Botkin Environmental Science Earth as Living Planet 8th txtbk

416 CHAPTER 19 Water Pollution and Treatment
Primary Treatment
Incoming raw sewage enters the plant from the municipal
sewer line and first passes through a series of screens to
remove large floating organic material. The sewage next
enters the “grit chamber,” where sand, small stones, and
grit are removed and disposed of. From there, it goes to
the primary sedimentation tank, where particulate matter
settles out to form sludge. Sometimes, chemicals are
used to help the settling process. The sludge is removed
and transported to the “digester” for further processing.
Primary treatment removes approximately 30 to 40% of
BOD by volume from the wastewater, mainly in the form
of suspended solids and organic matter. 28
Secondary Treatment
There are several methods of secondary treatment. The
most common treatment is known as activated sludge,
because it uses living organisms—mostly bacteria. In this
procedure, the wastewater from the primary sedimentation
tank enters the aeration tank (Figure 19.16), where
it is mixed with air (pumped in) and with some of the
sludge from the final sedimentation tank. The sludge
contains aerobic bacteria that consume organic material
(BOD) in the waste. The wastewater then enters the final
sedimentation tank, where sludge settles out. Some of this
“activated sludge,” rich in bacteria, is recycled and mixed
again in the aeration tank with air and new, incoming
wastewater acting as a starter. The bacteria are used again
and again. Most of the sludge from the final sedimentation
tank, however, is transported to the sludge digester.
There, along with sludge from the primary sedimentation
tank, it is treated by anaerobic bacteria (bacteria that can
live and grow without oxygen), which further degrade the
sludge by microbial digestion.
Methane gas (CH 4 ) is a product of the anaerobic
digestion and may be used at the plant as a fuel to run
equipment or to heat and cool buildings. In some cases,
it is burned off. Wastewater from the final sedimentation
tank is next disinfected, usually by chlorination, to eliminate
disease-causing organisms. The treated wastewater is
then discharged into a river, lake, or ocean (see A Closer
Look 19.4), or in some limited cases used to irrigate farmland.
Secondary treatment removes about 90% of BOD
that enters the treatment plant in the sewage. 28
The sludge from the digester is dried and disposed
of in a landfill or applied to improve soil. In some instances,
treatment plants in urban and industrial areas
contain many pollutants, such as heavy metals, that are
not removed in the treatment process. Sludge from these
plants is too polluted to use in the soil, and sludge must
Primary treatment
Secondary treatment
Incoming raw
sewage from
sewer line
Screens
Sludge
Aeration tank
Discharge of treated
water to surface
water (rivers, lakes,
oceans)
Grit
chamber
Sedimentation
tank
Air
Activated
sludge
return
Final
sedimentation
tank
Disinfection
Sludge is disposed of
in a landfill or used to
improve agricultural
land or for mine
reclamation
Digester
Sludge
drying beds
FIGURE 19.16 Diagram of sewage treatment processes. The use of digesters is relatively new,
and many older treatment plants do not have them.