The Central Wastewater Treatment Plant in Nashville.

Wastewater-treatment plants are essential for cleaning "used" water before releasing it back into the environment. Methods used to create clean water go from simple physical mechanisms, allowing solids to settle in ponds, to chemical (using chlorine) and even using ultraviolet light to kill harmful organisms.

► Take a look at the processes that occur at a wastewater-treatment plant.

• Water Science photo gallery