Growing Sewers

Why didn’t they put in the right size sewers to start with, so they wouldn’t have to rip up our street now?

Think of the sewer system as a tree growing over time.  The sewer “tree” kept growing, but the “trunk” pipe was still the same size as it was originally.  A larger trunk pipe needs to be installed to carry the increased amount of sewage that needs to fit through it.
Originally, the tree wasn’t expected to get as large as it has grown (and will continue to grow).  The pipe was sized for how large the tree was anticipated to grow.  Since the tree is serving more homes than anticipated, the sewer pipe needs to be made larger.  If larger pipes are not installed, the sewage will back-up onto the ground or into homes.
When the “trunk” sewer was built, designers estimated how much flow would come to that pipe.  Best available data was use to estimate growth in the number of homes and businesses that would send sewage to that pipe.  However, things do not always grow the way you might expect.  More homes and/or businesses than expected may be built.  Also, over time the sewer pipes age, and as a result cracks and breaks develop. This lets groundwater seep in so the amount of flow increases.
Sometimes a pump station is added that sends sewage to an area.  Thus, a whole new area is being served that was not in the original growth plan.    You could think of this as a new branch added to the tree.  As a result, more flow is now in this sewer than was anticipated originally.  Thus, larger sewer pipes need to be installed so that the increased sewage flow can get through it.
If we don’t enlarge these sewer pipes, sewage will overflow out of the system.  The sewage will take the path of least resistance, and end up in our streets, streams or maybe even our basements.

More Sanitary Sewer Information