From a treatment view, liquids can be regarded as similar to air so the more the UV radiation is able to penetrate the liquid, the merrier efficient is its action. The degree of efficiency thus greatly depends on the liquid and more particularly its absorption coefficient at 254 nm. As an example, natural water's transparency to 254 nm may vary by as much as a factor of 10 or more from place to place.
Often UV radiation may supplement or replace conventional chlorination measures. UVC has advantages over chlorinating techniques, because it produces far fewer noxious by- products and it is unaffected by the pH of the water or its temperature. Disinfection with UV radiation is reached by floating the water through the reactor. No additional chemicals are needed. Characteristics of the water (like smell, taste and colour) are not influenced by radiation. There is definitely no mutagen impact of UV radiation.
If an UV lamp has to be immersed in a liquid, it should be enclosed in a quartz or UV-C transparent PTFE sleeve. Installations for disinfecting liquids may have the following forms:
- One or more lamps enclosed in a quartz container or one of similar material (with a high transmittance at 254 nm), which is surrounded by the liquid to be disinfected. A multiple of such configurations can be used inside one outer container.
- A quartz tube (with high transmittance at 254 nm) transporting liquid surrounded by a cluster of lamps in reflectors or by an integral reflector UV lamp.
- Irradiation by means of lamps installed in reflectors or integral reflector UV lamps mounted above the surface of the liquid.
FIELDS OF APPLICATION
Purification of drinking water
UV installations are suitable for industrial, commercial and residential markets. Effectiveness of disinfection is considerable influenced by the quality of water. Separate treatment technologies often are required to improve the water quality before disinfection:
- Sediment filters, to remove particles that »shadow« microbes or absorb UV-light
- Carbon filters, which remove organic compounds and undesirable odours
- Water softeners to reduce hardness
UV is often used in conjunction with Reverse Osmosis (RO) applications. Disinfection prior to the RO systems increases the durability of the RO membrane by reducing the accumulation of bacterial bio films.
Spas and swimming pools
With UVC as a supplement, chlorination methods need less chlorine for the same result. The reason that UVC is not suitable for sole use is that swimming pool water circulation has to take into consideration solids, inorganic compounds, hence filtration and chemical processes are also needed. A standard technique is to circulate part of the water through a continuous flow UVC device, thus creating a partial closed loop system; this in tandem with the chlorinator produces effective disinfection. It can lower the chlorine dose up to 70%.
Water coolers, dispensers
Water vending devices store and dispense water that is non-chlorinated. The machines must be licensed by local health service departments. One of the requirements for the license is that the vending machine is equipped with a disinfection unit to reduce the number of bacteria and other microorganisms. Bottled water coolers, which also dispense non-chlorinated water, are not required to contain a disinfection unit. However, without an active disinfection system, also bottled water cooler reservoirs are subject to bio film growth. Such bio films act like a breeding place for bacteria, protected by the gel-like substance. Bacteria contamination, regardless of whether it is non-harmful or even beneficial, is not a quality to be associated with drinking water. To avoid bio film growth simple UV reactors can be used.
Chlorine has been used to disinfect waste water for over a century. However, while chlorine is very effective, it is also associated with environmental problems and health effects. UV irradiance has proven to be an environmentally responsible, convenient and cost-effective way to disinfect public waste water discharges. The required UV dose levels depend on the upstream processes, and range, taking into account flow rates and UV transmittance of the water, between 50 and 100 mJ/cm².
Industrial (municipal) drinking water
A few fatal waterborne outbreaks of cryptosporidiosis in North America have proven the fact that existing disinfection and filtration technologies could not guarantee to eliminate cryptosporidium oocysts from the water. Cryptosporidium parvum is a human pathogen, capable of causing diarrhoeal infections, sometimes even leading to death. The organism can be shed as an environmentally resistant form (oocyst) and persists for months. Cryptosporidium is almost completely resistant against chlorine. Recent studies have verified that UV can achieve significant inactivation of cryptosporidium at very modest doses. Exposures as low as 10 mJ/cm² will result in a more than 4- log reduction of concentration.
Fish ponds owners are often troubled by phototrophic microorganisms. These are typical water organisms widely distributed in both fresh and salt water. To destroy algae or to inhibit their growth, either a high dose of UV 254 nm radiation is needed or a long irradiation time. These conditions can be met relatively easily by creating a closed loop system whereby the water is presented to the UVC source a number of times per day. The lamp is encased in a quartz tube. In practice, it has been found that, for instance, a UV 5W lamp in series with a filter can keep a 4.5m³ water pond clear.
Concerning aquariums two problems can occur: one is that they become swamped with algae; the second is that parasites may cause fish diseases. Warm water provides an excellent condition for microorganisms and the lighting features used also promotes algae growth. The same system as used for ponds is advocated. A low pump speed will create a long dwell time across the lamp, so helping both bacteria kill rate and algae agglomeration. Using UVC in ponds and aquariums is also beneficial because it can destroy parasites introduced by new fish; the latter can be catastrophic in many cases. UVC treatment provides an effective solution particularly for suspended zoospores. Multiplication does not take place and aquariums can be free of parasites within a very short time. Even affected fish soon cease to display symptoms of morbidity.