WO2012177907A1 - System and method for oxygen supply for wastewater treatment plant having biological treatment system and supercritical water oxidation treatment of sludge - Google Patents
System and method for oxygen supply for wastewater treatment plant having biological treatment system and supercritical water oxidation treatment of sludge Download PDFInfo
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- WO2012177907A1 WO2012177907A1 PCT/US2012/043586 US2012043586W WO2012177907A1 WO 2012177907 A1 WO2012177907 A1 WO 2012177907A1 US 2012043586 W US2012043586 W US 2012043586W WO 2012177907 A1 WO2012177907 A1 WO 2012177907A1
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- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F3/00—Biological treatment of water, waste water, or sewage
- C02F3/02—Aerobic processes
- C02F3/12—Activated sludge processes
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- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F11/00—Treatment of sludge; Devices therefor
- C02F11/06—Treatment of sludge; Devices therefor by oxidation
- C02F11/08—Wet air oxidation
- C02F11/086—Wet air oxidation in the supercritical state
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- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F3/00—Biological treatment of water, waste water, or sewage
- C02F3/02—Aerobic processes
- C02F3/12—Activated sludge processes
- C02F3/26—Activated sludge processes using pure oxygen or oxygen-rich gas
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- C—CHEMISTRY; METALLURGY
- C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
- C10L—FUELS NOT OTHERWISE PROVIDED FOR; NATURAL GAS; SYNTHETIC NATURAL GAS OBTAINED BY PROCESSES NOT COVERED BY SUBCLASSES C10G, C10K; LIQUEFIED PETROLEUM GAS; ADDING MATERIALS TO FUELS OR FIRES TO REDUCE SMOKE OR UNDESIRABLE DEPOSITS OR TO FACILITATE SOOT REMOVAL; FIRELIGHTERS
- C10L5/00—Solid fuels
- C10L5/40—Solid fuels essentially based on materials of non-mineral origin
- C10L5/46—Solid fuels essentially based on materials of non-mineral origin on sewage, house, or town refuse
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
- F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
- F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
- F25J3/04—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream for air
- F25J3/04521—Coupling of the air fractionation unit to an air gas-consuming unit, so-called integrated processes
- F25J3/04527—Integration with an oxygen consuming unit, e.g. glass facility, waste incineration or oxygen based processes in general
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- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F11/00—Treatment of sludge; Devices therefor
- C02F11/06—Treatment of sludge; Devices therefor by oxidation
- C02F11/08—Wet air oxidation
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- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F3/00—Biological treatment of water, waste water, or sewage
- C02F3/02—Aerobic processes
- C02F3/06—Aerobic processes using submerged filters
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
- F25J2215/00—Processes characterised by the type or other details of the product stream
- F25J2215/50—Oxygen or special cases, e.g. isotope-mixtures or low purity O2
- F25J2215/54—Oxygen production with multiple pressure O2
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E50/00—Technologies for the production of fuel of non-fossil origin
- Y02E50/10—Biofuels, e.g. bio-diesel
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E50/00—Technologies for the production of fuel of non-fossil origin
- Y02E50/30—Fuel from waste, e.g. synthetic alcohol or diesel
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02W—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO WASTEWATER TREATMENT OR WASTE MANAGEMENT
- Y02W10/00—Technologies for wastewater treatment
- Y02W10/10—Biological treatment of water, waste water, or sewage
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02W—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO WASTEWATER TREATMENT OR WASTE MANAGEMENT
- Y02W10/00—Technologies for wastewater treatment
- Y02W10/30—Wastewater or sewage treatment systems using renewable energies
Definitions
- the present invention relates to lowering oxygen supply costs for wastewater treatment plants employing fluid oxidizing processes such as the use of a supercritical water oxidation (SCWO) reactor or wet air oxidizing reactor (WAO) for handling sludge produced in the wastewater treatment plant (WWTP). More particularly, the present invention relates to a system and method for tailoring an air separation unit to provide gaseous oxygen and/or air to the biological systems within the wastewater treatment plant concurrently with a source of high pressure liquid oxygen for the destruction of sludge produced from the wastewater treatment plant.
- SCWO supercritical water oxidation
- WAO wet air oxidizing reactor
- Traditional methods of wastewater treatment involve bringing wastewater streams into contact with bacteria either in an aerobic and/or anaerobic type process in what is known as activated sludge treatment.
- These bacteria consume parts of the substrate material or waste contained in the wastewater, which are typically organic compounds containing carbon, nitrogen, phosphorus, sulfur, and the like.
- a portion of the waste is consumed to further the metabolism of the bacterial cells or maintain the physiological functioning of the bacterial cells.
- a portion of the waste is also consumed as part of the process of synthesis of new bacterial cells.
- the organisms that grow inside the basin require oxygen. This oxygen can be provided from air or from a high purity oxygen stream.
- the delivery mechanism may be through the use of surface aerators which agitate the surface of the water in the basin creating fine drops that are thrown into the air and absorb oxygen from it. More commonly, air is compressed to typically 5-15 psig in rotary lobe or centrifugal blowers and then fed to diffusion devices at the base of the aeration basin to form fine bubbles from which oxygen is dissolved into the fluid within the basin. For high purity oxygen systems, oxygen rather than air is used to feed the basin. Oxygen has several advantages over air. It can be used to intensify the biological process such that the volume of the basin, and thus its footprint and capital is reduced. The capital associated with dissolution devices for the oxygen are also typically much lower than that of the compressor and diffusers in the air based system.
- the power required for dissolution of the oxygen and mix the aeration basin is also typically significantly lower than the power for the surface aeratpors or compressors of the aeration system.
- the drawback of oxygen is that the benefits must be weighed against the cost of the oxygen delivered to the basin.
- the activated sludge treatment process yields a certain amount of sludge which must be continuously removed from the treatment basin to maintain the steady state sludge balance which is critical to the effective functioning of the activated sludge treatment system.
- SCWO supercritical water oxidation
- the SCWO process and technology appears to have several advantages over the other methods of sludge disposal in that it is a robust process with respect to the ability to oxidize a wide range of organic materials and receive a sludge stream feed with a wide range of solids content.
- This ability to oxidize sludge streams having a wide range of organic materials of varying solids content can lead to reduction in capital and operating expenses in comparison with the conventional pretreatments of sludge such as; digesting, dewatering, thickening, etc.
- Yet another significant advantage of the SCWO technology is the ability to recover energy from the SCWO process. In particular, excess energy could be produced from the SCWO process by raising steam from the heat recovery of the exit stream and driving a steam turbine generator. Such energy recovery, however, is likely only economical when employing larger scale plants.
- This disclosure provides several embodiments to the present invention. Specifically, a method for supplying oxygen to a wastewater treatment plant (WWTP) incoiporating a fluid-phase sludge oxidation system is described. The method includes the steps of:
- ASU air separation unit
- An additional step of converting a portion of the heat to on-site generated electrical power and utilizing the on-site generated electrical power to supply at least part of the electrical power consumed by the ASU is often desirable.
- heat is produced by the fluid-phase oxidizing reactor it can also be used to regenerate thermal swing adsorption (TSA) pre-purifiers within the ASU.
- TSA thermal swing adsorption
- a fluid-phase oxidizing reactor that is normally a wet air oxidizing (WAO) reactor.
- WAO wet air oxidizing
- SCWO supercritical water oxidizing
- the ASU it is desirable for the ASU to be located proximate to or on-site with the WWTP such that the ASU and the WWTP can utilize common storage tanks, common operations support, monitoring and maintenance personnel and other infrastructure for production and use of the oxygen within the ASU, the WWTP, and the associated fluid-phase oxidizing reactor.
- the ASU includes a liquefier and the high pressure oxygen stream is a liquid oxygen stream produced from a cold box within the air separation unit that is pumped to elevated pressure so that the liquid oxygen stream can be vaporized prior to introduction to the fluid-phase oxidizing reactor. During this operation, a portion of the heat produced is used to vaporize the liquid oxygen stream prior to introduction to the fluid-phase oxidizing reactor.
- the high pressure oxygen stream may also be in the form of gaseous oxygen produced from a cold box within the ASU and subsequently compressed to a pressure equal to or greater than the elevated pressure necessary for the fluid- phase reactor to operate efficiently.
- the low pressure oxygen stream or compressed air stream or both streams from the air supply system and associated ASU are often directed to an aeration basin or an aerobic digester or an ozone generator or any combination of the three unit processes within a WWTP. It is also possible for the high pressure oxygen stream, which is often in the form of gaseous oxygen produced at the required pressure to feed the SCWO directly from the cold box within the ASU by pumping liquid within the cold box and warmthing against incoming streams to the cold box will allow for recovering refrigeration energy.
- MBR membrane bioreactor
- the ASU is configured to produce oxygen at high purity levels that are greater than or equal to 99 percent pure oxygen that produce excess liquid and gaseous products which can be stored on site for subsequent transfer in bulk or cylinder form to local industrial or medical gas uses located proximate the air separation unit for use with the WWTP and the fluid-phase oxidizing reactor. It is also possible to configured the ASU to produce oxygen at lower purity levels, normally in a range of between about 94 percent to 99 percent pure oxygen which is also appropriate for use by the WWTP and the fluid-phase oxidizing reactor.
- the method described in the initial embodiment can further include the step of directing a portion of the refrigeration capacity from the high pressure oxygen stream or the air separation unit to cool the carbon dioxide off-gas and produce cool carbon dioxide gas or liquid products.
- the fluid-phase oxidizing reactor produces ash and other solid particles containing phosphorous and other minerals from the reaction of a high pressure oxygen stream with the sludge stream within the reactor and the methods herein further include disposing, capturing or selling the phosphorous and other minerals preferably to a customer base also proximate to the
- a waste water treatment plant (WWTP) system is described that is integrated with an oxygen or air or other fluid gaseous or liquid supply and provides; an air separation unit (ASU) for separating air by cryogenic rectification to produce at least one low pressure oxygen stream at a pressure of less than 100 psig, at least one high pressure oxygen stream at an elevated pressure of equal to or greater than 300 psi, and at least one compressed air stream.
- ASU air separation unit
- the system also includes ; one or more secondary wastewater treatment systems for the biological treatment of wastewater fluidically coupled to the air separation unit, such that the secondary wastewater treatment systems produce a sludge stream;
- a sludge stream circuit including one or more pumps to increase the pressure of a portion of the sludge stream to form sludge containing water at a pressure greater than about 300 psi;
- a fluid-phase oxidizing reactor fluidically coupled to the (ASU) and one or more secondary wastewater treatment systems for reacting the high pressure oxygen stream and the pressurized sludge containing water stream to produce heat;
- control unit for operatively controlling the flow of the low pressure oxygen stream or the compressed air stream or both streams from the air separation unit to the one or more secondary wastewater treatment systems so that the control unit further operatively controls the flow of the high pressure oxygen stream and the pressurized sludge containing water stream to a fluid - phase oxidizing reactor.
- the control unit further is operatively coupled to the air separation unit (ASU) to control the production of the low pressure oxygen stream(s), said the high pressure oxygen stream(s) and the compressed air stream.
- ASU air separation unit
- the pressure of the sludge containing water stream is between about 300 psi and 3000 psi for the case where the fluid-phase oxidizing reactor is a wet air oxidizing reactor and for the case where the pressure of the sludge containing water stream is greater than about 3000 psi, the reactor is a supercritical water oxidizing reactor.
- the combined WWTP/ASU system is preferably located so that the ASU air separation unit is proximate to or on-site with a secondary wastewater treatment system and the associated fluid-phase oxidizing reactor.
- the remainder of the system elements have been mostly previously described above, P T/US2012/043586
- the gas supply/air separation system that is coupled with the waste water treatment system. More specifically, for a waste water treatment facility that includes one or more primary treatment units coupled to the air separation unit, the low pressure oxygen stream or compressed air produced from the air separation unit is directed to the primary treatment systems. For a wastewater system with one or more tertiary wastewater treatment systems that require oxygen or ozone, the tertiary wastewater treatment systems are coupled to the air separation unit and a portion of the low pressure oxygen stream produced from the air separation unit is directed to the primary wastewater treatment systems.
- the system described also can include a steam turbine generating set for coupling to the fluid- phase oxidizing reactor, so that the steam turbine generator set is configured to convert a portion of said heat to on-site generated electrical power, said steam turbine generating set further coupled to said air separation unit to supply at least part of said electrical power consumed by said air separation unit.
- the system allows for heat being produced by the fluid-phase oxidizing reactor to be used to regenerate one or more thermal swing adsorption (TSA) pre-purifiers in the air separation unit.
- TSA thermal swing adsorption
- the system further includes a carbon dioxide purification system coupled to an ASU to cool and purify the carbon dioxide off-gas and produce cool carbon dioxide gas or liquid products.
- a sludge stream exiting a pretreatment stage of the waste water treatment plant is sent to a supercritical water oxidizing (SCWO) reactor where sludge is pumped to an elevated pressure of greater than 3000 psig, mixed with high pressure oxygen and/or air produced in an air separation unit (ASU), so that the oxygen and/or air remains pressurized and is heated and then reacted for combusting the sludge within the SCWO reactor.
- SCWO supercritical water oxidizing
- the process stream exiting SWCO reactor is cooled and reduced in pressure such that energy recovery from the cooling and pressure drop for making steam is accomplished that also provides for heating and/or generating electricity using a steam turbine generator set.
- the ASU produces a combination of ' oxygen and compressed air streams including a low pressure gaseous oxygen stream, a high pressure gaseous oxygen stream, and, as needed, a compressed air stream that is fed to either an aeration basin of the WWTP or to the SCWO reactor or to both the WWTP and the SCWO reactor. Any excess of the combination of oxygen and compressed air streams are utilized by surrounding consumers when the WWTP and the SCWO reactor do not consume all of the streams produced by the ASU. Excess liquid products are stored on site for subsequent transfer in bulk form to service local industrial gas market(s) thereby further reducing costs associated with operating the SCWO reactor.
- the oxygen can be either used for the aeration basin or sold to industrial gas users proximate to the combined WWTP and ASU facility.
- Another embodiment includes the case of utilizing larger scale air separation units (ASUs) that produce 200 or more tons per day of oxygen of lower purity oxygen as a gas at purity levels in the range of between 94 percent to 99 percent pure oxygen, providing gaseous oxygen in this purity range to the aeration basin of the WWTP is acceptable and the lower purity oxygen reduces the power consumption requirements of producing the oxygen from the ASU.
- ASUs larger scale air separation units
- delivery of high pressure oxygen for use in the SCWO reactor may be achieved directly from the cold-box of the ASU by pumping some of the cryogenic liquid oxygen from the distillation column in the ASU and transferring heat to a high pressure incoming air stream to the ASU such that remaining cryogenic liquid oxygen continues to be processed in the ASU to produce higher purity liquid oxygen thereby meeting needs of local industrial gas markets.
- auxiliary boiler is provided for heating in the event that the SCWO reactor is off-line or is shutdown and thereby enables stable energy production from the combined ASU/WTTP system.
- a system that includes an aeration basin and a supercritical water oxidizing (SCWO) reactor or a wet oxidation reactor within a waste water treatment plant (WWTP) where the oxygen gas is generated from an air separation unit (ASU) and is injected into the aeration basin and the SCWO or the wet oxidation reactor and the oxygen is also available for injection into a primary wastewater flow at a front end of the WWTP or to an ozone generator to produce an ozone/oxygen gas mixture.
- SCWO supercritical water oxidizing
- WWTP waste water treatment plant
- the heat exchangers should be spiral wound coil type and the heat exchangers should be comprised of steel and the steel should be steel is stainless steel or other alloys of steel with heat capacity greater than that of aluminum
- the combination of an air separation unit (ASU) and a waste water treatment plant (WWTP) with a supercritical water oxidizing reactor (SCWO) requires one or more control systems, with an arrangement between the ASU, an aeration basin as well as either the SCWO reactor or a wet air oxidation reactor within the WWTP and includes one or more programming logic controllers (PLC) and associated computer operating system(s) and software to coordinate all processes and flows to and from each operating unit of each facility.
- PLC programming logic controllers
- Model predictive computational software and hardware combinations that model the primary units of the system including the ASU, the WWTP, the SCWO reactor, the aeration basin, and the wet air oxidation reactor or any combination of these primary units for providing orders to the machinery of and within the primary units of the system is provided.
- FIG. 1 is a schematic illustration of the oxygen supply system for a wastewater treatment plant that is coupled to or processes its sludge by means of an SCWO reactor in accordance with the present invention.
- FIG. 2 is a schematic illustration of a supercritical water oxidation (SCWO) system and process of the type used to process sludge from a wastewater treatment plant.
- SCWO supercritical water oxidation
- FIG.3 is a schematic illustration of the gas supply streams, gas pressures, energy and waste water treatment process flows for the combination of a WWTP/gas supply system.
- ASUs air separation plants or units
- These plants may be non-cryogenic, for example based on Vacuum Pressure Swing Adsorption, or cryogenic where the bulk separation is effected by distillation.
- Cryogenic facilities are characterized in that they can produce both gaseous and liquid products (oxygen, nitrogen, argon) at a range of pressures and flows depending on the required products. These facilities have been built on ever larger scales, greater than 10 tons per day to several thousand tons per day. Because the feed is air, the primary operating cost is for the energy required to compress the air to the plant and to compress/pump the product gases.
- This equipment includes compressors, turbines, distillation columns, refrigeration units, heaters, pumps, heat exchangers, etc. all required for the energy intensive needs for separating, purifying, and producing gases and liquids from air.
- cryogenic air separation systems have been developed over the last 60 years as well, to meet customer demands for specialized needs including that of low and high pressure gaseous and liquid oxygen and other industrial gases including nitrogen, argon, xenon, neon, etc.
- These cryogenic ASU's are designed differently depending on the production requirements. Specifically, for the present disclosure, the need for delivering oxygen in various forms (high pressure for the SCWO, low pressure for the activated sludge treatment) to a WWTP with an SCWO reactor and liquid products for sale to local industries has been established.
- the SCWO When the SCWO is combusting sludge it provides an exothermic reaction with an abundance of heat.
- This heat can be used to generate steam.
- this steam can be used to generate power via a steam turbine.
- the power can be used directly to provide all or a portion of the energy required to power the air separation plant and the WWTP.
- steam can be integrated with the ASU, for evaporation of the liquid to the gas state for example.
- the exothermal heat generated from the SCWO or the wet-oxidation reactors might be used for helping run other operating units within the ASU including for example pre-purification regeneration systems, etc.
- the excess heat and/or steam made from the heat could be utilized.
- the proper choice of the ASU depends on a number of considerations. Among them, the SCWO process' requirement for very high pressure oxygen is predominant. As noted earlier, there are several approaches to providing oxygen at these very high pressures.
- One approach is to produce oxygen at low pressures in a "gaseous only" facility.
- a "gaseous only” facility uses relatively little power for refrigeration and delivers high yields of gaseous oxygen with only minor amounts of liquid oxygen.
- For a typical 100 tpd (ton per day) gaseous oxygen facility about 95+ tpd gaseous oxygen is produced and only 1-2 tpd of liquid oxygen and nitrogen (normally sent to storage tanks for future consumption or "as needed” demand) is produced.
- Some nitrogen and argon is also delivered and can be captured and cooled to liquid.
- These "gaseous only” facilities deliver product oxygen at relatively low pressures (just above ambient pressures from 1-3 arm.). A fraction of this low pressure gaseous oxygen can be sent to the secondary waste water treatment facility without further compression. The remaining low pressure gaseous oxygen can be compressed to the pressure required by the SCWO system. This is not recommended due to cost and operational concerns associated with very high pressure gaseous oxygen compression equipment 12 043586
- a second approach also is to produce oxygen at low pressures in a "gaseous only" facility.
- a fraction of this low pressure gaseous oxygen can be sent to the secondary waste water treatment facility without further compression.
- the remaining gaseous oxygen then can be liquefied using a separate liquefier.
- the resulting liquid oxygen then can be pumped to the pressures required by the SCWO facility, warmed against some heat source, typically the atmosphere, warm water from the ASU plant's compression train intercoolers or waste heat from the SCWO system itself.
- Such a system has the virtues of simplicity and easy control but is very energy intensive since the refrigeration of liquefaction is disposed of without recovery.
- a third ASU choice is available.
- the ASU can be adapted to pump the fraction of oxygen product required for the SCWO system internally using self-produced liquid oxygen to the required pressure.
- This high pressure supercritical oxygen then can be wanned against incoming process feed air thereby recirculating the refrigeration and retaining it within the ASU's insulated process space.
- This system thereby is much more energy efficient and hence more economical.
- “Pumper” facilities also use compressors, but primarily for compressing air using standard compressors. Working with cold gases or liquids and pumping oxygen up to high pressures without oxygen compressors is a more reliable, economical more desirable method of oxygen delivery for the needs of a SCWO system.
- the drawback is that the very high pressure oxygen required for the SCWO system requires special heat exchange equipment within the ASU. Ideally, for the SCWO system to be efficient, the oxygen should be provided at levels near the pressure and temperature of supercritical water (373.94 degrees Celsius and 216.75 arm.). Designing heat exchangers to operate at these conditions is not a trivial task.
- the plate fin brazed aluminum types normally used in cryogenic ASUs are incapable of accommodating the pressures required.
- this preferred ASU system is one often referred to as a "gaseous pump cycle plant" or "pumper.
- ASU large enough to meet all of these needs is required. It can be equipped with internal pumps to provide very high pressure oxygen to the SCWO system in a manner similar to that of a "pumper”; it can produce low pressure gaseous oxygen for use in the secondary wastewater treatment facility and it can be equipped with powerful refrigeration services to permit the production of large volumes of cryogenic liquid products for the local geography.
- Such ASU plants are capable of providing more or less liquid oxygen, nitrogen and argon, typically between 10 and 50% of the plant feed air volume.
- the supercritical water provides a high degree of solubility of organic matter and also provides high diffusivities.
- an oxidant in this case oxygen from the ASU facility
- the rate of reaction of the organic material within the SCWO reactor is greatly enhanced and essentially instantaneous in the presence of oxygen, leading to the nearly complete destruction of any organic matter.
- SCWO supercritical water oxidation
- the cost of the oxygen to be used as the oxidant in the SCWO reactor represents up to 80% of the total operating costs of an SCWO system.
- the economic feasibility of an SCWO process and commercial success will be improved through optimization of the ASU provided to produce the oxygen for SCWO processes and WWTP process.
- Control systems for managing the desired combination of the SCWO or Wet Air Oxidation (WAO) Combination with the WWTP are not trivial and require coordinating different process streams and monitoring different process parameters as needed between the two plants.
- Oxygen supply with dissolved oxygen sensor(s) are used to maintain set points and control oxygen flow into the aeration basin.
- One or more blowers are normally used to ensure that enough air and/or oxygen is sent to the aeration basin.
- the ASU plant is normally equipped with control valves and/or other switches that provide for flow of the oxygen (or compressed air or other gases) to the desired locations.
- Ozone provides for tertiary waste water treatment and is used in abundance in some facilities to lower chemical oxygen demand (COD). Control of the amount of oxygen and the pressure needed to send the gas form the ASU to an ozone generator on the WWTP site, is also part of the present disclosure.
- a feedback/feed forward arrangement between the ASU and the aeration basin as well as to either the SCWO or WAO reactor along with the necessary PLC (programming logic controller) and associated computer operating system and software greatly benefits the necessary coordination.
- Model predictive computational software and hardware combinations for modeling plants and providing orders to the machinery of the plants is needed as well.
- FIG. 1 there is shown a schematic illustration of a WWTP system that includes a combination of the use of an air separation unit (ASU) [7], that provides for the complete oxidation and destruction of sludge from wastewater treatment plants by the use of a supercritical water oxidation reactor (SCWO) [6].
- ASU air separation unit
- SCWO supercritical water oxidation reactor
- An oxygen supply system for a wastewater treatment plant that is coupled to and processes its sludge by means of an SCWO reactor is provided as one embodiment of the present disclosure.
- the illustrated wastewater treatment plant (WWTP) comprises an incoming stream of polluted water [1] that is first introduced into a primary clarifier [2], followed by secondary treatment [3], such as biological treatment in an activated sludge basin, and subsequently by a secondary clarifier [4].
- a treated wastewater [19] stream leaves the secondary clarifier for disposal or for additional advanced tertiary treatment and reuse.
- a portion of the sludge stream [18] is returned to the activated sludge basin [3].
- the remainder [17] is sent to a digester, thickener, or some other form of sludge pretreatment in the process stage [5].
- Sludge stream [15] from the primary clarifier [2] may also be sent directly to the sludge pretreatment [5] process stage.
- the pretreatment portion that is prior to the SCWO will typically include maceration and may also include sludge thickening, dewatering, etc.
- the sludge leaving the pretreatment stage [5] will typically contain solids in the range of 3-25% by mass and preferably in the range of 10-20% by mass.
- the sludge stream [20] exiting the pretreatment stage [5] is then sent to the SCWO stage [6] reactor where it is pumped to an elevated pressure (greater than 3000 psig), mixed with high pressure oxygen [8] produced in the air separation unit (ASU) [7], heated, and then reacted in the SCWO reactor.
- ASU air separation unit
- the resulting stream [13] exiting the SCWO reactor is then cooled and dropped in pressure.
- any energy recovery [14] associated with cooling this stream can be used to raise steam for use in heating or else as a means to generate electricity by, for example, a steam turbine generator set (not shown in FIG 1, but provided in FIG 2).
- the air separation system [7] can produce both a low pressure gaseous oxygen stream [9] that is fed to the aeration basin of the wastewater treatment plant [3] and also produces a high pressure oxygen stream [8] that is fed to the SCWO reactor stage [6]. It is also possible for the air separation system [7] to produce additional liquid products (oxygen, nitrogen, argon, etc.) that are stored on-site for subsequent transfer in bulk form to service the local industrial gas market(s).
- the optimum choice of the air separation processes employed by the ASU depends on the scale of the air separation system or plant and the amount of liquid products to be produced and was earlier discussed. Since there is often a need to produce very high purity oxygen (>99% purity) to meet the needs of the local industrial gas markets, production of all oxygen products in smaller scale air separation units (i.e. scale of less than about 100 tons per day (tpd) of oxygen production) is typically at the very high purity levels. This high purity liquid oxygen is then stored on site and subsequently used for both the SCWO reactor and off-site customers requiring higher purity oxygen for their use.
- the high purity gaseous oxygen stream [9] as shown here is used for oxygenation of the wastewater in the aeration basin however it is not essential to use high purity gaseous oxygen for the aeration basin. Either compressed air or lower purity oxygen is also acceptable, and can be supplied by the ASU for both the aeration basin as well as for the SWCO reactor.
- Products exiting the SCWO reactor include both gas phase products such as carbon dioxide and perhaps nitrogen as well as liquid phase products such as water. Ash and other solid particles also exit the SWCO reactor represented as insoluble materials that are also a by-product of the SCWO process.
- the solid, gas and liquid products exiting the SCWO reactor [13] are preferably separated and either recovered for re-use (such as recovery of phosphorus from the ash) or sent to disposal [not shown]. Recovery and re-use of these products also helps offset the costs of operating the SCWO reactor.
- air is normally supplied through a multistage compressor and separated within the ASU process to provide both a supply of high pressure oxygen to an SCWO reactor and can also further supply lower pressure gaseous oxygen requirements to the biological basin(s) of the waste water treatment plant (into the aeration basis [3]).
- ASU ASU separation unit
- the ASU is preferably designed to support the export of additional liquid products (i.e. nitrogen, oxygen, argon, etc.) to the local industrial gas markets to further offset the effective cost of oxygen delivered to the wastewater treatment plant and the SCWO reactor.
- additional liquid products i.e. nitrogen, oxygen, argon, etc.
- the air separation unit represented as [7] in FIG. 1 preferably comprises both a cryogenic cold- box that cools and separates the air, pressurization means for the high pressure oxygen stream, either internal or external to the cold-box, and cryogen storage systems. .
- the amount of liquid oxygen that can be generated from the air separation system is a function of the process cycle chosen for the ASU (as earlier described). Table 1 below characterizes the details associated with typical activated sludge within a WWTP and the associated air or oxygen requirements for biological oxygen demand - BOD - (in kg/day) for an aeration basin and for a SCWO reactor.
- the acronyms provided in Table 1 for the environmental loads on the WWTP include; the SRT, which represent the solids retention time, the WAS which represents the waste activated sludge, the BOD which represents the biological oxygen demand, the COD, which represents the chemical oxygen demand, the TSS which represents the total suspended solids, the VSS which represents the volatile suspended solids, and the TKN which represents the total Kjehldahl nitrogen for the WWTP.
- liquid oxygen - LOX- it is possible to provide a fraction of the liquid oxygen - LOX- to be typically between about 5% to about 60%, but most likely about 50% of the total oxygen flow produced from the cold-box, with the remaining oxygen being provided in gaseous form. This ratio of liquid to gaseous oxygen production would match the typical demand of the
- a liquefier can be added to the air separation system that takes all or a portion of the gaseous products, including oxygen, from the plant and liquefies it through a liquefaction cycle typically employing nitrogen as the working fluid.
- the liquefier can be a stand-alone unit essentially independent from the air separation system except for a conduit to supply the gas to be liquefied, or preferably the liquefier may be integrated with the air separation system with the liquid from the liquefier being returned to the ASU and allowing more liquid products to be withdrawn from the plant
- the WWTP and SCWO require higher or lower ratios of liquefied oxygen to gaseous oxygen
- differently designed ASU's are possible as is the need for proper location and sizing of the liquefier.
- the preferred air separation system is co-located at a wastewater treatment plant that also includes an SCWO reactor.
- the air separation system may also include an internal or external pumping arrangement to pressurize the liquid oxygen from the cryogenic storage systems to the very high pressures, vaporize the high pressure liquid, and optimally deliver the high pressure oxygen to the SCWO system.
- the air separation system or plant can be designed to produce liquid oxygen that is subsequently pumped to a high pressure and supplied to the SCWO reactor directly from the ASU.
- alternative arrangement requires special high pressure heat exchangers such as spiral wound systems using stainless steel rather than aluminum.
- a minimum liquid oxygen production to be stored on-site is required to provide back-up capability for the SCWO reactor in the event of a disruption in the direct supply of high pressure oxygen from the ASU to the SCWO reactor, to provide storage for load leveling, and also to provide liquid products for sale to the local market. Since there is often a need to produce very high purity oxygen (>99% purity) to meet the needs of the local industrial gas markets, production of all oxygen products in smaller scale air separation units (i.e. scale of less than about 100 tons per day (tpd) of oxygen production) is typically at the very high purity levels.
- This high purity liquid oxygen is then stored on site and subsequently used for both the SCWO reactor and off-site customers requiring higher purity oxygen for their use.
- the high purity gaseous oxygen stream [9] as shown here is used for oxygenation of the wastewater in the aeration basin.
- FIG. 2 is a schematic illustration of a typical SCWO system used for the complete oxidation and destruction of sludge from wastewater treatment plants.
- SCWO is a process that achieves the much desired goal of complete elimination of organic materials in the sludge stream. With the appropriate reaction temperatures, pressures, and residence times, almost any organic pollutant can be completely destroyed by the SCWO process, with residence times in the SCWO reactor often less than 1 minute.
- the SCWO process consists of four main steps, namely:
- the SC WO process has the advantage of presenting simple and fast reaction rates and of being a homogeneous reaction without mass transfer limitations.
- reagents are fed into a SCWO reactor [23] that consists of an oxidant [21 ] and an aqueous based sludge waste stream [22] from a wastewater treatment plant (stream [20] as provided in Figure 1).
- SCWO reactor [23] that consists of an oxidant [21 ] and an aqueous based sludge waste stream [22] from a wastewater treatment plant (stream [20] as provided in Figure 1).
- oxidants can be used in the SCWO process including hydrogen peroxide, gaseous or liquid oxygen, or even compressed air.
- the choice of oxidants is often dictated strictly by economics.
- Use of hydrogen peroxide is not economically feasible due to its high cost, and the costs (per unit of oxygen) associated with compression of air to the very high pressures required by the SCWO process is also prohibitive.
- the most economical choice is to pump liquid oxygen through an LOX pump [25] to the high pressures required for the SCWO reaction.
- the sludge and other organic materials in the aqueous medium are normally pressurized up to the SCWO working pressures (i.e. 22.1 MPa or 217 arm) using a pump [24] and fed into the SCWO vessel and eventually the SCWO reactor [23].
- the heating value of the sludge feed [22] is controlled by concentrating or diluting the sludge feed [22] to a desired solids level, or by adding additional waste or fuel if the heating value of the sludge feed is low.
- the oxygen is supplied at high pressures to match the operating pressure of the SCWO reactor, which requires pressures of about 3200 psia.
- the sludge feed and oxygen are then mixed in a mixer [26] and then fed directly to the SCWO reactor [23], or through a heat exchanger (not shown) to preheat the reactants before introduction to the SCWO reactor.
- the oxygen and the aqueous based sludge streams are mixed they react exothermieally within the reactor vessel [23].
- the heat released by the oxidation reaction is sufficient to heat the reagents up to the operation temperature, which is typically above 600 Celsius, more preferably about 650 Celsius for optimally short reaction times, a temperature at which all the organic matter in the sludge is rapidly oxidized. Heat recovery to generate steam for cogen oration of electricity begins with this process.
- the SC WO reactor is preferably designed to support the harsh operational conditions and the oxidative atmosphere.
- the main operational parameters to be controlled in the SCWO reactor are reaction temperature, residence time, and extremely high pressure.
- reaction temperature When the reaction temperature is increased, the efficiency of the SCWO process is higher and the residence time necessary for the total oxidation of the reagents is reduced.
- residence times necessary for complete conversion of all organic matter in the sludge stream are less than or equal to about 50 seconds.
- liquid or supercritical oxygen will instantaneously combust with any fuel source, including sewage sludge.
- residence times in the SCWO reactor can vary from a few seconds to about 1 minute.
- the reaction pressure should be maintained above the critical pressure of water (i.e. 22.1 MPa).
- the SCWO process therefore provides for attempts to remove these solid insoluble particles by a solid-fluid separation as shown by the use of a separator [27].
- the separation means may be any conventional means including hydrocyclones, filtration systems, etc., making possible the recovery of potentially high value particles.
- the gaseous products of the reaction including carbon dioxide and perhaps nitrogen along with other fluid phase products (i.e. supercritical water) leave the SCWO vessel [28] at temperatures around 650 degrees Celsius and a pressure of about 23 MPa.
- This exit stream [29a] must be cooled and depressurized down to ambient temperatures and pressures and then separated in gas/liquid separator [37] into liquid aqueous products and gas products.
- Energy recovery from the depressurization and cooling of the exit stream [29a] can be used to preheat the reagents or even to produce electricity, as described in more detail below.
- Other revenue opportunities and/or cost avoidance opportunities from operation of the presently disclosed system includes recovery and re-use of the products from the SCWO reactor.
- Such valuable products might include water, carbon dioxide, nitrogen, as well as the residual 'ash' from the SCWO reactor.
- the "ash" stream [29b] from the remaining post oxidation contains all minerals (e.g. phosphorus and other minerals) present in the sludge stream in their highest oxidation state. These minerals can be further processed to generate additional income streams .
- a state-of-the-art steam generator [30] may be included to supply power for the on-site needs and possibly also provide excess power so that electricity could be sold back to the grid, depending on the efficiency of the heat exchange equipment [31], [32], and [33], representing heat exchangers, pumps, and intercoolers or other heat exchangers, respectfully and the steam turbine generator set [30-33] employed.
- An alternative arrangement includes providing steam to directly drive the compression train of the ASU.
- low-pressure steam or hot water may be produced by the energy recovery system associated with the SCWO unit, which can provide energy to the ASU, to the WWTP, or to other customers.
- the carbon dioxide and other products exiting the SCWO reactor can be recovered and re-used elsewhere in the plant prior to or after depressurization of the steam [34] or purified and sold to help offset the operating costs of the SCWO process. Separation of the SCWO products leads to potential recovery of gas [35] such as carbon dioxide during a stripping process that results in treated water effluent [36] returned back to the municipality for use by its citizenry.
- gas [35] such as carbon dioxide during a stripping process that results in treated water effluent [36] returned back to the municipality for use by its citizenry.
- the additional biomass may include wastewater sludge obtained from other WWTPs and/or from industrial wastes. Where such excess power generation is a key consideration to building and operating the disclosed system, it may be advantageous to also provide an auxiliary boiler that can provide heat in case the SCWO reactor is off-line or shutdown.
- the auxiliary boiler would enable stable energy production from the site.
- Table 2 details representative of ASU output streams in terms of flow rate (tons per day); temperatures (°F) and pressures (psig) to meet the oxygenation requirements of a wastewater treatment plant processing about 100 million gallons per day (MGD) of municipal wastewater together with oxygen requirements for the destruction of the sludge in the SCWO reactor.
- An air separation system integrated with the wastewater treatment plant and a state-of-the-art steam generator would to supply its own power needs and still have excess power to be sold, depending on the efficiency of the heat exchange equipment and steam turbine generator set employed.
- One alternative arrangement includes providing steam to directly drive the compression train of the ASU, or to provide power in the WWTP for pumping or other purposes.
- Another use for excess heat and/or steam would include using it within a pre-purification unit within a ASU.
- FIG. 3 is included to provide a schematic illustration yielding a general overview of how the gas supply streams, gas pressures, energy and waste water treatment process flows are exchanged between the combination of a WWTP [10] in combination with a gas supply system [1].
- the WWTP [10] includes at least a primary treatment section [20].
- This primary treatment [20] is often, but not always, followed by secondary treatment using biological (air and/or oxygen basins), [30] leading to further tertiary treatment [40] involving color removal and disinfection of the waste water.
- Most WWTPs today do not provide the fluid phase oxidation reactors, such as the supercritical oxidizing reactors or the wet air oxidation reactors (SCWO or WAO) as presented in [50] on the schematic.
- SCWO or WAO wet air oxidation reactors
- the gas supply system provides either singularly or combined, an air compression unit [2], an air separation unit [3], as well as the possibility of cryogen storage [4], pumping [5], compression [6], vaporization [7]), and refrigeration [8] capabilities.
- the primary treatment [20], secondary treatment [30], and tertiary treatment [40] can all be supplied with lower pressure (typically less than 300 psi and often less than 100 psi) gaseous oxygen (GOX) designated as [A].
- the same areas of the WWTP [10] can be supplied with compressed air [B], as well as low pressure nitrogen gas and/or waste gases [C].
- the SCWO and WAO in the fluid oxidation region of the WWTP [50] utilize the high pressure GOX shown as [D].
- the matching of pressures of fluid flow from the gas supply system (1) to the WWTP is useful and critical to the functional operation of the overall combined system of the WWTP and the gas supply system/ASU.
- energy flows can proceed either from the sludge combustion by the fluid-phase oxidation reactors [50] or other exothermal processes [20, 30 and or 40] generated within the WWTP toward the gas supply system [1].
- the feasibility of exchanging energy from the ASU [3] toward the fluid-phase sludge oxidation system [50], most likely in the form of refrigeration [8] for condensing and recovering carbon dioxide or other components from the sludge oxidation products is also shown.
- oxygen from a co-located or on-site ASU greatly reduces the energy consumption for aeration compared to conventional compressed air-based aeration schemes.
- Use of oxygen also generally improves the overall performance of the biological systems in the wastewater treatment plant.
- Providing an oxygen supply system that promotes the dual use of oxygen in the aeration basin of the wastewater treatment plant and also to the SCWO process represents a significant reduction in energy costs for the wastewater treatment plant.
- the on-site ASU provides additional flexibility to the aeration system of the wastewater treatment plant in that the ASU can be configured and/or controlled to deliver compressed air or oxygen or both air and oxygen to the aeration basin based on selected operating conditions and performance criteria.
- the WWTP may not include primary treatment, or the biological treatment step may be a membrane bioreactor rather than a conventional activated sludge process, or other types of secondary biological treatment may be employed such as anaerobic or anoxic zones in addition to an aerobic biological zone.
- the aspects and features of the presently disclosed system would be equally applicable to a 'wet oxidation' process in lieu of the SCWO process.
- the 'wet oxidation' process is similar in many respects to the SCWO process but operates under subcritical operating conditions for water and/or oxygen.
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Abstract
A method for supplying oxygen to a wastewater treatment plant with a supercritical water oxidation reactor (6) is disclosed. The preferred method involves the constructing and operation of an air separation system (7) proximate the location of the wastewater treatment plant and SCWO reactor (6) to separate oxygen from air and produce a first output of high pressure oxygen (8) and a second output of gaseous oxygen (9). The gaseous oxygen from a second output (9) of the air separation system (7) is supplied to one or more biological systems (3) in the wastewater treatment plant whereas the high pressure oxygen (8) is supplied to the SCWO reactor (6). An aqueous-based sludge stream at supercritical pressures is combined with the high pressure oxygen and reacted in the SCWO reactor (6) to destroy all organic materials present in the sludge stream.
Description
SYSTEM AND METHOD FOR OXYGEN SUPPLY FOR
WASTEWATER TREATMENT PLANT HAVING BIOLOGICAL TREATMENT SYSTEM AND SUPERCRITICAL WATER OXIDATION TREATMENT OF SLUDGE
PRIORITY STATEMENT
The present disclosure and accompanying claims are taken from Provisional application number 61/499,958 entitled "System and Method for Oxygen Supply for Wastewater Treatment Plant Having Biological Treatment System and Supercritical Water Oxidation Treatment of Sludge" filed on 6/22/2011 the priority date for which is June 22, 2011 and for which priority from this application is taken.
In addition, U.S. Patent No. 6,460,373, entitled "CRYOGENIC RECTIFICATION SYSTEM FOR PRODUCING HIGH PURITY OXYGEN", which issued on October 8, 2002, U.S. Patent No. 6,173,586, entitled "CRYOGENIC RECTIFICATION SYSTEM FOR PRODUCING VERY HIGH PURITY OXYGEN", which issued on January 16, 2001, U.S. Patent No.
5,682,766, entitled "CRYOGENIC RECTIFICATION SYSTEM FOR PRODUCING LOWER PURITY OXYGEN AND HIGHER PURITY OXYGEN", which issued on November 4, 1997 and U.S. Patent No. 5,881,570, entitled "CRYOGENIC RECTIFICATION APPARATUS FOR PRODUCING HIGH PURITY OXYGEN OR LOW PURITY OXYGEN", which issued on March 16, 1999 are all hereby incorporated by reference.
FIELD OF THE INVENTION
The present invention relates to lowering oxygen supply costs for wastewater treatment plants employing fluid oxidizing processes such as the use of a supercritical water oxidation (SCWO) reactor or wet air oxidizing reactor (WAO) for handling sludge produced in the wastewater treatment plant (WWTP). More particularly, the present invention relates to a system and method for tailoring an air separation unit to provide gaseous oxygen and/or air to the biological systems within the wastewater treatment plant concurrently with a source of high pressure liquid oxygen for the destruction of sludge produced from the wastewater treatment plant.
BACKGROUND
Supercritical Water Oxidation (SCWO) in a Waste Water Treatment Plant iWWTP) Traditional methods of wastewater treatment involve bringing wastewater streams into contact with bacteria either in an aerobic and/or anaerobic type process in what is known as activated sludge treatment. These bacteria consume parts of the substrate material or waste contained in the wastewater, which are typically organic compounds containing carbon, nitrogen, phosphorus, sulfur, and the like. Typically, a portion of the waste is consumed to further the metabolism of the bacterial cells or maintain the physiological functioning of the bacterial cells. In addition, a portion of the waste is also consumed as part of the process of synthesis of new bacterial cells. In the aerobic process, the organisms that grow inside the basin require oxygen. This oxygen can be provided from air or from a high purity oxygen stream. In the case of air, the delivery mechanism may be through the use of surface aerators which agitate the surface of the water in the basin creating fine drops that are thrown into the air and absorb oxygen from it. More commonly, air is compressed to typically 5-15 psig in rotary lobe or centrifugal blowers and then fed to diffusion devices at the base of the aeration basin to form fine bubbles from which oxygen is dissolved into the fluid within the basin. For high purity oxygen systems, oxygen rather than air is used to feed the basin. Oxygen has several advantages over air. It can be used to intensify the biological process such that the volume of the basin, and thus its footprint and capital is reduced. The capital associated with dissolution devices for the oxygen are also typically much lower than that of the compressor and diffusers in the air based system. Due to the absence of nitrogen, the gas evolving from the basin is significantly reduced with a consequential lowering of the generation of aerosols and the stripping of volatile organic compounds to the atmosphere from the incoming wastewater. The power required for dissolution of the oxygen and mix the aeration basin is also typically significantly lower than the power for the surface aeratpors or compressors of the aeration system. The drawback of oxygen is that the benefits must be weighed against the cost of the oxygen delivered to the basin.
The activated sludge treatment process yields a certain amount of sludge which must be continuously removed from the treatment basin to maintain the steady state sludge balance which is critical to the effective functioning of the activated sludge treatment system. In addition to this biological sludge, there may be a settling step upstream of the activated sludge process that generates additional sludge that must be disposed. It is estimated that the costs associated with sludge handling and disposal processes can be between 20-60% of total operating costs associated with the overall wastewater treatment process.
Existing methods for dealing with the removal of sludge include transporting the sludge to landfills, utilization of sludge for land application or agricultural purposes, and incineration of the sludge. Most sludge disposal operations require some prior treatment of the sludge; a process known in the art as solids handling. Sludge handling processes are often costly and time consuming operations and typically involve one or more of the following steps:
(a) the concentration of the sludge in a thickener, usually requiring the use of polymers; (b) digestion of the sludge in order to stabilize the bacteria and to further reduce the volume and pathogen content of the sludge;
(c) dewatering of the sludge to reach approx 15-25% solids content; which involves the passage of the sludge through centrifuges or other solid-liquid separation type devices;
(d) storage of the sludge; and
(e) transportation to sites for landfill, land application by farmers, or other end uses.
Moreover, each of the conventional sludge disposal options have adverse economic impacts and operating considerations, as briefly outlined below. The availability of landfills that will accept sludge is waning, as well as the cost to dispose of sludge in a landfill greatly increasing. Incineration of sludge has been practiced for years but is costly, consumes significant volumes of natural gas or other suitable fuels, and discharges exhaust emissions containing numerous pollutants.
An alternative sludge disposal technology is the supercritical water oxidation (SCWO) process. When water is heated and compressed to sufficiently high temperature and pressure it becomes a supercritical fluid, distinct from either a gas or a liquid state. In this state, the supercritical fluid provides a high degree of solubility of organic matter combined with high diffusivities. When supercritical water is combined with an oxidant in a SCWO reactor, the rate of reaction of the organic material within the SCWO reactor is greatly enhanced, leading to the nearly complete destruction of any organic matter.
The SCWO process and technology appears to have several advantages over the other methods of sludge disposal in that it is a robust process with respect to the ability to oxidize a wide range of organic materials and receive a sludge stream feed with a wide range of solids content. This ability to oxidize sludge streams having a wide range of organic materials of varying solids content can lead to reduction in capital and operating expenses in comparison with the conventional pretreatments of sludge such as; digesting, dewatering, thickening, etc. Yet another significant advantage of the SCWO technology is the ability to recover energy from the SCWO process. In particular, excess energy could be produced from the SCWO process by raising steam from the heat recovery of the exit stream and driving a steam turbine generator. Such energy recovery, however, is likely only economical when employing larger scale plants.
The SCWO process, however, has realized only limited commercial success to date. This is due at least in part to the fact that the SCWO process requires very high pressure equipment which is both costly and subject to excessive fouling and corrosion primarily as a result of the high temperatures, the oxidizing environment in the SCWO reactor, as well as the presence of inorganic salts and solid particles that precipitate out of solution as the process fluid is raised in temperature and pressure.
Another drawback or disadvantage of the SCWO process is the need for high pressure oxygen containing products that serve as the oxidant. This high pressure oxygen requirement could be produced by compressing a vast amount of air, but the operating and maintenance costs associated with the compressed air are typically too high. In existing SCWO
installations, the oxygen requirements have been typically satisfied through delivery of liquid oxygen via truck from an air separation plant located remote from the plant to the SCWO site where liquid oxygen is then pumped to the necessary pressure and subsequently supplied to the SWCO reactor.
In summary, what is needed, and is therefore a portion of the subject of the present disclosure, are systems and methods that can be utilized to make the sludge disposal process using a SCWO within a WWTP more attractive from an economic standpoint and that are also simple to implement without adversely impacting the robustness and flexibility of the SCWO process within the WWTP.
SUMMARY OF INVENTION
This disclosure provides several embodiments to the present invention. Specifically, a method for supplying oxygen to a wastewater treatment plant (WWTP) incoiporating a fluid-phase sludge oxidation system is described. The method includes the steps of:
separating air in an air separation unit ( ASU) by cryogenic rectification to produce at least one low pressure oxygen stream at a pressure of less than 100 psig, at least one high pressure oxygen stream at a pressure of equal to or greater than 300 psi, and a compressed air stream;
directing the low pressure oxygen stream or the compressed air stream or both streams from the ASU to one or more biological systems in the WWTP ;
dissolving the low pressure oxygen stream or the compressed air stream or both streams in oxygen utilizing systems within the WWTP , including at least one biological system to produce a sludge stream;
increasing the pressure of the sludge stream to form sludge containing water at an elevated pressure greater than about 300 psi;
introducing the high pressure oxygen stream and the sludge stream containing water at said the elevated pressure to a fluid-phase oxidizing reactor; and
reacting the high pressure oxygen stream and the sludge stream containing water at the elevated pressure in the fluid-phase oxidizing reactor to produce heat.
An additional step of converting a portion of the heat to on-site generated electrical power and utilizing the on-site generated electrical power to supply at least part of the electrical power consumed by the ASU is often desirable. When heat is produced by the fluid-phase oxidizing reactor it can also be used to regenerate thermal swing adsorption (TSA) pre-purifiers within the ASU.
Included in this method is the ability to provide pressure for the pressurized sludge containing water stream that is between about 300 psi and 3000 psi leading to the use of a fluid-phase oxidizing reactor that is normally a wet air oxidizing (WAO) reactor. When the pressure of the pressurized sludge containing water stream is greater than about 3000 psi, the fluid-phase reactor is normally a supercritical water oxidizing (SCWO) reactor. Further to the initial embodiment, it is desirable for the ASU to be located proximate to or on-site with the WWTP such that the ASU and the WWTP can utilize common storage tanks, common operations support, monitoring and maintenance personnel and other infrastructure for production and use of the oxygen within the ASU, the WWTP, and the associated fluid-phase oxidizing reactor.
In an additional associated embodiment to the method presented, the ASU includes a liquefier and the high pressure oxygen stream is a liquid oxygen stream produced from a cold box within the air separation unit that is pumped to elevated pressure so that the liquid oxygen stream can be vaporized prior to introduction to the fluid-phase oxidizing reactor. During this operation, a portion of the heat produced is used to vaporize the liquid oxygen stream prior to introduction to the fluid-phase oxidizing reactor. The high pressure oxygen stream may also be in the form of gaseous oxygen produced from a cold box within the ASU and subsequently compressed to a pressure equal to or greater than the elevated pressure necessary for the fluid- phase reactor to operate efficiently. The low pressure oxygen stream or compressed air stream or both streams from the air supply system and associated ASU are often directed to an aeration
basin or an aerobic digester or an ozone generator or any combination of the three unit processes within a WWTP. It is also possible for the high pressure oxygen stream, which is often in the form of gaseous oxygen produced at the required pressure to feed the SCWO directly from the cold box within the ASU by pumping liquid within the cold box and wanning against incoming streams to the cold box will allow for recovering refrigeration energy.
It is also possible to direct the low pressure oxygen stream or compressed air stream or both streams to a membrane bioreactor (MBR) system for scouring or cleansing membranes of the MBR system or mixing and aerating MBR system streams within the WWTP.
Often, the ASU is configured to produce oxygen at high purity levels that are greater than or equal to 99 percent pure oxygen that produce excess liquid and gaseous products which can be stored on site for subsequent transfer in bulk or cylinder form to local industrial or medical gas uses located proximate the air separation unit for use with the WWTP and the fluid-phase oxidizing reactor. It is also possible to configured the ASU to produce oxygen at lower purity levels, normally in a range of between about 94 percent to 99 percent pure oxygen which is also appropriate for use by the WWTP and the fluid-phase oxidizing reactor. When the fluid-phase oxidizing reactor produces carbon dioxide as an off-gas the method described in the initial embodiment can further include the step of directing a portion of the refrigeration capacity from the high pressure oxygen stream or the air separation unit to cool the carbon dioxide off-gas and produce cool carbon dioxide gas or liquid products.
The fluid-phase oxidizing reactor produces ash and other solid particles containing phosphorous and other minerals from the reaction of a high pressure oxygen stream with the sludge stream within the reactor and the methods herein further include disposing, capturing or selling the phosphorous and other minerals preferably to a customer base also proximate to the
WWTP/ASU facility. In order to generate more heat and potentially more electrical and/or mechanical energy it is useful to provide an additional step to the initial embodiment of adding supplemental biomass or other combustible waste materials to the sludge stream that is subsequently fed to the fluid-phase reactor within the WWTP.
In an additional embodiment, a waste water treatment plant (WWTP) system is described that is integrated with an oxygen or air or other fluid gaseous or liquid supply and provides; an air separation unit (ASU) for separating air by cryogenic rectification to produce at least one low pressure oxygen stream at a pressure of less than 100 psig, at least one high pressure oxygen stream at an elevated pressure of equal to or greater than 300 psi, and at least one compressed air stream.
The system also includes ; one or more secondary wastewater treatment systems for the biological treatment of wastewater fluidically coupled to the air separation unit, such that the secondary wastewater treatment systems produce a sludge stream;
a sludge stream circuit including one or more pumps to increase the pressure of a portion of the sludge stream to form sludge containing water at a pressure greater than about 300 psi;
a fluid-phase oxidizing reactor fluidically coupled to the (ASU) and one or more secondary wastewater treatment systems for reacting the high pressure oxygen stream and the pressurized sludge containing water stream to produce heat;
a control unit for operatively controlling the flow of the low pressure oxygen stream or the compressed air stream or both streams from the air separation unit to the one or more secondary wastewater treatment systems so that the control unit further operatively controls the flow of the high pressure oxygen stream and the pressurized sludge containing water stream to a fluid - phase oxidizing reactor. The control unit further is operatively coupled to the air separation unit (ASU) to control the production of the low pressure oxygen stream(s), said the high pressure oxygen stream(s) and the compressed air stream.
For the system detailed above the pressure of the sludge containing water stream is between about 300 psi and 3000 psi for the case where the fluid-phase oxidizing reactor is a wet air oxidizing reactor and for the case where the pressure of the sludge containing water stream is greater than about 3000 psi, the reactor is a supercritical water oxidizing reactor.
As for the case of the method for the initial embodiment earlier described, the combined WWTP/ASU system is preferably located so that the ASU air separation unit is proximate to or on-site with a secondary wastewater treatment system and the associated fluid-phase oxidizing reactor. The remainder of the system elements have been mostly previously described above,
P T/US2012/043586
with regard to the methods of employing a liquefier, cold box, vaporizers, and compressors, etc. for the gas supply/air separation system that is coupled with the waste water treatment system. More specifically, for a waste water treatment facility that includes one or more primary treatment units coupled to the air separation unit, the low pressure oxygen stream or compressed air produced from the air separation unit is directed to the primary treatment systems. For a wastewater system with one or more tertiary wastewater treatment systems that require oxygen or ozone, the tertiary wastewater treatment systems are coupled to the air separation unit and a portion of the low pressure oxygen stream produced from the air separation unit is directed to the primary wastewater treatment systems. The system described also can include a steam turbine generating set for coupling to the fluid- phase oxidizing reactor, so that the steam turbine generator set is configured to convert a portion of said heat to on-site generated electrical power, said steam turbine generating set further coupled to said air separation unit to supply at least part of said electrical power consumed by said air separation unit. The system allows for heat being produced by the fluid-phase oxidizing reactor to be used to regenerate one or more thermal swing adsorption (TSA) pre-purifiers in the air separation unit. In addition, for situations where the fluid-phase oxidizing reactor produces carbon dioxide as an off-gas, the system further includes a carbon dioxide purification system coupled to an ASU to cool and purify the carbon dioxide off-gas and produce cool carbon dioxide gas or liquid products.
In a further embodiment, a sludge stream exiting a pretreatment stage of the waste water treatment plant is sent to a supercritical water oxidizing (SCWO) reactor where sludge is pumped to an elevated pressure of greater than 3000 psig, mixed with high pressure oxygen and/or air produced in an air separation unit (ASU), so that the oxygen and/or air remains pressurized and is heated and then reacted for combusting the sludge within the SCWO reactor.
In addition, the process stream exiting SWCO reactor is cooled and reduced in pressure such that energy recovery from the cooling and pressure drop for making steam is accomplished that also provides for heating and/or generating electricity using a steam turbine generator set.
Here it is possible that the ASU produces a combination of' oxygen and compressed air streams including a low pressure gaseous oxygen stream, a high pressure gaseous oxygen stream, and, as needed, a compressed air stream that is fed to either an aeration basin of the WWTP or to the SCWO reactor or to both the WWTP and the SCWO reactor. Any excess of the combination of oxygen and compressed air streams are utilized by surrounding consumers when the WWTP and the SCWO reactor do not consume all of the streams produced by the ASU. Excess liquid products are stored on site for subsequent transfer in bulk form to service local industrial gas market(s) thereby further reducing costs associated with operating the SCWO reactor.
In a further embodiment, if the scale of the ASU is less than 100 tons per day production of oxygen and the oxygen is produced at purity levels greater than 99 percent purity of the oxygen not fully utilized by the SCWO the oxygen can be either used for the aeration basin or sold to industrial gas users proximate to the combined WWTP and ASU facility.
Another embodiment includes the case of utilizing larger scale air separation units (ASUs) that produce 200 or more tons per day of oxygen of lower purity oxygen as a gas at purity levels in the range of between 94 percent to 99 percent pure oxygen, providing gaseous oxygen in this purity range to the aeration basin of the WWTP is acceptable and the lower purity oxygen reduces the power consumption requirements of producing the oxygen from the ASU. For these larger scale ASUs, delivery of high pressure oxygen for use in the SCWO reactor may be achieved directly from the cold-box of the ASU by pumping some of the cryogenic liquid oxygen from the distillation column in the ASU and transferring heat to a high pressure incoming air stream to the ASU such that remaining cryogenic liquid oxygen continues to be processed in the ASU to produce higher purity liquid oxygen thereby meeting needs of local industrial gas markets.
If more heat is needed in order to increase the power generation to the ASU from the WWTP, additional sources of biomass or other combustible waste materials are added to the sludge going to the SCWO reactor and the excess power generation is useful for operating the combined ASU/WWTP system. In some instances an auxiliary boiler is provided for heating in
the event that the SCWO reactor is off-line or is shutdown and thereby enables stable energy production from the combined ASU/WTTP system.
Further it is a part of this disclosure to allow for the same method as previously described using an aeration basin and producing a sludge stream and reacting the sludge containing water with oxygen and/or compressed air all within a wet air oxidation (WAO) reactor. Providing an oxygen supply system allows for promotion of a dual use of oxygen in the aeration basin of the WWTP and also to the WAO reactor all of which provides a reduction in energy and air and/or oxygen costs for the combined ASU and WWTP system.
It is also possible to include a system that includes an aeration basin and a supercritical water oxidizing (SCWO) reactor or a wet oxidation reactor within a waste water treatment plant (WWTP) where the oxygen gas is generated from an air separation unit (ASU) and is injected into the aeration basin and the SCWO or the wet oxidation reactor and the oxygen is also available for injection into a primary wastewater flow at a front end of the WWTP or to an ozone generator to produce an ozone/oxygen gas mixture.
To ensure proper heat transfer within the combination of an air separation unit (ASU) and a waste water treatment plant (WWTP) with a supercritical water oxidizing (SCWO) reactor including one or more a heat exchangers that operate at pressures high enough to supply oxygen directly to a supercritical water reactor the heat exchangers should be spiral wound coil type and the heat exchangers should be comprised of steel and the steel should be steel is stainless steel or other alloys of steel with heat capacity greater than that of aluminum
In yet another embodiment, the combination of an air separation unit (ASU) and a waste water treatment plant (WWTP) with a supercritical water oxidizing reactor (SCWO) requires one or more control systems, with an arrangement between the ASU, an aeration basin as well as either the SCWO reactor or a wet air oxidation reactor within the WWTP and includes one or more programming logic controllers (PLC) and associated computer operating system(s) and software to coordinate all processes and flows to and from each operating unit of each facility. Model predictive computational software and hardware combinations that model the primary units of the system
including the ASU, the WWTP, the SCWO reactor, the aeration basin, and the wet air oxidation reactor or any combination of these primary units for providing orders to the machinery of and within the primary units of the system is provided.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other aspects, features, and advantages of the present invention will be more apparent from the following, more detailed description thereof, presented in conjunction with the following drawings, wherein:
FIG. 1 is a schematic illustration of the oxygen supply system for a wastewater treatment plant that is coupled to or processes its sludge by means of an SCWO reactor in accordance with the present invention. FIG. 2 is a schematic illustration of a supercritical water oxidation (SCWO) system and process of the type used to process sludge from a wastewater treatment plant.
FIG.3 is a schematic illustration of the gas supply streams, gas pressures, energy and waste water treatment process flows for the combination of a WWTP/gas supply system.
DETAILED DESCRIPTION
Use of Air Separation Units or Plants to Supply High Pressure Oxygen, Compressed Air and/or Low Pressure Oxygen
The use of air separation plants or units (ASUs) that provide atmospheric gases and liquids for industrial use is well known. These plants may be non-cryogenic, for example based on Vacuum Pressure Swing Adsorption, or cryogenic where the bulk separation is effected by distillation. Cryogenic facilities are characterized in that they can produce both gaseous and liquid products (oxygen, nitrogen, argon) at a range of pressures and flows depending on the required products. These facilities have been built on ever larger scales, greater than 10 tons
per day to several thousand tons per day. Because the feed is air, the primary operating cost is for the energy required to compress the air to the plant and to compress/pump the product gases. The need to establish energy reduction measures to reduce power consumption requirements has resulted in continued optimization of the equipment utilized within these ASUs. This equipment includes compressors, turbines, distillation columns, refrigeration units, heaters, pumps, heat exchangers, etc. all required for the energy intensive needs for separating, purifying, and producing gases and liquids from air.
Specialty cryogenic air separation systems have been developed over the last 60 years as well, to meet customer demands for specialized needs including that of low and high pressure gaseous and liquid oxygen and other industrial gases including nitrogen, argon, xenon, neon, etc. These cryogenic ASU's are designed differently depending on the production requirements. Specifically, for the present disclosure, the need for delivering oxygen in various forms (high pressure for the SCWO, low pressure for the activated sludge treatment) to a WWTP with an SCWO reactor and liquid products for sale to local industries has been established.
Combination of Utilizing a SCWO in a Waste Water Treatment Plant (WWTP) with an Air Separation Unit (ASU)
By utilizing all or at least most of the oxygen produced by the ASU-cryo system described above to provide the oxygen needs for a WWTP together with a SCWO (or even a wet-oxidation process that is similar in many respects to the SCWO process for treating sludge but operates under subcritical operating conditions for water and/or oxygen), allows for an economical and synergistic combination of industrial plant processes.
When the SCWO is combusting sludge it provides an exothermic reaction with an abundance of heat. This heat can be used to generate steam. Preferably, this steam can be used to generate power via a steam turbine. The power can be used directly to provide all or a portion of the energy required to power the air separation plant and the WWTP. Alternatively, steam can be integrated with the ASU, for evaporation of the liquid to the gas state for example. Additionally, the exothermal heat generated
from the SCWO or the wet-oxidation reactors might be used for helping run other operating units within the ASU including for example pre-purification regeneration systems, etc. There are certainly additional scenarios where the use of the excess heat and/or steam made from the heat could be utilized. In summary, there is certainly recognition that optimal synergy results from the combination of the ASU with the WWTP .
Achieving sludge disposal technology using the supercritical water oxidation (SCWO) process that includes the provision of a compatible air separation unit (ASU) capable of providing oxygen is the subject of this disclosure. Further to the earlier discussion providing the oxygen in an efficient and economically attractive manner on a relatively large scale requires an
understanding of the proper choice of the ASU for the WWTP facility and the surrounding geographical area.
The proper choice of the ASU depends on a number of considerations. Among them, the SCWO process' requirement for very high pressure oxygen is predominant. As noted earlier, there are several approaches to providing oxygen at these very high pressures.
One approach is to produce oxygen at low pressures in a "gaseous only" facility. Such a facility uses relatively little power for refrigeration and delivers high yields of gaseous oxygen with only minor amounts of liquid oxygen. For a typical 100 tpd (ton per day) gaseous oxygen facility, about 95+ tpd gaseous oxygen is produced and only 1-2 tpd of liquid oxygen and nitrogen (normally sent to storage tanks for future consumption or "as needed" demand) is produced. Some nitrogen and argon is also delivered and can be captured and cooled to liquid. These "gaseous only" facilities deliver product oxygen at relatively low pressures (just above ambient pressures from 1-3 arm.). A fraction of this low pressure gaseous oxygen can be sent to the secondary waste water treatment facility without further compression. The remaining low pressure gaseous oxygen can be compressed to the pressure required by the SCWO system. This is not recommended due to cost and operational concerns associated with very high pressure gaseous oxygen compression equipment
12 043586
A second approach also is to produce oxygen at low pressures in a "gaseous only" facility. A fraction of this low pressure gaseous oxygen can be sent to the secondary waste water treatment facility without further compression. The remaining gaseous oxygen then can be liquefied using a separate liquefier. The resulting liquid oxygen then can be pumped to the pressures required by the SCWO facility, warmed against some heat source, typically the atmosphere, warm water from the ASU plant's compression train intercoolers or waste heat from the SCWO system itself. Such a system has the virtues of simplicity and easy control but is very energy intensive since the refrigeration of liquefaction is disposed of without recovery. To avoid this waste of expensive refrigeration, a third ASU choice is available. This is a cryogenic ASU facility that is designed to provide predominantly gaseous oxygen at high pressures. The ASU can be adapted to pump the fraction of oxygen product required for the SCWO system internally using self-produced liquid oxygen to the required pressure. This high pressure supercritical oxygen then can be wanned against incoming process feed air thereby recirculating the refrigeration and retaining it within the ASU's insulated process space. This system thereby is much more energy efficient and hence more economical.
"Pumper" facilities also use compressors, but primarily for compressing air using standard compressors. Working with cold gases or liquids and pumping oxygen up to high pressures without oxygen compressors is a more reliable, economical more desirable method of oxygen delivery for the needs of a SCWO system.
As noted above, the drawback is that the very high pressure oxygen required for the SCWO system requires special heat exchange equipment within the ASU. Ideally, for the SCWO system to be efficient, the oxygen should be provided at levels near the pressure and temperature of supercritical water (373.94 degrees Celsius and 216.75 arm.). Designing heat exchangers to operate at these conditions is not a trivial task. The plate fin brazed aluminum types normally used in cryogenic ASUs are incapable of accommodating the pressures required.
However, these conditions can normally be accomplished using stainless steel instead of aluminum as the choice of material construction, and employing high pressure, tubular heat exchanger designs such as spiral wound coil heat exchangers In response to the needs associated with the SCWO process, this preferred ASU system is one often referred to as a "gaseous pump cycle plant" or "pumper.
Additional considerations influencing the type of ASU used to support a SCWO system are the needs of the local geographical region for oxygen, nitrogen and argon products. These needs can be furnished by the cryogenic ASU serving the SCWO by means of liquid products supplied by the plant in addition to the products supplied to the secondary wastewater treatment facility and the SCWO system.
Here an ASU large enough to meet all of these needs is required. It can be equipped with internal pumps to provide very high pressure oxygen to the SCWO system in a manner similar to that of a "pumper"; it can produce low pressure gaseous oxygen for use in the secondary wastewater treatment facility and it can be equipped with powerful refrigeration services to permit the production of large volumes of cryogenic liquid products for the local geography. Such ASU plants are capable of providing more or less liquid oxygen, nitrogen and argon, typically between 10 and 50% of the plant feed air volume. Economic feasibility studies indicate that such ASU plants are normally capable of at least 200 tpd of oxygen delivery might also be capable of providing at least 40-100 tpd of high pressure gaseous oxygen to the SCWO process, similar volumes to the secondary treatment facility and the balance to meet the needs of the local geography. It is well known that increasing the scale of the plant leads to economies of scale for all products reducing both the costs of oxygen supplied to the wastewater treatment facility and the cost of products supplied to the local geographies. There are many reasons for these scale economies. Among them are use of common storage tanks and other infrastructure for production and use of oxygen and other products and using common operations support, monitoring and maintenance.
As previously stated, when water is heated and compressed to sufficiently high temperatures and pressures (normally above 647 degrees Kelvin and 217 atm. pressure) it becomes a supercritical fluid, distinct from either the gas or liquid state of water. In this state, the supercritical water provides a high degree of solubility of organic matter and also provides high diffusivities. When supercritical water is combined with an oxidant (in this case oxygen from the ASU facility) in a SCWO reactor, the rate of reaction of the organic material within the SCWO reactor is greatly enhanced and essentially instantaneous in the presence of oxygen, leading to the nearly complete destruction of any organic matter.
However, the supercritical water oxidation (SCWO) process, has realized only limited commercial success to date due to lack of optimal designs leading to economic feasibility. As the desirable SCWO process requires very high pressure equipment, so too does the desirable ASU plant, and this match of capital equipment needs has been heretofore unrecognized or achieved. Until now the SCWO process need for high pressure oxygen containing products that serve as the oxidants has been considered a drawback or disadvantage. It is possible to provide high pressure oxygen by compressing a vast amount of air, but the operating and maintenance costs associated with the compressed air are typically too high. In existing SCWO installations, the oxygen requirements have been previously satisfied through delivery of liquid oxygen via truck to the SCWO site where liquid oxygen is then pumped to the necessary pressure. This is a practical solution but requires the oxygen to be produced as a liquid from a remote air separation facility and then transported to the site.
As described before, the use of SCWO process requires high pressure oxygen that is typically produced from the cryogenic separation of air. To economically deliver a supply of very high pressure oxygen at the volumes typically needed by commercial scale SCWO reactors, the air separation equipment and process must be designed using the appropriate process cycles and equipment designs and properly scaled to minimize capital costs and power requirements.
Recognizing a dual use of oxygen opportunity where SCWO processes are coupled with wastewater treatment facilities overcomes some of the economic challenges of delivering only
high pressure oxygen. By also supplying lower pressure oxygen gas to the aeration basin in the wastewater treatment plant, the air separation cycle within the ASU can be designed to be more effective in unit power consumption in terms of kWh per ton of oxygen produced compared to the air separation cycle where only very high pressure oxygen or only liquid oxygen is required. Further the increased scale of the plant lowers the unit production cost of the oxygen.
Once the system is designed with the proper cycle and equipment designs for high pressure oxygen supply, the incremental cost in capital and power to also provide low pressure oxygen for the biological wastewater treatment is modest. This reduces the cost to provide the advantages of oxygen aeration to the biological system, such as reduced aeration power or increased treatment capacity and removes the capital expense associated with the air compression and diffusion.
As also previously stated, the cost of the oxygen to be used as the oxidant in the SCWO reactor represents up to 80% of the total operating costs of an SCWO system. As a result, the economic feasibility of an SCWO process and commercial success will be improved through optimization of the ASU provided to produce the oxygen for SCWO processes and WWTP process.
Control Methods for the SCWO or Wet Air Oxidation /WWTP Combination with an Air Separation Unit
Control systems for managing the desired combination of the SCWO or Wet Air Oxidation (WAO) Combination with the WWTP are not trivial and require coordinating different process streams and monitoring different process parameters as needed between the two plants. Oxygen supply with dissolved oxygen sensor(s) are used to maintain set points and control oxygen flow into the aeration basin. One or more blowers are normally used to ensure that enough air and/or oxygen is sent to the aeration basin. The ASU plant is normally equipped with control valves and/or other switches that provide for flow of the oxygen (or compressed air or other gases) to the desired locations.
For the SCWO and the WAO (wet air oxidation) reactors, the need to detemiine temperatures of the sludge and/or the reactors themselves and how this is proportional to the oxygen concentration and purity and volume usage, is needed. For example, if starving oxygen or the air or the combination
into the aeration basin or the SCWO occurs, there needs to be adequate protection to control flow rates and process conditions using thermocouples and control valves, etc. A control loop around the SCWO, and the aeration basin so that high and low pressure oxygen can be regulated, is also necessary. Another issue that can be addressed by the combination of the ASU and WWTP is that ozone needs for the WWTP can also be met. Ozone provides for tertiary waste water treatment and is used in abundance in some facilities to lower chemical oxygen demand (COD). Control of the amount of oxygen and the pressure needed to send the gas form the ASU to an ozone generator on the WWTP site, is also part of the present disclosure.
A feedback/feed forward arrangement between the ASU and the aeration basin as well as to either the SCWO or WAO reactor along with the necessary PLC (programming logic controller) and associated computer operating system and software greatly benefits the necessary coordination. Model predictive computational software and hardware combinations for modeling plants and providing orders to the machinery of the plants is needed as well.
Turning now to Fig. 1, there is shown a schematic illustration of a WWTP system that includes a combination of the use of an air separation unit (ASU) [7], that provides for the complete oxidation and destruction of sludge from wastewater treatment plants by the use of a supercritical water oxidation reactor (SCWO) [6]. An oxygen supply system for a wastewater treatment plant that is coupled to and processes its sludge by means of an SCWO reactor is provided as one embodiment of the present disclosure. The illustrated wastewater treatment plant (WWTP) comprises an incoming stream of polluted water [1] that is first introduced into a primary clarifier [2], followed by secondary treatment [3], such as biological treatment in an activated sludge basin, and subsequently by a secondary clarifier [4]. A treated wastewater [19] stream leaves the secondary clarifier for disposal or for additional advanced tertiary treatment and reuse. A portion of the sludge stream [18] is returned to the activated sludge basin [3]. The remainder [17] is sent to a digester, thickener, or some other form of sludge pretreatment in the process stage [5]. Sludge stream [15] from the primary clarifier [2] may also be sent directly to the sludge pretreatment [5] process stage. The pretreatment portion that is prior to the SCWO will typically include maceration and may also include sludge thickening, dewatering, etc.
Ideally the sludge leaving the pretreatment stage [5] will typically contain solids in the range of 3-25% by mass and preferably in the range of 10-20% by mass. The sludge stream [20] exiting the pretreatment stage [5] is then sent to the SCWO stage [6] reactor where it is pumped to an elevated pressure (greater than 3000 psig), mixed with high pressure oxygen [8] produced in the air separation unit (ASU) [7], heated, and then reacted in the SCWO reactor. The resulting stream [13] exiting the SCWO reactor is then cooled and dropped in pressure. Any energy recovery [14] associated with cooling this stream can be used to raise steam for use in heating or else as a means to generate electricity by, for example, a steam turbine generator set (not shown in FIG 1, but provided in FIG 2). In one preferred embodiment, as shown in FIG.l, the air separation system [7] can produce both a low pressure gaseous oxygen stream [9] that is fed to the aeration basin of the wastewater treatment plant [3] and also produces a high pressure oxygen stream [8] that is fed to the SCWO reactor stage [6]. It is also possible for the air separation system [7] to produce additional liquid products (oxygen, nitrogen, argon, etc.) that are stored on-site for subsequent transfer in bulk form to service the local industrial gas market(s). The optimum choice of the air separation processes employed by the ASU depends on the scale of the air separation system or plant and the amount of liquid products to be produced and was earlier discussed. Since there is often a need to produce very high purity oxygen (>99% purity) to meet the needs of the local industrial gas markets, production of all oxygen products in smaller scale air separation units (i.e. scale of less than about 100 tons per day (tpd) of oxygen production) is typically at the very high purity levels. This high purity liquid oxygen is then stored on site and subsequently used for both the SCWO reactor and off-site customers requiring higher purity oxygen for their use. The high purity gaseous oxygen stream [9] as shown here is used for oxygenation of the wastewater in the aeration basin however it is not essential to use high purity gaseous oxygen for the aeration basin. Either compressed air or lower purity oxygen is also acceptable, and can be supplied by the ASU for both the aeration basin as well as for the SWCO reactor.
Products exiting the SCWO reactor include both gas phase products such as carbon dioxide and perhaps nitrogen as well as liquid phase products such as water. Ash and other solid particles also exit the SWCO reactor represented as insoluble materials that are also a by-product of the
SCWO process. The solid, gas and liquid products exiting the SCWO reactor [13] are preferably separated and either recovered for re-use (such as recovery of phosphorus from the ash) or sent to disposal [not shown]. Recovery and re-use of these products also helps offset the costs of operating the SCWO reactor. Within the ASU [7], air is normally supplied through a multistage compressor and separated within the ASU process to provide both a supply of high pressure oxygen to an SCWO reactor and can also further supply lower pressure gaseous oxygen requirements to the biological basin(s) of the waste water treatment plant (into the aeration basis [3]). By providing this on-site, dual-use of oxygen in both liquid and gaseous forms, the effective cost of oxygen per unit for the SCWO process is greatly reduced. The reduction in oxygen cost is attributed to:
(i) increasing the scale of oxygen production on the on-site or co-located air
separation unit (ASU);
(ii) Removing the power and capital needs associated with all or a portion of the air otherwise required for the activated sludge treatment
(iii) using common storage tanks and other infrastructure for the production and uses of oxygen in the ASU, wastewater treatment plant, and SCWO reactor; and
(iv) using common operations support, monitoring and maintenance for the ASU, the waste water treatment plant, and the SCWO reactor system.
Further, the ASU is preferably designed to support the export of additional liquid products (i.e. nitrogen, oxygen, argon, etc.) to the local industrial gas markets to further offset the effective cost of oxygen delivered to the wastewater treatment plant and the SCWO reactor.
The air separation unit represented as [7] in FIG. 1 preferably comprises both a cryogenic cold- box that cools and separates the air, pressurization means for the high pressure oxygen stream, either internal or external to the cold-box, and cryogen storage systems. . The amount of liquid oxygen that can be generated from the air separation system is a function of the process cycle chosen for the ASU (as earlier described).
Table 1 below characterizes the details associated with typical activated sludge within a WWTP and the associated air or oxygen requirements for biological oxygen demand - BOD - (in kg/day) for an aeration basin and for a SCWO reactor. Here the acronyms provided in Table 1 for the environmental loads on the WWTP include; the SRT, which represent the solids retention time, the WAS which represents the waste activated sludge, the BOD which represents the biological oxygen demand, the COD, which represents the chemical oxygen demand, the TSS which represents the total suspended solids, the VSS which represents the volatile suspended solids, and the TKN which represents the total Kjehldahl nitrogen for the WWTP.
TABLE 1*
A Typical Complete Mix Activated Sludge Nitrifying Plant for a Municipal Waste
Water Treatment Facility
* : Metcalf & Eddy, Example 8-2, p. 707, for typical case ""Wastewater Engineering, Treatment and Reuse," Fourth Edition, International Edition 2004, Metcalf & Eddy, Inc.,
Revised by George Tchobanoglous, Franklin L. Burton, and H. David Stensel, McGraw- Hill, New York. ISBN 007- 124140-
In one embodiment, it is possible to provide a fraction of the liquid oxygen - LOX- to be typically between about 5% to about 60%, but most likely about 50% of the total oxygen flow produced from the cold-box, with the remaining oxygen being provided in gaseous form. This ratio of liquid to gaseous oxygen production would match the typical demand of the
WWTP/SCWO system for high pressure and low pressure oxygen. In a second embodiment; in those instances where there is a greater need for liquid products, a liquefier can be added to the air separation system that takes all or a portion of the gaseous products, including oxygen, from the plant and liquefies it through a liquefaction cycle typically employing nitrogen as the working fluid. The liquefier can be a stand-alone unit essentially independent from the air separation system except for a conduit to supply the gas to be liquefied, or preferably the liquefier may be integrated with the air separation system with the liquid from the liquefier being returned to the ASU and allowing more liquid products to be withdrawn from the plant In cases where the WWTP and SCWO require higher or lower ratios of liquefied oxygen to gaseous oxygen, differently designed ASU's are possible as is the need for proper location and sizing of the liquefier.
The preferred air separation system is co-located at a wastewater treatment plant that also includes an SCWO reactor. The air separation system may also include an internal or external pumping arrangement to pressurize the liquid oxygen from the cryogenic storage systems to the very high pressures, vaporize the high pressure liquid, and optimally deliver the high pressure oxygen to the SCWO system. Alternatively, the air separation system or plant can be designed to produce liquid oxygen that is subsequently pumped to a high pressure and supplied to the SCWO reactor directly from the ASU. As also earlier described, alternative arrangement requires special high pressure heat exchangers such as spiral wound systems using stainless steel rather than aluminum. Also, a minimum liquid oxygen production to be stored on-site is required to provide back-up capability for the SCWO reactor in the event of a disruption in the direct supply of high pressure oxygen from the ASU to the SCWO reactor, to provide storage for load leveling, and also to provide liquid products for sale to the local market.
Since there is often a need to produce very high purity oxygen (>99% purity) to meet the needs of the local industrial gas markets, production of all oxygen products in smaller scale air separation units (i.e. scale of less than about 100 tons per day (tpd) of oxygen production) is typically at the very high purity levels. This high purity liquid oxygen is then stored on site and subsequently used for both the SCWO reactor and off-site customers requiring higher purity oxygen for their use. The high purity gaseous oxygen stream [9] as shown here is used for oxygenation of the wastewater in the aeration basin.
FIG. 2, as provided is a schematic illustration of a typical SCWO system used for the complete oxidation and destruction of sludge from wastewater treatment plants. From the environmental perspective, SCWO is a process that achieves the much desired goal of complete elimination of organic materials in the sludge stream. With the appropriate reaction temperatures, pressures, and residence times, almost any organic pollutant can be completely destroyed by the SCWO process, with residence times in the SCWO reactor often less than 1 minute. In a broad sense, the SCWO process consists of four main steps, namely:
(1) reagent preparation and pressurization;
(2) reaction;
(3) salt separation;
and
(4) depressurization and heat recovery.
From a technical perspective, the SC WO process has the advantage of presenting simple and fast reaction rates and of being a homogeneous reaction without mass transfer limitations.
Referring back to FIG. 2, the wastewater application of a SCWO process of the present disclosure is more fully described. According to this diagram, reagents are fed into a SCWO reactor [23] that consists of an oxidant [21 ] and an aqueous based sludge waste stream [22] from a wastewater treatment plant (stream [20] as provided in Figure 1). Various oxidants can be used in the SCWO process including hydrogen peroxide, gaseous or liquid oxygen, or even compressed air. The choice of oxidants is often dictated strictly by economics. Use of hydrogen peroxide is not economically feasible due to its high cost, and the costs (per unit of oxygen)
associated with compression of air to the very high pressures required by the SCWO process is also prohibitive. The most economical choice is to pump liquid oxygen through an LOX pump [25] to the high pressures required for the SCWO reaction. The sludge and other organic materials in the aqueous medium are normally pressurized up to the SCWO working pressures (i.e. 22.1 MPa or 217 arm) using a pump [24] and fed into the SCWO vessel and eventually the SCWO reactor [23]. The heating value of the sludge feed [22] is controlled by concentrating or diluting the sludge feed [22] to a desired solids level, or by adding additional waste or fuel if the heating value of the sludge feed is low.
It is desirable for the oxygen to be supplied at high pressures to match the operating pressure of the SCWO reactor, which requires pressures of about 3200 psia. The sludge feed and oxygen are then mixed in a mixer [26] and then fed directly to the SCWO reactor [23], or through a heat exchanger (not shown) to preheat the reactants before introduction to the SCWO reactor. When the oxygen and the aqueous based sludge streams are mixed they react exothermieally within the reactor vessel [23]. The heat released by the oxidation reaction is sufficient to heat the reagents up to the operation temperature, which is typically above 600 Celsius, more preferably about 650 Celsius for optimally short reaction times, a temperature at which all the organic matter in the sludge is rapidly oxidized. Heat recovery to generate steam for cogen oration of electricity begins with this process. The SC WO reactor is preferably designed to support the harsh operational conditions and the oxidative atmosphere.
The main operational parameters to be controlled in the SCWO reactor are reaction temperature, residence time, and extremely high pressure. When the reaction temperature is increased, the efficiency of the SCWO process is higher and the residence time necessary for the total oxidation of the reagents is reduced. At SCWO reaction temperatures around 650 degrees Celsius, residence times necessary for complete conversion of all organic matter in the sludge stream are less than or equal to about 50 seconds. At 650 degrees Celsius liquid or supercritical oxygen will instantaneously combust with any fuel source, including sewage sludge. Depending on the reaction temperature, residence times in the SCWO reactor can vary from a few seconds to about 1 minute. Preferably, the reaction pressure should be maintained above the critical pressure of water (i.e. 22.1 MPa). Further increasing the pressure generally does not improve the process
efficiency. On the other hand, when operating the reactor at slightly lower pressures than the critical pressure of water, the sludge destruction rate or conversion rates decrease slightly, but the process is still effective as long as the reaction temperatures remain high and the residency times are extended. Because of reduced salt solubility in the SCWO process, eventually stable solid particles are typically become present in SCWO process. These particles are often corrosive and tend to cause equipment fouling, plugging, and erosion. The SCWO process therefore provides for attempts to remove these solid insoluble particles by a solid-fluid separation as shown by the use of a separator [27]. The separation means may be any conventional means including hydrocyclones, filtration systems, etc., making possible the recovery of potentially high value particles.
The gaseous products of the reaction, including carbon dioxide and perhaps nitrogen along with other fluid phase products (i.e. supercritical water) leave the SCWO vessel [28] at temperatures around 650 degrees Celsius and a pressure of about 23 MPa. This exit stream [29a] must be cooled and depressurized down to ambient temperatures and pressures and then separated in gas/liquid separator [37] into liquid aqueous products and gas products. Energy recovery from the depressurization and cooling of the exit stream [29a] can be used to preheat the reagents or even to produce electricity, as described in more detail below. Other revenue opportunities and/or cost avoidance opportunities from operation of the presently disclosed system includes recovery and re-use of the products from the SCWO reactor. Such valuable products might include water, carbon dioxide, nitrogen, as well as the residual 'ash' from the SCWO reactor. The "ash" stream [29b] from the remaining post oxidation contains all minerals (e.g. phosphorus and other minerals) present in the sludge stream in their highest oxidation state. These minerals can be further processed to generate additional income streams .
A state-of-the-art steam generator [30] may be included to supply power for the on-site needs and possibly also provide excess power so that electricity could be sold back to the grid, depending on the efficiency of the heat exchange equipment [31], [32], and [33], representing heat exchangers, pumps, and intercoolers or other heat exchangers, respectfully and the steam turbine generator set [30-33] employed. An alternative arrangement includes providing steam to
directly drive the compression train of the ASU. In addition to high-pressure steam, low-pressure steam or hot water may be produced by the energy recovery system associated with the SCWO unit, which can provide energy to the ASU, to the WWTP, or to other customers.
Additionally, the carbon dioxide and other products exiting the SCWO reactor can be recovered and re-used elsewhere in the plant prior to or after depressurization of the steam [34] or purified and sold to help offset the operating costs of the SCWO process. Separation of the SCWO products leads to potential recovery of gas [35] such as carbon dioxide during a stripping process that results in treated water effluent [36] returned back to the municipality for use by its citizenry.
As mentioned above, if the scale of the wastewater treatment plant and SCWO system is sufficiently large, electricity generation from the facility is also potentially profitable. In many large municipal wastewater operations, the energy production can exceed the energy required by the air separation system and the WWTP and thus provide a source of renewable and sustainable energy to the grid. If the energy from the SCWO system is sufficient to power the ASU during normal operating conditions, alone is a very significant cost reduction. ASUs require large amounts of power for compression, refrigeration, distillation, heat exchangers, etc. and were historically built in areas where energy costs are low - especially near hydroelectric power plants, for example. In order to increase the power generation from the WWTP additional sources of biomass or other combustible waste materials could be added to the sludge stream going to the SCWO system. The additional biomass may include wastewater sludge obtained from other WWTPs and/or from industrial wastes. Where such excess power generation is a key consideration to building and operating the disclosed system, it may be advantageous to also provide an auxiliary boiler that can provide heat in case the SCWO reactor is off-line or shutdown. The auxiliary boiler would enable stable energy production from the site.
In contrast, for larger scale air separation systems or plants (i.e. with a scale of 200 tpd or more of oxygen production), it is feasible to design the air separation process to produce oxygen gas at lower purity levels that are typically in the range of between about 94% to 99% pure oxygen. The use of this oxygen in the aeration basin of the wastewater treatment is also very feasible.
Production of this lower purity oxygen allows for ultimate reduction in the power requirements per unit of oxygen for the ASU. Also, in the larger scale ASU plants, delivery of high pressure oxygen for use in the SCWO reactor can be achieved directly from the ASU cold-box by pumping some of the cryogenic liquid from the distillation column in the ASU and transferring heat to a high pressure incoming air stream to the air separation system. The remaining cryogenic liquid will continue to be processed in the ASU to produce higher purity liquid oxygen to meet the needs of the local industrial gas markets.
Table 2 below details representative of ASU output streams in terms of flow rate (tons per day); temperatures (°F) and pressures (psig) to meet the oxygenation requirements of a wastewater treatment plant processing about 100 million gallons per day (MGD) of municipal wastewater together with oxygen requirements for the destruction of the sludge in the SCWO reactor.
Table 2. ASU Output Streams
For municipal waste water treatment plants of this size an air separation system integrated with the wastewater treatment plant and a state-of-the-art steam generator would to supply its own power needs and still have excess power to be sold, depending on the efficiency of the heat exchange equipment and steam turbine generator set employed. One alternative arrangement includes providing steam to directly drive the compression train of the ASU, or to provide power in the WWTP for pumping or other purposes. Another use for excess heat and/or steam would include using it within a pre-purification unit within a ASU.
Figure 3 is included to provide a schematic illustration yielding a general overview of how the gas supply streams, gas pressures, energy and waste water treatment process flows are exchanged between the combination of a WWTP [10] in combination with a gas supply system
[1]. Generally, the WWTP [10] includes at least a primary treatment section [20]. This primary treatment [20] is often, but not always, followed by secondary treatment using biological (air and/or oxygen basins), [30] leading to further tertiary treatment [40] involving color removal and disinfection of the waste water. Most WWTPs today do not provide the fluid phase oxidation reactors, such as the supercritical oxidizing reactors or the wet air oxidation reactors (SCWO or WAO) as presented in [50] on the schematic. These separate functions [20], [30], [40], and [50] within the WWTP [10] can be fed the proper gas streams at the proper pressures as shown in the column (not numbered) separating the WWTP [10]) and the gas supply system [1]). The gas supply system provides either singularly or combined, an air compression unit [2], an air separation unit [3], as well as the possibility of cryogen storage [4], pumping [5], compression [6], vaporization [7]), and refrigeration [8] capabilities. The primary treatment [20], secondary treatment [30], and tertiary treatment [40] can all be supplied with lower pressure (typically less than 300 psi and often less than 100 psi) gaseous oxygen (GOX) designated as [A]. Likewise the same areas of the WWTP [10] can be supplied with compressed air [B], as well as low pressure nitrogen gas and/or waste gases [C]. The SCWO and WAO in the fluid oxidation region of the WWTP [50] utilize the high pressure GOX shown as [D]. The matching of pressures of fluid flow from the gas supply system (1) to the WWTP is useful and critical to the functional operation of the overall combined system of the WWTP and the gas supply system/ASU.
As also shown, energy flows can proceed either from the sludge combustion by the fluid-phase oxidation reactors [50] or other exothermal processes [20, 30 and or 40] generated within the WWTP toward the gas supply system [1]. Likewise, the feasibility of exchanging energy from the ASU [3] toward the fluid-phase sludge oxidation system [50], most likely in the form of refrigeration [8] for condensing and recovering carbon dioxide or other components from the sludge oxidation products is also shown.
INDUSTRIAL APPLICABILITY
Numerous technical advantages result from the use of oxygen supplied from a centralized on-site production facility versus independent air compression for provision of oxygen for the activated sludge treatment. For example, use of oxygen in lieu of air tends to reduce odors emanating from the wastewater treatment plant, can reduce sludge bulking and foaming in the aeration basin, and reduces the evolution of volatile organic compounds (VOC) from the aeration basin. The oxygen gas may also be used outside of the main biological aeration basin - for example, it may be injected into the primary wastewater flow at the front end of the WWTP for odor control, or may be fed to an ozone generator to produce an ozone/oxygen gas mixture for disinfection of the effluent. Other applications of the on-site production of oxygen and/or compressed air in the WWTP will be apparent to those skilled in the art.
In addition, use of oxygen from a co-located or on-site ASU greatly reduces the energy consumption for aeration compared to conventional compressed air-based aeration schemes. Use of oxygen also generally improves the overall performance of the biological systems in the wastewater treatment plant. Providing an oxygen supply system that promotes the dual use of oxygen in the aeration basin of the wastewater treatment plant and also to the SCWO process represents a significant reduction in energy costs for the wastewater treatment plant.
Alternatively, one could continue using an air-based aeration scheme but eliminate the compressors at the aeration basin by simply supplying compressed air that is provided directly from the ASU (prior to separation). The on-site ASU provides additional flexibility to the aeration system of the wastewater treatment plant in that the ASU can be configured and/or controlled to deliver compressed air or oxygen or both air and oxygen to the aeration basin based on selected operating conditions and performance criteria.
Having thus described several aspects of several embodiments of this invention, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. For example, the WWTP may not include primary treatment, or the biological treatment step may be a membrane bioreactor rather than a conventional activated sludge process, or other types of secondary biological treatment may be employed such as anaerobic or anoxic zones in addition
to an aerobic biological zone. Furthermore, many of the aspects and features of the presently disclosed system would be equally applicable to a 'wet oxidation' process in lieu of the SCWO process. The 'wet oxidation' process is similar in many respects to the SCWO process but operates under subcritical operating conditions for water and/or oxygen.
It should be understood that illustrative embodiments are described in accordance with various features and aspects of the present invention. However, the embodiments described herein are not necessarily intended to show or incorporate all aspects and features of the invention, but rather are used to describe a few illustrative embodiments. Thus, aspects and features discussed herein are not intended to be construed narrowly in view of the illustrative embodiments. In addition, it should be understood that aspects and features of the invention described may be used alone or in any suitable combination with other aspects and features also described. From the foregoing, it will be apparent that other embodiments and various modifications may be made to the present invention without departing from the scope thereof. The foregoing description of the invention is intended merely to be illustrative and not restrictive thereof. The scope of the present invention is defined by the appended claims and equivalents thereto.
Claims
1. A method for supplying oxygen to a wastewater treatment plant incoiporating a fluid- phase sludge oxidation system comprising the steps of:
separating air in an air separation unit (air separation unit) by cryogenic rectification to produce at least one low pressure oxygen stream at a pressure of less than 100 psig, at least one high pressure oxygen stream at a pressure greater than 300 psi, and a compressed air stream;
directing said low pressure oxygen stream or said compressed air stream or both streams from said air separation unit to one or more biological systems in said wastewater treatment plant;
dissolving said low pressure oxygen stream or said compressed air stream or both streams in oxygen utilizing systems within said wastewater treatment plant , including at least one biological system to produce a sludge stream;
increasing the pressure of said sludge stream to form sludge containing water at an elevated pressure greater than about 300 psi;
introducing said high pressure oxygen stream and said sludge stream containing water at said elevated pressure to a fluid-phase oxidizing reactor; and
reacting said high pressure oxygen stream and said sludge stream containing water at said elevated pressure in said fluid-phase oxidizing reactor to produce heat.
2. The method of claim 1, wherein the pressure of said pressurized sludge containing water stream and the high pressure oxygen stream are between about 300 psi and 3000 psi and said fluid-phase oxidizing reactor is a wet air oxidizing reactor.
3. The method of claim 1, wherein the pressure of said pressurized sludge containing water stream and the high pressure oxygen stream are greater than about 3000 psi and said fluid- phase reactor is a supercritical water oxidizing reactor.
4. The method of claim 1, wherein said air separation unit is located proximate to or on- site with said wastewater treatment plant and wherein said air separation unit and wastewater treatment plant utilize common storage tanks and other infrastructure for production and use of said oxygen within said air separation unit, said wastewater treatment plant and said fluid-phase oxidizing reactor.
5. The method of claim 1, wherein said air separation unit is located proximate to or on- site with said wastewater treatment plant and wherein said air separation unit and wastewater treatment plant utilize common operations support, monitoring and maintenance personnel for said air separation unit, said wastewater treatment plant and said fluid-phase oxidizing reactor.
6. The method of claim 1 , wherein said air separation unit includes a liquefier and said high pressure oxygen stream is a liquid oxygen stream produced from a cold box within said air separation unit that is pumped to said elevated pressure wherein said liquid oxygen stream is vaporized prior to introduction to said fluid-phase oxidizing reactor.
7. The method of claim 1, wherein a portion of the heat produced by said fluid-phase oxidizing reactor is used to vaporize said liquid oxygen stream prior to introduction to said fluid- phase oxidizing reactor.
8. The method of claim 1, wherein said high pressure oxygen stream is in the form of gaseous oxygen produced from a cold box within said air separation unit and subsequently compressed to a pressure equal to or greater than said elevated pressure.
9. The method of claim 1, wherein said high pressure oxygen stream is in the form of gaseous oxygen produced at the required pressure to feed said supercritical water oxidizing reactor directly from said cold box within said air separation unit by pumping liquid within said cold box and warming against incoming streams to said the cold box thereby recovering refrigeration energy.
10. The method of claim 1 , wherein said low pressure oxygen stream or said compressed air stream or both streams are directed to an aeration basin of said wastewater treatment plant.
11. The method of claim 1, wherein said low pressure oxygen stream or said compressed air stream or both streams are directed to an aerobic digester of said wastewater treatment plant.
12. The method of claim 1, wherein said low pressure oxygen stream or compressed air stream or both streams are directed to an ozone generator to produce ozone for use in said wastewater treatment plant.
13. The method of claim 1 , wherein said low pressure oxygen stream or said compressed air stream or both streams are directed to a membrane bioreactor system within said wastewater treatment plant for scouring or cleansing membranes of said membrane bioreactor system or mixing and aerating the membrane bioreactor system streams.
14. The method of claim 1 , further comprising the step of converting a portion of the heat to on-site generated electrical power and utilizing the on-site generated electrical power to supply at least part of the electrical power consumed by said air separation unit.
15. The method of claim 1, wherein the heat produced by said fluid-phase oxidizing reactor is used to regenerate thermal swing adsorption pre-purifiers in said air separation unit.
16. The method of claim 1, wherein said air separation unit is configured to produce oxygen at high purity levels greater than or equal to 99 percent pure oxygen and wherein excess liquid and gaseous products produced within said air separation unit are stored on site for subsequent transfer in bulk or cylinder form to local industrial or medical gas uses located proximate said air separation unit other than in said wastewater treatment plant and said fluid- phase oxidizing reactor.
17. The method of claim 1 , wherein said air separation unit is configured to produce oxygen at purity levels in the range of between about 94 percent to 99 percent pure oxygen for use by said wastewater treatment plant and said fluid-phase oxidizing reactor.
18. The method of claim 1 , wherein said fluid-phase oxidizing reactor produces carbon dioxide as an off-gas and the method further comprises the step of directing a portion of the refrigeration capacity from said high pressure oxygen stream or said air separation unit to cool said carbon dioxide off-gas and produce cool carbon dioxide gas or liquid products.
19. The method of claim 1 , wherein said fluid-phase oxidizing reactor produces ash and other solid particles containing phosphorous and other minerals from the reaction of said high pressure oxygen stream and said sludge stream and the method further comprises disposing, capturing or selling said phosphorous and other minerals.
20. The method of claim 1 , further comprising the step of adding supplemental biomass or other combustible waste materials to said sludge stream.
21. A wastewater treatment system with integrated oxygen supply comprising:
an air separation unit for separating air by cryogenic rectification to produce at least one low pressure oxygen stream at a pressure of less than 100 psig, at least one high pressure oxygen stream at an elevated pressure greater than 300 psi, and a compressed air stream;
one or more secondary wastewater treatment systems for the biological treatment of wastewater fluidically coupled to said air separation unit, said secondary wastewater treatment systems producing a sludge stream;
a sludge stream circuit including one or more pumps to increase the pressure of a portion of said sludge stream to form a sludge containing water stream at a pressure greater than about
300 psi;
a fluid-phase oxidizing reactor fluidically coupled to said air separation unit and said one or more secondary wastewater treatment system for reacting said high pressure oxygen stream and said pressurized sludge containing water stream to produce heat;
a control unit for operatively controlling the flow of said low pressure oxygen stream or said compressed air stream or both streams from said air separation unit to said one or more secondary wastewater treatment systems; said control unit further operatively controlling the pressure and flow of said high pressure oxygen stream and said pressurized sludge containing water stream to said a fluid oxidizing reactor; and
said control unit further operatively coupled to said air separation unit to control the production of said low pressure oxygen stream(s), said high pressure oxygen stream(s) and said compressed air stream.
22. The system of claim 21, wherein the pressure of said pressurized sludge containing water stream and high pressure oxygen stream are between about 300 psi and 3000 psi and said fluid-phase oxidizing reactor is a wet air oxidizing reactor.
23. The system of claim 21, wherein the pressure of said pressurized sludge containing water stream and high pressure oxygen stream are greater than about 3000 psi and said reactor is a supercritical water oxidizing reactor.
24. The system of claim 21, wherein said air separation unit is located proximate to or on-site with said secondary wastewater treatment system and said fluid oxidizing reactor.
25. The system of claim 21, wherein said air separation unit includes a liquefier and said high pressure oxygen stream is a liquid oxygen stream produced from a cold box within said air separation unit and pumped to said elevated pressure, said system further comprising a vaporizer coupled to said high pressure oxygen stream to vaporize said liquid oxygen stream prior to introduction to said fluid oxidizing reactor.
26. The system of claim 21, wherein said high pressure oxygen stream is in the form of gaseous oxygen produced from a cold box within said air separation unit and subsequently compressed to a pressure equal to or greater than said elevated pressure.
27. The method of claim 21, wherein said high pressure oxygen stream is in the form of gaseous oxygen produced at the required pressure to feed said supercritical water oxidizing reactor directly from the cold box within said air separation unit by pumping liquid within said cold box and warming against incoming streams to said cold box thereby recovering refrigeration energy
28. The system of claim 21 , wherein said secondary wastewater treatment system is an aeration basin.
29. The system of claim 21, wherein said secondary wastewater treatment system is an aerobic digester.
30. The system of claim 21 , further comprising a one or more primary wastewater treatment systems coupled to said air separation unit and wherein said low pressure oxygen stream or compressed air produced from said air separation unit is directed to said primary wastewater treatment systems.
31. The system of claim 21 , further comprising a one or more tertiary wastewater treatment systems requiring oxygen or ozone, said tertiary wastewater treatment systems coupled to said air separation unit and wherein said low pressure oxygen stream produced from said air separation unit is directed to said primary wastewater treatment systems.
32. The system of claim 21, further comprising a steam turbine generating set for coupled to said fluid oxidizing reactor, said steam turbine generator set configured to convert a portion of said heat to on-site generated electrical power, said steam turbine generating set further coupled to said air separation unit to supply at least part of said electrical power consumed by said air separation unit.
33. The system of claim 21, wherein said heat produced by said fluid-phase oxidizing reactor is used to regenerate one or more thermal swing adsorption pre-purifiers in said air separation unit.
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