How Water Treatment Works

A treatment plant runs five steps: coagulation, flocculation, sedimentation, filtration and disinfection. The first three clump the dirt and let it sink. The last two catch what’s left and kill what’s alive.

About 90 percent of Americans drink water from a public system, and the EPA counts more than 148,000 of them, from big city filtration plants down to a well serving one trailer park. The size changes. The sequence mostly doesn’t.

The five steps, in order

The CDC lays the process out in this order, and nearly every plant in the country follows it.

Step	What happens	What it removes
1. Coagulation	Chemicals are mixed into the raw water	Nothing yet. It lets fine particles stick together.
2. Flocculation	Slow, gentle stirring	Nothing yet. Particles grow into heavy clumps called floc.
3. Sedimentation	Water sits still in a large basin	Most of the floc, which sinks
4. Filtration	Water passes through sand, gravel and charcoal	Remaining particles and many germs
5. Disinfection	Chlorine, chloramine, chlorine dioxide, UV or ozone	Whatever is still alive

1. Coagulation

Dirt in river or lake water is too fine to settle on its own. Some of it would take days to sink. So the plant adds a coagulant, and the CDC names the usual ones as “specific types of salts, aluminum, or iron.” Alum, which is aluminum sulfate, is the classic.

2. Flocculation

This is a slow stir. The coagulated particles bump into each other and grow into flocs, clumps big enough to see. Stir too hard and you tear them apart. Stir too gently and they never meet.

3. Sedimentation

The water moves into a basin and sits. The CDC puts it plainly: “Flocs are heavier than water, so they settle to the bottom.” Clear water comes off the top and the sludge is pulled from the bottom.

It’s also the easiest step to copy at home. More on that below.

4. Filtration

The clear water runs down through beds of sand, gravel and charcoal. According to the CDC, these filters “remove germs, including parasites, bacteria, and viruses.” Some plants add ultrafiltration membranes or reverse osmosis on top.

5. Disinfection

Last comes the chemical that kills what got through. Chlorine, chloramine and chlorine dioxide are the common ones. Some plants use UV light or ozone instead.

The difference matters once the water leaves the plant. Chlorine and chloramine leave a small amount behind that keeps working inside the pipes. UV and ozone don’t, and the CDC notes they give no protection on the trip to your house.

After disinfection, most plants adjust the pH so the water doesn’t eat the pipes. Many add fluoride.

The chemistry, briefly

Three reactions carry most of the load.

Coagulation is about charge. Tiny clay and silt particles carry the same electrical charge, so they push each other away. Aluminum and iron salts cancel that charge and let them clump.

Chlorine is an oxidizer. It damages the cell walls and inner machinery of bacteria and viruses. It also reacts with anything else in the water, so cloudy water uses it up faster.

pH controls both. Coagulants and chlorine each work best in a certain range, and plants check and adjust it more than once.

That second point is why the bleach dose for cloudy water is roughly double. The dirt eats the chlorine before the germs do.

Why your tap still gets a boil notice

The plant is rarely the problem. The pipes are.

The EPA went through every boil water advisory it could find for 2021 and counted 4,036. Eighty percent came from main breaks, repairs and loss of pressure. Only 6.3 percent came from confirmed or likely fecal contamination.

When pressure drops, a pipe can pull in groundwater from outside. The water left the plant clean and may not be clean at your faucet. That’s why a notice after a main break or a storm is about germs, and why boiling is the right answer to it.

Large scale vs. small scale

Every step a filtration plant runs has a kitchen version. They’re cruder, and they only work on the water in front of you.

Plant step	Home version	What to know
Coagulation and flocculation	A pinch of alum in cloudy water	The CDC travel guidance uses about 1/4 teaspoon per quart
Sedimentation	Let the bucket sit, then pour off the top	Takes hours, not minutes
Filtration	Cloth, coffee filter, or a certified filter	Cloth only removes grit. Germs need a certified filter.
Disinfection	Boiling, bleach or UV	The step you can’t skip

The home version of the first four steps only makes water clearer. The last step is what makes it safe, and the purifying water guide covers each method and the one thing each fails at. If you’re here for a class, the water purification science project builds the first four steps in a plastic bottle.

Drinking water plants vs. sewage treatment plants

People mix these up. A drinking water plant cleans water on the way into town. A sewage treatment plant cleans used water from toilets and drains on the way out, before it goes back to a river or lake. Different plants, different rules, different end product.

Your tap is fed by the first kind. If your source river runs past the second kind upstream, the first kind is the reason that doesn’t reach your glass.

One number

Four out of five boil notices trace back to a pipe, not the plant. When one comes, the fix is heat: boil it for one minute, three above 6,500 feet, or drink from what you have stored.