Install UV after sediment, iron and other pretreatment, near the end of the treatment train. Size it for peak flow, provide reliable power and leave enough clearance to replace the lamp and quartz sleeve.
Pretreatment comes first
Sediment, iron and turbidity can interfere with UV transmission. Treat those problems upstream before relying on UV for microbiological control.
Size by peak flow
A chamber that is undersized for simultaneous household demand can reduce delivered UV dose. Use the manufacturer rated flow under the applicable performance standard.
Plan for power and service access
UV systems need continuous electrical power. Leave enough room to remove the lamp and quartz sleeve without dismantling nearby plumbing.
Install a bypass carefully
A bypass is useful for service, but users should understand that water flowing through bypass is not being disinfected.