You Asked, We Answered
What is a UV-C dose and how is it calculated?
The UV Dose is a sum of the UV irradiance in W/m^2 multiplied the exposure time in seconds, i.e the Contact Time or 'CT'. Selecting the correct UV-C dose is dependent on water quality, UV-C light has to penetrate the water and therefore the clarity is taken into consideration, this is measured in UVT (Ultraviolet Transmittance) and is compared to distilled water at 100% clarity. Lesser water quality requires more power to make the UV-C light penetrate the water, as such (and assuming a UVT test is not available) we use an assumed table to match varying water qualities to the UV-C dose required.
Below are some examples of common water sources and the UV-C dose required.
Drinking Water: UVT typical 90 - 95% = Dose required 16mJ/cm^2
Rain Water Harvesting: UVT typical 80 - 85% = Dose required 30mJ/cm^2
Borehole water: UVT typical 95% = Dose required 16 - 40mJ/cm^2
River water: UVT typical 80% = Dose required 30mJ/cm^2
Please Contact Us should you have any queries regarding flow rates, water quality and dose rates required.
What is a 'log' reduction?
The percentage of micro-organisms inactivated by any disinfection process are commonly known as 'LOG' reductions. The below illustrates the comparison between the log and the percentage reduction.
1 Log = 90%
2 Log = 99%
3 Log = 99.9%
4 Log = 99.99%
5 Log = 99.999%
6 Log = 99.9999%
Is there a requirement for pre-treatment of water before the UV-C Disinfection system?
Depending on your water source pre-treatment may be required.
If you are considering using UV-C light to disinfect your mains water supply, this water should be clean and relatively free of particulate load. Most UV-C disinfection systems suggest a maximum particulate size of 10 microns, therefore an inline pre-filter is advisable.
If your water source is from a borehole where there may be iron and manganese present, it is advisable to install and iron and manganese filter as well as a particulate filter to prevent rust stains and discolouration in the water.
If you are recovering rain water for use in washing machines, dishwashers, toilets etc. a dedicated series of pre-filtration will be required before the UV-C disinfection unit to mechanically remove the particulate load and increase the UVT. Please contact us if you have any queries.
Will UV-C inactivate Legionella pneumophila?
Legionella pneumophila is a thin, aerobic, pleomorphic, flagellated, non-spore-forming, Gram-negative bacterium of the genus Legionella. L. pneumophila is the primary human pathogenic bacterium in this group and is the causative agent of Legionnaires' disease, also known as legionellosis. UV-C Photons at a dose of 9.4 mj/cm^2 have proven a 4 log reduction (99.99%) of Legionella pneumophila sero group 1. - Cervero-Aragó et al. 2014
Will UV-C inactivate Pseudomonas aeruginosa?
Pseudomonas aeruginosa causes urinary tract infections, respiratory system infections, dermatitis, soft tissue infections, bacteremia, bone and joint infections, gastrointestinal infections and a variety of systemic infections, particularly in people with low immune responses. UV-C Photons at a dose of 10 mj/cm^2 have proven a 3 log reduction (99.9%) of Pseudomonas aeruginosa. - Abshire & Dunton 1981
Will UV-C inactivate Cryptosporidium?
Cryptosporidium is a microscopic parasite that causes the diarrheal disease cryptosporidiosis. Both the parasite and the disease are commonly known as “Crypto.” There are many species of Cryptosporidium that infect animals, some of which also infect humans. A dose of UV-C at 6mj/cm^2 inactivates Cryptosporidium spp. at a 4 log reduction (99.99%). - Qian et al. 2004
Will UV-C Inactivate E.Coli?
Escherichia coli (E. coli) is a bacteria that normally lives in the intestines of both healthy people and animals. In most cases, this bacteria is harmless. It helps digest the food you eat. However, certain strains of E. coli can cause symptoms including diarrhoea, stomach pain and cramps and low-grade fever. Some E. coli infections can be dangerous. The most familiar strains of E. coli that make you sick do so by producing a toxin called Shiga. This toxin damages the lining of your small intestine and causes your diarrhoea. These strains of E. coli are also called Shiga toxin-producing E. coli (STEC). A UV-C dose of 10mj/cm^2 will give 99.99% inactivation of the 6 main E.coli strains whilst a 14mj/cm^2 dose will give 99.99% inactivation of 97% of E.coli strains. Chen et al. 2015, Sommer et al. 1998, Bowker et al. 2011, Sholtes et al. 2016
What is Biofilm?
A biofilm is a collection of organic and inorganic, living and dead material collected on a surface. It may be a complete slime/film or, more commonly in water systems, small patches on pipe surfaces. Biofilms in drinking water pipe networks can be responsible for a wide range of water quality and operational problems. Biofilms can be responsible for loss of distribution system disinfectant residuals, increased bacterial levels, reduction of dissolved oxygen, taste and odour changes, red or black water problems due to iron or sulphate-reducing bacteria, microbial-influenced corrosion and reduced materials life (Characklis and Marshal, 1990).
Viruses and parasites like Cryptosporidium can be trapped in biofilms and bacteria such as legionella pneumophila will propagate. When a biofilm matures and starts seeding (releasing further bacteria into the water system) a risk of either consumption, infection or further contamination is present. Pseudomonas aeruginosa, is a notorious producer of slime and can be effectively inactivated with UV-C.