem. Otherwise, besides how lousy the toilet may flush, no.

Those who do have problems we just tell to use liberal ammounts of water in general just to keep the pipes flushed out.

As far as municipal sewers go, it’s good because many treatment plants are already operating at or near capacity. I’m no expert on treatment plants but when they go over capacity I’m pretty sure they just let the extra go if you know what I mean.

Also its cheaper to treat less water. Theres a whole business in stopping water infiltration (clean ground water seeping into sewer lines) because of the treatment costs involved.

At first I thought low flush toilets were bad but now I can understand the big picture.

This question puzzled me for some time until I finally figured it out. The sewage doesn’t have to go immediately to the disposal pland. There is a minimum required slope to the system, every time you fluch or take a shower or wash the dishes, water comes along to keep the solids moving downslope, even if they haven’t disintegrated. In the main lines there is an almost continuous flow of water because, although there are peaks and valleys in usage, over a large area someone is almost always doing something.

My old toilet had a dam, but I was told (by a Roto-rooter expert I know ;)), that old toilets aren’t designed for low flow and might not work efficiently.
It didn’y flush right until I removed the dam.

Well, for gravity sewers, they are unnecessary.

For low-pressure sewers, they already exist. They’re called “grinder pumps.”