TM 5-814-8
add 15 to 30 percent additional BOD load on the
Table 7-2. Typical raw sludge solids contentContinued
Solids Content
liquid treatment system. Generally, thermal sys-
(percent dry solids
tems are only practical for larger plants, greater
by weight)
Sludge Type
t h a n 10 mgd, or for special applications where
Combined Trickling Filter and
high bacteriological kills are necessary for land
3.0-6.0
Primary
disposal.
0.5-1.5
Activated Sludge
d. Chemical conditioning. Where mechanical
3.0-5.0
Combined Activated and Primary
dewatering is utilized, some form of chemical
conditioning is common. Most plants find that
7 - 3 . Conditioning and stabilization
lime and/or ferric chloride produce the best re-
For most military installations, disposal of sludge
sults and are most economical. Where disposal of
nondigested sludges occur, high lime treatment
in landfills or on the land will be cost-effective
(pH of 11.5 for over 2 hours) will render a
and must be utilized. The rare exceptions are
areas where incineration can be justified by the
stabilized sludge. Lime, unlike ferric salts, is a
bactericide which assists in treating the sludge.
excessively long hauling distances required for
reaching an acceptable disposal site or by the
7-4. Thickening
presence of industrial wastes that preclude land
disposal. These land disposal methods require
Most military facilities recycle secondary sludges
some previous stabilization step to avoid environ-
to the primary tanks. Since most plants are
mental degradation.
trickling filters, the resulting sludge mixture is in
a. Anaerobic digestion. Anaerobic digestion,
the 5 percent dry solids range and thickening is
although sometimes difficult to control, is a very
therefore not warranted. At new activated sludge
desirable and proven stabilization step. It con-
installations, thickening may be necessary due to
serves energy when the system produces a com-
the low solids content; flotation will usually be
bustible gas that can be used for sludge heating
cost-effective for these applications. Gravity
and for other purposes. The process will function
thickening is appropriate for combined sludges.
well in most climates and renders a stabilized
a. Gravity. Gravity thickening is accomplished
sludge. For military installations, anaerobic diges-
in a tank equipped with a slowly rotating rake
tion shall be used unless highly variable solids
mechanism that breaks the bridge between sludge
loads are expected or unless local factors justify
particles, thereby increasing settling and compac-
use of alternative processes. The most important
tion. The primary objective of a thickener is to
factor for sizing digester capacity is the volatile
provide a concentrated sludge underflow. The
solids loading. TM 5-814-3 should be referred to
design of a mechanical thickener is generally
for acceptable design criteria.
based upon a solids loading rate. Typical solids
b. Aerobic digestion. Aerobic digestion is a
loading rates are in the range of 10 to 30 lbs/sq
stabilization process applicable to facilities where
ft/day. Gravity thickeners should be designed to
blowers are installed or are required for the liquid
maintain aerobic conditions in the unit. Anaerobic
treatment operations. Since most military plants
conditions may cause floating sludge and odor
do not have blower systems, aerobic digestion will
problems with the unit. Thickener performance
not be feasible. Other disadvantages are high
can be improved by the addition of coagulant to
power requirements and low efficiencies for mili-
the influent feed. Polyelectrolytes are the most
tary installations located in extreme northern
common type of coagulant aid used in thickening.
climates. Aerobic digestion may have application
b. Dissolved air flotation. Thickening through
at small package plant facilities or where wide
dissolved air flotation is becoming increasingly
load variations cause difficulties with anaerobic
popular and is particularly applicable to gelati-
digestion.
nous sludges such as activated sludge. Flotation
c. Thermal conditioning. "Cooking" sludge un-
thickeners can be loaded at higher levels than
der elevated temperature and pressure is a ther-
gravity thickeners because of a more rapid sepa-
mal conditioning and stabilization process receiv-
ration of the solids from the sewage. Loadings are
ing more attention in the U.S. It eliminates
typically in the range of 10 to 55 lbs/sq ft/day
chemicals needed to condition a sludge prior to
depending on the sludge and the degree of
dewatering and also increases dewatering rates.
conditioning. In flotation thickening, small air
Disadvantages are that it is a fuel consumer
bubbles released from solution attach themselves
to and become enmeshed in the sludge floes. The
unless heat recovered from a combustion process
air-solid mixture rises to the surface of the basin,
is available, and supernatant recycle flows can
7-2
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