The present invention relates to a method and installation
for biological drying of domestic waste.
Large quantities of domestic waste and industrial waste
are produced in the Netherlands, Europe and worldwide every year. Globally, and
also in Europe, this waste is for the greater part dumped at landfills. In order
to limit the environmental drawbacks of and the space taken up by waste at landfills,
European countries, particularly the Netherlands and Germany, have switched to separation
at source and reuse of specific flows, such as for instance glass, paper, tin and
kitchen and garden waste (KGW).
This does not however alter the fact that, in addition
to plastic and synthetic materials, a great deal of paper, organic KGW-like material
and moisture is still present in the remaining waste (residual waste) after this
separation at source. This residual waste is usually incinerated integrally in waste
incineration plants (so-called WIPs).
The residual waste therefore still comprises a large fraction
of KGW (about 35%), other constituents (11%) and some smaller fractions (glass,
ferrous material) which together, as poorly combustible fraction, form more than
half the residual waste. This has resulted in many cases, for instance in many plants
in Germany, the United Kingdom and some in the Netherlands, in the decision to separate
the poorly combustible fraction, generally by screening. The residual waste presented
is herein separated into a low-calorific (about 5.5 GJ/ton), poorly combustible
passing fraction with many wet organic components, referred to as Organic Wet Fraction
(OWF), and a high-calorific (about 12 GJ/ton) passing fraction with many dry, readily
combustible components such as plastics, synthetic materials and paper, the so-called
RDF (Refuse Derived Fuel). A ratio generally results of about 40% OWF and 60% RDF.
The advantage of this separation is that a smaller, readily
combustible RDF fraction remains, for which a lower WIP capacity is required. The
drawback is that a poorly combustible OWF fraction remains, which is tipped, optionally
after fermenting and/or composting.
Due to the environmental drawbacks dumping of untreated
OWF is prohibited in a number of countries, such as Germany. Fermenting and/or composting
is then necessary. The combination of separating residual waste into OWF and RDF
and fermenting and/or composting is also referred to as Mechanical Biological Treatment
Fermenting and/or composting must result in a well-stabilized
OWF such that one of the environmental drawbacks of dumping, i.e. the release of
methane gas, is largely obviated. This requires prolonged composting of the OWF
or of the fermentation residue. In the composting of OWF it is further necessary
to aerate the material. Due to the structure of the OWF and the limitations of the
available aerating techniques the aeration does not progress optimally. It is partly
as a result hereof that volatile organic carbon compounds such as fatty acids, aldehydes,
ketones and other hydrocarbons, usually referred to as VOC and TOC, escape during
the composting, particularly at the start of the process. Volatile organic carbon
(VOC) comprises the volatile carbon-containing compounds. Total organic carbon (TOC)
stands for the total fraction of organic compounds.
These organic substances are harmful to the environment
and must therefore be removed. In order to remove these substances from the airflow
expensive and energy-demanding techniques must be applied, such as regenerative
thermal post-combustion. Due to these problems with the processing of OWF, MBT is
an expensive process, often resulting in a cost disadvantage relative to integral
combustion in a WIP. In the case of MBT the dumping of stabilized waste can moreover
still be deemed a useless application, and therefore a drawback.
The present invention therefore has for its object to provide
a method and installation with which high-calorific waste can be obtained and wherein
the above stated drawbacks of the traditional waste treatment processes are obviated.
In the research which resulted in the present invention
it was established that, surprisingly, it is possible to biologically process waste
integrally, so without preseparation into RDF and OWF, by making use of composting
processes such that a high-calorific residual flow is generated. It has further
been found that this integral process derived from composting, also referred to
as Integral Biological Treatment (IBT), can be improved in far-reaching manner by
a number of specific measures.
The invention provides for this purpose a method for biological
drying of waste, in particular domestic waste, by composting the waste in a series
of parallel treatment spaces for a determined number of days, wherein the waste
is in a startup phase, an active phase or a cooling phase of composting in the different
treatment spaces, comprising of
- a) supplying air from a treatment space, in which the waste is in the active
composting phase, to a treatment space in which the waste is in the startup phase
for the purpose of increasing the temperature in this startup treatment space;
- b) discharging the air after a time from a treatment space in which the waste
is in the startup phase of composting to a treatment space in which the waste is
in the active composting phase for the purpose of reducing the odour and the volatile
hydrocarbons in the air; and
- c) discharging the air and the dry waste from a treatment space in which the
waste is dry.
In this application 'composting' is understood in the broadest
sense of the word. In the narrower sense composting consists of the treatment by
means of an optionally controlled fermenting process in which organic materials
such as branches, leaves, kitchen waste, etc. are converted into a stable humus
(compost) by micro-organisms during a fermenting process in the presence of oxygen.
Substantially heat, carbon dioxide and water are released here. The heat contributes
toward the evaporation of the present and released water. In this application composting
is also understood to mean the treatment of a mixture of organic constituents and
non-organic constituents (such as plastics and other synthetic materials, leather,
aluminium foil, laminate, rubble, etc), wherein the full or partial conversion of
the organic constituents present by micro-organisms produces the heat for drying
the whole mixture to a greater or lesser extent.
The waste treated in this manner does not have to be subjected
beforehand to a separating step but can be dried integrally. A coarse size-reducing
step is however preferably applied beforehand in order to obtain a more uniform
size, whereby the organic wet fraction present therein becomes more accessible to
the composting micro-organisms.
The invention is not limited to biological drying of integral
waste, although this is recommended, whereby residual waste no longer need be separated
into 60% combustible fraction and 40% organic wet fraction. After treatment according
to the invention only about 70% of the untreated waste still remains as combustible
fraction. The organic wet fraction forms an integral part hereof and no longer has
to be dumped.
The air supplied from an active treatment space to a startup
treatment space in the method according to the invention is moist and warm and still
contains sufficient oxygen, whereby the startup treatment space is brought to temperature
much more rapidly and the overall production time can be considerably shorter than
in the traditional processes. According to the invention it has been found that
the composting process can be completed sooner when startup takes place with this
moist and warm air. With the traditional installations the temperature increases
more slowly and generally does not reach the level which is reached in an installation
according to the invention, whereby more time is required to complete the drying
Figure 1A shows the temperature curve during
the different phases of the method according to the invention.
The method according to the present invention results for
instance after only 13 part-days in waste sufficiently dried to enable processing
thereof in high-calorific incinerators. With the traditional methods about 20 part-days
are necessary for this purpose (
The air in a treatment space in the startup phase of composting
comprises on day 1 the highest concentration of volatile hydrocarbons (VOC). In
the existing installations this heavily loaded air is discharged together with air
from other treatment spaces. Although the heavily contaminated air is hereby diluted
to some extent, the overall hydrocarbon content in the discharged air is still relatively
high, 200 mg/m3 in the example of table 1. This air is further purified
in a washer installation, although the air quality can often not be improved sufficiently
herewith to enable direct emission. An additional process, known as regenerative
thermal post-combustion, is therefore necessary.
In the method according to the invention this air does
not immediately enter the washer or the outside air but, conversely, the active
treatment spaces. Since the composting process is already well underway here, the
VOC and TOC in this contaminated air are for the greater part degraded by bacteria
populations already developed there. The level of harmful substances hereby decreases
in the treatment spaces, where the composting proceeds on production days 2-6, from
for instance 800 mg/m3, a normal value during the first 24 hours, to
28 mg/m3 and lower. Table 1 shows the progression of the hydrocarbon
content during 6 days of composting.
Progression of TOC
content during 6 days of composting
Average to washer
average TOC mg/m3
percentage contribution TOC
In the method according to the invention the VOC content
in the air to be discharged to the washer can decrease to for instance 28 mg/m3
or lower. The method according to the invention hereby has a two-fold advantage.
Firstly, the treatment time becomes considerably shorter through the use of warm
startup air and, in addition, the environmental impact of the discharged air is
After the composting process in a determined treatment
space has ended, the air can be discharged from this space to a washer for the purpose
of further reducing the odour and hydrocarbons remaining therein. This preferably
takes place by means of biological decomposition in for instance a biofilter. The
great advantage hereof is that regenerative thermal post-combustion, which is necessary
when the hydrocarbon content is still too high, can now be wholly or partially avoided.
Up to 20% of the production costs can hereby be saved. The environmental advantage
is that no, or less, energy (obtained by burning fossil fuels) is necessary for
the post-combustion, and that the CO2 emission of the overall process
is therefore lower.
Air from the active treatment spaces has for instance an
oxygen content of 19%, a relative humidity of 99% and a temperature of 45°C.
Fresh outside air may optionally be added to the discharge air from the active treatment
spaces in order to optimize the temperature, the humidity and/or the oxygen content
The method steps of supplying warm startup air from active
treatment spaces to startup treatment spaces and discharging contaminated startup
air from a startup space to active spaces in order to reduce the VOC and TOC herein
can also be applied in normal composting installations in which the waste is not
treated integrally but in which a fraction of the waste or for instance KGW is composted
The essence of the invention lies in the reuse of air from
active treatment spaces for the purpose of preheating startup air in a space where
the composting is just beginning and cleaning the highly contaminated air from a
startup space which has been operating for 1 to 2 days in the active spaces.
In addition, the drying process can be improved still further
by aerating the waste in the treatment space alternately from above and from below.
In the traditional method the waste is aerated in only one direction (usually from
below). For this purpose air is blown through the waste from the floor. According
to the invention it has now been found that, when the airflow in the treatment space
is reversed from time to time, whereby it not only passes through the waste from
bottom to top but also from top to bottom, a much more homogenous drying of the
waste can be achieved.
Reversal of the airflow prevents the cooling of the air
which occurs with blowing only from below and which may result in condensation.
A zone with a higher moisture content is hereby created at the bottom of the treatment
space. Since it is precisely drying of the waste which is being attempted, it is
undesirable that the waste become moist again through condensation.
By reversing the airflow the available air is moreover
mixed more intensively and for a longer time with the composting residual waste,
whereby the hydrocarbon contents in the end product can be reduced even further.
Furthermore, the drying efficiency increases and the drying time is shortened still
Nor is this technique limited to the biological drying
of integral waste, but can also be applied in the already used methods or for composting
or biological drying of other materials.
According to another aspect of the invention, the washing
of the emission air can be further optimized by bringing it into contact in counterflow
with a washing liquid after discharge, for instance from a space in which the drying
process has been completed. It has been found that a rapid absorption of the gases
to be removed, such as ammonia, is hereby realized and fewer odour-removing micro-organisms
are flushed out of the channels.
The invention further relates to an installation for performing
the method, comprising a plurality of treatment spaces for the waste, each of which
is connected via closing means to shared means for supplying air to the treatment
spaces and shared means for discharging air from the treatment spaces, wherein the
air supply means and the air discharge means are in mutual connection, preferably
via a fan.
Alternatively, the supply and discharge means can be the
same means. The position of the closing means then determines at any given moment
whether there is supply of air or discharge of air.
In an advantageous embodiment of the installation according
to the invention the supply means take the form of a tube, here also referred to
as surplus channel. Part of the air from the active treatment spaces is received
herein to be supplied to a startup treatment space, here also referred to as startup
Air is discharged from the active tunnels via the discharge
means, which for instance take the form of a tube, also referred to as startup channel,
and guided to the surplus channel, or contaminated startup air is guided from a
startup tunnel to one or more active tunnels.
The supply and discharge means for contaminated startup
air from a startup tunnel and warm and moist startup air carried from an active
tunnel to a startup tunnel can take the form of different types of conduit, such
as tubes, cylinders, pipes etc.
The closing means preferably take the form of actively
controllable or pressure-controlled valves. The valve, which discharges contaminated
air from a startup tunnel to the startup channel which is there diluted with air
from active tunnels and guided to active tunnels via the surplus channel, is preferably
pressure-controlled. As soon as the pressure in the startup tunnel reaches a determined
value, air will be discharged. Other valves, such as valves at locations where opening
or closing depends on factors not related to the pressure, will however be actively
According to the invention each treatment space preferably
further comprises means for discharging air to the outside, which means are connected
to the treatment space via a supply opening with interposing of closing means. In
these discharge means the air usually undergoes a further treatment before the air
is emitted. This further treatment comprises of wetting the air with water in order
to absorb harmful substances such as ammonia therein and to optimally wet the air
for the purpose of the functioning of the biofilter. In addition, air can be brought
into contact with a biofilter prior to emission in order to further reduce the odour
and the VOC and TOC.
According to the invention the washing and the biological
treatment are advantageously combined in that the washer channel is provided with
a perforated inner wall on which bacterial growth is provided. In a preferred embodiment
water is sprayed into the washer channel by means of sprayers directed toward the
supply opening for the air leaving the tunnel. By spraying counter to the airflow
the wetting is optimal and a part of the harmful substances, such as ammonia, is
already immediately absorbed, and less water is required. Flushing away of bacterial
growth is further prevented.
Traditionally the water is sprayed from above into a washer
channel. Water flows can hereby occur in the bottom of the channel which flush away
the bacterial growth there, whereby the biological cleaning of the discharge air
The new method of spraying water into a washer channel
can also be used, independently of the other parts of the installation according
to the invention, in traditional waste treatment plants which make use of washing
of the discharge air prior to emission.
In the installation according to the invention each treatment
space can be provided with means for supplying fresh air from outside. These means
are preferably formed by a tube connected to all treatment spaces. Fresh air may
be required to regulate the temperature of the startup air or to be added to the
startup air for the purpose of absorbing extra moisture or dosing extra oxygen for
the purpose of the composting process.
In the installation according to the invention each treatment
space preferably comprises means for recirculating the air therein. Drying is enhanced
by the flowing of the air.
According to the invention the installation is preferably
further provided with means for reversing the flow direction of the recirculated
air through the waste. It has been found that the drying efficiency hereby increases
and the drying time may be shorter.
In a preferred embodiment the recirculation means are formed
by an opening for respectively supplying or discharging recirculated air at the
top of the treatment space and openings for respectively discharging or supplying
recirculated air in the floor, wherein the opening at the top is connected to the
openings in the floor via means for respectively drawing in or blowing out air.
This system of periodic reversal of airflows in a treatment
space can likewise be applied in combination with other waste treatment plants,
techniques and types of waste, and is not limited to the installation according
to the invention.
Created during the drying is percolate liquid which is
discharged via the blow and suction openings in the aerating floor. According to
the invention this takes place on the rear side of the tunnels through the use of
a controlled closing valve. The closing valve is opened periodically for a short
time. Heretofore the discharge was positioned on the front side of the tunnels and
the discharge took place via water seals to a gutter. The drawback hereof is that
a considerable extra building investment is needed for this purpose. The gutters
are further contaminated by accumulation of sludge, whereby odour nuisance occurs.
The gutter must moreover be covered with a grating, over which the power shovels,
which supply and discharge the waste, drive in and out continuously. Such gratings
are considered to be an additional obstacle.
This new method of draining percolate water can also be
applied in waste treatment plants other than that according to the invention.
The present invention will be further elucidated with reference
to the accompanying figures, in which:
shows a perspective view with broken away parts of an installation according
to the invention and a detail of the washer channel;
shows a partly cross-sectional perspective view of one of the treatment
spaces with the different supply and discharge channels for air with different valve
shows a series of tunnels 1a to 1h. Of these, tunnel
1a will be deemed a startup tunnel and tunnels 1b to 1h are
active. Each of the tunnels is connected to the so-called startup channel
3 via a discharge channel 21. Warm, moist air from tunnels
1b to 1h, in which the composting is in the active phase, is discharged
via this channel and then guided to surplus channel 4. It is fed herefrom
to startup tunnel 1a.
Each tunnel is further connected to a fresh air channel
5. The relative humidity, the oxygen content and the temperature of the air
from surplus channel 4 can hereby be adjusted. Inside a tunnel
1 the air present therein can be recirculated via channels 6 and
further shows washer channel 8 which is connected to channels
1a-1h via discharge channels 9. Water is sprayed into the washer channel
via supply pipes 10, which are fed from a shared conduit 11. A detail
of the washer channel shows the perforated internal lining 12 on which a
bacterial mass (not shown) develops.
The operation of the different supply and discharge channels
and the valves becomes apparent from
shows a cross-section of tunnel 1a with waste 13. Tunnel
1a is provided with an aerating floor 14. This floor comprises a number
of aerating tubes 15 located at regular mutual distances and having outflow
openings 16. The supply or discharge of air and the discharge of percolate
water takes place via these openings. Percolate water is removed from the tube system
via a closing valve to be opened periodically (not shown). Aerating tubes
15 are connected to conduits 17 for air. Conduits 17 debouch
in a shared distribution pipe 18, which is connected to channels
6 and 7 for recirculated air.
The recirculated air is alternately blown upward through
the floor or suctioned off via the floor. Fan 18 always rotates in the same
direction. The direction of the airflow is determined by the position of valves
C, D and G.
shows how air from channel 6 is blown through the waste heap
14 via the floor. Valve G closes discharge opening 20 and fan
18 carries air which is drawn out of tunnel 1 via channel
19 to channel 6.
When valve G closes channel 6, valve C is opened
and valve D is closed, as in
, the air is carried into tunnel 1a via opening 20 and fed
back to the tunnel through channel 7 and valve C via the floor.
Valve A in the feed from the central fresh air channel
5 comes into operation when fresh air must be supplied to a tunnel
1 in order to adjust the temperature, the humidity or the oxygen content
of the tunnel air.
When a tunnel is in the startup phase, the air therein
meanwhile contaminated by the VOC and TOC present in the waste will have to be discharged
after a time. This takes place via valve E in channel 21, whereby the air
is received in startup channel 3. Valves E in channels 21 (shown schematically
by upward arrows) in active tunnels 1b-1h are then for instance closed. This
air is carried into surplus channel 4 via fan 22 and fed via valves
B in supply channels 23 (designated with downward arrows) to active channels
1b-1h, where the VOC and TOC can be removed. Valve B in startup tunnel
1a is then for instance closed.
When tunnel 1a is started up, the air therein is
first recirculated for a time by means of blowing, as shown in
. Valve D is then wide open and valve G closes discharge opening
20. Valves E and B are closed and valves A and F are almost closed in order
to compensate possibly occurring air pressure fluctuations. During this recirculation
the oxygen content is measured. When this content falls below 12%, fresh air is
supplied via valve A. Discharge of air to startup channel 3 further takes
place via valve E. Valve B to surplus channel 4 and valve F to washer channel
8 are then closed.
In a subsequent phase of the startup warm air is supplied
from surplus channel 4. Excess air is discharged via startup channel
3 to the surplus channel. This air can be supplied to active tunnels
1b-1h by opening valves B in discharge channels 23 in tunnels
In all situations the recirculated air continues to blow
when valve D is open and valve G closes opening 20, or begins to suction
when valve D is closed and valve C is open, and valve G closes channel
6. Blowing in particular will usually take place during startup.
When the waste has been sufficiently dried, the air from
the relevant tunnel can be supplied to washer channel 8 via a discharge channel
9. The discharged air is herein brought into contact with washing water
23 via an atomizer or sprayer 24 spraying in a direction opposite
to the direction of flow of the discharge air. Further cleaning of the emission
air takes place with a bacterial mass adhered to the perforated inner wall
The valve technique in the installation according to the
invention ensures that the airflows within the system can be utilized efficiently.
The drying process progresses more quickly and fully through the use of warm startup
air. The emission air is much cleaner than in the existing systems due to the additional
cleaning of contaminated air from a startup channel in active tunnels and the spraying
of washing water counter to the airflow in the washer. A regenerative post-combustion
can hereby be wholly or partially dispensed with. Only a minimal quantity of fresh
air need be added. Finally, the drying time can be shortened and the drying process
improved by the continuously reversing recirculation flow.