TM 5-814-8
presses sludge between two porous belts that
where it concentrates and is removed. The pri-
mary variables are recycle ratio, feed solids con-
forces water from the sludge through compres-
sion. The pressing operation is continuous and is
centration, air-to-solids (A/S) ratio, and solids and
hydraulic loading rates. Air pressures between 40
usually preceeded by a chemical addition phase
where flocculants are added to improve the
to 60 psi are commonly employed. The recycle
dewatering characteristics of the sludge. With the
ratio is related to the air-to-solids ratio and the
proper conditioning, belt presses can achieve a
feed solids concentration (72). Experience has
shown. that in some cases dilution of the feed
cake solid in the 20 to 30 percent range for
activated sludge and up to 35 to 40 percent cake
sludge to a lower concentration increases the
concentration of the floated solids. The use of
solids for metal hydroxide sludges.
d. Plate presses. Filter presses are an alterna-
polyelectrolytes will usually increase the solids
tive to vacuum filters and belt presses. Filter
capture and the thickened sludge concentration.
presses have higher capital and operating costs
c. Centrifuges. Centrifugation is employed both
for the thickening and the dewatering of sludges.
than either of the previous alternates, but pro-
The process of centrifugation is an acceleration of
duce a drier cake (solids concentrations in the
range of 25 to 40 percent). These units may be
the process of sedimentation by the application of
desirable at some installations to minimize fuel
centrifugal forces. There are three types of centri-
requirements when a combustion process follows
fuges available; the solid bowl, the basket type
or to reduce haul costs when long distances are
and the disc-nozzle separator. The basic difference
involved.
between the types of centrifuges is the method in
which solids are collected in and discharged from
7-6. Incineration
the bowl. Sludge solids settle through the liquid
pool and are compacted by centrifugal force
Sludge incineration reduces the volume handled in
against the walls of the bowl and are then
the transportation and ultimate disposal steps
conveyed by the screw conveyor to the drying or
and sterilizes the residue. High investment and
beach end of the bowl. The beach area is an
operating costs, and stringent air pollution crite-
i n c l i n e d section of the bowl where further
ria are significant considerations in determining
dewatering occurs before the solids are discharged
the need for incineration. Fuel is also a factor and
over adjustable weirs at the opposite end of the
without sufficient dewatering (to at least 35
.
bowl (80) Typically, centrifuges can thicken an
percent solids) the furnaces will be energy con-
activated sludge to a concentration of 5 to 10
sumers. Rarely has incineration been used at
percent without chemical addition.
military treatment facilities and it shall be evalu-
ated only for special applications or land scarce
7-5. Dewatering
areas. Fluidized bed furnaces may be considered
a. Drying beds. When stabilized sludge is de-
for some industrial wastes. Multiple hearth units
posited in a wet condition on the land, no
are predominantly used to burn sewage sludge.
dewatering is practiced. For facilities that require
Mixing sludge with refuse for burning takes
dewatering prior to disposal and have sufficient
advantage of the net heat generated by refuse
land area, drying beds are cost-effective and
should be used. Usually drying beds will be
feasible up to plant capacities of about 1 mgd.
7-7. Other processes
Sufficient storage should be provided in digesters
Many other sludge handling, processing and dis-
to allow operational flexibility.
posal operations have been tried and are in use at
b. Vacuum filters. Vacuum filtration is the
other than military installations and some pro-
most widely applied mechanical dewatering
cesses are currently in the technical development
method in the U.S. This method is well estab-
stage. These include pyrolysis, heat drying,
lished for removing moisture from sludge and can
comporting, freeze dewatering, drying lagoons,
achieve from 15 to 25 percent solids concentra-
rail and barge transport systems, fertilizer pro-
tion in the cake after dewatering. Vacuum filters
duction and others. Most of these are not practi-
shall be used for mechanical dewatering unless
cal or feasible for military facilities. Authority to
other methods are cost-effective for special appli-
deviate from using the proven processes pre-
cations.
sented in this section must be obtained from
c. Belt presses. The belt press is a recently
developed piece of dewatering equipment that
HQDA (DAEN-ECE-G) WASH DC 20314.
7-3
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