Ultra violet (UV) light is a natural part of sunlight and is a particular selection of wavelengths from the light spectrum. It damages the DNA molecules in bacteria, viruses, and other micro-organisms preventing them from replicating and surviving to cause harm. UV light inactivates within seconds micro-organisms and viruses by a photo chemical reaction within their vital DNA. This natural process allows environmentally friendly and reliable water disinfection without chemical additives. The process was developed for water filtration in the fifties and sixties and has grown in use and popularity since. The technology itself dates back to the early 1900’s.
UV systems are now in use in almost every industry worldwide including Hospitals, hotels, schools, power stations, drinking water treatment, dairies, cheese factories, fish farms, food and beverage, water fountains, electronics manufacture, pharmaceutical, cosmetics etc.
Germ killing UV light can be produced by a variety of lamps or arc tubes. The best suited are medium or low pressure mercury lamps. No micro-organisms are known to be immune to UV light. It is effective against bacteria, moulds, fungal spores, viruses and yeasts.
Ultra Violet rays are invisible and form part of the electromagnetic spectrum. They range from 10-400 nm and are situated between x-rays and visible light. The main UV spectrum is however between 200 or 400 nm and is subdivided as follows:
UV-C 200-280 nm
UV-B 280-315 nm
UV-A 315-400 nm
UV-C rays are retained by the ozone layer. The UV-B and UV-A rays reach the surface of the earth causing sunburn and sun tan and are also suitable for radiation therapy.
UV systems are normally sized to achieve at least a 99.99% kill with most sized for 99.999% kill. This is calculated by the dosage of UV light measured in milliwatt seconds per square centimeter (mWs/cm²).
NSF standards for UV systems have set the minimum UV light requirements at 38 mWs/cm² for class A point of use (POU) and point of entry (POE) devices that treat visually clear water. UV systems that do not meet class A standards should be designed to achieve a minimum 30 mWs/cm² dosage to ensure satisfactory water disinfection.
UV-C rays are germicidal since micro-organisms are sensitive to this wavelength range. Maximum sensitivity occurs at 260 nm.
UV lamps need to be replaced periodically, usually every 9-12 months to ensure adequate UV dose.
UV systems can also now be used to control protozoan parasites such as cryptosporidium and Giardia with high does rates between 80-140 mWs/cm².
UV System Advantages
- No toxic by-products produced.
- Cannot overdose the treated water.
- Treatment is not sensitive to temperature and pH differences.
- Has no smell or taste residual.
- Requires very little contact time (seconds) versus minutes for chemical disinfection.
- Simple installation, small amount of space required.
- Low running and maintenance costs.
- Does not effect minerals in the water.
UV System Disadvantages
- Not suitable for water with high levels of suspended solids, turbidity, colour or soluble organic matter.
- Unit performance is affected by microbial loading and chemical characteristics such as nitrites, sulfites, iron and hardness levels.
- No disinfection residual ie no free chlorine.
Typical UV Systems Include:
- A stainless steel treatment chamber.
- An inlet and outlet.
- A quartz lens and lamp.
- An electrical connection containing transformer, a UV lamp fail alarm and other components.
Other Options may include:
- Inbuilt surge protection or voltage regulator.
- Treatment chamber made from poly for use in corrosive environments.
- Multiple UV lamps in the one chamber
- Mechanical wipers to clean the quartz lens; either manual or automatic.
- Sensors to monitor the UV intensity connected to alarms.
- Lamp shut down when no flow is registered for a period of time.
- Telemetry systems for remote monitoring.
The size of the model of UV system required depends upon 3 main factors:
- The Water quality to be treated.
- The flow rate or quantity of water to be treated.
- The features or options required or selected.
Once this information is known or determined a particular model or number of suitable models can be selected and quoted.
Although the maintenance requirement is low the units UV chamber and quartz lens do need to be inspected and cleaned every 6-12 months. The lamp should also be replaced, o-rings lubricated valves, switches and sensors checked and tested.