I
TM 5-813-3/AFM 88-10 Vol 3
ity solids concentrators. Two or more of these proc-
solids content in the lagoon may be as low as one per-
esses can be used within the same system to obtain a
cent or as high as 17.5 percent. A liner of impervious
higher degree of solids concentration than would be at-
material may be required within the lagoon by state
tainable using only one process. In order to enchance
authorities if ground water contamination is a con-
the performance of some dewatering devices, the
cern. Water should be removed from the lagoon by re-
sludge can be "conditioned" prior to dewatering.
cantation. The decanted water may be returned to nat-
Among the methods of conditioning which have been
ural watercourses if state authorities permit, and is
sometimes returned to the treatment plant for recy-
used are: application of heat and pressure, freezing,
cling. At least two lagoons must be provided so that
lime treatment, and application of organic polymers.
fresh alum sludge can be placed in one while the alum
(e) Alum recovery. Recovery of alum from alum
sludge in the other is allowed to concentrate. After
sludge is possible by treatment of the sludge with sul-
furic acid followed by sedimentation or filtration to re-
sufficient drying, the sludge should be removed from
the lagoon and placed in a landfill or spread on suit-
move raw water sediment. Recovered alum can be re-
cycled, so long as inert material, iron and manganese,
able ground. The minimum solids content which
toxic metals, and color, do not become unacceptably
should be attained before alum sludge can be removed
from lagoons is generally about 10 percent. In colder
concentrated in the recycled alum solutions. Of these
climates, the freeze-thaw cycles to which the liquid in
materials, iron and manganese usually pose the great-
the lagoon is subjected will aid materially in concen-
est problem.
trating the solids. Sludge lagoons should be enclosed
(3) Lime-soda softening sludge. In most cases,
lime-soda softening sludge will be managed by lagoon-
by fencing adequate to exclude children and animals.
(b) Discharge to sanitary sewers. Alum sludges
ing. In order to use this method, large areas of land
may also be discharged to sanitary sewers if disruption
must be available within a reasonable distance from
of wastewater treatment processed is not anticipated.
the treatment plant. Lagoon capacity should be at
If this procedure is chosen, precautions must be taken
least 3.5 acre-feet per million gallons daily per 100
to insure that the sludge does not create a hydraulic
mg/L of hardness removed. Sized on this basis, the
overload in the sewers or form significant deposits in
storage capacity will be sufficient for 2-1/2 to 3 years,
the sanitary sewer. Inasmuch as a large portion of
after which the accumulated sludge must be removed
alum sludge is not biodegradable, the addition of alum
for disposal on farm land or in a landfill. If the sludge
sludge to wastewater will increase the sludge produc-
must settle through ponded water, a solids concentra-
tion of 20 to 40 percent can be anticipated. If the
tion at the wastewater treatment plant. Disposal of
ponded water is regularly decanted, the solids content
alum sludge to storm sewers is equivalent to disposal
in natural water courses and should not be attempted.
of the sludge will be about 50 percent. As in the case of
(c) Sludge beds. Another method of dewatering
alum sludge lagoons, softening sludge lagoons should
is application of the sludge to special sludge beds.
be constructed in groups of at least two or three to al-
These beds are usually composed of 6 to 12 inches of
low for alternate filling, drying, and removal of the
sand ranging in size up to 0.5 mm, with an underdrain
dried sludge. Lagoon depths will vary from 3 to 10
system of graded gravel 6 to 12 inches deep. Drain
feet. Lagoons will be fenced. The area dimensions of
pipes 6 to 8 inches in diameter are placed in the gravel
sludge lagoons should be such that the settled sludge
can easily be removed by cranes or draglines. The
to carry away the water from the beds. Sand beds can
sludge removed from lagoons can be placed in landfills"
usually achieve a 20 percent solids concentration in
or used as soil conditioner, although in some cases the
alum sludge within 100 hours at a loading rate of 0.8
applied sludge has plugged the upper soil layer until
pounds per square foot. However, the results are high-
broken down by winter freezing. The sludge maybe re-
ly dependent on the characteristics of the sludge and
calcined for lime recovery, usually after removal of
local climatic conditions. Warm, dry climates are best
most of the magnesium hydroxide by recarbonation
suited to the use of sand drying beds. The water pass-
and centrifuging. Other methods of dewatering soften-
ing into the drain pipes should be suitable for disposal
ing sludge prior to recalcining or landfilling include
into natural watercourses, The dewatered sludge is us-
vacuum filtration and centrifugation. Vacuum filters
ually removed from the sand bed by mechanical
and centrifuges produce sludge cake having solids con-
means, but a minimum solids content of approximate-
tent of 50 to 60 percent. Lime-soda softening sludge
ly 20 percent must be attained before mechanical han-
will not be discharged to sanitary sewers, and drying
dling is practical. After removal, the dewatered sludge
beds are not recommended because of clogging diffi-
is usually hauled to a landfill.
(d) Mechanical dewatering devices. Several me-
culties and potential dust nuisance.
(4) Diatomite sludge. Diatomite sludge dewaters
chanical devices have been used for dewatering of
rather easily, so any of several techniques, particularly
alum sludge, including pressure filters, centrifuges,
freeze-thaw devices, vacuum filters, and dual-cell grav-
vacuum filtration and lagooning, may be used to in-
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