Wastewater Treatment Systems
In the past, treatment of wastewater involved opening the door and throwing the contents of the wash basin out into the street. Practices such as this led to epidemics of such diseases as typhus, cholera and dysentery. Fortunately, today's modern wastewater treatment systems make these sanitation borne nightmares a thing of the past.
In modern wastewater treatment, there are several methods of sewage processing. One of the most popular is the use of aerobic microorganisms to digest organic wastes such as human excrement and food waste. In this system, the untreated sewage is placed in an environment with aerobic bacteria living in (or on) a nutrient media along with heavy aeration. The bacteria consume organic waste in the sewage stream and produce sludge, which is then filtered out of the waste stream and sent for disposal, most popularly by anaerobic digestion or by agricultural application.
The resulting liquid after the sludge has been removed is usually sent to a large basin, perhaps with aeration. Or it may be mechanically filtered. At any rate, at this stage the goal is to remove nearly all organic matter present to reduce the "decomposibility" of the water.
After the BOD (biochemical oxygen demand, a measure of "decomposibility") and turbidity (cloudiness) has reached acceptable standards the water is then usually sterilized with chlorine or UV light and aerated. As a side note, those wonderful fountains in front of the waterworks isn't just for show. They are aerating the final product before release. At this point, the water is as pure as rain...and ready to be released in the environment.