Note: Descriptions are shown in the official language in which they were submitted.
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WELL-MIXED FLOW BIOREACTOR FOR AEROBIC TREATMENT
OF AQUEOUS WASTES AT HIGH ORGANIC AND SOLIDS LOADINGS
FIELD OF THE INVENTION
The present invention generally relates to the treatment of aqueous
wastes rich in organic content, either soluble or particulate, and is more
specifically
concerned with aerobic treatment wherein a well-mixed flow bioreactor is
capable of
handling and treating aqueous wastes at high organic loadings, enabling the
production of flocs and their subsequent removal in a settling tank, thereby
yielding a
significant purification performance of the aqueous waste. The present
invention also
relates to a well-mixed flow bioreactor enabling an efficient and
environmentally sound
treatment of aqueous wastes at high organic loadings and an environmentally
sound
disposal of the treated liquid effluent. The bioreactor of the present
invention is also
1 S designed to accommodate the treatment of aqueous wastes containing high
concentrations of suspended solids. In one embodiment, the invention relates
to a
well-mixed flow aerobic bioreactor, without mechanical part moving therein,
for the
treatment of aqueous wastes of high organic and solids contents which enables
the
development of the biodegrading flora, the formation of flocs and a
substantial
reduction of the amount of nitrogen and phosphorus initially present in the
aqueous
waste.
In many countries, aqueous waste management, either domestic,
industrial or agricultural, has been and is still carried out without much
concern for the
environment. However, septic tank sludge, animal waste such as liquid piggery
waste,
leachate from landfill sites or wastewater from food processing industry have
been the
target of restrictions and protective measures to reduce the impact of their
disposal
into the environment. Some solutions have been applied to solve some of the
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2
problems associated with these human activities. In some cases, however, the
lack
of an acceptable solution has hampered or halted the growth of some
industries. The
best example is the pig industry, which has brought many producing countries
into
investing significant sums of money into research and development of
technologies
for treating liquid piggery waste. In conventional fattening piggeries, the
raw slurry
containing animal urine and feces is usually collected and stored in ponds or
large
concrete structures where it is allowed to decompose freely until it is used
as fertilizer.
Odor production and emissions are left uncontrolled, fertilizer quality is
highly variable
and/or potentially lost to the atmosphere, and the volumes are diluted by
precipitations. Furthermore, management of the fertilizer through intensive
farming
techniques brings a lot of environmental concern with respect to pollution
when it is
disposed of onto land (soil compaction, excess landspreading dosages, surface
water
runoff, groundwater contamination). Clearly, there is an important need for an
efficient
and environmentally sound apparatus and method of treating aqueous wastes rich
in
organic content, such as liquid piggery waste remains.
Besides measures proposed for pollution source reduction; such
as pig-on-litter systems, better nutrient assimilation through enzyme
complement to
the animal diet, volume reduction through better water management in the
piggeries,
different strategies for odor control and several types of physical-chemical
treatment
processes for liquid piggery waste have been studied. These include
olygolysis, or
electrolytic treatment (Ranalli et al., 1996, J. Env. Sci. Health A., X1:1705-
1721 ),
thermal dewatering technologies (Sirven process) or phase separation using
membranes, chemical precipitation, centrifrages or other devices. Such
technologies
either propose only a partial treatment or require major capital investment.
Biological processes for treating liquid piggery waste have also
been developed. For example, pig slurry treatment has been approached using
aerobic or anaerobic technologies or a combination thereof. Potential
applications for
these technologies have been looked at either as regional facilities in areas
where the
pig industry is concentrated or as local facilities installed at the
production site.
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3
Regional facilities using anaerobic technologies exists, for example,
in the Netherlands and in Denmark. Such facilities use the Promest and Memon
systems (Rulkens and Ten Have, 1994, Wat. Sci. Technol., 30:157-165) or the
Ecosun
and Lindtrup processes. Because of high energy costs in these countries,
regional
facilities using anaerobic digestion were thought to be economically feasible
since the
energy input is very low and the potential recovery of combustible biogas
exists.
However, energy yields are low and difficult to optimize, raising the question
as to
whether large scale projects for the sole purpose of treating animal waste
should be
further developed or be abandoned.
Anaerobic processes (Massy and Droste, 1997, Can. Agric. Eng.;
X9_:35-41; Massy et al., 1997, Can. Agric. Eng., 39:25-33; Masse et al., 1996,
Can. J.
Civ. Eng., 2$:1285-1294) or combined anaerobiGaerobic treatment strategies (Le
Hy
et al., 1989, Wat. Sci. Technol., 21:1861-1864) in local facilities at the
production site,
are also being studied. Although, these anaerobic processes can be simple in
design,
produce little residual solids and have low energy requirements, they also
have several
disadvantages. They are sensitive. to toxic constituents of the influent,
provide poor
clarification, do not assimilate or degrade all nutrients, and produce noxious
and
potentially explosive gases.
Aerobic treatment of aqueous waste is well known and has been
used for many years. Its most notorious application is in municipal and
industrial
wastewater treatment. Numerous processes and apparatuses based on aerobic
treatment of aqueous waste have been described (for example, U.S. Pat. Nos.
2,907,463, issued to Light et al. on Oct. 6, 1959; 4,522,722 issued to
Nicholas on June
11, 1985; 5,798,673 issued to Huntington on Jan. 17, 1989; and Canadian Patent
No.
1,117, 042 issued to Spector on Jan. 26, 1982). Many variations of such
aerobic
treatment are used and adapted around the world depending on the quality of
the
wastewater, the way by which the wastewater is fed into the system, the
process
efficiency, and the local regulations governing the quality of the effluent.
Most of these
systems are large in dimensions, command high capital costs and a sign~cant
input
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4
in energy, and have not been designed for compactness and high organic
loadings.
Some efforts have been dedicated to adapting various designs that
use aerobic or sequential aerobiclanaerobic processes for the treatment of
aqueous
wastes at high organic loadings. More specifically for the treatment of liquid
piggery
waste, nitrificatioNdenitrification and aerobic sequential batch reactor
processes have
been studied and developed (Fernandes and McKyes, 1991, Trans. ASAE, X4:597-
602; Martinez, 1997, J. Agric. Eng. Res., X6:51-62; Su et al., 1997, J. Env.
Sci. Health
A., x:391-405). Treatment at the production site proposed by some promoters
has
been considered a potential solution to the problem. Technologies such as
nitrification/denitrification processes for nitrogen reduction in France
(Agroclar,
D~nitral, Val-pure, Technolyse, Temois), or different composting methods of
liquid
manure in France (Guemevez, Isateri, and Lisia-post) and in Belgium (Menart)
have
been used or are currently used in such applications. However, most of the
technologies proposed are either expensive to build and/or operate, are
complex to
operate, or treat the waste material only partially.
There thus remains a need to provide an aerobic treatment of
aqueous wastes containing high levels of suspended solids, organic matter and
nutrients that can be oxydized and assimilated into more mineralized, stable
andlor
innocuous forms during the process. There also remains a need for improving
the
concentration of the fertilizing value in the sludge in a small fraction of
the influent
volume for improving the odor control, for improving the proper handling of
the
biological floc for best settling properties, and for improving the quality of
the aqueous
effluent. There also remains a need to provide a one-step method for the
treatment
of aqueous wastes at high organic and solids loadings and to provide a
significant
purification performance thereof while yielding a sludge having added value.
The present invention seeks to meet these and other needs.
The present description refers to a number of documents, the
content of which is herein incorporated in reference.
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SUMMARY OF THE INVENTION
An object of the present invention is therefore to provide an aerobic
bioreactor wherein efficient biodegradation of aqueous organic wastes at high
organic
loadings takes place. It is also an object of the inventiori to provide a
bioreactor and
5 method of treatment of aqueous organic wastes at high organic loadings which
yield
a treated water which is substantially free of phosphorus and nitrogen.
Furthermore,
it is an object of the present invention to provide a bioreactor and method
using same
which yield a product sludge having a better fertilizing value than the
influent waste.
An additional object of the present invention is to provide a
bioreactor which can treat the aqueous waste without any prior treatment
thereof and
which can remove nutrients such as nitrogen and phosphorus from the process
effluent.
A further object of the present invention is to provide a well-mixed
flow aerobic bioreactor, which is free of mechanical parts moving therein,
enabling an
efficient treatment of the waste material and allowing the concentration of
the fertilizing
value in the sludge in a fraction of the influent volume, while controlling
odor
emissions.
Yet, a further object of the invention is to provide a well-mixed flow
aerobic bioreactor of compact size wherein agitation and aeration are supplied
by an
air lift pump and air diffusers and which is adapted for the treatment of
aqueous
wastes at high organic loadings and methods of use thereof. In a preferred
embodiment, the present invention provides a bioreactor wherein agitation and
aeration are supplied by at least one air lift pump, and at least one
diffuser, which can
maintain homogeneity of inert and biological suspended solids at
concentrations of at
least 2 to 10 times (1 % to 5 % mixed liquor suspended solids) that of
conventional
municipal wastewater treatment systems. Indeed, in most activated sludge
processes,
reactor solids concentration are in the range of 2,000-7,000 mg SS/L. The
present
invention provides the means to handle reactor solids concentrations of about
24,000
mg SS/L (from 3,000-53,000 mg SSIL). It will be appreciated by the person of
ordinary
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6
skill that pure oxygen can enable a handling of reactor solids of even higher
concentrations. It should also be recognized that the type and concentration
of
specific gases (nitrogen, ammonia, etc.) introduced into the bioreactor can be
adapted,
by the person of ordinary skill, to meet specific needs of the substrate to be
treated
and/or of the level of performance of purification to be achieved.
Another object of the present invention is a well-mixed flow aerobic
bioreactor, wherein aqueous waste at high organic loadings (above 0,30 Ib
total
CODIfP.d or above 0,09 Ib filtered COD/ft3.d) is biodegraded while high
concentrations
of volatile suspended solids (in excess of 30 000 mg VSS/L), i.e. the
biocatalyst, are
present in the bioreactor.
Broadly stated, it is an object of the present invention to provide a
bioreactor, wherein raw aqueous wastes containing grains, sand particles,
animal hair
and other small debris, can be handled without plugging, thereby overcoming or
reducing the need for a separation of the suspended solids from the aqueous
waste
as in necessary in the systems of the prior art.
It is also an object of the invention to provide adequate control over
the shear stress imposed on the biological catalyst to insure swift and proper
separation of the biological solids from the liquid effluent by passive
settling. The
instant invention provides the means to minimize foam formation and wherein
excess
foam can be controlled chemically vegetable oil, animal fat and the like
and/or
mechanically (i.e. commonly known foam breakers). It will be understood that
foam
problems are linked to the substrate or aqueous waste which is treated. In the
case
of piggery waste with which foam problems can be encountered, minimizing foam
formation is important. It will be recognized that foam problems are generally
only
encountered at the start up of the bioreactor. Once the bioreactor has been
stabilized,
foam control is usually not necessary.
Furthermore, it is an object of the present invention to provide a
bioreactor wherein air is supplied at low pressure and high flow-rate thus
preventing
ammonia stripping and allowing assimilation of ammonia nitrogen by the
microbial
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7
flora. The bioreactor of the present invention provides the means to release
the
oxygen-containing medium (such as air) into the tank of the bioreactor at a
pressure
which is slightly superior to the hydrostatic water pressure in the bioreactor
at the site
where the oxygen-containing medium is delivered (a function of the height of
the
mixed liquor column). By providing a relatively low air pressure, which
minimizes or
avoids a substantial migration of ammonia to the surface of the mixed liquor
and
eventually a degasing thereof of its ammonia (i.e. stripping of the ammonia)
the
bioreactor of the present invention allows a substantial assimilation of the
ammonia
by the flora.
In a preferred embodiment, the invention provides a bioreactor
wherein mixed liquor suspended solids and air are drawn into air lift pumps
(such as
Venturi tubes) for increased contact times, providing internal recycling of
the mixed
liquor (from as high as 500-1,000 cycles/day), as calculated from the flow
rate of the
mixed liquor from the channel into the discharge area of the bioreactor and
from the
hydraulic retention time and the sludge retention time (when applicable), good
oxygen
transfer rate (i.e. 3% for a hydrostatic pressure of 6 feet, at the point of
entry of the
oxygen containing medium into the bioreactor) and high oxidation levels
(removal of
between 80-99°~ of filtered COD at loadings commonly handled by the
bioreactor of
the present invention and preferably above 90% of filtered COD). The gas
pressure
of the oxygen-containing medium, as it enters the bioreactor, can be adapted
in
relationship to the water pressure at the point of entry of the oxygen-
containing
medium, provided that it is slightly higher than the water pressure and
provided that
it is not high enough to be accompanied by ammonia stripping, foaming and high
shear forces. For a hydrostatic pressure of 10 feet, the oxygen transfer rate
should
be preferably in the range of 4-5%, which is satisfactory to provide the
advantages
of the present invention.
In a preferred embodiment, it is an aim of the invention to provide
a bioreactor wherein flow patterns and mass transfer can be controlled in
different
sections of the bioreactor and wherein high dissolved and high rendered oxygen
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8
concentrations can be maintained at the bottom of the bioreactor. It will thus
be
recognized that the bioreactor of the present invention enables a modulation
of the
level recycling of the mixed liquor as well as of the level of residual oxygen
at different
levels or different sections within the bioreactor (i.e. by changing the flow
rate within
the air lift pump or modulating the flow rate of the air diffuser).
It is also an object of the present invention to provide a bioreactor
wherein efficient aerobic treatment is effected at mixed liquor temperatures
in the mid-
range from about 10°C to about 30°C, and preferably ranging from
about 15°C to 25°C.
The operation of the bioreactor in the mid-temperature range is a sign~cant
advantage
as compared to the bioreactors of the prior art which operate between
25°C to about
40°C, and for which energy often has to be supplied by an exogenous
source. The
mesophyllic temperature range of the bioreactor of the present invention
therefore
provides a significant advantage over such bioreactors of the prior art.
Yet another object of the invention is to reduce the volume of the
waste by concentrating organic carbon and sorbed nutrients into the sludge
while .
producing a large liquid fraction (from 45 to 75 % of the influent volume)
that can be
easily polished or directly disposed of in a sewer or water course.
The invention also relates to a process for the aerobic
biodegradation of aqueous organic wastes at high organic loadings. Further,
the
invention relates to a process wherein the biological catalyst develops as a
free
suspension from the facultative flora present in the waste to be treated. If
the waste
lacks a proper biocatalyst, start-up can be initiated using inoculum such as
activated
sludge from wastewater treatment plants or such as commercial bacteria
products.
The invention also relates to a bioreactor which optimizes and favors the
growth of a
mesophyllic flora (optimal temperature for growth being in the mid-range) .
Oxidation and assimilation are stimulated through the supply of air,
which generates off gases rich in carbon dioxide. The off-gases may also
contain
other compounds in various amounts if their precursors are present in the
influent
waste and if the conditions for volatilization prevail in the bioreactor. Some
non-limiting
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9
examples of volatile compounds include low molecular weight fatty acids,
alcohols,
nitrogen gas, nitrous oxide, ammonia and reduced sulfur compounds.
Volatilization
conditions can be adapted to specific volatile compounds by the person of
ordinary
skill. However, temperature, pH and high oxidative conditions prevailing in
the
bioreactor favour the escape of oxidized gaseous compounds rather than that of
reduced gaseous compounds. Also, the low speed of air admission into the
bioreactor,
the downward suction of the aerated liquor by the air lift pumps and the high
internal
recirculation of the liquor in the reactor prevent the stripping of gaseous
compounds
from the liquor. Thus, the bioreactor and method of the present invention
minimize the
release of reduced gas contaminants. In any event, gas contaminants can be
captured and eliminated by a filter, if required. Of importance, the
bioreactor of the
present invention virtually eliminates odorous gas effluents.
The invention thus relates to a bioreactor wherein the constituents
of the influent material are controllably converted into water, gas, and
biological solids.
The off gas can be removed from the bioreactor and the liquid effluent
separated
from the biological solids (sludge) in a settling tank or basin. In most
embodiment, the
gas produced by the bioreactor is substantially odor-free.
The influent organic waste material is passively introduced or
continuously pumped, at an injection point opposite to the reactor overflow,
directly
into the bioreactor from the source or from an upstream homogenizing tank or
basin.
The organic waste then comes into contact with the biological catalyst freely
suspended in the bioreactor and is oxidized with the addition of air which
also provides
agitation. Air is injected through air lift pumps, and, whenever higher
aeration is
needed, through the addition of an appropriate number of air diffusers. The
contact
time between the substrate and the catalyst, oxygen transfer and dispersion
are
maximized by the pneumatic Venturi tubes which continuously recirculate the
mixed
liquor by discharging it into at least one channel installed above the liquid
surface. The
channels) empty(ies) its(their) content into the bulk of the liquid at a
discharge area
located at the end opposite to the reactor overflow. The incoming waste is
thus
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instantaneously homogenized with the recirculated mixed liquor. Pneumatic
mixing
provides low shear conditions that preserve the integrity of the biological
flocs.
Consequently, the flocs are more amenable to a downstream swift and passive
settling
step. After settling, the clarified effluent leaves the settling unit. The
biological sludge
5 rich in assimilated carbon and sorbed nutrients along with the inert solids
fraction that
may have been present in the influent material can be collected at that time.
The
settling unit can be looped with the bioreactor through devices that recycle
biological
solids as necessary and return any scum that may form at the surface of the
clarifier.
In certain embodiments, the flow of the liquids to and from the bioreactor can
be
10 carried out by gravity.
The quality of the sludge and aqueous outflows of the settling unit
will be dependent on the quality of the influent waste. Final treatment and/or
disposal
options will depend on local environmental standards (sewer and/or water
receiving
bodies) as well as on biomass reuse capabilities of the local or regional
community
and can be adapted by the person of ordinary skill. In a preferred embodiment,
the
bioreactor of the present invention provides the production of a sludge from
the
settling unit, such that the sludge is a better ingredient in the preparation
of
commercial fertilizers than the influent material provided to the bioreactor.
The present invention also provides a method of treating aqueous
wastes containing high levels of organic material comprising an aerobic
biodegradation
inside a well-mixed flow bioreactor, whereby reduction levels of 90 % or above
are
obtained for (1 ) total suspended solids; (2) organic material; (3) nitrogen
and
phosphorus contamination; and whereby reduction levels in excess of 99.s % are
obtained for pathogenic bacteria.
As used herein, the designation "organic wastes" is meant to cover
preferably organic wastes having high organic content. Non-limiting examples
of such
organic wastes include animal slurries such as liquid piggery waste, septic
tank
sludge, landfill site leachate and agro-food or other industrial wastewater.
Preferably,
the organic loadings of the bioreactor will be between about 0.17 to 0.46 Ib
total
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11
CODIft3.d or between about 0.06 to 0.17 Ib filtered CODIft3.d. It shall be
understood
that the organic content of the aqueous wastes which can be efficiently
treated in
accordance with the present invention is dependent on the concentration of
volatile
suspended solids (VSS) or biocatalyst in the bioreactor. For example, an
increase in
the concentration of VSS should allow the treatment of aqueous wastes having
higher
organic concentration. It follows that the concentration of VSS can be adapted
as a
function of the organic content of the aqueous waste.
The term "biodegradation° denotes the fact that the treatment or
degradation of the organic waste in the bioreactor is enabled by a biological
catalyst
which is freely suspended and develops from the facultative aerobic microbial
flora
present in the waste to be treated. Of course, it will be understood that the
biological
catalyst can be added to the waste to be treated and that the microbial flora
to be
added can be adapted to the type of pollutant to be removed from the aqueous
waste.
In addition, inoculation of the bioreactor will often take place naturally by
the microbial
flora contained in the aqueous waste. It shall also be understood that a
particular type
of aqueous waste generally contains a microbial flora which is usually best
adapted
to the degradation of the substrate in which it lives. The bioreactor of the
present
invention and method of degradation using same, favour and select for the
development of such a microbial flora, thereby enabling an efficient and
complete
biodegradation of the organic content, provided that an adequate sludge
retention time
is applied. The microbial flora refers generally to bacteria and higher life
forms such
as protozoa which develop in the mixed liquor. The bioreactor also enables the
production of flocs. Thus, the present bioreactor and method of using same are
adaptable to the treatment of different types of aqueous wastes at high
organic
content, provided that the microbial flora has the necessary growth conditions
inside
the bioreactor. The adequate sludge retention time can be determined by the
person
of ordinary skill as a function of the food mass ratio (F/M) of the FIM inside
the
bioreactor; of the hydraulic retention time and of the level of purification
pertormance
of the bioreactor which is targetted.
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12
As used herein the term "flocs", well known to a person of ordinary
skill to which the present invention pertains, refers to a flocculant mass
formed by the
aggregation of inert and biologically active particles. The present invention
enables the
production of flocs without a dependance upon added flocculating agents. Under
certain conditions, the addition of flocculating agents although less
preferred (see
below) could also be envisaged.
As used herein, "sludge or biological sludge" is well known in the
art and denotes that the sludge is of biological origin and that it provides a
biomass.
Non-limiting examples of sludge thickening include, centrifugation, drainage
on porous
bed, filtration on membrane or by press. Non-limiting examples of sludge
stabilization
include, composting, lime treatment, and aerobic or anaerobic digestion.
Sludge
disposal usually pertains to disposal in a landfill site unless sludge land
farming is
possible andlor permitted. Thermal destruction of sludge is yet another method
to
dispose of the sludge.
Since flocculating agents are generally not required in the method
of the present invention, a decrease in the value of the sludge by these
flocculating
agents is avoided. Furthermore, the method of the present invention produces a
sludge which has a higher fertilizing value than the influent waste. in some
applications
of the method of the present invention, the product sludge has a potential as
an
ingredient in the preparation of animal feed. Therefore, the product sludge
has a
potential value on the market.
The recitation "organic loading" can be expressed as a function of
the volume of the reactor or as a function of the concentration of biocatalyst
in the
bioreactor. It is usually expressed as unit mass of COD or BOD per unit volume
of
reactor per day (Ib COD or BOD/ft3.d) or as a unit mass of COD or BOD per unit
mass
of biocatalyst per day (Ib COD or BOD/Ib VSS.d).
Depending on the sludge retention time and on the hydraulic
retention time, the bioreactor and method of the present invention enable a
reduction
of total COD of the influent waste in excess of 90 % after the clar~cation
step. The
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13
reduction can be as high as about 98 %. Thus, the bioreactor of the present
invention
and method of treating aqueous waste of high organic content of the present
invention
provide a very efficient reduction in organic content of such aqueous wastes.
The terms "COD" and "BOD", as well known in the art, relate to the
chemical oxygen demand and biochemical oxygen demand, respectively. The COD
is a chemical oxidation method for the measure of all the matter which is
chemically
oxidizable. It is always higher than the carbonaceous BOD because it includes
both
bio- and non-biodegradable matters. The carbonaceous BOD represents the
quantity
of oxygen used by bacteria to oxidize the biodegradable matter present in a
sample
of aqueous waste in a period of, normally, five days.
In this context, aqueous wastes having a carbonaceous BOD
between about 100 to about 100,000 mg BOD~/L, preferably 3,000 to 30,000 mg
BOD~IL, and defining an example of aqueous wastes having high organic content,
are
encompassed as being within the scope of the present invention. As a
reference,
domestic waste waters have organic contents between 100-400mg BOD~IL and
typically about 250mg BODS/L. Using the bioreactor of the present invention to
treat
such a type of aqueous waste would require an increase in the sludge retention
time
in order to obtain the optimal F/M for the targeted purification performance.
As used
herein the BOD values are pertinent to indicate the type of aqueous wastes
which are
within the scope of the present invention. It should not be used to
characterize the size
of the treatment units, however. In this context, the size of the unit should
be based
on the volumetric loading to the reactor (i.e. unit mass of COD or BOD per m3
of
reactor volume per day).
The recitation "freely suspended microbial flora" refers to the
biomass being in a suspended growth process as opposed to the biomass being in
an
attached rowth process.
The recitation "aqueous organic waste" refers to the fact that the
solvent is water as opposed to oil or the like.
The bioreactor and method of the present invention provide the
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14
significant and novel advantage of enabling the treatment of influents
containing
suspended solids in concentrations as high as 6.5%. The bioreactor and method
of the
present invention maintain thourough mixing conditions and thereby the
homogeneity
of the suspended solids in the reactor.
S The biocatalyst, as characterized by the content of volatile
suspended solids, is generally present in the aqueous wastes and contributes
to the
concentration of suspended solids. The bioreactor and method of the present
invention allow the treatment of aqueous wastes containing concentrations of
VSS in
the range of 0 to 50,000 mgJL, preferably in the range of 10,000 to 30,000
mg/L.
It shall be understood that regardless of the suspended solids in the
influent material, the reactor can develop and maintain concentrations of VSS
up to
50,000 mg/L. The VSS range will vary according to the BOD concentration of the
influent, the hydraulic retention time and the sludge retention time. Of
course, a
person of ordinary skill will be able to adapt these parameters in order to
obtain a
desired performance of the bioreactor and aqueous waste treatment method. The
bioreactors and methods of the prior art in contradistinction to those of the
present
invention can rarely operate at VSS concentrations superior to 10,000 mg/L. In
fact,
in general, the preferred range of operation thereof is 1,500 - 6,000 mg
VSS/L, for
municipal applications and slightly higher for industrial applications. The
bioreactor of
the present invention is thus the first to provide the means to operate at VSS
concentrations superior to 10,000 mglL, as high as 50,000 mg/L, and preferably
between 10,000-30,000 mglL.
It is to be understood that the sludge retention time will have to be
adapted by the person of ordinary skill as a function of the specific type of
treatment
and aqueous waste treated. In certain situations, in which no sludge recycling
is used,
the sludge retention time will be equal to the hydraulic retention time.
However, in
order to ensure an efficient clarification, there exists an optimal sludge
retention time
which is dependent on the type of aqueous waste and the hydraulic retention
time..As
mentioned above, this optimal sludge retention time can be determined by
CA 02315838 2000-06-21
wo ~ns6s~ ecncA9staim6
conventional means by the person of ordinary skill to which the present
invention
pertains.
It shall also be understood, that the bioreactor tank of the present
invention can be under or above the ground level or alternately at
intermediate levels.
5 The person of ordinary skill, will be able to adapt the system to the
correct level:
While the method of treatment of aqueous wastes at high organic
and solids content by the well-mixed flow aerobic bioreactor of the present
invention
is demonstrated with liquid piggery waste, as mentioned, other aqueous wastes
at high
organic and solids content can be treated in accordance with the present
invention.
10 Non-limiting examples thereof include other animal waste slurry, septic
tank sludge,
landfill site leachate and agro-food or other industrial or domestic
wastewaters.
Broadly, the bioreactor of the present invention is the first bioreactor
enabling the presence of a substantial level of dissolved or residual oxygen
at the
bottom of the reactor (at least 1.5 mg /L and preferably 2.0 mg/L). Thus, the
15 bioreactor of the present invention is the first to enable the obtention of
a substantially
homogenously oxygenated (or aerobic) mixed liquor.
Having thus generally described the invention, reference will now
be made to the accompanying drawings, showing by way of illustration a
preferred
embodiment thereof, and in which:
Fig. 1 is a schematic of a typical set-up for aqueous waste
treatment in accordance with the present invention showing the bioreactor of
the
present invention in relationship with peripheral units that can be used in
accordance
with the invention;
Fig. 2 is a top plan view of the structure and configuration of the
bioreactor;
Fig. 3 is a cross-sectional view on line 3-3 of Fig. 2; and
Fig. 4 is a cross-sectional view on line 4-4 of Fig. 2.
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16
Other objects, advantages and features of the present invention will
become more apparent upon reading of the following non-restrictive description
of
prefen-ed embodiments with reference to the accompanying drawing which is
exemplary and should not be interpreted as limiting the scope of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Broadly, the aqueous waste treatment process of the present
invention and apparatus therefor consist of a bioreactor 2 and of two standard
units
(Figure 1 }. A homogenizing tank 4 can be placed upstream of the bioreactor 2,
the
subject of the present invention, and a settling tank 6 installed downstream.
The
homogenizing tank 4 or basin is optional but preferred for waste containing
settleable
solids. The aqueous waste 8 flows passively or can be pumped into the
homogenizing
unit 4 either batchwise or continuously by a pump 10. A pump 12 is necessary
between the homogenizing tank or basin 4 and the bioreactor 2 if the outflow
from the
1 S homogenizing unit 4 is located below the inflow of the bioreactor 2.
The bioreactor 2 achieves biological oxidation of the influent waste
14 and flocculation of suspended solids to provide a mixed liquor 16 that
overflows
into the settling tank 6 in which it can be easily separated to generate an
aqueous
effluent 18 and a biological sludge 20 to be further treated or disposed of.
The product
sludge 20 can be managed according to the needs of the user and disposed of in
different ways known to the person of ordinary skill. The aqueous effluent 18,
depending on its quality and on local environmental standards, can be polished
in
further pur~cation steps or disposed of in a sewer system or in receiving
water bodies
as commonly known in the art.
An oxygen containing medium such as an air supply has to be
provided for the oxidation to take place. In a preferred embodiment of the
present
invention, an oxygen containing medium 21 (air) is supplied to the bioreactor
2 by a
positive displacement blower 22 and off-gas 24 can be treated, if required, as
well
known in the art.
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17
The agitation and aeration design of the bioreactor of the present
invention minimizes foam formation. Excess foam building at the liquid surface
can be
controlled by various mechanical devices or by the addition of chemical foam
suppressor. In a preferred embodiment of the present invention, excess foam is
suppressed by the addition of a foam suppressor 26 (i.e. vegetable oil) with a
dosing
pump 28.
The bioreactor maintains homogeneity of the material to be treated
and reduces the organic content of the influent waste by 90 % to over 98 % (as
expressed in COD) depending upon the hydraulic retention time and the sludge
retention time. The bioreactor is designed to handle influents containing
suspended
solids of up to 6.5 % and organic loadings above 0,11 Ib filtered COD/ft3.d.
Furthermore, the design of the bioreactor enables its operation at high
volatile
suspended solids concentrations (up to 50,000 mg VSS/L).
The settling tank 6 passively removes most of the suspended solids
leaving the biological unit. The amount of suspended solids removed in tank 6
depends on the operating conditions of the biological unit. If scum is present
at the
surface or the settling unit, this can be returned to the bioreactor (shown as
30 in Fig.
1 ). Recycling of the sludge (shown as 32 in Fig. 1 ) is optional and it can
be achieved
if the hydraulic retention time does not allow the appropriate sludge
retention time in
the bioreactor. In such a case, the old sludge provides the biological
catalyst for the
next round of biotreatment of aqueous wastes.
The bioreactor 2 (figures 1-4) is either under or above ground level.
At least one channel 34 installed above the operating volume extends over a
substantial portion of the length of the bioreactor 2, preferably over 80% of
the length
and more preferably over 95 % of its length. Fresh influent material 14 can be
fed
continuously at any point by an influent pipe 15 or into the channel discharge
area 36
of bioreactor 2 where it comes into or is injected directly in the bulk of the
mixed liquor.
The channel 34 has a slope of at least 1 % to insure a flow of the mixed
liquor towards
the discharge area 36 located at the end opposite to the overflow 16 and
overflow pipe
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18
17 of the bioreactor 2. Alternatively, the influent 14 can be injected into
channel 34.
In a preferred embodiment, for treatment of aqueous waste at high organic and
solids
loadings, the influent is injected into the channel discharge area 36, and
more
preferably, below the surface of the mixed liquor, thereby avoiding odor
emissions.
As best shown in Figs. 2 and 3, aeration and agitation are entirely
pneumatic and provided by Venturi tubes 38 connected to the walls of the
channel 34
where they empty their contents into the channel 34. Additional aeration and
agitation
provided by air diffusers 40 befinreen the Venturi tubes at the bottom of the
bioreactor
2: These diffusers should be selected according to mass transfer efficiency
and
oxygen depletion rates at set operating conditions. Furthermore, the air
diffusers
should be of the non-clogging type. They should also supply medium and coarse
air
bubbles into the mixed liquor. This bubble size allows a suitable air transfer
to the
mixed liquor while preventing the breaking of the biological flocs. Such air
diffusers 40
also minimize foam formation. The general arrangement of the Venturis tubes 38
can
be in staggered or parallel rows and the number of Venturis tubes 38 and rows
will
vary according to the size of the bioreactor and the number of channels. Each
Venturi
tube 38 is equipped with a suction device 42 sitting on cross supports 44.
As shown in Figs. 2-4, oxygen containing medium such as air is
supplied to the Venturi tubes 38 and diffusers 40 by individual calibrated
pipes 46
linked to a manifold 48. If more flexibility in the liquid flow pattern of the
reactor is
required, the air flow through each pipe 46 can be controlled by individual
valves 50.
The appropriate air source has to be supplied by a positive displacement
blower 22
(Fig. 1 ) which provides the bioreactor 2 with the oxygen containing medium 21
through
the manifold 48. The positive displacement blower 22 should be sized as a
function
of the oxygen demand and of the hydrostatic pressure to provide approximately
2,500
ft3 of air per pound of GODS per day. Heat transfer may also be considered as
a
selection criteria for the blower. Indeed, as commonly known the gas pressure
and
the temperature of the gas (or oxygen-containing medium) as it enters the
bioreactor
are intimately linked.
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19
The mixed liquor 16 leaves the bioreactor 2 passively through an
overflow pipe 17 towards the settling unit 6. The overflow pipe 17 and its air
vent 52
are located at the end of the bioreactor 2 opposite to the channel discharge
area 36.
Peripheral flows such as for scum recycle 30 or sludge recycle 32 (see Figs. 1
and 2-
3) from the settling unit 6 can be provided by a scum recycle pipe 54 and
sludge
recycle pipe 56, installed at the upstream end of the channel 34 and in the
channel
discharge area 36, respectively. Foam suppressor 26 can be added to the mixed
liquor
at any appropriate point of bioreactor 2 by a foam suppressor injection pipe
60. Off-
gas can exit the bioreactor 2 at any appropriate point and as necessary by an
off-gas
pipe 58 at the top of bioreactor 2. The off gas pipe 58 can be connected to a
filter or
gas treating device as commonly known in the art.
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wo 99n5657 - PCT/CA98/01076
The present invention is illustrated in further detail by the following
non-limiting examples.
5
The liquid piggery waste produced by 1fi0 pigs was treated at a
fattening piggery with the bioreactor of present invention using a hydraulic
retention
time of 12 days. The system also included a homogenizing tank as well as a
settling
tank (without sludge recycle). Characterization of the influent material and
of the
10 supernatant leaving the settling tank before polishing yielded the
following results:
Analytical parameterInfluent Settling Removal
concentrationtank efficiency
(m /L) Supernatant(%)
(mglL)
Total COD 61 850 1 430 97.7
Filtered COD 18 260 1 080 94.1
15 Total sus nded solids37 570 510 98.6
Total nitro en (Kjeldahl)4 100 190 95.4
Ammonia vitro en 2 450 12 99.5
Total phosphorus 1 200 120 90.0
Pathogenic bacteria7.7x10' 5:2x10'' 99.9
CFU/100 mL CFUI100
mL
20
Conditions were as follows: Influent flow-rate of 1,120 Ud; food to mass ratio
(f/M) of
0.29 Ib total COD/lb VSS.d or 0.09 Ib filtered CODIIb VSS.d working at 0.30 Ib
total
COD/ft3.d or 0.09 Ib filtered CODIft3.d; no sludge recycle. Hydrostatic
pressure at the
site of the air diffusers was 6 feet of water, oxygen transfer rate was
between 2.7% to
3.2%; residual or excess oxygen in the mixed liquor was on average of 2 mg/L.
Mass
balance around the reactor and the settling tank indicated that each unit mass
of liquid
piggery waste was converted into 45 % of supernatant and 55 % of sludge
containing
3.5 % suspended solids, by weight.
Thus, the bioreactor and method exemplified enable a removal
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21
efficiency of the different tested parameters which is almost maximum. During
the two
months trial run described in example 1, almost 20 % of the nitrogen and 90 %
of the
phosphorus were captured in the sludge. The biological sludge contained, on a
dry
basis, 3.6 % total nitrogen, 6.3 % plant available phosphoric acid (P205), 2.6
% soluble
potassium (K20), 14.8 % humic acid and 15.4 % amino acids. Thus, the sludge
produced had an increased commercial value as compared to the influent waste
of the
bioreactor.
The liquid piggery waste produced by 300 pigs was treated at a
fattening piggery with the present invention using a hydraulic retention time
of 6.5
days. The system had an homogenizing tank as well as a settling tank, without
sludge
recycle. Characterization of the influent material and of the supernatant
leaving the
settling tank before polishing yielded the following results:
Analytical parameterInfluent Settling Removal
concentrationtank efficiency
(mg/L) supernatant(%)
(m /L)
Total COD 22 440 2 010 91.0
Filtered COD 14 620 1 360 90.7
Total suspended solids7 090 490 93.1
Total vitro en (Kjeldahl)2 220 140 93.7
Ammonia vitro en 1 990 82 95.9
Total phosphorus 630 88 86.0
Conditions were as follows: Influent flow-rate of 2,140 Ud; food to mass ratio
(f/m) of
0.35 Ib total COD/lb VSS.d or 0.24 Ib filtered COD/lb VSS.d; working at 0,22
Ib total
CODIft3.d or 0,14 Ib filtered COD/ft3.d; no sludge recycle. Hydrostatic
pressure at the
site of the air diffusers was 6 feet of water; oxygen transfer rate was
between 2.7% to
3.2%; residual or excess oxygen in the mixed liquor was on average.of 2 mglL.
No
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22
mass balance can be calculated around the bioreactor and the settling tank for
this
example because results were obtained under non-permanent regime conditions.
Interestingly, the bioreactor was able to treat aqueous waste at a very high
efficiency
even though the non-insulated bioreactor {and above ground) was submitted to
ambient temperature levels that reached below zero °C and in which the
influent
oxygen-containing medium temperature was as low as -20°C.
There has thus been described a compact bioreactor for treating
aqueous organic wastes at high organic loadings. The system concentrates into
the
biological sludge over 85 % of the nitrogen and over 92 % of the phosphorus
contained into the bioreactor outflow and it can allow discharge of the
aqueous effluent
to a water course depending on local regulations. The aqueous effluent is odor-
free
and the output solids may be processed through a stabilization step if a
stable and
odor-free product is needed for further utilization. Off-gases can be treated
if
necessary through a filter.
The system provides very high reductions in total suspended solids
(above 93 %), organic material (BOD or COD, above 90 %), nitrogen (above 93 %)
and phosphorus (above 8fi %) contaminations that are otherwise discharged into
the
environment, treated only partially, or treated through more expensive
technologies
based upon biological, physicochemical or thermochemical processes.
Although the present invention has been described herein by way
of preferred embodiments thereof, it can be. modified, without departing from
the spirit
and nature of the subject invention as defined in the appended claims.
