23 JULY 2003
chemical sludge. Following is a summary of the generation, composition, and charac-
teristic of each of these types of sludge.
2-18.104.22.168 Primary Sludge. Primary sludge is typically generated by solids separation or
sedimentation and gravity settling to remove settleable solids. This sludge consists
primarily of organic solids, grit, and inorganic fines. This sludge is typically pumped to
downstream processing facilities for thickening, conditioning, and dewatering prior to
2-22.214.171.124 Biological Sludge. Biological sludge, a term typically associated with
municipal-type sludge but which also applies to industrial and HTRW sludge, is gener-
ated by biological treatment processes, such as activated sludge, and fixed film biore-
actors. This sludge consists primarily of conversion products from organics in the
primary effluent and suspended particles that escaped the initial treatment. This type of
sludge is generally more difficult to thicken and dewater than primary and chemical
2-126.96.36.199 Chemical Sludge. Chemical sludge is generated when chemicals, such as
aluminum or iron salts, lime, or polymers, are added to precipitate suspended solids.
The iron and aluminum salts, lime, and polymers are primarily used to cause the
suspended solids to flocculate and coagulate. Parameters that affect the characteristics
of chemical sludge include: the wastewater composition (chemistry), pH, mixing, and
reaction time. Chemical sludge may also consist of suspended solids, in addition to
potentially toxic material loadings and industry-specific components (i.e., heavy metals
from metal processing industries).
2-3.1.2 In addition to these three primary sludge sources, mixed sludge can also
exist. Mixed sludge may consist of a combination of primary, biological sludge, and
chemical sludge and will have properties that are proportional to the respective compo-
sition of each original type of sludge.
2-3.1.3 Selection of the appropriate sludge dewatering process depends on several
factors, including ultimate disposal or use, potential side streams, and local, state, and
Federal laws. Several analyses can also be used to determine the optimum sludge
dewatering process, including an initial screening of dewatering processes, an initial
cost evaluation, laboratory (bench-scale) testing, field (pilot-scale) testing, and a final
evaluation based on detailed design parameters. Additional criteria to consider include
integration with proposed or existing wastewater treatment equipment and technologies,
operation and maintenance costs, reliability of the dewatering device, existing site and
environmental constraints, and compatibility with the ultimate disposal method. A
general block diagram showing typical solids handling treatment and disposal methods
is shown in Figure 2-4.