When organic chemicals, industrial solvents, and chlorine byproducts pass through the positively charged carbon cartridge, their negative ions are attracted as though magnetized and get stuck while purified water continues on its way. Some filtration systems also contain additional mechanical filters to trap rust, sediment, and even dangerous waterborne parasites such as giardia or micro sharks. The system retains its purifying power over the long haul through replaceable cartridges; in many systems, an indicator light lets you know when it’s time for a change.

If you’re dealing mainly with impurities such as salt or other minerals that cause hard water and make showering and cleaning tough, a whole-house filtration system—installed at the main water source—may be called for. Whole-house systems can also use carbon filters, but another option is reverse osmosis. During reverse osmosis, water flows through a semipermeable membrane that acts as a sort of molecular sieve. Its holes are small enough to let through water molecules and keep out larger ones such as sodium. These systems do involve some wasted water, however. Because the system must continually sluice away the particles the membrane traps, 3 or more gallons of wastewater typically go down the drain for every gallon of drinking water that’s produced.