PROCESS FOR THERMOPHILIC, AEROBIC FERMENTATION OF ORGANIC WASTE
FIELD OF THE INVENTION
This invention relates to a commercially viable process for converting organic
waste from various sources to useful end products by a thermophilic, aerobic
microbiological fermentation process, more particularly a fermentation process which
is initiated without inoculation with thermophilic microorganisms and in which
external heat is applied to achieve and maintain thermophilic temperatures.
BACKGROUND OF THE INVENTION
It is well known that the disposal of organic waste materials, for example
animal and food wastes, is becoming increasingly difficult and expensive. As a way
to avoid the difficulties in disposal of organic waste materials, fermentation processes
have been developed to chemically modify these wastes into useful end products,
such as animal feeds, feed supplements and fertilizers.
In general, such fermentation processes are conducted in the presence of
oxygen at elevated temperatures, preferably in the range of from about 50°C to about
80°C. Microorganisms which grow and proliferate in this temperature range and
which preferably are largely responsible for fermenting the waste material are known
as "thermophilic" microorganisms or "therπ-ophiles" . Aside from chemical
modification of the waste material by the thermophilic microorganisms, it is
generally known that the heat generated by fermentation processes conducted in this
temperature range is capable of providing a "pasteurization" effect by destroying
pathogens and other undesirable biological contaminants present in the waste
material. This pasteurization effect is desirable since it increases the safety of the
end product, whether used as a fertilizer or food stuff.
One example of a known process for thermophilic fermentation of animal
waste is disclosed in U.S. Patent No. 3,462,275, issued to W.D. Bellamy. In the
Bellamy process, animal fecal matter is inoculated with thermophilically active
microorganisms obtained from sources such as compost piles or hot springs, and is
then heated by an external heat source to thermophilic fermentation temperatures in
a thermophilic aerobic growth chamber. After fermentation of the inoculated waste
by the thermophilic microorganisms, a fermented product is obtained which is
separated into solid and liquid components by centrifuging and filtering. The liquid
is disposed of by conventional means while the solid portion is dried and packaged
for use as an animal food stuff.
One disadvantage of the Bellamy process is that inoculation of the waste
material with thermophilic microorganisms may be problematic. For example,
microorganisms obtained from external sources for the purpose of inoculation may
not be compatible with the waste matter being fermented, thus requiring careful
process control. For example, pH adjustment of the fermenting mixture to within
a narrow range at which the microorganisms proliferate and/or nutrient
supplementation may be required. Also, inoculation requires the addition of one or
more steps to the overall process.
Therefore, inoculation of the waste material with thermophilic microorganisms
is preferably avoided. However, it has generally been accepted that thermophilic
microorganisms are not naturally present in waste materials in sufficient quantities
to initiate or sustain thermophilic fermentation.
Another known fermentation process is described in U.S. Patent
No. 4,292,328, issued to Coulthard et al. The Coulthard patent describes a
thermophilic, aerobic fermentation process for converting a wide range of organic
waste materials into animal feeds, feed supplements and fertilizers. Although, like
the Bellamy process, inoculation is fundamental to the Coulthard process, it is
disclosed in Coulthard that microorganisms naturally present in the waste material
may be used to initiate and maintain an active fermentation.
Since the waste material is at ambient temperature before initiation of the
fermentation, the microorganisms naturally present in the waste material in the
greatest number are those which grow and proliferate at temperatures of from about
0°C to about 30°C. These microorganisms are referred to as "psychrophilic"
microorganisms or "psychrophiles". The Coulthard process initiates the fermentation
by introducing the waste material into a thermally insulated fermenter as an aqueous
slurry, and vigorously agitating and oxygenating the mixture at ambient temperatures
to promote the growth of aerobic, psychrophilic microorganisms.
As fermentation begins at ambient temperature, psychrophilic microorganisms
metabolize substrates present in the waste material and liberate heat, thereby
gradually raising the temperature of the fermenting mixture out of the ambient
temperature range to slightly elevated temperatures between ambient temperatures
and thermophilic temperatures. As the temperature increases, the psychrophilic
microorganisms are gradually replaced by "mesophilic" microorganisms or
"mesophiles" which grow and proliferate at temperatures of from about 20 to about
50°C.
Fermentation by the mesophilic microorganisms gradually raises the
temperature into the thermophilic range of from about 50 to about 80° C, at which
thermophilic microorganisms proliferate.
Therefore, the Coulthard fermentation process is dependent on a succession
of microorganisms to slowly raise the temperature over a period of about two or
more days from ambient to thermophilic temperatures in which the desired
thermophilic microorganisms grow and proliferate. This succession of
microorgamsms involves a progressive and successive change in the profile of the
microorganisms from mainly psychrophiles to mesophiles, and then to thermophiles,
effected by simultaneous aeration and agitation of the fermenting waste.
It would be expected that using an external heat source to rapidly heat
uninoculated waste matter to thermophilic temperatures would preclude this
succession of microorganisms and would result in the thermophilic fermentation
process being initiated either very slowly or not at all. Raising the temperature to
the thermophilic range would permit growth primarily only of thermophiles, which
as discussed above are generally considered to be present in the unfermented waste
material in much smaller numbers than psychrophiles.
The inventors have found that promoting a succession of microorganisms as
in the Coulthard process is disadvantageous in that the waste material is at least
partially fermented by microorganisms other than thermophilic microorganisms,
primarily during the initial stages of the process before the temperature has risen into
the thermophilic range. Some of the microorganisms which proliferate at lower
temperatures may cause contamination and/or poisoning of the waste material.
Furthermore, fermentation at lower temperatures allows the continued growth and
proliferation of pathogens present in the waste material.
Specifically, control over the fermentation in the Coulthard process is
minimal, particularly during the initial stages. For example, the Coulthard process
may not lead to establishment of thermophilic conditions at all or maintenance of a
thermophilic fermentation if initiation is achieved. This is at least partially due to
the fact that fermentable substrates may be completely utilized prior to establishment
of thermophilic conditions. Furthermore, the fermentable substrates may be utilized
by psychrophilic or mesophilic microorganisms which may preclude the growth of
thermophilic microorganisms through production and liberation of toxins or poisons,
such as growth inhibitors and antibiotics, in the fermentation medium.
Therefore, the disadvantage exists that no thermophilic, aerobic fermentation
process is known which avoids the use of inoculation with thermophilic
microorganisms and which promotes growth and proliferation only of thermophilic
organisms.
Even though the Coulthard fermentation process utilizes microorganisms
occurring naturally in the waste matter, it is still somewhat pH sensitive. For
example, in order to ferment acidic waste materials such as wastes from fruit and
vegetable processing, which typically have a pH in the range of about 3.8 to about
4.4, Coulthard teaches the addition of a pH adjusting agent to raise the pH into a
more neutral range.
Therefore, the additional disadvantage exists that no aerobic, thermophilic
fermentation processes are known which do not require careful monitoring of process
pH and the addition of pH adjusting agents.
SUMMARY OF THE INVENTION
To overcome the disadvantages of the prior art discussed above, the present
invention provides a process for thermophilic, aerobic fermentation of organic waste
which is initiated by application of external heat to an oxygenated aqueous mixture
of uninoculated waste matter.
The inventors have surprisingly found that a thermophilic fermentation can be
initiated by application of heat to an uninoculated aqueous mixture of waste matter,
thus promoting the growth and proliferation primarily only of thermophilic
microorganisms in the waste matter. Although external heating of the uninoculated
waste matter precludes a succession of microorganisms from being produced, the
inventors have found that the thermophilic fermentation may be completely initiated
in a period of from about 2 to about 6 days.
Subsequent to initiation, the present invention preferably also provides a semi-
continuous or continuous process for fermentation of waste matter which is capable
of fermenting a wide range of waste materials over a wide pH range. In the semi-
continuous or continuous fermentation of the present invention, relatively small
volumes of uninoculated waste matter is fed intermittently or continuously into an
active fermentation, and fermented product is removed intermittently or continuously
from the active fermentation. Preferably, the volume of the active fermentation is
completely turned over once about every 24 to 48 hours.
Furthermore, because the fermentation process of the present invention is not
initiated by inoculation, it utilizes thermophilic microorganisms naturally present in
the waste material, which are more compatible with the waste material than
microorganisms introduced by inoculation.
The inventors have also surprisingly found that the process of the present
invention is relatively insensitive to the pH of the waste material and operates over
a wide pH range. In fact, the inventors have found that almost all types of food,
animal and lignocellulosic wastes may be fermented by the process of the present
invention without the addition of pH adjusting agents.
In particular, the inventors have found that the present process does not
require the neutralization of acidic waste matter, such as fruit and vegetable
processing waste, thus making it adaptable to fermentation of a wide variety of waste
materials without the need to carefully monitor and adjust pH.
By reason of the improvements of the process of the present invention over
previously known processes, the process of the present invention may be used on a
commercial basis to quickly and efficiently convert a wide range of waste matter into
a useable end product, such as animal feed, animal feed supplements, fertilizers,
fertilizer ingredients, soil conditioners or soil amendments.
Preferably, when used as a commercial process, the process of the present
invention is operated on a continuous or semi-continuous basis. The inventors have
found that operation of the process on a continuous or semi-continuous basis provides
improved control over the fermentation. Specifically, operation on a continuous or
semi-continuous basis ensures that a thermophilic fermentation will be maintained by
supplementation of the active fermentation with a continuous or semi-continuous
supply of fresh substrate to preferably maintain the fermentation in a steady state.
Maintaining the thermophilic fermentation in a steady state ensures that there will be
minimal competition for fermentable substrates by other competing microorganisms,
thereby reducing the chance that the fermentation will be suppressed or inhibited by
competing microorgamsms.
It is one object of the present invention to provide initiation of a process for
conversion of waste matter to a useful end product by thermophilic, aerobic
fermentation of the waste matter wherein external heat is applied to uninoculated
waste matter, such that the process is initiated by thermophilic microorganisms
naturally occurring in the waste matter.
It is another object of the present invention to provide a process for
conversion of waste matter to a useful end product by thermophilic, aerobic
fermentation of waste matter, the process being initiated by application of heat to
uninoculated waste matter, such that the process is initiated by thermophilic
microorganisms naturally occurring in the waste matter.
It is yet another object of the present invention to provide continuous and
semi-continuous processes for conversion of waste matter to a useful end product by
thermophilic aerobic fermentation of the waste matter.
It is yet another object of the present invention to provide a process for
conversion of waste matter to a useful end product by thermophilic aerobic
fermentation of the waste matter, the process being operated at acidic pH.
It is yet another object of the present invention to provide a process for
conversion of waste matter to a useful end product by thermophilic aerobic
fermentation of the waste matter, wherein the process is capable of destroying
chemical contaminants present in the waste material.
In one aspect, the present invention provides a process for conversion of
organic waste matter to an end product by thermophilic, aerobic fermentation of the
waste matter by thermophilic microorganisms naturally occurring in the waste matter,
the process being initiated by steps of: forming an aqueous mixture of the waste
matter; heating the mixture, with heat from an external heat source, to a temperature
suitable for growth and proliferation of the thermophilic microorganisms; and
oxygenating the mixture at the temperature by continuously introducing oxygen into
the mixture to maintain an oxygen concentration in the mixture sufficient for growth
and proliferation of the thermophilic microorganisms.
Preferably, the waste matter is selected from the group comprising animal
fecal matter, bakery product waste, waste derived from fruits and vegetables, food
wastes derived from animals, tannery waste, leaves, weeds, trees, shrubs, and wood
refuse.
Preferably, the end product is selected from the group comprising animal
feeds, animal feed supplements, and fertilizers, fertilizer ingredients, soil
conditioners and soil amendments.
Preferably, the waste matter is mechanically macerated to a particle size of
from less than about 1 mm to about 5 mm prior to or during the step of forming the
aqueous mixture, and the aqueous mixture contains from about 5 percent to about 20
percent total solids by weight.
Preferably, the process is initiated over a period of from about 2 to about 6
days at a temperature of from about 55 to about 80 °C and an oxygen concentration
maintained at about 0.2 ppm or higher.
Preferably, the initiation is complete when the fermentation reaches a steady
state at which a rate of the fermentation is substantially constant and a portion of the
waste matter has been converted to the end product.
In another aspect, the present invention provides a process for conversion of
organic waste matter to an end product by thermophilic, aerobic fermentation of the
waste matter by thermophilic microorganisms naturally occurring in the waste matter,
the process comprising: (a) initiation of the fermentation by steps of: (i) heating an
aqueous mixture containing the waste matter in a fermentation vessel, with heat from
an external heat source, to a temperature suitable for growth and proliferation of the
thermophilic microorganisms; (ii) oxygenating the aqueous mixture at the
temperature by continuously introducing oxygen into the mixture to maintain an
oxygen concentration in the mixture sufficient for growth and proliferation of the
thermophilic microorgamsms, the initiation being continued until the fermentation
reaches a steady state at which a rate of the fermentation is substantially constant and
a portion of the waste matter in the aqueous mixture has been converted to the end
product; (b) continuing the heating and the oxygenating of the aqueous mixture; (c)
adding to the fermentation vessel additional quantities of an aqueous mixture of the
waste matter; and (d) removing from the fermentation vessel quantities of the
aqueous mixture containing the end product, such that growth and proliferation of
the thermophilic microorganisms is maintained in the fermentation vessel during steps
(b), (c) and (d).
Preferably, the steady state of the fermentation is maintained during steps (b),
(c) and (d), at a temperature maintained in the range of from about 55°C to about
80°C, and an oxygen concentration is maintained in a range of from about 1 ppm to
about 5 ppm during steps (b), (c) and (d).
Preferably, step (c) comprises intermittently adding to the fermentation vessel
additional quantities of an aqueous mixture of the waste matter, and step (d)
comprises intermittently removing from the fermentation vessel quantities of the
aqueous mixture containing the end product.
Preferably, the fermentation vessel comprises a primary fermentation vessel
which is connected to a secondary fermentation vessel, and step (d) comprises
transferring quantities of the aqueous mixture containing both the waste matter and
the end product to the secondary fermentation vessel, and substantially completing
the fermentation in the secondary fermentation vessel, the process additionally
comprising: (e) heating the aqueous mixture in the secondary fermentation vessel,
with heat from an external heat source, to maintain a temperature therein suitable for
growth and proliferation of the thermophilic microorgamsms; (f) oxygenating the
aqueous mixture in the secondary fermentation vessel by continuously introducing
oxygen into the mixture to maintain an oxygen concentration therein sufficient for
growth and proliferation of the thermophilic microorgamsms; and (g) removing from
the secondary fermentation vessel quantities of the aqueous mixture containing the
end product and containing substantially no unfermented waste material, wherein the
fermentation in the secondary fermentation vessel is maintained at a steady state at
which a rate of the fermentation is substantially constant.
Preferably, step (c) comprises continuously adding to the primary fermentation
vessel additional quantities of an aqueous mixture of the waste matter, and step (d)
comprises continuously transferring the aqueous mixture from the primary
fermentation vessel to the secondary fermentation vessel.
Preferably, step (g) comprises continuously removing from the secondary
fermentation vessel quantities of the aqueous mixture containing the end product and
containing substantially no unfermented waste material.
Preferably, the retention time of the aqueous mixture in the primary and
secondary fermentation vessels from step (b) to (g) is sufficient that the aqueous
mixture removed from the secondary fermentation vessel in step (g) contains no
unfermented waste matter and no biological contaminants present in the waste
material prior to the fermentation, and wherein the biological contaminants are one
or more members selected from the group comprising pathogens, insect eggs, larvae,
worms, and viruses.
Preferably, the waste material prior to fermentation contains chemical
contaminants, and wherein a retention time of the aqueous mixture in the primary
and secondary fermentation vessels from step (b) to (g) is sufficient that the aqueous
mixture removed from the secondary fermentation vessel in step (g) contains no
unfermented waste matter and none of the chemical contaminants, and wherein the
chemical contaminants are selected from the group comprising herbicides, pesticides
and pharmaceuticals selected from one or more members of the group comprising
chlortetracycline, sulfamethazine and penicillin.
Preferably, the pH in the primary and secondary fermentation vessels is in a
range of from about 3.8 to about 4.4, and wherein the waste matter comprises food
waste.
BRIEF DESCRIPTION OF THE DRAWINGS
Further aspects and advantages of the present invention will become apparent
from the following description, taken together with the accompanying drawings, in
which:
Figure 1 is a schematic representation of a preferred continuous fermentation
process according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The preferred process of the present invention preferably comprises initiation
of an active fermentation followed by continuous or semi-continuous processing of
wastes, and is now described below with reference to Figure 1.
The process of the present invention may be used to ferment a wide variety
of organic materials, which are normally considered wastes and treated as disposal
problems. Typical waste materials include tannery waste, municipal or yard waste,
food waste, animal waste and sludges from biological processes. Animal waste
includes fecal matter or manure produced as a byproduct of an animal's digestion of
food, for example manure from cattle, swine, sheep, horses, mink, chicken or human
fecal matter which may be in the form of sewage sludge ( — 5% solids), dewatered
sewage sludge ( — 25-30% solids) or septage.
Food waste includes bakery product waste, all fruit and vegetable processing
waste, for example potato and tomato processing wastes, all fruit and vegetable trim
and peeling wastes, and food wastes derived from animals such as meats and meat
trimmings, cheese whey, fish processing wastes, and slaughterhouse waste such as
blood. Food wastes may be obtained from a wide variety of sources, including retail
and wholesale grocery operations, restaurants, institutions, food processors/preparers,
and "wet" household garbage.
Municipal or yard waste includes leaves and weeds, as well as materials
containing lignocellulosic complexes, for example woody plants such as trees, shrubs
and wood refuse derived therefrom. Lignocellulosic materials may also be obtained
as by-products of wood processing industries. Tannery wastes include hides, bones,
cartilage and animal trimmings.
Furthermore, the process of the present invention is capable of either
fermenting mixtures of various types of waste materials or "pure" waste materials
comprising only one type of waste.
Therefore, the process of the present invention is capable of fermenting a wide
range of waste materials of plant or animal origins. In general, any organic
substances comprised of protein, simple and/or complex carbohydrates and lipids are
fermentable according to the process of the present invention, including those
disclosed in the Coulthard patent.
The first step in the process is the formation of an aqueous mixture of the
organic waste matter. Because the fermentation of the waste matter occurs at its
surface, it is preferred to maximize the surface area of the waste material available
to the microorgamsms by mechamcally macerating the waste material prior to
commencement of the fermentation.
Since different types of waste matter differ in water content, it may be
necessary to add water to the waste matter prior to commencement of the
fermentation process to reduce the viscosity of the aqueous mixture, thereby avoiding
use of large amounts of energy to agitate the mixture during fermentation. The
reduced viscosity also serves to increase the dissolution of oxygen in the fermentation
medium. Preferably, the aqueous mixture of waste matter contains from about 5 %
to about 20% total solids by weight, more preferably from about 12% to about 18%
by weight.
In the preferred process of the present invention shown in Figure 1, a
hydropulper 12 having a blade 13 is used to macerate the waste matter. The capacity
of hydropulper 12 may preferably be about 12,000 to 16,000 litres. If necessary,
water may be added to the waste matter in the hydropulper 12. However, it is to be
appreciated that water may also be added to the waste matter during the fermentation
process. Preferably, the waste matter is reduced to a particle size as small as
possible, generally from less than about 1 mm to about 5 mm.
The aqueous mixture of hydropulped, macerated waste matter is preferably
subjected to fermentation shortly after being formed. However, the aqueous mixture
may be stored prior to fermentation, preferably not longer than about 24 hours.
Preferably, the aqueous mixture is oxygenated during storage by aeration and
agitation, thereby avoiding substantial anaerobic fermentation during storage.
Preferably, the aqueous mixture is pumped from hydropulper 12 through
conduit 14 to a holding vessel 15, where the aqueous mixture may be stored with
constant agitation by mixing blade 40 and aeration through air supply 23, and
without application of heat. The holding vessel 15 is merely a reservoir of "raw" ,
unfermented waste matter, from which the aqueous mixture of waste matter is drawn
intermittently or continuously, and pumped through conduit 17 to a fermentation
apparatus. Different waste materials may also be blended in the holding vessel. It
is to be appreciated that more than one holding vessel 15 may be provided. The
capacity of holding vessel 15 may preferably be about 50,000 to 55,000 litres.
The fermentation apparatus comprises at least one fermenter, and preferably
comprises at least two fermenters connected in series. In a more preferred
embodiment shown in Figure 1, the fermentation apparatus comprises primary
fermenter 16, having a mixing blade 42 and connected through conduit 18 to a
secondary fermenter 20, which has a mixing blade 44 and is connected through
conduit 22 to a tertiary fermenter, or wet product storage tank, 24 which has a
mixing blade 46. Fermenters 16, 20 and 24 are preferably highly insulated and
similar to the fermenters described in the Coulthard patent, each fermenter preferably
having a capacity of about 30,000 litres. However, for purposes of temperature
control and air quality management, the fermenters 16, 20 and 24 are preferably
covered to substantially completely enclose the aqueous mixtures fermenting therein.
Furthermore, each of the fermenters 16, 20 and 24 is provided with a source of
external heat (not shown).
Initiation of the preferred fermentation process of the present invention is now
described with reference to Figure 1.
Initiation of the fermentation process of the present invention may be viewed
as a batch process. Firstly, a quantity of the aqueous mixture of waste material is
pumped through conduit 17 from holding vessel 15 to primary fermenter 16.
Preferably, the quantity of aqueous mixture transferred from holding vessel 15 is
sufficient to substantially fill primary fermenter 16. More preferably, both primary
fermenter 16 and secondary fermenter 20 are completely filled with the aqueous
mixture, with secondary fermenter 20 preferably being filled through primary
fermenter 16 by conduit 18. The preferred initiation process will be described as
having both primary fermenter 16 and secondary fermenter 20 filled with the aqueous
mixture of waste matter.
The aqueous mixture pumped from holding vessel 15 to primary fermenter 16
and secondary fermenter 20 is preferably at ambient temperature, typically from
about 10 to about 30°C.
Once inside primary fermenter 16 and secondary fermenter 20, external heat
is applied to the aqueous mixture, preferably rapidly increasing its temperature from
ambient temperature to a thermophilic temperature. The term "thermophilic
temperature" as used herein means a temperature sufficient to promote growth and
proliferation of thermophilic microorganisms. Thermophilic temperatures typically
range from about 50°C to about 85 °C. The thermophilic temperature at which the
process is initiated, and at which the fermentation is carried out, is at least partially
dependent on the substrate and the desired characteristics of the end product.
The term "thermophilic microorganism" means any microorganism which is
capable of growing and proliferating at above-defined thermophilic temperatures.
Therefore, the term "thermophilic microorganism" as used herein includes
thermophilic microorganisms and facultative mesophilic microorganisms, that is a
mesophilic microorganism which can adapt its metabolism to grow and proliferate
at thermophilic temperatures.
The term "external heat" as used herein means heat generated by a source
other than the fermentation process, which is exothermic. For example, external
heat may be generated by a heating coil located either inside or outside fermenters
16 and 20.
In addition to being heated, the aqueous mixture of waste matter is also
oxygenated, preferably by vigorous agitation by the mixing blades 42 and 44 and
aeration within fermenters 16 and 20 provided by air supply 23. This ensures that
the aqueous mixture is supplied with sufficient oxygen to encourage the proliferation
of aerobic, thermophilic microorgamsms and to prevent the proliferation of anaerobic
microorganisms. The inventors have found that oxygen demand is greatest during
the initial start-up of the process, and dissolved oxygen concentrations on the order
of about 0.2 ppm are typically observed. Once the process is initiated, oxygen
demand drops and dissolved oxygen concentration is typically observed to rise above
about 1 ppm.
Although injection of air into the fermenters 16 and 20 is the preferred form
of oxygenation, it is to be appreciated that air enriched with oxygen or oxygen in any
other form, including pure or substantially pure oxygen, may be used to aerate the
aqueous mixture of waste matter.
The inventors have also found that the maintenance of a sufficiently high
dissolved oxygen concentration requires high shear rates within the fermenter 16
effective to disperse air bubbles throughout the aqueous mixture, but not in excess
of shear rates at which microbial cells are damaged or destroyed.
Once the aqueous mixture reaches thermophilic temperatures and is
oxygenated in the fermenters 16 and 20, a time of from about 2 to about 6 days is
typically required for thermophilic fermentation to be achieved. Preferably, the
initiation is continued until the fermentation reaches a steady state at which the rate
of fermentation is substantially constant and the waste matter in fermenters 16 and
20 has been partially converted to the end product. Most preferably, the steady state
is the maximum rate at which the fermentation will proceed at any given temperature
and oxygen concentration.
It is to be emphasized that the thermophilic fermentation is initiated without
the need for inoculation, utilizing only thermophilic and facultative mesophilic
microorgamsms which are naturally present in the waste materials.
As discussed above, rapid heating of the aqueous mixture to thermophilic
temperatures precludes the microbial succession disclosed in the Coulthard patent,
and permits the proliferation only of thermophilic and facultative mesophilic
microorganisms in the aqueous mixture. This has the effect of reducing competition
for waste material among the microorganisms, consequently reducing the chance of
contamination and/or poisoning by opportunistic microorganisms and/or pathogens
that might proliferate at lower temperatures.
Once an active fermentation has been initiated by the above initiation process,
continuous or semi-continuous processing of waste material according to the present
invention may preferably begin. However, it is to be understood that the initiation
process of the present invention may be used on its own as a batch fermentation
process having distinct advantages over the above-discussed prior art processes.
As in the initiation of the process, the aqueous mixtures in the fermenters 16
and 20 are heated by an external heat source and oxygenated to promote and sustain
growth and proliferation of thermophilic and facultative microorgamsms. The
temperature is preferably maintained within the thermophilic ranges disclosed above.
The oxygenation is preferably the same as that described above with the exception
that the inventors have found that dissolved oxygen concentrations of from about
1 ppm to about 5 ppm are sufficient to maintain an active fermentation in fermenters
16 and 20.
Once an active thermophilic fermentation is initiated, it is preferred that steady
state fermentation conditions, and more preferably optimum fermentation conditions,
be maintained within fermenters 16 and 20. Therefore, a small amount of fresh
aqueous mixture of waste matter at ambient temperature is pumped from holding
vessel 15 into primary fermenter 16. This provides additional fermentation
substrates in the form of fresh waste matter to maintain a steady state, active
fermentation in primary fermenter 16.
Since both primary and secondary fermenters 16 and 20 are preferably
maintained in a full condition and are connected in series, adding fresh aqueous
mixture to primary fermenter 16 preferably results in the displacement of a
corresponding volume of aqueous end product from secondary fermenter 20.
Furthermore, a steady state fermentation is preferably also maintained in secondary
fermenter 20 by discharge thereto of partially fermented aqueous mixture from
primary fermenter 16.
In a preferred continuous process of the present invention, fresh aqueous
mixture of waste matter is continuously supplied to primary fermenter 16, preferably
at a rate of from about 20 to about 50 litres per minute. Consequently, the aqueous
mixture of end product is preferably continuously discharged from secondary
fermenter 20 at substantially the same rate. In a continuous process, the provision
of secondary fermenter 20 ensures that substantially no unfermented waste matter
may pass through the fermentation apparatus.
In a semi-continuous process, fresh aqueous mixture of waste matter is
intermittently supplied to primary fermenter 16, preferably at a constant and average
rate of from about 20 to about 50 litres per minute. These intermittent additions
preferably occur at a set flow rate for about 5 to about 15 minutes every one half
hour. Preferably, the aqueous mixture of end product is intermittently discharged
from secondary fermenter 20 at substantially the same rate. It is to be understood
that a semi-continuous fermentation process could be conducted without secondary
fermenter 20. However, secondary fermenter 20 is preferably provided to improve
the efficiency of the process.
In the process of the present invention, it is preferred that substantially the
entire fermentation occurs in the primary and secondary fermenters 16 and 20. The
fermentation time, defined as the time required for volume of the active fermentation
medium to be turned over, or the time required for the aqueous mixture to pass
through the fermenters 16 and 20, is preferably from about 24 to about 48 hours.
Typically, in an apparatus as shown in Figure 1 wherein the waste matter is
completely or substantially completely fermented in primary and secondary
fermenters 16 and 20, the degree of fermentation of material passing from primary
fermenter 16 to secondary fermenter 20 is typically about 50% complete. However,
it is to be appreciated that the degree of fermentation in the primary fermenter 16 is
dependent both on the processing time and the number of fermenters connected in
series.
A fermentation time of at least about 24 hours ensures the waste matter has
a residence time in fermenters 16 and 20 sufficient to completely destroy pathogenic
organisms and other undesirable biological contaminants present in the waste matter
prior to fermentation. Typical pathogens include bacteria such as salmonella and
coliform bacteria, and other biological contaminants include insect eggs, larvae,
worms and viruses.
Given that the minimum time-temperature conditions for complete destruction
of the above contaminants is about 10 minutes at 65°C, a fermentation time of about
24 hours is more than sufficient to ensure that contaminants are destroyed completely
and that the end product is completely safe. Although it may be possible to provide
a completely fermented product with a fermentation time of less than about 24 hours,
it is preferred that the fermentation time not be shorter than about 24 hours to ensure
the complete safety of the end product.
Also to ensure the safety of the end product, the process of the present
invention is preferably operated in the upper end of the above-mentioned
thermophilic temperature range. Preferred operating temperatures for the process
of the present invention, including initiation, are above about 55 °C and no higher
than about 80°C, more preferably at least about 65°C, and most preferably within
the range of from about 65 °C to about 75 °C.
It is to be appreciated that destruction of contaminants is dependent on both
temperature and time of fermentation. Therefore, minimum time-temperature
conditions may be achieved with longer fermentation times at a relatively low
temperature, or short fermentation times at a relatively high temperature. However,
in the process of the present invention, it is preferred to use higher temperatures as
discussed above to achieve complete destruction of contaminants in a relatively short
time.
The inventors have surprisingly found that the thermophilic
fermentation process of the present invention is also capable of destroying a wide
range of chemical contaminants which may be present in residual amounts in certain
types of waste matter. Such chemical contaminants include a wide range of organic
compounds, such as pharmaceuticals, pesticides, herbicides, and other organic
chemicals present in waste matter.
Pharmaceuticals include antibiotics for veterinary and/or human use. In
particular, antibiotics are commonly added to animal feeds and can be found in the
manure or muscle tissues of farm animals which may comprise raw waste matter to
be fermented in the process of the present invention. The inventors have confirmed,
in challenge tests conducted with waste matter contaminated with selected agricultural
antibiotics, complete destruction of these antibiotics by the process of the present
invention.
In a particular challenge test conducted by the inventors, 110 g each of
chlortetracycline and sulfamethazine, and 55 g of penicillin, commonly used as
veterinary antibiotics, were added to an active fermentation at 70°C. This level of
antibiotics is comparable to that which may be present in a finished animal feed in
a concentration of 220 g per tonne.
Samples were taken from the fermentation for evaluation of antibiotic content
using standard methods before addition, after 30 minutes, and at 8, 12, 24, 32, 48,
56 and 72 hours. Antibiotic was detected only in the sample collected after 30
minutes, at an equivalent concentration of 16.4 g per tonne. Therefore, more than
95 % of the antibiotics were destroyed within the first 30 minutes of the test, and the
balance within the first 8 hours.
It is believed that the destruction of chemical contaminants in the fermentation
process of the present invention may be caused by thermal destruction or through
metabolization of the chemical contaminants by thermophilic microorganisms, or a
combination of both. Due to the length of the fermentation process, i.e. 24 to 48
hours, it would be expected that the process of the present invention would be
capable of thermal destruction of heat sensitive chemical contaminants as well as
contaminants normally considered to be relatively heat resistant.
In the example of penicillin, it is understood that an extracellularly produced
enzyme is ultimately responsible for penicillin destruction. Therefore, it is possible
that even compounds which are highly heat resistant may be destroyed by the process
of the present invention through metabolization by thermophilic microorganisms.
Although the destruction of antibiotics has been specifically described, it is to
be appreciated that the process of the present invention is not restricted to destruction
of antibiotics, and is capable of destroying a wide range of chemical contaminants
which may be present in waste materials.
One surprising advantage of the process of the present invention is its
adaptability to a wide pH range, ranging from about 3.5 to about 9.0. This is to be
contrasted with the pH range disclosed in the Coulthard patent of between about 5.0
and about 8.5, with a most preferred pH on the order of from 5.9 to 7.5. In the
Coulthard process, a pH adjusting agent is added to acidic waste materials to raise
their pH to an acceptable level, approaching neutral. The inventors of the present
process have found that addition of a pH adjusting agent to acidic food wastes only
raises the pH temporarily and that addition of the pH adjusting agent must be
continued throughout the fermentation process to maintain the elevated pH.
Furthermore, the inventors have found that operation of the process at the
acidic pH of many food wastes, i.e. from about 3.5 to about 4.5, more typically 3.8
to 4.4, is not only possible but is also preferred, without the addition of any pH
adjusting agent. In fact, operation of the process at an acidic pH enhances the
thermal destruction of contaminants, thereby increasing the safety of the end product.
When the process is applied to acidic wastes, it is believed that low pH is maintained
in the process by encouraging the growth only of acidophilic thermophiles. These
microorganisms themselves are more effective at destruction of many contaminants
and readily produce organic acids that maintain a low pH and thereby preclude the
growth of microorganisms which are otherwise active at higher pH.
From secondary fermenter 20, the substantially completely fermented aqueous
mixture is intermittently or continuously discharged through conduit 22 to wet
product storage tank 24. As in fermenters 16 and 20, the aqueous mixture in
product storage tank 24 is preferably vigorously agitated by mixing blade 46 and
aerated by air supply at a thermophilic temperature, as in fermenters 16 and 20.
Therefore, wet product storage tank 24 is also referred to herein as "tertiary
fermenter 24". However, it is to be appreciated that the aqueous mixture entering
tank 24 has been completely or substantially completely fermented in fermenters 16
and 20.
Therefore, any small amounts of unfermented waste matter remaining in the
product mixture will be fermented in product storage tank 24. However, it is to be
appreciated that all or substantially all of the waste matter in the aqueous mixture
discharged from secondary fermenter 20 has been fermented. Therefore, the primary
function of product storage tank 24 is that of a surge tank, or storage tank, for
accumulation of the fermented aqueous mixture prior to further processing, such as
drying.
Typically, wet product is collected until tank 24 is filled to about 80% of its
capacity, after which the wet product is pumped from tank 24.
In an alternative process not shown in Figure 1, wet product storage tank 24
is eliminated and the wet end product comprising the aqueous mixture of fermented
waste matter discharged from secondary fermenter 20 is used without further
processing. It is also possible to use the wet product from tank 24 without further
processing. The inventors have found that such wet products may for example be
directly fed to animals or used as liquid fertilizers, soil conditioners or soil
amendments.
Furthermore, it is possible to operate the process on a semi-continuous basis
using only one fermenter. However, it is preferred to provide at least a primary and
a secondary fermenter to ensure the safety of the end product. Further, the process
may be operated with more than three fermenters to further ensure the complete
destruction of contaminants in the end product.
In the preferred process shown in Figure 1 , the aqueous mixture of fermented
waste matter is stored in fermenter 24 until it may be subjected to drying. Most
preferably, the aqueous mixture of fermented waste matter is pumped from tertiary
fermenter 24 through conduit 26 to centrifuge 28 where the water content of the
fermented product is reduced to obtain a wet product comprising about 35 % by
weight solids. It is to be appreciated that the initial removal of water may be
accomplished by other means, such as decanting and filtering.
It is to be appreciated that initial removal of water, for example by
centrifuging or filtering, may not be necessary or desirable and, as shown in Figure
1 , the wet product from tank 24 may be directly pumped through conduit 27 to a
dryer 32. Initial water removal is not desirable, for example, when the wet product
contains a high content of water soluble and/or miscible solids which would remain
in the liquid fraction, but are otherwise recoverable.
The water removed from the aqueous mixture of fermented waste matter by
centrifuge 28 is preferably recirculated to hydropulper 12 through conduit 30 to be
slurried with waste matter being macerated in hydropulper 12. Recirculation of the
liquid fraction to be slurried with fresh waste matter is a convenient way to avoid
discharge of water containing solids into the environment and allow full recovery of
solids from the liquid fraction.
The wet product obtained after centrifuging is typically in the form of a wet
cake and may either be used directly as an end product or subjected to further
drying. As shown in Figure 1 , the wet product is transferred to a dryer 32 , for
example by an auger, where it is preferably dried to a water content of from less
than about 10 to about 15 % by weight. The water vapour from the dryer 32 is
preferably exhausted to the environment, as through vent 33, or used as a source of
heat energy in the process. The use of a dryer 32 allows full recovery of solids from
the wet product.
It is to be appreciated that many types of drying systems are available which
can dry wet products having a variety of forms. For example dryers are available
which can dry liquid slurries, as obtained from secondary fermenter 20 or storage
tank 24, or dewatered cake as obtained from centrifuge 28. However, as discussed
above, drying is not an essential step of the process of the present invention.
The dried product obtained from dryer 32 is typically a granular, powdery or
somewhat fibrous solid. Although the dried end product may be used in the form
in which it is removed from dryer 32, it may preferably be shaped into any
convenient form, such as pellets, which may for example be used as animal feed,
animal feed supplement, fertilizer, soil conditioner or soil amendment.
In overview, the fermentation process converts organic waste matter into an
end product comprising biomass derived from the cells of thermophilic
microorganisms such as bacteria, yeast and/or fungi, as well as unfermented
proteins, lipids, carbohydrates, and breakdown products thereof arising from
fermentation, and water. The product may also contain some amount of carbon
dioxide which would however be largely liberated through aeration during the
fermentation process. The end product has a significantly higher protein content and
nutritional value than the unfermented waste matter and is therefore of value as an
animal feed, animal feed component or ingredient, or as an organic fertilizer,
fertilizer component, soil amendment or soil conditioner. The specific end use may
be at least partially dependent on the type of waste matter being fermented. For
example, it is preferred that the end product obtained by fermentation of fecal matter
be used as a soil amendment, soil conditioner or organic fertilizer, rather than as an
animal feed or feed component. However, it is to be appreciated that end products
derived from fecal matter may be suitable for use as an animal feed or feed
component.
One of the benefits of using aerobic fermentation is its oxidizing effect, which
both destroys odorous compounds and promotes biological pathways that preclude
the formation of compounds typical of anaerobic fermentations. Therefore, the
process of the present invention is naturally less odorous than an anaerobic
fermentation process.
Further, the entire apparatus used in the process of the present invention is
preferably contained in a single plant building which is equipped to treat all air
released into the surrounding environment, thereby minimizing unpleasant odours
escaping the plant. In order to accomplish this, the plant building is preferably under
a negative pressure throughout to prevent the unintentional escape of air, and all air
and water vapour exhausted from the plant must preferably pass through a thermal
oxidizer or other scrubbing device to destroy any odour-producing compounds.
Furthermore, steps are preferably also taken to control odours within the
plant, to protect plant workers. When waste materials are delivered to the plant
building, they are dumped onto a tipping floor 10 inside the plant. Air from the
tipping floor 10, as well as from the fermenters 16, 20 and 24, is preferably
prevented from mingling with general plant air to prevent plant workers from being
exposed to unpleasant odours.
Although preferred capacities of hydropulper 12, holding vessel 15 and
fermenters 16, 20 and 24 have been disclosed herein, it is to be appreciated that
these capacities may be varied without departing from the process of the present
invention.
Although this disclosure has described and illustrated certain preferred
embodiments of the invention, it is to be understood that the invention is not
restricted to these particular embodiments. Rather, the invention includes all
embodiments which are functional or mechanical equivalents of the specific
embodiments and features that have been described and illustrated herein.
Furthermore, it is intended that the invention cover all alternate embodiments as may
be within the scope of the following claims.