In addition to the conventional processes for cleaning water -- such as the use of chemicals like chlorine and ozone or filtration systems -- the importance of ultraviolet light as a reliable, environmentally friendly and economically viable alternative is ever increasing.
Communal wastewater, after being cleaned in clarification plants, is reintroduced into the water cycle, into rivers, lakes and coastal waters. Special treatment is necessary if, at subsequent points, drinking water is extracted or wastewater is fed back to bathing waters. If the wastewater has a high germ count then the hygienic conditions for bathing become doubtful as there are significant dangers of infection for humans.
Project "Clean Isar" -- A Decision for UV
The city of Munich and the State of Bavaria started a project to improve the hygienic water quality of the Isar river, to ensure safe bathing during the summer months. One part of the Isar is diverted into the Isar canal and used for energy generation. As a result, the remaining Isar between the cities of Bad Toelz and Freising carries very little water. By introducing cleaned wastewater from the clarification plants in this area the degree of contamination was high, in spite of sufficient treatment, and the water was heavy with pathogens.
The Europe-wide, unique "Clean Isar" project included the decision to use UV wastewater disinfection as the final cleaning stage in the sewage treatment plants which fed the wastewater back into the river. The aim was to significantly reduce the germ count of the waste waters and so improve the hygienic water quality of the Isar in the bathing season, while meeting the severe EU bathing water regulations. In 2000, the Bavarian State showed evidence from the pilot scheme for wastewater disinfection in Bad Toelz that bathing water quality could be achieved in the Isar. Hygienic considerations are decisive for the bathing water quality. Contamination of the water, for example by pathogens, can cause illnesses in the bathers. Bacteria and viruses play an important role, particularly, faecal germs, streptococci, salmonella and enteric viruses.
The treatment of water with UV radiation is a very effective physical process for reliably disinfecting water and breaking down pollutants. The energy-rich radiation, at wavelengths of around 254 nanometers is absorbed by the DNA and destroys its structure. As a result, the UV rays, in a matter of seconds, destroy the cells of the pathogens in the water, such as bacteria, micro-organisms, viruses, fungi or parasites. In addition, chlorine-resistant parasites such as Giardia and Cryptosporidia can be killed by UV light. Pathogens cannot develop any resistance to UV light. An important benefit is that this cleaning process takes place completely without chemicals and hence there are no chemical residuals.
UV light is versatile in its application -- it can be used to treat water, air and surfaces. Drinking water treatment in waterworks, wastewater treatment in sewage treatment plants or process water treatment in industry (typically, the recycling and re-use of process water) are the interesting applications of UV in the water industry.
UV disinfection becoming more accepted
The physical method of disinfection with UVC light is a reliable and economical technique, which is often used for the disinfection of water. For example, the city of New York has chosen this technology for drinking water disinfection for new waterworks which are planned for the Catskill/Delaware catchment area about 160 kilometer north of New York city. This will be the largest UV disinfection plant for drinking water in the world. From 2010, the new waterworks will treat up to 2.2 billion gallons (8.3 million cubic meters) of water per day for more than nine million people in New York city and its environs.
In recent years, the acceptance of UV disinfection in commercial applications has continued to grow. UV disinfection has been successfully used for wastewater treatment in the USA for decades and is now becoming increasingly specified in Europe, as in the Isar project.
The town of Freising has also setteled for wastewater disinfection through UV radiation to provide hygienic wastewater and clean water in the Isar. The sewage treatment plant in Freising treats around 5 million m³ of wastewater (per day)f rom the town of Freising, its commercial companies (Dairy, breweries) and eight neighbouring townships. Within the scope of the "Clean Isar" project, the Freising plant was completely modernised and enlarged. Today the clarification plant is designed for an 110,000 inhabitant value. In addition, the water rights application meets stricter limits and the escapage loss has been drastically reduced. The plant expansion includes a new biogical treatment plant, settling tanks, a fish pond, new CHP plants and a UV disinfection plant.
Freising decided upon a plant from the Canadian manufacturer TROJAN technologies. This was installed in September 2005 and began operation in the summer of 2006. An important plus point was the sophisticated wiper technology of the TrojanUV3000Plus system, which continuously removes any dirt or deposits from the quartz safety tubes.
This possible coating, which is often ferrous, leads to a reduction in the UV intensity in the water.
The UV plant in Freising is fitted with 196 UV lamps, each of 250 Watt output. The UV lamps are contained in UV-transparent Quartz glass tubes, which are tightly sealed. Two open channels work with two corresponding banks, which are each fitted with six modules. The plant is self-regulated according to the wastewater throughput. Depending on the throughput and the water transmission, one channel is operated with the necessary number of modules. The lamp power can be controlled between 60% and 100% intensity. An SPS controller automatically regulated the operating mode. A UV sensor continually monitors the UV lamp intensity and an on-line measurement of water transmission matches the power to the demand.
The Freising plant is designed for a maximum of 652 litres per second (2350 m³/hour). Maximum input is 611 liters per second. This ensures that the capacity of the UV plant is sufficient to handle the maximum expected wastewater input. The plant delivers a UV dose of 306 Joule/m², which ensure certain UV disinfection.
An external, independent laboratory takes monthly samples and tests the water quality. The samples are taken before and after the UV plant and look out for faecal germs and streptococci. The regional authority also monitors the wastewater quality. This takes the form of unannounced, monthly sampling to check bathing water quality.
Willi Frankl, manager of the Freising sewage works says of his experiences, "The plant stands or falls on the UV lamps. The operating life of the lamps is a decisive factor. The later I have to change over lamps, the more economical the UV plant."
The energy efficiency of the UV disinfection plants in communal clarification plants can be improved by using particularly efficient UV lamps. The challenger for today's UV lamps is to significantly increase their efficiency and operating life.
Different lamp technologies for water treatment
The disinfection of water can involve the use of compact medium pressure UV lamps, low pressure UV lamps and high power amalgam lamps.
Medium pressure lamps give a broad band spectrum over the complete range from 200 to 400 nanometers. Their high radiation flux allows very good disinfection from compact units. Similarly, with high throughput, it is possible to design very compact disinfection systems. For example, these lamps are used on board ships where space is very limited to disinfect ballast water. Typical lamp operating life is 1500 to 5000 hours.
Low pressure lamps emit radiation at a wavelength of 254 nanometers. Classical UV low pressure lamps offer exceptional efficiency: up to 40% of the electrical power is converted into UV radiation for disinfection. However, the power density is limited and a large number of lamps is often required. When synthetic quartz glass is used as the lamp material, additional UV radiation at 185 nanometers is emitted and this can be used for oxidation processes. The first lamp change is normally after 8000 hours. After this the lamps exhibit a drop-off in UV intensity of up to 50 percent.
However, there are significantly longer life lamps. For example, the specialist light source manufacturer, Heraeus Noblelight, has developed new high power, amalgam lamps, which operate for up to 16,000 hours with virtually constant UV output and so provide significantly more power than conventional standard low pressure lamps, with their usual operating life of 8000 hours. A unique longlife coating doubles the life of amalgam lamps. This new technology not only improves the useful life of the lamps but also ensure a virtually constant UV output over the length of this working life.
In addition, with the new coating, high power amalgam lamps can eliminate the transmission loss of quartz glass which is associated with conventional lamps, so that a virtually constant disinfection action is achieved over the complete operating life. With conventional UV lamps, mercury diffuses into the quartz, so that such lamps deliver only 50% of the original UV power after 8000 hours. The new technology allows up to 16000 hours of operation at virtually constant UV output and consequently significantly more power overall than conventional lamps.
In comparison with previous lamp technology, UV amalgam lamps offer the best combination of efficiency and operating life. Thanks to the higher UV output and the long operating life, system builders now need fewer lamps in any new disinfection plants they design. As a result there is significant potential for saving in the number of lamps, in system components, in energy demands and in maintenance and service costs. End users, such as waterworks and clarification plants, profit from the long operating life. The time between lamp changes is virtually doubled while the operating time provides the lamps with more UV power than conventional low pressure lamps.
Outlook: New areas of application for UV radiation
The increasing environmental pollution caused by medications in wastewater is becoming more of a problem. In order to destroy the very complex pharmaceuticals such as steroids and antibiotics, it is sensible and effective to use a combination of UV radiation and powerful oxidation substances such as hydrogen peroxide. UV radiation at a wavelength of 185 nanometers makes even higher energies available and allows oxidization processes, which can break down health-threatening chemicals in water.
Heraeus Noblelight GmbH is one of the technology- and market-leaders in the production of specialist light sources.
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