Nature already had the answer.

For over a century we have cleaned water with chemicals. They work, but they don't disappear. Every day, the medicines and toxins we consume pass straight through treatment plants that were never built to stop them. In the first global survey of river pollution, pharmaceutical residues exceeded safe levels at 43% of the sites tested across 104 countries. The technology to break them down already runs in many countries. What holds it back is investment, not capability. The same is true in the air, where combustion in industry leaves harmful emissions like NOx behind.

 
Nature already had another way. The crisp freshness after a thunderstorm is ozone at work: three oxygen atoms instead of two, formed by lightning and UV light. It is one of the most powerful oxidisers in nature, able to break down many of the pharmaceutical residues and pathogens that conventional treatment leaves behind, then turn back into the oxygen we breathe.

No chemicals added. No new problem left behind.