Wastes Treatment Plants
municipal sewer treatment plants use bacterial digestion to treat
wastewater. There are many methods and devices used to do this,
but they all use bacterial digestion to remove organic waste from
Raw sewage coming into the treatment plant
(the influent) contains:
2. GRIT - stones, sand, coffee
grounds, cigarette butts, pieces of rubber, plastic and cloth,
and other non biodegradable matter.
3. SUSPENDED SOLIDS - particles of
organic waste that are able to be settled out of the water.
4. DISSOLVED SOLIDS - particles
of organic waste that are very fine and soluble, and can not be
settled out of the liquid.
job of the treatment plant is to remove as much
of the grit, dissolved and suspended solids as possible.
When these are removed, the cleaned water (the effluent) is discharged
into a lake, river or ocean, or simply allowed to trickle down
into the ground.
All municipal sewage treatment plants are regulated by state and
Federal agencies, and the performance of every treatment plant
is measured by testing the treated effluent as it is discharged
from the facility. The most important measurement of water quality
is the Biological Oxygen
Demand ( BOD ).
BOD is an indirect measurement of the amount of organic matter
in the water. This test shows how much oxygen would be required
(demanded) by aerobic bacteria if they were to digest all the
organic material in the water. This test is based on the fundamental
chemical reaction that describes bacterial digestion:
Organic waste + Oxygen ----Bacteria---->
Water + Carbon Dioxide
BOD is expressed in parts per million, or "ppm". When
the BOD of water is high, it means that the water has a lot of
organic matter in it - and that bacteria would demand - or use
- a lot oxygen or digest that amount of organic material. Likewise,
a low BOD measurement shows that most or all
of the organic material has been removed from the water.
As raw sewage influent enters the treatment plant, the first step
is to filter out any sand, grit, stones, cigarette butts and other
hard, insoluble matter. This is done by a mechanical filter, screen
assemblies and settling basins.
The next step in the process is to separate the suspended organic
solids from the water. This is done in a device called a primary
clarifier. In this clarifier, the suspended solids are settled
to the bottom, sometimes with the aid of flocculating agents.
After settling out as much of the suspended solids a possible,
these solids are pumped out of the bottom of the primary clarifier
as sludge. This process removes
most of the organic waste from the water. However, the liquid
portion that remains behind is still far from clean. It still
contains all the dissolved solids, and so further processing of
this liquid is required.
After this point, the sludge and the liquid portions of the waste
are treated separately. First, we will follow the liquid portion
through the remaining treatment processes, until it is clean and
ready for discharge. The remaining liquid contains dissolved
organic matter that must be removed by bacterial digestion before
the water is actually clean. This is done in another device,
often called the oxidation tank (pond).
This device is easy to identify by its bubbling and gurgling action,
caused by air being forced through it (aeration).
This aeration promotes the bacterial digestion process, and within
several hours, the bacteria will digest most of the remaining
When the digestion is deemed to be complete, the liquid waste
then goes into a "final" clarifier or finishing pond.
Here, the liquid is held motionless, and any remaining
solids (including the bacteria) are settled to the bottom, to
be pumped off and treated with the sludge. The remaining liquid
will be quite clear, as almost all of the organic matter has been
removed from it. At this point, the water must meet state
and Federal guidelines and be clean enough for discharge.
Often, the water receives on final treatment before
it is discharged to a lake, ocean, stream, river, etc. The
final treatment is disinfection to kill any and all the bacteria
that being discharged into the environment. There are several
different methods of disinfection, but chlorination (using
chlorine gas ) is the most common disinfection process
used in the world today.