Shadowing: the physics of the problem
UV disinfection is line-of-sight. A microorganism is only inactivated if 254 nm light reaches it, and a particle of silt between the lamp and the microbe casts a shadow exactly where it counts. Worse, microbes attach to particles and hide in their crevices and pores; a bacterium embedded in the far side of a 20 µm particle can ride through the brightest chamber untouched. Turbidity that looks trivial (water you would happily call clear at 1 – 2 NTU) measurably degrades UV performance, and a visible haze can defeat it outright.
The particle-size arithmetic makes the point sharper. Cryptosporidium oocysts are 4 – 6 µm; Giardia cysts 8 – 14 µm. Particles in that size range are simultaneously the hardest for upstream media filters to catch, the most effective shadows, and the vehicle of choice for the exact pathogens UV is usually installed to kill: the chlorine-resistant Cryptosporidium and the chlorine-tolerant Giardia.
Why 5 micron is the number
The standard specification, a 5 µm sediment cartridge immediately ahead of the UV chamber, is aimed at that arithmetic: it strips the particle sizes that shelter protozoa and larger microbial clumps, at a pressure-drop and cartridge cost low enough to be untroubling. Manufacturers state it as an installation condition, and validated-performance claims assume it. Where the duty is health-critical or the claim must survive scrutiny, step up to absolute-rated 5 µm (or finer). A nominal cartridge that passes a third of 5 µm particles lets through much of what it is there to stop, as the micron-ratings article explains. On dirty sources, the cartridge is the final stage of sediment control, not the whole of it: media filtration does the bulk work upstream, per the coarse-to-fine principle.
Prefiltration also protects the hardware itself: less sediment means less material scouring and coating the quartz sleeve, so the sleeve stays transparent between cleanings and the rated dose survives in practice.
What it does not fix
A prefilter removes particles; it does nothing for dissolved light-absorbers (colour, organics, iron in solution), which lower UVT and starve the chamber's outer edges of intensity. Filtered water can still be poor UV water: tea-coloured surface water passes a 5 µm cartridge essentially unchanged. Where colour or iron is present, the treatment for it (carbon, oxidation and catalytic media) belongs upstream, and a UVT measurement belongs in the design file. The prefilter is necessary; it is not sufficient.
Installation notes worth the minute
- Mount the cartridge where it can be changed without disturbing the UV chamber, with gauges (or at least a gauge point) to watch its ΔP.
- Change it on differential pressure like any sediment cartridge. A blinded prefilter throttles flow, and low flow through the UV is the one direction of error that is safe, but the household response (bypassing the filter "temporarily") is not.
- Never plumb a bypass around the prefilter–UV pair. A bypass that exists will eventually be open.
Rule of thumb: no prefilter, no disinfection claim. Fit five micron ahead of every UV chamber (absolute-rated where health is the point), and treat colour and iron upstream, because the lamp can only kill what its light reaches.