Ultraviolet Disinfection

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What Is Ultraviolet Disinfection?

The small part of electro-magnetic spectrum having wave lengths included between 100 and 400 nm (thousandths of micron) is defined as space of the ultra-violet irradiation. The UV-C are part of the subspace characterized by the wave lengths included between 100 and 280 nm. Electro-magnetic waves with different wave length and width induce interactions with the matter of different nature, the UV-C irradiation with l= 254nm is particularly interesting thanks to its marked germicidal power.

Water is passed through a system to flow beside a lamp or series of lamps to expose the water (and organisms in the water such as bacteria, algae, fungi and viruses and other organism and other chemical products) to the UV-C which, if sized and designed correctly in the system, will lower the rate of these organisms/ products in the water.

How They Operate

Water flows in one end, then along the length of the chamber close to the quartz sleeve inside, then out the other side.

In nature the UV-C rays are part of the secondary cosmic irradiation which, interacting with the high strata of the terrestrial atmosphere, generates ozone and with a lower energy, it reaches ground as tanning UV(A+B) irradiation. Artificially UV-C rays are produced with the help of special fluorescent lamps containing mercury fumes, these lamps are made of a very pure quartz (>99.99% SiO2) transparent to the UV-C light which they emit in an almost monochromatic form (>95% of Ć = 254nm).

The high germicidal power of this wave length must be sought in DNA and in the link of its fundamental components. DNA is a macromolecule present in all living organisms in which all information necessary for life and reproduction reside. The alteration, induced by the UV-C irradiation, of some chemical links present among nucleoids is able to change the information contained and conveyed by DNA, these alterations prevent its normal activity, and this irreversibly leads to the cellular death.

In nature the UV-C rays are part of the secondary cosmic irradiation which, interacting with the high strata of the terrestrial atmosphere, generates ozone and with a lower energy, it reaches ground as tanning UV(A+B) irradiation. Artificially UV-C rays are produced with the help of special fluorescent lamps containing mercury fumes, these lamps are made of a very pure quartz (>99.99% SiO2) transparent to the UV-C light which they emit in an almost monochromatic form (>95% of Ć = 254nm).

System Position
Dirty water reduces the UV’s efficiency (called UVT – Ultra Violet Transmittance) so they must be positioned after primary filtration and ideally at the cleanest point in the system to minimise the size of the units. Often they are placed after biofiltration and ozonation also. UV sterilisers should generally be plumbed with a by-pass to allow them to be serviced while not disturbing normal operation. These valves can also be used to adjust the flow to ensure within the design specification or requirements of the system. Flow meters can help ensure this optimal operation or otherwise the design can ensure the flow rate is never exceeded.
Sizing a UV
UV effectiveness is proportional to intensity and exposure time and can be expressed in a number of ways such as (mJ/cm2).  There are several key things
  • Flow rate
  • UVT
  • Solids filtration
  • What the target organism/product is 
These are used to extrapolate the desired unit relative to the dose rate and flow rate. There is much published information for destruction rates for various organisms/products, these should be used where possible whilst using a high degree of precautionary principle to ensure there system functionality. The UVT should be tested several times under worst case scenarios where possible (or known values used) prior to sizing a vessel.
Service and Maintenance
  • Regular replacing the globes as per manufacturer guidelines (Replace Lamp once every 8-20 months when used 24hr per day, Quartz sleeve only if cracked)
  • Clean quartz sleeves every 2-6months
Compatibility
UV systems are generally an industry standard in aquatic systems due to the myriad of benefits and low impacts of their cons. They are great as a minimum to maintain basic water quality control for algae. They do not remove the source of the algae and bacteria but rather it is to help prevent build up.
Pros and Cons
PROS:
  • Eliminate algae and other unwanted organisms (Bacteria, Fungi, Pathogen)
  • Can destruct chlorine, ozone in other applications (intake/waste treatment)
  • Controls disease outbreaks
  • Zero water use
  • Ease of use
  • Can reduce water turbidity by reducing bacterial and algal blooms
  • User friendly and relatively safe
  • Chemical Free
  • Environmentally Friendly
  • Unlimited in size of application
CONS:
  • Can seem expensive depending on the size of the system.
  • Regular cleaning required every 4 months or so
  • Lamps and quartz sleeves are fragile when cleaning
  • Bypasses, plumbing and fixtures need to be included in upfront costs
  • UV light is only effective if unit is sized appropriately and at the specified UVT
  • Adds heat
  • Lamp must be replaced – 8-20 months depending on supplier (include in OPEX)
  • Power Consuming (on for 24h)
  • Problems of penetration in water (50mm) (1-3mm for 100% kill) Expensive and bulky for large scale applications.
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