Ultraviolet radiation - state-of-the-art method of water disinfection

Abstract

Development of medicine and sharpening of the problem of environment pollution resulted last years in much more strict requirements to the level of waste and natural water disinfection. Therefore, chlorination being the main conventional method of water disinfection, is not able any more to meet all modern sanitary and environmental challenges.

At present time, disinfection of water with ultraviolet (UV) radiation is the real alternative to chlorination. High efficiency and reliability of UV method is confirmed by the many years' experience of its application in operating water treatment plants with different capacities.

Municipalities are the main field of the application of UV equipment. UV radiation is used for disinfection of underground and surface clean water as well as biologically treated wastewater. Application of UV disinfection allows to significantly reduce or totally exclude chlorine as well as to avoid the formation of unwanted disinfection by-products in water. Application of UV technology on water treatment plants provides a highly efficient barrier for microorganisms resistant to chlorine like viruses and protozoa. For the last years UV has been widely introduced for disinfection of water in swimming pools, local water supply systems (both individual and collective), for disinfection of hot water and in industrial water supply systems (process, reused, ultra-pure water).

Modern UV equipment covers wide range of flow capacities from 1 to 3000 m3/h per one unit. This equipment may be used in both self-flow and pressurized water systems. UV equipment has compact design, low power consumption and long lifetime of germicidal lamps (up to 12 000 hours). For the past ten years more than 1000 municipal and industrial UV installations have been launched in Russia . Among them are the largest in Europe UV stations for disinfection of drinking water (with a flowrate of 405 000 m3/d) and sewage (290 000 m3/d).

Disinfection of clean and waste water is the actual problem in most of the regions and counties since it is directly connected with human health. The fact that water can be the cause of mass disease has been known from ancient time. Despite present achievements of medicine the epidemiological problem of waterborne disease is still urgent. Development of hygienic science and sharpening of the problem of environment pollution bring about a constant tendency of making sanitary requirements for drinking water more and more strict all over the world and in Russia, in particular. The same is for hygienic requirements for quality of treated wastewater.

At the same time the bioassay tests of water quality stimulated by introduction of new strict regulations show that chlorination - the main conventional method of water disinfection - is not always able to meet new water quality norms on microbiological parameters such as viruses and protozoa cysts. Furthermore, chlorine generates toxic chlororganic compounds which content is limited by water quality regulations of many countries.

Thus, one of the global and perspective tendencies now is to modernize technological processes of water treatment by changing chlorination technique and applying new alternative methods of water disinfection. This is especially actual in treatment of water from surface sources in many countries and in Russia.

Ultraviolet (UV) method as a mean of bacteria destruction has been known for more than 100 years. It is based on exposure of treated water to UV radiation of germicidal spectrum produced by gas-discharge mercury lamps. First UV installations for water disinfection were launched in Germany and France in 1910. In 80s attempts to solve a problem of chlororganic compounds in water and search for alternative to chlorine stimulated the growth of interest in UV method in several developed European countries and USA. During this period efficient UV light sources and UV equipment with competitive technical and cost-effective characteristics were developed.

During the last ten years, the application of UV disinfection in drinking water and wastewater treatment processes has gained a mass character. In Russia alone there are more than 1000 water treatment plants operate now on base of UV equipment manufactured by Research and production enterprise - LIT Technology, Moscow . By this the major fields of UV application are drinking water supply and sewage systems at both municipal and industrial companies.

Disinfection of underground water

is the most typical field of application of ultraviolet radiation. Last years' researches showed that filtration of water through the ground does not always secure against ingress of viruses into underground water source. In this regard there were carried out a lot of experimental works connected with UV application for viruses destruction in water. UV radiation dose of 16 mJ/cm² was found to be able to provide log 3 reduction of hepatitis A viruses and log 2 reduction of rotaviruses concentration. To achieve higher level of inactivation UV radiation dose can be increased. By this even relatively high UV radiation doses do not form any by-products in water negative to living organisms.

At present time in Russia there are a lot of UV systems of high flow capacities installed at water treatment plants of municipal and industrial enterprises that use underground water sources.

Application of UV radiation in treatment of clean water from surface sources

is relatively new field of application. However, technological schemes developed recently and operating experience show that application of UV allows to solve typical problems in this field. First of all it is connected with the fact that application of UV allows not only to strengthen reliability and performance of a treatment plant against viruses and protozoa cysts, but also makes it possible to exclude primary chlorination and thereby to reduce formation of chlororganic compounds.

Experimental investigations of UV radiation effect (254 nm wavelength) on pathogen cysts (Lamblia cysts and Cryptosporidium oocysts) in dechlorinated tap water were carried out. It was found that complete destruction of these pathogens at initial concentration of 100 units/cm3 is achieved with UV radiation dose of 16 mJ/cm2. Since the content of Lamblia cysts and Cryptosporidium oocysts in natural surface water sources very rarely amounts to several tens per one liter, UV radiation doses of 16-40 mJ/cm2 are quite enough to provide high epidemic safety of potable and recreation water according to protozoa index. These facts were confirmed by numerous experimental studies carried out at UV systems in operating water treatment plants.

Full-scale UV disinfection systems are now used on tens of surface water treatment plants in Russia. Among the largest of them are the UV plant in City of Otradnyi with flow capacity of 75 000 m3/day and the largest in Europe UV system in City of Togliatti with flowrate of 405 000 m3/day.

UV disinfection of wastewater

is one of the most prospective fields of UV method application. Wastewater is the main source of microbe pollution of environment, surface and seawater, underground water supply reservoirs, drinking water and soil. In practically all developed countries of the world there is a tendency to eliminate chlorination and to replace it by UV disinfection systems. Application of UV radiation allows not only to provide efficient disinfection of wastewater but also to escape from dangerous and complicated chlorination stations from water treatment plant and to exclude toxic chlorine and chlororganic compounds from wastewater.

UV disinfection of wastewater is widely introduced in Russia on many mid- and large-scale wastewater treatment plants. Among them is the largest in Europe wastewater UV system with flow capacity of 290 000 m3/day on municipal WWTP of Togliatti city (see Fig. 3). Recently the first stage of UV system at municipal sewage treatment plant in the city of Samara has been put into operation. Total flow capacity of this WWTP is one million m3/day and full-scale UV system is expected to be launched in earlier 2004.

Application of UV for disinfection of reused water in swimming pools

allows to reduce concentration of residual chlorine down to the level when there is no allergic effect on swimmers (from 1.2 mg/l down to 0.1-0.3 mg/l). For swimming pools with open cycle of water UV radiation is used as an independent disinfection method without any chlorination at all.

Besides application fields mentioned above, UV radiation is widely used for disinfection of water in local water supply systems of both individual or collective type (hospitals), for disinfection of hot water and in industrial applications (process, reused, ultra-pure water) like food and beverage industries, pharmaceutics, cosmetics, microelectronics, fish farms, irrigation.

At present time there is a variety of UV systems with different designs and wide range of flow capacities designed for disinfection of clean and wastewater of different quality.

• Modern UV systems ensure precalculated UV radiation dose that guarantees conformity of treated water to any sanitary and bacteriological standards.

• There are different design modifications that allow to incorporate UV disinfection system into any process chart of existent water treatment plants.

• Latest advances in light source technology resulted in creation of effective germicidal lamps with high UV radiation power output (5-6 times higher than previously), enhanced UV-C efficiency of 35…45% and extended lifetime of more than 12 000 hours. All this made UV technology cost-effective and highly competitive in comparison to other disinfection methods.

• State-of-the-art control systems designed on the base of microprocessor technology provided high reliability, simple maintenance, low power consumption of UV systems and extended lifetime of germicidal lamps.

Conclusion

Enhanced technical and maintenance parameters of manufactured UV equipment along with recent advances in technological aspects of UV method provided conditions for widespread application of UV equipment for disinfection of water in different municipal and industrial fields.