UFC 3-280-03
23 JULY 2003
Because of the costs associated with the thermal conditioning, it is typically
not advantageous to use it where other methods, such as chemical conditioning, are
applicable.
2-4.4.5
Chemical Conditioning. Chemical conditioning is the most commonly used
pretreatment method for filter presses. Factors that can affect chemical conditioning are
the characteristics of the sludge, sludge handling and processing conditions, and sludge
coagulation and flocculation. Because sludge can consist of various types of primary,
secondary, and chemical solids of various inorganic and organic content, sludge
characteristics significantly affect the sludge conditioning. The characteristics that most
commonly affect conditioning and dewatering are sludge particle size and distribution,
particle surface charge and degree of hydration, particle interaction, solids
concentration, ratio of types of sludge (i.e., primary to secondary sludge), biopolymer
production, and alkalinity (EPA 1987, WEF 1992).
a. A primary objective of conditioning is to increase the particle size by com-
bining the smaller particles into larger, more easily handled particles. Because sludge
particles are negatively charged and typically repel rather than attract one another,
conditioning is required to neutralize the repulsive effects so the particles can collide
and increase in size.
b. Sludge handling and processing conditions also affect sludge conditioning.
For example, aged or unstabilized biological sludge that has been stored for a long
period, which could be as little as a few days, causing it to turn anaerobic, requires
more conditioning chemicals for dewatering than does fresh sludge. Long raw sludge
storage or long transport periods can also increase the demand for chemicals and the
degree of hydration and fines content of any sludge stream. Hydroxide sludges may be
stored for weeks without inhibiting their dewaterability, provided the sludge feed is
mixed prior to dewatering, which provides a consistent sludge feed. In addition to stor-
age time, any preconditioning, such as the addition of chemicals for precipitation before
the conditioning chemicals are used, must also be considered. The sequence in which
the conditioning chemicals are added to the sludge must be considered. For example,
when using a two-chemical conditioning system, such as ferric chloride and lime or two
polymers, it is better to add the conditioning chemical with the anionic charge before the
chemical with the cationic charge. If using both cationic and non-ionic polymer, the
cationic charged polymer should be added before the non-ionic polymer. Although the
types of polymers and how to apply them are specified, the minimum lag time between
the two types should be several minutes to ensure adequate reaction. However, the lag
time should not exceed an hour or longer, because it can decrease the dewatering
performance.
c. Sludge coagulation and flocculation are the fundamental objectives of
conditioning. This two-step process consists of coagulation, where the sludge particles
are destabilized by decreasing the magnitude of the repulsive interactions between
particles, followed by flocculation, in which colloidal and finely divided suspended
particles are agglomerated by gentle mixing (EPA 1987). Following are design
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