Under our houses, shops and factories there are a set of pipes separate to the ones that bring us water. These pipes are fed from drains connected to the bathroom, kitchen, laundry e.t.c. Everything that goes down these drains heads toward the water reclamation facility where the wastewater is cleaned and treated.
It's important to remember drains aren't bins. Putting the wrong thing down the sink or toilet can cause blocked drains and/or a breakdown in the wastewater system. Read about how to mind your wastewater here.