Ozone was the most effective pretreatment to deactivate NDMA precursors.

Medium pressure UV was as effective as chlorine and more effective than low pressure UV.

Direct ozone reactions were more important than hydroxyl radical reactions.

Precursor deactivation by medium pressure UV involved indirect photolysis.

Ozone, medium pressure UV and (less so), chlorine, increased NDMA formation from polyDADMAC.