Overview of Sewage Treatment

The basic process of wastewater treatment in a treatment plant is shown in the diagram below.  The solids are allowed to settle out of the water and the remaining organic matter is broken down by microorganisms.  
As in a water treatment plant, water in the wastewater treatment plant moves through a variety of chambers, each of which has a specific function.


The raw wastewater enters the treatment plant and is immediately passed through a bar screen.  This removes the largest solids from the water.  These solids may be hauled to a landfill or ground up and allowed to pass through the rest of the treatment process.  

After passing through the bar screen, the influent enters the grit chamber.  Here, the wastewater passes into a wide basin, which slows the wastewater's velocity.  The slower flow causes grit to settle out.  Grit is the heaviest material in wastewater and includes substances such as sand, coffee grounds, eggshells, gravel, and cinders.  Grit cannot be broken down by biological processes, so it is hauled to a landfill.  

Next, the remaining wastewater reaches the comminutor, also known as the grinding pump.  In the comminutor, water is passed through a rotating cutting screen.  This cutting screen shreds any large chunks of organic matter in the wastewater into smaller pieces.  This makes it easier for the the microorganisms to use the organic matter as food.  

The final stage of pretreatment is aeration.  In the aeration basin, oxygen is added to the water.  In addition, microorganisms are seeded (added to the wastewater) here, and they begin to use the oxygen to break down organic matter.  As the microorganisms eat, they multiply rapidly and consume the B.O.D. (Biochemical Oxygen Demand; the organic matter or "food") very quickly.  The microorganisms usually only require two hours in the aeration basin to consume all of the organic matter.  

The air which is forced into the wastewater in the aeration chamber also serves another purpose.  It keeps the microorganisms suspended in the water so that they do not settle out.  

Primary Treatment

After the wastewater leaves the aeration chamber, it enters the clarifier.  The clarifier is a type of sedimentation basin in which the heavier solids sink to the bottom and the lighter materials float to the surface.  

One of the primary purposes of the clarifier is to remove the microorganisms from the water.  After digesting organic matter in the aeration basin, these microorganisms now have food, grit, and other particles stuck to their outer enzyme coating.  So they are heavy and sink to the bottom (or floc out) in the clarifier.  The supernate, a clear liquid, rises to the top and is allowed to flow out of the clarifier.

The sludge at the bottom of the clarifier contains a great deal of microorganisms.  Some of this sludge is removed and either digested or sent to a landfill (to "waste").  But some of the sludge is reused to seed the aeration chamber with microorganisms.  


After leaving the clarifier, the supernate is chlorinated and allowed to sit in a contact chamber while the chlorine reacts with microorganisms in the water.  This process disinfects the water, killing the disease-causing microorganisms.  Now the water has been thoroughly treated and can be released into natural bodies of water.