Environmental Engineering Reference
In-Depth Information
10.2 sourCes of sludge
Wastewater sludge is generated in primary, secondary, and chemi-
cal treatment processes. In primary treatment, the solids that float or
settle are removed. The floatable material makes up a portion of the
solid waste known as
scum
. Scum is not normally considered sludge;
however, it should be disposed of in an environmentally sound way. The
settleable material that collects on the bottom of the clarifier is known
as
primary sludge.
Primary sludge can also be referred to as
raw sludge
because it has not undergone decomposition. Raw sludge from a typi-
cal domestic facility is quite objectionable and has a high percentage of
water, two characteristics that make handling difficult.
Solids not removed in the primary clarifier are carried out of the
primary unit. These solids are known as
colloidal suspended solids.
The secondary treatment system (e.g., trickling filter, activated sludge)
is designed to change those colloidal solids into settleable solids that
can be removed. Once in the settleable form, these solids are removed in
the secondary clarifier. The sludge at the bottom of the secondary clari-
fier is called
secondary sludge.
Secondary sludges are light and fluffy
and more difficult to process than primary sludges—in short, secondary
sludges do not dewater well.
The addition of chemicals and various organic and inorganic sub-
stances prior to sedimentation and clarification may increase the sol-
ids capture and reduce the amount of solids lost in the effluent. This
chemical addition
results in the formation of heavier solids, which trap
the colloidal solids or convert dissolved solids to settleable solids. The
resultant solids are known as
chemical sludges.
As chemical usage
increases, so does the quantity of sludge that must be handled and dis-
posed of. Chemical sludges can be very difficult to process; they do not
dewater well and contain lower percentages of solids.
10.3 sludge CharaCTerisTiCs
The composition and characteristics of sewage sludge vary widely
and can change considerably with time. Notwithstanding these facts,
the basic components of wastewater sludge remain the same. The only
variations occur in the quantity of the various components as the type of
sludge and the process from which it originated changes. The main com-
ponent of all sludges is
water.
Prior to treatment, most sludge contain 95
to 99% water (see Table 10.1). This high water content makes sludge han-
dling and processing extremely costly in terms of both money and time.
Sludge handling may represent up to 40% of the capital cost and 50%
of the operation cost of a treatment plant. As a result, the importance
of optimum design for handling and disposal of sludge cannot be over-
emphasized. The water content of the sludge is present in a number of
different forms. Some forms can be removed by several sludge treatment
processes, thus allowing the same flexibility in choosing the optimum
sludge treatment and disposal method. The various forms of water and
their approximate percentages for a typical activated sludge are shown
in Table 10.2. The forms of water associated with sludges include:
