Liquids can be treated in a similar way to air: the further the UV radiation is able to penetrate the liquid, the more effective it is. The degree of efficiency strongly depends on the liquid and its absorption coefficient at 254 nm. For example, the transmittance of natural water at 254 nm can vary by a factor of 10 from place to place.

UV radiation often replaces or supplements conventional chlorination as it produces fewer harmful by-products and is not affected by the pH value of the water or its temperature. Under the effect of UV radiation, germs are killed in the water as it flows through the irradiation chamber when the UV lamp is switched on. The disinfecting effect of the UV light is instantaneous. No chemicals need to be added and the quality of the water (color, smell, taste) is not affected. A mutable effect of the UV light is excluded.

When a UV lamp is immersed in a liquid, it should be enclosed in quartz glass or a UVC-transparent PTFE sleeve. Designs for disinfecting liquids can look as follows:

• A lamp, enclosed by a quartz glass container or similar material (high transmission at 254 nm radiation), surrounded by the liquid to be disinfected. A variety of such arrangements can be used within an enclosing container.
• A quartz glass tube (with high transmission at 254 nm radiation), which transports the liquid and is surrounded by a group of lamps with reflectors or UV lamps with integrated reflectors.
• Irradiation with the help of lamps installed in reflectors, or UV lamps with integrated reflectors mounted above the liquid.

Decentralized Drinking Water Treatment

UV installations are suitable for industrial, commercial, and domestic use. The quality of the water is important for the performance of UV systems. Separate treatment technologies are often required to improve the water quality before disinfection. For example:

• Sediment filters (to remove particles that shade microbes)
• Carbon filters (removes organic compounds and unwanted odors)
• Water softeners

UV radiation is used in conjunction with reverse osmosis applications. Disinfection before RO systems increases the durability of the RO membrane by reducing the accumulation of bacterial biofilms.

Pools and Swimming Pools

With UVC as a supplement to the traditional method of water treatment, significantly less chlorine is needed for the same result. UVC cannot be used alone, as solids and inorganic compounds in swimming pool water circulation must be considered, which require filtration and chemical processes with residual effects. A standard technology is to disinfect a portion of the water in a side-stream through a UVC device. In combination with chlorination, chlorine dosage can be reduced by up to 70%.

Process Water (Beverages)

Drinking water dispensers store and dispense water. These dispensers must be approved by the health authority. For approval, it is required that the dispenser is equipped with a disinfection unit to reduce the number of bacteria and other microorganisms. For drinking water dispensers that use bottled water, a disinfection unit is not required. However, such devices are also at risk of bacterial biofilms developing in their reservoirs. These biofilms act as breeding grounds for bacteria, protected by a gel-like substance. To prevent biofilm growth, simple UV reactors can be used.

Wastewater

Chlorine has been used for over a century to disinfect water. Despite its effectiveness, it is associated with environmental issues and health impacts. It has been shown that UV irradiation is an environmentally friendly, convenient, and cost-effective way to disinfect public wastewater. The required UV dose depends on the preceding processes and ranges from 50-100 mJ/cm², considering the flow rate.

Drinking Water Treatment in Water Treatment Plants

Severe outbreaks of cryptosporidiosis caused by drinking water in North America show that existing disinfection and filtration technologies cannot guarantee the elimination of Cryptosporidium oocysts. Cryptosporidium parvum is a human pathogen that causes diarrhea infections, which can be fatal. The organism can be deposited as an environmentally resistant form (oocyst) and can live for months. Cryptosporidia are nearly completely chlorine-resistant. Studies have shown that UV light achieves significant inactivation of Cryptosporidia even at moderate doses. A low irradiation dose of just 10 mJ/cm² already leads to a reduction in concentration by more than 4 log levels.

Fish Ponds

Fish pond owners often face problems with phototropic microorganisms (typical water organisms found in both fresh and saltwater). When algae need to be destroyed or inhibited in their growth, a high dose of UVC or long irradiation is necessary. This can easily be achieved by installing a closed-loop system where the water is exposed to the UVC source several times a day. The lamp is protected by a quartz immersion tube. In practice, it has been found that, for example, a 5W UV lamp connected in series with a filter can keep a 4.5m² pond clear of water.

Aquariums

In aquariums (whether fresh or saltwater), two problems always arise: algae and parasites. Warm water provides excellent conditions for microorganisms, and aquarium lighting contributes to algae growth. Here, we recommend the same system as for fish ponds. A low pump speed achieves a long retention time along the lamp, which benefits both a high bacterial kill rate and algae aggregation. UVC in aquariums also destroys parasites, thus eliminating fish diseases. This treatment provides an effective solution, especially for floating ciliates. Microorganisms cannot multiply, and aquariums are freed from parasites in a short time, with affected fish soon showing no disease symptoms.