Sewage is the wastewater released by residences,
businesses and industries in a community. It is 99.94 percent water,
with only 0.06 percent of the wastewater dissolved and suspended solid
Typically, sewage treatment is achieved by the initial
physical separation of solids from the raw wastewater stream followed
by the progressive conversion of dissolved biological matter into a
solid biological mass using indigenous, water-borne bacteria. Once the
biological mass is separated or removed, the treated water may undergo
additional disinfection via chemical or physical processes. This 'final
effluent' can then be discharged or re-introduced back into a natural
surface water body (stream, river or bay) or other environment
(wetlands, golf courses, greenways, etc.). The segregated biological
solids undergo additional treatment and neutralization prior to proper
disposal or re-use.The cloudiness of sewage is caused by suspended
particles which in untreated sewage ranges from 100 to 350 mg/l. A
measure of the strength of the wastewater is biochemical oxygen demand,
or BOD5. The BOD5 measures the amount of oxygen microorganisms require
in five days to break down sewage. Untreated sewage has a BOD5 ranging
from 100 mg/l to 300 mg/l. Pathogens or disease-causing organisms are
present in sewage. Coliform bacteria are used as an indicator of
disease-causing organisms. Sewage also contains nutrients (such as
ammonia and phosphorus), minerals, and metals. Ammonia can range from
12 to 50 mg/l and phosphorus can range from 6 to 20 mg/l in untreated
Sewage treatment is a multi-stage process to renovate
wastewater before it reenters a body of water, is applied to the land
or is reused. The goal is to reduce or remove organic matter, solids,
nutrients, disease-causing organisms and other pollutants from
wastewater. Each receiving body of water has limits to the amount of
pollutants it can receive without degradation. Therefore, each sewage
treatment plant must hold a permit listing the allowable levels of
BOD5, suspended solids, coliform bacteria and other pollutants. The
discharge permits are called NPDES permits which stands for the
National Pollutant Discharge Elimination System.
Process / Stages
Wastewater entering the treatment plant includes 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 a 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 up to the
aeration tanks (item 3). From here on, gravity takes over to move the
wastewater through the treatment process.
One of the first steps that a water treatment facility
can do is to 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 tanks.
Each tank is divided into two sections. In the first section, air is
pumped through the water.
4. Removing sludge
As organic matter decays, it uses up oxygen. Aeration
replenishes the 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 tanks and taken to landfills.
Wastewater then enters the second section or
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 called digesters.
5. Removing scum:
As sludge is settling to the bottom of the
sedimentation tanks, lighter materials are floating to the surface.
This 'scum' includes grease, oils, plastics, and soap. Slow-moving
rakes skim the scum off the surface of the wastewater. Scum is
thickened and pumped to the digesters along with the sludge.
6. Killing bacteria:
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 other solid particles that
remained in the water. Water is sometimes filtered through carbon
particles, which removes organic particles. This method is used in some
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
protects fish and other marine organisms, which can be harmed by the
smallest amounts of chlorine.
7. Wastewater Residuals:
The treated water (called effluent) is then discharged to a local river or the ocean
Another part of treating wastewater is dealing with the
solid-waste material. These solids are kept for 20 to 30 days in
large, enclosed tanks called 'digesters.' Here, bacteria break down
(digest) the material, reducing its volume, odors, and getting rid of
organisms that can cause disease. The finished product is mainly sent
to sludge drying beds, landfills, but sometimes can be used