Note: Descriptions are shown in the official language in which they were submitted.
CA 02592059 2007-06-15
BYOMECHANICAL DEVICE FOR PRODUCING A BIOMASS
BA CKGROLTND OF THE INVEN'x'zON
FIELD OF TH)< fNVEN"I'YON
The present invention relates generally to processing of waste materials, and
more particularly to systems and processes for handling organic waste
znaterials.
DESCRIPTION OF THE PRIOR ART
Landfilling has traditionally been the method of waste handli.ng, but
landfillin,g can cause environmentally unacceptable pollution discharges to
the water
and, as real estate values increase, is considered to be an undesirable use of
land.
Thus, current waste management strategies seek to limit the amount of refuse
directed
to landfills. Recycling and cornposting programs have becorne widely accepted
for
both commercial and resiciential waste to reduce the demands on landfills.
Generally, recycling programs require separating the waste by type, either at
a
point of collection (source separated) or further along, such as at a transfer
station.
Recyclable components c:m include glass, metals, and plastics, while
compostable
components can iraclude, lor example, agricultural wastes, plant matter, food
stuffs,
wood, cardboand, and paper. Once separated, waste materials are commonly
referred
to as "source separated," and source separated matexnials that are collected
together
from separate collection points constitute a "single stream."
Compost facilities have been built to process non-recyclable waste, either in
the form of municipal solid waste with provisions for eontanaination removal,
or
source separated organic waste. An alternative to composting for non-
recyclable
waste streams are refuse-to-energy plants where material is burned to create
energy.
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Refuse-to-energy plants first process waste by grinding and then burni.ng the
ground
material. Although efforts are made to separate out hazardous materials from
the
waste stream, these plants have had a history of emissions and operational
problems
related to contaminants. The residual ash created from this burning has also,
in some
cases, been found to be hazardous.
Anaerobic digestion presents another alternative for handling organic waste
materials. '1'he primary objective of anaerobic digestion is the production of
a mixture
of hydrocarbon gases ("biogas'), which may be utilized as an energy source to
generate electricity and/or heat. Any solid material remaining at the
completion of the
anamobic digestion process is typically disposed of by conventional
landfilling or
composted into a soil amendment.
Because of the high capital costs associated with anaerobic digestion
equipment, and the environmental issues associated with refuse-to-energy
plants,
composting has become the dominant method in the United States for the
management and re-use of'organic waste materials generated in nual and
suburban
settings. The growing use of composting as a preferred alternative to disposal
of
organic waste material has also created some environmental problems. These
problems include emissioiLs of noxious gases and ozone pre-cursors, runoff
from the
compost facility, and high energy consumption during material processing.
These
problems may become particularly acute if the organic waste material contains
large
amounts of food waste or other high moisture content waste.
Cornmercial-scale composting is also subject to a variety of financial
considerations including capital investment related to accozxumodating peak
seasonal
feedstock deliveries, compost process time, and controlling the timing of
compost
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production to match the seasonal demand of the agricultutal industry and other
compost buyers. Further, the compost produced by these facilities is a low-
value
product, therefore municipalities have to pay to have the waste accepted.
SCTMMARX
An exemplary sysiem for processing organic waste material to a biomass
product comprises a rotatable drnun and aa air system. The drum is sloped
relative to
the horizontal, i.e., level ground, and includes a feed end and a discharge
end and
breaker bars extending inwardly fxom an interior surface of the drum. The drum
also
includes an aerotolerant anaerobic bacteria to facilitat.e fermentatiori of
the organic
waste material. The air system is configured to blow air into the discharge
end of the
drum. In some embodiments, the air system can also include an air scrubber
configured to remove volatile organ.ic compounds, such as volatile fatty
acids, from
air discharged from the feed end of the drum. In some of these embodiments,
the air
system fiirther includes a blower configured to recirculate air from the air
scrubber to
the discharge end of the daum. Loading equxpznent provided to load the waste
material into the drum can., in some iuaabodiments, also serve to collect the
air from
the feed end of the drum and therefore comprise part of the air system.
An exemplary method for processing organic waste material to a biomass
product comprises fermenting a biodegradable fraction of the waste material
with an
aerotolerant anaerobic bacteria in a rotating drum, and controlling the
envimnment
within the rotating drum. Controlling the environment includes maintaining the
oxygen content of the air within the drum below the ambient oxygen
concentration,
mainta'~ an acidic pH of the waste material within the drum proximate to a
discharge end thereof, and maintaining the moisture content of the waste
material
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within the drum in the range of 40% to 60%. Maintaining the oxygen content of
the
air within the drum can include, in some embodiments, removing air from a feed
end
of the drum and recirculating the air to the discharge end of the drum.
Controlling the
environment within the rotating drum can further include maintaining the
temperature
within the drum from about 130 F at a feed end of the drum to about 165 F at
the
discharge end. In some embodiments, the method further comprises adjusting a
carbon to nitrogen ratio of the waste material to within a range of about 20:1
to about
40:1 prior to fermenting the biodegradable fraction of the waste material.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is a schematic representation of an exemplary biomixer according to an
embodiment of the invention.
FIG. 2 is a perspective view of a discharge end of the biomixer of FIG. 1.
FIG. 3 is partial cross-section of the biomixer of FIG. 1.
FIG. 4 is a perspective view of a feed end of the biomixer of FIG. 1.
FIG. 5 is a schematic representation of an exemplary biomass production
facility including a biomixer according to an embodiment of the invention.
DETAILED DESCRIPTION
Apparatus and methods for the mechanical/biological treatment of organic
waste materials are provided herein. These apparatus, termed "biomixers,"
employ a
combination of mechanical shearing and biological activity in a controlled
environment to produce biomasses suitable for anaerobic digestion and other
purposes. An embodiment of the invention is directed to a rotating drum that
includes
bacteria capable of facilitating a fermentation process. Additionally, the
embodiment
includes an air system to move air through the rotating drum. Adjustments to
the air
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flow through the druin can be used to control the fermentation process
therrein. The
air system can also be used to recover volatile fatty acids from the
environment of the
drum.
An exemplary bioznixer 100 for praducing a biomass is described wxtlt
reference to FIG. 1. The biomixer 100 comprises a rotatable drum 105 that in
operation is sloped relative to the horizontal so that waste matezial
(represented by
arrow 110) introduced at a feed end 115 traverses the biomixer 100 to a
discharge end
120. FIG. 1 also shows aai air system for moving air (represented by arrows
125)
through the biomixer 100 and, in some embodiments, for recirculating and/or
recovering volatile fatty acids from the air 125. Components of the air system
that are
shown in FIG. 1 include an air injector 130, such as a blower, and an air
collection
device 135, such as a hood. The air system is discussed in nzore detail
elsewhere
herein.
A suitable drum 105 for the biomixer 100 comprises a cylinder approximately
8 feet to 16 feet in diameter with a length of up to about 15 times the
diameter. One
suitable material for the drum is mild steel. A drum 105 as described can be
supported by two dnnn support brackets (not shown). Drums with larger ratios
of
length to diameter may require additional drnmt support brackets. A drum drive
unit
(not shown) is provided to rotate the drum in a range of about 1/4 to 2
revolutions per
minute.
As shown, the drum 105 can be maintained at an angle relative to the
horizontal with the discharge end 1201ower than the feed end 115. Under
operating
conditions, the slope of the drum 105 is about 3/16 of an inch per foot of
length, in
some embodiments, but can be increased or decreased to adjust the rate with
whickt
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waste material traverses the drum 105. To protect against excessive corrosion
of the
105 drum, cathodic protection, liners, andlor coatings can be applied to the
inside of
the drum. The dratn 105 can also include access manholes, sampling ports,
monitoring ports, and discharge ports, such as the discharge ports 200 in the
perspective view of the discharge end 120 of the biomixer 100 shown in FIG. 2.
Some embodiments employ four air actaated discharge ports 200, sized
approximately 30 inches by 36 inches.
As also shown in FIG. 2, an air inlet 210 located near the center of the
discharge end 120 of the drum 105 communicates with the air injector 130 (FIG.
1).
Air 125 exiting the drum 105 from the feed end 115 (FIG. 1) can be recovered
and
scrubbed of volatile fatty acids and optionally returned to the drum 105 by
the air
injector 130. As discusse<i below, this recirculated air can have an
advantageously
lower oxygen concentration compared to the atmospheric oxygen level.
FIG. 3 shows a partial cross-section of the biomixer 100. As can be seen,
some embodiments include breaker bars 300 that extend inwardly from an
interior
surface 310 of the drum 105. The breaker bars 300 are provided to agitate the
waste
material and provide additional shearing. Fuzthex, the breaker bars 300 can
serve to
protect the d.nnn 105. Specifically, material that packs between breaker bars
300
helps to control corrosion and abrasion of the drum 105. In some embodiments
the
breaker bars 300 extend about 2 inches in length,l, as measured from the
interior
surface 310 and have a thickness, t, of about %z an inch. In some embodiments
the
breaker bars 300 are spaced, sp, about 4 inches to about 6 inches apatt. In
some cases
lining of the dnun 105 with a plastie material, stainless steel, or a special
coating will
allow the breaker bars 300 to be spaced as much as 4 to 6 feet apart. It will
be
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appreciated that the breaker bars 300 can extend substantially the length of
the drum
105 (i.e., perpendicular to the plane of the drawing in FIG- 3). Further
embodiments
of the biomixer 100 also include spikes (not shown) extending inwardly from
the
interior sufface 310 near the feed end 115. The spikes are useful to break
open any
bags in the waste material.
As shown in FIG. 4, an opening 400 is provided at the feed end 115 of the
drum 105 for introducittg waste materials into the drum 105. The opening 400
can be
aligned with suitable loading equipmerit (not shown) that minimizes spillage
ofwaste
material. Air 125 is also discharged from the feed end 115 of the drum 105
through
the opening 400. In some embodiments, the loading equipment seals against the
opening 400 and includes separate discharge ports for discharging the air 125.
Thus,
in some of these embodiments, the loading equipment also seirves as at least
part of
the air collection device 135 (FIG. 1).
In some embodunemts, the drnun 105 is loaded to about balf full at the feed
end
l 15, leaving a few feet of headroom at the discharge end 120. When loaded in
this
way, approximately two tl drds of the volume of the drum 105 is filled by the
waste
material, leaving the remaining third empty to allow the waste material to
tumble as
the drnun 105 rotates. A suitable retention time is about 1.5 days, but can
range finm
about one to about three days. A suitable rotation rate is about half of a
revolution per
minute, though the revolution rate can be increased during loading and/or
unloading
to approximately one revolution per minute to accelerate the unloading process
and to
hel.p move the waste matwial down the length of the drum 105 to facilitate
loading.
As noted above, the rotating drum 105 includes bacteria capable of
facilitating
a fermentatioxt process. In order to introduce the bacteria into the drum 105,
the
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biological content of the waste material can be adjusted, for instance, by
addition of
select bacteria prior to being loaded into the biomixer 100. The added
bacteria can
either be a cultured bacteria, or can be a bacteria that is recovered from a
biomass
previously produced by the biotnixer 100. In the latter case, a small fraction
of the
biomass produced by the biomixer 100 is recirculated back into the waste
material
being introduced into the biomixer 100. In some embodiments the small fraction
of
biomass added to the waste material is ten percent or less of the mass of the
incoming
waste material.
The added bacteria can include any bacteria capable of facilitating a
fermen.tation process, sucli as aerotolerant anaerobic bacteria. Aerotolerant
anaerobic
bacteria are specialized anaerobic bacteria characterized by a fermentative-
type of
metabolism. Tlrese baoteria bive by ferneentation alone, regardless of the
presence of
oxygen in their environrnent. Exemplary aerotolerant anaerobic bacteria
include
species in the genera Desulfomonas, Buryrivibrio, Eubacrerfum, Lactobacillus,
Closiridium, and Ruminococcus.
In operation, a biodegradable t'action of the waste material, typically
eonsisting primarily of paper and other organic components, is converted in
the
biomixer 100 to a partially hydrolyzed biomass by mechanical breakdown and
fennentation. The paper fraction o#'the waste materia[ becomes wet and is
broken
into increasingly smaller pieces by the mechanical action. Other organic
components
are likewise sheared by the tumbling action within the slowly rotating
biomixer 100.
At the same time, aerotolerant anaerobic bacteria in the low oxygen
environment
within the biomixer 100 Facilitate femtentation of the biodegradable
fractiozt. The
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fermentation results in the partial hydrolysis of the biodegradable fraction
into volatile
fatty acids and their precursors.
As noted above, the environment in the biomixer 100 is controlled to
facilitate
the fecznentation process caused by the aerotolerant anaerobic bacteria. The
environment is prima'rily cietermined by the composition of the waste
material,
including the choice of aerotoleran.t anaerobic bacteria, the rate of air flow
throagh the
drum 105, and the oxygen concentration of the air. In some embodiments the
oxygen
concentration of the discharged air (as it leaves the feed end 115) is below
3.0% and
can be as low as about 0.5%. Within the biornixer 100 an oxygen level gradient
can
vary from about 0.5% near the feed end 115 to about 5.0% at the discharge end
120.
Recirculating the discharged air back into the biornixer 100 helps maintain
the low
oxygen concentration within the biomixer 100.
As the waste mateilal traverses the biomixer 100 towards the discharge end
120, both the production of volatile fatty acids from the waste material
mcreases, and
the pH of the waste material drops to about 5.5 or lower. A pH range from the
feed
end 115 to the discharge end 120 can vary from about 8 to about 4.5. If
necessary,
the pH of the waste material can be made more basic prior to being introduced
to the
dntm 105 in order to raise the endpoint pH at the discharge end 120. While
this can
serve to protect the biomixer 100 from corrosion damage, raising the endpoint
pH
may also reduce the efficiency of the fermentation process.
Also, as the waste material traverses the biomixer 100, and the fermentation
process increases, the temperature of the waste material also increases. A
suitable
temperature for the fermeiitation process is about 145 F but the temperatnre
can range
&ozn about 130'F at the feed end 115 to about 165'F at the disohaxge end 120.
While
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the moisture content at the feed end 115 can be about 60 1o, heating the waste
material
causes some of the moisture to evaporate and be removed from the biomixer 100
by
the air flowing therethroulg . However, even though some moisture is lost as
the
waste material traverses the biomixer 100, other mass is also lost, for
example,
through the volatilization of volatile fatty acids. The overall result is that
the moisture
content of the waste material will range from about 60 /u at the feed end 115
to as low
as about 40% at the discharge end 120, though a typical final moisture content
is
around 50%.
It will be appreciated that the biomixer 100 can also include sensors to
measure moisture, oxygen content, pH, and the ternperature of the environment
at
different locations within the biomixer 100. Conditions within the biomixer
100 can
also be determined from readings made at locations outside of the biomixer
100. For
instance, sensors can measure the moisture, oxygen content, and temperatwre of
the
air entering and exiting the biomixer 100. Samples of the waste material can
also be
withdrawn for compositional analysis through sampling ports in the drum 105.
Based on readings from the sensors and/or other measured process
information, various paratneters can be controlled to keep the moisture level,
oxygen
content, pH, and tezttperature in the biomixer 100 within desired ranges.
These
parameters can include thc; rotation speed of the drum 105, the rates of
loading and
unloading, the slope of the drum 105 relative to the horizontal, the air
pressure at the
discharge end 120, the moisture and oxygen content of the air being introduced
into
the biomixer 100, the pH .md moisture content of the material being loaded
into the
biomixer 100, and so forth In particular, the rate at which air is blown into
the drum
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105 can be controlled, for exatnple, by being cycled on a timer or by an
electronic
controller configured to control the air flow rate based upon sensor
measurements.
For example, the composition of the waste material can be optionally adjusted
as needed to obtain a more optimal mixture for processing within the biomixer
100.
For instance, drier material such as paper can be added where the moisture
content of
the incoming waste material is too high. Alternatcly, wetter materials or
water can be
added to the incoming waste material to increase the moisture content. A
suitable
moisture content for the waste material being introduced into the biomixer 100
is
about 60% but can vary between about 50% to about 65%.
Similarly, other materials can be added as needed to adjust factors such as
the
pH, the carbon to nitrogen ratio, and the biological content of the waste
material. For
instance, additional carbon or nitrogeni can be added in the forrzx of seleet
waste or
chemicals. A suitable carbon to nitrogen ratio is about 30:1, but can vary
within a
range of about 20:1 to about 40:1. A neutral or slightly acidic pH in the
range of 5 to
6 is also preferred. The pH can be made more basic, for example, by isolating
and
removing low pH waste or by adding select high p)`I waste. Similar techniques
can be
employed to lower the pH. The pH can also be adjusted by adding commercially
available acids or bases.
The rate of mixing, temperatm-e, oxygen content, and retention time in the
drum 105 can also be controlled. Some of these parameters can be controlled,
for
instance, by adjusting the rotational speed of the drum 105, the slope of the
drum 100
relative to the borizontal, the air flow through the drum 105, and the rates
of feedi.ng
into, and discharge from, ihe drum 105. Additionally, the temperature,
moisture
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content, and oxygen content of the air that is introduced into the drum 105
can also be
controlled.
FIG. 5 is a schematic representation of an embodiment of a biomass
production facility 500 including a biomixer 100_ The biomass production
facility
500 comprises a receiving building 510 and a biomass processing building 520.
In
this embodiment the biomixer 100 spans a distance between the buildings 510,
520
but, except for the feed and discharge ends 115, 120, is not itself housed
within a
building. Bladders, hoods, or other means, can be employed to seal around the
biomixer 100 to keep odors from escaping from the buildings 510, 520. Each of
the
buildings 510, 520 can also include a negative pressure system that includes a
biofilter
to remove odors from the air. In the alternative, the entire facility can be
housed in a
single building rather than having the biomixer 100 span the distance between
two
buildings.
The receiving building 510 can anclude a tipping floor for receiving waste
materials such as source separated organic waste or municipal solid waste. The
receiving building 510 can also include sorting and screening areas and
equipment.
Sordng, which can be petibrned mechanically, by hand, or through a combination
thereof, can be employed to rezuove hazardous materials such as batteries,
problematic and/or unprocessablc itcros such as construction materials, and
recyclable
materials. Screening, such as witlh a trommel, can be employed to classify the
waste
material according to size into waste streams of "overs" and "unders."
In some embodiments, the overs are directed to the biomixeor 100 while the
unders are directed to other processing, such as anaerobic digestion. It will
be
appreciated tlxat certain source separated waste may require little or no
sorting or
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screening and may be loaded directly into the biomixer 100. In other
instances, where
the overall moistme content of the incoming waste material is above the
desired
moisture content range for processing in the biomixer 100, removing the unders
from
the incoming waste material is beneficial as the unders typically contain a
disproportionate amount of the moisture of the waste materxal. Thus, removirtg
the
unders from the incoming waste mate,rial serves both to lower the moisture
content of
the waste material enterina the biomixer 100 to a more acceptable range, and
reduces
the total amount of waste material that needs to be processed.
The receiving building 510 also includes loading equipment 530 to transfer the
waste material into the biomixer 100. The loading equipment 530, in some
embodiments, can include a mixer for adjusting characteristics of the waste
material,
such as moisture content and composition, prior to loading the waste material
into the
biomixer 100. The mixer can be used, for example, to blend amen.dmeztts into
the
waste material to adjust the pH or to add bacteria. The loading equipment 530,
in
some embodiments, employs a gravity conveyor system and/or a mechanical ram to
load the waste material into the biomixer 100- Loading waste material into the
biotnixer 100 can either ba actuated manually or automatically based on a
timed or
volume-based cycle. As provided above, in some embodiments, the loading
equipment 530 can also serve as at least part of the air collection device 135
(FIG. 1)
of the air system. For instance, air can be discharged from the biomixer 100
through a
feed ram opening or through dedicated discharge ports in the loading
equi,pment 530.
The air system can also comprise an air scrubber 540 configured to receive the
air recovered from the biomixer 100 through the air collection device 135, for
example, through a hood or the loading equipment 530 as in FIG. S. The air
scrubber
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540 can be located outside of both buildings 510, 520, as in FIG. 5, or
located within
either of the buildings 510, 520. The air scrubbcr 540 is configured to
recover
volatile organics from the air, thus suitable systems for the air scrubber 540
include
water scrubbing systems. As the solubility of volatile fatty acids is higher
in alkaline
solutions, capture of volatile fatty acids can be improved by raising the pH
of the
water in the water scrubbing system.
Air collected fzom within the biomixer 100 can include a relatively ltigh
concentration of volatile organic compounds such as volatile fatty acids. The
air
scxubber 540 produces both cleansed air and coneentrated volatile organic
compounds
in water. The volatile organic compound concentrate, preferably once
saturated, can
be directed to an energy rccovery process or returned to the biomixer 100
through the
loading equipment 530_ The cleansed air from the air scrubber 540, having come
initially from the low-oxygcn environment within the biomixer 100, can have a
lower
oxygen content than the ambient concentration. In some embodiments, as shown
in
FIG. 5, the cleansed air is returned to the discharge end 120 of the biomixer
100 to
help keep the oxygen concentration low within the biomixer 100. The oxygen
content
of the air entering the biornixer 100 can be further adjusted by adding
ambient air to
raise the oxygen Ievel, or by adding an inert gas such as nitrogen to further
dilute the
oxygen level. Alternatively, rather than return the cleansed air to the
biomixer 100,
where a negative pressure system including a biofilter is employed to remove
odors
from the air within either of the buildings 510, 520, the cleansed air can
also be
directed into the biofilter imd released to the atmosphere.
The biomass processing building 520, in some ewbodiments, incaudes biomass
pxooessing equipment 550 for screening and/or sorting the biomass produced by
the
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biomixer 100. The bionnass processing equipment 550 can include a trommel. for
example, to classify the biomass into overs and unders for further processing.
For
instance, the overs can be directed to a composting facility while the unders
are
directed to an anaerobic digester. Suitable mesh sizes for the trommel range
from
about 1 to 3 inches.
While in some embodiments sorting and/or screening is perforrned on the
waste material in the receiving building 510 prior to processing in the
biomixer 100,
in other embodiments sorting and(or screening is instead performed on the
biomass
product by the biomass processing equipment 550 in the biomass processing
building
520. In still other embodiments sorting and/or screening is performed on both
the
waste material and the biomass product. As shown in FIG. 5, air from the
biomass
processing equipment 550 can also be directed to the air scrubber 540 as
further
processing of the biomass releases additional volatile fatty acids into the
air.
In the foregoing specification, the present invention is described with
reference to specific embodiments the.reof, but those skillcd in the art will
recognize
that the present invention is not limited thereto. Various features and
aspocts of the
above-lescribed present inivention may be used individually or jointly.
Further, the
presemt invention can be utilized in any number of environments and
applications
beyond those described herein without departing from the broader spirit and
scope of
the specification. The specification and drawings are, accordingly, to be
regarded as
illustrative rather than restrictive. It will be recognized that the terms
"comprising,"
"including," and "having," as used herein, are specifically intended to be
read as
open-ended terms of art.
