Wastewater treatment is a multistage process of getting the shit out of the water. Here is the process as it is implemented in most cases. Some plants may differ.
Primary wastewater treatment is largely a mechanical process of removing suspended pollutants. After preliminary screening to remove large particles, wastewater flows or is pumped into settling tanks. Here, the water is slowed considerably and gravity goes to work drawing heavier solids to the bottom. A system of rakes and scrapers remove floating matter from the surface and sludge from the bottom of the tanks. The average settling time is two hours under normal conditions. During heavy rains that time is often reduced considerably to accommodate the increased flow. On average, primary treatment removes 65% of the pollutants in wastewater.
Secondary treatment is actually a number of related processes with one main purpose -- to encourage microorganisms to digest dissolved pollutants. Wastewater leaves primary treatment and is pumped over plastic 'honeycomb' filters that infuse the water with microbes. Then the water is infused with liquid oxygen to increase the microbes' metabolism. The mixture is kept in an agitated state to keep everything in suspension and to further encourage digestion. This aeration and agitation process lasts about 2 hours on average (less during heavy rains).
After this stage, the water is brought to final settling tanks where any remaining biosolids precipitate out of the solution. A polymer is introduced to attach to living and dead microbes and bring them to the top for skimming off. From here, the wastewater moves to disinfection basins, also known as chlorine contact basins.
In these basins, wastewater is injected with a sodium hypochlorite (bleach) solution. The bleach is kept in with the wastewater for 30-45 mintues to ensure all remaining microorganisms are killed off.
Lastly, the water is treated with sodium bisulfite to neutralize the bleach before the water returns to the river.
So what happens to the shit? The sludge from primary and secondary treatment is sent to tanks where it is further digested by microorganisms. The thickened, concentrated sludge is then oxidized (baked) under pressure at 370°. Then, the resulting material is further dehydrated in centrifuges that reduce the sludge to 35-50% dry solid cake. At this point, the sludge is ready for the landfill. Water removed from these processes is further treated, usually by returning the water to primary processing area.
Information gathered from my experience working for the Louisville Jefferson County Metropolitan Sewer District in Kentucky and from MSD's website (www.msdlouky.org). I was unsure whether this writeup belonged here or in its own node. I can move it if the management thinks it is wise to do so.