Note: Descriptions are shown in the official language in which they were submitted.
CA 02913574 2015-12-01
LOW-PRESSURE AERATION TREATMENT OF BIOLOGICAL WASTEWATER
FIELD
[0001] This description relates to a method and apparatus for the
treatment of
wastewater. More particularly, it relates to the use of a low pressure
aeration system to improve
the oxygen transfer efficiency and means to generate this low pressure in a
pressurized aeration
tank.
BACKGROUND
[0002] Wastewater treatment facilities prepare wastewater, such as sewage or
industrial waste,
for return to the water cycle. Treatment facilities generally practice several
steps in treating the
wastewater, such as screening, degritting, sedimentation, floatation,
clarification, filtration,
biological treatment, disinfection and/or sludge stabilization, to name a few.
[0003] Biological treatment is used to remove soluble and colloidal organics
that pass through
the primary treatment of the water. Common biological waste treatment
processes fall into three
major groups: aerobic, anaerobic and biological nutrient removal. Aerobic
wastewater treatment
that maintains a population of microorganisms in suspension and in the
presence of oxygen is
typically called the "activated sludge process". The activated sludge process
involves the
introduction of air or oxygen into a mixture of primary treated sewage or
industrial wastewater
that contain microorganisms such as bacteria and protozoans, to develop a
biological floc
(biomass), thereby reducing the organic content in the wastewater.
[0004] In the activated sludge process, soluble and colloidal organics,
nutrients and other
substances in the wastewater are consumed by the microorganisms, and a
sufficient supply of
oxygen is required by the microorganisms to maintain the required microbial
activity.
Biochemical oxygen demand, or BOD, is the amount of dissolved oxygen needed by
the
microorganisms to break down organic materials present in a given water sample
at certain
temperature over a specific time period. A BOD value for wastewater may be
expressed as the
mg 02 consumed per litre of sample, during 5 days of incubation at 20 C
(BOD5), and is a
measure of the organic loading of the wastewater. Once the wastewater has
received sufficient
CA 02913574 2015-12-01
treatment the solid material, or sludge, is separated from the liquid and
these two components
may be further processed, discarded or recycled back into the treatment
system.
[0005] Thus, biological treatment reduces the organic content in the
wastewater stream.
Oxygen transfer efficiency is critical to this process. The oxygen transfer
efficiency, that is, the
efficiency with which oxygen (02) dissolves into a liquid, is predominantly
dependent on bubble
size, pressure, temperature and contact time. Increasing the pressure of the
system will increase
the oxygen transfer efficiency according to Henry's law, and therefore some
treatment facilities
use pressurized tanks to decrease footprint, as a pressurized tank can achieve
better oxygen
transfer efficiency than a similarly-sized non-pressurized tank.
[0006] An aerobic wastewater treatment facility typically includes: (a) an
aeration tank that
holds the wastewater and microorganisms (the "mixed liquor"), (b) an oxygen
source such as
atmospheric or pressurized air or oxygen and equipment to disperse the oxygen
through the
mixed liquor, and (c) a means for separating the mixed liquor into biomass
(sludge) and the
treated water, after the treatment is completed.
[0007] The most common means for dispersing oxygen into the wastewater in an
aeration tank
is by forcing air through fine or coarse bubble diffusers into the tank using
a blower. The air
bubbles are introduced at the bottom of the aeration tank holding the mixed
liquor and travel
upwards through the mixed liquor because they have a lower density than the
mixed liquor, and a
small percentage of the oxygen is dissolved in or transferred to the liquid.
[0008] Other methods of introducing oxygen into mixed liquor have been
described, including:
a) use of pure oxygen or an oxygen generator to introduce oxygen into mixed
liquor;
b) use of primary and secondary pressure vessels (at 50 ¨ 70 psi) combined
with compressed
air and splash plates/ejectors to atomize air into the mixed liquor. This is
typical of a dissolved
air flotation (DAF) system which is a mechanism used for creating bubbles upon
release to
atmospheric pressure in order to float solids to the top of the water (DAF is
a separation
technology like a clarifier and is not used for aerobic treatment);
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c) use of surface aerators, which is common with pond/lagoon treatment.
Surface aerators
provide agitation at the surface which exposes the water to air;
d) use of jet aerators, which transfers oxygen by simultaneously
introducing high kinetic
energy liquid and air through a series of jet nozzles; and
e) use of vertical shaft aerator that combines mixing and aeration
submerged in a tank. The
shaft draws in air and the mixing energy from the rotating aerator combines to
provide a mixed
tank with oxygen.
[0009] Pure oxygen or oxygen generation present a safety hazard at site
particularly with the
use of compressed pure oxygen. It is not as cost effective as other methods,
nor is it readily
available in certain regions. Further, the oxygen tanks or generators take up
additional space and
the tanks require filling when depleted.
[0010] Use of pressures above 15 psi requires pressure vessel certification
and design. In order
to cost-effectively meet certification requirements, tanks are typically
cylindrical in design.
Compressed air is typically used as the mechanism to pressurize the tanks and
requires a
compressed air system separate from the pressure vessel, to accommodate this.
[0011] CA 2,598,524 entitled Aerating Wastewater for Re-Use, describes a
method and
apparatus that uses recirculation of the mixed liquor between primary and
secondary pressurized
aeration tanks that are pressurized with compressed air, and a splatter plate
in the secondary tank,
to aerate the mixed liquor. The mixed liquor is transferred to a settling tank
or clarifier, and the
solids that settle are recycled back to the aeration tank.
[0012] US 4,369,111 entitled Activated Sludge System, describes a method and
apparatus that
uses a pressurized aeration tank pressurized with compressed air to 4.0-4.5
bar (58-65 psi), and a
series of redans (such as plates) and the compressed air, to aerate the mixed
liquor. The mixed
liquor is transferred to a floatation basin where a drop in pressure takes
place, producing gas
bubbles that enhance the floatation.
[0013] CA 2208847 entitled Method and Apparatus for the Treatment of
Concentrated
Wastewater, describes a pressurized process for the treatment of high-solid
wastewater having
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relatively high BOD and phosphorus concentration that includes anaerobic and
aerobic
treatment.
[0014] CA 1194623 entitled Method and Apparatus for Treating Organic
Wastewater
describes an apparatus that consists of three pressurized vessels linked in
series by piping, and
maintained at equal pressure (up to 35 psi) by means of a common manifold.
Liquid flows from
one vessel to another by gravity. CA 2630328 entitled Liquid Aeration
Apparatus and
Wastewater Treatment Apparatus relates to septic tank systems that uses an
attached growth,
fixed film process, and more particularly to a vessel used therein that has an
air diffuser and a
liquid intake at specific positions on the vessel.
[0015] US 6752926 entitled Method and Apparatus for Treatment of Wastewater
describes a
closed bioreactor into which oxygen is provided by diffusion through a non-
porous hydrophobic
membrane in a recirculation line. The bioreactor is operated as a closed unit
at elevated
pressures. EU 0058225 entitled Pressurized Aeration Tank for Activated-Sludge
Process Sewage
Treatment describes a pressurized aeration tank (42-85 psi) that uses an axial-
flow impeller
pump to mechanically agitate the mixed liquor.
[0016] Package plants are pre-manufactured wastewater treatment
facilities, for use in
areas with a limited number of people and small wastewater flows. They are
commonly used in
small communities or in isolated locations, for example, trailer parks,
hospitals, prisons,
construction camps and remote camps for resource industries (oil & gas,
mining, forestry).
There is a continuing need to reduce their footprint and the costs associated
with building and
operating the plants. Plants intended to be used in remote locations have the
added complications
that monitoring, maintenance and repair are difficult, and they may need to be
operational under
extreme weather conditions. Minimizing footprint can also be a concern for
municipal
wastewater treatment plants. They are often in residential areas with limited
room for expansion.
There is therefore an interest in wastewater treatment systems and methods
which have a smaller
footprint, are cost-effective, and are easy to maintain.
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SUMMARY
[0017] Motivated by the considerations of reducing cost, footprint, energy
consumption and
= maintenance requirements, described herein is a wastewater treatment
method and apparatus that
uses the activated sludge process and an aeration tank that is pressurized a
low pressure. Also
described are low-maintenance and energy-efficient components of the method
and apparatus
that promote aeration of the mixed liquor, and the maintenance of the low
pressure, in the tank.
[0018] In one aspect, described herein is an apparatus for the biological
treatment of
wastewater in an activated sludge process comprising:
a) a primary separator that produces a pretreated wastewater with large
solids removed;
b) a conduit for transporting the pretreated wastewater to a pressurized
aeration tank which
has a headspace pressure of between about 1 and about 10 psi;
c) means for aerating and agitating a mixed liquor in the aeration tank;
and
d) a conduit for transporting the mixed liquor to a secondary separator,
for separating the
mixed liquor into an activated sludge component and a clarified liquor
component.
[0019] The aeration tank may have a rectangular or square base. In some
embodiments the
aeration tank is cuboid.
[0020] In some embodiments the aeration tank is pressurized to the headspace
pressure by a
pump that pumps the pretreated wastewater into the aeration tank.
[0021] In some embodiments the primary separator is a screening tank. In some
embodiments
the secondary separator is a membrane separator, and the apparatus further
comprises a pump
that pumps the mixed liquor to the membrane separator.
[0022] In some embodiments the apparatus further comprises a return path along
which at least
a portion of the activated sludge component is recirculated back to the
aeration tank. The return
path may comprise at least one air eductor that aerates the portion of the
activated sludge
component that is recirculated back to the aeration tank.
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[0023] In some embodiments the means for aerating and agitating a mixed liquor
in the
aeration tank is a diffused air system. The diffused air system may comprise
at least one
multiplier nozzle.
[0024] In another aspect, described herein is a method for the biological
treatment of
wastewater using the activated sludge process, which comprises the steps of:
a) delivering a pretreated wastewater stream into a pressurized aeration
tank that has a
headspace pressure of between about 1 and about 10 psi;
b) aerating and agitating a mixed liquor in the aeration tank; and
c) maintaining the mixed liquor in the pressurized aeration tank for a
period of time
sufficient for microorganisms in the wastewater to consume soluble and
colloidal organics,
nutrients and other substances in the mixed liquor.
[0025] In some embodiments the aeration tank has a rectangular or square base.
In some
embodiments the aeration tank is cuboid.
[0026] The method may further comprise the steps of: (a) generating the
headspace pressure in
the tank by pumping the wastewater stream into the aeration tank, and (b)
maintaining the
headspace pressure at less than about 10 psi with at least one backpressure
regulator.
[0027] The method may further comprising the steps of:
a) after the period of time, transporting the mixed liquor from the
pressurized aeration tank
to a secondary separator;
b) separating the mixed liquor to produce an activated sludge component and
a clarified
liquor effluent;
c) aerating at least a portion of the activated sludge component to
produce aerated activated
sludge; and
[0028] delivering the aerated activated sludge to the pressurized aeration
tank.
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[0029] In some embodiments the portion of the activated sludge is aerated with
at least one air
eductor.
[0030] In another aspect described herein is a method for the biological
treatment of sewage
using the activated sludge process, which comprises the steps of:
a) passing primary raw sewage through a screening tank to produce a
pretreated sewage
stream;
b) pumping the pretreated sewage stream into an aeration tank that is
pressurized to a
pressure of between about 1 and about 10 psi;
c) aerating and agitating a mixed liquor in the aeration tank;
d) discharging the mixed liquor from the aeration tank;
e) separating the discharged mixed liquor into an activated sludge
component and a clarified
liquor component;
0 aerating at least a portion of the activated sludge component, to
form an aerated activated
sludge; and
g) delivering the aerated activated sludge to the aeration tank.
[0031] In one embodiment the step of separating the discharged mixed liquor is
performed by
membrane separation. In some embodiments the step of aerating a portion of the
activated sludge
component is performed by directing the portion of the activated sludge
through at least one air
eductor. In some embodiments the aerated activated sludge is introduced into
the aeration tank
through at least one multiplier nozzle disposed in the aeration tank.
[0032] In another aspect described herein is a method for pressurizing an
aeration tank used in
the biological treatment of wastewater with the activated sludge process to a
pressure of between
about 1 psi and about 10 psi in the headspace, comprising the steps of:
a) pumping the wastewater into the aeration tank to pressurize the tank
to a pressure greater
than about 1 psi, and
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b) maintaining the pressure in the headspace of the aeration tank below
about 10 psi with at
least one backpressure regulator.
[0033] In some embodiments the aeration tank has a rectangular or square base.
In some
embodiments the aeration tank is cuboid.
[0034] In another aspect, described herein is a method of increasing the
dissolved oxygen
content of a mixed liquor in an aeration tank comprising:
a) drawing external air into a pressurized flow stream with air eductors,
to generate an
aerated pressurized flow stream;
b) injecting the aerated pressured flow stream into the mixed liquor
through injection
nozzles, to physically mix the aerated pressurized flow stream with the mixed
liquor; and
c) circulating the aerated pressurized flow stream through the injection
nozzles at a high
rate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] Figure 1 is a drawing of an embodiment of the steps of the wastewater
treatment method
described herein.
[0036] Figure 2 is a drawing of an embodiment of the wastewater treatment
apparatus described
herein.
[0037] Figure 3 is a drawing of an embodiment of the wastewater treatment
method described
herein.
DETAILED DESCRIPTION
[0038] A wastewater treatment plant that has a reduced footprint, is low-
maintenance, and
energy-efficient is desirable. The inventors have designed a wastewater
treatment plant which
uses the activated sludge process, motivated by the considerations of reducing
cost, footprint,
energy consumption and maintenance requirements. The method and apparatus
described herein
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can treat many kinds of wastewater including sewage, liquid agricultural and
industrial waste,
and is particularly suitable for use as a package plant.
[0039] Non-pressurized aeration tanks are the industry standard in wastewater
treatment
facilities, however, compared to high-pressure aeration tanks they have a
significantly larger
footprint and are not ideal for use in treatment plants that need to be small,
portable or that are
limited by expansion space. While it would be desirable to use a high-pressure
aeration tank to
decrease footprint while achieving the same oxygen transfer efficiency, these
types of tanks are
more expensive to manufacture than are non-pressurized tanks, they require
certification, and
they are more onerous to maintain. For remote areas, it is important to
minimize the need to
perform routine and urgent maintenance on a wastewater treatment plant.
[0040] High-pressure aeration tanks used in the activated sludge process are
operated at a
pressure of greater than about 15 psi in the headspace. These tanks require
the use of ancillary
devices to maintain these high pressures, such as compressed air/oxygen tanks,
compressors or
pressure boosting pumps, which also add to cost and increase maintenance
burden. Therefore, it
is desirable to avoid using high-pressure aeration tanks if reducing cost and
maintenance burden
are the objectives.
[0041] Described herein is a wastewater treatment method and apparatus that is
pressurized to
improve oxygen transfer efficiency (as compared to non-pressurized/atmospheric
tanks), but that
does not use a high-pressure aeration tank. Tanks intended for use at
pressures above about 15
psi need to be certified. The tank used in the presently-described wastewater
treatment method
and apparatus is designed for use at pressures of below about 15 psi, and is
used in the methods
described at above atmospheric pressure and below a pressure of about 10 psi
in the headspace.
[0042] High-pressure aeration tanks are typically cylindrical, because tanks
with curved sides
have greater structural stability than tanks with a planar sides, such as
cuboid tanks. A tank with
= 25 planar sides would need to be reinforced to meet certification
requirements, thus a cylindrical
tank is less expensive to manufacture for high-pressure applications. Because
the low-pressure
tank used herein does not need to be certified, a cuboid tank with a
rectangular or square base,
which has several advantages over a cylindrical tank, may be used. For
essentially equivalent
footprints, a cuboid tank can hold a greater volume of wastewater, and the
surface area of the
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wastewater in the tank is greater, therefore an equalization or holding tank
is not needed
(reducing footprint some more). Circular tanks may be used in the methods and
apparatus herein
if there is existing tankage that can be used for aeration, if space is not a
problem, or if other site
conditions require it.
[0043] Another benefit of using a low-pressure aeration tank is that
compressed air tanks or
compressors do not need to be used to pressurize the tank, saving on cost and
maintenance.
However, in the methods and apparatus described herein it is still necessary
to pressurize the
tank to above atmospheric pressure and up to a pressure of about 10 psi.
[0044] The aeration step in the activated sludge processes, whether in a
pressurized or non-
pressurized tank, is commonly one of the most energy-demanding processes of
the entire system,
consuming as much as 50 to 90% of the total energy costs of a typical
treatment facility.
Reducing energy consumption during aeration is usually the best initial step
to minimize energy
costs. Further, using a low-maintenance means of aeration is desirable in
wastewater treatment
plants that are intended for remote use.
[0045] Accordingly, the inventors have determined that there is a lower
pressure that can be
used in an aeration tank in the activated sludge process which provides a
significant
improvement in oxygen transfer efficiency over a non-pressurized tank and a
meaningful
reduction in footprint size. The inventors have further identified means for
pressurizing and
aerating this low-pressure tank that are cost-effective, low maintenance and
that have a small
footprint. The wastewater treatment method and apparatus described herein are
therefore ideal
for package plants (particularly those used in remote locations), or for
retrofitting aeration tanks
in municipal or industrial plants to increase capacity and efficiency without
requiring additional
space and/or construction of new tanks.
[0046] METHOD
[0047] Fig. 1 shows a schematic of an embodiment of the wastewater treatment
method
described herein. A wastewater stream 12 is subjected to a primary separation
step to provide a
pre-treated wastewater stream 16 that is delivered to an aeration tank, where
it undergoes
biological treatment at a low pressure. A pump may be used to deliver the pre-
treated wastewater
CA 02913574 2015-12-01
stream to the aeration tank. The mixed liquor is agitated and aerated in the
aeration tank. Mixed
liquor 22 is subjected to a secondary separation step to separate it into a
clarified liquor effluent
component 29 containing suspended solids (biosolids) and an activated sludge
28 component. A
pump may be used to transfer the mixed liquor to the secondary separator.
[0048] All, or a portion of, the activated sludge 28 may then be recirculated
back to the
biological treatment step, and the recirculated or return activated sludge may
be aerated along the
recirculation path to form an aerated activated sludge 32 that is delivered to
the aeration tank to
aerate and agitate the mixed liquor therein. The clarified liquor effluent
component 29, and any
waste activated sludge 28 may then be further treated and/or released to the
environment.
[0049] "Wastewater", as used herein refers to water that is no longer needed
or suitable for its
n-lost recent use, and that must therefore be treated before release back to
the water cycle, or
before re-use. Non-limiting examples of wastewater as contemplated herein are
sewage (e.g.,
household waste), liquid agricultural and industrial waste.
[0050] Primary separation is performed to remove large particulate matter
which could damage
components of the treatment facility, if not removed. Primary separation
methods that may be
used include, without limitation, screening, grit removal, sedimentation and
floatation. Preferred
is the use of screening for the primary separation step.
[0051] Biological treatment is performed in a pressurized aeration tank at a -
low pressure",
which as used herein means a pressure of between about 1 psi and about 10 psi,
preferably
between about 3 psi and about 9 psi, and most preferably between about 5 psi
and about 7 psi.
[0052] The pressure in the aeration tank may be generated at least in part, by
the action of a
pump, which pumps the pretreated wastewater 16 into the aeration tank, and/or
by the use of air
eductors, compressed air or oxygen, compressors, pressure boosting pumps,
blowers and the like.
The pressure in the tank may be kept below a maximum predetermined pressure by
at least one
backpressure regulator, for example a pressure control valve, a pressure
vacuum relief valve or a
pressure reducing valve.
[0053] The secondary separation step separates the mixed liquor 22 from the
aeration tank into
an activated sludge component 28 and a clarified liquor effluent 29. Methods
that may be used to
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perform this separation include, without limitation, sedimentation,
floatation, filtration and
membrane separation. Preferred is membrane separation.
[0054] A pump or gravity flow may be used to transfer the mixed liquor from
the biological
treatment step to the secondary separation step. A pump is required for
external membranes but
not necessarily for submerged membranes, in membrane separation.
[0055] The activated sludge 28 component may be divided into two portions, a
portion that is
recirculated back to the aeration tank for biological treatment (return
activated sludge) and a
portion that may be further treated and/or released the environment (waste
activated sludge). The
portion that is recirculated back to the biological treatment step may be
aerated before it enters the
aeration tank. The consistency of activated sludge after secondary separation
may be between
about 0.5% to about 1.2% solids, depending on separation technology. In some
embodiments, if a
membrane is used, it is in the 0.8 to 1.2 % range. Methods of aerating the
return activated sludge
en route to the aeration tank include, without limitation, the use of air
eductors or jet aeration.
Alternatively or in addition, the mixed liquor in the aeration tank may be
aerated with diffusers,
aspirating impeller mixers, jet aeration pumps or from a compressed air/oxygen
gas source.
[0056] In a preferred embodiment of the method, the return activated sludge is
circulated back
to the aeration tank through air eductors, and membrane separation is used for
the secondary
separation. In this preferred embodiment therefore, pressure from the membrane
filtration step is
available for the air eductors. For large plants, aspirating impeller mixers
or blower/diffusers
may be preferred.
[0057] APPARATUS/SYSTEM-
[0058] Fig. 2 shows a schematic of an embodiment of the wastewater treatment
apparatus/system described herein. In the apparatus/system, wastewater 12 is
delivered to a
primary separator 14, which removes large suspended solids and particulate
matter. The pre-
treated wastewater stream 16 from the primary separator 14 is delivered via a
conduit to a
pressurized aeration tank 20, where it undergoes biological treatment. The
pressurized aeration
tank 20 is operated under low pressure, which pressure is maintained between
about 1 psi and
about 10 psi.
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[0059] After biological treatment the mixed liquor 22 in the pressurized
aeration tank is
directed along a conduit to a secondary separator 26 which separates this
mixture into an
activated sludge component 28 and a clarified liquor effluent 29. All, or a
portion of, the
activated sludge component is recirculated to the pressurized aeration tank
20. This return
activated sludge may be aerated before it is reintroduced into the aeration
tank. The return
activated sludge is delivered to the aeration tank 20 via a conduit, where it
is added to the mixed
liquor and aerated therewith.
[0060] Primary Separator
[0061] The primary separator 14 may be a screenings tank, a grit removal
facility, a
sedimentation tank, a floatation tank, or another type of primary separator
known to persons of
skill in the art. More than one primary separator may be used in the apparatus
described herein.
[0062] Aeration Tank
[0063] Aeration tank 20 is a sealed pressure reservoir that is operated under
low pressure to
increase oxygen transfer efficiency, as compared to a non-pressurized tank.
Since the aeration
tank is pressured to below 15 psi, it does not need to be a certified pressure
vessel.
[0064] Typically, aeration tanks used in the activated sludge process are
operated at
atmospheric pressure. The pressurized aeration tank 20 used in the methods and
apparatus
described herein operates at low pressure, between about 1 psi and 10 psi,
preferably between
about 3 psi and about 9 psi, most preferably between about 5 psi and about 7
psi. Operating at
this slightly pressurized condition provides improved oxygen transfer
efficiency, which increases
the dissolved oxygen level. "Oxygen transfer efficiency" or -standard oxygen
transfer rate"
refers to the efficiency with which oxygen (02) dissolves into the mixed
liquor. As is known,
oxygen transfer efficiency increases as pressure increases. "Dissolved oxygen"
("DO" in mg
02/L or ppm) is the amount of 02 dissolved in the mixed liquor.
[0065] Aeration tank 20 may be of any shape. Contemplated shapes for the base
of the tank
include circular, oval, rectangular and square. The tank may be without
limitation, ovoid,
cylindrical or cuboid shaped. In a preferred embodiment the tank is cuboid
shaped.
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[0066] Preferred for use herein is a tank 20 with a square base, as a tank
having a square base
with width W can hold a greater volume of liquid than a cylindrical tank of
diameter D, when W
= D. Further, the top of the liquid inside a square tank has a greater surface
area as compared to
the liquid in a cylindrical tank (when W = D). A significant amount of foam
accumulates at the
top of the pressurized aeration tank, and it is important to leave a headspace
above the liquid to
accommodate this foam. Commonly, sewage treatment plants that use the
activated sludge
process include a separate equalization tank which acts as a holding tank to
receive the raw flow
and hold it until there is enough head space in the aeration tank to add more
liquid. The gained
surface area from the square aeration tank over the cylindrical tank may
obviate the need for a
separate equalization tank.
[0067] In other embodiments of the apparatus, pressurized aeration tank 20 may
not have a
square base. For example, existing aeration basins operated at atmospheric
pressure may be
circular, oblong, or rectangular tanks constructed of reinforced concrete. The
apparatus and
system described herein contemplate retrofitting of an existing aeration basin
or tank, so that it
can be pressurized and operated at a low pressure, to increase oxygen transfer
efficiency. The
means of retrofitting an existing tank so that it can operate at low pressure
depends on type of
water/wastewater treatment system in which the tank is operated. As currently
used
water/wastewater treatment facilities commonly use tanks which operate at
atmospheric
pressure, in many embodiments it is this type of tank that will be
retrofitted. This may be
accomplished, for example, by adding a gasketed cover to the tank, and
otherwise sealing off
areas where air/gas can escape to the atmosphere. The cover can be made, for
example, of
aluminum, steel or fiberglass. Tank structure should be evaluated,
particularly if a steel tank is
being retrofitted, to assess whether the tank can withstand the low pressure
that will be applied.
In some embodiments the walls of the tank may need to be reinforced.
[0068] The aeration tank may be pressurized to a pressure of between about 1
and about 10 psi
by one or more pumps, air eductors, compressed air or oxygen, compressors,
pressure boosting
pumps, blowers and the like. A backpressure regulator 19 may be used to
release pressure
should the pressure exceed a maximum predetermined limit. Regulator 19 may be,
for example
a pressure control valve which functions as a back-up pressure control valve,
or a pressure
vacuum relief valve, which may be vented to another tank and/or to atmosphere.
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[0069] Preferred for use herein to pressurize the aeration tank is a pump that
transfers liquid
from the primary separator, in combination with pressure from air eductors
that receive fluid
under pressure from a membrane separator.
[0070] Aeration/Agitation of Tank
[0071] Aeration and agitation of the pressurized aeration tank 20 may be
accomplished by
using a diffused air system, compressed air/oxygen, diffusers, aspirating
impeller mixers, jet
aeration pumps, or other methods known to those of skill in the art to aerate
the mixed liquor.
Existing water treatment facilities that are retrofitted with a low-pressure
aeration tank may
continue to use the previously established aeration methods, provided that
pressure is regulated
appropriately.
[0072] Preferred for use herein for aeration and agitation is an eductor
system that draws air
into a pressurized flow stream in combination with a plurality of multiplier
nozzles located near
the bottom of the tank. In some embodiments, a conventional blower may be used
to introduce
atmospheric air into the bottom of the pressurized aeration tank through
diffusers. Conventional
blowers typically have a maximum operating pressure of about 13.5 psi and
therefore cannot be
used in tanks that operate at a high pressure. However, they may be used in
some embodiments
of the methods and apparatus described herein, given that the aeration tank is
operated at a
pressure less than about 10 psi.
[0073] Secondary Separator
[0074] After biological treatment, the mixed liquor is separated into an
activated sludge
component and a clarified liquor component. The secondary separator 26 may be
a membrane
separator, a settling tank, a clarifier, an air flotation separator or another
type of secondary
separator known to persons of skill in the art. Preferred for use in the
methods and apparatus
herein is a membrane separator.
[0075] In embodiments not necessarily intended for use in a package plant,
membrane
separation may not be used, or may be used in conjunction with other treatment
steps to clarify
the effluent from the mixed liquor. For example, existing activated sludge
wastewater treatment
CA 02913574 2015-12-01
facilities that are retrofitted to have a low-pressure aeration tank may
continue to use clarification
methods already in place to separate the activated sludge from the clarified
liquor.
[0076] After separation, the clarified liquor component 29 may be further
treated or disposed
of. Al!, or a portion, of the recovered activated sludge component 28 may be
recirculated back to
the pressurized aeration tank, the remainder being sent for further treatment
or disposal.
[0077] The return activated sludge may be aerated by an aerator 30, on route
back to the
aeration tank, to create an aerated pressurized flow stream. In a preferred
embodiment the aerator
is one or more air eductors, which aerate the return activated sludge.
Existing wastewater
treatment facilities that are retrofitted to have a low-pressure aeration tank
may continue to use
aerators that are already in place to aerate the return activated sludge. This
type of system could
have a reduced capacity blower that is supplemented with additional air from
an eductor or other
means of air injection.
[0078] The return aerated pressurized flow stream may be circulated through
injection nozzles
at a high rate, before it enters the aeration tank. This high rate may be more
than 4 times the rate
of transfer flow from the screening tank to the pressurized aeration tank, and
in some
embodiments 4-10 times higher, 4-7 times higher or 4-5 times higher than the
transfer flow rate.
[0079] Having thus described the basic apparatus and method herein, specific
embodiments
will now be described. A specific embodiment used as a sewage treatment
package plant is
shown in the accompanying Fig. 3. This embodiment is designed to be mounted on
a skid and
transported to the site of usage. It is therefore designed to have a small
footprint, to be low-
maintenance, and to avoid the use of hazardous components such as compressed
air/oxygen.
[0080] In the embodiment shown in Fig. 3, raw sewage 12 is delivered to a
screenings tank 14,
which removes large suspended solids and particulate matter such as rags,
paper, plastics, metals
and the like. The pre-treated sewage stream 16 exiting from the screenings
tank is delivered via
a conduit to a pressurized aeration tank 20, where it undergoes biological
treatment. The
pressurized aeration tank 20 is operated under low pressure, which pressure is
generated, at least
in part, by the action of at least one pump 18.
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CA 02913574 2015-12-01
[0081] Pressurized aeration tank 20 shown in Fig. 3 has a square base, which
is a preferred
embodiment as it can hold a greater volume of liquid than can a cylindrical
tank, for essentially
the same footprint. The gained surface area from using a square aeration tank
over the cylindrical
tank enables combining of the volumes for equalization and treatment in one
tank, again
reducing footprint.
[0082] In the embodiment shown in Fig. 3, at least one pump 18 pumps the pre-
treated sewage
16 into the aeration tank. The pump is selected to be able to provide
sufficient pumping pressure
to pressurize the aeration tank to the predetermined operating pressure range
for the aeration
tank. Air eductors 30 act as a secondary means for pressurizing or maintaining
pressure in the
aeration tank, while also drawing in oxygen from the atmospheric air. At least
one backpressure
regulator 19 releases pressure from the pressurized aeration tank should the
pressure exceed a
maximum predetermined limit.
[0083] Aeration and agitation of the mixed liquor 22 in the pressurized
aeration tank 20 is
accomplished by using a diffused air system. Air is introduced through porous
diffusers or
through air nozzles near the bottom of the tank. Air diffusers may be of
different types, including
without limitation bubble diffusers, tubular diffusers, jet aerators,
aspirator devices and U-tubes.
[0084] In the embodiment shown in Fig. 3, a plurality of multiplier nozzles 34
located near the
bottom of the aeration tank increase overall mixing efficiency by creating a
vortex as the aerated
liquid passes through the nozzle. In one embodiment, the nozzles are
constructed in a conical
shape that is designed to provide dynamic mixing under pressure which yields
greater mass
transfer.
[0085] A conventional blower may be used to introduce atmospheric air into the
bottom of the
pressurized aeration tank of Fig. 3, through a diffuser. Conventional blowers
typically have a
maximum operating pressure of about 13.5 psi. If the aeration tank is operated
at a pressure of
about 9.5 psi, adding about 2-3 psi for the weight of the liquid on top of the
diffusers results in a
total psi at the diffusers of about 11.5 ¨ 12.5 psi, which is below the upper
limit for operability of
the blower.
17
CA 02913574 2015-12-01
[0086] In this embodiment a bleed branch with flow meter may be provided in
the discharge
air line of the blower to control the amount of air that is pumped to the
pressurized aeration tank.
The high temperature air from the blower may be routed through the membrane
separation tank
(see below), in a finned pipe. Heat can therefore be transferred to the
treated water, which will
enhance the screen and membrane cleaning. The cooled air may then be delivered
to the diffuser
from the inlet side of the aeration tank, as more air is needed at the inlet
side of the tank.
[0087] After biological treatment the mixed liquor 22 in the pressurized
aeration tank is
pumped along a conduit via pump 24 to a membrane filtration separator 26,
which separates this
mixture into an activated sludge component 28 and a clarified liquor effluent
29. Membrane
filtration has a smaller footprint than other means of clarifying the mixed
liquor, such as for
example, a settling tank, clarifier or air flotation separator.
[0088] The membrane is a semi-permeable and selective barrier that separates
the water in the
rnixed liquor from the activated sludge in the mixed liquor. Several different
membrane
configurations may be used, for example plate-and-frame, spiral wound, tubular
and hollow-
fiber. Preferably the membrane is a self-cleaning membrane. In one embodiment,
the membrane
is a flat sheet membrane system where the filtration unit is external from any
tankage.
[0089] In the embodiment of Fig. 3, at least a portion of the activated sludge
component 28
recovered from the membrane filtration step is recirculated (returned) back to
the pressurized
aeration tank 20. The recirculation flow rate used depends on the pressure in
other parts of the
system and the specific requirements of the membrane system. For example, a
minimum fluid
pressure is needed to force the liquid through the membrane in the membrane
separator, and a
minimum fluid pressure and flow is needed in the piping to ensure that
sufficient 02 is drawn
into the eductors 30 (see below) to aerate the return activated sludge. The
flow rate of pump 24 is
therefore determined by the flow rate needed to meet these specified
pressures.
[0090] Along the recirculation path in the embodiment of Fig. 3, the return
activated sludge
passes through at least one air eductor 30, where it is aerated. Aerated
return activated sludge 32
emitting from the eductor 30 is delivered to the aeration tank 20 via a
conduit and is introduced
into the tank though the distribution pipe located at the bottom of the tank.
As noted above, the
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CA 02913574 2015-12-01
distribution pipe has a plurality of multiplier nozzles 34 attached thereto,
through which the
aerated activated sludge passes before entering the tank and mixing with its
contents.
[0091] The at least one air eductor of the embodiment of Fig. 3 uses a Venturi
effect to draw
atmospheric air into the pressurized flow stream of the return activated
sludge 28. In essence, a
constriction in the eductor increases the velocity of the activated sludge 28
as it passes
therethrough, decreasing pressure and creating a vacuum that draws air into
this fluid stream.
The eductor(s) is therefore a passive means of aerating the return activated
sludge, bootstrapping
off of the increased pressure of the fluid that is required by the membrane
separation step. The
use of eductors enables quieter operation of the plant, as no blowers or
compressors are used.
[0092] Air eductors have a high recirculation rate, and provided that the
motive fluid (in this
case the return activated sludge 28) has a sufficient pressure, they can
inject aerated liquid into a
pressurized vessel. In the embodiment of the apparatus shown in Figure 3, the
clarified liquor
being recirculated to the tank flows through the eductors at between about 15
to about 25 psi and
is therefore injected into the pressurized aeration tank at a pressure of
about 5 to about 7 psi.
[0093] An important feature of the wastewater treatment plant described in the
embodiment
shown in Fig. 3 is that the aeration tank is pressurized by the pumping of
fluid into the tank. As
discussed above, a first source of pressure in this embodiment is pump 18,
which pumps the fluid
into the aeration tank. A second source of pressure in the aeration tank is
from the air eductor(s)
30. A pump 24 produces flow of the fluid stream necessary for the Venturi
action in the
educator that draws air into the stream. This stream is fed into the aeration
tank and can
contribute to the pressurization. The use of pumping and low pressure air,
only, to pressurize the
system is an important feature of this embodiment. As noted above, pressure in
the aeration tank
is limited to less than 10 psi by using backpressure regulators.
[0094] Another important feature of the wastewater treatment apparatus shown
in Fig. 3 is that
it reduces bubble size and increases contact time by replacing blowers and
diffusers with
eductors and injection/multiplier nozzles. The eductors draw air into a
pressurized flow stream
and the injection nozzles then further improve oxygen transfer efficiency by
physically mixing
air-rich activated sludge with the mixed liquor in the pressurized aeration
tank. The combination
of the eductors, injection nozzles and a high recirculation rate through the
injection system leads
19
CA 02913574 2015-12-01
to a higher pressure which increases oxygen transfer efficiency over what can
be obtained with
conventional blowers and diffusers, thereby increasing the dissolved oxygen
available to the
microbes.
[0095] The embodiment of the wastewater treatment plant shown in Fig. 3,
therefore, has a
number of potential advantages over other systems including that it has a
small footprint, it is
simple in design, energy consumption is low, it avoids using hazardous
components such as
compressed air/oxygen and high pressure, and it is low-maintenance.
EXAMPLE.
The following is a representative example of and embodiment of the low-
pressure aeration
wastewater treatment system described herein.
[0096] Raw sewage is fed into a screening tank through a 0.6mm screen that has
a capacity of
34 m3/hr. The tank has a volume of 0.8 m3 and comes with a gravity drain. The
screened
sewage from this tank is pumped by three inlet transfer pumps sized for a flow
of 17m3/hr with a
discharge pressure of 16 psi. The screened sewage is transferred from the
screening tank to the
aeration tank which has a volume of 56 m3.
[0097] Air is introduced into the bottom of the aeration tank through four
multiplier nozzles
(Mazzei N45-DT mixing nozzles), which receive aerated liquid from two upstream
eductors. The
aeration tank is designed to be operated at up to 5-7 psi in the headspace
when the water level is
at a maximum and a level of dissolved oxygen is between about 1.5 ¨ 2.5 mg/l.
[0098] Two backpressure regulators are used to maintain the pressure of the
tank to between 5-
psi, and a maximum of 9.5 psi. A motorized pressure control valve (AT Controls
OC/OS series
butterfly valve) is equipped as a back-up pressure control valve. A pressure
vacuum relief valve
Enardo Model 860 is equipped on the tank for safety protection. It is vented
to screen and then to
the atmosphere through the venting line of the screen.
[0099] The hydraulic residence time (HRT) in the aeration tank is 4.2 hours,
with a 5 hour
equalization time. The aeration tank can also be used to dampen peak flows and
provide a means
of diluting and distributing batch discharges of high-strength contaminant in
the water.
CA 02913574 2015-12-01
[00100] The mixed liquor from the aeration tank is pumped by a variable
frequency drive pump
(Gorman-Rupp Model T4A3) to a membrane separator at a rate of 160 m3/hr and
discharge
pressure rating of 72 psi. The pressure of the fluid entering the membrane
separator is about 70
psi, and exiting about 26 psi. The membrane separator is a Pleiade 2000 series
membrane system
manufactured by Orelis Environment. The system entails 75.6 m2 of
ultrafiltration membranes
that are flat sheet and stacked between plates. The unit is an external
membrane system that is
not submerged in a tank containing fluid.
[00101] A portion of the liquid that passes through the membrane separator is
recirculated back
to the pressurized aeration tank. Generally, about 15 to 25 times more (on a
volume basis) of the
activated sludge is recirculated back to the aeration tank than is moved
forward in the process for
further treatment. The recirculated flow has a pressure of about 24 psi, and
is fed into two 4"
eductors (Mazzei Model 4091 Air Injector) with suction check valves, inlet
filter and discharge
distribution nozzles. The air and liquid are mixed, and this mixture then
flows through the outlet
pipe section of the eductors with a pressure of about 10 psi, and is
discharged back into aeration
tank through the multiplier nozzles at a pressure of about 6 psi.
[00102] While the activated sludge wastewater treatment method and apparatus
have been
described in conjunction with the disclosed embodiments which are set forth in
detail, it should
be understood that this is by illustration only and the method and apparatus
are not intended to be
limited to these embodiments. On the contrary, this disclosure is intended to
cover alternatives,
modifications, and equivalents which will become apparent to those skilled in
the art in view of
this disclosure.
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