$~~~~$Early civilizations often built drainage systems in urban areas to handle rainwater that ran down the street during a storm. The Romans constructed elaborate systems that also drained wastewater from the public baths. However, as the population of the cities grew, the old drainage systems became overloaded. During the Industrial Revolution, manufacturing waste was added to sewage, which increased the need for more efficient sewage treatment. In the mid-19th century, the first steps were taken to treat wastewater.
$~~~~$Sewage or wastewater treatment in modern times is the process of removing harmful physical, chemical, and biological elements from wastewater and house sewage. The whole process starts with screening out large objects such as paper and wood, and removing heavy materials like dirt. The screened wastewater is then ready to go through a series of concrete tanks for further treatment. In the second step, the sewage passes into the primary tanks. Here, human waste, called sludge, settles to the bottom while oils and grease float to the top, where they are collected. At the same time, organic matter like eggshells or coffee grounds in the sewage is broken down into smaller substances. The remaining sewage then enters the secondary tanks for the third stage of treatment. The solids that were not treated in the primary tanks are removed here through decomposition, which digests the material. Then, the liquid sewage is filtered through sand. This filtering process gets rid of almost all bacteria, as well as other solid particles that remain in the water. Finally, the wastewater flows into the last tanks, where the chemical chlorine is added to kill the remaining bacteria. After the bacteria are destroyed, the chlorine is eliminated from the water, and the treated clean water is discharged into a river or the ocean.