Note: Descriptions are shown in the official language in which they were submitted.
2ls3683
Method for the microbiological decomposition of organic
waste
Description:
The invention relates to a method for the
microbiological decomposition of organic waste according to
5 the preamble of claim 1. From DE AS 15 92 729 a method is
known in which waste is subjected to a composting process
in a closed container under aerobic conditions, air being
passed through the waste material discontinuously and in
bursts, and the duration of the rest times between the
aeration periods being so controlled that the temperatures
in the compost material are held below 75C.
Although it exhibited very good composting results,
this discontinuous method is being displaced by methods in
which the composting material is continuously aerated. The
continuous aeration methods have, however, the disadvantage
that the air blown in seeks the path of least resistance
through the waste heap so that the aeration of the waste in
the heap does not take place uniformly and compaction zones
form in which, possibly, the oxygen content falls so low
that the conditions for an aerobic composting process are
no longer afforded and anaerobic processes can occur.
Further, the methods which work with continuous air supply
have the disadvantage that the temperature distribution
within the composting waste materials is uneven, because
the supplied fresh air cools the lower layers so that the
temperature is higher in the upper layers. Similarly, with
continuous aeration, humidity is drawn upwards out of the
lower layers, so that the lower layers easily dry out and
the composting process there is interrupted. The
continuously working methods have thus overall the
disadvantage that the composting process develops unevenly
in the various regions of the waste heap. If the entire
waste material is to achieve a particular degree of
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composting, the waste heap must be left in the composting
container longer than would be necessary with a composting
process which is everywhere even.
On the other hand, the discontinuously working,
Blaubeuer's aeration method has the disadvantage that the
aeration is controlled in accordance with the temperature
of the composting material, so that the composting
conditions cannot be optimally set.
The object of the invention is thus to so modify the
method indicated in the introduction, working with
discontinuous aeration, that effective control of the
composting process is possible.
This object is achieved in accordance with the
invention with a method according to claim 1.
Advantageous embodiments of the invention are apparent
from the subclaims.
The invention will now be described and explained in
more detail with reference to the drawing.
The arrangement employed for the composting is
schematically represented in Fig. 1. It consists of a
container 1, which is provided with a suitable aeration
floor, for example a perforated floor or grid mesh 3.
Preferably, the aeration floor consists of longitudinal or
angled profiles arranged one next to another in the
longitudinal direction with small spacing, or alternatively
of round rods. With a perforated floor or a grid mesh, the
composting material can be caught in the holes and block
them, so that each hole must be individually cleared. With
the aeration floor constructed with longitudinal profiles
or rods there is, in contrast, always a possibility, using
a suitable pusher, to push away the composting material
2ls~68~
from the aeration floor in the longitudinal direction at
the end of the composting process, at the same time
scraping out material which is caught in the gaps between
the longitudinal profiles.
The waste material is heaped on this aeration floor 10
through the opened cover 4. For evenly distributing the
waste material in the container 1, a broaching blade 6 may
be arranged inside the container 1. This broaching blade 6
is movable in the vertical direction on two rails 7, 8. The
raising of the broaching blade 6 is effected with the aid
of lifting spindles 11, 12. Beneath the perforated floor 3
there is an aeration box 2 into which air can be forced
with the aid of a blower 15. The air forced in then flows
through the holes of the perforated floor into the waste
material 10. There is arranged in the exhaust air channel
9, which is located at the upper side of the container 1,
a sensor 13 with which the oxygen concentration in the
exhaust air can be determined. A second sensor may also
provided with which, for example, the CO2 content can be
determined. Within the container 1, temperature sensors 16
are provided with which the temperature of the waste
material at various heights above the perforated floor is
measured.
The output signal of the oxygen sensor 13 and the
signals of the temperature sensors 16 are supplied to a
control 14 which in correspondence with the measured value
of the oxygen content and possibly also corresponding to
the measured value of the temperature, controls the rest
times between the aeration periods. The control 14 directly
affects the blower 15 and switches it on or off.
The exhaust air flows through a water separator 18 and
then enters a biological filter 19. The biological filter
19 likewise has a perforated floor 21 over which the
biologically active material 20 is deposited. The exhaust
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air flows through this material 20 before it reaches the
outlet-22 to the atmosphere.
5The method according to the invention proceeds as
follows:
After filling with the waste material, which may be
treated with an inoculation substrate, for example an
already composted organic material, the container is closed
and the composting process begun. After the composting
process has commenced, which can be determined by measuring
the temperature within the waste heap, air is forced in
with the aid of the blower so that the compost material is
aerated from below. This aeration occurs in bursts; within
a time range of approximately 1 to 5 minutes a quantity of
air is forced in which approximately corresponds to the
volume of the container. Simultaneously with the forcing in
of the air, or a few seconds later, the exhaust air above
the waste heap is drawn off with a blower. Over the
duration of the air supply the oxygen content in the drawn
off exhaust air is monitored. In the composting process
there is only slight gas exchange between the reaction gas
such as CO2, formed in the waste material and the space
above the waste material. For this reason, during the
aeration periods, a relatively high oxygen content of for
example 20~ is initially measured in the exhaust air. When
the air supplied in bursts has passed through the waste
heap and there carried along the organic reaction gases,
the oxygen content in the exhaust air sinks to a value
which may be between 16 and 19~. When the gaseous
decomposition products are flushed out, the oxygen content
increases again until the oxygen content of fresh air is
attained. The more oxygen is consumed by the
microbiological decomposition process in the waste heap
during the intervals between aeration the lower the oxygen
content of the exhaust air determined during the aeration
2l53683
period falls. The lowest oxygen content in the exhaust air
determined during the aeration period is thus a measure of
the activity of the composting process and can thus be
employed for the control of the composting process. The
desired value for the oxygen content in the exhaust air is
around 17 to 18~. If the oxygen content sinks below this
value, the interval between the aeration periods is
shortened. If, for example, the aeration is carried out in
each case for 3 minutes with an interval of 20 minutes and
after a certain time the composting process in the waste
material has so accelerated that more oxygen is consumed
and as a result the measured oxygen content in the exhaust
air sinks below the value of 17~, the aeration is no longer
carried out every 20 minutes but for example every 10
minutes or, if the oxygen content approaches the lower
limit of 16~, every 5 minutes. Conversely, if the oxygen
content in the exhaust air falls less strongly, the space
of time between the individual aeration periods is
increased. With advancing decomposition of the organic
material the oxygen consumption of the microorganisms
becomes ever smaller, so that the intervals between
aeration periods become ever longer. When they reach around
one hour, this indicates that the composting process is
complete and the process can be terminated, the composted
material removed and replaced by new waste material.
With an alternative form of the method in accordance
with the invention, the oxygen content in the space between
the heaped waste material and the container wall is
measured with a sensor 24. Between this cushion of air and
the quantity of gas within the waste heap a certain level
of gas exchange takes place due to convection and
diffusion, so that a part of the oxygen in this air cushion
enters the heap and is there consumed by the composting
process. As a result, the oxygen content in the space
between the waste heap and the container wall sinks in the
rest periods between the aeration periods. The more active
2153~83
the stage of the rotting process the more rapidly the 2
content sinks and the more often must the supply of air be
effected.
Finally, the oxygen content of the gas within the
waste heap can also be measured. Since the decomposition
process in the waste material can have strong local
variations several probes (25, 26) must be provided in
various positions of the waste heap and averages formed
from their data.
