Water disinfection using UV-C

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Updated - October 28, 2024

Water disinfection can be carried out in various ways. The safest and simplest solution is irradiation with UV-C (wavelength 200 ... 280 nm). This method is used in laboratories, clinics, industry, aquariums and households alike. Microorganisms such as bacteria, viruses, yeasts and fungi are killed 99.9 % at a suitable flow rate per time unit and irradiation intensity.

Studies

Among water specialists, UV-C purification is occasionally discussed with regard to the inactivation of viruses. The argument is that viruses are smaller than bacteria and would not be inactivated by UV-C.

The Study situation However, this confirms the inactivation, also of viruses, of course always depending on the dose rate.

Theory …

However, sterilization is not the same Filtering. Contaminating particles are only removed using appropriate filters! In addition, contaminated water reduces UV-C transmission and thus the effectiveness of UV-C radiation.
For example, at a distance of 1 cm from the radiation source (corresponding to 80 % of the 100% radiation power, after 17 mm still 70 %) ultrapure water has a transmission of 99 .. 100 %, drinking water 85 .. 98 %, rainwater 75 .. 85 % and Wastewater 50 .. 75 %.

Radiation dose

The radiation dose is calculated from the time (t) multiplied by the intensity (I) of the UV-C lamp and is in mJ/cm² measured.

The dose required to kill microorganisms is, for example:

pathogen	Dose mJ/cm2
Clostridium tetani	4,90
Hepatitis A	11,00
Infective pancreatic necrosis	60,00 *
Klebsiella pneumoniae	11,00
Legionella pneumophilla	2,04
Poliovirus	12,00
Pseudomonas aeruginosa	11,00
Rotavirus	35,00
Saccharomyces cerevisiae	6,00
Salmonella typhi	14,00
Salmonella typhimurium	13,00
Spores of the bac. subtilis	12,00
Staphylococcus aureus	36,00
Streptococcus faecalis	4,50

As a rule, the dose information for the devices is in J/m² or mJ/cm² specified. In terms of the designated effectiveness of at least 90% in killing bacteria and viruses, they refer to the respective flow rate.

*Values above 40 mJ/cm² require the use of a second equivalent stage for flow rates above 4 ... 7 liters/min. require the use of a second equivalent stage in order to also reliably detect these pathogens, taking into account possible photoreactivation on the part of the photolyases. These enzymes may repair the cell damage to the DNA caused by the UV-C light. To counteract this effect effectively, doses of around 44 mJ/cm²or 440 J/m² necessary.

Halving the flow rate results in a doubling of the dose, which in turn increases the disinfection capacity by a factor of 10. The 90 percent disinfection then increases to 99 %. Tripling the dose requires 99.9 % disinfection.

This means that the above-mentioned photoreactivation at half the flow rate is already taken into account with one unit.
However, if you intend to sterilize highly contaminated water and require higher flow rates, you are well advised to use a second and possibly third stage.

... and the practice

Pressure pumps installed in mobile homes, e.g Shurflo, deliver up to 7 liters/min. One Purway 500 UV-C sterilization system (12 or 24 V DC) is for up to 400 liters/h at 400 J/m² and operates at 254 nm with a UVT (UV transmission) of 90 %. The flow rate is 6.7 liters per minute. Due to the small pipe cross-sections of 10 ... 12 mm, the Purway system delivers a maximum of 1.1 liters/min. This results in an efficiency of > 99.99 %.

It makes sense to install the system with a so-called monitoring unit. This warns when the lamp's service life has been reached in order to ensure a consistently high disinfection capacity.
It should be noted that the dose rate of the lamp decreases over time. Therefore, either the total dose rate should be oversized accordingly or a faster lamp replacement interval should be planned.

It is also recommended to order a replacement UV-C lamp. This is less in view of the expected service life, but rather because of the premature defect, which is highly probable according to Murphy's law...

Choice of installation location

You will not need germ-free water in the shower or bathroom. But probably in the kitchen. The cold water pipe just before the tapping point is therefore a good idea. There is usually enough space on the back wall of the sink cabinet to integrate the 32 cm long UV unit into the water hose.

In addition to two matching hose nozzles and hose clamps, you also need a 12/24 V DC connection. This is where the water pump comes in handy. A two-wire cable, laid along the water hoses towards the pump and connected there parallel to the power supply, is sufficient if a suction pump is activated via the tap.

Special feature pressure pump

For intermittently running pressure pumps, a delay circuit must be installed in parallel, which in turn takes over the power supply to the disinfection system so that it remains switched on for an adjustable period of time even after the pressure pump has been switched off and water extraction has been resumed.

This is accomplished by one Control electronics, which delays the switching off of the supply voltage for the disinfection system for an adjustable period of time.
It is connected directly to the on-board voltage of 12 or 24 V. At the signal input, a cable is connected in parallel to the outgoing plus line of the pressure switch of the pressure pump. When the pump starts, the delay time is started by the activated plus level and the pressure pump is switched on and the disinfection system is kept switched on for the set period of time.

ps If you need personal support in the implementation for a fee, you are welcome to Ticketing make!