Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD AND APPARATUS FOR TREATING WASTE
The present invention relates to a method and apparatus for
treating waste, and particularly for treating solid and
liquid waste in two stages.
Abattoirs, farms, food processing plants and the like
regularly produce solid organic waste which needs to be
treated in order to render it suitable for discharge into
the environment.
One method which is known for digesting such waste is
described in British patent number 2 230 004. This document
describes a two stage digestion process which uses an
installation comprising a fluids digestion vessel and a
solids digestion vessel which are connected together. The
solids digestion vessel is in the form of a tower or other
fixture which is located in or on the ground, and the fluids
digestion vessel is an adjacent tank. Bacterially active
waste is fed from the fluids digestion vessel into the
solids digestion vessel until at least some of the solids
have been sufficiently digested to be environmentally
acceptable. The digested solids are then removed, perhaps
for use as a soil conditioning agent, and a further batch of
solids waste is added to the solids digestion vessel.
A problem with this process is that the organic waste
material which is to be digested must be loaded onto a
transport container where it is produced, and subsequently
transferred from the transport container into the solids
digestion vessel. Typically the waste producer uses a skip
into which the waste is tipped for transport. The
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untreated waste often smells offensive, and it may be
hazardous to health.
It is an object of the present invention to provide a
process for digesting solid waste material which reduces
the above mentioned problems, and apparatus for use in
carrying out the process.
According to a first aspect of the present invention there
is provided a process for digesting a solid waste
material, which process comprises:
a. collecting waste material containing biodegradable
solids in a solids digestion vessel at a first
location;
b. transporting the solids digestion vessel to a second
location at which is sited a source of organic
material undergoing anaerobic bacterial digestion in
a fluid phase digestion stage to produce a fluid
fraction containing active bacteria;
c. feeding at least part of the fluid fraction from the
fluid phase digestion stage into the solids digestion
.vessel to provide at least part of a fluid phase in
the solids digestion stage; and
d. recovering at least part of the environmentally more
acceptable solids fraction from the solids digestion
stage.
Preferably the process also includes the step of
transporting the solids digestion vessel away from the
second location. The solids digestion vessel is
preferably transported back to the first location,
although it may alternatively be transported to another
site.
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The fluid which is transferred from the fluid source may
also transfer heat and/or nutrients to the solids
digestion vessel.
By using a mobile solids digestion vessel the waste may be
loaded directly into the vessel at source, thereby
reducing the amount of materials handling necessary in the
process. Furthermore, the vessel may be substantially
sealed during transport so as to reduce the escape of
odour into the environment.
An additional benefit is that the producer of the waste
may readily have the treated waste returned to him in the
same vessel. The treated waste may then be unloaded at
leisure, and a new load of waste loaded into the vessel.
This reduces the handling required for the solid waste
products, and it ensures that the waste producer will
receive back his ow-n treated waste, and not someone else's
treated waste of unknown origin and therefore unknown
composition.
It is preferred that the solids digestion vessel is
connected to the fluid source by a pipe or other conduit
means so that the feeding of fluid from the fluid source
to the vessel may be conveniently carried out. It would
however be possible to feed the fluid to the solids
digestion vessel by other means, for example in one or
more buckets. For convenience hereinafter the invention
will be described with reference to at least one
connection for feeding fluid from the fluid source to the
solids digestion vessel.
The solids digestion vessel may be either wholly or
partially mobile by being mounted on, or part of, a
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vehicle, for example a container lorry or a rail bulk
wagon.
The bacterial digestion is anaerobic, but aerobic
digestion may additionally be used in the process. Some
waste, for example abattoir waste, is preferably
anaerobically digested for several days, and then
subjected to aerobic digestion for some more days. For
example abattoir waste may be anaerobically digested for
seven days, and then aerobically digested for seven more
days.
The preferred digestion times will be influenced by the
nature of the waste which is being digested and the use to
which the digested solids waste is to be put. For
composting for example, it is preferred that anaerobic
digestion is carried out for one to four days, and aerobic
digestion from 10 to 25 days. For soil conditioning
agents, typically anaerobic digestion will be maintained
for four to nine days followed by 7 to 14 days of aerobic
digestion. For convenience the invention will be
described hereinafter with reference to anaerobic
digestion.
Different types of bacteria predominate at different
temperatures in anaerobic digestion of organic waste.
Mesophilic bacteria operate in a relatively low
temperature range, usually less than 40 C, and they have
a relatively slow rate of digestion. Thermophilic
bacteria work effectively in a higher temperature range,
around 40 to 65 C (the thermophilic range). Digestion in
the higher temperature thermophilic range occurs more
rapidly than in the lower temperature mesophilic range,
and it gives better control or elimination of pathogens
and parasitic organisms.
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The solids digestion vessel is preferably provided with a
heater for warming the waste material during the journey
from the first location to the second location. This pre-
heating of the waste helps to speed up the digestion
process, thereby reducing the retention time. The heating
may be particularly effective on long or slow journeys by
allowing bacteria to operate in the thermophilic
temperature range. The heater may also be used during the
process, when the vessel is connected to the fluid source,
to maintain a thermophilic temperature. The heater
preferably heats the waste to a temperature in the range
to 80 C, and particularly preferably to the optimum
thermophilic range. The optimum temperature range which
will be used in practice will depend on the tolerance to
15 temperature of the bacteria which are used to digest the
waste. The heater may be independently powered, or it may
be powered directly or indirectly (for example by the
vehicle battery) from the dynamo of the vehicle engine.
When the vessel is connected to the fluid source, its
20 heater may be connected to an external power source to
avoid draining the vehicle battery.
A plurality of vessels may each be connected to a single
fluid source, thereby reducing the number of fluid source
containers required whilst keeping the solid wastes
substantially separate from each other. Each vessel may
be directly connected to the fluid source, or the vessels
may be connected to a common manifold which in turn is
connected to the fluid source.
It is preferred that the total volume of the or all of the
solids vessels is approximately equal to the volume of the.
fluid source.
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The fluid source may comprise a conventional liquid
digester; for example a completely stirred tank reactor.
The rate of completion of digestion of the solid waste
material may be increased by agitation during the process.
It is therefore preferred that the vessel is provided with
means for agitating its contents. The agitating means may
comprise any suitable means for separating or moving
solids; for example a paddle for stirring, means for
blowing gas through the solids, means for pushing liquid
back up through the solids, or means for vibrating the
waste'solids. In a preferred embodiment the agitation
means comprise means for blowing gas through the solids,
because this may be achieved by recycling gas produced in
the digestion process.
The agitating means is preferably mounted in the vessel
and can be connected to an external supply of power or gas
according to its mode of operation. A benefit of passing
gas or liquid up through the solid waste to agitate it is
that this may be achieved without the use of dedicated
agitation means, thereby allowing a simplification of the
design of the apparatus.
Anaerobic digestion produces a methane rich gas in the
solids and fluid containing vessels. This gas can be used
for agitation of the contents of either vessel or both
vessels. It may also be used to equalise the pressure in
both vessels, and to transfer heat from one vessel to
another. Excess of gas may be used for energy generation,
for example in a combined heat and power plant (CHP), or
it can be flared off or vented to the atmosphere.
At least some of the fluid phase in the vessel is
preferably returned from the vessel to the fluid source so
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that the fluid fraction recycles through the solids
digestion stage. This recycling of liquid aids the
transfer of heat, nutrients and active bacteria between
the vessels.
Make up water, optionally containing nutrients for the
bacteria and/or liquid organic wastes, may be added to the
fluid source and/or the vessel as required.
All of the fluid connections between the fluid source and
the vessel may be carried in a single umbilical or other
connector. For example a single connector could carry two
liquid and two gas pipelines, for two-way flow of liquid
and gas between the fluid source and the connector. The
use of a single connector for all the fluid connections is
convenient. It is also safer than independently making
all of the fluid connections, where forgetting to make one
connection could lead to a loss of waste material and/or
gas. For safety reasons it is preferred that the
connector unsymmetrical so as to ensure that the
connection may only be made when the connector is in a
single correct orientation.
The flow of liquid and/or gas may occur by virtue of
gravity or gas pressure, or one or more pumps may be used.
The environmentally more acceptable solids fraction is
preferably recovered by stopping the passage of fluid from
the fluid source to the vessel, and draining fluid from
the vessel. The drained fluid is preferably returned to
the fluid source. When the treated solids are adjudged
sufficiently drained, the vessel may then be moved to a
location remote from the fluid source, typically back to
the first location, and the treated solid waste removed
from the vessel. It would however be possible to remove
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the treated solid waste from the vessel at the second
location. If it is convenient to handle the treated solid
waste as a slurry rather than a solid, more of the fluid
may be retained with the solid waste, or extra water may
be added to the solid waste.
If the solid waste is to be aerobically digested after the
anaerobic digestion stage this is preferably accomplished
by stopping the inflow of fluid into the solids digestion
vessel, and allowing fluid to drain out. Air is then
pumped into the solids digestion vessel so that it
percolates over the surface of the solid particles. The
air may conveniently be introduced through a gas inlet
pipe that has been used previously for feeding methane
rich gas into the vessel. A layer of wood chips or other
suitable odour absorbing material may optionally be laid
over the top of the solid waste to reduce odorous
emissions during the aerobic digestion stage.
The invention also provides apparatus suitable for
carrying out the above described process.
Accordingly, a second aspect of the present invention
provides anaerobic digestion apparatus comprising a fluids
digestion vessel, a solids digestion vessel, and means for
feeding fluid between the two vessels, characterised in
that the solids digestion vessel is detachable from the
fluids digestion vessel and transportable.
The solids digestion vessel is preferably provided with
wheels for ease of transport, for example by being mounted
on a trailer.
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The invention will now be further described by way of
example, with reference to the following drawings in
which:
Figure 1 is a diagrammatic flow diagram of the solid
and fluid digestion stages of a process in accordance
with a first embodiment of the invention;
Figure 2 is a diagrammatic flow diagram of the solid
and fluid digestion stages of a process in accordance
with an alternative embodiment of the invention; and
Figure 3 is a diagrammatic flow diagram of another
embodiment of a process in accordance with the
present invention.
The fluids digestion vessel shown in Figure 1 is a
conventional liquid digester. Fluid waste is passed into
the fluids digestion vessel through a sieve to remove
large particles, and the fluid is maintained at a
temperature of around 55 C by a heating and cooling coil.
Cooling fluid is fed through the coil if the rate of
digestion is too high so that the temperature is raised
above a preset limit. Other temperatures could also be
used; for example a temperature of around 35 C could be
used for digestion by mesophilic bacteria. The fluid
waste is subject to anaerobic digestion in the presence of
bacteria to provide an output which is biologically
active, but in which much of the environmentally
unacceptable material has been converted into acceptable
material and methane gas. Some of the gas produced by the
digestion process may be returned to the fluids digestion
vessel via a sparger to help agitate the fluid. The fluid
digestion stage will typically require the presence of a
bacterial inoculum to initiate digestion, and this may be
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provided by the addition of partially treated sewage
sludge in the initial material. However, the fluid
digestion is preferably operated continuously, and
material is recycled in the process, so that it is usually
only necessary to provide an inoculum during start up of
the process, or following a period of shut down.
A solids digestion vessel, here shown as a container lorry
trailer, has been loaded with solid waste material at a
remote location. The solids digestion vessel has been
transported by road to the site of, and adjacent to, the
fluids digestion vessel. A heater is mounted in the
solids digestion vessel, and maintains the temperature of
the solid waste at around 55 C. The heater is powered by
the lorry battery in transit, and by an external power
source when the vessel is connected to the fluids
digestion vessel.
Connections are made between the solids digestion vessel
and the fluids digestion vessel for the passage of fluid
from the fluids digestion vessel to the solids digestion
vessel, and vice versa so that the fluid fraction recycles
through the solid waste. The fluid input to the solids
digestion vessel is preferably via one or more spargers
located at the top of the solids digestion vessel to
assist uniform distribution of fluid across the surface of
the solid waste. The fluid percolates down the solids
digestion vessel and covers the surfaces of the solids
within the vessel, thereby subjecting the solids to
bacterial digestion by the bacteria within the fluid. The
fluid need not flow over the solids under gravity, but may
be pumped to increase the flow rate. It is also within
the scope of the invention for the fluid to be pumped so
as to flow up or through or across the solids digestion
vessel if desired.
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Connections are also made for the passage of gas from the
fluids digestion vessel to the solids digestion vessel and
vice versa. Gas preferably enters the solids digestion
vessel via one or more spargers so as to agitate the
contents of the solids digestion vessel over most or all
of its volume. Excess of gas may be used for energy
production, stored, discharged to atmosphere, or flared
off. If the excess of gas is used for energy production,
some or all of the energy produced may be used directly or
indirectly to provide heat for the heating and cooling
coil. For CHP it is preferred that the daily output of
gas exceed about 1000 cubic metres.
The pH of the solid waste and/or the liquid waste may be
adjusted to suit the bacteria which are carrying out the
digestion, by the incorporation of suitable materials, for
example ground chalk or lime. These materials may be
added to the solids waste when it is loaded into the
vessel at a first location remote from the fluids
digestion vessel.
The solids digestion is carried out until the solid waste
has been digested to a sufficient extent. This will
usually be achieved when the solid is acceptable for
direct discharge or use in the environment. After the
solids digestion has been carried out sufficiently, the
passage of fluid into the solids digestion vessel is
halted, for example by disconnecting the connecting pipe,
and fluid may be allowed to drain from the solids until
the solid waste has reached a suitable consistency for
handling. It is not essential for this further drainage
to occur, and all connections may be disconnected
immediately after sufficient digestion has occurred if
desired.
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After disconnection of the solids digestion vessel from
the fluid source, the solids digestion vessel is
transported to another location where it may be emptied
and refilled with a further batch of solid waste.
A plurality of solids digestion vessels may be
transported, separately or together, to the location of
the fluid source as shown in Figure 2. Here, three solids
digestion vessels have been loaded with solids waste and
transported to the location of the fluids digestion
vessel. Each solids digestion vessel has been connected
to the fluids digestion vessel by means of an umbilical
connection which houses four pipe lines comprising: fluid
in; fluid out; gas in; gas out. When the solids in a
solids digestion vessel have been sufficiently digested,
the umbilical connection is removed, and the solids
digestion vessel is transported to another location,
optionally after the removal of some or all digested solid
waste material.
Referring now to Figure 3, a fluids digestion vessel is
permanently located underground, and it is connected to
three solids digestion vessels. Each solids digestion
vessel is detachable from the fluids digestion vessel and
transportable.
Each solids digestion vessel is connected in the same way
to the fluids digestion vessel, via a sump, and to a
biogas line via a gas meter M. For simplicity all of the
connections are illustrated for the left hand solids
digestion vessel only.
Fluid waste is continuously recycled between the fluids
digestion vessel and an adjacent standpipe, by means of a
pump. A band heater is controlled by a thermostat to keep
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the fluid temperature in the standpipe at around 59 C,
causing the temperature of fluid in the fluids digestion
vessel to be around 55 C.
Fluid waste (liquor) is pumped out of the standpipe and
distributed between the three solids digestion vessels.
The liquor is shown here entering the top of the solids
digestion vessels; however it could also be arranged to
flow in through the bottom, to provide agitation and to
unblock solids from the grid at the bottom of the vessel.
The top of the fluids digestion vessel is connected to the
top of each solids digestion vessel by gas pipes. This
allows equilibration of pressure in each vessel by passage
of gas in either direction. Excess of biogas is collected
in a gas holder (top left of diagram) or vented or
connected to apparatus for gas use (top right of diagram) .
Liquor which passes down through the solid waste in each
solids digestion vessel is collected in a sump, from where
it is pumped back into the fluids digestion vessel for the
cycle to start again.
Occasionally, for example once a day, the solid waste is
agitated by pumping gas through the liquor outlet piping
in the bottom of the solids digestion vessel. This is
done by closing off the valve in the liquor outlet which
is closest to the sump, disconnecting the gas outlet from
the biogas line, and connecting a gas pump between the gas
outlet and the liquor outlet. Operation of the gas pump
locally recycles gas through the grid at the bottom of the
solids digestion vessel, causing agitation of the solids
therein. On completion of the agitation the reverse
procedure is followed to restore the fluid connections to
their usual operative status as shown in the diagram.
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A single gas pump may be used sequentially on any or all
of the solids digestion vessels, or each vessel may be
provided with a dedicated gas pump.
As an alternative to the use of a gas pump, agitation of
the solids may be achieved by pumping liquor back from the
sump through the liquor outlet pipes in each solids
digestion vessel. This avoids the use of pressurised gas
and is therefore preferred for safety reas ons . The
pumping may be steady or in pulses to achieve more
vigorous agitation.
The gas outlet at the top of each solids digestion vessel
may be connected to any one of three pipes (illustrated
for the vessel on the left in Figure 3). One pipe is a
simple vent to atmosphere or for sampling or collection.
A second pipe connects to the biogas line via a meter M.
Each gas meter M may be used for measuring the rate of
local gas production when liquid pumping is stopped. The
third pipe is used for connecting the gas pump for
agitation of the solid as described above. The three
connections are shown separately for the purposes of
illustration, but they could be combined together in a
single housing and different connections made by means of
a three way valve.
All of the fluid connections are snap tight valve
connections. The valves are open when the connection is
correctly made, but each half of the connection is sealed
by a valve when not connected. It is preferred that the
gas pipes are of different diameter than the liquid pipes,
to avoid accidental connection of a gas pipe to a liquid
pipe.
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It is particularly preferred that the internal diameter of
the gas piping is greater than that of the liquid piping
for ease of flow. For example the gas pipes may be 25 mm
and the liquid pipes may be 19 mm internal diameter.
Typically each solids digestion vessel is filled two-
thirds to three-quarters full of solids waste at a remote
location, for example an abattoir. After transportation
to the site of the fluids digestion vessel it is filled
with hot liquid from the fluids digestion vessel or the
standpipe. The remaining connections are then made, and
agitation is provided as required.
If aerobic digestion is required after completion of
anaerobic digestion, this may readily be carried out by
draining liquid waste from a solids digestion vessel,
disconnecting the pipe connections, and blowing air
through the liquor outlet pipe at the bottom of the solids
digestion vessel.
The apparatus shown in Figure 3 will digest a maximum of
about 15,000 tonnes per annum of solids waste, and can
economically digest 5,000 tonnes per annum or less. This
compares to a fixed installation which would not
economically be used to digest less than around 10,000
tonnes per annum.
The invention therefore provides a convenient method of
treating solid and liquid wastes, which reduces the amount
of handling of the untreated solid waste.
