CSO’s: Combined Sewer Overflows – A Bad Combination

[note: CSO’s are alternatively called Combined Sewer Overflows/Outlets/Outfalls]

It might be obvious by now that the titles of this blog series are following alphabetical order. I picked that strategy as a writing exercise for no other reason that finding a topic on sewage to fit all letters of the alphabet (Q, X, Y, Z, for example) will force me out of my “comfort zone” and do further research.

I’m a microbiologist by training, but have no particular expertise in sewage treatment. It’s just (likely) a fact there is not a microbiology undergraduate program in Canada that doesn’t cover treatment of wastewater as part of the curriculum. This is probably true at universities around the world. Sewage treatment not only makes life more pleasant and civilized, it is one of the main planks in public health, and increasingly an important factor in reducing human impact on (what remains of) the natural environment.

Sewage in most advanced countries has two “streams”.  The first and most obvious is the sanitary sewer – waste water from commercial, residential, and industrial sources. Water from toilets, often informally referred to as “black water”, requires most attention in terms of disinfection and disease control.  In addition, “black water” is so rich in nutrients that it’s unimpeded flow to natural waterways can cause considerable ecological damage and create “dead zones” (see next entry “D”, in this blog series. Water from hand washing, bathing, clothes washing, and dishwashing is referred to as “grey water”.  This water is also “dirty” with bacteria and viruses (removed and often killed during the cleaning processes), but the soaps and detergents are the bigger environmental problem and should also be removed by treatment before the waste water is discharged back into the environment. Grey water may also contain other chemicals such as bleach (Sodium hypochlorite), paints and paint solvents, etc. “Black” and “grey” water are almost always mixed at the source in the sewage line, sent to larger sewage mains beneath our streets and transported to a sewage treatment plant. Thus, we have a much larger volume of water to treat, and are treating multiple problems (micro-organisms, excess nutrients, soaps, detergents, and other chemicals) in a single stream of waste. Most municipalities that make the effort to treat their sewage follow this model, though a very few do treat “black” and “grey water” separately (e.g. Los Angeles,). The mixing of grey and black water however is NOT what is referred to as “combined sewer system”.

Because of the extensive system of asphalt- and concrete roads, bridges, parking lots, and buildings, our cities have a greatly reduced soil with which to absorb rainwater and return it to the water table. To solve this problem, we have a system of storm sewers remove excess rainwater that lands on roads, building tops, and excess run-off from lawns. This water would normally be quite clean (it is rainwater after all), and ideally this water could be piped directly to a body of water. However, our streets often have litter and many contaminants that have dripped from cars (oil, brake fluid, power steering fluid, windshield washer fluid, etc), and our lawns and gardens have fertilizers, pesticides, and herbicides that make decontamination desirable before the water is sent to a receiving water (lake, river or stream). Therefore, as a matter of saving money, the storm water systems are often combined with the sanitary sewer system. Other than increasing the volume of water that is now treated by sewage treatment plants (often called pollution control plants) this would not ordinarily be a problem. However, under very heavy rainfalls, excess storm sewer water can overwhelm the combined sewer system. The pollution control plants can no longer handle the volume being delivered – with the unfortunate consequence that they don’t handle the excess water. It goes directly to the receiving waters, in the case of London, the Thames River. A good pictorial of this overflow can be viewed here.

London dumps raw sewage into the Thames River quite often and in considerable quantities. These events occur after heavy rainfalls and rapid snowmelts when the storm sewer adds too much volume too quickly to our combined sewer system. About 2.5% of London’s untreated sewage gets dumped into the Thames River annually. That amounts to hundreds of millions of liters of diluted but “raw” sewage. This is the primary (but not only) factor why Ecojustice awarded London a grade of (C-), the second lowest grade of 12 Great Lakes municipalities surveyed. London has some high quality (and expensive) Pollution Control Plants but their potential is wasted when we allow CSOs to bypass them. In a newspaper article assessing the Ecojustice report, one London City councillor appeared to “pooh-pooh” (pun intended) the idea that Londoners would be willing to spend a bit more on sewage treatment to prevent future CSOs. I disagree. I believe there is a silent majority who aren’t aware of the problem – and don’t even want to think about it. After all, “Sewage Ain’t Sexy”. . . But that’s also why we need to treat it to the best of our ability.


Next Post: “Dead Zones”

Blog submitted by Peter Ferguson


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