Wet air oxidation of wastewater sludge solids

Abstract

This invention provides a method and system for treating sludge solids from wastewater streams. The wastewater treatment includes wet air oxidation (WAO) treatment upstream of anaerobic sludge digestion. The disclosed method and system provide a reduction in the amount of sludge solids to be disposed of, which is beneficial for increasing the amount of fuel gas generated through the treatment system.

Description

[0001] Cross-references to related applications

[0002] This application claims priority and benefit to U.S. Provisional Patent Application No. 63 / 590,038, filed October 13, 2023, the entire disclosure of which is incorporated herein by reference. Technical Field

[0003] This disclosure relates to methods and systems for treating sludge solids from wastewater streams. More specifically, this disclosure relates to a wastewater treatment method comprising a wet air oxidation (WAO) treatment step upstream of an anaerobic sludge digestion step. The disclosed methods and systems provide a reduction in the amount of sludge solids to be disposed of, which is beneficial for increasing the amount of fuel gas generated through the treatment system. Background Technology

[0004] Effective handling of municipal sewage and industrial wastewater requires the treatment of sludge solids and organic waste. Traditionally, wastewater sludge solids have been transported to landfills or agricultural sectors for incineration or disposal. However, these types of disposal have recently come under scrutiny due to the presence of perfluoroalkyl and polyfluoroalkyl substances (“PFAS”) (commonly known as “permanent chemicals”) in the sludge. It has been noted that the presence of PFAS in substances applied to farmland can lead to PFAS in both downstream crops and livestock, thereby contaminating the domestic food supply with these chemicals. Given the increasing awareness of the dangers and prevalence of PFAS contamination, some states have begun to ban the use of wastewater sludge in farmland and limit the amount of sludge allowed in landfills. The costs associated with wastewater sludge disposal have increased significantly due to these restrictions and / or outright bans.

[0005] Therefore, in order to comply with these changes in regulations surrounding sludge disposal, wastewater treatment methods that reduce the amount of wastewater sludge that needs to be disposed of are required. Summary of the Invention

[0006] This document discloses a method and system for treating wastewater sludge via wet air oxidation (WAO). The method and system include a secondary treatment unit 101, a wet air oxidation (WAO) unit 102, and a digester 103.

[0007] According to a first aspect of this disclosure, a method for treating wastewater is provided. The method includes directing a wastewater stream to a secondary treatment unit 101, wherein the secondary treatment unit 101 provides biological treatment of the wastewater stream and produces a treated sludge stream 28; directing the treated sludge stream to a solids thickening device 106, wherein the solids thickening device 106 separates the treated sludge stream 28 into a concentrate stream 30 and a sludge solids stream 32; directing the sludge solids stream 32 to a wet air oxidation (WAO) unit 102, wherein the WAO unit dissolves the sludge solids stream 32 by wet air oxidation to provide a WAO effluent stream 34; separating the WAO effluent stream 34 into a solid slurry stream 36 and a decanted liquid stream 38; and directing the decanted liquid stream 38 to a digester 103, wherein the decanted liquid stream 38 undergoes anaerobic digestion to produce a digester gas stream 44, wherein the digester gas stream 44 comprises methane, carbon dioxide, and hydrogen.

[0008] In a second aspect of this disclosure, unless otherwise stated, this aspect may be combined with any other aspect listed herein, a method for treating wastewater includes the steps of removing a recirculated sludge stream 42 from a digester 103 and directing the recirculated sludge stream 42 into a solids thickening unit 106.

[0009] In a third aspect of this disclosure, unless otherwise stated, this aspect may be combined with any other aspect listed herein, and the method of treating wastewater includes the step of processing digester gas stream 44 to provide a source of methane as a product for the manufacture of fuels or chemicals.

[0010] In the fourth aspect of this disclosure, unless otherwise stated, this aspect may be combined with any other aspect listed herein, and the method of treating wastewater includes the step of directing a solid slurry stream 36 to a dryer 108 to produce a sludge disposal stream 40.

[0011] In the fifth aspect of this disclosure, unless otherwise stated, this aspect may be combined with any other aspect listed herein, the method of treating wastewater includes the steps of removing supernatant stream 46 from digester 103 and directing supernatant stream 46 to secondary treatment unit 101.

[0012] In the sixth aspect of this disclosure, unless otherwise stated, this aspect may be combined with any other aspect listed herein, and the method of treating wastewater includes the step of directing a concentrate stream 30 into a secondary treatment unit 101.

[0013] In the seventh aspect of this disclosure, unless otherwise stated, this aspect may be combined with any other aspect listed herein, and the method of treating wastewater includes the step of clarifying the wastewater stream 20 in a primary treatment unit 104 located upstream of the secondary treatment unit 101.

[0014] In the eighth aspect of this disclosure, unless otherwise stated, this aspect may be combined with any other aspect listed herein, and the method of treating wastewater includes the step of clarifying the treated sludge stream 28 in a secondary clarifier 105 located upstream of the solids thickener 106.

[0015] In the ninth aspect of this disclosure, unless otherwise stated, this aspect may be combined with any other aspect listed herein, and the method provides a reduction of at least 50% in volatile solids (VSR) from wastewater stream 20 to digester gas stream 44.

[0016] In the tenth aspect of this disclosure, unless otherwise stated, this aspect may be combined with any other aspect listed herein, and the method provides a reduction of at least 80% in volatile solids (VSR) from wastewater stream 20 to digester gas stream 44.

[0017] According to the eleventh aspect of this disclosure, a wastewater treatment system 100 is provided. The system includes a secondary treatment unit 101 configured to provide biological treatment of wastewater stream 20; a wet air oxidation (WAO) unit 102 located downstream of the secondary treatment unit 101, wherein the WAO unit is configured to dissolve a sludge solid stream 32 by wet air oxidation to provide a WAO effluent stream 34; a device for separating the WAO effluent stream 34 into a solid slurry stream 36 and a decanted liquid stream 38; and a digester 103 located downstream of the WAO unit, wherein the digester is configured to subject the decanted liquid stream 38 to anaerobic digestion to produce a digester gas stream 44, wherein the digester gas stream 44 comprises methane, carbon dioxide, and hydrogen.

[0018] In the twelfth aspect of this disclosure, unless otherwise stated, this aspect may be combined with any other aspect listed herein, the wastewater treatment system includes a recirculated sludge stream 42 configured to flow from the digester 103 to a solids thickening unit 106 located upstream of the WAO unit 102.

[0019] In the thirteenth aspect of this disclosure, unless otherwise stated, this aspect may be combined with any other aspect listed herein, wherein the solids thickening device 106 is configured to provide a sludge solids stream 32 and a concentrate stream 30.

[0020] In the fourteenth aspect of this disclosure, unless otherwise stated, this aspect may be combined with any other aspect listed herein, wherein the sludge solids stream 32 is configured to flow from the solids thickening unit 106 to the WAO unit 102.

[0021] In the fifteenth aspect of this disclosure, unless otherwise stated, this aspect may be combined with any other aspect listed herein, the wastewater treatment system includes a dryer 108 configured to process a solid slurry stream 36 to produce a sludge disposal stream 40.

[0022] In the sixteenth aspect of this disclosure, unless otherwise stated, this aspect may be combined with any other aspect listed herein, the wastewater treatment system includes a supernatant stream 46 configured to flow from the digester 103 to the secondary treatment unit 101.

[0023] In the seventeenth aspect of this disclosure, unless otherwise stated, this aspect may be combined with any other aspect listed herein, wherein the concentrate stream 30 is configured to flow from the solids concentration apparatus 106 to the secondary processing unit 101.

[0024] In the eighteenth aspect of this disclosure, unless otherwise stated, this aspect may be combined with any other aspect listed herein, the wastewater treatment system includes a primary treatment unit 104 located upstream of the secondary treatment unit 101.

[0025] In the nineteenth aspect of this disclosure, unless otherwise stated, this aspect may be combined with any other aspect listed herein, the wastewater treatment system includes a secondary clarifier 105 located downstream of the secondary treatment unit 101 and upstream of the WAO unit 102.

[0026] In the twentieth aspect of this disclosure, unless otherwise stated, this aspect may be combined with any other aspect listed herein, and the wastewater treatment system provides a reduction of at least 80% in volatile solids (VSR) from wastewater stream 20 to digester gas stream 44.

[0027] In the twenty-first aspect of this disclosure, unless otherwise stated, this aspect may be combined with any other aspect listed herein, wherein the system digester 103 is configured to receive only the decanted liquid stream 38 as the inlet stream. Attached Figure Description

[0028] The advantages of the inventive concept will become apparent upon consideration of the following detailed disclosure, particularly when taken in conjunction with the accompanying drawings, wherein:

[0029] Figure 1 A flowchart of an exemplary wastewater treatment system according to this disclosure is shown. Detailed Implementation

[0030] This document discloses methods and systems for treating sludge solids from wastewater streams. While certain embodiments of the methods and systems are described in detail in this disclosure, this disclosure should be considered exemplary and is not intended to be limited to the disclosed embodiments.

[0031] As used herein, the term “COD” or “chemical oxygen demand” refers to a measure of the amount of oxygen required to completely oxidize organic and inorganic pollutants in wastewater. COD measurements include bioinstable, bioinhibitory, and biorecalcitrant compounds.

[0032] As used herein, the term "unit" generally refers to a unit operation. A unit operation can be one or more fundamental operations in a process. A unit may have one or more subunits (or subsystems). Unit operations may involve physical changes or chemical transformations, such as separation, crystallization, evaporation, filtration, polymerization, isomerization, other reactions, or combinations thereof. A unit may include one or more individual components.

[0033] As used herein, the terms “wastewater” and “wastewater stream” encompass any water to be treated flowing into a wastewater treatment system; such as surface water, groundwater, and wastewater streams from industrial, agricultural, and municipal sources; the water to be treated contains contaminants including biodegradable materials, inorganic or unstable organic compounds that can be broken down by bacteria, biodegradable compounds, and / or bioinhibitory compounds.

[0034] The methods and systems of this disclosure will be further described with reference to the accompanying drawings. It should be understood that the features shown in the drawings are not necessarily drawn to scale. In the drawings, the direction of fluid flow is indicated by arrows. Fluid can be guided from one unit to another, for example, with the aid of valves and fluid flow systems. As those skilled in the art will understand, such fluid flow systems may include compressors and / or pumps, as well as control systems for regulating fluid flow.

[0035] refer to Figure 1 The diagram shows a flow chart of a wastewater treatment system 100. The system 100 includes a secondary treatment unit 101, a wet air oxidation (WAO) unit 102, and a digester 103.

[0036] Generally, wastewater flow 20 undergoes multiple treatment stages upon initial entry into system 100. The primary treatment stage may include mechanical methods (e.g., bar screens) for removing large objects, and sand or gravel channels where sand, gravel, and stones settle. According to this disclosure, system 100 may include a primary treatment unit 104 located upstream of secondary treatment unit 101 and a secondary clarifier 105 located downstream of secondary treatment unit 101. Figure 1 As shown, the primary treatment unit 104 is configured to receive the inlet wastewater stream 20. The primary treatment unit 104 may include a settling tank and provides an initial clarification stage, wherein significant amounts of insoluble substances in the water are removed or neutralized, including, for example, fats, oils, and greases. The primary effluent stream 22 exits the primary treatment unit 104 and is directed to the secondary treatment unit 101. In embodiments of a system including the primary treatment unit, the primary sludge stream 48 exits the primary treatment unit 104 and is directed to the WAO unit 102. It should be understood from this disclosure that the primary treatment stage and the primary treatment unit 104 are optional, and the inventive concept also encompasses directing the wastewater stream 20 directly to the secondary treatment unit 101, wherein no initial treatment is performed upstream of the secondary treatment unit 101.

[0037] Secondary treatment unit 101 provides biological treatment for the wastewater. Within secondary treatment unit 101, primary effluent 22 is treated with bacteria. Biological treatment typically employs biomass with bacterial or microbial communities to at least partially hydrolyze or transform biodegradable materials, including, for example, sugars, fats, organic molecules, and compounds that generate oxygen demand in water. Activated sludge effluent 24 exits secondary treatment unit 101 and is directed to secondary clarifier 105. Secondary clarifier 105 may include gravity separation, such as settling tanks or flotation, or physical barriers, such as membrane separation units. Secondary clarifier 105 produces treated water effluent 26 and treated sludge effluent 28. Treated water effluent 26 is discharged from the system for disposal or further use, for example, discharge into a freshwater channel, recycling, or reuse.

[0038] System 100 may include a solids thickener 106 located downstream of the secondary clarifier 105. The solids thickener 106 functions to optimize the concentration of sludge solids upstream of the WAO unit 102. The solids thickener 106 may include a centrifuge or a gravity belt thickener. It should be understood that a centrifuge may be replaced by any device suitable for separating the inlet slurry into separate high-solids (i.e., slurry) and low-solids (i.e., liquid) portions. The solids thickener 106 is configured to separate the treated sludge stream 28 into a concentrate stream 30 and a sludge solids stream 32. According to this disclosure, the concentrate stream 30 may be recycled back to the secondary treatment unit 101 for further processing.

[0039] Continue to refer to Figure 1 System 100 includes a wet air oxidation (WAO) unit 102 located downstream of a solids thickening unit 106. According to this disclosure, the WAO unit 102 is configured to process a sludge solids stream 32. The WAO unit 102 includes one or more dedicated reactor vessels and provides aqueous phase oxidation of unwanted components through an oxidant at elevated temperature and pressure. The oxidant may contain molecular oxygen from an oxygen-containing gas, including, for example, pressurized oxygen-containing gas supplied by a compressor. The oxidant may flow through a heat exchanger in the sludge solids stream 32. Figure 1The sludge solids stream 32 is added before, during, or after (not shown in the diagram). Therefore, the inlet sludge solids stream 32 is treated in WAO unit 102 to convert the volatile organic compounds in sludge solids stream 32 into soluble, biodegradable chemical oxygen demand (measured as bio oxygen demand, "BOD"), which includes carbon dioxide, water, and biodegradable short-chain organic acids (such as acetic acid). The oxidation of sludge solids in WAO unit 102 maximizes the amount of soluble BOD in the system. Inorganic components, including sulfides, thiols, and cyanides, may also be oxidized. In some aspects, the oxidation process in the WAO unit is carried out at temperatures ranging from 130°C to 320°C, including 150°C to 300°C, including 150°C to 250°C, including 150°C to 200°C. In some aspects, the oxidation process in the WAO unit is carried out at pressures ranging from 8 bar to 220 bar (including 10 bar to 200 bar).

[0040] According to this disclosure, the WAO effluent stream 34 exits the WAO unit 102 and is fed into a decanter 107. The decanter 107 produces a solids slurry stream 36 and a decanted liquid stream 38. It should be understood that the decanter 107 can be replaced by any device suitable for separating the inlet slurry into separate high-solids (i.e., slurry) and low-solids (i.e., liquid) portions. The decanted liquid stream 38 contains soluble COD, including carbon dioxide, water, and biodegradable short-chain organic acids (such as acetic acid). The solids slurry stream 36 contains nitrogen and phosphorus. The solids slurry stream 36 can be directed to a dryer 108 to produce a sludge disposal stream 40. In some aspects, the dissolved solids content, COD / BOD, or other identifiable characteristics of the decanted liquid stream 38 and / or the solids slurry stream 36 can be monitored. If the level of any of these identifiable characteristics is not within the desired range or is not at the desired level, the operating parameters of the WAO unit can be adjusted. For example, if the COD of the solids slurry flow 36 deviates from the desired level or acceptable range, the operating temperature of the WAO unit can be adjusted to provide optimized conditions.

[0041] According to this disclosure, the decanted liquid stream 38 exits the decanter 107 and is directed to the digester 103. The digester 103 may include an aerobic or anaerobic digester. In some aspects, the digester 103 includes an anaerobic digester. In some aspects, the digester 103 is configured to receive only the decanted liquid stream 38 as an inlet stream. The outputs from the digester 103 include a recirculated sludge stream 42, a digester gas stream 44, and optionally a supernatant stream 46. The supernatant stream 46 may be recycled to a secondary treatment unit 101 for further treatment.

[0042] The digester gas stream 44 exiting digester 103 contains methane, carbon dioxide, and hydrogen. Generally, the digester gas stream 44 contains a product fuel source, or in other words, an energy source. For example, the digester gas stream 44 can provide fuel for unit operations, including, for example, boilers or generator sets (“generator sets”). If desired, the digester gas stream 44 can be further processed to provide output from the system.

[0043] According to this disclosure, the recirculated sludge stream 42 is directed back into the system for separation in a solids thickening unit 106 located upstream of the WAO unit 102. Therefore, in some aspects, the inlet line of the solids thickening unit 106 includes a treated sludge stream 28, a recirculated sludge stream 42, and optionally a primary sludge stream 48.

[0044] It is worth noting that conventional wastewater treatment facilities do not include a WAO (wastewater aerobic digestion) unit and therefore rely entirely on anaerobic digestion to treat the sludge stream after the initial clarification and centrifugation stages. To effectively treat the large amounts of organic compounds in the inlet sludge stream, the anaerobic digesters in these conventional systems may require a residence time of 7 to 10 days, or even up to 100 days in some cases. Similarly, conventional systems rely on microorganisms to perform different stages of anaerobic digestion, including hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Furthermore, the sludge disposal stream produced by conventional processes contains not only contaminants such as PFAS (phosphorus oxidases) but also a large amount of volatile organic compounds. Moreover, the total volume of sludge requiring treatment is substantial.

[0045] On the contrary, the inventors were surprised to find that including a wet air oxidation process upstream of the digester resulted in significant processing efficiency, as detailed in the following paragraphs.

[0046] First, by separating organic compounds from the sludge stream via wet air oxidation, the required residence time in the digester is reduced to approximately 8 to 12 hours. This is because in conventional wastewater treatment facilities, anaerobic digestion processes typically proceed in two steps: first, organic matter is converted into carboxylic acids, and then completely converted into methane and carbon dioxide by bacteria. The rate-limiting step in this conventional process within the digester is the conversion of organic material into carboxylic acids. However, according to the wastewater treatment system 100 described herein, the conversion of organic material is completed simultaneously with sludge dissolution. Specifically, only the methane production stage of anaerobic digestion occurs after wet air oxidation. Therefore, the wastewater treatment system 100 operates in a continuous manner, as opposed to batch processes. Consequently, the volumetric requirement of the digester in the disclosed wastewater treatment system is significantly lower than that of digesters in conventional processes.

[0047] Furthermore, compared to conventional wastewater treatment processes, the final volume of the effluent sludge requiring expensive disposal is significantly reduced in the disclosed system 100. This is because the volume of organic compounds (such as carbon) from the raw inlet wastewater stream 20 is significantly higher than in conventional wastewater treatment processes, where these organic compounds are converted into methane in the digester gas stream 44 leaving the system. In other words, the wastewater treatment system disclosed herein converts most of the carbon from the raw inlet wastewater stream into usable fuel, unlike the sludge waste that requires disposal.

[0048] This effect can be observed by measuring the efficiency of sludge processing in the digester stage. Specifically, the improvement in the disclosed system can be quantified by measuring the reduction in volatile solids (VSR) occurring from the raw wastewater stream 20 to the digester gas stream 44 exiting the digester 103. For example, in conventional wastewater systems, the observed reduction in VSR is approximately 20% when taking into account solids formed in the digester and requiring disposal. However, in the disclosed system, which includes a wet air oxidation treatment step located upstream of the digester and means for returning the recycled sludge stream 42 to the system, a significantly higher proportion of organic compounds from the raw inlet wastewater stream 20 has been found to be converted into methane in the digester gas stream 44 exiting the system. In some aspects, the reduction in VSR occurring from the raw wastewater stream 20 to the digester gas stream 44 is at least 50%, including at least 70%, including at least 75%, including at least 80%, including at least 90%.

[0049] As noted, a high proportion of organic compounds from the raw inlet wastewater stream 20 are converted into methane and carbon dioxide in the digester gas stream 44. If desired, the digester gas stream 44 can be processed to provide a product fuel source, i.e., for energy production or chemical manufacturing. The digester gas stream 44 can be used as a fuel source for one or more unit operations of the treatment system, or alternatively, it can be sold as energy to external facilities. Therefore, compared to conventional wastewater treatment processes, Figure 1 The wastewater treatment system 100 described significantly reduces the volume of solid sludge that requires expensive disposal, which helps to increase the amount of product fuel that can be used or sold to external facilities.

[0050] Finally, compared to conventional wastewater treatment systems, the effluent sludge requiring disposal contains a significantly reduced volume of water. Given that PFAS are water-soluble and are therefore considered to be primarily transported in the aqueous phase of the sludge, the reduced volume of water in the sludge disposal stream 40 results in a reduction of soluble contaminants such as PFAS compared to conventional systems.

[0051] The terminology set forth herein is used only for the description of embodiments and should not be construed as limiting this disclosure as a whole. All references to a singular feature or limitation of this disclosure shall include the corresponding plural feature or limitation, and vice versa, unless otherwise specified or clearly implied by the context of the combination of references. Unless otherwise stated, “a,” “an,” “the,” and “at least one” are used interchangeably. Furthermore, as used in the specification and appended claims, the singular forms “a” and “the” include their plural forms, unless the context clearly indicates otherwise.

[0052] With regard to the term "includes / including" as used in the specification or claims, it is intended to be included in a manner similar to the term "comprising," as explained when the term is used, serving as a transitional word in the claims. Furthermore, with regard to the use of the term "or" (e.g., A or B), it is intended to mean "A or B or both." The term "A or B only but not both" will be used when the applicant intends to indicate "only A or B but not both." Therefore, the use of the term "or / or" herein is inclusive, not exclusive.

[0053] All combinations of methods or process steps used herein may be performed in any order unless otherwise specified or clearly implied by the context of performing the referenced combination.

[0054] All ranges and parameters disclosed herein, including but not limited to percentages, parts, and ratios, should be understood to encompass any and all subranges assumed and included therein, as well as every number between the endpoints. For example, the stated range “1 to 10” should be considered to include any and all subranges that begin with a minimum value of 1 or greater and end with a maximum value of 10 or less (e.g., 1 to 6.1 or 2.3 to 9.4), as well as every integer contained within that range (1, 2, 3, 4, 5, 6, 7, 8, 9, and 10).

[0055] The methods and systems disclosed herein may include, constitute, or substantially constitute the essential elements of this disclosure, as well as any additional or optional elements described herein, or may otherwise be used for wastewater treatment applications.

[0056] According to this disclosure, various inventive concepts can be utilized in combination with each other. Furthermore, any particular feature relating to a specific disclosed aspect of the methods and systems of this disclosure should be interpreted as applicable to all disclosed aspects of the methods and systems of this disclosure, unless the incorporation of such particular feature would contradict the express terms of the disclosed aspect. Additional advantages and modifications will be apparent to those skilled in the art. Therefore, this disclosure, in its broader aspects, is not limited to the specific details, representative devices, or illustrative examples shown and described therein. Therefore, deviations from these details may be made without departing from the spirit or scope of the inventive concept.

[0057] Example

[0058] The following examples are included for illustrative purposes and do not limit the scope of the general inventive concept described herein.

[0059] Example 1

[0060] like Figure 1 The wastewater stream is treated on a pilot scale in the wastewater treatment system, including, for example, the use of a wet air oxidation (WAO) unit 102 located upstream of digester 103. As detailed in Table 1, the oxidation conditions are limited in an attempt to maximize the amount of soluble sludge solids while maintaining the highest possible soluble COD / BOD.

[0061] Table 1: Effect of oxidation temperature on the volume of sludge solids

[0062]

[0063] As shown in Table 1, different operating temperatures for the WAO unit were observed to evaluate the effect of oxidation temperature on the volume of sludge solids. Specifically, according to this disclosure, the objective of the analysis was to minimize COD in the solid phase and maximize COD in the liquid phase.

[0064] While each operating temperature listed in Table 1 provides effective wastewater treatment according to this disclosure, the WAO operating temperature of 175°C was found to provide optimized conditions. In this sample, the soluble COD in the inlet sludge was 1,280 mg / L. After oxidation at 175°C, the soluble COD increased to 17,500 due to the formation of organic acids, and the pH of the solution decreased to 4.0. The amount of volatile suspended solids in the slurry decreased from 24,200 mg / L to 4,570 mg / L. Thus, suspended solids were reduced by 75% while maintaining a high soluble COD / BOD ratio.

Claims

1. A method for treating wastewater, the method comprising: (a) The wastewater stream is directed to a secondary treatment unit, wherein the secondary treatment unit provides biological treatment of the wastewater stream and produces a treated sludge stream; (b) The treated sludge stream is directed to a solids thickening device, wherein the solids thickening device separates the treated sludge stream into a concentrated liquid stream and a sludge solids stream; (c) The sludge solids stream is directed into a wet air oxidation (WAO) unit, wherein the WAO unit dissolves the sludge solids stream by wet air oxidation to provide a WAO effluent stream; (d) Separate the WAO effluent into a solid slurry stream and a decanted liquid stream; as well as (e) The decanted liquid stream is directed into a digester, wherein the decanted liquid stream undergoes anaerobic digestion to produce a digester gas stream, and wherein the digester gas stream contains methane, carbon dioxide and hydrogen.

2. The method of claim 1, further comprising removing the recirculated sludge stream from the digester and directing the recirculated sludge stream into the solids thickening device.

3. The method according to claim 1 or claim 2, further comprising processing the digester gas stream to provide a product methane source.

4. The method according to any one of claims 1 to 3, further comprising directing the solid slurry stream to a dryer to produce a sludge disposal stream.

5. The method according to any one of claims 1 to 4, further comprising removing the supernatant stream from the digester and directing the supernatant stream to the secondary processing unit.

6. The method according to any one of claims 1 to 5, further comprising guiding the concentrated liquid stream into the secondary processing unit.

7. The method according to any one of claims 1 to 6, further comprising clarifying the wastewater stream in a primary treatment unit located upstream of the activated sludge unit.

8. The method according to any one of claims 1 to 7, further comprising clarifying the treated sludge stream in a secondary clarifier located upstream of the solids thickening unit.

9. The method according to any one of claims 1 to 8, wherein the method achieves a reduction of at least 50% in volatile solids (VSR) from the wastewater stream to the digester gas stream.

10. The method according to any one of claims 1 to 9, wherein the method achieves a reduction of at least 80% in volatile solids (VSR) from the wastewater stream to the digester gas stream.

11. The method according to any one of claims 1 to 10, wherein the wet air oxidation of the sludge solids stream in the WAO unit is carried out at a temperature of 130°C to 320°C.

12. A wastewater treatment system, the system comprising: (a) A secondary treatment unit configured to provide biological treatment of wastewater streams; (b) A wet air oxidation (WAO) unit located downstream of the secondary treatment unit, wherein the WAO unit is configured to provide a WAO effluent by dissolving the sludge solids stream through wet air oxidation. (c) A tool for separating the WAO effluent into a solid slurry stream and a decanted liquid stream; and (d) A digester located downstream of the WAO unit, wherein the digester is configured to subject the decanted liquid stream to anaerobic digestion to produce a digester gas stream, and wherein the digester gas stream comprises methane, carbon dioxide and hydrogen.

13. The system of claim 12, further comprising a recirculated sludge stream configured to flow from the digester to a solids thickening unit located upstream of the WAO unit.

14. The system of claim 13, wherein the solids thickening device is configured to provide the sludge solids stream and the concentrated liquid stream.

15. The system of claim 13 or claim 14, wherein the sludge solids stream is configured to flow from the solids thickening device to the WAO unit.

16. The system according to any one of claims 12 to 15, further comprising a dryer configured to process the solid slurry stream to produce a sludge disposal stream.

17. The system according to any one of claims 12 to 16, further comprising a supernatant stream configured to flow from the digester to the secondary processing unit.

18. The system according to any one of claims 14 to 17, wherein the concentrated liquid stream is configured to flow from the solids concentration device to the secondary processing unit.

19. The system according to any one of claims 12 to 18, further comprising a primary processing unit located upstream of the secondary processing unit.

20. The system according to any one of claims 12 to 19, further comprising a secondary clarifier located downstream of the secondary processing unit and upstream of the WAO unit.

21. The system according to any one of claims 12 to 20, wherein the system achieves a reduction of at least 80% in volatile solids (VSR) from the wastewater stream to the digester gas stream.

22. The system according to any one of claims 12 to 21, wherein the digester is configured to receive only the decanted liquid stream as an inlet stream.

23. The system according to any one of claims 12 to 22, wherein the WAO unit is configured to dissolve the sludge solids stream by wet air oxidation at a temperature of 130°C to 320°C.

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