UFC 3-280-03
23 JULY 2003
ferrous sulfide, ferrous chloride, and aluminum sulfate, have been used. Ferric chloride
is added before lime is added because it hydrolyzes in water and forms positively
charged iron complexes that neutralize negatively charged sludge solids and allows
them to aggregate. Ferric chloride also reacts with the bicarbonate alkalinity in the
sludge to form hydroxides that act as flocculants. This reduction in alkalinity also lowers
the pH. The lime is then added to raise the sludge pH so that the hydroxides form more
efficiently from the ferric chloride reaction. Lime also reacts with bicarbonate to form
calcium carbonate, which provides the sludge with additional structural integrity and
porosity needed to increase the rate of water removal during the pressure filtration. The
optimum dosages of both ferric chloride and lime for conditioning depend on the type of
sludge and associated characteristics. Applicability and typical dosages for these
chemicals for various sludge applications are presented in Tables 2-4 and 2-5.
a. The lime and ferric chloride conditioning system typically consists of sludge
transfer pumps, a lime slurry preparation and feed system, a ferric chloride solution
preparation and feed system, a conditioning tank equipped with a mixer, and a press
feed pump system. It is usually desirable to use a sludge storage tank prior to the
conditioning system to provide an inventory of the amount and type of sludge to be
conditioned. This storage tank should be equipped with a mixer so that the sludge will
have uniform concentration characteristics, allowing proper dosages and conditioning
chemicals to be used to. A schematic of a typical lime and ferric chloride system is
shown in Figure 2-6. A more detailed description of lime slurry and ferric chloride
preparation and feed systems is presented in Subparagraph 2-4.6.1.
b. Ferric chloride should be added to the conditioning tanks a minimum of 10
seconds before the lime slurry is added to assist floc formation. To provide additional
reaction time with the sludge, the ferric chloride solution can also be injected directly
into the sludge transfer pipeline. Injection methods include direct injection without
mixing, mixing at the point of entry, or injection with air. If direct injection is used, a 10 to
20-second retention period is required before lime is added to the conditioning tank. If
the mechanical mixing method at the point of entry is used, an additional tank and
mixing equipment are required.
c. The conditioning tank should be equipped with a mixer (typically vertical
gate or turbine mixers) to ensure that the sludge and conditioning chemical are
thoroughly mixed and to develop the floc. To provide adequate time for mixing and floc
formation, the minimum retention time in the conditioning tank should be 5 to 10
minutes (WPCF 1983). A long retention time, 20 to 30 minutes, may be used to ensure
complete mixing with the lime to minimize potential lime scale buildup in piping and filter
media. However, if the retention time is longer than that required to form a good sludge
floc, the floc can deteriorate and break down. The retention time may be affected by the
changing sludge feed rates required throughout the filter cycle and by the use of
multiple presses, if required.
d. The sludge conditioning system must be designed to meet the feed
requirements of the filter press. Two methods can be used to achieve this goal. The first
uses sludge transfer pumps designed to maintain a nearly constant level of sludge in
the conditioning tank, while meeting the requirements of the conditioned sludge feed
2-36
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