Water Wednesday: “Mommy, Where Does It Go When I Flush?”

By Chrislyn Johnson

Last spring, when I was potty training my 3-year-old, he asked me where it goes after we flush the toilet. I thought about this before I answered him, because I have often overwhelmed the poor child with my answers. He once asked me “What is water?” and I told him it was two hydrogen atoms and one oxygen atom.

For most people, it is enough to be told that when you flush the toilet, it goes to the sewage treatment plant. Since I worked in wastewater regulation for a little while, I know it goes far beyond that, and I have trouble answering this seemingly simple question with a simple answer.

Once it goes down the drain, the water travels through a sometimes aging, sometimes modern, infrastructure of pipes to a wastewater treatment plant. Treatment options vary, from open lagoons to all-inclusive mechanical plants, all with the same goal: to treat sewage so it can be released into the environment. Many modern facilities do this with an “activated sludge” process that uses bacteria to naturally break down the waste.

As it enters the plant, the solids are separated out by a grit screen and settling basins. Heavier solids like plastics, eggshells, and intact items are settled out and removed; then taken to the landfill. The next step is the primary clarifier, where the sewage moves slowly along so heavier particles and sludge can settle out. At the same time, grease and oils dumped down the drain float to the top and are skimmed off the surface.

After the clarifier, the water is moved to the main part of the treatment: the aeration basin. Bacteria feast on the nutrients to break down the sewage and remove chemicals in the wastewater as it bubbles and roils with oxygen. Depending on the plant, an additional tank is sometimes added to help remove nitrogen. Since the treated water goes back into rivers and streams, this additional step is helpful in removing nitrogen before it can cause problems. Nitrogen can cause algal blooms that not only can be toxic, but also consume a lot of oxygen during decomposition, which kills the fish.

Following the aeration and nitrogen removal processes, the water then flows into a secondary clarifier. Water trickles out from weirs at the top of the large, circular tanks of the clarifier. The water is disinfected, either by chemical means (such as chlorination, similar to bleach), or through newer alternatives like ultraviolet (UV) lights. Once disinfected, the treated water is released into a nearby river or stream.

Whereas the water treatment is nearly finished in the secondary clarifier, the sludge often has a few more steps to completion. The bacteria slowly settle to the bottom of the clarifier into what is called the sludge blanket. Some of the sludge blanket from the clarifier is recycled and added back into the incoming wastewater to begin the treatment reaction in the aeration basin. Depending on the type of plant, the remainder of the sludge travels to the digesters for either aerobic or anaerobic digestion (where the bacteria eat each other).

Aerobic digestion uses oxygen to further break down the sludge. It is nearly odorless, but also costly since the process has to be manually oxygenated. The other common alternative is anaerobic digestion, which is not so odorless since it produces methane. However, the methane can be captured and used to generate electricity to operate the plant. The waste heat from the generators even can be used to keep the anaerobic digesters at the correct operating temperature. After leaving the digesters, water is removed from the sludge, which can then be disposed of or used as a soil conditioner. With clean water going back to the stream or river, and sludge going back to the earth, the cycle is complete.

I thought about this intricate series of steps that mimics the breakdown processes wastes would undergo in nature, given sufficient time and space. I thought about how fortunate we are to live in a country where water quality is a high priority, and we can make a daily difference to protect our local waterways (see graphic below).

I also thought about my son’s level of understanding, as he impatiently asked me again, “Where it go?” With all of this in mind, I looked down at my innocent little boy and told him, “It goes to the sewage treatment plant, honey.”

Click image to see larger version.

Click image to see larger version.

About the Author: Chrislyn Johnson is a Life Scientist with EPA Region 7’s Water, Wetlands, and Pesticides Division. She holds degrees in biology and photography from the University of Central Missouri and loves all things nature. She also enjoys access to flush toilets.

Scientific American
U.S. Census Bureau
World Health Organization

Editor's Note: The views expressed here are intended to explain EPA policy. They do not change anyone's rights or obligations. You may share this post. However, please do not change the title or the content, or remove EPA’s identity as the author. If you do make substantive changes, please do not attribute the edited title or content to EPA or the author.

EPA's official web site is www.epa.gov. Some links on this page may redirect users from the EPA website to specific content on a non-EPA, third-party site. In doing so, EPA is directing you only to the specific content referenced at the time of publication, not to any other content that may appear on the same webpage or elsewhere on the third-party site, or be added at a later date.

EPA is providing this link for informational purposes only. EPA cannot attest to the accuracy of non-EPA information provided by any third-party sites or any other linked site. EPA does not endorse any non-government websites, companies, internet applications or any policies or information expressed therein.