I was pointing you to evidence in the form of the PET UV transmission spectrum. The spectrum shows that PET strongly absorbs light below 300nm, with some PET variants having an even higher UV cutoff. SODIS requires UV light shorter than 300nm. That means SODIS will be ineffective using PET containers.
Instead you need a container that will not filter out (absorb) the wavelengths necessary for pyrimidine dimer formation.
no association with milk implied.