Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a process for treatment
and disposal of sewage sludge.
Sewage sludge contains moisture-holding organic
matter and nutrients that can be beneficial when returned to
nutrient and organic-poor, but well drained agricultural land.
Sewage sludge in its originally derived form is un-
acceptable for land disposal for several reasons, including:
1~ Raw sewage sludge contains all of the disease
organisms common to man, as well as undesirable plant seeds.
This material, in an untreated form, presents a health hazard `- ~ -
and can create undesirable plant growths;
2~ Raw or und~gested sewage sludge is highly putresc-
ible and can be a nuisance, particularly from an odor stand-
point, i.f it is allowed to decompose under controlled con-
ditions on the land. Decaying matter can also encourageundesirable insect propagation;
3) I~ applied to agricultural land in an indiscriminate
manner and at excessive application rates, the nitrogen in
the sludge can contaminate ground water in the application
area.
The known methods of processing sewaye sludge in-
volve thermal conditioning in such a manner as to improve -
the dewaterability of the sludge so that the solid and liquid
phases are readily separated, as by sedimentation, filtration,
centrifugation, etc. and the solid and liquid phases are then
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disposed of or subjected to further treatment separately
see Vnited States Patents 3,256,179; 3,697,417; and United
States Patent 3,824,186).
The invention relates to a process for condition-
ing sewage sludge for disposal on land which comprises heat-
ing the sludge at a temperature between about 65C. and about
150C., for a time between about 30 seconds and about 60 :. .
minutes, the time varying inversely with the temperature;
and, without any separation of solid and liquid phases, treat-
ing the resulting fluid, sterilized sewage sludge by bio- .
logical digestion at a temperature between about 32C.
and 60C.
The biological digestion can consist of an aerobic
process or an anaerobic process, or a combination of both.
A further aspect of the invention lies in the
further step of transporting the fluid sludge after bio- ~ ~
logical digestion to a land application site and distribut- , , .
ing it at a load rate of less than about 300 pounds of
nitrogen per acre per year. ;
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The term "sewage sludge" as used herein is intended
not only to include conventional sewage wastes derived from
flush toilets commonly used in western countries, but also to
include similar concentrated wastes of human or animal origin,
S such as night soil produced in oriental countries. The sewage
sludge is ph~sically an aqueous slurry or suspension of waste
solids. ,
The object of the present invention is to condition
sewage sludge so that it is devoid of undesirable micro-
organisms and reduced in septic and odor-producing properties
and there~ore acceptable for land disposal. This is realized
by:
a~ ~eating the sludge to a sufficient temperature for
a long enough period of time to inactivate pathogenic organisms
lS and plant seeds in the sludge and to degrade organic matter to
more readily digestible substances, but restricting the time
and temperature of heating to a range which does not substan-
tially decreasc the water-holding characteristics of the treated
solids,
b~ Treating the resulting 1uid sterilized sewage
sludge without dewatering by biological digestion at a tempera-
ture between about 90F. and 140F. ¢32 and 60C.~.
~he biologically digested sewage sludge is then
suitable for distribution on land without previously separating ;-
or dewatering the sludge solids.
While the individual steps of sterilization of sludge
by heat and biological decomposition of sludge are separately
known in the art, the particular combination and sequence of
those steps herein described give advantages which have not
previously been realized.
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In the process o~ the instant invention sludg~ is
~terilized before blological degradation. Although sludge has
previously been sterllized or pasteurized arter anaeroblc
digestion/ thl~ i8 the ~lrst time that heat treatment prior to
sludge digestion has been proposed. The advantages of doing
heat treatment sterilization be~ore digestion are:
a) Heat treatment renders the organic matter ln the
Rludge more bi~degradable. Heat treatment before biologlcal
dlgestion, therefore, improves digestion by giving greater
decomposition in a shorter time.
Experiments conducted in which sewage sludge was
seeded with 25% by volume of well-digested sludge and anaerobl-
cally dlgested to produce combustible gas (methane), both in it~
raw state and a~ter cooking showed that dige~tion o~ a seeded
cooked sludge consistently produced mo~a gas in a given time
than the digestion o~ uncooked raw sludge, Summary results of a
number o~ batch experiments (an average o~ two or more experi-
ments) are as ~ollows:
% Addltional Total ~as -
Day~ of Productlon Compare~ to
Di~e~tion gg~g~g,~lg~sg~geparation Uncooked Slud~e Trea m~n~ ;`
Cooking Time - Temperature `
95F.Minutes F.
16 3 176 +10 l
25. 16 3 212 + 9 -
16 60 212 +11 `
16 3 248 +20
18 60 248 +26 .
The data indicate that up to 26;~ more gas was obtained
in an 18 day digestion period when sludge was cooked ~or 60
mlnutes at 248F~ before digestion. Nearly as good results were
obtained (20~ additional gas) by cooking for 3 minutes at 248Fo
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- Ir sludge ls heated ~or sterilization purposes after
biological digestlon, biologically stable organic matter may be
converted to biodegradable matter which is available for
decomposition later, causing a possible nuisance at the ultimate
dlsposal ~ite. Also, heat treatment a~ter digestion creates
soluble nutrients which are more readily available ~or drainage
to groundwater or runo~f.
b) ~eat treatment of ~ludge creates a warm sludge whicn
i~ conducive to e~icient biological a~tion. No other external
~10 heating ls required. I~ sludge is sterilized after biological
action, the residual heat remaining in it i5 lost rather than
put to beneficial use in controlled decomposition.
c) By introducing a sterlle nutrient sludge into a
biological treatment step, the biology of the system can be
controlled by se~ding o~ the desired organisms and ad~usting
environmental conditions to sustain the most desirable type Or
biological growth. When unsterilized raw sludge is introduced
to a biological dlgestion the biota present are uncontrollable
~in¢e the feed stock itsel~ is a mixed culture of many types o~
mioro-organisms.
d) The process proposed, i.e., heat treatment, to
lnactivate undesirable organlsms, ~ollowed by biological
digestion to reduce potential nuisance (odor, etc.) forming
conditions, leaves a fluid sludge, desirable ~or direct ~ertilizer
application on land. Dewatering i9 unnecessary because the
material is biologically stable and relatively nuisance free,
and undesirable since much of the nutrient value of the material
is in soluble ~orm as ammonia.
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The process proposed may take several variations as -
described below.
1. A sludge heating step in which the temperature
to which the sludge is heated and the holding time is suffi~
cient to render the sludge free of disease-producing micro-
organisms and viable plant seedsO The time-temperature range
of "cooking" for sterilization purposes may ~e from about 60
minutes at about 150F. ~65C.~ to about 30 seconds at about
300F. ~150C,~, the time varying inversely with the tempera-
ture. A preerred condition is about 250F. (120C.) for
about 10 minutes. The "cooking" process also renders the
organic matter in the sludge more susceptible to subsequent
biological degradation ~digestion~O Longer than minimum
"cooking" times will increase the solubilization and sub-
sequent biological degradation of the organic matter.
The heating step is preferably carried out in the
absence of substantial amounts of oxygen ~air) so that no
appreciable reduction in chemical oxygen demand of the sewage
sludge is brought about through oxidation,
2. The sterile sewage sludge is discharged at a
warm temperature, between about 100 and 150F., to a biological `
degradation step in which seed organisms in a tank degrade the
putrescible organic matter to gases and biologically stable
organic matter. The biological treatment step may be either
anaerobic or aerobic. In the anaerobic process the warm,
sterile sludge is discharged to a covered tank ~either fixed
or floating cover~, having an average retention time of about
5 to 20 days. During initial operations the
anaerobic tank may be seeded with soil bacteria.
Rich agricultural loam containing soil organisms
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can be used ~or this purpose. The anaerobic decomposition of the
sterile putrescIble sludge results in the production of me~hans
gas whlch can be burned and used as fuel in the sludge heat
treatment process. The tank may be equipped with mechanical or
gas diffusion mlxing to encourage optimum mixing of seed and
sludge ~eed. The anaeroblc process ~unctions efficiently in the
range of 90 to 140F. (~2 to 60C~). A pre~erred temperature is
95-100F. The temperature of the bio-digestion may be controlled
at the desired level by controlllng the temperature o~ the
heated sludge discharged to it.
Modi~ications o~ the biological stabilization btep ara
possible and appropriate in certain situations. For example5
aerobic digestion is preferred over anaerobic digestion in a
case where the sludge contains signi~icant quantities o~ toxic
metals such as copper which in small concentration will inhibit
anaerobic decomposltion~ but not aerobic digestlonO A~roblc
digestion o~ the sterile sludge may be achieved by aerating the
sludge introduced at about 100 to 150Fo for a period varying
~rom one to ten days. The aeroblc decomposltion is carried out
in an open tank in which oxygen is dif~used, either by mechanica~
aeration or by di~using an oxygen containing gas throughout the
contents of the tank. Putrescible organic matter in the
introduced sludge is decomposed to carbon dioxide and water.
The aerobic decomposition process is greatly accelerated by the
warm temperatures Or the sludge being fed to the processO
Aerobic decomposition proceeds at a very slow rate at
temperatures less than 65F~ To obtain reasonably rapid rate~
o~ decomposition, temperatures exceeding 90Fo (pre~erred ~ ~
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temperature 110F.~ are desirable in aerobic digestion. me
aerobic digestion may also be seeded by introducing soil
organisms during inltial operations. Another possible modi~i-
cation i3 heat treatment followed in sequence by anaerobic and -
aerobic diges~on. This scheme gives the advantages o~ energy -
recovery (gas ~or fuel) of the anaeroblc process and provides
ror aerobic "polishing" o~ the anaerobic sludge e~fluent which
i9 in a chemically reduced state. For example, an anaerobically
digested sludge containing sulfur compounds will have the sulfur
present in the ~orm o~ sul~ides or mercaptans, which not only
could be undesirably odorous, but al~o create undesirably high
oxygen demands on the soil to which it is applied. Aerobic -
treatment o~ this sludge oxidizes the sul~ides and improves the
odor as well as reduces the oxygen demands at the ~inal soil
application site.
The latter two stage biological treatment requlres less
power for aeration in the aerobic stage than in the modl~ication
using aerobio digestion only, since the anaerobic stage removes
the ma~or part o~ the oxygen demand by anaerobic decompositionO
3. The liquid sludge exiting ~rom the biolo~ical
digestion is either distributed directly on the land, or is
transported to a storage tank or a lagoon in which it is retained
until it i9 convenient to apply to the land. The sludge does not
have an unpleasant odor at this point and may undergo ~urther
"maturlng" in the storage basin. The sludge applied to the land
is applied at a loading rate controlled by the amount of nltrogen
remaining in the sludge. The loading rate should be less than
about 300 pounds o~ nitrogen per acre per year, usually in the
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range of 80 to 300 pounds o~ nitrogen per acre per year (~5 to
135 kilograms of. nitrogen per 4000 square meters per year), the
amount varying depending upon the.number of years of application~
the crop utilizatlon o~ nitrogen, and soil ractors. Sewage
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sludge containin~ a high percentage o~ nitrogen must be applied
at~a lower rate than sludge containing a low nitrogen concen-
tration. I~ the nitrogen application is excessive, ~ trogen
compounds will appear in the surface and ground drainage waters
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~rom the area. Since most o~ the-nitrogen in biologically
treated sludge will be present in the soluble ammonia ~orm, a ~:
desirable ~ertilizing nutrient, application of the ~luid sludge
to land without dewaterlng is most desirable ~rom an agricultural
standpoint. HoweverJ i~ the nitrogen content is too grsat ~or
safe disposal, a substantial portion o~ it can be removed by
15 . dewatering the sludgeJ conveniently by decanting the supernatant
llquor i~ obtainable from the storage basinO There~ore, i~
desired, supernatant liquor can be removed from the storage
basins or lagoon~ to reduce the amount of nitrogen ln the
residual.sludgeJ which then will require less land area ~or
ultimate disposal than i~ the entire volume o~ treated sludge
were applied to the land. In some cases it may be attractive
to add water to the treat.ed slud~e to further elutriate soluble
nitrogen by decantatlon be~ore applying the residual sludge to
the land. ~ :
The foll~wing example will further illustrate the
invention without the latter being limited thereby. .
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EXAMPLE
Raw sludge having 3~6,~ total solids containing 67%
volatlle matter is heated to 250F. ~or 10 minutes. Heating i9
accomplished in countercurrent heat exchangers and a reactor.
Cold sludge at approximately 60F. is pumped into the system
through a heat exchanger and then to a reactor which is main- ~;
tained at the desired temperature either by inJecting steam into
the reactor or by steam or other liquid heated heat exchangers
~ust prior to the reactor. The heated sludge exits the system,
passing countercurrent to the incoming cold sludgeO The heated
sludge leaves the system at a temperature o~ 90 to 150F, and
enters an anaerobic digestion tank. Su~icient combustible gas
. is produced in the anaerobic digestion to provide the ~uel
required for the heat treatment step. Under usual conditlons
10 to 15 days' detention time are provided in the ana~robic
digestion tank, during which time approximately 40% o~ the
volatile solids in the ~ludge are converted to burnable gas,
or exampleJ 50 gallons per mlnute (72,200 gallons per day)
o~ ~ludge oontalnlng 14,400 lbso o~ volatile solids is intro-
duced) and 40% o~ these solids having a ~uel value o~ 10,000 BTU
per pound Or volatile solids are converted to gasJ the amount o~
fuel available is 14,400 x 1OJOOO = 57.6 million BTU per day.
The heat required to raise th~ temperatllre o~ the incoming sludge
~rom 60F. to 100F. is approxlmately 72J200 x 803~ x 40 = 24
million BTU. There~oreJ a 40% conversion of the ~uel value in
the gas to heat trans~erred to the incoming sludge will glve
su~icient heat to maintain desirable temperatures without the
necessity for providing additional ~uelO The warm sludge exiting
the heat exch~ngers passes through a closed digestlon tank
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equipped wlth gas collection equipment where lt mixes with
previously seeded and digested sludge and the digestion proceedsO
Approxlmately 40~ volatile solids reduction to gas is achieved
in a 10 to 15 day detention time. A~ter passing through the
digester, the stable sludge normally will pass to a storage
lagoon where it will remain until land and weather conditions
are acceptable ~or spreading the sludge on the land. .¦
In cases where aerobic digestion is used instead of
anaerobic digestlon, an oxygen-bearing gas is supplied to the
digesting mlxture, which digestion is carried out either in an
open or closed tank held at about 110Fo Approximately 40%
reduction Or vola.tile solids will be obtained in a 10 day
retention period when surficient oxygen is supplied to maintain
a residual dissolved oxygen content Or at least 0~5 mg/10 I~ 5
aerobic dige~tion ls used, there wlll not be any recovery Or l~
combustible gas and all the ~uel required to maintain a desired ..
digestion temperature will have to be supplied rrom an outside
sourcoO The aeroblcally digested sludge will again go to a
storage l.agoon to await convenient distribution to th~ land~
Ir aeroblc digestion is to rOllow anaerobic dlgestion, . ..
lt normally wlll be advantageous to provide at least 15 days'
detention time in anaeroblc digestion to obtain maximum volatile .
solids reduction Or approximately 50%, followed by an aerobic :
~ "polishing" Or 1 to ~days' detention time, in which the dissol~ed .. ~. oxygen content la maintalned at at least 0.5 mg/l.
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