Water treatment plants?

Do water treatment plants use an actual sand and pebble filtration system? I'm putting together a lesson for my cl and the epa's site says the final step in a water treatment plant is a sand and pebble filtration system, however, their language is unclear. I can't tell if that means that's the final step and we're using sand and pebbles to simulate it or if water treatment plants actually use this kind of filtration system. Thanks!