Environmental Engineering Reference
In-Depth Information
32.5.1 S
ludgE
S
ourcES
, c
haractEriSticS
,
and
p
ropErtiES
There are several sources of sludge or solids. Sludge can originate from chemical processes such
as precipitation or coagulation; physical processes such as sedimentation; or biological processes
such as in the wastewater treatment plants. Solids are constituted of organic and inorganic solids.
Municipal sludge is mainly of biological origin and is produced in wastewater treatment plants.
Such solids are mostly organic (from 50 to 90% is organic component). The sources of solids vary
according to the type of the plant and its method of operation. The principal sources of solids and
the types are presented in Table 32.13.
To treat and dispose of the solids originated in wastewater treatment plants effectively, it is
important to know the characteristics of the solids to be processed. The characteristics vary
depending on the origin of the solids, the amount of aging that has taken place, and the type of
processing to which they have been subjected. Solids characteristics are gathered in Table 32.14.
32.5.2 g
EnEral
c
ompoSition
Typical data on the chemical composition of untreated solids are reported in Table 32.15. Many
chemical constituents, including nutrients, are important in considering the ultimate disposal of the
processed solids and liquid removed during processing. The measurement of pH, alkalinity, and
organic acid content is important in process control of sludge digestion. The content of heavy metals,
pesticides and hydrocarbons has to be determined when land application methods are considered.
The thermal value of solids is important where incineration is considered.
Waste sludge is generally composed of solids and water. Solids are only up to 10% of sludge
(often referred to as total dry solids or just total solids); the rest is water. Solids are then generally
divided in organic solids (often referred to as volatile solids) and inorganic solids.
One of the most important factors in sludge digestion is the organic component in solids (volatile
solids). From this parameter depends how much solids can be removed. The higher the organic
content in solids, the better the removal rate will be, and in case of anaerobic digestion, this means
higher biogas production. Also, the thermal value of sludge increases with the content of organic
solids in sludge. The inorganic components of sludge (inorganic solids) mainly stay inactive
throughout the sludge digestion process.
taBle 32.13
sources of solids from conventional Wastewater treatment Plants
unit operation
or process
types of solids
remarks
Screening
Coarse solids
Coarse solids are removed by mechanical and hand cleaned bar screens. In small
plants solids are often comminuted for removal in subsequent treatment units
Grit removal
Grit and scum
Scum removal facilities are often omitted in grit-removal facilities.
Preaeration
Grit and scum
In some plants, scum removal facilities are not provided in preaeration tanks.
If the preaeration tanks are not preceded by grit removal facilities, grit
deposition may occur in preaeration tanks.
Primary
sedimentation
Primary solids and
scum
Quantities of solids and scum depend upon the nature of the collection system
and whether industrial wastes are discharged to the system.
Biological
treatment
Suspended solids
Suspended solids are produced by the biological conversion of BOD. Some
thickening may be required to concentrate the waste sludge stream from the
biological treatment system.
Secondary
sedimentation
Secondary biosolids
and scum
For good wastewater treatment it is necessary to remove solids from secondary
settling tanks.
