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How do water treatment plants remove harmful bacteria?

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Bacteria A scanning electron micrograph of gram-positive *Mycobacterium tuberculosis* bacteria, the cause of tuberculosis and scrofula.

Water treatment plants remove harmful bacteria through a multi-stage process designed to ensure public safety and prevent the spread of waterborne diseases.1

Physical Removal

Before disinfection, plants use physical processes to reduce the number of microbes and suspended particles:
* Coagulation and Flocculation: Chemicals are added to clump small particles together into larger masses called "floc," which then settle to the bottom in a sedimentation basin.2
* Filtration: Water passes through porous media, such as sand or anthracite, which traps remaining suspended particles and microorganisms. This step is crucial because turbidity can shield bacteria from later disinfection efforts.23

Disinfection

Disinfection is the final, essential step that kills remaining pathogens. Common methods include:21
* Chemical Disinfectants: The application of chlorine or chlorine compounds is standard.21
* Alternative Agents: To avoid risks associated with chlorine, some plants utilize ozone, hydrogen peroxide, or ultraviolet radiation. Ultraviolet light is particularly effective at destroying pathogens without leaving chemical residuals or affecting water taste.24