Fig. 4. Hierarchy of chemicals, considering their transformation products, towards desirable intrinsic substance properties, from safe-by-design chemicals (top, green), to biologically mineralizable substances (yellow), to substances that are removable via advanced treatment technologies (orange), to non-readily treatable substances (red, bottom). The arrows on the right indicate the sequence of considerations that should be made when assessing the profile of a chemical. Formation of transformation products is particularly critical for biological treatments and oxidation processes, labelled with “!Transformation products!” in the figure.