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.
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.