What Happens After You Flush? Wastewater Treatment in Layman’s Terms, By a Guy Who’s Seen It All

It’s that time again - time to begin reflecting on the past year. The economy, health care, and politics will undoubtedly be points of discussion and reflection at many a holiday table. Believe it or not, wastewater treatment has elements of all three of these topics, and I think it deserves equal time (though I may be a bit biased).

Not much can be bought for a penny these days, but for less than a penny per flush, sewer customers receive unlimited access to thousands of miles of piping and pumps throughout the city. That same penny will also turn the “water” coming into a treatment plant from something most don’t want to discuss (much less see), into a recycled product that can be safely disposed of back into the environment and ready for reuse. Now that’s good economics.

It’s not too hard to imagine the relationship between health and wastewater. The Romans understood the importance of disposing of their wastewater some distance away from where they lived, although they may not have completely understood the reason why. (Brings to mind several old sayings, but they probably aren’t appropriate here.) Throughout time, civilizations have succumbed to the effects of improperly handled wastewater. Today, with the help of four disinfection projects with which KAI has been involved, local wastewater effluent is disinfected, including through the use of high-tech ultraviolet (UV) technology.

As for the politics of wastewater, I’ll leave that open for discussion around the table with friends and family this holiday season.

Most folks know little about wastewater other than round and round it goes, where it stops nobody knows. At KAI, our Construction Management (CM) group knows exactly where it stops because we get into the thick of it, so to say, and get our hands dirty. (And by the way, the direction water spins while going down the drain has little to do with the spin of the earth and the Coriolis force… but that is a discussion for another time.) The reason we at KAI like to reflect on wastewater is that we have completed not one, not two or three, or even four, but five wastewater projects in the past year. I could reflect on the projects, what went well, what didn’t, and the lessons we learned. I could also write about new construction strategies, or contracts, or even business philosophies, but today I want to reflect on good old used water, wastewater, or sewage, as most refer to it.

The U.S. Geological Survey estimates that each person uses between 80 and 100 gallons of water per day. With a population of around 1.4 million people in the St. Louis City/County region, we consume in excess of 100 million gallons of water per day. A large portion of that becomes wastewater which requires treatment prior to being discharged into a local stream. Add to that number, infiltration of leaking pipes and the rain water of combined sanitary/storm sewers and we are talking about a bunch of wastewater. According to the U.S. Census Bureau, the average household consists of 2.6 people. The average household then generates more than 200 gallons of wastewater per day using the USGS consumption rates; in St. Louis, they dispose of that water for less than $.01 per gallon. Not a bad deal, if you ask me.

Few people would want to see the effects of untreated wastewater discharged directly into a stream. Try to discuss it and you will find most are squeamish and unwilling to talk about it. What then does a household receive for the $.01 that it costs to twist a lever and watch that penny go down the drain? The simple answer is a whole lot, but the real answer is much more complicated. In grade school I went on a field trip to watch Twinkies being made; we also went to the St. Louis Federal Reserve and watched as countless bundles of money moved past the thick glass observation windows. In high school we toured the Chrysler Assembly plant and watched car after car go by, each receiving a part as it passed a waiting assembly line worker. Each trip undoubtedly taught me something, because I remember each trip all these years later. Who then teaches us about the production line required to treat the 1.6 gallons per flush that we all take for granted? The answer is likely no one.

With a simple twist of a lever, 1.6 gallons of wastewater begin a journey in which it ultimately ends up back in a stream. There are a few stops along the way and the spiral down the drain is just the beginning. As it leaves the commode, it flows into the piping within our homes and for most of us, out to a pipe somewhere close by where it is combined with the wastewater of our neighbors. It then continues its journey downhill combining with the wastewater of other neighborhoods. With each added neighborhood, the pipe carrying the wastewater becomes bigger and bigger.

There is at least one obstacle to this whole process though -- gravity. Although water doesn’t spiral down the drain because of the effects of the Coriolis force, it does go downhill because of the laws of physics. It is unlikely that the miles of pipe it takes to get a single flush to a wastewater treatment plant is all downhill. (Remember, gravity still rules and the saying, “it’s all downhill from here” probably didn’t originate in the wastewater industry.) So, when gravity has done its job and carried the wastewater to a new low, pumps are required to lift it up to a higher elevation to once again begin a downhill journey to the local wastewater treatment plant.

The trip to the wastewater plant contains many twists and turns, but ultimately that flush, combined with the single flushes of others, does make it. Out of sight and out of mind to most (except the lucky few of us who get to work in this industry) are the miles of piping and pumps it takes to carry our discarded waste to a treatment plant for recycling. Also hidden from view are the countless pipes, pumps, and the equipment that lay below the treatment plant.

The real work starts when the wastewater arrives at the treatment plant. Through physical and mechanical processes, the incoming water (influent) is treated by grinding, chopping, skimming, separation, and breaking down the waste. One may be able to make the claim that the wastewater business was the first organic business because the breaking down of waste really is an organic process and relies on naturally occurring bacteria to do the dirty work at many plants. As the bacteria breaks down the waste, all it requires in exchange is oxygen, which the treatment plant provides in abundance using large blowers. A final settling to remove any remaining solids and the sludge that sinks to the bottom and the treated wastewater (effluent) is ready to discharge into a nearby stream or river. Or is it?

Customers of the St. Louis wastewater system have reached an important milestone of not only removing the yuck from wastewater but also removing the unseen pathogens. The wastewater treatment process has evolved over the years in response to growing concerns of water quality. After all, many cities draw what is to become their drinking water out of the same streams the wastewater treatment plants discharge effluent to. Who doesn’t enjoy fishing, swimming and other activities in the cool water of a local stream? This is where the treatment plants KAI has worked on come in. Four of the five wastewater projects we have completed in the past year have been projects that involve disinfection of the treated effluent.

UV disinfection of wastewater effluent is fast becoming the preferred disinfection system for water, as nothing is added to the water except ultraviolet light. In the warm seasons, the water is treated with ultraviolet light to address undesirable pathogens such as e. coli and cryptosporidium. An interesting fact about UV treatment is that in many respects the pathogen elimination is instantaneous. However, it is critical to the UV treatment process that all of the effluent water is exposed to the ultraviolet light in order to kill the pathogens. Because of this, the “dose” of ultraviolet treatment is administered by careful control over the flow of effluents at the point of treatment. As KAI managed the construction of the UV system, UV channel dimensions required very tight tolerances to assure that the flow to each channel was balanced and equal. Flow monitoring is performed at several stages as sewage progresses through primary, secondary, and advanced treatment processes. Finally, the sparkling clean water makes its way into a nearby body of water, ready to be used and enjoyed once more.

Now that you know more than you ever thought possible about what happens after you flush, I hope you have a renewed appreciation for what goes on behind (and under) the scenes at your local wastewater treatment plant!