No, like literally. I apologize for the clickbait, but have you ever wondered what happens after you flush? Probably not, because that's gross to think about. However, the U.S. has some pretty great sanitation and I am very thankful for our water remediation.
So what happens? To start, water makes its way to a collection system where it begins a multistep process that takes it from murky waste to clean, reusable water. According to the Environmental Protection Agency, the U.S. treats 34 billion gallons of wastewater every day, which is about 51,130 Olympic pools. 🤯
Step 1: Pretreatment
Stuff that wasn't supposed to be flushed is filtered out by a screening system. It's like a TSA check, but instead of making sure you aren't bringing weapons into the airport, these screens make sure you aren't letting diapers clog a multimillion dollar system. What remains graduates to a "grit chamber." Here the water is slowed down and gravity does its thing, sinking large particles like eggshells, gravel, seeds, and sand. This typically stops anything over 150 microns, which is a little larger than a particle of table salt.
Primary Clarification
Eventually the water makes its way to primary clarification. This is a large tank where the water's velocity is slowed again, allowing particles to settle, float, or continue on to the next step. It's pretty effective: 50-60% of suspended solids are removed and 25-40% of biological oxygen demand (BOD) is decreased. BOD is a measurement of how much dissolved oxygen bacteria need while breaking down pollutants. A high BOD can cause anoxic dead zones where fish and wildlife suffocate, or it can create toxic algae blooms.
Sludge, Methane, and Scum
The solids that settle are called "primary sludge." These are scraped into collection areas. The sludge has the potential to be turned into fertilizer if its excess water is removed and it goes through a process called anaerobic digestion. This is a fancy way to say the sludge is put in a tank without oxygen while bacteria break everything down over 15-30 days.
Methane is produced during this process, which can be converted to steam and harnessed as energy for the treatment plant. The basic functions are really similar to our own stomachs.
From here it moves to dewatering via centrifuge: it spins fast to dry. After this process, what remains can be used as fertilizer. Although this may become less popular due to CECs (contaminants of emerging concern) such as PFAS, aka the forever chemicals. PFAS is not treated for during this process, and its strong carbon-fluorine bond (one of the strongest bonds in organic chemistry) is not only hard to treat but hard to detect.
Getting back on track, the grease, fats, and oils that float in the previously mentioned collection areas are called "scum." This is highly concentrated and hard to break down. Sometimes it can be pumped into the anaerobic tank to boost methane production. Other times it can be sold to companies for industrial lubricant and biofuel production. The remaining option is the landfill.
Biological Treatment
The middle layer, known as effluent, flows on to biological treatment. Bacteria break down sugars, fats, and detergents. This step further reduces the BOD, bringing it down 80-95%. Aeration systems supply oxygen bubbles throughout the basin and help keep the bacteria healthy. This phase strips out nitrogen, phosphorus, and ammonia, converting them into harmless compounds before they can damage natural ecosystems.
Final Clarification
Funnily enough, it moves on to final clarification, which sounds like the last step but isn't. This is yet another settling tank where clumps sink to the bottom, but this time it's the good stuff. Some of the bacteria that consumed the pollutants get pushed out of the previous tank into this one. Here they settle, are referred to as activated sludge, and can be pumped back into the other tank to repeat the process.
Filtration
Next, the water runs through a filter with layered substrates. Think layers of sand, anthracite coal, or activated carbon (which is often derived from coconut husk). At peak performance, micro particles and heavy metals down to 0.01-0.1 microns can be filtered out. For reference, a human hair is usually 50-70 microns thick. This catches a lot of the remaining bacteria and matter.
The Final Step: Disinfection
The final step is to kill off any stubborn pathogens, viruses, and bacteria. There are three main choices for this: chlorine, UV, and ozone.
Chlorine
Chlorine is the traditional, cost-effective method. The chemical is injected into the water, creating hypochlorous acid (HOCl). This is the heavyweight bacteria, virus, and pathogen fighter, and it is hard to keep alive. A pH under 7.5 has to be maintained in order to do so. HOCl acts like a molecular Trojan horse, sneaking into cells past their protective walls, and once inside it attacks the proteins and enzymes needed to process food and create energy. With the cell's internal structures damaged, it dies.
The water then goes through dechlorination to chemically neutralize the chlorine, which could otherwise strip the protective slime coats off of fish and destroy their gills. To get chlorine to absolute zero, plants inject sulfur-based reducing agents that split into non-harmful chemicals buffered by natural water chemistry.
UV
UV is the chemical-free alternative. UV lamps scramble the cellular data of these pathogens instead of killing them. These little mutated microorganisms are then sterile and unable to reproduce. This process is really cool because there is no byproduct left behind, but you need very clear water for it to be effective.
Ozone
Ozone is a highly reactive, unstable molecule. It's like that crazy hairy dude from Barnyard that breaks everything. It takes out viruses and bacteria instantly by rupturing their cell walls. It even breaks down trace odors and organic compounds. The drawback is that it's super energy intensive and leaves no lasting disinfectant in the water as an additional line of pathogen defense (UV doesn't either).
Once one of these three processes is complete, the water is officially ready to be returned to the environment: successfully completing its journey from flush back to the fishes.
Thanks for nerding out with me a bit!