Ultraviolet treatment is a physical process with one governing quantity: the dose an organism receives as it passes the lamp. Dose is the intensity of the light multiplied by the time spent in it, which is why two installations with identical lamps can perform completely differently. Understanding that product explains almost every ultraviolet success and failure in the field.
What the light does to an organism
Ultraviolet at the germicidal wavelength is absorbed by the genetic material inside bacteria, viruses and protozoa, damaging it so the organism can no longer reproduce. An organism that cannot reproduce cannot establish an infection, which is why the correct term is inactivation rather than killing. Nothing is filtered out and nothing is added: the organism physically remains in the water, rendered harmless. That also means there is no residual protection downstream, unlike a chemical disinfectant that travels with the water and keeps working in the pipes.
Dose is intensity multiplied by time
Intensity is what the lamp delivers and how much of it reaches the water through the quartz sleeve. Time is set by flow rate: the faster water moves through the chamber, the less exposure it gets. That is why every unit carries a flow rating, and why exceeding it quietly undoes the treatment at precisely the moment the household is using most water. It is also why a lamp at the end of its life, or a sleeve with a film on it, fails without any change in appearance. Both reduce intensity while everything looks normal.
Clarity is the third variable
Light has to travel through the water to reach the organisms in it. Suspended particles shade them, and dissolved material absorbs the wavelength that does the work, which is measured as transmittance. Water that looks clear can still transmit ultraviolet poorly if it carries tannins or dissolved organics, and water with iron or hardness will progressively coat the sleeve and cut intensity from the outside. That is the whole reason pretreatment is mandatory rather than advisable, and why the ultraviolet stage is placed last in a treatment train.
What it inactivates and what it does not
Bacteria and viruses are readily inactivated at the doses domestic equipment delivers, and protozoan cysts, which resist chlorine strongly, are also susceptible, which is one of ultraviolet's real advantages over chemical disinfection. What it does nothing about is chemistry: hardness, iron, manganese, nitrate, arsenic, chlorine byproducts, taste and odor all pass through unchanged. Proving it works means testing the water at the tap after installation and periodically after that. Our uv bulb replacement guide covers the annual lamp change that keeps the dose real.
Questions people ask about uv treatment for water
What decides the dose an organism receives?
Lamp intensity multiplied by exposure time. Flow rate sets the time, and lamp age, sleeve cleanliness and water clarity all set the intensity reaching the water.
Does UV kill organisms or just disable them?
It damages their genetic material so they cannot reproduce. They remain physically in the water but can no longer cause infection.
Does UV leave any protection in the pipes?
None. There is no residual, so anything entering the plumbing downstream of the lamp is untreated. That is the main difference from chemical disinfection.
Is UV effective against cysts that resist chlorine?
Yes, and that is one of its advantages. Protozoan cysts that are highly chlorine resistant are susceptible to ultraviolet at achievable doses.