How much of the water we drink is recycled?

In summer 2022, much of the United Kingdom experienced extreme temperatures above 40 °C. Coupled with months of low rainfall, rivers and reservoirs were left unusually low. Discussing water security, the head of the Environment Agency, Sir James Bevan, raised an unpalatable solution, to “reprocess the water that results from sewage treatment and turn it back into drinking water – perfectly safe and healthy, but not something many people fancy.”While the thought of a second-hand drink is unappetising, our neighbourhood water supply is in fact recycled constantly, whether as part of the natural or urban water cycles. Treatments include flocculation, micro- and ultrafiltration, disinfection, and the use of activated carbon and ozone.

“It’s difficult to say which is best, as their efficiency is highly situation-dependent,” notes Neuzeret. “It actually doesn’t matter how often water has been recycled, or from what source, what matters is how it is recycled, and to what quality.”

What is certain, he says, is that water accumulates pollutants, and dilution reduces them. So as part of the natural water cycle, wastewater can be treated and released into a body of water, such as a river, before being recaptured as surface water, retreated, then piped into homes. But this option is diminishing.

“Climate change means that in some areas, especially in summer, water released from treatment plants accounts for almost all of a river’s flow, reducing its ability to dilute wastewater,” says Neuzeret.

Wastewater can also be treated for direct use. According to Neuzeret, there are two types of water treatment plants. The first, to produce non-drinking water, typically uses activated sludge to remove macropollutants such as phosphates and nitrates. “Some micropollutants are stopped, but a lot get through, and are released into the natural environment,” adds Neuzeret.

Predictably, treatment for drinking water is more rigorous, with several processes targeting micropollutants, and often involving chlorination to remove potential bacteria and viruses. So how do we know how effective water treatments have been?Neuzeret’s research is focused on micropollutants from a wide range of sources, including heavy metals released by industry, pesticides used in agriculture and residues of the medicines consumed by people, alongside our cosmetics, solvents and detergents.

“While it is widely acknowledged that the number of micropollutant compounds is well above 100 000, this variety means that we can’t talk about average samples. Situations can be completely different from one treatment plant to another,” explains Neuzeret.

In the EU-funded ToxMate project, coordinated by Neuzeret, pollution-sensitive organisms were used as waterborne sensors. The team established a protocol for real-time water quality monitoring by studying how pollutants changed the behaviour of three species of aquatic invertebrates – shrimp, leeches and aquatic snails.Innovations like these will help us ensure our water is not simply reused, but fully recycled. However Neuzeret points at another potential solution to our growing thirst. “We only use small amounts of potable water from our taps, around 7 % in France for drinking and cooking, the rest is used for washing ourselves, our clothes and our dishes. We could be systematically collecting this ‘grey water’ at home to reuse it for toilet flushing, clothes washing or garden irrigation.”

So how many times has your glass of water been drunk before? The majority of Earth’s available water has already existed for a few billion years, albeit in a variety of forms (gas, liquid, solid) and locations (rivers, lakes, oceans, groundwater). So it follows that drinking water will already have continually undergone various forms of recycling, long before it reaches our taps.

One decade-old thought experiment calculated how many times the atoms in an average water molecule could have once been concentrated urine from a variety of vertebrate species, coming up with the answer of: at least 10.

Bottoms up. But perhaps we’ll stick to wine.

Click here to find out more about Neuzeret’s research: Monitoring water toxicity in real time