Photocatalysis is a destructive process: UV light excites electrons in the TiO₂ nanoparticle across its band gap, leaving a hole behind. That hole oxidizes surface-adsorbed water into a hydroxyl radical, a species reactive enough to tear apart almost any organic molecule it touches — non-selectively, down to CO₂ and H₂O. Turn the UV off and the same particles are chemically inert; nothing gets removed.
Membrane filtration is a purely physical sieve: it separates by size, not chemistry. It has no idea what a molecule is made of — it only asks whether it is bigger than a pore. Dissolved contaminants are molecular-scale and sail through every time; only particulate matter gets caught, and even that isn't destroyed, just relocated onto the membrane face.
- Watch the dissolved-organics counter under photocatalysis with the UV lamp on vs off.
- Switch to membrane mode and watch the same dissolved molecules pass straight through while particulates pile up.