How does UV work?
Ultraviolet or UV energy is found in the electromagnetic spectrum between visible light and x-rays and can best be described as invisible radiation.
In order to kill microorganisms, the UV rays must actually strike the cell. UV energy penetrates the outer cell membrane, passes through the cell body and disrupts its DNA preventing reproduction. UV treatment does not alter water chemically; nothing is being added except energy.
The degree of disinfection by ultraviolet radiation is directly related to the UV dose applied to the water. The dosage, a product of UV light intensity and exposure time, is measured in microwatt second per square centimeter (µws/cm²), millijoule per square centimeter (mJ/cm²) or Joules per square meter (J/m²). The accompanying table lists dosage requirements to destroy common microorganisms. Most UV units are designed to provide a dosage greater than 30mJ/cm² after one year of continuous operation.
Organism Susceptibilities
The resistance of different micro-organisms to ultraviolet radiation varies considerably. The approximate doses of UV 254nm energy required for the inactivation of various micro-organisms are shown below:
Doses of UV (at 254nm) in mWsec/cm² (mJ/cm²) required for the inactivation of 90%
| Organisms | Dose | Yeasts | Dose |
| Bacterium coli in water | 5.4 | Bakers yeast | 3.9 |
| Bacillus anthracis | 4.52 | Brewers yeast | 3.3 |
| S. enteritidis | 4.0 | Common yeast cake | 6.0 |
| B.megatherium (veg) | 1.13 | Saccharomyces ellipsoideus | 6.0 |
| B.megatherium (spores) | 2.73 | Saccharomyces sp. | 8.0 |
| B. parathyphosous | 3.20 | Saccharomyces cerevisiae | 6.0 |
| B. subtilis | 7.10 | Torula sphaerica (found in milk & cheese) | 2.3 |
| B. subtilis (spores) | 12.00 | Various algae | |
| Corynebact, diphteriae | 3.37 | Diatoms, Blue algae, Green algae | 360 - 600 |
| Eberthella typhosa | 2.14 | Protozoa | |
| Escherichian coli | 3.0 | Paramecium | 64 - 100 |
| Micrococcus candidus | 6.05 | ||
| Legionella pneumophila (Legionnaires disease) | 3.80 | Mould spores | |
| Micrococcus piltonencis | 8.10 | Aspergillus amstelodami (meat) | 66.70 |
| Micrococcus sphaeroides | 10.0 | Aspergillus flavus | 60.0 |
| Neisseria catarrhalis | 4.40 | Aspergillus glaucus | 44.0 |
| Phytomonas tumefaciens | 4.40 | Aspergillus niger (bakeries) | 132.0 |
| Proteus vulgaris | 2.64 | Cladosporium herbarum(cold stores) | 60.0 |
| Pseudomanas aeruginosa | 5.50 | Mucor mucedo (meat, fat, bread, cheese) | 65.0 |
| Pseudomanas fluorescens | 3.50 | Mucor racemodus A | 17.0 |
| S. typhimurium | 8.0 | Mucor racemodus B | 17.0 |
| Sarcina luta | 19.70 | Oospara lactis | 5.0 |
| Serratia marcescens | 2.40 | Penicillium digitatum | 44.0 |
| Dysentry bacilli | 2.20 | Penicillium expansum | 13.0 |
| Shigella paradyscenteriae | 1.68 | Penicillium chrysogenum (fruit) | 50.0 |
| Spirillum rubrum | 4.40 | Penicillium roqueforti | 13.0 |
| Staphylococcus albus | 1.84 | Rhizopus nigricans | 111.0 |
| Staphylococcus aureus | 2.60 | Scopulariopsis brevicaulis(cheese, etc.) | 80.0 |
| Streptococcus hemolytics | 2.16 | ||
| Streptococcus lactis | 6.15 | ||
| Streptococcus viridans | 2.0 | ||
| Tubercle bacillus | 10.0 | Viruses | |
| Most viruses are inactivated by doses of UVC at 254nm of between 1.0-10.0 mWsec/cm² |
Calculating UV Dose
UV Dose Calculation UV Dose = Retention Time x Intensity
Average Retention:
• By flow rate / reactor volume
• Min. velocity
• Max. velocity
• Head loss
Intensity:
• Lamp output
• Lamp age
• Quartz sleeve transmissivity (coating)
• Water quality (UV transmittance)
UV units for water treatment
Special low-pressure mercury vapor lamps produce ultraviolet radiation at 254 nm, the optimal wavelength for disinfection and ozone destruction. The UV lamp never contacts the water; it is either housed in a quartz glass sleeve inside the water chamber or mounted external to the water which flows through UV transparent Teflon tubes. Some ultrapure water systems use 185 nm UV units for reducing TOC (total organic carbon).
Maintenance requirements for UV units Lamp Replacement
UV lamps do not burn out as normal florescent lamps do. Instead the UVc wavelength reduces throughout the year's lifetime of the lamp, and after 1 year tends to reduce dramatically. All of our systems are sized to achieve the desired intensity or dose at THE END OF THE LAMP LIFETIME. The UVc light wavelength is invisible to the naked eye. The blue/violet light that comes from the UV lamp does not necessarily mean that the UV lamp is still destroying bacteria.
Monitoring Performance
We can fit our systems with a UV intensity monitoring system, which will measure the itensity of the UV lamps between them and the wall of the UV chamber. This can be configured so that f the UV level level drops to below a pre determined level (i.e. if the lamps are nearing their the end of their lifetime, or if the water clarity has dropped in the chamber) then an alarm or a remote signal can be activated. Water should also, ideally be sampled and tested for bacteria counts regularly.
Cleaning
As water passes through the UV unit, minerals, debris and other material in the water will deposit out and onto the quartz. This will limit the penetration of UV rays through the sleeve and into the water. To maintain high clarity, the glass around the lamp must be cleaned regularly. Cleaning frequency depends on the water quality.
Health & Safety
UVc light is dangerous, do not, under any circumstances look directly at the UV lamp.
