Each UV lamp (UV sterilizer) consists of two basic elements: a tube (housing made of stainless steel), UV emitters and a system supplying the emitters. The task of the tube is to properly position the emitter or a set of emitters so that the flowing water is irradiated with UV rays generated by the emitters in the most effective way possible. In order to protect the emitters from possible mechanical damage, direct contamination or to ensure their trouble-free replacement, they are placed in special quartz protective tubes. The UV lamp works when the power supply system is turned on and provides the appropriate voltage necessary to activate the lamp causing it to glow.

This is how the lamp works in relation to its technical features, however, in order to obtain the appropriate effectiveness of water disinfection (deactivation of bacteria, viruses, yeast or mold), it is extremely important to provide the appropriate dose of radiation related to the desired wavelength. To bring the issue closer, we present the basic concepts below.

WATER DISINFECTION
The common definition of this word is: "a procedure aimed at destroying microorganisms and their spores. Disinfection destroys vegetative forms of microorganisms but does not always remove spores."

UV RADIATION
UV radiation in most cases meets the definition of "disinfection" because it causes a rapid photochemical reaction in the DNA of microorganisms. As a result, microorganisms are killed or irreversibly lose their ability to reproduce. It should be remembered that bacteria and viruses are the least resistant to UV radiation (i.e. they are deactivated the fastest), yeasts are slightly more resistant, and molds show increased - the highest resistance.

WAVELENGTH
UV radiation ensures proper water disinfection only and exclusively when the appropriate wavelength is maintained. This wavelength is the radiation range within the range of 254÷265 nm (UVC range). The main absorption of microorganism DNA occurs at a wavelength of 260 nm. Therefore, it is very important that when purchasing a lamp, we are sure that our device emits the appropriate wavelength to ensure full deactivation of microorganisms. We are not able to check on our own whether the device emits the appropriate wavelength or not - in this case, we are left with only a subjective assessment of the UV lamp manufacturer's credibility, or the requirement to provide appropriate confirmation certificates.

RADIATION DOSE
It is nothing more than a measure of energy delivered to water in order to ensure its disinfection. As one might guess, the higher the dose, the greater the disinfection efficiency. The measure of radiation dose is J/m² = 0.1 mJ/cm² = 1000 mWs/cm². Some microorganisms require a lower dose and others, unfortunately, a higher one - generally, it should be assumed that the minimum dose ensuring proper disinfection should not be less than 400 J/m².

RADIATION INTENSITY
This is the radiation intensity expressed in W/m² depends mainly on the power of the radiators and the surface being illuminated. Remember that the radiation intensity should refer only to effective radiation, i.e. within the wavelength range between 254 and 265 nm. The higher the radiation intensity, the higher the dose and the greater the disinfection capacity.

EXPOSURE TIME
The exposure time expressed in seconds depends solely on the water flow rate and tells us how long a given water molecule is exposed to. The longer the time, the greater the dose and the disinfection capacity increases. It should be remembered that the greater the flow, the faster the water flows through the lamp and the shorter the exposure time.

WATER CLARITY
Water clarity, also called "transmittance" or "transmission" of water, is nothing more than a correction factor in determining the radiation dose. This factor expressed in [%] tells us how transparent (clear) the water is and to what extent it dampens UV radiation. The clearer the water, the higher this factor (theoretically max 100%). A lower factor determines the need to use more powerful radiators to increase the radiation intensity and as a result obtain the assumed dose.

Generally, we can divide the emitters into two types: low-pressure and medium-pressure. In both cases, we are talking about the pressure of the mercury vapor of the emitters emitting UV radiation, and not about the working pressure (pressure of the water surrounding the emitter). 

Regardless of the type of radiators, it is absolutely required that both the radiators and the power supply systems are manufactured by reliable, proven and certified manufacturers. Additionally, when choosing a given lamp in relation to the radiators used, we should pay attention to their service life and the costs of their replacement. However, remember that this period is a contractual period, because it depends to a large extent on the number of times the lamp is turned on and off. 

Theoretically, each disinfectant should cause by-products, but it is important to determine the critical value in this case. Studies have shown that UV radiation causes the formation of by-products at radiation in the range of 1000 J, while the real doses are at the level of 400 J/m². In this case, special attention should be paid to emissions below 220 nm, which easily convert nitrates into nitrites. Wavelengths below 250 nm, which can be carcinogenic, are also problematic. This particular problem of medium-pressure emitters has been solved by using appropriate quartz protective tubes that cause absorption of waves in the wrong range. The disadvantage of this solution is the loss of approx. 40% disinfection capacity.

In this case, the answer will be diplomatic - it depends. The use of a UV lamp alone usually meets our expectations in terms of effective disinfection in such a way that the water flowing out of the device is bacteriologically clean. However, whether it will still be bacteriologically clean at the point or points of water collection depends solely on the condition, length and type of the installation or water supply network. Therefore, if we are sure about the condition of our installation or network, we can usually give up the need for chemical disinfection. However, if our water supply network is extensive or in poor condition (e.g. soft sediments) or there is a potential risk that the connected recipients may contaminate previously disinfected water (e.g. through a domestic hydrophore installation not protected by an anti-contamination valve) - let's not give up on chemical disinfectants. It is not fair to say that UV lamps will solve all problems related to water disinfection. With full responsibility, we can say that a well-selected lamp is completely effective, but at the same time it is noted that it works locally and in no way protects us from secondary contamination. Only chemical disinfectants that maintain their effectiveness at every point of the installation or network (such as sodium hypochlorite or chlorine dioxide) can protect us from secondary contamination of the network. Of course, the most correct and technically effective solution is a double system, i.e. a UV lamp at the entrance to the installation or at the exit from the water treatment plant, along with additional chemical disinfection (using smaller doses of disinfectant). 

Each UV lamp manufacturer praises their devices as unique, with high efficiency and disinfection capacity, and at the same time low energy consumption. However, the main problem is that due to many technical factors, it is very difficult for a typical user to obtain confirmation of the declared data (e.g. dose, wavelength, etc.). Therefore, Germany, Austria, Switzerland and the Netherlands were the first to join the project of standardizing UV lamp parameters in 1997. The main point in the certification process is the indication of the biosimetric dose (radiation dose), which guarantees the proper effectiveness of the device. Most UV device manufacturers provide the efficiency of the lamps based on their own calculations. The biosimetric dose is an empirically determined value, which means that the device must neutralize a specified number of microorganisms with an efficiency of at least 99.99% at a radiation dose of 400 J/m² and this with a specific amount of water and known UV transmittance. Then, after meeting these conditions, the device manufacturer can count on the appropriate DVGW or OVGW certificate. Such a certificate is the most reliable indicator for the customer that the device they intend to purchase meets the declared performance properties.

CASING PIPE CLEANING SYSTEM
Water flowing through the lamp contains suspension and other compounds that, over a longer period of time, can accumulate on the internal surfaces of a given system or installation, causing the formation of so-called deposits. This is a completely natural phenomenon and in many cases unavoidable, and applies to basically all elements of a given installation - pipes, fittings, fittings, water meters and UV lamps. While in other cases such a situation does not have a significant impact on the correct functioning of the given elements, in the case of UV lamps it is very unfavorable because the sediment deposited on the casing pipes will simply suppress the UV radiation emitted by the radiator. As a result, a smaller dose of radiation will reach the water, which may result in an insufficient level of disinfection. The solution to this problem is periodic cleaning of the casing pipes, which involves physically removing them from the lamp and cleaning them. Unfortunately, this involves periodic decommissioning of the lamp, performing service activities, during which cases of breakage of the casing pipe itself occur. Therefore, renowned UV lamp manufacturers also offer a cleaning system. These are flexible rings placed on quartz casing pipes, which, in combination with an appropriate mechanism, periodically move along the pipes, causing them to clean themselves (the principle of operation is similar to car windshield wipers). Depending on the version, the cleaning mechanism itself can be started manually or automatically (periodically from a UV radiation sensor). There are also mixed cleaning systems on the market, i.e. mechanical-chemical. Their principle of operation is identical to the mechanical system, with the difference that the cleaning ring itself contains a special gel in its inner surface that supports cleaning. The disadvantage of this solution, however, is that the gel must be replenished periodically and that it may affect the quality parameters of the water. 

It should be remembered that while in the case of low-pressure lamps the cleaning system can be treated as an optional equipment, in the case of medium-pressure lamps it should be a mandatory element of the equipment, because the higher operating temperature of the radiators causes scale to precipitate on the protective tubes. 

DEFLECTOR
The deflector is nothing more than specially shaped rings located at the inlet to the lamp, ensuring even distribution of water jets over its entire internal surface. The advantage of this solution is that due to the even flow, each water molecule is exposed to the same dose of radiation. This device was invented after periodic problems with the disinfection capacity of lamps. In this case, hydraulic modeling proved helpful, allowing to assess the distribution of water jets in a typical lamp (without a deflector) together with their flow speed. As a result of the turbulence caused by the fittings and fittings installed behind a given installation, turbulent water movement occurs, which in some cases produces water jets with different flow speeds. As a result, the jet with a lower flow speed received a higher dose of radiation than the jet with a higher speed. As a result, microorganisms could appear in the water flowing out of the lamp, which were not removed due to the improper disinfection efficiency of the faster jet. 

RADIATION INTENSITY MEASUREMENT
Measurement of radiation intensity is possible when our UV lamp is equipped with a special sensor built into the lamp housing. This system allows the user to continuously monitor the correct operation of the lamp in terms of emitted radiation, which can be directly translated into the disinfection capacity of the device.    

The installation of the lamp depends only on the shape of the housing, which determines the installation itself. As a rule, all devices available on the market have a threaded connection (smaller diameters) or a flange connection. In each of these cases, the installation of the lamp to the installation is typical and should not cause any difficulties. Regardless of the shape of the device itself, remember to follow the iron rules:

• let's keep free space on the side of the radiators so that they can be replaced without having to completely dismantle the lamp,
• let's check the lamp's operating pressure to make sure it is not lower than the maximum pressure in the installation,
• at the inlet and outlet of the lamp, we should use shut-off fittings. It is best if the fittings do not cause additional turbulence in the water stream, such as smooth-flow fittings (ball valves, gate valves). Due to the shape of the disc, dampers can unfortunately cause unfavourable turbulence in the water stream, resulting in uneven water flow rates (uneven flow rates = uneven exposure time = uneven radiation = too low local radiation dose + partial lack of disinfection effectiveness),
• it is desirable to use a bypass ensuring emergency water flow through the installation in the event of the water supply to the lamp being cut off, e.g. during possible inspections,
• in the case of a UV lamp used periodically and exposed to low temperatures (e.g. summer houses, mobile disinfection system), ensure the possibility of complete drainage of the device,
• The UV lamp should be self-venting (the air bubble will reduce the effectiveness of disinfection). Let's take this into account when installing the device. 

The UV lamp should be selected by the manufacturer or supplier of the device, however, in order to make the selection correctly, it is necessary to provide basic output data characterizing the given installation.

WATER FLOW
Water flow directly affects the exposure time, which in turn affects the radiation dose, which translates into disinfection effectiveness. Therefore, correctly determining this parameter is the most important element when selecting a lamp. Most systems are characterized by irregular water flow during the day, so the UV lamp should be selected for the maximum flow occurring periodically during the day. The minimum or average flow is irrelevant in this case.

WATER CLARITY - TRANSMITTANCE
This is an individual feature of each water. Although many suppliers and manufacturers of UV lamps rely on estimated data assigned to a given type of water, we, however, due to the correctness of the lamp selection and the associated operating costs, encourage actual measurement of water clarity.

WATER QUALITY
Defining the water quality problem is closely related to the answer to the question of why we use a UV lamp and what dose of UV radiation we should use in a specific case. If we use a lamp to neutralize a specific group of microorganisms, we should precisely indicate their type and, on this basis, select a dose of radiation that guarantees their neutralization. However, if we use UV disinfection for preventive purposes, we should refer to the type of installation/water and use the appropriate dose in this respect. Depending on the given application, it is recommended to use the following doses:

• drinking tap water - above 400 J/m²
• food industry - above 600 J/m²
• cosmetics industry - above 600 J/m²
• pool water - above 600 J/m²
• pharmaceutical industry - above 1200 J/m²
• yeast deactivation - above 1500 J/m²
•  

When buying a UV lamp, we should act rationally and refer to our ideas and hopes regarding the technical level of the lamp being purchased. We encourage a binary assessment of technical parameters when comparing lamps as well as an assessment of the distributor's credibility. The approach to determining the output parameters necessary for selecting a lamp is also important when assessing the distributor/manufacturer. A certain nonchalance in this respect or trivialization of the output data (e.g. omitting the transmittance measurement) should trigger a "yellow light" - remember that in the event of a UV lamp malfunction, the responsibility will usually fall on the buyer who accepted the output data before selecting the lamp. 

• avoid cheap lamps - the difference in price is mainly due to the quality of the components used,
• precisely define the output data for lamp selection such as: flow, transmittance (clarity) and water quality level,
• avoid lamp suppliers/manufacturers who offer and select lamps only based on the "flow" parameter,
• check the production range of lamps of a given manufacturer (the larger the better),
• check the manufacturer's references (the more the better),
• check whether a given lamp manufacturer has certificates for its products,
• assess the credibility and approach of the technical staff - their vocabulary, knowledge of the subject, level of offer, etc.,
• check whether the offer specifies the dose of UV radiation,
• check the transmittance level to which the UV radiation dose was determined (remember that the absolute majority of advertising brochures from UV lamp manufacturers and distributors refer to purely theoretical transmittance, i.e. 98-100%),
• check whether the offer includes the lamp aging factor (the dose should refer to the final operating period of the radiators), check the declared operating time of the radiators, bearing in mind that this value depends on the number of times the lamp is turned on and off,
• check the time it takes for the offered lamp to reach its operating parameters (warm-up time),
• check whether the lamp manufacturer also offers a lamp cleaning system, a flow deflector or a radiation intensity measurement,
• check service availability.