By the year 2050, it's been estimated two-thirds of the global population will live in areas that rely on drinking water that contains the runoff from farms and wastewater from cities and factories. So safe and effective purification methods will be even more critical in the coming years.

“Public water quality has received a lot of attention in recent years as some disturbing discoveries have been made regarding lead levels in cities across the country. Now, a new study from the Johns Hopkins University pinpoints other chemicals in water that are worth paying attention to—and in fact, some of them may be created, ironically, during the water treatment process itself.

To rid water of compounds that are known to be toxic, water treatment plants now often use methods to oxidize them, turning them into other, presumably less harmful chemicals called "transformation products." Though earlier studies have looked at the byproducts of water treatment processes like chlorination, not so much is known about the products formed during some of the newer processes, like oxidation with hydrogen peroxide and UV light, which are especially relevant in water reuse.”

They discovered that the phenols converted into products including 2-butene-1,4-dial, a compound that is known to have negative effects, including DNA damage, on human cells. Interestingly, furan, a toxic compound in cigarette smoke and car exhaust, is also converted into 2-butene-1,4-dial in the body, and it may be this conversion that's responsible for the toxicity of those fumes.

Source: University of California, Berkeley - Article