UV disinfection is the stage where bacteria, viruses and protozoan cysts lose the ability to reproduce under radiation at a wavelength of about 254 nm. No reagent is introduced into the water, its chemical composition is left unchanged, and no residual disinfecting action remains downstream of the chamber at all.
Principle
The chamber is a stainless steel tube with inlet and outlet connections; a quartz sleeve runs along its axis, and a low-pressure mercury lamp sits inside the sleeve. The water travels in the gap between the sleeve and the chamber wall, never touches the lamp and has no electrical contact with it. A ballast is mounted outside, and industrial versions also carry a UV intensity sensor and a lamp hour counter.
The lamp emits in a narrow band around 254 nm, and that radiation is absorbed by the nucleic acids of the cell. Bonds form between adjacent bases in the DNA strand, after which the organism can no longer copy its genome correctly. It is not destroyed and it is not removed from the water — it loses the ability to reproduce. Hence the precise term: inactivation, not removal.
The outcome is set by the UV dose — the product of radiation intensity and the time the water spends in the chamber; the dose is measured in millijoules per square centimetre. The time follows from the flow rate, while the intensity depends on lamp power, lamp hours run, chamber geometry and on how much radiation gets through the body of the water. The required value is set by the regulation in force at the site and by the organism the stage was installed for, not chosen on general grounds.
How much radiation reaches the wall furthest from the lamp is described by the transmittance of the water — the fraction of 254 nm radiation that passes through a 1 cm layer. That is why the capacity in the catalogue is stated with a condition: «at T = 90 %» and «at T = 85 %» are different figures for one and the same chamber. The lower the transmittance, the smaller the dose collected at a given flow, and the lower the flow at which the chamber still delivers the required dose. Transmittance is reduced by turbidity, colour, dissolved organics and iron compounds — and colour does so while barely changing how the water looks.
Disinfection happens only inside the chamber and only while the water is passing through it. Neither reagent nor reaction product is left behind, so downstream the water is protected by nothing: any ingress of microflora further down the line — through a tank that is not sealed, a dead leg of pipework, the draw-off point itself — is compensated by nothing. From this follows the rule of placement: the chamber goes last in the scheme and as close to the point of use as the layout allows.
Principle
«The lamp is lit» is no proof that the dose is being delivered. The germicidal output of a mercury lamp falls with hours run long before the lamp stops glowing: by the end of the life declared by the manufacturer it is a markedly smaller fraction of the initial figure, and there is no outward sign of this at all. The rated life is counted in burning hours rather than in calendar months, and every start consumes it further. This is why critical schemes carry an hour counter and an intensity sensor: without them the only evidence that the stage is working is the glow, and the glow says nothing about the dose.
Water that looks clear can have a low transmittance. The eye responds to turbidity, while 254 nm radiation is also absorbed by colour — dissolved organics, humic substances, iron compounds — which barely clouds the water. A chamber that suits the flow for borehole water after iron removal may fall short of the dose several times over on surface water of the same apparent clarity. Transmittance is a measured quantity, and in the calculation it has to be a number rather than an assumption about the source.
Resistance to chlorine and resistance to UV are different properties, and the two methods do not share a list of resistant organisms. Protozoan cysts that survive chlorination at ordinary doses are inactivated by UV comparatively easily; certain groups of viruses, conversely, need several times the dose that bacteria do. Substituting one method for the other «at equivalent efficiency» is therefore incorrect: what changes is not the depth of disinfection but its nature. To this is added the fact that some micro-organisms possess mechanisms for repairing damaged DNA — one more reason to set the dose with a margin rather than at the minimum.
Practice
The stage alters neither the taste, nor the smell, nor the mineral content of the water, and forms none of the chlorinated by-products. For food production and for sites where any hint of reagent is unacceptable, that is the main reason it is chosen.
Disinfection takes place during the passage of the water through the chamber. The contact tank that reagent methods cannot work without is not needed here, and the space the stage occupies is measured as a length of pipework.
The stage loses no water at all: it has no concentrate, no backwash line and no discharge point, and the whole of the incoming flow goes to the consumer. What remains on the operating side is electricity, the lamp and periodic cleaning of the quartz sleeve.
The stage does require an uninterrupted power supply and continuous supervision. With the lamp out the water keeps flowing — and keeps flowing undisinfected, with no sign of it at the tap; alarm and intensity monitoring therefore belong to the workability of the scheme rather than to its convenience.
The chambers are supplied as finished units in three sub-series for different flows and connections. We do not manufacture them: our part of the work is the water analysis and its transmittance, the dose calculation against peak flow, the selection of the chamber and of its place in the scheme, supply, commissioning and subsequent service.
Limits
Practice
The cases are typical examples, not site reports.
Scheme
UV is never a stand-alone stage at all. It goes last and works only on water that has already been prepared mechanically: suspended solids shield micro-organisms from the radiation, while iron, manganese and hardness salts leave a deposit on the quartz sleeve. The composition of the pre-treatment is decided by the analysis, but its presence is not an improvement to the scheme — it is the condition on which the design dose is delivered at all.
The position of the chamber follows from the absence of residual action. Everything downstream of it — the storage tank, the distribution pipework, the dead legs — is capable of re-contaminating water that has already been disinfected. Hence the rule: the chamber goes after the tank rather than before it, and as close to the point of use as the layout allows, while the tank itself is separately provided with a means of sanitising.
Downstream of reverse osmosis UV is installed for a reason of its own. Permeate is almost free of salts and therefore weakly buffered, the surfaces of the tank and the pipework beyond the membrane are colonised by biofilm faster than on raw water, and the line downstream of the unit is closed precisely by UV. The membrane does retain micro-organisms, but that in itself does not make the water biologically stable.
The pipework has to provide a bypass and room for servicing. The lamp is withdrawn along the axis of the chamber, and that calls for a clear space as long as the chamber itself; in a cramped plant room on finished pipework that length is discovered only at the first service. A bypass makes it possible not to stop the supply while the lamp is changed, but for those minutes the water runs undisinfected, and the order of work is agreed in advance rather than on the spot.
Scheme
Operation
Energy
Electricity is the only resource the stage consumes continuously. Power in the catalogue runs from 14 W for the lamps of domestic chambers to 280 W in the middle sub-series, and for some models the power consumed is stated as a figure of its own; industrial versions make up their power with several lamps. Referred to the volume treated at rated flow this comes to some 0.01–0.04 kWh/m³ — a small figure against the pumping equipment of the scheme. What matters more is that consumption does not depend on flow at all. The lamp burns identically at full load and at a trickle, so on sites with uneven draw-off the consumption per cubic metre rises several times over, and standing idle with the lamp lit consumes its life to no purpose.
Consumables
What goes to drain
The stage loses no water at all: it has no concentrate, no backwash water and no discharge line, and the whole of the incoming flow goes to the consumer. Two kinds of waste do arise, and neither is an effluent. The first is spent lamps: low-pressure mercury lamps contain mercury, and they cannot be handled as ordinary glass either during replacement or at disposal; for the same reason a lamp broken inside the chamber means contaminated water and calls for a stop, a drain-down and a flush. The second is the solution left after chemically cleaning the quartz sleeve: a small volume of weak acid carrying the deposit, which is neutralised and taken away by an agreed route rather than tipped into the line downstream of the chamber.
Sizing
What we account for
What a wrong choice costs
Sizing
Questions
Because the dose depends not only on the flow but also on how much radiation gets through the body of the water. T is the transmittance: the fraction of 254 nm radiation that passes through a 1 cm layer. On turbid or coloured water the same chamber at the same flow delivers a smaller dose, so the capacity is stated together with the condition under which it was obtained. A figure without that condition would not be a characteristic but a promise: on water of a different transmittance it does not hold.
Lamp life is stated by the manufacturer in burning hours and for low-pressure lamps runs to thousands of hours — about a year of round-the-clock operation. Waiting for the lamp to go out will not do, because the germicidal output falls long before that: the lamp glows, the stage is nominally working, yet the dose is already below the design value and there is no outward sign of it. The replacement date is set by the hour counter, and at critical sites it is checked in addition by a UV intensity sensor.
At the point of installation, yes; across a distribution network, no. UV disinfects the water as it passes through the chamber and leaves nothing behind in it: residual action is absent altogether. It therefore protects the water neither in a storage tank nor in the pipework downstream, and an extended network calls for a reagent with a residual concentration. The lists of resistant organisms differ as well: protozoan cysts are inactivated more readily by UV than by chlorination, while certain groups of viruses, conversely, need an elevated dose.
As a rule yes, and the question is settled by an analysis rather than by inspection. The radiation is absorbed not only by turbidity but also by colour — dissolved organics and iron compounds, which barely cloud the water. Iron, manganese and hardness salts also leave a deposit on the quartz sleeve, and it lowers the dose gradually and invisibly to the operator. Mechanical filtration is therefore installed ahead of the chamber, and where iron and manganese are present, the corresponding stage for removing them.
After it, where the layout allows. Disinfected water is not protected from re-contamination: a tank has surfaces, air and stagnant zones, and microflora develops in it regardless of the state in which the water arrived. With reverse osmosis permeate this happens particularly fast, since it is weakly buffered and almost free of salts. A chamber at the tank outlet covers both the tank and the pipework feeding it; the tank itself is nonetheless included in the sanitising routine, because the inactivated microflora does not leave the water on its own.
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