February 2022

Who Knows Where the Poo Goes? teaser image
There are many everyday modern advancements and technologies that society takes for granted. The  light refraction of contact lenses is rarely considered as are the combustive properties of fuel in engines.  Light floods a room with the flip of a switch, vast collections of knowledge can be instantaneously  accessed, and houses are able to withstand earthquakes, hurricanes, and floods. But who really knows  how these designs work? Who knows how refrigerators cool? Who knows how skyscrapers are  constructed? …Who knows where the poo goes?

Since graduating with my bachelor's degree, I have been working fulltime at an engineering consulting  firm where I work with a team to design wastewater treatment plants for municipalities. This career has  opened my eyes to not only the intricacies of treating contaminated water but to how little the general  population considers what happens to the water they use and waste. Therefore, I would like to discuss  the general steps of wastewater treatment and how these processes are critical for releasing this water  back into the environment.

Wastewater Collection
Wastewater is water that has been contaminated by domestic, industrial, or commercial processes as  well as surface runoff. Typically, this water will be filled with many solids and harmful pathogens that  must be removed by wastewater treatment plants before being sent back into the environment.

For collection purposes, most wastewater treatment plants are located at a lower elevation than that of  the municipality they service. This allows for the flow of water to occur naturally by gravity through  sewers with the minimal use of pumps. Once the wastewater reaches the plant, it is introduced into a  lift station that pumps the water to a higher elevation so it can again flow by gravity throughout the  plant itself.

Primary Treatment 
Primary treatment involves the removal of solids from the incoming wastewater to the plant. Therefore,  the first process wastewater goes through upon entering the plant is the headworks. This facility starts  with screens that remove large floating objects such as rags, diapers, and sticks. The next section of the  headworks focuses on grit removal, and this is where finer solids such as sand, dirt, and gravel are  removed from the wastewater.

Next, the wastewater flows to primary clarifiers or sedimentation tanks where the particles that have  not been removed by screening and grit removal can slowly settle to the bottom of large tanks. This  allows the smaller sinking particles to be scraped off the bottom of the tanks and pumped out of the  water as sludge. This sludge can either be sent to a landfill or be further treated to be used in land  applications (e.g., as a fertilizer). The primary clarifiers also allow for the finer floating solids such as  algae to be skimmed off the top of the water as scum.

Secondary Treatment
The wastewater at this point in the treatment process still contains bacteria and organic materials.  Secondary treatment agitates and aerates the flow as to provide oxygen to these bacteria, allowing  them to break down the organic materials within the wastewater. This process occurs in aeration basins and takes several hours to complete.

Next, the flow enters another set of sedimentation tanks called the secondary clarifiers. This is where  the remaining sludge that is rich with bacteria is removed from the wastewater in the same way primary sludge is removed from the primary clarifiers. This collected bacteria-rich sludge is sent back to the  aeration tanks to help in breaking down organic materials. Finally, the wastewater is sent to a disinfection process to kill any remaining pathogens such as E. coli and Salmonella. The current standard for disinfection is to chlorinate the water. While this process is  cheap, well-defined, and extremely effective with the ability to kill 99% of pathogens, chlorine is toxic to  aquatic life. This means the wastewater must also undergo a dechlorination process before it is put back into the environment. Other disinfection processes that are growing in popularity among wastewater  treatment plants are UV radiation and ozone treatment.

Not many people consider how their water waste is treated nor how fortunate they are to have access  to such processes that protect the environment and help clean water for future consumption, processes  to which many other countries do not have access. Wastewater treatment plants require many facilities, technologies, and chemicals to operate properly, and they typically cost millions of dollars to build. They truly are processes that are so essential to our everyday lives yet so frequently taken for granted.  Hopefully, one day, we can all appreciate each modern technology that makes our lives so much simpler and easier to navigate. May we continue to learn, appreciate, and count our numerous blessings.

May we all know where the poo goes.

About the Author

image of author Abigail Graves

Abigail Graves

Originally from small beer-town Shiner, TX, Abigail is currently a master’s student in Chemical Engineering with an emphasis in water resources. She earned a bachelor’s degree in Chemical Engineering from Texas A&M, yet chemistry is still her least favorite science. She works fulltime at an engineering consulting firm specializing in wastewater treatment plant design. She is married, has five dogs and loves anything nerdy, but will break some ankles on the basketball court if needed.

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