How Tap Water Is Treated: From River or Well to Your Faucet

Most US tap water from rivers, lakes and reservoirs goes through five main steps, as the CDC describes them: coagulation, flocculation, sedimentation, filtration and disinfection. Chemicals make tiny particles clump together, the clumps settle out, filters catch what’s left, and a disinfectant like chlorine kills remaining germs. Many utilities then adjust the water’s chemistry to keep it from corroding pipes. Groundwater from wells often needs less treatment. Throughout the process, EPA rules require utilities to test the water and meet specific limits.

Here’s what each step does, why it matters, and how you can find out what your own utility does.

Where the water starts

The CDC notes that water from lakes, rivers and reservoirs (surface water) usually needs more treatment than groundwater, because it typically carries more sediment, germs, chemicals and toxins. Groundwater is filtered naturally as it moves through soil and rock, though it can pick up minerals and contaminants of its own, like arsenic, radium or nitrate.

That difference shapes everything that follows. EPA’s Surface Water Treatment Rules apply to all public systems using surface water, or groundwater that’s directly influenced by surface water. Our guide to groundwater vs. surface water quality compares the two in more detail.

Step 1: Coagulation

Raw river or lake water carries particles too small to settle out quickly on their own, such as silt, clay and organic matter.

In coagulation, plant operators add chemicals such as aluminum or iron salts. The CDC explains that these chemicals help bind dirt and other small particles together, so they can be removed in the next steps.

Step 2: Flocculation

Next, the water is mixed gently. The small clumps from coagulation grow into larger, heavier particles called floc. The CDC notes that operators may add more chemicals at this stage to help the floc form.

Step 3: Sedimentation

The water then sits in settling basins. Because floc is heavier than water, it sinks to the bottom, separating solids from the water. The settled material, called sludge, is removed.

Coagulation, flocculation and sedimentation together are sometimes called “clarification.” In a USGS study of one conventional treatment plant, clarification also accounted for part of the removal of trace organic chemicals, though filtration and disinfection did more of that work.

Step 4: Filtration

The clearer water from the top of the settling basins passes through filters. The CDC describes filters made of sand, gravel and charcoal with different pore sizes, which remove germs (including parasites, bacteria and viruses), dissolved particles and chemicals.

Some plants use additional or alternative filtration:

Activated carbon removes odors and many organic chemicals.
Ultrafiltration uses membranes with very small pores.
Reverse osmosis removes additional dissolved substances. The CDC notes plants often use it for recycled water or seawater.

How filtration is checked: turbidity

Turbidity is a measure of cloudiness, and it’s the main way regulators check whether filtration is working. Cloudy water can shield germs from disinfection.

For plants using conventional or direct filtration, EPA requires that filtered water measure 0.3 NTU or less in at least 95% of readings each month, and never exceed 1 NTU.

Step 5: Disinfection

Disinfection kills or inactivates the germs that make it through earlier steps. The CDC lists chlorine, chloramine and chlorine dioxide as common chemical disinfectants.

Some plants also use ultraviolet (UV) light or ozone. The CDC notes these work well inside the plant but don’t keep protecting water once it leaves, so they’re paired with a chemical disinfectant.

Why your water still has a little chlorine in it

Utilities intentionally leave a small amount of disinfectant in the water, called a residual, so it keeps protecting the water as it travels through miles of pipe to your tap. Federal rules for filtered surface water systems say:

The residual entering the distribution system can’t fall below 0.2 mg/L for more than 4 hours.
The residual can’t be undetectable in more than 5% of distribution samples in a month, for two months in a row.

Some utilities use chlorine and others use chloramine for this, and each has trade-offs for taste and byproducts. Our guide to chlorine and chloramine in tap water explains the difference.

Treatment targets for germs

EPA’s Surface Water Treatment Rule requires treatment to remove or inactivate 99.9% of Giardia and 99.99% of viruses. Later rules added a requirement for filtering systems to remove at least 99% (2-log) of Cryptosporidium, a parasite that resists chlorine. Our guide to Cryptosporidium and Giardia covers why these parasites get special attention.

The trade-off: disinfection byproducts

When chlorine reacts with natural organic matter in water, it forms disinfection byproducts such as trihalomethanes (TTHMs) and haloacetic acids (HAA5), which EPA regulates. Removing more organic matter during coagulation and filtration means fewer byproducts later. See disinfection byproducts explained.

Step 6: pH adjustment and corrosion control

Treated water can still harm health if it eats away at pipes on the way to your home. Lead and copper don’t usually come from the treatment plant. They dissolve from service lines, solder and fixtures.

The CDC notes that plants commonly adjust pH after disinfection to improve taste, reduce pipe corrosion and help the disinfectant keep working. Under EPA’s Lead and Copper Rule, many systems use corrosion control treatment. EPA’s technical guidance describes two common approaches:

Adjusting pH and alkalinity so water is less corrosive.
Adding a corrosion inhibitor, most often orthophosphate, which helps form a protective layer inside pipes.

Corrosion control reduces lead risk but doesn’t eliminate it, especially in homes with lead service lines. See lead in tap water: how to test and protect.

Optional and specialized steps

Depending on the source, utilities may add other treatment:

Fluoride. Many community systems adjust fluoride for dental health.
Targeted treatment. The CDC notes that some supplies need special methods for contaminants like nitrate, radionuclides or toxins from blue-green algae. Others use ion exchange, granular activated carbon or reverse osmosis for PFAS.
Softening or iron and manganese removal where source water is high in minerals.

Testing: how you know it worked

Treatment is only half the system. EPA rules set legal limits for more than 90 contaminants and specify testing schedules and methods utilities must follow. Key examples:

Turbidity monitoring at the plant, as described above.
Disinfectant residual checks at the plant and throughout the pipe network.
Bacteria. Under EPA’s Revised Total Coliform Rule, in effect since April 1, 2016, systems test for total coliform and E. coli at sites throughout the distribution system on a set schedule. Certain results require the system to assess and fix problems, and violations require public notice. See coliform and E. coli in tap water.
Chemicals and metals on schedules that depend on the contaminant and system size, including lead and copper sampling at customer taps.

Results are reported to state regulators and summarized for customers each year in a Consumer Confidence Report.

How to find out what your utility does

Read your Consumer Confidence Report. It names your water source and lists detected contaminants. Our guide to reading your water quality report walks through it.
Check your utility’s compliance record. Search your water system to see its TapWaterSafety grade and recent EPA violations.
Ask your utility. Most will tell you which disinfectant they use and what corrosion control they apply.

Bottom line

Conventional treatment works in layers. Coagulation, flocculation and sedimentation remove most particles, filtration catches what’s left, disinfection kills remaining germs, and a residual disinfectant protects water in the pipes. Corrosion control helps keep lead and copper in pipes from dissolving. Each step is backed by EPA testing requirements, and the results are public, so you can check how your own utility is doing.