23 JULY 2003
2-188.8.131.52 Acid Wash Systems. Acid washing is primarily used when sludge is
conditioned with lime. It is essentially used for larger or continuously operating
dewatering systems; however, its technical and economical feasibility should be
evaluated for small or intermittently operated systems. The acid wash is typically an
internal washing system that uses a dilute hydrochloric acid solution to remove lime
scale buildup on the filter media by pumping and circulating the dilute acid throughout
the closed press. Typically, continuously operating presses are acid washed at least
every 30 to 40 cycles or as often as once a week (WPCF 1983).
Equipment for this system usually includes a bulk acid storage tank, acid
transfer pump, water metering system, dilute acid washwater storage tank, acid wash
pump, and associated valves and piping. The concentrated acid should be transferred
directly from the storage container to the acid wash solution dilution tank by an acid-
resistant (non-metallic) pump, such as a drum pump if the acid is shipped directly in
small containers. The dilute acid storage tank should be of sufficient capacity to fill the
press and allow for circulation. Typically, the capacity of the dilute acid storage tank
should be approximately 1.5 times the capacity of the press (e.g., 70 L/m3 [0.5 gallon
per cubic foot] of press). A low-pressure (e.g., 140 to 210 kPa [20 to 30 psi] maximum)
and acid-resistant pump should be used to transfer and recirculate the dilute acid.
Piping and plumbing should be provided to isolate the press from the sludge stream and
allow the acid wash to recirculate to the storage tank where the spent acid solution can
be drained. A throttling valve installed in the return line to the acid tank is often required
to ensure complete top-to-bottom press filling and washing of the filter-media. A sche-
matic of a typical acid wash system is shown in Figure 2-9.
Typically, a 38% concentration hydrochloric acid wash that is delivered in
carboy containers (barrels), by tank truck, or in tank car shipments, is used. A final
solution strength of 5% up to a maximum of 25% is typically used.
Compressed Air Systems. Compressed air is required for several functions
and types of equipment within the filter press system, including opening and closing the
press for pneumatically operated, hydraulically controlled units, core and air blowdown,
plate shifting, inflation of diaphragms in variable-volume filter press systems, operation
of pneumatic controls, and operation of sludge feed pumps that have air diaphragms.
Although air requirements will be specific to each press, normally multiple air
compressors are required, with air filters and silencers to remove moisture and oil
condensates, air aftercoolers, air receivers, and associated valves and accessories,
such as pressure gauges and automatic and manual drains.
Although specific to each application, two grades of air may typically be
required for a filter press: plant grade air and instrument grade air. Instrument grade air
will be dry, and moisture- and oil-free. This type of air will normally be required for
equipment such as pneumatic controls. Plant air typically is unfiltered air used for core
and air blowdown or for inflating diaphragms in variable-volume units. To supply these
two grades of air, a dual air supply system may be used in conjunction with a primary
compressed air supply. The side of this dual system that supplies the instrument grade