Aiming at the design of the commonly used open channel underwater irradiated sewage ultraviolet disinfection system, the selection of the ultraviolet lamp
type, the determination method of the ultraviolet dose and the determination of the design flow of the open channel underwater irradiated sewage ultraviolet disinfection system are discussed, and the disinfection open channel and its Design method of water level controller. Aiming at the design of the commonly used open channel underwater irradiation sewage ultraviolet disinfection system, the selection of the ultraviolet lamp type, the determination method of the ultraviolet dose and the determination of the design flow rate of the open channel underwater irradiation sewage ultraviolet disinfection system are discussed, and the disinfection open channel and its Design method of water level controller. With the recognition of the shortcomings of chlorine disinfection, people are seeking alternative technologies for chlorine disinfection. has been widely used in the United States and Canada because of its advantages such as rapid and efficient sterilization, safety, easy operation and small footprint. At present, most of the UV sewage disinfection systems use open channel underwater irradiation. In our country, the application of ultraviolet disinfection lamps
in sewage treatment has just started, and there are not many reports on the research and system design of sewage ultraviolet disinfection technology with underwater irradiation in open channels. 1 System composition The composition of the open channel underwater irradiation ultraviolet disinfection system is shown in Figure 1. The ultraviolet lamp is placed in parallel on the support and immersed in water. The modules are independent of each other. Each module can be configured with 2, 4, 6, 8 or 16 UV tubes. If the ultraviolet disinfection system adopts automatic cleaning, cleaning facilities must be installed. Sewage flows through the ultraviolet lamp by gravity, and a water level controller is installed downstream of the channel. 2 System design 2.1 UV lamp type selection UV disinfection lamps have three types of low-pressure low-intensity lamps, low-pressure high-intensity lamps and medium-pressure high-intensity lamps. Both the low-pressure low-intensity lamp and the low-pressure high-intensity lamp emit monochromatic light with an output wavelength of 253.7nm and the life of the lamp tube is 8000~12000h, while the medium-pressure high-intensity lamp emits polychromatic light with a wavelength of 230~300nm and the lamp life is 5000h. However, the photoelectric conversion rate of medium-voltage high-intensity lamps is low (only about 15%), energy consumption is high, and electricity bills are high; while the photoelectric conversion rates of low-voltage low-intensity lamps and low-voltage high-intensity lamps are 30% to 40%. At present, sewage treatment plants mostly use low-voltage high-intensity lamps and medium-voltage high-intensity lamps, and technical and economic comparisons should be made according to actual conditions during design. 2.2 Determination of UV dose The number of UV lamps depends on the UV dose required to inactivate microorganisms. The relationship between the inactivation of microorganisms and the dose of ultraviolet radiation can be expressed by a mathematical model. In addition, certain components in the sewage will cause fouling on the surface of the quartz sleeve of the lamp tube and affect the UV transmittance. The fouling coefficient is related to the cleaning method of the lamp: manual cleaning is 0.7, pure mechanical cleaning is 0.8, and mechanical plus chemical cleaning is 1.0. Therefore, when determining the UV dose, the UV dose obtained by the above method should also be included in the lamp aging coefficient and fouling coefficient to obtain the UV dose required to reach the microbial disinfection standard during the life of the lamp. 2.3 Design flow and illumination time The UV disinfection system must be designed according to the maximum flow during the life of the UV lamp, otherwise, the disinfection requirements will not be guaranteed when the flow increases. At the same time, the disinfection requirements at the minimum flow must also be met. Many smaller sewage treatment plants have a flow close to zero at night. During this period, the sewage around the quartz sleeve will heat up and deposits on the sleeve. It may also expose the quartz sleeve to the water surface and expose it to the air. The material on the tube will be dried out, thereby forming a deposit. Therefore, the maximum and minimum flow rates must be determined during design. Lighting time is also an important design parameter. Different kinds of microorganisms need different light time to inactivate under the same UV intensity. The current design is to use high-intensity ultraviolet energy and take a short light time, for example, the light time is 6-10s. Of the two water level controllers, it may be better to use a weir that allows the UV lamp to remain fully submerged at zero flow. Generally, weirs are used for small disinfection systems with less than 20 UV lamps, while flap gates are used for large UV disinfection systems. 3 Conclusion The reasonable design of the open channel underwater irradiation sewage ultraviolet disinfection lamps system is the key to the application of ultraviolet disinfection technology in sewage treatment. Choosing the appropriate type of ultraviolet lamp will help save investment and operating costs, and correctly determine the ultraviolet dose, maximum and minimum flow and Reasonable design of the water level controller is an important aspect to ensure the disinfection effect.
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