Between flush and faucet
Preliminary Treatment: All the random debris in sewage is filtered out, typically using bars and grates. Essentially, the treatment process starts by removing the largest objects and moving on to the smallest.
Primary Treatment: The waste then is sent to settling tanks, where heavy solids, such as fecal matter, sinks to the bottom and forms a sludge. Meanwhile, oils and soapy chemicals float to the top and form scum.
Secondary Treatment: The next step is to remove organic matter and other biological contaminants from this clarified water. In aeration tanks, oxygen bubbles into the water, allowing microorganisms to gradually digest contaminants. After gorging, these microorganisms clump together and settle to the bottom of the tank.
Tertiary Treatment: Wastewater that will be used for non-potable purposes, such as irrigating crops or golf courses, must then be disinfected. Typically, chlorine is used to kill bacteria, making the water safe for many purposes, but not for drinking.
Advanced Purification: Additional steps are taken to purify the water enough to consume. It is disinfected with ozone, digested by microbes and filtered through activated carbon. Reverse osmosis clears away chemicals and salt, and finally, advanced oxidation with UV light cleanses remaining contaminants.
Graphic by John Osborn D'Agostino