1After a heavy rain, a river that supplies a city's water can arrive at the treatment plant turbid. Runoff has carried clay and silt from fields and streets into the current, and a coin dropped into a glass of that water could vanish from sight before it reached the bottom. Before the water can be made safe to drink, it has to be made clear, because the particles that block the view can also shelter germs from the chemicals meant to kill them.
2The trouble is size. Much of what the rain carried in is clay so fine that it is nonsettleable. Each particle also carries a slight negative electrical charge, so the particles push one another away instead of clumping. Operators add a coagulant, a chemical that cancels the charge, and stir the water gently. Like a snowball rolling downhill, each small clump gathers more particles as it tumbles, until the clumps, called floc, are large enough to see.
3Next, the water flows slowly through long, quiet basins. Here the floc does what the fine clay could not: it settles, sinking to the floor of the basin as a layer of sludge. The water that enters a basin is still murky, but by the time it reaches the far end, hours later, it has clarified.
4Even clear water is not finished. It passes through beds of sand and gravel, and sometimes charcoal, which trap particles too small for the basins to catch. Then comes disinfection. At many plants, chlorine or a similar chemical does the heavy lifting, killing bacteria and viruses that made it through the filters.
5Operators also leave a little disinfectant, called a residual, in the water. The reason is the pipes. Between the plant and a kitchen faucet may lie miles of water main, some of it decades old, with joints and cracks where contamination can enter. The residual guards against recontamination along the way. Treated water, in other words, is not a finished product so much as a promise the plant has to keep all the way to the tap.