A drinking-water plant can receive cloudy water after it has travelled through a watershed. Some particles settle easily, while others stay suspended. A utility chooses a series of treatment steps based on its source water and the contaminants it needs to control. The route from a reservoir to the tap often begins by making tiny particles easier to remove.
Help particles settle, then filter the water
The U.S. Centers for Disease Control and Prevention describes a typical sequence. During coagulation, treatment staff add substances that help small particles stick together. Gentle mixing then helps form larger clumps in a process called flocculation. When the water moves through a settling stage, heavier clumps can drop out, leaving clearer water above.
Filtration does another part of the work. Water can pass through layers of materials such as sand and gravel, or through other filter systems selected for the source. Different filters target different things. Think of the settling stage as taking out what will drop to the bottom, and filtration as catching material still carried in the water.
Control germs at the plant and along the way
Disinfection uses treatments such as chlorine compounds, ozone or ultraviolet light to kill or inactivate remaining germs. Different methods serve different purposes. A utility may use a chemical disinfectant that leaves a residual in the water to help protect it as it travels through the distribution pipes. Ultraviolet treatment acts where the water passes the light.
EPA’s surface-water rules apply to public systems using surface water and certain groundwater sources influenced by surface water. They generally require filtration and disinfection. Some systems can use disinfection alone for surface sources that meet specified water quality and watershed protections. Treatment varies with source, equipment and local requirements.
Find the report for your own system
The U.S. EPA links to Consumer Confidence Reports for community water systems. Start with the name of the utility that serves your address and check the report’s service area and reporting year. Find the water source and the monitoring results, then use the report’s definitions to read the units and any reported limits. The results belong to the system and period named in the report.
The treatment sequence explains how a utility turns source water into drinking water. For a current disruption or advisory, look to the utility or local authorities for the latest information for your location.
Sources & further reading
- Centers for Disease Control and Prevention | How water treatment works
Explains the stages in a common treatment train and disinfection methods.
- U.S. EPA | Surface Water Treatment Rules
Defines the federal scope and when systems may use disinfection without filtration.
- U.S. EPA | Consumer Confidence Reports
Links to local public-system reports and instructions for reading them.