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2-4.3.4 Other Sludge Transfer Processes. Other sludge transfer processes include
those following the filtration cycle, such as air blowdown or core blowing, which blow
compressed air through the filter press systems to remove liquid sludge.
2-184.108.40.206 Air Blowdown. Air blowdown is a recommended, but optional, feature that
can be used prior to cake discharge to aid in the release of the sludge cake, improve its
dryness, and drain the remaining liquid from filtrate ports. The air blowdown system
consists of piping and valves that connect the filtrate ports with a common discharge
pipe to form a manifold, through which compressed air is blown after the filtration cycle
and cake discharge to remove any residual liquid in the cake or filtrate ports. The liquid
removed during this process is typically returned to the sludge storage or conditioning
tank. Criteria to be considered for air blowdown are air flow rate, pressure, and duration.
Typical air usage requirements are based on the filter area of 0.02 to 0.07 L/sm2 (0.2 to
0.8 scfm/ft2) at an operating pressure of 280 kPa (40 psi) for a duration of 1 to 3
2-220.127.116.11 Core Blowing. Core blowing is another recommended, but optional, feature.
Liquid sludge remaining in the sludge feed ports is removed by compressed air before
the press is opened at the end of the filter cycle. Although the amount of remaining
liquid sludge is typically small and has little effect on the moisture content of the sludge
cake, it has a tendency to run down the face of the filter media, blinding localized areas,
and subsequently forming a non-uniform cake. Core blowing reduces the potential for
non-uniform cake formation and minimizes the frequency of filter media washing.
Criteria to be considered for core blowing include the pressure of air required
and duration, typically operating pressures ranging from 550 to 690 kPa (80 to 100 psi)
for a duration of 1 to 3 minutes. Although core blowing is a recommended option that
provides a desirable effect, the cost of equipment, piping, and building space should be
considered before a designer selects this feature, especially for smaller systems or one
not treating HTRW.
Pretreatment Requirements. Sludge pretreatment typically includes sludge
degritting and grinding, and sludge conditioning. Grit is typically removed and ground at
the headworks of the treatment facility to reduce wear and maintenance on downstream
processes. Sludge is conditioned before dewatering to enhance water removal and to
improve solids capture by chemical or physical treatment of the sludge. The most
commonly used conditioning methods use chemical or thermal treatment. Chemical
conditioning uses inorganic chemicals (i.e., ferric chloride and lime) or organic
polyelectrolyte (polymers), or both. Thermal conditioning enhances the dewatering
characteristics of the sludge by applying both heat and pressure. Although the thermal
methods and other methods involving sludge thickening and stabilization can be used,
chemical conditioning is used most often. Following is a brief summary of degritting and
grinding, sludge thickening, and sludge stabilization, and a detailed discussion of
chemical sludge conditioning.