I think I will go with Eddie on this. I'm the guy that has to keep these " just enough" sewers going. I only have 35 years experience, you others may have more. I have seen many more sewers give trouble because they were too flat. I have NEVER seen a 45 degree down hill sewer clogged. The flat sewer may work on paper, but how do you keep it perfectly flat? There are always dips and sags in the back fill. Every puddle will have solids puddled in it. This falls under "urban legends" the myth busters would have a great time with it.
Well to further the hijack
I would never argue with your experience, but I think you are referring to house sewer laterals like Eddie was also.
I'm interested in the municipal sewer system. I do basement waterproofing work and many of the older homes, probably 50% have what's called combination drains in the basement. basically it's a wye off the 4 inch cast iron house sewer under the floor. off the wye is a 4" cast gas trap and then a 4" cast iron back flow valve. the back flow valve is in a small pit in the floor and the sub-floor footing drains spill into the pit and drain thru the back flow valve. So it "combines" the storm water from under the house with the sanitary sewage.
Environmentally it's a terrible system , cause it overwhelms the treatment plant with storm water, some of the oldest streets in the city actually have the catch basins drain into the sanitary sewer, they separate them when they redo the roads where possible.
It's a great system for the homeowners, when they function properly, as there is no need for a sump pump, worry of power failure or water discharging out the side of the house onto the yard.
The problem is because there is so much storm water in the system during heavy rain events, many people living in the lower areas experience sewer back ups. sometimes the back-flow valves are stuck open, but even if they work, they rarely close tight plus when the flap is shut the home's storm water can't get out so the basement floods. Often times the back flow valves are capped and a sump pump is installed
Anyway, I need to be able to advise the homeowners of the possibilities and alternatives based on the risk exposure, so it helps that I know locations and sizes of the street and main sewers in any particular neighborhood. I have the detailed maps.
Sorry kind of long winded here, but anyway maybe that idea of the liquids and solids seperating is an urban legend, but not the almost flat main municapal sewers. Those are often referred to as engineering marvels, and rightly so.
This thread made me dig deeper to further understand the subject, I've always tried to explain to my customers about the way their basement water is handled thru the sewer system, I've always told them that it travels all the way to the plant at a very precise 1%, as that's what I believed.
But that doesn't work, as that's 50+ feet of slope in just a mile, we do not have that much topograhy to work with. So I learned that those large trunk sewers are set at somewhere around 0.1% in the very flat areas, that is an engineering marvel.
Sorry for the long rant, it's just something I'm interested in.
JB
