Here's a step-by-step guide
describing what happens at each stage of the treatment
process and how pollutants are removed help keep our
The Primary Treatment Process
Wastewater entering the treatment plant include items like
wood, rocks, and even dead animals. Unless they are removed,
they could cause problems later in the treatment process.
Most of these materials are sent to landfill.
The wastewater system relies on the force of gravity to move
sewage from your home to the treatment plant. So
wastewater-treatment plants are located on low ground, often
near a river into which treated water can be released. If
the plant is built above the ground level, the wastewater
has to be pumped to the aeration tanks (item 3). From here
on gravity takes over to move the wastewater through the
One of the first steps that a water treatment facility can
do is just shake up the sewage and expose it to air. This
causes some of the dissolved gases (such as hydrogen
sulfide, which smells like rotten eggs) that taste and smell
bad to be released from the water. Wastewater enters a
series of long, parallel concrete tank. Each tank is divided
into two sections. In the first section, air is pumped
through the water.
As organic matter decays, it uses up oxygen. Bubbling oxygen
through the water also keeps the organic material suspended
while it forces 'grit' (coffeegrounds, sand and other small,
dense particles) to settle out. Grit is pumped out of the
tank and taken to landfills.
4. Removing sludge:
Wastewater then enters the second section of sedimentation
tanks. Here, the sludge (the organic portion of the sewage)
settles out of the wastewater and is pumped out of the
tanks. Some of the water is removed in a step called
thickening and then the sludge is processed in large tanks
As sludge is settling to the bottom of the sedimentation
tanks, lighter materials are floating on the surface. This
'scum' includes grease, oils, plastics, and soap.
Slow-moving rakes skim the scum off the surface of
wastewater. Scum is thickened and pumped to the digesters
along with the sludge.
Many cities also use filtration in sewage treatment. After
the solids are removed, the liquid sewage is filtered
through a substance, usually sand, by the action of gravity.
This method gets rid of almost all bacteria, reduces
turbidity and color, removes odors, reduces the amount of
iron, and removes most solid particles that remained in the
water. Water is sometimes filtered through carbon particles,
which removes organic particles. This method is used in some
6. Killing bacteria:
Finally, the wastewater flows into a 'chlorine contact'
tank, where the chemical chlorine is added to kill bacteria,
which could pose a health risk, just as is done in swimming
pools. The chlorine is mostly eliminated as the bacteria are
destroyed, but sometimes it must be neutralized by adding
other chemicals. This fish and other marine organisms, which
can be harmed by the smallest amounts of chlorine. The
treated water (called effluent) is then discharged to a
local river or the ocean.