Waste water treatment Science communication


What happens after you flush the toilet?

If you have ever been curious about where stuff goes when you flush the toilet, I will take you on a tour of a wastewater treatment plant

You probably won’t find this in TripAdvisor, but an interesting site in Linköping, Sweden is its wastewater treatment plant. When construction on a new ozonation process is complete, this plant, owned by Tekniska Verken, will be the most high-tech one in Sweden.

Wastewater treatment plants were designed to remove solids, organic matter, nitrogen, and phosphorus. They were not designed to remove household chemicals (such as flame retardants that wash off clothing in the laundry) and pharmaceutical resides. When we take medications like antibiotics, analgesics, anti-depressants, oral contraceptives, antihistamines, etc., our bodies produce residues that enter waste streams through our excretion. There is growing scientific evidence that pharmaceutical residues from wastewater can have negative effects on wildlife.

The idea that “dilution is the solution to pollution” means that substances in wastewater are not harmful in small concentrations. In the case of Linköping, treated wastewater is about 1/27th of the flow of the River Stångån, which suggests limited opportunity for dilution. We do not fully understand how pharmaceutical residues interact with each other and in the environment, and what concentrations are “safe”. Under the precautionary principle, there is an argument to remove these residues before they reach the environment.

Ozonation is often used to purify drinking water before it reaches our homes, but it is not a typical step in sewage treatment. The bubbling of ozone (O3) through wastewater breaks down pharmaceutical residues and kills pathogens.

So what happens after you flush the toilet? The answer depends on where you live. If you live in suburban or urban areas, your toilet is probably connected to a centralized sewage treatment plant. Below I describe the process at Linköping. Other treatment plants may not perform all steps or may use different techniques.


Step 1: Sewers transport wastewater to the plant.

sewage inflowRaw sewage from toilets and sinks

Step 2. Mechanical screens remove items that cannot be removed during treatment: tampons, wipes, condoms, toys, etc. Depending on where you live, these items are incinerated or sent to a landfill.

mechanical removal of solidsView of mechanical separation step - this glass lid covers rotating mechanical "fingers"

mechanical removal of solidsPeeking inside. I placed the camera lens against the outer glass lid

Step 3. Sand and other grit (e.g. coffee grounds) are removed by gravity. Chemicals are added to bind and remove phosphorus from the water.

sand and grit removalAfter entering the treatment plant, sand, grit, and other particles are removed

Step 4. Aeration: Air is added to the water to stimulate microbial decomposition of sewage organic matter (e.g. feces, food from in-sink garbage disposals).

Step 5. Sedimentation: Gravity removes suspended solids. At Linköping, the removed solids are later used in biofuel production.

sewage sedimentationSelfie in front of a sedimentation tank

Step 6. Activated sludge. Microbes and protozoans busily decompose organic matter and remove it from the wastewater. Gravity removes suspended solids.

activated sludgeIn the activated sludge process, bacteria and protozoa consume organic matter. Bubbles are from aeration

Step 7. Ozonation. When this step is in operation later in 2017, ozone will be bubbled through the water and break down pharmaceutical residues. Testing by Tekniska Verken found 90% removal efficiency of 42 different substances by ozonation.

ozonationThe new ozonation process was still under construction when we visited

Step 8. Nitrogen removal. Plastic “macaroni” are added to provide surfaces on which bacteria can grow; there are filters at the end of this step to prevent the macaroni from entering surface waters. Different groups of bacteria convert ammonia to nitrate (nitrification) and convert nitrate to harmless N2 gas (denitrification), which is released to the atmosphere.

coupled nitrogen-denitrificationBacteria convert dissolved nitrogen in wastewater to N2 gas. Specks in water are “macaroni”

Movie of nitrogen removal process

Step 9: Clarifier. Aluminum chloride is added to remove phosphorus.

Step 10: Treated wastewater is discharged to the environment

sewage outflowTreated water enters a side channel before flowing into the RIver Stångån

Another interesting aspect of the Tekniska Verken facility is that by-products of sewage treatment (such as solids removed by gravity) are used to produce biogas. Public buses in the area are fueled by sewage-derived biogas and biogas is available for purchase for private cars and taxis.

biogas from sewageWaste treatment by-products are converted to biogas in a nearby facility

Wastewater treatment plants combine physical, chemical, and biological processes to do things that most people consider disgusting. The biological processes are basically industrial-scale versions of what bacteria and protozoans do naturally. The nitrogen removal step, for example, harnesses the abilities of bacteria to convert nitrogen released from our bodies into N2 gas, which makes up about 80% of our atmosphere. For my Ph.D. research, I studied denitrification in lake sediments, so it is interesting to see denitrifiers in "captivity".

The bottom line is that municipal wastewater treatment is not a like a giant Brita® filter; what is not removed during treatment enters a nearby lake or river. Something to think about the next time you consider pouring medications, paint, solvents, pesticides, etc., down the drain.

If you want to learn more about what your local wastewater treatment plant does and does not remove and where the treated wastewater goes, contact your city or county. Most facilities offer tours and you can get a first-hand look at the process. And it doesn’t smell too bad!

 vatten så klartClear water, of course

For further reading: Policy Brief about advanced wastewater treatment


Michelle McCrackin