Since August 20, 493 residents of Baoyi Town, Shou County, Anhui Province have successively experienced symptoms of fever, vomiting, abdominal pain and diarrhea. According to the preliminary investigation by the joint investigation team of provinces, cities and counties, it was determined that they were caused by Shigella infection. The Shigella infections were scattered in several villages in Baoyi Town. The preliminary investigation was related to the pollution of the tap water supply source under the flood disaster.
The news can’t help but remind people of the 2009 Chifeng tap water pollution incident in Inner Mongolia. According to Sohu News, a heavy rain fell in Chifeng City, Inner Mongolia Autonomous Region on July 23. In the following days, thousands of residents in the new urban area After drinking tap water, symptoms such as diarrhea, vomiting, dizziness, fever, etc. appeared. For a time, the number of diarrhea patients in various medical clinics increased sharply. On July 26, the cause of the incident was notified by the Chifeng City Construction Committee as: heavy rain sewage invaded the source of drinking water pollution. The heavy rain in the urban area caused the drainage of surface water to be difficult. A large amount of sewage invaded the water source well of Kowloon Water Supply Company, which was responsible for the water supply of the newly built urban residents, and contaminated the drinking water. The water samples were collected by the health department on July 26. The total number of bacteria and colonies seriously exceeded the standard, and the Salmonella detected at the same time was the main cause of this water pollution incident.
What is Shigella
Shigella Castellani is a type of gram-negative short bacilli with a size of 0.5～0.7×2～3μm. It is a common pathogen of human bacillary dysentery. Humans are susceptible to Shigella, and 10 to 200 bacteria can cause disease in 10 to 50% of volunteers.
What is Salmonella
The Salmonella that caused the Chifeng water pollution incident has a cell size (0.6～0.9)×(1～3) micron without spores. It was isolated from Salmonella choleraesuis in 1885 during the cholera epidemic, so it was named as Salmonella. Typical symptoms include fever, nausea, vomiting, diarrhea, and abdominal cramps, which usually get better within 72 hours after fever. Infants, the elderly, and immunocompromised patients may develop severe and life-threatening bacteremia due to Salmonella entering the bloodstream, and a small number of them will also have meningitis or osteomyelitis.
Ensuring the safety of drinking water has always been one of the key areas of water treatment. Common drinking water safety involves the control of chemical pollutants and the safety control of microbial disinfection. How to balance chemical and biological safety and ensure water quality is safe and suitable for drinking is a challenge to the design of water treatment processes.
When the water quality of the water source is at risk, a single chemical disinfection design has the risk of substandard or excessive disinfection by-products. On the basis of conventional chlorine disinfection, combined with ultraviolet disinfection, the risk can be minimized.
According to the national standard stipulated in “Urban Water Supply and Drainage Ultraviolet Disinfection Equipment” (GB-T19837), when disinfecting drinking water or drinking water, the effective dose of UV is not less than 40mj/cm2.
- For Shigella, the UV dose can reach 99.99% removal at 4.2mj/cm2
- For Salmonella, 99.99% removal can be achieved when the UV dose is 7.1mj/cm2
It can be seen that according to the national standard design, ultraviolet disinfection can effectively eliminate the biological risks caused by Shigella and Salmonella in drinking water.
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