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Combined Sewer Overflow
By Bobbi Holm

Combined Sewer Overflow - Sounds nasty, doesn’t it?  There’s a lot of talk in Omaha recently about the combined sewer overflow problem.  Just what is a combined sewer overflow?  Well, it begins with a combined sewer system.  A combined sewer system collects runoff, domestic sewage, and industrial wastewater in the same pipe.  Most of the time, the combined sewer takes all this wastewater to a treatment plant to be treated and discharged into a body of water.  But, and it’s a big but, when we have runoff from rain or snowmelt, the sewage treatment facility may not be able to handle all the water.  When that happens, the system is designed to overflow at certain relief points along the pipeline, discharging untreated wastewater directly into a river or stream.  These discharges are called combined sewer overflows (CSOs) and the discharge relief points are combined sewer overflow outfalls.

When these combined systems were built, some dating back to the 1800’s, this seemed like an acceptable system.  (Considering that initially, waste was dumped untreated into the Missouri River, it was a huge improvement.)  There were fewer people and there was less runoff.  And even then, governments were trying to save tax dollars and building a single system was much cheaper.  However, we’re paying for that now.

Omaha has approximately 800 miles of combined sewers with 29 permitted CSO outfalls.  Eighteen of these dump into the Missouri River, and 11 flow into Papillion Creek tributaries.  Turns out these 29 “permited” outfalls is their way of beginning to attack the problem.  With the permit come certain requirements.  One requirement is the development of a long term control plan (LTCP).  Omaha submitted a Substantively Complete LTCP (not my term, I assure you) in October of 2007 and must have a final LTCP ready by October 2009.

And we need a plan.  You ask, how often can it happen that untreated sewage gets dumped into our streams and river?  More often than anyone wants.  I’d vote for never.  According to the Substantively Complete LTCP, a computer model (based on the 2002 combined sewer system and using representative precipitation data) estimates that the busiest CSO outfall would have 58 overflows per year and the total annual volume of combined sewer overflows in the entire system would be 3,247.8 million gallons.   That’s three and a quarter billion gallons.  3 billion and change!!  Of course, not all that volume was sewage because it was substantially diluted with runoff, but it was contaminated with sewage and that’s close enough for me.  (Runoff itself isn’t exactly clean, but that’s another story.)

For lots of information, go to http://www.omahacso.com/.  We’ll look at possible solutions down the road.