Quick take
Install UV after the major pretreatment stages. At minimum, control sediment/turbidity; iron, manganese and hardness may also need treatment to prevent sleeve fouling and maintain UV transmission.
Sediment and turbidity come first
CDC states UV works better with prefiltration. Suspended particles can shield microorganisms or reduce UV transmission.
Iron and manganese can foul the sleeve
Mineral deposits and staining on the quartz sleeve reduce the amount of UV light reaching the water.
Hardness can create scale
Hard-water scale can coat the sleeve over time, increasing cleaning frequency and reducing performance if neglected.
UV belongs near the end of the treatment train
A common private-well layout is sediment/chemical treatment → softening if needed → final prefiltration → UV → house.