How Is Tap Water Treated? From the Plant to Your Faucet

How Tap Water Is Treated: The Short Answer

If your water comes from a river, lake or reservoir, it most likely goes through five steps at the treatment plant: chemicals clump the dirt together, gentle mixing grows those clumps, they settle out, the clearer water runs through filters, and a disinfectant is added to kill what's left. Plants also commonly adjust the water's pH, partly so it's less likely to eat at pipes on the way to you.

If your water comes from wells, the list is often much shorter. And for everyone, the plant is only half the trip. The water still has to cross miles of city pipe, your service line and your home's plumbing before it reaches the glass.

This guide walks through each step, what it does and doesn't do, what can change after the water leaves the plant, and how to find out what your own utility actually does. That last part matters most if you're deciding whether you need anything at home.

It Starts With Where Your Water Comes From

Public water systems supply drinking water to about 90 percent of Americans, and there are more than 148,000 of them, according to the EPA. They don't all treat water the same way, and the biggest reason is the source.

Water from lakes, rivers and reservoirs usually needs more treatment than water pumped from underground. The CDC explains that surface water carries more sediment, germs and chemicals than groundwater does. Federal rules reflect that difference:

• Surface water systems must filter and disinfect under the EPA's Surface Water Treatment Rules. A small number are allowed to skip filtration because their source meets strict water quality and watershed protection criteria. The same rules cover wells that are directly influenced by surface water.
• Groundwater systems fall under the Ground Water Rule, which is risk-based. States survey these systems, and a system found at risk of fecal contamination has to take corrective action. A system that relies on treatment for virus protection has to show that the treatment reliably removes or inactivates 99.99 percent of viruses.

So a town drawing from a clean, deep aquifer may run little more than disinfection and pH adjustment, and some run less. A city drawing from a river runs the full sequence below.

The Steps at a Typical River or Lake Treatment Plant

What follows is the conventional sequence the CDC describes. Individual plants vary, so treat this as the basic path rather than a blueprint for your own utility.

Coagulation and Flocculation

River and lake water carries dirt and other small particles. Operators add chemicals that help those particles bind together. Then the water is mixed gently, which lets them grow into bigger, heavier clumps called floc.

What it doesn't do: this step doesn't make the water safe to drink. It's preparation, turning a cloud of fine particles into something the next steps can actually catch.

Sedimentation

The water moves into large, calm basins. Floc is heavier than water, so it sinks to the bottom, and the clearer water at the top flows on.

What it doesn't do: settling only takes out what's heavy enough to sink. Everything else moves on to the filters.

Filtration

Next, the water passes through beds of sand, gravel or charcoal with different pore sizes. According to the CDC, these filters remove germs, including parasites, bacteria and viruses, along with dissolved particles such as dust and some chemicals. Plants that use activated carbon also reduce bad smells.

This is also where federal rules put a hard number on one parasite. For Cryptosporidium, the EPA requires systems that filter to remove at least 99 percent (the rules call this 2-log removal), and systems with higher-risk sources face additional treatment requirements.

What it doesn't do: filtration at a plant treats the water once, at the plant. It can't protect the water from anything it picks up afterward.

Disinfection

Disinfection is often the last major step. Plants add chlorine, chloramine or chlorine dioxide to kill any germs still present. The CDC notes that plants deliberately leave a low level of disinfectant in the water as it leaves, so it keeps killing germs in the pipes between the plant and your tap.

Some plants also use ultraviolet light or ozone, either instead of or alongside chemical disinfectants. According to the CDC, both work well inside the plant, but unlike a chemical residual they don't keep killing germs as the water travels through the pipes.

What it doesn't do: that leftover disinfectant is intentional, so the water reaching you isn't supposed to be disinfectant-free. Disinfection also creates byproducts of its own, which the EPA regulates alongside the germ rules. Our guide to disinfection byproducts in drinking water covers how they form and why levels can rise at the far end of a system.

pH Adjustment, Corrosion Control and Fluoride

Plants commonly adjust the water's pH before it leaves. The CDC gives three reasons: it improves taste, it reduces corrosion of pipes, and it helps disinfectants keep killing germs as the water travels. Some communities also add fluoride at this stage; the CDC says the right amount keeps teeth strong and reduces cavities.

Corrosion control gets less attention than the other steps, but it's the one that matters most for lead. One requirement of the EPA's Lead and Copper Rule is corrosion control treatment, meaning utilities must make their water less corrosive to the pipes and fixtures it touches on the way to your faucet. Lead usually isn't in the water when it leaves the plant. The EPA notes that lead contamination often comes from corrosion of the plumbing itself, such as lead service lines, lead solder and brass fixtures, and corrosion control is how a utility limits how much of that metal dissolves into the water.

What Happens Between the Plant and Your Faucet

A utility's treatment is measured and regulated, but the water still has a long trip ahead. Three things can change along the way.

The Disinfectant Keeps Working

That leftover disinfectant is why some tap water tastes or smells of chlorine. Many utilities use chlorine. Others use chloramine, made by adding ammonia to chlorine, because it lasts longer in the pipes. More than one in five Americans uses water treated with chloramine, according to the EPA, and some utilities switched to it specifically to meet disinfection byproduct limits. EPA says water with chloramine that meets its standards is safe for drinking, cooking and bathing. If you notice the taste, which disinfectant your utility uses decides which filter will remove it.

Miles of Distribution Pipe

U.S. distribution systems span almost one million miles of pipe, the EPA estimates. As those pipes age, corrosion and wear can lead to breaches, contamination drawn in when pressure drops, and main breaks. That's also why a main break or a loss of pressure can lead to a boil water advisory. The water may have been fine when it left the plant; the risk entered in the pipe.

Your Service Line and Home Plumbing

The last stretch belongs to you. The pipe from the water main to your house is the service line, and in older homes it may be lead. The EPA notes that lead pipes are more likely in homes built before 1986, and that in homes without a lead service line, the most common sources are brass or chrome-plated brass faucets and lead solder. How long water sits in the pipes is one of the factors the EPA lists in how much lead gets in, which is why the first water out of the tap in the morning can differ from water that's been running. Treatment at the plant can't remove what the water picks up after it leaves, although corrosion control is meant to limit it.

How to Find Out What Your Own Utility Does

You don't have to guess. Community water systems must send customers an annual water quality report, also called a Consumer Confidence Report, by July 1 each year. The EPA requires it to name the lake, river, aquifer or other source of your water and to list the regulated contaminants that were found. Our guide on how to read your water quality report goes through it column by column.

The report won't answer everything, so these are worth a call or an email to your utility:

• Is our water surface water, groundwater or a blend? This tells you roughly which treatment steps your water went through.
• Do you disinfect with chlorine or chloramine? This decides what kind of filter will remove the taste.
• Do you add a corrosion control treatment, and have you had any lead results above the action level?
• What do your records show for the service line at my address? Lead, galvanized steel, copper or unknown each point to a different next step.

If you're on a private well, none of this applies. Nobody treats that water but you, and our comparison of well water vs city water covers what changes.

What Home Treatment Can Add

City treatment is designed to make water safe to drink and to keep it that way through the pipes. It isn't designed around your taste preferences, and it can't see inside your house. Most of what people choose to treat at home falls into the gaps described above:

Two answers narrow the choice more than anything else: whether your utility uses chlorine or chloramine, and whether your home has lead anywhere in its plumbing. Once you know those, you can decide whether a filter at the kitchen tap covers what you care about or whether you want treatment at the point where water enters the house.

If you'd rather measure than assume, a test of your own tap water shows what's actually coming out of your faucet after the plant, the pipes and your plumbing have all had their turn.