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    How to apply ultraviolet disinfection process to reclaimed water treatment

    In order to ensure the biological and chemical properties of reclaimed water and tail water, the selection of disinfection process is very important. At present, the main disinfection methods are chlorine, chlorine dioxide and ultraviolet disinfection. This paper will introduce the ultraviolet disinfection process from the disinfection effect and other aspects. In practical application, the disinfection process should be selected according to the different objectives of sewage treatment.

    At present, all domestic sewage discharge and reuse standards have put forward requirements for disinfection, such as the discharge standard of pollutants for urban sewage treatment plant (GB 18918-2002) stipulates the discharge standards of fecal coliform group under different requirements; the standard for water quality control of reclaimed water for urban wastewater reuse as cooling water (GB 50335-2002) specifies the control standard for fecal coliform group; and the standard for the control of fecal coliform group in urban sewage treatment plant (GB 18918-2002) The standard of total coliform number and total residual chlorine (mg / L) is stipulated in gbl8920-2002; fecal coliform group standard is specified in water quality for landscape environment of urban wastewater reuse (gbl8921-2002); and the standard for reclaimed water reuse of urban wastewater (GB / t19772-2005) is for reclaimed water reuse Standard for fecal coliform.

    (1) Disinfection effect

    The results showed that UV had good inactivation effect on pathogenic microorganisms in water. The disinfection dose of secondary treatment effluent was 15-22mws / cm2, and that of reclaimed water was 24-30mws / cm2. Ultraviolet rays can inhibit DNA replication by forming thymidine dimer on microbial DNA. However, ultraviolet disinfection has no ability of continuous sterilization. Some microorganisms can be reactivated by light after being exposed to visible light. The researchers suggest that if the wastewater is reused after treatment, ultraviolet disinfection should be used in combination with chlorine / chlorine dioxide.

    (2) Microbial reactivation in water after disinfection

    The mechanism of ultraviolet disinfection is that the DNA of microorganism will form thymidine dimer after being irradiated by ultraviolet ray, which can not replicate itself and die or be destroyed by the immune system of bacteria. Current studies have shown that the activity and value-added ability of photoreactivation can be improved. UV chlorine disinfection has good inactivation effect and can control light reactivation. When the UV irradiation dose was 5.4 MJ / cm 2, adding 2.5 mg / L chlorine and reacting for 10 min, the coliform group in the effluent was less than 20 CFU / L, and there was no light revival phenomenon.

    (3) Formation of by-products

    There are many organic substances in the sewage. When chlorination is used, not only more effective chlorine is consumed, but also trihalomethane (THMs) carcinogenic by-products are produced. UV disinfection does not produce by-products. With the increase of irradiation dose, formaldehyde content in water will slightly increase, but still lower than 300 μ g specified in GB 18918-2002.

    Industry insiders said that for southern China, where there is abundant rainfall and the goal of sewage treatment is to meet the discharge standard, both UV disinfection and chlorine dioxide disinfection can be competent. For the north of China, water resources are in short supply, so it is imperative to reuse sewage. At this time, the combination of ultraviolet and chlorine dioxide disinfection process or chlorine dioxide disinfection alone can be used. For the newly-built projects and upgrading projects with strict land occupation requirements, the patented integrated sedimentation and disinfection tank process can be adopted, and the sedimentation tank and disinfection tank can be built together, or the original sedimentation tank can be transformed into a sedimentation and disinfection tank.

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