We are flushing paracetamol down the toilet and into our water supply – here’s how it could be removed

Many people use drugs including paracetamol on a regular basis to treat headaches. But only part of each drug is taken into the bloodstream, while the rest is released into the wastewater through our urine when we go to the toilet.

Paracetamol is an ingredient in the tablet. Most of the paracetamol is absorbed into the blood. Around 5% of the paracetamol is immediately excreted in urine in its original form.

Over around 24 hours, up to 95% of one dose of paracetamol – including the amount that was previously absorbed into the blood stream – is excreted in urine, after being degraded in the liver.

Mainly as a result of this physical process, paracetamol is increasingly being detected in rivers around the world. In the UK, maximum concentrations around 1 microgram per litre have been measured in the River Thames and in different estuaries. Similar levels were found in rivers and lakes of other European countries, including Serbia and Spain.

In the Nairobi river in Kenya, paracetamol concentrations have reached up to 16 micrograms per litre, which is high enough to cause cellular damage in water organisms such as clams. In Asian surface waters, high paracetamol levels have been reported as well.

And even though 81% of wastewater in the EU is collected and treated in municipal wastewater treatment plants, these facilities are not yet equipped to deal with micropollutants such as paracetamol. The prefix “micro” refers to the concentrations these substances can reach in the environment, and these are typically comparable to the size of a sugar cube in an Olympic swimming pool.

In humans, an overdose of paracetamol can lead to serious liver damage. Once in the environment, the drug does not suddenly lose its effect. Water organisms tend to be more sensitive to pharmaceuticals than humans. Liver toxicity has also been observed in certain fish species after three weeks of exposure to paracetamol at concentrations below 1 microgram per litre. In another study, even a few micrograms per litre led to malformations of fish embryos and reduced their survival rate by 90%.

Wastewater is one of the major ways through which micropollutants can enter the environment. However, private households are not the only source of micropollutants in the wastewater stream – they can also come from hospitals, where medications are used in much higher quantities.

These can end up in the hospital wastewater, which often goes into the same sewer pipes that households are connected to (and some industries as well), and is transported to municipal wastewater treatment plants. In Oslo, wastewater coming from two hospitals was responsible for 12% of the paracetamol input into the local wastewater treatment works – the highest share among the 20 tested pharmaceuticals. A more extensive study from the US also found that paracetamol was the most prevalent pharmaceutical in hospital wastewater and, even after a high degree of removal in wastewater treatment plants, still posed a high ecological risk.

For that reason, many hospitals are now being encouraged to install some kind of on-site pre-treatment for their wastewater before it goes into the sewers. The other option is to treat it in separate plants, which might be hard if they are already connected to the municipal sewage network.

Given the increasing pharmaceutical consumption, the concentrations of micropollutants in wastewater, including paracetamol, are on the rise. Insufficient wastewater treatment could therefore lead to increasing levels in the environment. However, the concentrations of paracetamol in surface water have decreased since the late 90s due to advances in wastewater treatment.

In some European countries, such as Sweden and Ireland, a large portion (up to around 80%) of drinking water is sourced from lakes and rivers. Protecting our freshwater resources is essential in order to keep using them.

How can we clean up?

The EU is now working on introducing treatment at wastewater treatment plants to tackle levels of paracetamol and other medicines in the water supply. Large treatment works will need to upgrade their facilities by 2045.

Several techniques could be used for this. One is ozonation. You might know ozone from the atmospheric ozone layer, where it fulfils the role of shielding humans from UV light and thereby protecting cells from damage. But using its ability to readily react with other molecules, ozone can be used to treat wastewater.

This process doesn’t tackle all micropollutants equally. Paracetamol belongs to those substances that are easily removed with ozone, while others, such as the blood pressure medication irbesartan, require more ozone to be degraded fully. Unfortunately, there are also micropollutants that can’t be tackled with ozonation at all, such as the “forever chemicals” Pfas.

During ozonation, ozone doesn’t only react with micropollutants, but also with natural organic molecules in the wastewater, which means that higher ozone doses are required to clean the water. More ozone is required to remove micropollutants from wastewater than from pure water (for instance tap water), because there are other elements in wastewater that also react with ozone, while tap water is already “clean”. This process is called “ozone-scavenging” and can result in increased costs for wastewater treatment plants.

Another issue is that ozonation can sometimes even increase toxicity in the wastewater. Because by reacting with ozone, the micropollutants are technically not removed, but degraded. The molecule of a degraded micropollutant looks slightly different and can have a higher toxicity than the original molecule (at least in some cases). But ozonation can be used in combination with other treatments.

Toxicity here does not primarily refer to humans, but it can harm organisms in the environment such as algae, microbes, crustaceans, or fish, leaving them unable to swim or making them infertile in some cases. Although, if micropollutants were to pass through drinking water treatment in high enough levels, it could also have serious health implications for humans.

Pharmaceutical consumption trends show that people take more medications than ever before, and the pharmaceutical industry is rapidly growing. So it is becoming increasingly important to tackle micropollutant levels in our wastewater and upgrade wastewater treatment plants to keep our water clean.

Isabell Fritz receives funding from The Swedish Research Council (Vetenskapsrådet) and Formas – a Swedish Research Council for Sustainable Development.