Here in suburbia, people don't often think about the pipes that connect their properties to municipal drainage systems. I want to clarify a few points about those pipes. If your home is served by a septic tank on the premises, today's blog isn't relevant but perhaps you'll find it interesting anyway.
Like most municipalities, the City of Raleigh operates two drainage systems that are meant to be seperate. One is sewer and the other is stormwater. The sewer system for 500,000 people feeds into a single treatment plant on the Neuse River, 20 miles southeast from my house. As you might imagine, the pipes get rather large in diameter on their way to the plant.
The stormwater system starts with curb drains and, in some cases, feeds from building gutters. In the case of my neighborhood, these are short-range pipes — less than a mile long — that drain into Tributary #2 of Hair Snipe Creek, Crabtree Creek, and the Neuse River. It's important to understand that nothing in the stormwater system is treated. Whatever enters the stormwater system goes directly into surface water — a significant problem because the stormwater runoff is polluted with vehicle oils, lawn chemicals, and so forth. To make matters worse, people sometimes use the stormwater system for disposal of liquid waste because they mistakenly believe that the stormwater system feeds into the sewer system.
In reality the sewer and stormwater systems are not 100% separate, and the inadvertent connections create serious health risks in the form of sewage spills:
- Sewer pipes become clogged by debris and grease. Sewage flows downhill, and it's got to go somewhere. The stormwater system becomes a sink.
- Although sewer pipes are fabricated to be impervious, they degrade over time and are subject to damage by trees and accidents. Liquids leak out, and liquids leak in. In the case of outbound leakage, again the stormwater system becomes a sink.
- In the case of inbound leakage which engineers call "I&I" (infiltration and inflow), stormwater runoff adds to the volume of liquid passing through the sewer system and can easily overwhelm the capacity of the pipes or the treatment plant.
- Prodigious rainfall that the stormwater system cannot handle causes localized flooding. Pooling water then enters the sewers through manhole covers, and the sewer system is overrun.