Waste water treatment could actually be increasing the amount of some toxins, by transforming common pain-killing drugs into more dangerous forms.
During the treatment process, say University of New South Wales scientists, interactions with bacteria can change them into something more dangerous.
Some drugs can occur in two forms, known as enantiomers - chemically very similar, but with drastically different effects on the human body.
Where both versions are known to be safe, drugs are manufactured and dispensed as mixtures of the two forms. But some are dispensed as single enantiomers, precisely because the other form is known to be toxic.
The researchers monitored three common pharmaceuticals during wastewater treatment. These included the anti-inflammatory drug naproxen, which is manufactured and dispensed as a single enantiomer, known as S-naproxen. Its counterpart, R-naproxen, is known to be highly toxic to the liver and is not publicly available.
And they found that, during the treatment process, some of the safe version was converted to the unsafe form.
"We found that some of the S-naproxen had turned into R-naproxen - so even though we’re measuring a major reduction in the concentration of naproxen, the overall toxicity could be increasing," says study supervisor Dr Stuart Khan of the UNSW Water Research Centre.
And the chemicals won't show up in eco-toxicological assessments. Because the toxic version of naproxen isn't a registered pharmaceutical, it won't be tested for - and it should be, says Khan.
"We can’t just look at what’s disappearing during the wastewater treatment process, but we need to consider what it’s turning into," he says. "And is this breakdown product an even greater concern than the original compound?"
It’s not clear eactly how the transformation's taking place, but Khan believes it's enzyme-driven, with microorganisms in the treatment plant converting the non-toxic into the toxic form.
Something similar can occur in the human gut - indeed, it was precisely this type of inversion that turned thalidomide into such a lethal drug in pregnant women.
Khan and his team are now working to see whether similar processes are taking place with other pharmaceuticals.