Ultraviolet light for water treatment

Drinking water is essential for life, but is a valuable and scarce commodity. Lessthan 0.01% of the planet‘s 1.4 billion cubic kilometers of water is easily accessiblefreshwater. Several global mega-trends, like population growth, urbanization andclimate change, are driving water scarcity and public concerns on water quality.According to the World Health Organization (WHO), more than two million people –many of them children – die each year of diarrhoeal diseases that are caused bywater borne pathogens. Access to safe water is one of the big challenges of ourtimes and ultraviolet light for water treatment has become an essential technologyto meet it.



Drinking water is essential for life, but is a valuable and scarce commodity. Lessthan 0.01% of the planet‘s 1.4 billion cubic kilometers of water is easily accessiblefreshwater. Several global mega-trends, like population growth, urbanization andclimate change, are driving water scarcity and public concerns on water quality.According to the World Health Organization (WHO), more than two million people –many of them children – die each year of diarrhoeal diseases that are caused bywater borne pathogens. Access to safe water is one of the big challenges of ourtimes and ultraviolet light for water treatment has become an essential technologyto meet it.

UV Water Disinfection – a Safe Method and Economic AlternativeThe first UV water decontamination was installed in Paris, France as early as 1910.Quartz glass lamps – a development that goes back to the chief developer at Heraeus,Richard Küch (1860 – 1915) - are still used today, but modern high-tech UV lampsand their early predecessors are worlds apart. Today’s UV disinfection is a wellestablishedtechnology. The method is very safe and based on profound scientificknowledge. The real challenge today is to further increase the efficiency and servicelife of the lamps.

Contaminated water can be treated with high energy UV radiation which inactivatesviruses or micro-organisms such as bacteria, yeasts, fungi or even parasites. UV watertreatment has several benefits over other disinfection processes, notably chemicalssuch as chlorine and ozone, or filtration. It does not use chemicals, which makesit environmentally friendly.

The method is not pH-dependent and does not affectthe water’s qualities, like taste, odor or color. Disinfection byproducts (DBPs) withcarcinogenic or toxic effects are not formed. An all-important advantage is the fact,that pathogens cannot build any resistance to UV light. Thus, UV inactivates evenGiardia and the chlorine-resistant Cryptosporidia. UV disinfection has low overallcapital and operating costs, and is easy to maintain and operate.

Removal of Harmful Chemicals – Advanced Oxidation with UV Micropollutants, which include such chemicals as endocrine disrupting compounds,pharmaceuticals and personal-care-products have come into public focus in recentyears and are a serious threat for drinking water quality. In order to decompose thegenerally complex structures (e.g. of steroids or antibiotics) UV radiation is combinedwith powerful chemical oxidants such as ozone or hydrogen peroxide. A processknown as advanced oxidation process (AOP).

Fertilizers, herbicides and pesticidesfrom agriculture are other examples of micropollutants that can be successfullytreated with this method, as is shown in Andijk at Holland’s largest drinking waterreservoir Ijsselmeer.

Spectrum

Ultraviolet radiation covers the wavelength range from 100 to 380 nanometers.The disinfection process uses wavelengths in the UVC range from 240 to 280nanometers, while the oxidation process uses the wavelengths down into the VUVrange below 200 nanometers.

UVC light for technical applications is usually generated by mercury lamps becauseof their high efficiency in terms of electrical energy conversion into UVC light.Commonly, there are two types of mercury lamps used: low pressure and mediumpressure lamps.

Sketch of a typical low
pressure lamp

Sketch of a typical medium
pressure lamp

Low Pressure Lamps
A low pressure lamp comprises of a quartz tube with pinched filaments, is filled withrare gas (some mbar) and mercury or amalgam. The filaments are coated with emitterpaste that facilitates escaping electrons from the filament. A voltage applied acrossthe lamp provides an electrical discharge. The power density of the discharge is low,therefore only a small portion of mercury is evaporated and enters the gas phase. Therare gas acts as a buffer gas and is necessary for maintaining the electrical discharge.Mercury atoms are ionized and excited in the discharge by electron impact. Excitedatoms emit very effectively photons with two wavelengths: 254 nm and 185 nm inthe UV range (often called spectral lines). 185 nm emission can be filtered out bychoosing quartz of suitable transmittance.

Disinfection process

The 254 nm emission is well absorbed by DNA ofall microorganisms and viruses. This absorptionleads to a destruction of the genetic structureof DNA and inhibits the transcription of itsinformation. The microorganisms and viruses arebiologically inactivated, thus 254 nm emission issuitable for disinfection purposes.