For 3 billion years, Earth has been using, cleaning and then reusing its water again and again. Water never wears out or breaks. It just gets dirty and needs to be cleaned. The ability of water to be used and reused is known as recycling.

Waste Disposal throughout History
Even in ancient times, the disposal of sewage and wastewater was an important health issue. The ancient civilizations of Assyrians, Babylonians, Greeks and Romans built extensive sewer systems to dispose of human waste. Interestingly, the storm water sewer systems built by the Romans thousands of years ago are still being used today in parts of Europe.

By the beginning of the 20th Century, human waste was removed from cities and towns through miles and miles of wastewater pipeline. However, the pipelines often dumped the untreated waste into surrounding bodies of surface water. This practice greatly contributed to the spread of diseases like cholera and caused the death of numerous plants and animals. Clearly, the only solution was to figure out a way to clean and purify the wastewater before it reached the main water supply.

Modern Water Treatment
Just like nature is able to clean dirty water and make it usable again, modern treatment facilities use similar processes to clean wastewater. The four major wastewater cleaning components are:

1. Pretreatment: Metal screens are used to remove large objects and chunks of debris when water passes through.

2. Primary treatment: Gravity is used to cause solid matter to settle to the bottom of large basins.

3. Secondary treatment: Microorganisms are added to digest any remaining pollutants. Air is also added to speed up digestion. After the microorganisms have "eaten," they settle to the bottom of large basins.

4. Tertiary treatment: Remaining small solid particles are mixed with a chemical to form larger particles called floc. The floc is then trapped in filters containing anthracite coal and/or sand. Chlorine or another type of disinfectant is added to water to ensure that the water is safe before being released back to into the environment through a network of underground pipelines. The recycled water pipelines are separate from drinking water pipelines.