Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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813-017-OX
154/
TITLE OF THE INVEI~TION
PROCESS AND APPARATUS FOR
BIOLOGICAL TREATMENT OF SLUDGE
1 BACKGRO~ND OF THE INVENTION
Field of the Invention
This invention relates to the biological treatment
of sludge and more particularly to the treatment of
sludge containing organic materials by hydrolysis and
fermentation.
Description of the Prior Art
In processes of the type disclosed in German
Patent 26 21 524 hydrolysis and acid fermentation take
place in a reactor~ The effluent from this reactor
enters a setting tank, from which precipitated residual
sludge is removed as one of two components and the
fermentation liquid is removed as the other
component~ Both components separately undergo a
digestion process in a second step~ In the case of
ordinary sludge the digestion of the residual sludge
provides only about 20% of the total gas formed by the
digestion stage. The relatively large apparatus cost
for this small benefit makes this process for
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1 converting the residual sludge to a form suitable for
disposal uneconomical. The digestion of the residual
sludge appears unavoidable because it is loaded with
foul-smelling noxious materials.
Accordingly, a need has con-tinued to exist for an
economical process of treating residual sludge to
prepare it for disposal.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to
lo provide an economical method for treating sewage
sludge~
A further object is to provide a digestion process
for the treatment of residual sludge.
A further object is to provide a method of
treating sewage sludge to prepare it for disposal.
Further objects of the invention will become
apparent from the description which follows:
The objects of the invention are attained by a
process comprising:
20(a) fermenting said sludge anaerobically whereby a
fermentation liquid and a residual sludge are produced;
(b) continuously withdrawing said fermentation
liquid and subjecting said liquid to a digestion;
(c) continuously withdrawing said residual sludge;
1 (d~ washing said residual sludge with a washing
liquid;
(e) adding said washing liquid from step (d) to
said fermentation liquid in step (b).
An apparatus for carrying out the process of the
invention comprises
a fermentation reactor wherein said sludge is
fermented, producing a fermentation liquid and a
residual sludge;
lo a residual sludge washing tank;
means for withdrawing said residual sludge from
said fermentation reactor and conveying it to said
residual sludge washing tank;
a digestion tank;
means for withdrawing said fermentation liquid
from said fermentation reactor and conveying it to said
digestion tank;
means for supplying a washing liquid to said
residual sludge washing tank;
means for withdrawing said washing liquid from
said residual sludge washing tank and conveying said
washing liquid to said digestion tank.
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1 BRIEF DESCRIPTION OF THE DRAWING
A more complete appreciation of the invention and
many of the attendant advantages thereof will be
readily attained as the same becomes better understood
by reference to the following detailed description when
considered in connection with the accompanying
drawings, wherein:
The sole figure is a schematic diagram of an
installation suitable for carrying out the process of
lo this invention.
DETAILED DESCRIPTION OF THE INVENTION AND
THE PREFERRED EM~ODIMENTS
According to the invention, the residual sludge
withdrawn from the fermentation reactor is washed and
thereby leached and then discarded. The washing step
frees the residual sludge from the fermentation liquid
which makes a significant contribution the the bad
odor. The wash liquid con~aining the fermentation
liquid washings is added to the fermentation liquid
withdrawn from the fermentation reactor and subjected
to a digestion process along with it.
Fresh water can be used as the wash liquid.
However, it is preferred to use for this purpose the
digested fermentation liquid (referred to as the
digested liquid in the following~, which, on the one
hand, is already at the current process temperature,
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1 and, on the other hand, is sliyhtly alkaline, while the
sludge liquid is acid. Thus, in the washing, a
neutralization ta~es place.
~ Iydrolysis and acid fermentation take piace in the
same tank, and the residual sludge settles out. Only
after the residual sludge remains in this tank for a
long time is the fermentation essentially complete,
i.e., the coatiny of the sludge particles which can be
biologically removed by microbes, becomes completely
lo removed from the particles, leaving as little residue
as possible. The known process, referred to above, is
inconsistent with this experience. Thus, it produces a
residual sludge, before the sludge digestion, which is
only incompletely washable even if it were to be
washed. A washed sludge of this type according to the
invention could in any case be prepared by addition of
chemical-s such as, e.g., calcium carbonate, or the
residual sludge could even be aerobically fermented.
Although such a post-treatment would already provide a
more economical process, this post-treatment can be
entirely or almost entirely omitted for the usual
sludges, if the sludge withdrawn from the fermentation
reactor is already thoroughly fermented there~
The biological treatment of sludge proceeds very
slowly. Accordingly, in order to obtain a sufficient
residence time of the sludge in the fermentation
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1 reactor, the controlling apparatus need only be
actuated on an hourly cycle~ Therefore, it is suffi-
cient to withdraw samples of the sludge being processed
at rather large intervals and to determine its degree
of decomposition by measuring its digestion. According
to the results of this analysis, the residence time of
the sludge in the fermentation reactor is shortened or
lengthened by increasing or decreasing the amount of
sludge continuously withdrawn. The labor of handling
lo the samples is conveniently shortened by determining
only the initial digestion rate of the samples and
taking this rate as representative. Thus, the
incubation of the samples with bacterial cultures from
the digestion reactor for the fermentation liquid makes
it possible to keep the time lag short in order to
attain precise process control.
An example of an apparatus for carrying out the
process conveniently is shown in the drawing. It shows
the plan of an installation for complete treatment of
sludge containing organic material wherein digested
fermentation liquid capable of being discharged into
streams is produced and discardable sludge and
digestion gas is produced.
Sludge tank 1 contains sludge, e.g., from a common
clarification apparatus. The sludge is continuously
withdrawn by a pump 2, warmed to about 37C in heater
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1 3, adjusted to a p~l of 5 to 6 by addition of chemicals
4, and fed into a fermentation reactor 5, After an
average hydraulic residence time in fermentation
reactor 5 of about 12 hours, during which the
fermentation gases C02 + H2 are evolved, the mobile
liquid portion of the acid fermented sludge is with-
drawn at the top of the fermentation reactor 5 as
fermentation liquid and sent via pump 6 to a decanter
7. In the decanter 7 the turbidity of the mobile
lo liquid is reduced. The sludge residue is returned to
the fermentation reactor through a pipe 8. Addition of
chemicals 9 neutralizes the moderately turbid fermen-
tation liquid so that its further treatment takes place
under neutral or slightly alkaline conditions. At a
temperature of 37C, controlled by heater 10, the
digestion liquid is taken to a fermentation reactor
11. The liquid is continuously digested therein with
evolution of digestion gas C~4 + CO2, and after an
average residence time of 12 hours, leaves tank 11 as
digestion liquid. The greater portion of the dis-
charged material 12 is post clarified until it is dis-
chargeable into a stream or, e~g., sprinkled on fields.
The residual sludge which precipitates directly in
fermentation reactor 5 and which is composed for the
most part of non-decomposable solids, is taken via pipe
14 to a sludge container 15 in which it is mixed with a
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portion of the digestion liquid supplied throuyh pipe
13 and is thoroughl~ mixed with a stirring apparatus
16, and is thus washed. During the washing most of the
granular solid particles are freed from the Eermenta-
tion liquid which surrounds them. From the tank 15 the
mixture goes to a precipitator or thickener apparatus
17. The treated and thickened sludge (discharge stream
18) is buried or used in some other way. The wash
liquid 19 which contains fatty acids and odoriferous
lo materials goes to the inlet side of pump 6 and is
Eurther treated together with the fermentation liquid
which is drawn off at the top of tank 5.
Samples are withdrawn from the sludge discharge
stream 18 at intervals of several hours and incubated
in laboratory 20 with bacterial cultures from the
digestion tank 11. The initial fermentation rate is
measured. If it is too large, that is if the sludge
has been withdrawn too soon from the fermentation
reactor 5, the inactive interval of the intermittently
activated pump 22 is increased via the control
indicated by line 21. Thus, in a cycle of one to
several hours the adjustment operation is performed to
regulate the variable amount withdrawn in the described
control loop.
Because of the relatively short residence time,
the reactors 5 and 11 are small relative to the tanks
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g
1 used in conventional apparatus. In a conventional
single-step apparatus for the treatment of sludye from
100,000 inhabitants it is necessary to have a reactor
volume of about 2,500 cubic meters. In this case, an
installation according to the process of the prior art
mentioned above requires three reactors with a total
volume of about 700 m3. On the contrary, in the
process of the invention only two reactors are needed
having a total volume of about 400 m3. Installation
costs are correspondingly decreased, and thus the
economics are also improved without any decrease in
environmental protection.
Having now fully described the invention, it will
be apparent to one of ordlnary skill in the art that
many changes and modifications can be made thereto
without departing from the spirit or scope of the
invention as set forth herein.
