Note: Descriptions are shown in the official language in which they were submitted.
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Integrated Multi-Zone Wastewater Treatment System and
Method
Field of the Invention
The method and apparatus of this invention relate to the treatment of a liquid
or
slurry of waste stream (e.g., wastewater or sludge) originated from municipal
or
industrial activities, or the treatment of groundwater or landfill leachate
contaminated with organic and/or inorganic chemicals. The organic material may
contain sources of BOD and COD as well as hazardous chemicals such as
aromatic hydrocarbons, including benzene, toluene, ethylbenzene, xylenes,
phenols, cresols, polycyclic aromatic hydrocarbons (PAHs), and halogenated
(e.g., chlorinated) hydrocarbons, such as tetrachloroethylene,
trichloroethylene,
1,1,1-trichloroethane and similar xenobiotics, and inorganic material notably
nitrogen and phosphorus.
Background of the Invention
The treatment of wastewater and contaminated groundwater require the removal
of organic and inorganic contaminants, usually present in solid and/or
dissolved
form, before their discharge into the receiving waters. The organic
contaminants
include sources of COD/BOD such as proteins, lipids and polysaccharides as
well as hazardous compounds such as aromatic and aliphatic hydrocarbons.
Examples of the latter group include gasoline and diesel fuel, polycyclic
aromatic
hydrocarbons, phenols, chlorophenols, alkylated benzenes, tetrachloroethylene
(PCE) and trichloroethylene (TCE). The nitrogenous and phosphorus
compounds, which are among the most undesirable inorganic contaminants of
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wastewater and contaminated groundwater, also need to be removed during the
treatment process.
Biological treatment processes use a variety of microorganisms such as
bacteria,
protozoa and metazoa for an efficient and complete biodegradation of
contaminating compounds in wastewater and contaminated groundwater and
landfill leachates. The removal of carbonaceous, nitrogenous and phosphorus-
containing compounds is carried out by bacteria, whereas protozoa and metazoa
mostly contribute to the reduction of turbidity since they graze on bacteria
as a
food source. The organic charge in the wastewater is often measured by
chemical oxygen demand (COD) or biochemical oxygen demand (BOD). These
parameters define the overall oxygen load that a wastewater will impose on the
receiving water. During biological treatment processes, organic substances are
removed since these substances serve as the source of carbon in the microbial
metabolism. Nitrogen and phosphorus are also consumed by microorganisms as
essential nutrients to support microbial growth during assimilatory processes,
while excess amounts of nitrogenous compounds is removed during dissimilatory
microbial nitrogen metabolism where they are transformed to molecular nitrogen
and released into the atmosphere. The remaining phosphorus may be removed
by the "luxury phosphorus uptake" process where special groups of
microorganisms accumulate phosphorus and store it as poly-phosphorus
compounds, thus removing it from the system during sludge disposal.
Nitrogen and phosphorus have been recognized as major contributors to
eutrophication, a process that supports the growth of algae and other
undesirable
organisms in the receiving waters and diminishes the concentration of
dissolved
oxygen, thus threatening aquatic life. Therefore, stringent criteria have been
introduced demanding the reduction of these nutrients below certain levels
that
are established by environmental agencies, before the effluent of treatment
plants can be safely disposed to the receiving waters.
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In general, the success of biological treatment systems depends on the
concentration of biomass and the mean cell residence time (MCRT), as well as
the ability of treatment system to separate sludge from the treated liquid.
These
parameters control the efficiency of treatment and the quality of effluent.
Suspended-growth biological treatment systems that are based on activated
sludge processes have difficulties in maintaining an adequate concentration of
active biomass, and to effectively separate solids from liquid. They also
produce
large amounts of sludge and have a slow adaptation to fluctuating influent
conditions. These problems have been addressed in the design of fixed-film
treatment systems that use attached microbial biomass, immobilized on a
support material. The immobilized cells grow and produce microbial biofilm,
containing a consortium of microorganisms that changes with time and within
the
thickness of the biofilm. Fixed-film systems can operate in aerobic, anoxic or
anaerobic modes depending on the nature of the contaminating compounds.
These systems offer microbial diversity and prevent the washout of biomass.
They also offer a higher MCRT and ease of operation relative to the separation
of
biomass from liquid. However, fixed-film systems have a lower rate of
contaminant removal compared to suspended-growth treatment systems since
the rate of removal in these systems is controlled by mass transfer and
diffusion
within the microbial biofilm. Aerobic fixed-film systems treating high organic
concentrations have a limited capacity because of oxygen transfer limitations.
Moreover, the build-up of a thick and heavy biofilm, resulting from high
concentrations of organic substances may cause clogging, seriously disrupting
the operation of the treatment system. Therefore, fixed-film systems have a
limited application in the treatment of high organic load wastewaters. Another
disadvantage of fixed-film treatment systems is that they usually operate in
plug-
flow mode and do not offer the homogenous environment provided by completely
mixed reactors. In these systems, there is a high concentration of
contaminants
at the influent end of the reactor and the microorganisms will be subjected to
the
full concentration of contaminants that may be toxic.
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All of the technologies discussed above were originally designed for secondary
treatment, i.e. removal of carbonaceous compounds and solid-liquid separation
and not to remove nutrients, notably nitrogenous and phosphorus compounds or
halogenic substances from wastewater or contaminated groundwater and landfill
leachate that require the presence of different environments with different
levels
of dissolved oxygen concentration and oxidation-reduction potential. These
technologies also cannot stabilize the produced sludge and need supplementary
vessels for this process.
In order to meet stringent discharge criteria including nitrogen and
phosphorus
removal, wastewater treatment plants usually upgrade their performance by
using add-on technologies such as biological nutrient removal (BNR) systems.
The theories of biological nitrogen and phosphorus removal mechanisms
demonstrate that nitrogen removal needs the presence of aerobic and anoxic
environments, while the removal of phosphorus demands the presence of
anaerobic and aerobic environments in the treatment system.
Conventional wastewater treatment technologies, originally developed for the
removal of carbonaceous compounds (BOD) and suspended solids,
accommodate nutrient removal by providing additional aerated, anoxic or
anaerobic units in series, along with various internal recycle streams to
achieve
the required removal of nitrogen and/or phosphorus. These modifications have
increased the complexity of the treatment systems and complicated their proper
design and optimization. Nitrification, the first step in the biological
nitrogen
removal mechanism that involves the conversion of ammonia nitrogen to nitrate
nitrogen, requires an aerobic environment and is achieved in all aerobic
reactors
if the right operating conditions such as the liquid pH, carbonate
concentration,
and sludge retention time exist. Denitrification, i.e. the transformation of
nitrate
nitrogen to molecular nitrogen, can be accomplished by the addition of an
anoxic
= activated sludge reactor or fixed-film system. An easily degradable
carbon
source must be present for the denitrification process. If the treatment
system
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cannot supply the required carbon source, then methanol, ethanol, acetic acid
or
a different compound must be added. In these combined processes, the
wastewater or contaminated groundwater is first fed into the anoxic
denitrification
reactor. The effluent from the anoxic reactor is fed into the aerobic reactor.
A
sufficient retention time in the aerobic reactor is needed to ensure a
complete
oxidation of carbonaceous compounds as well as adequate growth and
proliferation of slow-growing nitrifi,ers to carry out the nitrification
process and
convert ammonia-nitrogen to nitrate-nitrogen. Sequencing batch reactor (SBR)
systems have been used for biological nitrogen removal by incorporating anoxic-
aerobic sequencing bioreactors. Instead of using an external carbon source for
denitrification, it is quite possible to design a nitrification-
denitrification system
' that uses the carbon present in the raw wastewater as well as the carbon
released from the endogenous respiration of microbial sludge. In these
systems,
nitrification and denitrification occur in a singe vessel with alternating
aerobic and
anoxic zones. Alternatively, aerobic and anoxic zones may be present in
separate vessels positioned in series. Sufficient recycle is required to
prevent the
effluent from containing excessive ammonia concentrations. Several systems
have been developed along with these design elements. The two most
successful have been the denitrifying oxidation ditch and the Bardenpho
process.
In the denitrifying oxidation ditch, an anoxic zone is added inside the
aerobic
reactor. The influent is added to the anoxic zone and the effluent is
withdrawn
from the aerobic reactor. Solids are then separated from the liquid by
settling in a
clarifier. One of the most common design modifications for enhanced nitrogen
removal is known as the Modified Ludzack-Ettinger (MLE) process. In this
process, an anoxic tank is added upstream of the oxidation ditch along with
mixed liquor recirculation from the aerobic zone to the tank to achieve higher
levels of denitrification.
Another variation of aerobic/anoxic systems for nitrification/denitrification
is the
four stage Bardenpho process that has two 'aerobic and two anoxic vessels.
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Carbon from the untreated wastewater and from endogenous decay of
microorganisms is used for denitrification by returning the aerobically
treated
wastewater to the initial anoxic zone.
In order to achieve phosphorus removal as well as nitrogen removal, the
incorporation of an additional anaerobic zone in the treatment system is
necessary. Two such configurations are the A2/0 process and the five-stage
Bardenpho process. The A2/0 system includes anaerobic, anoxic and aerobic
zones. In this process, the untreated wastewater is first added to the
anaerobic
zone where soluble phosphorus is released and VFAs are uptaken by the
phosphorus accumulating microorganisms (PA0s). The effluent of the anaerobic
tank is fed into the anoxic zone for the reduction of nitrate and its
conversion to
nitrogen. The effluent of the anoxic zone flows to the aerobic zone for BOD
removal and nitrification. The separation of solids and liquid takes place in
a
clarifier. Two recycle streams are present in this process: one from the
clarifier to
the anaerobic zone to return a portion of the separated sludge, and the second
one from the aerobic to the anoxic zone carrying nitrate for the
denitrification
process. The ability of the A2/0 process to provide anaerobic dehalogenation
has raised interest in this process for the treatment of groundwater and
landfill
leachate contaminated with hazardous chemicals such as chlorinated aliphatic
compounds, recognized as common contaminants of soil and groundwater
around the world.
In the Bardenpho ,system, there are two aerobic and two anoxic zones. Similar
to
the A2/0 process, there are also two recycle streams between the clarifier and
the anaerobic zone, and between the first aerobic and anoxic zones. In this
configuration, a more complete removal of nitrogen is achieved. Moreover, the
anaerobic zone will not receive nitrate in the recycle stream, thus a better
phosphorus removal process can also take place. The five-stage Bardenpho
process has a high nutrient removal capacity and can remove high
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concentrations of nitrogen and phosphorus from wastewater and contaminated
groundwater.
As learned from the above description, most conventional nutrient removal
systems are multi-vessel/multi-zone processes that have complicated designs
and large footprints, and require high operator attention and maintenance
requirements.
An alternative nutrient-removal, wastewater treatment technology that uses a
multi-zone system with different environmental conditions is the Integrated
Biologically Active Clarifier (IBAC). This technology that is currently in
commercial operation in Quebec, Canada, combines biological treatment, solid-
liquid separation and sludge stabilization in a single vessel. This treatment
system has three vertically-stacked biological zones having aerobic, anoxic
and
anaerobic environments as well as a clarification zone. The vertical stacking
of
the treatment zones causes the settlement of heavy solid material including
high-
density biological flocs to the anaerobic zone that is located at the bottom
of the
reactor where anaerobic biodegradation occurs. The system does not use any
'recycle streams either for sludge or mixed liquor. The mixing and liquid
recirculation is provided by the introduction of air into the aerobic zone.
This
technology suffers from a series of problems that seriously upset the
operation of
the treatment system. They include the periodic rise of sludge in the
clarification
zone due to excessive gas production in the bottom anaerobic zone,
inconsistent
nutrient removal, and poor settleability of solids. In addition, this
technology does
not permit proper control and optimization of the biological treatment and
solid-
liquid separation processes due to the occurrence of all different processes
in a
single vessel and the existing interactions among them.
Several wastewater and groundwater treatment technologies for the removal of
organic carbon and nutrients are described in the patent literature. Examples
of
techniques dealing with the removal of carbonaceous material, nitrogen and/or
=
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phosphorus form wastewater can be found in US patents 4,488,968; 4,948,510;
= 5,128,040; 5,160,043; 5,288,405; 5,518,618; 5,601,719; 5,651,891;
5,972,219;
6,139,743; 6,372,138; and 6,413,427.
US patent 5,518,618 relates to a system for the treatment of nitrogenous
wastewater by biological nitrification and denitrification, wherein the
wastewater
along with microorganisms immobilized on a carrier material flow alternately
in a
downward flow in an oxygen-depleted chamber and in an upward flow in an
oxygen-rich chamber. The aerated reactor chamber may be partly divided into a
riser and a downcomer which allow, as a result of the air supply, mass
circulation
to take place in this chamber. The oxygen-depleted reactor chamber contains a
degassing chamber and a settling chamber at the top in order to separate the
liquid from the solid sludge. The aerated chamber is supplied with a gas
delivery
system for producing an upward flow of waste water and a gas discharge located
above it. The aerated chamber is also supplied at the top with an overflow to
the
oxygen-depleted reactor chamber. This treatment system does not have any
anaerobic zone to promote phosphorus removal and does' not stabilize sludge.
US patent 6,139,743 relates to a suspended-growth treatment system that
includes a multiplicity of tanks, some with several compartments, including an
anaerobic/anoxic reaction tank, an aeration tank and a settling tank. The
settled
sludge is further returned into the anaerobic compartment. The treatment
system
is capable of reducing the carbon, nitrogen and phosphorus compounds in the
wastewater. However, it requires that an external carbon source be provided to
each compartment of the anaerobic/anoxic reaction tank to support the nitrogen
and phosphorus removal processes. This practice adds to the operational cost
of
treatment. The treatment system has a large footprint that requires large
areas
for its setup and operation. Like other suspended-growth treatment
technologies,
there is also a high production of sludge and an associated high cost for its
disposal.
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US patents 5,128,040 and 5,651,891 use a series of aerobic and
anoxic/anaerobic tanks for the removal of BOD, improvement of solid
settleability
and reduction of nitrogen and phosphorus in wastewater. Suspended-growth
processes, e.g. activated sludge, or attached-growth processes, e.g. trickling
filter or rotating biological contactor may be used for BOD reduction and
nitrification in these treatment systems. One of the specifications teaches
the
use of two fermentation tanks for the production of VFAs, required in the
denitrification and phosphorus removal process, and its return to the
anoxic/anaerobic tank. These inventions improve the efficiency of solids
removal
in trickling filter/solids contact processes. However, they have a limited
phosphorus removal capacity, use a multiplicity of tanks, need a very large
area
for operation and have several recycle streams, considerably increasing their
complexity, maintenance requirement and cost of operation.
US patent 5,6017,19 discusses a similar treatment system for the removal of
BOD, nitrogen and phosphorus from wastewater containing a series of aerobic,
anoxic and anaerobic vessels, a sludge fermenter, a secondary clarifier, and a
supplemental substrate source with several recycle streams. The inclusion of
an
anaerobic fermentation stage in the treatment process has also been discussed
in several other specifications such as US patents 4,999,111, 5,013,441, and
4,874,519.
US patent 5,480,548 relates to a step feed activated sludge process including
anaerobic-anoxic-aerobic zones for biological nitrogen and phosphorus removal.
The system contains multiple step feed points arranged in a series of
consecutive
treatment stages and a series of recycle lines carrying the effluents of
anoxic and
anaerobic zones as well as the return activated sludge to different zones. The
system is purported to achieve reductions of phosphorous and ammonia greater
than 90% and 97%, respectively, and a 50% reduction in total nitrogen.
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US patent 4,056,465 relates to a modified activated sludge system where BOD-
containing wastewater and recycled sludge are initially mixed under anaerobic
conditions and in the absence of nitrate or nitrite thereby promoting the
= production of the desired type of microorganisms. The effluent of this
stage is
then sent to an aerobic tank where BOD removal, phosphorus uptake and
nitrification take place. Nitrates and nitrites are removed by interposing an
anoxic
treating zone between the anaerobic zone and the aerating zone.
US patent 4,948,510 describes a process containing a plurality of basins which
may be individually controlled to achieve anaerobic, anoxic or aerobic
conditions.
The basins are reconfigurable in that the flow of effluent to a basin,
transfer of
mix liquor between basins and effluent discharge from a basin can be varied to
create a treatment cycle which has features of both continuous and batch
processes while minimizing recycle rates and hydraulic level changes.
US patent 6,063,273 discloses an apparatus for the biological purification of
wastewater, the apparatus having a column containing an upflow anaerobic
sludge bioreactor (UASB) at the bottom and an aerobic reactor at the top. The
two reactors are separated from one another by a partition in which openings
are
provided to allow the anaerobic effluent through into the aerobic reactor. The
partition forms a buffer zone preventing the biomass from the anaerobic zone
to
mix with the biomass from the aerobic zone.
Several other treatment technologies, mostly based on activated sludge
processes, using multi-vessel or multi-zone aerobic/anoxic/anaerobic systems
for
the complete or partial removal of nitrogen and phosphorus from municipal or
industrial effluents have been reported. Examples of such technologies are
found in US patents 4,056,465; 4,271,026; 4,488,967; 4,500,429; 4,522,722;
4,948,510; and 5,137,636.
Examples of treatment systems and apparatus for groundwater and landfill
leachate contaminated with hazardous material have been discussed in US
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patents 4,290,894; ' 4,442,005; 5,080,782; 5,389,248; 5,413,713; 5,578,202;
5,766,476; 5,804,432; 5,922,204; 6,159,365; and 6,461,509.
US patent 5,922,204 discusses a treatment system containing a plurality of
reactors connected in series. By means of process kinetics control, the
treatment
system accomplishes biological denitrification and, metal precipitation in the
first
reactor followed by biological sulfate reduction and the production of
saturated
and unsaturated hydrocarbons in the second reactor, followed by
methanogenesis and finally aerobic respiration in the final reactor.
US patent 5,413,713 discloses a method for increasing the rate of anaerobic
bioremediation in a bioreactor by recirculating to the bioreactor a given
portion of
a pollution stream that has flowed through a passageway containing material on
whose surface anaerobic microorganisms can attach or become immobilized.
The flow rate of recirculating liquid is adjusted to the level that it would
slough
from the surface and return to the stream at least a portion of the attached
or
immobilized microorganisms from the surface.
US patent 5,080,782 relates to a bioremediation vessel containing a gas
injection system, a nutrient addition system, and a continuously regenerating
culture of microorganisms which biodegrade hazardous substances. The
bioremediation vessel contains a support material for the immobilization of
microorganisms while a fraction of microorganisms slough off the support
medium and disperse into the treated ground water.
US patent 6,461,509 discusses a treatment system that uses an aerobic packed-
bed bioreactor to degrade organic material in the contaminated water. An
additive consisting of at least one of a vegetable extract and a nutrient
medium,
is added to the contaminated water immediately before entering the packed bed
to stimulate the production of exo-enzymes by the bacteria.
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US patent 5,578,202 relates to a water processing system for processing highly
contaminated water that has a plurality of processing chambers defined by
partition walls. The system comprises an anaerobic chamber, an aerobic
chamber, a buffering chamber, and a recycle for recycling part of the water
received in the buffering chamber back to the anaerobic chamber, and a filter
material forming a buoyant filter layer in an upper part of the water received
in the
anaerobic chamber. The filter layer can be highly resistant against clogging,
and
can be easily maintained with the result that the overall system can be made
both
simple and economical.
US patent 6,159,365 discusses an encased packaged modular type unit for =
treatment of contaminated water comprising a separation compartment to
separate solids and, if present, oil and grease in contaminated water, a
fluidized
bed reactor assembly containing aeration zone(s), internal recirculation
zone(s),
clear effluent zone(s), and mixing/degassing zone(s). The fluidized bed
contains
suspended viable biomass, a physico-chemical reagent or a mixture of both
biomass and physico-chemical reagent for the removal of contaminating
compounds, and a compartment wherein excess sludge is thickened and
removed.
Most of prior art solutions suffer from complicated designs, high maintenance
requirements or large footprints as well as a limited capacity to address the
treatment of groundwater or landfill leachate contaminated with a mixture of
contaminating compounds of organic and inorganic nature. Examples of such
contaminations include mixtures of hydrocarbons (e.g. diesel fuel, jet fuel or
gasoline) with nitrate and phosphorus, commonly resulting from the combined
agricultural and airport or military activities. Other examples include
mixtures of
aromatic hydrocarbons and halogenated hydrocarbons (e.g. PCE and TCE), and
sometimes metals. These kinds of contaminations require the simultaneous
presence in the treatment system of diversified groups of microorganisms as
well
as different environmental conditions including different levels of dissolved
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oxygen concentration and redox potential for their complete treatment.
Provisions
have to be made for adequate biomass growth and maintenance of all different
microbial groups, effective solid-liquid separation, sludge stabilization, and
proper
optimization and control of environmental conditions in the multiple zones of
the
treatment system.
Summary of the invention
It is an object of the invention to provide a wastewater treatment apparatus
capable of efficiently removing organic contaminants as well as suspended
solids
and inorganic contaminants, notably nitrogen and phosphorus, from wastewater
and contaminated groundwater.
It is another object of the present invention to provide a treatment method
for
efficiently removing suspended solids and organic material as well as some
inorganic contaminants notably nitrogen and phosphorus contained in
wastewater and contaminated groundwater and to maintain low solids generation
in comparison with flow-through treatment systems or those having sludge re-
circulation.
In accordance with one aspect of the invention, there is provided a wastewater
treatment system comprising
a first processing vessel having a wastewater inlet, the vessel comprising
an aerobic zone, at least one oxygen-depleted zone in fluid communication with
the aerobic zone, and a first clarification zone in fluid communication with
the at
least one oxygen-depleted zone, wherein the aerobic zone comprises aeration
means for supplying air or oxygen to the aeration zone and disposed so that
operation of the aeration means causes recycling of wastewater between the
aeration zone and the at least one oxygen depleted zone,
a second processing vessel comprising an anaerobic zone in a lower part
thereof,
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a first passage and pump means communicating the at least one oxygen-
depleted zone with the anaerobic zone, and
an outlet for processed wastewater.
In accordance with another aspect of the invention, there is provided a
wastewater purification process comprising the steps of:
feeding wastewater into a first processing vessel having an aerobic zone,
a microaerophilic zone and an anoxic zone, providing aerobic
conditions in said aerobic zone so that the wastewater circulates
between the aerobic zone, the microaerophilic zone and the anoxic
zone,
providing solid support for microbial biomass in said aerobic zone,
feeding part of content of the anoxic zone to an anaerobic zone in a
second processing vessel for sludge present in the content to settle in
the anaerobic zone,
removing effluent from the first processing vessel and
removing the sludge settled in the anaerobic zone.
In one embodiment, the apparatus of the invention has two separate but
interlinked tanks containing four different zones, namely aerobic,
microaerophilic,
anoxic and anaerobic, for the biological treatment, as well as two
clarification
= zones and a filtration unit for separation of solids from liquid. The
interconnected
microaerophilic and anoxic zones are also termed oxygen-depleted zones. The
aerobic zone is an airlift reactor that contains air diffusers at the bottom
of the
zone to introduce air into the zone. The air bubbles mix the liquid and its
content
of microorganisms, and provide oxygen for the aerobic biological processes
that
take place in this zone. Aeration also produces circulation of liquid between
the
aerobic zone and its adjacent microaerophilic and anoxic zones that are
located
at the sides and under the aerobic zone, respectively. The aerobic zone
contains
suspended microorganisms of heterotrophic and autotrophic groups that grow
inside the circulating liquid, known as mixed liquor. A part of or the
complete
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volume of the aerobic zone also contains loose carrier material or stationary
objects to support the attachment of microbial biomass and the formation of
microbial biofilm. The walls of the zone are not considered "stationary
objects"
for attachment of microbial biomass. Thus, the aerobic zone contains ' both
suspended-growth and attached-growth microbial biomass. Most of the organic
carbonaceous contaminants are removed in this zone. The nitrification process,
a part of the biological nitrogen removal process, also takes place in the
aerobic
zone. The microaerophilic and anoxic zones are employed for the
denitrification
processes to transform the nitrogenous compounds into nitrogen gas and to
completely remove nitrogen. The anaerobic zone along with the aerobic,
microaerophilic and anoxic zones are employed for the removal of phosphorus
compounds. Dispersed microbial biomass and light bioflocs accumulate inside
the circulating mixed liquor while heavier bioflocs and solid waste material
precipitate to the bottom of anoxic zone. A fraction of particulate organic
matter
entrapped in the bioflocs is hydrolyzed in the anoxic zone while the rest of
it is
transferred to the anaerobic zone. In the anaerobic zone, the solids are
digested
and transformed by fermentative bacteria during the fermentation processes to
short-chain volatile fatty acids or VFAs. The VFAs are returned to the aerobic
zone by a recycle stream between the anaerobic and the microaerophilic zones,
providing easily degradable carbon source for the denitrification process. The
recycle stream from the anaerobic to microaerophilic zone also carries
microorganisms including 'phosphorus accumulating microbes or PAOs that carry
out the removal of phosphorus by the luxury phosphorus removal process. The
influent wastewater continuously circulates between the aerobic,
microaerophilic
and anoxic zones exposing the contaminants to three different environments
that
are necessary for a complete treatment of the contaminated water. The
circulation of liquid causes the accumulation of suspended microbial biomass
and
light biological flocs inside the mixed liquor, increasing the mean cell
residence
time (MCRT) and enhancing microbial adaptation to the variations of influent
wastewater and contaminated groundwater characteristics, including the
concentration of contaminants and the incoming flow rate, as well as possible
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toxic shocks. The accumulation and further adaptation of microorganisms, both
in
the suspended and attached form, increases the specific as well as the
volumetric rates of contaminant biodegradation, producing a high rate
treatment.
Liquid circulation between the three zones also prevents the accumulation of
nitrates or nitrites that are produced in the aerobic zone and may inhibit
microbial
activities.
The wastewater treatment system of current invention enables the control of
biomass retention time (MCRT) at a level that can, maximize microbial
physiological activities while improving solids settleability. Compared to the
conventional technologies, the present system is expected to produce
considerably less biological solids or sludge, thus reducing the associated
costs
of solids handling and disposal.
The solid materials are separated from liquid by precipitation in two
clarification
zones. The colloidal materials as well as any additional suspended solids are
retained in the filtration unit creating a treated liquid to emerge from the
treatment
system. The treatment system is more compact and uses less space compared
to most conventional technologies.
In one aspect, the invention provides a wastewater treatment system
comprising:
a) a first processing vessel with inlet for receiving wastewater to be
treated comprising:
(i) an aerobic zone;
(ii) at least one oxygen-depleted zone in communication with the
aerobic zone;
(iii) an aerator for introducing air or oxygen-containing gas into
the aerobic zone;
(iv) said aerobic zone, said oxygen-depleted zone, and said
aerator being arranged such that the introduced air or oxygen-containing
gas causes circulation of wastewater through the aerobic zone and the
at least one oxygen depleted zone; and
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(v) a first clarification zone in communication with the at least one
oxygen-depleted zone; and
b) a second processing vessel separate from said first processing vessel
comprising an anaerobic zone in a lower part thereof;
c) first passage for permitting the transfer of material from the at least
one oxygen-depleted zone to the anaerobic zone of said second processing
vessel;
d) an outlet for processed wastewater; and
e) a solid disintegration unit downstream the anaerobic zone to
disintegrate biosolids removed from the anaerobic zone in order to solubilize
all
or part of the biosolids to produce biodegradable organic matter.
In one aspect, the invention provides a wastewater purification process
comprising the steps of:
feeding wastewater into a first processing vessel having an aerobic
zone, a microaerophilic zone and an anoxic zone while providing aerobic
conditions in said aerobic zone such that the wastewater circulates through
the
aerobic zone, the microaerophilic zone and the anoxic zone,
providing solid support for microbial biomass in said aerobic zone,
feeding part of the contents of the anoxic zone to an anaerobic zone in a
second processing vessel to permit sludge present in said contents to settle
in
the anaerobic zone,
removing effluent from the first processing vessel;
removing any sludge settled in the anaerobic zone of the second
processing vessel; and
disintegrating the sludge in order to solubilize all or part of the biosolids
to produce biodegradable organic matter.
In one aspect, the invention provides a wastewater treatment system
comprising:
an aerobic zone;
16a
CA 02583752 2012-07-20
one or more oxygen-depleted zones in communication with said aerobic
zone;
an aerator for supplying oxygen-containing gas to said aerobic zone,
said aerobic zone and said one or more oxygen-depleted zones being
arranged such that operation of said aerator causes introduced wastewater to
circulate through said aerobic zone and said one or more oxygen-depleted
zones;
an anaerobic zone isolated and apart from said aerobic zone and said
one or more oxygen-containing zones; and
a passage for permitting the transfer of part of the contents of said one
or more oxygen-depleted zones to said anaerobic zone; and
a solid disintegration unit downstream the anaerobic zone to disintegrate
biosolids removed from the anaerobic zone in order to solubilize all or part
of
the biosolids to produce biodegradable organic matter.
Brief Description of the Drawings
The invention will be described in more detail in the following description in
conjunction with the drawings in which like numerals denote like elements and
in
which:
Fig. 'I represents schematically an exemplary wastewater treatment
system of the invention,
Fig. 2 represents schematically another embodiment of a wastewater
treatment system of the invention,
Fig. 3 illustrates a relationship between the percentage of wastewater
departed from the system and the number of liquid circulations between the
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aerobic, microaerophilic and anoxic zones,
Fig. 4 illustrates a relationship between the number of liquid circulations
between the zones as in Fig. 3 and the wastewater organic load for 90% and
99% displacement, and
Figs. 5, 6, 7, and 8 represent alternative embodiments of the wastewater
treatment system of the invention.
Detailed Description of the Invention
As stated above, the treatment system of the present invention uses two
separate but interlinked tanks for the biological treatment and solid-liquid
separation processes. The first tank contains an aerobic zone, microaerophilic
and anoxic zone plus a clarification zone. The second tank contains an
anaerobic
zone, a solid-liquid separation zone and a filtration unit. An example of the
treatment system according to the invention is presented in FIG. 1.
It will be noted that in the embodiments described herein, the anaerobic zone
is
disposed in a different vessel than the aerobic zone and the oxygen-depleted
zone(s).
The wastewater is introduced into the. first tank 1 containing an aerobic zone
2,
microaerophilic zone 3 and anoxic zone 4. The separation of solids from liquid
takes place in two clarification zones 6 and 7, the first zone 6 attached to
the
microaerophilic zone 3, and the second zone 7 located in the second tank 19
disposed downstream of the first tank 1 and separated therefrom.
The biological treatment takes place in the four interactive zones, namely
aerobic
2, microaerophilic 3, anoxic 4 and anaerobic zone 5 that support the
multiplicity
of biological processes required for the removal of organic and inorganic
pollutants of wastewaters, wastewater solids and contaminated groundwater.
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The microaerophilic zone and the anoxic zone are considered, jointly, as an
oxygen-depleted zone.
The four zones are defined by their respective average concentrations of
dissolved oxygen (DO) and oxidation-reduction (redox) potential (ORP). The DO
concentration is highest in the aerobic zone 2 and is usually kept greater
than 2
mg/L while the ORP is greater than +200 my. In the microaerophilic zone 3, the
DO concentration is in the range of 0-1 mg/Land the ORP is in the range of 0
to
+200 my. The DO concentration in the anoxic zone 4 is not detectable by
commercial electrodes which read a steady zero concentration while the ORP
ranges from -100 to +100 my. However, the anoxic zone 4 may contain traces of
oxygen. The anaerobic zone 5 practically does not contain any oxygen with a
steady DO concentration of zero and an ORP of ¨300 to ¨100 my. Conventional
anaerobic zones or chambers may have ORP values below ¨300 my to promote
methanogenesis processes for the formation of methane gas. However, the
formation of these conditions is prevented in the treatment system of the
present
invention in order to prevent the transformation of volatile fatty acids to
methane.
Volatile fatty acids are needed in the treatment system of the present
invention to
serve as carbon source for the denitrification process that takes place in the
microaerophilic zone 3 and anoxic zone 4, and also as a source of carbon and
energy for the phosphorus accumulating organisms during the phosphorus
removal process.
The aerobic zone 2 is exemplified in FIG. 1 as an airlift reactor that uses
compressed air produced by a commercial air compressor 9, to introduce air
bubbles in the aerobic zone 2 and to raise the mixture of liquid (i.e.
wastewater or
contaminated groundwater or landfill leachate) and solids in this zone. The
influent wastewater is introduced into the treatment system at the top of the
aerobic zone 2. In an alternative embodiment of the invention, the wastewater
.can be introduced into the treatment system at the top of the microaerophilic
zone 3 as illustrated in FIG. 2. Aeration by the introduction of compressed
air
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achieves the following objectives: supply of oxygen to the microorganisms for
the
aerobic processes that take place in the aerobic zone 2; mixing of liquid and
solids in the aerobic zone and generation of a homogeneous environment thus
enhancing the mass transfer of oxygen, nutrients and intermediate metabolites
across the cellular membrane; and circulation of mixed liquor between the
aerobic, microaerophilic and anoxic zones. This tends to reduce, compared to
the prior art, the number of pumps and recycle streams in the treatment system
of the present invention. The compressed air is introduced in the treatment
system through air diffusers 10 located at the bottom of the aerobic zone 2.
The
upward flow of liquid in the aerobic zone carries the suspended solids and
flows
towards the adjacent microaerophilic zone 3 through the gates 11 located at
the
top of a cylindrical partition wall 30 separating the aerobic zone 2 from the
microaerophilic zone 3. The mixture of liquid and suspended solids including
dispersed microorganisms, bioflocs or other solid organic, inorganic or inert
material, known as the mixed liquor, flows downward in the microaerophilic
zone
3 and passes through the anoxic zone 4 that is located at the bottom of the
tank
1 and under the aerobic zone 2. The clarification zone 6 is disposed
concentrically relative to the nnicroaerophilic zone 3 and separated therefrom
by a
cylindrical wall 32 leaving channels for the passage of liquid at the bottom
of the
wall.
The action of air compressor 9 creates pressure difference across the air
diffusers 10 and directs the flow of mixed liquor towards the aerobic zone,
thus
creating a continuously circulating liquid between the aerobic,
microaerophilic
and anoxic zones. The flow pattern of mixed liquor in the anoxic zone is
further
controlled by the design of baffles 12 in this zone that are attached to the
dividing
wall 32 between the microaerophilic zone 3 and clarification zone 6. The
baffles
direct the mixed liquor towards the centrally-located aerobic zone 2 while
reducing fluid turbulence at the entrance of the clarification zone 6. The
baffles
12 make an angle ranging from 30 to 90 degrees with a horizontal line. The
resulting circulation exposes the contaminating compounds in the mixed liquor
to
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three different environments, i.e. aerobic, microaerophilic and anoxic during
each
cycle, ensuring a high biodegradation rate of organic material as well as
nitrification and denitrification and phosphorus accumulation by a diverse
group
of microorganisms. Mixed liquor circulates about 200 times between the three
zones before a quantity greater than 90% of its original liquid leaves the
system,
as presented in FIG. 3. In fact, for 99% liquid discharge from the treatment
system, the number of liquid circulations between the three zones varies in
the
narrow range of 375 to 425 turns regardless of the organic load, as presented
in
FIG. 4. The continuous circulation of mixed liquor is meant to achieve the
following:
- Accumulation of microbial biomass in the mixed liquor, producing a high
biomass concentration and a high mean cell residence time (MCRT) or
solids retention time (SRT).
- Repeated exposure of mixed liquor to aerobic, microaerophilic and anoxic
zones thus conditioning the wastewater and improving solids settleability
while controlling the growth of filamentous microorganisms.
- Increase of the specific and volumetric rates of biodegradation of
contaminating substances due to the accumulation of microbial biomass in
the mixed liquor and its increased concentration and adaptation.
- Partial removal of carbonaceous compounds by the facultative anaerobes
in the anoxic and microaerophilic zones, contributing to the increased rate
of contaminant removal while reducing the energy costs associated with
oxygen consumption.
- Low
ratio of feed to the circulating liquid, implying that the feed is rapidly
diluted and mixed with the mixed liquor, thereby ensuring maximum
resistance to shock loads.
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- Den itrification of the produced nitrates and nitrites shortly after
their
formation, preventing the accumulation of these inorganic contaminants
that may exert inhibitory effects on microbial metabolism.
The hydraulic retention time of mixed liquor in the aerobic zone is controlled
by
adjusting the number of gates 11 between the aerobic zone 2 and
microaerophilic zone 3, the size of apertures in the gates, and the input
power of
air compressor 9. Exemplary dimensions of the treatment system and the
corresponding operating conditions are presented in Table 1.
=
The aerobic zone 2 contains solid support material 8 in the form of loose
carriers
or stationary objects of natural or artificial origin to support the
attachment and
growth of microbial biomass and the formation of microbial biofilm. The
biofilm
retains a relatively high biomass concentration of various trophic groups and
ensures the growth and proliferation of slow-growing microorganisms, notably
the
autotrophic microorganisms that carry out the nitrification process and are
essential for the biological removal of nitrogen. The retention of nitrifying
microorganisms in the treatment system has posed a challenge to many
wastewater treatment systems, especially those that only use suspended-growth
microorganisms. The solid support material 8 should preferably have a non-
clogging nature and should not unduly affect the liquid flow pattern. In fact,
the
presence of support material may increase the availability of oxygen to
microorganisms by delaying the rise of oxygen through the system. The support
material is preferably placed at the lower half section of the aerobic zone
near the
air diffusers where the dissolved oxygen concentration is at its highest
value.
This will support the optimum growth and proliferation of nitrifying bacteria
that
are sensitive to the concentration of dissolved oxygen, ensuring an efficient
nitrification. The removal of ammonium ions that results from the
nitrification
process enhances the settleability of solids since ammonium is a monovalent
cation and its presence deteriorates the settling properties of biosolids.
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The design of the aerobic ,zone in the present invention that includes the
presence of both suspended-growth and attached-growth microorganisms is
expected to result in a higher solid retention time and sludge age in the
treatment
system compared to the prior art. In addition, the retention of nitrifying
bacteria
implies that the wastewater can be treated stably during the winter season
when
the activity of nitrifying bacteria is lower. Therefore, the treatment system
of the
present invention offers the benefits of both suspended-growth as well as
attaChed-growth microbial systems. They include homogeneity of microbial
environments in each zone, increased mass transfer. rates of oxygen, nutrients
and metabolic products across the cellular membrane, accumulation of a diverse
population of microorganisms at high concentrations, low production of sludge,
increased microbial adaptation, as well as increased specific and volumetric
biodegradation rates. Examples of the support material include, but are not
confined to, rocks, wood chips, plastic material, and pumice.
The aerobic biological processes for the biodegradation of contaminants take
place in the aerobic zone. These processes include the hydrolysis of high-
molecular weight, long-chain organic compounds and their conversion into low-
molecular weight compounds, followed by the degradation of low-molecular
weight compounds by aerobic biological processes and their subsequent
transformation into mostly carbon dioxide and water. The heterotrophic
microorganisms degrade the carbonaceous compounds while the autotrophic
microorganisms carry out the nitrification process for the conversion of
ammonia
nitrogen to nitrate. The transformation of nitrate to molecular nitrogen for
the
complete removal of nitrogenous compounds takes place in the microaerophilic
and anoxic zones. Dissolved oxygen concentration diminishes within the length
of the microaerophilic zone 3 and assumes values near zero at the bottom of
this
zone. The establishment of low dissolved oxygen concentrations in the
microaerophilic zone 3 may be enhanced by placing solid support material in
the
form of loose carriers or stationary objects of natural or artificial origin
in the
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entire volume or a part of this zone to support the formation of microbial
biofilm.
This arrangement is an alternative version of the present invention as
illustrated
in FIG. 5. The support material 40 should be non-clogging and should not
unduly
affect liquid circulation. The concentration of dissolved oxygen diminishes
within '
the thickness of the biofilm layer which promotes denitrification.
The presence of immobilized biomass aids a rapid recovery of treatment in case
of toxic shocks or biological upset and impedes the wash out of microbial
cells.
The accumulated biomass typically contains a mixed population of
microorganisms, facilitating the simultaneous removal of carbonaceous and
nitrogenous compounds. An additional advantage of biomass accumulation is an
increase of high mean cell residence time (MCRT), also referred to as the
solid
retention time (SRT) that enhances microbial adaptation to the components in
wastewater, thus improving their biodegradation ability and increasing the
rate of
biodegradation. Adapted microbial cells also have a higher tolerance to the
potential toxins in the wastewater and maintain high degradation capacities
for
the substrate at higher inlet concentrations.
=
It is desirable, according to the present invention, to maintain a high
concentration of microbial biomass at an optimum mean cell residence time
(MCRT). A high mean cell residence time (MCRT) and a high concentration of
microbial biomass are desirable in wastewater treatment systems to ensure high
specific and volumetric biodegradation rates of contaminants. However, the
mean cell residence time should be maintained within an optimum range in order
to prevent the aging of biomass and the associated deterioration of its
physiological activities and settling capacity. In the system of the present
invention, the mean cell residence time (MCRT) can be controlled by adjusting
the output power of air blower that controls the magnitude of the forces
exerted
on the solid particles in the mixed liquor. The adjustment of air blower's
output
power serves to control liquid circulation and enables, through an increased
settling opportunity, the occasional removal of a part of microbial cells,
thus
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preventing the accumulation of old and less efficient cells and ensuring the
maintenance of a high concentration of active microbial biomass in the
circulating
mixed liquor.
When operated at optimum conditions, the system of the present invention
should result in a reduced production of biological solids or sludge compared
to
conventional aerobic treatment technologies. The reduced amount of sludge
produced during the treatment process is due to a low cell growth, sloughed-
off
biofilm, occasional removal of biomass to control its retention time, and
precipitation of high-density flocs to the bottom of the tank wherein the
sludge is
partly digested and stabilized in the anaerobic zone 5 (of the second tank
19).
The remaining sludge can be transferred by pipes or tubes 24 out of the second
tank 19 and may be further concentrated in a sludge concentrator placed
downstream from the second tank 19. The design of the present invention is
thus expected to prevent the generation of excessive biological sludge and
produce a reduced amount of sludge compared to the prior art. In addition, the
design of the invention aims to ensure the retention of a high concentration
of
active biomass in both suspended as well as attached form in the biological
treatment zones.
The solids are separated from the liquid in two clarification zones. The first
zone
6 is adjacent to the microaerophilic zone 3 while the second zone 7 is
physically
separated from the first clarification zone and acts as a back-up ,clarifier
in the
event of biological upset or whenever the quality of effluent emerging from
the
= first tank does not conform to the treatment criteria. While mixed liquor
continuously circulates between the three zones of the system, a fraction of
liquid
freely flows towards the clarification zone 6 that is adjacent to the
microaerophilic
zone 3 and leaves the system in accordance with the laws of continuity since
there is no liquid accumulation in the system. As the liquid flows upward in
the
first clarification zone 6, the solid material flows downward and precipitates
to the
bottom of the anoxic zone 4. The effluent emerging from the first
clarification
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zone 6 will be .assessed by turbidity probes (not illustrated) that are placed
in the
effluent line 14 to determine its turbidity. In the event of a poor quality of
effluent,
it can be directed through line 15 towards the second clarification (solid-
liquid
separation) unit (zone) 7 that is housed in the second tank 19. Here, the
remaining solid material will precipitate to the anaerobic zone 5 while liquid
flows
' towards the exit port of the system 16. The liquid passes through a
filtration unit
17 located at the top of the second tank 19 to retain the fine suspended
solids
and remaining colloidal material, ensuring the emergence of a relatively clear
effluent 18 from the treatment system. The filtration unit contains packing
material such as activated carbon, peat moss and sand. Baffles 20 inside the
solid-liquid separation unit 7 facilitate the separation of solids from the
liquid and
prevent the rise of sludge that is accumulated at the bottom of the tank.
In an alternative embodiment of the invention, a treatment unit may be added
to
the treatment system of the present invention to enhance the removal of
phosphorus from the effluent and for the increased settleability of suspended
solids. The treatment unit may employ chemical coagulation, or. if phosphorus
removal is not an issue, a technique selected from zonation, ultrasound
treatment and pulsed electric fields. In the embodiment illustrated in FIG. 6,
the
effluent leaving the first tank 1 will pass through a chemical coagulation
treatment
unit 50 before entering the second tank 19. This provision aims to ensure
continued treatment at a relatively high efficiency in case of toxic shock or
during
the periods of operation upset. If chemical coagulation system is used, the
chemicals may be for example iron and/or aluminum salts.
The solids accumulated at the bottom of the anoxic zone 4 can be transferred
by
pipes 21 to the anaerobic zone 5 on an intermittent basis, a few times daily.
The
anoxic zone 4 supports the growth and proliferation of facultative anaerobic
bacteria that are involved in the denitrification and phosphorus removal
processes and contribute to the removal of COD and BOD. This zone, typically,
partly digests and solubilizes the high-molecular weight and particulate solid
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material and produces volatile fatty acids (VFAs) by fermentation processes.
These compounds, mainly propionic, butyric and acetic acids are low molecular-
weight fatty acids that are easily degradable and may be used as the source of
carbon by the denitrifying bacteria during the nitrate reduction processes
that
take place in the microaerophilic and anoxic zones. The anoxic zone also
conditions the mixed liquor during each cycle of mixed liquor circulation,
improving bioflocculation capacity of microbial biomass and the subsequent
= settleability of solids. The anaerobic zone further digests and
solubilizes the
accumulated solids and produces additional volatile fatty acids (VFAs) by
fermentation processes. The VFAs produced in the anaerobic zone are
transferred by a pump 22 in a recycle stream 23 to the top of microaerophilic
zone 3 to be mixed with the mixed liquor. This action likely eliminates the
need
for the addition of an external carbon source for the denitrification
processes, a
common practice in conventional wastewater treatment systems, thus avoiding
additional costs. The VFAs produced in the anoxic zone 4 and anaerobic zone 5
are also uptaken by the phosphorus accumulating organisms (PA0s) and they
are further converted to polyhydroxyalkanoates (PHAs) that are stored in the
intracellular spaces of the microorganisms as the source of carbon and energy
for the subsequent phosphorus accumulation process that is carried out in the
aerobic zone. The transfer of PAOs to the aerobic zone takes place via the
recycle conduit 23 that discharges into the circulating mixed liquor at the
top of
the microaerophilic zone 3. The phosphorus accumulated in the intracellular
spaces of microbial biomass will then be discharged from the treatment system
with the waste sludge. Furthermore, the anaerobic zone 5 is typically the
place
for sludge stabilization where the population of disease-causing pathogenic
microorganisms and the organic content of the sludge are reduced.
As an alternative embodiment, the treatment system of the present invention
may
include a sludge pre-treatment unit between the anoxic and anaerobic zones as
presented in FIG. 7. In this embodiment, the biosolids removed from the anoxic
zone will be largely disintegrated in the sludge pre-treatment unit 60 in
order to
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_ _
produce more easily degradable organic matter and to improve their further
degradation and stabilization in the anaerobic zone. The disintegration
techniques include but are not limited to, thermal oxidation, chemical
digestion,
zonation, ultrasound treatment, pulsed electric fields, enzymatic treatment
and
high pressure homogenizer.
In another alternative embodiment of the invention, the treatment system may
include a solid disintegration unit after (downstream) the anaerobic zone as
presented in FIG. 8. In this version, the biosolids removed from the anaerobic
zone will be further disintegrated in the solids disintegration unit 70 in
order to
solubilize all or part of the biosolids and to produce more easily
biodegradable
organic matter. The solubilized organic material produced in the solid
disintegration unit 70 may be entirely sent to a storage tank for further
processing
or a fraction of it may be sent back by a recycle stream 25 to the top of the
microaerophilic zone to be mixed with the mixed liquor. The disintegration
techniques include, but are not limited to, thermal oxidation, chemical
digestion,
zonation, ultrasound treatment, pulsed electric fields, enzymatic treatment
and
high-pressure homogenizer.
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Table 1. Example of dimensions and operating conditions of the treatment plant
Parameter Dimension
Diameter of aerobic zone (m) 1.2
Diameter of microaerophilic 2.0
zone (m)
Overall bioreactor diameter 2.2
(m)
Height of aerobic zone (m) 4
Height of clarification zone (m) 4
Influent flow rate (m3/d) 5.3
28