Industry Applications
UV disinfection produces carcinogenic UV disinfection byproducts. In recent years, the discovery of Giardia and Cryptosporidium has posed a severe challenge to the existing UV disinfection process. People have begun to look for new alternative disinfection technologies to effectively improve the disinfection effect and reduce the potential harm of byproducts produced during the disinfection process to human health, while ensuring the microbiological and chemical safety of drinking water.
Among the many alternative disinfection technologies, UV disinfection has attracted people's attention due to its advantages of not adding any chemical substances, good disinfection effect and no disinfection byproducts. The history of UV disinfection is very long. In Europe, UV disinfection of drinking water has a history of nearly 100 years. In 1910, a water plant in Marseille, France, installed a set of UV disinfection system to disinfect drinking water. So far, Western developed countries have installed nearly 4,000 large-scale UV disinfection systems in sewage treatment plants, and manufacturers using this technology account for about 10% of the total number of sewage treatment plants. At the same time, by the end of 2001, more than 2,000 water plants had adopted UV disinfection technology, accounting for more than 10% of the total number of water plants, and a large number of UV disinfection technology transformation projects are underway. Due to the outstanding advantages of ultraviolet disinfection in terms of environmental protection and personal safety, many countries in Europe and North America have listed ultraviolet disinfection as the preferred method for water terminals, user water intakes and small water supply systems. In particular, after the discovery of Cryptosporidium in tap water, the United States has included ultraviolet disinfection technology as a good means of tap water disinfection in water supply regulations [2].
1 Biological principles of ultraviolet disinfection
Ultraviolet rays are located between X-rays and visible light. In physics, ultraviolet rays are generally divided into vacuum ultraviolet region (<190nm), far ultraviolet region (190-300nm) and near ultraviolet region (300-400nm); according to the differences in their biological effects, ultraviolet rays can be divided into UV-A (320-400nm), UV-B (275-320nm), UV-C (200-275nm) and vacuum ultraviolet part. In water treatment, the UV-C part of ultraviolet rays is actually used. In this band, the ultraviolet rays around 260nm have been proven to have high sterilization efficiency [3].
The principle of ultraviolet sterilization is based on the absorption of ultraviolet light by nucleic acids. Ultraviolet sterilization is essentially a photochemical process. Each ultraviolet photon with a wavelength of 253.7nm has an energy of 4.9eV. Ultraviolet photons must be absorbed to be active. Nucleic acid is the basic substance and basis of life for all living organisms. Nucleic acid is divided into two categories: ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). Their common point is that they are polynucleotide chains connected by phosphodiester bonds according to the principle of purine and pyrimidine base pairing. When microorganisms are exposed to ultraviolet light, they absorb the energy of ultraviolet light, causing DNA damage. The two common forms of damage are cyclobutane pyrimidine dimer (CPD) and pyrimidine-pyrimidone photoproducts (PP). When DNA is exposed to ultraviolet light, adjacent pyrimidine bases are covalently cross-linked to form a cyclobutane tetracycle, which saturates the double bonds at positions 5 and 6 of the two bases to form CPD. Pyrimidine-pyrimidone photoproducts are formed by the formation of dioxyethane or azetidine 4 rings between the 5- and 6-carbon atoms of 5-pyrimidine or the 4-carbon atom of 3-pyrimidine and the oxygen atom or imino isomer at the 4-carbon. These are relatively stable chemical bonds, thus preventing DNA replication [4,5]. On the other hand, free radicals can be generated under ultraviolet irradiation to cause photoionization, causing microorganisms to be unable to replicate and reproduce, and they will die naturally or be eliminated by the human immune system without causing harm to the human body, thus achieving the purpose of disinfection.
2 Inactivation effect of ultraviolet disinfection on microorganisms in water
Ultraviolet disinfection has a high microbial inactivation effect, and has a good inactivation effect on a variety of microorganisms in water, and has a fast sterilization speed, most of which are within 1 second. In addition, ultraviolet disinfection technology also has a good inactivation effect on the pathogenic microorganisms Giardia and Cryptosporidium discovered in recent years. Cryptosporidium cysts are excreted into the environment through human and animal feces. They can survive for a long time in the environment. Cryptosporidium oocysts and Giardia cysts survive longer than other waterborne infectious microorganisms, and can cause multiple outbreaks of disease. The diseases caused by Cryptosporidium are very serious. The common symptoms are diarrhea, vomiting, low-grade fever, and flu-like symptoms. For patients with impaired immune function, such as AIDS patients, the disease is more serious and can lead to death. For example, in 1994, an outbreak of cryptosporidiosis occurred in Las Vegas, USA, and 20 AIDS patients died [6-9]. Recent studies have shown that when the radiation dose of low-pressure mercury lamps and medium-pressure mercury lamps is 30J/m2, more than 99.9% of Cryptosporidium can be inactivated. A large number of experiments have shown that both low-pressure mercury lamps and medium-pressure mercury lamps can effectively inactivate Cryptosporidium [10~12]. Ultraviolet disinfection also has a good effect on Legionella. Muraca compared the inactivation of Legionella by ozone, ultraviolet, LV and heating. Ultraviolet and heating (60 degrees) produced 5 log inactivation in 1 hour, while LV and ozone required 5 hours to achieve the same inactivation effect [13,14].
3 Comparison of ultraviolet disinfection with other disinfection methods
Comparison of five commonly used disinfection methods in terms of disinfection effect, cost and safety (see Table 2). As can be seen from the table, several chemical disinfectants take a long time to inactivate microorganisms, while ultraviolet disinfection can achieve the same inactivation effect in just a few seconds. Chemical disinfectants will produce some disinfection by-products that are harmful to human health, and the operation and management are also relatively complicated. Ultraviolet disinfection does not produce disinfection by-products during the sterilization process, and the operation is simple. Its infrastructure investment and operating costs are also lower than those of other chemical disinfection methods.
4 Advantages and disadvantages of ultraviolet disinfection applications
The ultraviolet disinfection process has advantages that other disinfection processes cannot match, and overcomes the shortcomings of existing traditional disinfection technologies. Many European countries as well as Canada and the United States in North America have revised their environmental legislation in the 1990s.
