When small amounts of sewage are discharged into a flowing body of water, a natural process of stream self-purification occurs. However, densely populated communities generate such large quantities of sewage that dilution alone does not prevent pollution. Instead of discharging sewage directly into a nearby body of water, it’s better to let it pass through a combination of physical, biological, and chemical processes that remove some or most of the pollutants. This takes place in sewage treatment plants.