Imagine you are on a conveyor belt. The conveyor belt is circling around a box of pizza. Every time you get close enough, you take a slice of pizza. If the conveyor belt moves too fast, you can’t grab the pizza. But if it’s too slow, you’ll starve and the pizza won’t be nice and hot anymore and it might even start to grow mold. This is how the activated sludge process was explained to me in my first Environmental Engineering lecture many moons ago.