U.S. patent application number 13/690956 was filed with the patent office on 2013-06-06 for method and system for ozone vent gas reuse in wastewater treatment.
The applicant listed for this patent is Malcolm E. Fabiyi, Randall B. Marx, Richard A. Novak. Invention is credited to Malcolm E. Fabiyi, Randall B. Marx, Richard A. Novak.
Application Number | 20130140232 13/690956 |
Document ID | / |
Family ID | 47324441 |
Filed Date | 2013-06-06 |
United States Patent
Application |
20130140232 |
Kind Code |
A1 |
Fabiyi; Malcolm E. ; et
al. |
June 6, 2013 |
METHOD AND SYSTEM FOR OZONE VENT GAS REUSE IN WASTEWATER
TREATMENT
Abstract
A system and method for ozone vent gas reuse is provided. The
disclosed system and method involve controlling or conditioning the
ozone vent gas stream or degassing unit vent gas stream and
directing the stream to a mechanically agitating contactor in an
aerobic section of the wastewater treatment system. The oxygen
content of the vent gas stream is controlled so as to ensure
sufficient oxygenation to the aerobic section of the wastewater
treatment system. Control may be effected by adjusting the oxygen
content of the vent gas stream in response to sensor or measurement
inputs characterizing the gas contents of the vent gas stream or
the dissolved oxygen levels. The volumetric flow of the vent gas
stream to the aerobic section may also be controlled by adjusting
the rotational speed of the mechanically agitating contactor in an
aerobic section of the wastewater treatment system.
Inventors: |
Fabiyi; Malcolm E.;
(Chicago, IL) ; Novak; Richard A.; (Naperville,
IL) ; Marx; Randall B.; (Glen Ellyn, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Fabiyi; Malcolm E.
Novak; Richard A.
Marx; Randall B. |
Chicago
Naperville
Glen Ellyn |
IL
IL
IL |
US
US
US |
|
|
Family ID: |
47324441 |
Appl. No.: |
13/690956 |
Filed: |
November 30, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61565941 |
Dec 1, 2011 |
|
|
|
Current U.S.
Class: |
210/604 ;
210/739; 210/96.1 |
Current CPC
Class: |
C02F 2209/22 20130101;
C02F 2303/04 20130101; C02F 3/26 20130101; C02F 2209/245 20130101;
C02F 3/1221 20130101; C02F 2209/03 20130101; C02F 2209/23 20130101;
C02F 1/78 20130101; Y02W 10/10 20150501; C02F 1/20 20130101; C02F
2209/003 20130101; C02F 2209/02 20130101; C02F 2209/38 20130101;
C02F 2301/046 20130101; C02F 3/14 20130101; C02F 2209/40 20130101;
Y02W 10/15 20150501; C02F 2303/18 20130101; C02F 2209/225 20130101;
C02F 2209/005 20130101; C02F 3/006 20130101; C02F 3/301 20130101;
C02F 2301/043 20130101 |
Class at
Publication: |
210/604 ;
210/739; 210/96.1 |
International
Class: |
C02F 1/78 20060101
C02F001/78 |
Claims
1. A method for supplying supplemental oxygen to a wastewater
treatment system comprising the steps of: directing an oxygen
containing feed stream to an ozone generator; operating the ozone
generator to produce an ozone containing gas stream; directing the
ozone containing gas stream to an ozone treatment system within the
wastewater treatment system to produce an ozone treated effluent
and; and an ozone vent gas stream; and directing the ozone vent gas
stream to a mechanically agitating contactor in an aerobic section
of the wastewater treatment system; wherein the oxygen content of
the ozone vent gas stream is controlled so as to ensure sufficient
oxygenation to the aerobic section of the wastewater treatment
system by adjusting the oxygen content of the ozone vent gas stream
in response to sensor or measurement inputs characterizing the gas
contents of the ozone vent gas stream or the dissolved oxygen level
in the aerobic section of the wastewater treatment section.
2. The method of claim 1 further comprising the step of directing
the ozone vent gas stream to an ozone destruct system configured to
destroy any ozone contained in the ozone vent gas stream.
3. The method of claim 1 wherein the step of directing the ozone
vent gas stream to the aerobic section of the wastewater treatment
system further comprises mixing or combining the ozone vent gas
stream with a make-up oxygen containing stream and directing the
combined stream to the aerobic section of the wastewater treatment
system.
4. The method of claim 1 wherein the step of adjusting the oxygen
content of the ozone vent gas stream further comprises adjusting
the flow of the oxygen containing feed stream to the ozone
generator.
5. The method of claim 1 wherein the step of adjusting the oxygen
content of the ozone vent gas stream further comprises adjusting
the power supplied to the ozone generator.
6. The method of claim 1 wherein the step of adjusting the oxygen
content of the ozone vent gas stream further comprises adjusting
the flow of a make-up oxygen containing stream to the ozone vent
gas stream.
7. The method of claim 1 wherein the volumetric flow of the ozone
vent gas stream is controlled so as to ensure sufficient
oxygenation to the aerobic section of the wastewater treatment
system by adjusting the rotational speed of the mechanically
agitating contactor in an aerobic section of the wastewater
treatment system.
8. A ozone vent gas reuse system for a wastewater treatment plant
comprising: an oxygen containing feed stream; an ozone generator
configured to receive the oxygen containing feed stream and produce
an ozone containing gas stream an ozone contactor for contacting an
effluent with the ozone containing gas stream to produce an ozone
treated effluent and an ozone vent gas stream; an ozone destruct
system configured to receive the ozone vent gas stream and destroy
any ozone contained in the ozone vent gas stream; a supplemental
oxygen delivery conduit coupling the ozone vent gas stream to a
mechanically agitating contactor in an aerobic section of the
wastewater treatment plant; and a control unit for controlling the
oxygen content of the ozone vent gas stream so as to ensure
sufficient oxygenation to the aerobic section of the wastewater
treatment system by adjusting the oxygen content of the ozone vent
gas stream in response to sensor or measurement inputs
characterizing the gas contents of the ozone vent gas stream or the
dissolved oxygen level in the aerobic section of the wastewater
treatment section.
9. The system of claim 8 wherein the ozone vent gas stream is mixed
or combined with a make-up oxygen containing stream and directing
the combined stream to the aeration basin of the wastewater
treatment system. and the combined stream is directed to the
aeration basin of the wastewater treatment plant.
10. The system of claim 8 wherein the control unit adjusts the
oxygen content of the ozone vent gas stream by adjusting the flow
of the oxygen containing feed stream to the ozone generator.
11. The system of claim 8 wherein the control unit adjusts the
oxygen content of the ozone vent gas stream by adjusting the power
supplied to the ozone generator.
12. The system of claim 8 wherein the control unit adjusts the
oxygen content of the ozone vent gas stream by adjusting the flow
of a make-up oxygen containing stream to the ozone vent gas
stream.
13. The system of claim 8 wherein the control unit further controls
the volumetric flow of the ozone vent gas stream by adjusting the
rotational speed of the mechanically agitating contactor in an
aerobic section of the wastewater treatment system so as to ensure
sufficient oxygenation to the aerobic section of the wastewater
treatment system.
14. A method for supplying supplemental oxygen in a wastewater
treatment system comprising the steps of: directing an oxygen or
ozone containing sludge stream within the wastewater treatment
system to a degassing unit; separating an oxygen containing off-gas
from the sludge stream to produce a supplemental oxygen containing
gas stream; and directing the supplemental oxygen containing gas
stream to an aerobic, anaerobic or anoxic section of the wastewater
treatment system wherein the oxygen content of the supplemental
oxygen containing gas stream is controlled in response to sensor or
measurement inputs characterizing the gas contents of the ozone
vent gas stream.
15. The method of claim 14 wherein the step of directing the
supplemental oxygen containing gas stream further comprises mixing
or combining the supplemental oxygen containing gas stream with a
make-up oxygen containing stream and directing the combined stream
to the aerobic, anaerobic or anoxic section of the wastewater
treatment system.
16. The method of claim 14 wherein the step of directing the
supplemental oxygen containing gas stream further comprises
directing the supplemental oxygen containing gas stream to a
mechanically agitating contactor in the aerobic, anaerobic or
anoxic section of the wastewater treatment system, the mechanically
agitating contactor configured to assist or enhance in the
dissolution of oxygen from the supplemental oxygen containing
stream into the contents of the aerobic, anaerobic or anoxic
section of the wastewater treatment system.
17. The method of claim 14 wherein the step of directing the
supplemental oxygen containing gas stream further comprises
directing the supplemental oxygen containing stream to a digester
for micro-oxygenation of the contents of the digester.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. provisional
patent application Ser. No. 61/565,941 filed Dec. 1, 2011, the
disclosure of which is incorporated by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to a method and system for
wastewater treatment, and more particularly, a method and system
for the reuse of the vent gas from an ozone generator or degassing
unit in a wastewater treatment plant to provide some or all of the
oxygenation requirements to a section of the wastewater treatment
plant.
BACKGROUND
[0003] Generally, the use of ozone gas in water treatment plants
has been around for many years and its use appears to be
increasing. For example, ozone disinfection is used in many medium
to large sized treatment plants. In addition, ozone treatment is
also commonly used for taste, odor control and color control. Other
uses of ozone in water treatment plants include treatment of sludge
in an aeration basin of a wastewater treatment plant as disclosed
in U.S. Pat. No. 7,309,432 issued Dec. 18, 2007 and U.S. Pat. No.
6,086,766 issued Jul. 11, 2000. Other more recent applications of
ozone include foam or bulking control as disclosed in U.S. Pat. No.
7,513,999 issued Apr. 7, 2009 as well as treatment of streams in
digesters, or other sections of wastewater treatment plant as
disclosed in U.S. patent application Ser. No. 13/685,330.
[0004] The use of ozone for tertiary treatment at wastewater plants
is increasing in popularity due to the rising demand for reuse
water. As a strong oxidant, ozone is an effective disinfectant that
produces discharge water free of known toxic disinfection
byproducts with the exception of those that result from ozonation
of high-bromine waters. Ozone is also cost-effective means to
achieve color removal without the addition of chemicals, or
generation of chemical sludge.
[0005] As ozonation of a filtered secondary effluent enables reuse,
ozone is popular in areas of "water stress" where usage rates are
high relative to water flows and storage in natural systems such as
lakes and rivers. In China, for example, the annual water usage
represents about 20% of the total available supply, however, due to
acquisition cost constraints and pollution, the nation's water
supply deficit has been estimated at over 40 billion cubic
meters.
[0006] Typically, the selection of ozone systems and technologies
for use in wastewater treatment plants are based on the total
capital and operating costs of the ozone treatment system balanced
against the benefits realized through the use of the ozone
treatments and in view of the regulatory mandates or requirements
in discharging effluents. Therefore, the selection of ozone
technology that offers the best economic value is of paramount
importance, regardless of the size of the wastewater treatment
plant.
[0007] The typical capital investment for an ozone generator and
contactor, and ozone destruct unit as well as the operating and
maintenance expenses associated with use of various ozone
technologies in wastewater treatment plants can be quite
significant. In addition, since ozone is extremely irritating and
toxic above certain concentrations, any residual ozone in the
off-gases from the contactor is typically destroyed using an ozone
destruct system or unit to prevent worker exposure to ozone
gases.
[0008] Oxygen is commonly used as a feed gas for generating ozone
gas, which is subsequently used for disinfection or oxidation of
water supplies. Oxygen may be generated on-site as a gas or liquid
or purchased in bulk as liquid oxygen. Numerous water and
wastewater treatment plants in the United States are using ozone
for water treatment, with a majority generating ozone from
purchased oxygen. The majority of the annual operating costs for
ozone systems within wastewater treatment plants include very high
power consumption associated with the production of ozone and costs
associated with the supply of oxygen.
[0009] Ozone can be produced from oxygen in the air or from
high-purity gaseous oxygen. This is achieved by several methods,
although the most common technique is flowing the oxygen containing
feed through a corona discharge with dielectric barrier. Ozone is
produced when a dry oxygen or air gas stream is subjected to a
high-voltage/high-density electrical current, which provides the
energy to drive the reaction. The field acts between two electrodes
separated by a dielectric, forming a gap across which the energy
discharge occurs. Oxygen-fed ozone generators will produce more
ozone for a given power input and produce higher ozone
concentrations in the product gas, as compared to operating on air.
Air-based ozonation systems also require additional capital
equipment including a drier as well as a compressor.
[0010] Liquid oxygen suitable for use in the generation of ozone
for the treatment of water should preferably have an oxygen content
of at least about 99.0 percent, by weight with a water content not
exceeding about 7.8 parts per million (ppm), equivalent atmospheric
dew point of -80.degree. F. and a total hydrocarbon content (e.g.
methane, ethane, acetylene, and other hydrocarbons) of less than
about 40 ppm. Other impurities such nitrogen, argon, and other
inert gases may also be present in small amounts.
[0011] Many current ozonation systems typically use oxygen as the
feed gas and only about 5-15% of the oxygen in the feed stream is
converted to ozone during ozone generation. The balance is
typically vented following destruction of the residual ozone and
serves no useful purpose, except that a small amount has been used
for re-aeration of the effluent. While several technologies have
been developed to recycle the oxygen present in the product gas,
few have been commercialized successfully (See U.S. Pat. Nos.
4,132,637 and 4,256,574). Also, recovery and resale of the ozone
vent gas is not a viable option due to the costs associated with
drying, purifying, compressing and shipping the waste gas.
[0012] What is needed therefore, are systems and processes
configured for use in wastewater treatment plants that mitigate the
capital and operating expenses associated with use of ozone
technologies, and in particular, effectively and efficiently re-use
of the ozone vent gases to meet the oxygenation requirements in
other sections of the wastewater treatment plant. In this manner,
the wastewater treatment plant can realize multiple benefits from
the use of an ozone treatment system thereby off-setting the high
operating costs of the ozone treatment system.
SUMMARY OF THE INVENTION
[0013] The present invention may be characterized as a method for
ozone vent gas reuse in a wastewater treatment system comprising
the steps of: (i) directing an oxygen containing feed stream to an
ozone generator; (ii) operating the ozone generator to produce an
ozone containing gas stream; (iii) directing the ozone containing
gas stream to an ozone treatment system within the wastewater
treatment system to produce an ozone treated effluent and an ozone
vent gas stream; and (iv) directing the ozone vent gas stream to a
mechanically agitating contactor in an aerobic section of the
wastewater treatment system. The ozone vent gas stream may be mixed
or combined with a make-up oxygen containing stream and the
combined stream is then directed to the aerobic section of the
wastewater treatment system. A key aspect of the present method of
ozone vent gas reuse is the control of the oxygen content in the
ozone vent gas stream. Preferably, the oxygen content of the ozone
vent gas stream is controlled so as to ensure sufficient
oxygenation to the aerobic section of the wastewater treatment
system by adjusting the oxygen content of the ozone vent gas stream
in response to sensor or measurement inputs characterizing the gas
contents of the ozone vent gas stream or the dissolved oxygen level
in the aerobic section of the wastewater treatment section.
[0014] Adjusting the oxygen content of the ozone vent gas stream
may be accomplished by one or more of the following techniques:
adjusting the flow of the oxygen containing feed stream to the
ozone generator; adjusting the power supplied to the ozone
generator; or adjusting the flow of a make-up oxygen containing
stream to the ozone vent gas stream. Alternatively, the volumetric
flow of the ozone vent gas stream to the aerobic section of the
wastewater treatment system may be controlled is controlled by
adjusting the rotational speed of the mechanically agitating
contactor in an aerobic section of the wastewater treatment
system.
[0015] The present invention may also be characterized as a ozone
vent gas reuse system for a wastewater treatment plant comprising:
(a) an oxygen containing feed stream; (b) an ozone generator
configured to receive the oxygen containing feed stream and produce
an ozone containing gas stream; (c) an ozone contactor for
contacting an effluent with the ozone containing gas stream to
produce an ozone treated effluent and an ozone vent gas stream; (d)
an ozone destruct system configured to receive the ozone vent gas
stream and destroy any ozone contained in the ozone vent gas
stream; (e) a supplemental oxygen delivery conduit coupling the
ozone vent gas stream to a mechanically agitating contactor in an
aerobic section of the wastewater treatment plant; and (f) a
control unit for controlling the oxygen content of the ozone vent
gas stream so as to ensure sufficient oxygenation to the aerobic
section of the wastewater treatment system by adjusting the oxygen
content of the ozone vent gas stream in response to sensor or
measurement inputs characterizing the gas contents of the ozone
vent gas stream or the dissolved oxygen level in the aerobic
section of the wastewater treatment section. Preferably, the oxygen
content of the ozone vent gas stream is controlled so as to ensure
sufficient oxygenation to the aerobic section of the wastewater
treatment plant by adjusting the flow of the oxygen containing feed
stream to the ozone generator; adjusting the power supplied to the
ozone generator; or adjusting the flow of a make-up oxygen
containing stream to the ozone vent gas stream; or any combinations
of the above-identified techniques.
[0016] Finally, the present invention may be characterized as a
method for supplying supplemental oxygen in a wastewater treatment
system comprising the steps of: (i) directing an oxygen or ozone
containing sludge stream within the wastewater treatment system to
a degassing unit; (ii) separating an oxygen containing off-gas from
the sludge stream to produce a supplemental oxygen containing gas
stream; and (iii) directing the supplemental oxygen containing gas
stream to an aerobic, anaerobic or anoxic section of the wastewater
treatment system. The oxygen content of the supplemental oxygen
containing gas stream is controlled in response to sensor or
measurement inputs characterizing the gas contents of the ozone
vent gas stream.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The above and other aspects, features, and advantages of the
present systems and methods will be more apparent from the
following, more detailed description thereof, presented in
conjunction with the following drawings, wherein:
[0018] FIG. 1 is a schematic illustration of an embodiment of a
system for ozone vent gas reuse in accordance with the present
invention; and
[0019] FIG. 2 is a schematic illustration of another embodiment of
a system for ozone related gas reuse in accordance with the present
invention.
DETAILED DESCRIPTION
[0020] Turning now to FIG. 1, there is shown a schematic
illustration of one embodiment of the present system and method for
ozone vent gas reuse in an aerobic section of a wastewater
treatment system.
[0021] As seen therein, the wastewater treatment system 10 includes
an intake conduit 14 adapted to direct an influent 13 of wastewater
to an aerobic section 20 of the wastewater treatment system 10. The
aerobic section 20 of the wastewater treatment system 10 can
include an activated sludge basin or other reactor intended
configured for the purpose of using microbial life and aerobic
processes to effect the removal of waste from water. The
illustrated system also includes one or more clarifiers 22
downstream of the aerobic section 20 adapted to separate at least
some liquid effluent from a sludge flow, an output conduit 24 for
transporting the liquid effluent 23; a waste activated sludge (WAS)
line 26 configured to send waste sludge to waste tank 29; and a
return activated sludge (RAS) line 28 adapted to transport and
return a portion of the separated sludge stream back to the aerobic
section 20 of the wastewater treatment system via intake conduit
14.
[0022] The effluent 23 is directed to a tertiary ozone treatment
system, illustrated as an ozone disinfection system 30 that
includes an oxygen containing feed stream 32, an ozone generator
34, an ozone containing stream 33, an ozone contactor tank 36, and
an ozone vent gas stream. The disinfected effluent 38 is removed
from the ozone contacting tank 36 after typically about 1 to 30
minutes of residence time and is available for various reuses. The
off-gases from the ozone disinfection system 30 comprise the ozone
vent gas stream 40 that is directed from the headspace 37 of the
ozone contactor tank 36 to an ozone destruct unit 42 to destroy
residual ozone in the ozone vent gas stream 40. From there, the
ozone vent gas stream 40 is directed to the aerobic section 20 of
the wastewater treatment system 10 via a supplemental oxygen
delivery conduit coupling the vent gas stream to one or more
aeration/oxygenation units 50 where it is used to oxygenate the
contents 44 of the aerobic section 20.
[0023] Although not shown, the present system also employs a
microprocessor based control unit operatively coupled to the ozone
generator 34, the one or more aeration/oxygenation units 50, the
oxygen feed stream 32 and a plurality of sensors or measurement
devices (not shown) characterizing the gas contents (e.g. oxygen
content, nitrogen content, carbon dioxide content, ozone content,
etc.), pressures, and/or temperatures in the ozone contacting tank
36, the ozone vent gas stream 40, and the aerobic section 20 of the
wastewater treatment system 10.
[0024] The reuse of ozone vent gas, containing elevated oxygen
concentration compared to the 20.9% oxygen concentration in air,
offers the following advantages: (i) lower power costs and overall
operating costs compared to air-based aeration; (ii) utilizes an
oxygen gas source that is essentially free as it would otherwise be
wasted from the ozone contactor; and (iii) highly flexible aeration
solution that can meet a higher range of oxygen uptake rates (OUR)
and dissolved oxygen (DO) requirements compared to air-based
aeration solutions.
[0025] In regard to the lower power costs and overall operating
costs of the ozone vent gas reuse solution compared to standard
air-based aeration solutions, it is well known that electrical
power is a major operating cost and aeration power typically
consumes more than half of the electrical power in a wastewater
treatment plant. Oxygen based aeration, including aeration with
ozone vent gas, typically uses less on-site power compared to air
based aeration systems. For example, while modern fine-bubble
diffuser systems used in many air-based aeration systems have a
typical clean water aeration efficiency (SAE) of about 4.2 kg/kWh,
a surface based oxygen aerator such as the I-SO.TM. oxygen-based
aerator, available from Praxair, Inc., has demonstrated an SAE with
high purity oxygen of up to 6.2 kg/kWh under typical
conditions.
[0026] Another component of operating cost for oxygen-based
aeration is the cost of purchased oxygen. However, when using the
ozone vent gas, the cost of purchased oxygen is essentially free as
the oxygen in the ozone vent gas was formerly a wasted product. An
additional advantage to oxygen-based aeration compared to air-based
aeration systems is the low capital cost of surface based or
floating oxygen aerators compared to the conventional air-based
submerged diffusers.
[0027] Finally, the installation and repair of surface based oxygen
aerators such as the Praxair supplied I-SO.TM. unit is much simpler
and generally less costly than installation, maintenance and repair
of many submerged diffusers. For example, the installation of the
surface based oxygen aerators are accomplished by mechanically
lowering the unit into a full aeration tank. Scheduled maintenance
usually consists of an annual oil change, which can be performed
while standing on the float for the surface based aeration unit.
Based on operating history, mechanical repairs to the surface based
aerator, such as replacement of the impeller or gear box are
projected to be required every 4 years or less often. In contrast,
the costs associated with installation, maintenance and repair of
submerged diffusers is somewhat higher as the aeration tank must
typically be emptied prior to conducting such installation,
scheduled maintenance or mechanical repairs.
[0028] An important aspect of the presents system and method for
ozone vent gas reuse is the ability to control the flow of the
ozone vent gas stream so as to optimize the aeration process using
ozone vent gas. An effective gas reuse control system can overcome
some of the challenges and problems typically associated with ozone
related gas reuse. For example, a common problem encountered when
considering ozone vent gas reuse for aeration purposes is that
there is insufficient oxygen content or purity in the ozone vent
gas stream to meet the targeted aeration process requirements.
Oxygen concentration in the vent gas affects both the quantity of
oxygen provided (concentration times gas volumetric flow) and the
efficiency of the aeration device (since the higher the
concentration of oxygen, the less energy needed to dissolve a given
mass of oxygen.) A control scheme suitable to ensure the oxygen
content in the ozone vent gas flow is sufficient to meet the
aeration requirements (from both quantity and efficiency
perspectives) is to incorporate an online oxygen gas purity
measuring system to estimate the real time oxygen partial pressures
in ozone vent gas stream. The estimated oxygen partial pressures
can be used for a variety of control purposes--for example, to
establish the required volumes, if any, of make-up oxygen to be
added to the ozone vent gas stream to meet the aeration process
needs, or to control the venting process from the ozone contacting
system to maintain high vent gas purities.
[0029] Alternatively, a control scheme may be established that sets
a minimum oxygen gas flow to the ozone production system that
produces sufficient oxygen in the ozone vent gas required for the
average influent flow to the aeration basin to be treated with the
ozone vent gas. The production of ozone within this oxygen gas flow
can be controlled by varying power input to the ozone generator, to
meet the needs of the ozone treatment process (such as
disinfection.) The control scheme can compensate for higher than
average conditions by adding supplemental or make-up oxygen gas to
the ozone vent gas stream. Alternatively, higher or lower aeration
needs for oxygen can be met, within constraints of oxygen flow in
the ozone contactor, by varying the oxygen gas flow rate to the
ozone generator, and adjusting ozone generator power as required to
maintain desired ozone reaction. For example, using a liquid phase
ozone sensor and implement a control loop to maintain a desired
ozone level by varying power input to ozone generator. The desired
ozone level and corresponding oxygen level in the ozone vent gas
stream could be a feedback control loop, with a feed-forward
control based on influent flow into the aeration basin.
[0030] Another problem or design challenge associated with ozone
vent gas reuse is that the ozone vent gas entrains excess air,
reducing the oxygen content and oxygen purity level. This is
usually caused by either low pressure conditions in the headspace
of ozone contacting tank which typically causes breather valves on
the ozone contacting tank to open and introduce excess air into the
headspace and reducing oxygen purity levels in the ozone vent gas
stream; or excess nitrogen gas stripping occurring in the ozone
contactor.
[0031] To solve the low pressure problem, the present system and
method contemplates coupling a source of oxygen gas to the ozone
collection tank via pressure correction valves and introducing
oxygen gas to the ozone collection tank in lieu of air in such low
pressure situations. Alternatively, the oxygen flow rate to the
ozone generator and ozone contacting system can be varied to
maintain a slight positive pressure in the contacting system, while
ozone production is controlled separately through control of ozone
generator power.
[0032] A more elegant solution to both problems employs a control
scheme that (i) varies the speed of a variable speed vent gas
blower in the discharge line to avoid low pressure conditions in
the ozone contactor tank and minimize air infiltration; (ii) vary
the oxygen flow to the ozone generator to maintain the oxygen
purity level in the headspace of the ozone contactor tank and ozone
vent gas stream; (iii) vary the power to ozone generator to
maintain the appropriate ozone residual in contactor tank. To
effect these control schemes, inputs to the controller would
presumably include ozone sensors, oxygen sensors and/or pressure
sensors in the headspace of the ozone contactor tank or ozone vent
gas conduits. Controlling the variable speed vent gas blower allows
the operator to maintain a slight positive pressure in the
headspace of the ozone contactor tank, or it can be controlled to
maintain oxygen purity in the vent gas within a certain range.
[0033] A still more interesting solution to address both problems
is to use a side stream ozone contactor in lieu of typical ozone
contactor tank with fine bubble diffusers. The side stream ozone
contactor enhances ozone dissolution as well as to introduce any
required supplemental oxygen gas flows. Using this side stream
contactor approach, it is possible to minimize post side stream
contacting gas dissolution by minimizing contact time and rapidly
expanding pipe dimensions post side stream contacting to facilitate
phase separation and minimize oxygen dissolution. The side stream
embodiment minimizes the air entrainment issue as it is a closed,
pressurized system and allows for use of a gas/liquid separator at
positive pressure to remove the vent gas. It also treats only part
of the liquid flow, supersaturating it, so we only strip nitrogen
from part of the flow into the vent gas. This greatly decreases the
amount of nitrogen in the vent gas.
[0034] Any control scheme that varies the percent ozone produced by
the ozone generator will alter the content and volume of the ozone
vent gas and the control thereof allows one to balance the loads
between need for ozone in the tertiary treatment and needs for
oxygen gas via the ozone vent gas stream. Such a control scheme
allows essentially, independent control of oxygen gas flow through
the ozone generator and ozone generator power.
[0035] In the preferred embodiments, the low-pressure ozone vent
gas stream is dissolved in the aerobic section of the wastewater
treatment plant using a plurality of Praxair's I-SO.TM. aeration
systems which are able to induce gas flows from the ozone vent gas
stream using a high strength vortex generated by the rotational
action of a helical impeller within a draft tube. The I-SO.TM.
system's capacity for ozone vent gas induction eliminates the need
for the compression of the ozone vent gas stream and associated
compression costs.
[0036] Using the I-SO.TM. surface based aeration system and one or
more of the above-identified control schemes, the ozone vent gas is
almost entirely recovered from the closed-tank ozone contactor
system, as long as optimum pressure in the ozone contactor tank
headspace is maintained. In effect, the number of aeration units
and operating conditions (i.e. rotational speed of the impellers)
of such aeration units receiving the ozone vent gas stream are used
to maintain appropriate pressure in the headspace of the ozone
contactor tank, which limits both air intrusion when pressure in
the headspace of the contactor tank is too low, and direct
ventilation or wasting of the ozone vent gas to the atmosphere when
the pressure in the headspace of the contactor tank is too
high.
[0037] Turning now to FIG. 2, there is shown a schematic
illustration of another embodiment of the present system and method
for ozone related gas reuse in a wastewater treatment system.
Similar to the embodiment of FIG. 1, the wastewater treatment
system 10 includes an intake conduit 14 adapted to direct an
influent 13 of wastewater to an aerobic section 20 of the
wastewater treatment system 10. The illustrated system also
includes one or more clarifiers 22 downstream of the aerobic
section 20 adapted to separate at least some liquid effluent from a
sludge flow, an output conduit 24 for transporting the liquid
effluent 23; a waste activated sludge (WAS) line 26; and a return
activated sludge (RAS) line 28 adapted to transport and return a
portion of the separated sludge stream back to the aerobic section
20 via intake conduit 14.
[0038] Within the RAS line 28, there is a sludge ozonation system
70 that includes a pump 72 to direct the RAS sludge to a plug flow
type ozonation reactor. The plug flow type ozonation reactor
includes a sufficient length of pipe 78 that assures a residence
time of the sludge in the ozonation reactor that is adequate for
ensuring effective dissolution of the ozone and reaction of the
ozone with the biosolids in the RAS line 28. The illustrated
embodiments also include one or more ozone gas injection systems
comprising a source of oxygen gas 32, an ozone generator 74 for
producing an ozone-enriched gas and one or more nozzles or venturi
type devices 76 for injecting the ozone-enriched gas into the
ozonation reactor through which the RAS sludge passes.
[0039] Upon exiting the plug-flow type ozonation reactor, the
ozonated sludge is then directed to a degassing unit 60 or
gas-liquid separator to remove the excess oxygen containing gas.
The excess oxygen containing gas 66 is then directed via a
supplemental oxygen delivery conduit coupling the stream to one or
more aeration/oxygenation units 50 where it is used to oxygenate
the contents 44 of the aerobic section 20 of the wastewater
treatment system 10 for reuse in the aeration process. The degassed
ozonated sludge 62 is returned to the aerobic section 20 via intake
conduit 14.
[0040] As with the earlier described embodiment, the disclosed
system and process also employs a microprocessor based control unit
operatively coupled to the ozone generator 74, the one or more
aeration/oxygenation units 50, the oxygen feed stream 32 and a
plurality of sensors (not shown) characterizing the gas contents
(e.g. oxygen content, nitrogen content, carbon dioxide content,
ozone content, etc.), pressures, and/or temperatures within the
degassing unit 60, the oxygen containing discharge stream 66, and
the aerobic section 20 of the wastewater treatment system 10.
[0041] As an example of the ozone vent gas reuse approach was
embodied in an expansion section of a municipal wastewater
treatment plant. The expansion section was designed to utilize the
ozone vent gas from an ozone contactor as a source of oxygen for
aeration in the secondary anoxic-anaerobic-oxic (AAO) process. The
expansion of the wastewater treatment plant increased the capacity
of the plant from about 120,000 m.sup.3/day to about 150,000
m.sup.3/day. The oxygen requirements for the incremental flow
(30,000 m.sup.3/day) in the expansion section was met completely by
using the ozone vent gas from a tertiary treatment ozonation system
applied to the entire effluent flow from the plant.
[0042] The oxygen source is liquid oxygen, which is vaporized at a
rate of approximately 16.5 mtpd. The vaporized oxygen is then mixed
with 0.5 mtpd of oxygen gas from bleed air (which contains about
78% nitrogen gas), to provide nitrogen in the ozone generator feed
gas. It has been found that a nitrogen content of about 1% to 5% in
the feed gas, leads to higher power efficiency for the ozone
generation. The 17 mtpd oxygen in the feed gas is used by an ozone
generator to make an ozone stream gas, which is roughly 9% ozone by
weight. This ozone gas is then fed into submerged diffusers in four
ozone-contacting tanks to maintain a concentration of 5 mg
ozone/liter in the ozone contacting tanks. In the ozone contacting
tanks, it is estimated that about 7 mtpd oxygen are lost through a
combination of ozone reaction, increase in dissolved oxygen in the
effluent and losses through the contactor. The remaining 10 mtpd of
oxygen gas is available for aeration or other purposes. The
oxygen-rich ozone vent gas stream (i.e. about 75-85% pure oxygen)
is reused and applied to an aeration zone in the expansion section
of the municipal wastewater treatment plant, specifically in the
reverse AAO process designed for nitrogen and phosphorus biological
nutrient removal.
[0043] The capital and operating cost savings associated with the
usage of ozone vent gas for aeration compared to air based aeration
of the 30,000 m.sup.3/day expansion section are projected to be
about 56% for capital with approximately the same relative savings
for operating costs related to aeration in the plant expansion.
Another advantage of the ozone vent gas reuse based aeration system
is that it uses floating aerators, so it is not be necessary to
drain the aeration basin for aerator maintenance.
[0044] While the inventions herein disclosed has been described by
means of specific embodiments and processes associated therewith,
numerous modifications and variations can be made thereto by those
skilled in the art. For example, the ozone vent gas stream may be
directed to primary influent stream to raise the dissolved oxygen
level for odor control or to supplement existing air-based aeration
systems and/or existing high purity oxygen based aeration systems
in the wastewater treatment plant. Still further, an ozone vent gas
recovery process in a wastewater treatment plant could be coupled
to enhance other unit operations, such as upstream anaerobic
treatments, membrane bioreactors, fixed film systems, sequencing
batch reactors, etc.
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