WO2022240349A1

WO2022240349A1 - Methods and systems for sludge treatment

Info

Publication number: WO2022240349A1
Application number: PCT/SE2022/050468
Authority: WO
Inventors: Peter AXEGÅRD; Fredrik BJÖRNERBÄCK
Original assignee: C-Green Technology Ab
Priority date: 2021-05-12
Filing date: 2022-05-12
Publication date: 2022-11-17
Also published as: EP4337616A1; SE2150615A1; SE545057C2

Abstract

There is provided a method of sludge treatment. The method comprises the steps of: - obtaining a sludge, preferably from a wastewater treatment plant; - dewatering the sludge to obtain dewatered sludge and sludge water; - heat-treating the dewatered sludge, optionally after dilution, which heat-treating step comprises separation into a heat-treated slurry and a second fraction, which has lower total suspended solids (TSS) than the heat-treated slurry; - subjecting the second fraction to wet oxidation to obtain a wet-oxidized fraction; - flashing the wet-oxidized fraction in at least one step to obtain at least one steam fraction and a cooled wet-oxidized fraction; - mixing sludge water from the dewatering step with the cooled wet-oxidized fraction to obtain a reject water mixture, wherein the temperature of the reject water mixture is higher than the temperature of the sludge water supplied to said mixing; and - subjecting the reject water mixture to a bacterial nitrogen removal treatment to obtain treated reject water.

Description

1

METHODS AND SYSTEMS FOR SLUDGE TREATMENT

TECHNICAL FIELD

[0001] The present disclosure relates to the field of sludge treatment.

BACKGROUND

[0002] WO 2017/ 003358 discloses a system and a method for heat-treatment of sludge, e.g. in a reactor capable of separating the heat-treated sludge (by means of sedimentation or fluidization) into fractions of different average particle size.

[0003] WO 2017/ 222462 discloses a method of treatment of sludge, comprising the steps of:

-preheating an incoming sludge with at least one steam fraction, preferably by direct steam injection, to obtain a preheated sludge;

-further heating the preheated sludge with a high-temperature steam fraction, preferably by direct steam injection, to obtain a heated sludge;

-hydrothermal carbonization (HTC) of the heated sludge to obtain a HTC-treated sludge;

-separating a particle-lean fraction from the HTC-treated sludge;

-wet oxidation of the particle-lean fraction to obtain a heated particle-lean fraction; -subjecting the heated particle-lean fraction to a first flashing to obtain the high- temperature steam fraction used in the further heating step;

-separating a particle-rich fraction from the HTC-treated sludge; and -subjecting the particle-rich fraction to flashing to obtain at least one steam fraction that is used in the preheating step and a cooled particle-rich fraction.

[0004] The reactor disclosed in WO 2017/ 003358 may be used for the HTC and separating steps of the method in WO 2017/222462.

[0005] WO 2020/112007 discloses a simplified method in which the HTC-treated sludge/slurry is not separated into a particle-lean and a particle rich fraction. Instead, WO 2020/112007 discloses wet oxidation of the whole HTC-treated sludge/slurry.

SUMMARY

[0006] An objective of the present disclosure is to facilitate energy-efficient nitrogen removal in sludge treatment. Another objective of the present disclosure is 2 to unburden wastewater treatment plants. Accordingly, the following itemized listing of embodiments of the present disclosure is provided.

1. A method of sludge treatment, comprising the steps of:

- obtaining a sludge, preferably from a wastewater treatment plant;

- dewatering the sludge to obtain dewatered sludge and sludge water;

- heat-treating the dewatered sludge, optionally after dilution, which heat- treating step comprises separation into a heat-treated slurry and a second fraction, which has lower total suspended solids (TSS) than the heat-treated slurry;

- subjecting the second fraction to wet oxidation to obtain a wet-oxidized fraction;

- flashing the wet-oxidized fraction in at least one step to obtain at least one steam fraction and a cooled wet-oxidized fraction;

- mixing sludge water from the dewatering step with the cooled wet-oxidized fraction to obtain a reject water mixture, wherein the temperature of the reject water mixture is higher than the temperature of the sludge water supplied to said mixing; and

- subjecting the reject water mixture to a bacterial nitrogen removal treatment to obtain treated reject water.

2. The method of item l, wherein the reject water mixture obtained in the mixing step has a temperature of 20-40 °C, such as 24-35 °C, such as 26-33 °C, such as 27-32 °C.

3. The method of item 1 or 2, wherein the temperature of the sludge water supplied to the mixing step is below 25 °C, such as below 20 °C, such as below 15 °C, such as below 12 °C.

4. The method of any one of the preceding items, wherein the temperature of the cooled wet-oxidized fraction supplied to the mixing step is 35-100 °C, such as 50-99 °C, such as 60-99 °C, such as 70-99 °C.

5. The method of any one of the preceding items, wherein the volumetric ratio of sludge water to cooled wet-oxidized fraction supplied to the mixing step is between 2.0:1 and 20:1, such as between 2.5:1 and 10:1, such as between 3:1 and 8:1. 3

6. The method of any one of the preceding items, wherein the bacterial nitrogen removal treatment comprises bacterial nitrification.

7. The method of item 6, wherein the bacterial nitrification comprises oxidation of ammonium to nitrite by ammonia-oxidizing bacteria (AOB).

8. The method of item 6 or 7, wherein the bacterial nitrification comprises oxidation of nitrite to nitrate by nitrite-oxidizing bacteria (NOB).

9. The method of any one of the preceding items, wherein the bacterial nitrogen removal treatment comprises bacterial denitrification.

10. The method of item 9, wherein said denitrification is carried out by heterotrophic bacteria.

11. The method of any one of the preceding items, wherein the bacterial nitrogen removal treatment comprises treatment with anammox bacteria.

12. The method of any one of the preceding items, wherein the bacterial nitrogen removal treatment is carried out in a Sequencing Batch Reactor (SBR).

13. The method of any one of the preceding items, wherein the heat-treating step is carried out after dilution of the dewatered sludge.

14. The method of item 13, wherein the diluted sludge has a dry matter content of 10% or lower.

15. The method of item 13 or 14, wherein the diluted sludge has a total organic content of 8.6% or lower, such as 8.0% or lower, such as 7.4% or lower.

16. The method of any one of items 13-15, wherein process water is separated from the heat-treated slurry and used in the dilution.

17. The method of item 16, wherein at least 90% of the process water that is separated from the heat-treated slurry is used to dilute the dewatered sludge.

18. The method of any one of the preceding items, wherein the heat-treated slurry is subjected to flashing in at least one step to obtain at least one steam fraction that is/are added in the heat-treating step.

19. The method of items 13 and 16, wherein said flashing is performed prior to the separation of the process water. 4

20. The method of item 17, wherein the temperature of the process water used to dilute the dewatered sludge is at least 45 °C, such as at least 50 °C, such as at least 60 °C.

21. The method of any one of the preceding items, wherein the dry matter content of the sludge is below 10%, such as below 8%, such as 3%s%.

22. The method of any one of the preceding items, wherein the sludge has a temperature below 25 °C, such as below 20 °C, such as below 15 °C.

23. The method of any one of the preceding items, wherein the dewatered sludge has a dry matter content above 10%, such as above 15%, such as above 20%, such as at least 25%.

24. The method of any one of the preceding items, wherein the at least one steam fraction obtained in said flashing of the wet-oxidized fraction is/are added in the heat-treating step.

25. The method of any one of the preceding items, wherein the sludge is obtained from a wastewater treatment plant and the treated reject water is returned to the wastewater treatment plant.

26. A method of sludge treatment, comprising the steps of:

- obtaining a sludge, preferably from a wastewater treatment plant;

- dewatering the sludge to obtain dewatered sludge and sludge water;

- heat-treating the dewatered sludge, optionally after dilution, to obtain a heat- treated slurry;

- subjecting the heat-treated slurry to wet oxidation to obtain a wet-oxidized slurry;

- flashing the wet-oxidized slurry in at least one step to obtain at least one steam fraction and a cooled slurry;

- separating the cooled slurry into a process water fraction and a solids fraction;

- mixing sludge water from the dewatering step with at least part of the process water fraction to obtain a reject water mixture, wherein the temperature of the reject water mixture is higher than the temperature of the sludge water supplied to said mixing; and

- subjecting the reject water mixture to a bacterial nitrogen removal treatment to obtain treated reject water. 5

27. The method of item 26, wherein the dewatered sludge is diluted before the heat- treating step with a part of process water fraction.

28. The method of item 26 or 27, wherein the reject water mixture obtained in the mixing step has a temperature of 26-40 °C, such as 26-35 °C, such as 26-33 °C, such as 27-32 °C.

29. The method of any one of items 26-28, wherein the temperature of the sludge water supplied to the mixing step is below 20 °C, such as below 15 °C, such as below 12 °C.

30. The method of any one of items 26-29, wherein the temperature of the process water supplied to the mixing step is 45-99 °C, such as 55-99 °C.

31. The method of any one of items 26-30, wherein the volumetric ratio of sludge water to process water supplied to the mixing step is between 1.5:1 and 20:1, such as between 2:1 and 10:1.

32. The method of any one of items 26-31, wherein the bacterial nitrogen removal treatment comprises bacterial nitrification.

33. The method of item 32, wherein the bacterial nitrification comprises oxidation of ammonium to nitrite by ammonia-oxidizing bacteria (AOB).

34. The method of item 32 or 33, wherein the bacterial nitrification comprises oxidation of nitrite to nitrate by nitrite-oxidizing bacteria (NOB).

35. The method of any one of items 26-34, wherein the bacterial nitrogen removal treatment comprises bacterial denitrification.

36. The method of item 35, wherein said denitrification is carried out by heterotrophic bacteria.

37. The method of any one of items 26-36, wherein the bacterial nitrogen removal treatment comprises treatment with anammox bacteria.

38. The method of any one of items 26-37, wherein the bacterial nitrogen removal treatment is carried out in a Sequencing Batch Reactor (SBR).

39. The method of any one of items 26-38, wherein the heat-treating step is carried out after dilution of the dewatered sludge. 6

40. The method of item 39, wherein the diluted sludge has a dry matter content of 10% or lower.

41. The method of item 39 or 40, wherein the diluted sludge has a total organic content of 8.6% or lower, such as 8.0% or lower, such as 7.4% or lower.

42. The method of any one of items 39-41, wherein a part of the process water fraction is used in the dilution.

43. The method of any one of items 26-42, wherein the sludge has a dry matter content below 10%, such as below 8%, such as 3%-5%.

44. The method of any one of items 26-43, wherein the sludge has a temperature below 25 °C, such as below 20 °C, such as below 15 °C.

45. The method of any one of items 26-44, wherein the dewatered sludge has a dry matter content above 10%, such as above 15%, such as above 20%, such as at least 25%.

46. The method of any one of items 26-45, wherein at least one steam fraction obtained in the flashing step is/are added in the heat-treating step.

47. The method of any one of items 26-46, wherein the sludge is obtained from a wastewater treatment plant and the treated reject water is returned to the wastewater treatment plant.

48. A system for sludge treatment, comprising:

- a dewatering device, such as a screw press, for dewatering sludge such that dewatered sludge and sludge water are produced;

- optionally a mixing device arranged to dilute dewatered sludge from the press with process water;

- a heating and separation arrangement arranged to heat the dewatered and optionally diluted sludge from the press or the mixing device and process it into a heat-treated slurry and a second fraction, which has lower total suspended solids (TSS) than the heat-treated slurry;

- a wet oxidation device arranged to subject the second fraction to wet oxidation and thereby produce a wet-oxidized fraction;

- a flashing arrangement arranged to subject the wet-oxidized fraction to flashing in at least one step and thereby produce a cooled wet-oxidized fraction; and

- an arrangement for bacterial nitrogen removal treatment arranged to treat a 7 mixture of sludge water from the press and the cooled wet-oxidized fraction from the flashing arrangement.

49. A system for sludge treatment, comprising:

- a heating arrangement arranged to heat dewatered and optionally diluted sludge to obtain a heat-treated slurry;

- a wet oxidation device arranged to subject the heat-treated slurry to wet oxidation and thereby produce a wet-oxidized slurry;

- a flashing arrangement arranged to subject the wet-oxidized slurry to flashing in at least one step and thereby produce a cooled wet-oxidized slurry;

- a separation device arranged to separate the cooled wet-oxidized slurry from the flashing arrangement into a process water fraction and a solids fraction; and

- an arrangement for bacterial nitrogen removal treatment arranged to treat a mixture of sludge water from the press and at least part of the process water fraction.

[0007] Many wastewater treatment plants produce a sludge that has a dry matter content of 3%~5%, which is then dewatered by a dewatering device. The water obtained from this device (“sludge water”) may then be purified by a bacterial nitrogen removal treatment. This treatment is however temperature-dependent, and the temperature of the sludge water is often too low for an efficient process, especially in wintertime. The sludge water may of course be heated, but the sludge water volumes are typically so large that such heating is not motivated from an economical or environmental perspective. Hence, the available bacterial nitrogen removal treatments have failed to realize their full potential as some wastewater treatment plants do not use them and other wastewater treatment plants only operate them during the warmer seasons.

[0008] The present inventors have realized that the wastewater treatment operations can be facilitated by mixing the sludge water with warmer process water that has undergone wet oxidation in a sludge treatment process. Thereby a reject water mixture having a temperature adapted to the bacterial treatment can be obtained. The mixing is particularly beneficial since the wet oxidation process, which 8 effectively reduces COD, typically fails to remove nitrogen compounds. The wet- oxidized process water, just like the sludge water, is thus in need of the bacterial treatment. The wet-oxidized process water is however primed for the bacterial treatment since the wet oxidation process converts organic nitrogen compounds to ammonia. Further, the wet-oxidized process water may carry at least some of the carbon that is needed for the bacterial reactions.

[0009] It follows from the discussion above that in one embodiment, the dewatering step of the above methods replaces a dewatering step otherwise carried out in the wastewater treatment plant. Omitting the wastewater treatment plant’s dewatering step improves the pumpability of the sludge and thereby facilitate transport of the sludge from a sludge outlet of the wastewater treatment plant to the location of the above method.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 illustrates an embodiment of the system according to the third aspect of the present disclosure for carrying out an embodiment of the method according to the first aspect of the present disclosure.

[0011] Figure 2 illustrates another embodiment of the system according to the third aspect of the present disclosure for carrying out another embodiment of the method according to the first aspect of the present disclosure.

[0012] Figure 3 illustrates an embodiment of the system according to the fourth aspect of the present disclosure for carrying out an embodiment of the method according to the second aspect of the present disclosure.

[0013] Figure 4 illustrates another embodiment of the system according to the fourth aspect of the present disclosure for carrying out another embodiment of the method according to the second aspect of the present disclosure.

DETAILED DESCRIPTION

[0014] As a first aspect of the present disclosure, there is provided a method of sludge treatment, which comprises the steps of:

- obtaining a sludge, preferably from a wastewater treatment plant;

- dewatering the sludge to obtain dewatered sludge and sludge water;

- heat-treating the dewatered sludge, optionally after dilution, which heat-treating 9 step comprises separation into a heat-treated slurry and a second fraction, which has lower total suspended solids (TSS) than the heat-treated slurry;

[0015] The heat-treating step may comprise hydrothermal carbonization (HTC).

[0016] The temperature of the heat-treated slurry obtained in the heat-treating step (i.e. before any wet-oxidation or flashing) is typically 180-250 °C, preferably 180-230 °C and more preferably 190-225 °C.

[0017] The reject water mixture obtained in the mixing step typically has a temperature of 20-40 °C and preferably 24-35 °C, such as 26-33 °C, such as 27-32 °C.

[0018] The temperature of the sludge water supplied to the mixing step is typically below 25 °C. The method is particularly advantageous when this temperature is below 20 °C and even more advantageous when it is below 15 °C, such as below 12 °C.

[0019] Accordingly, the temperature of the sludge obtained in the first step typically has a temperature below 25 °C, advantageously below 20 °C, more advantageously below 15 °C, such as below 12 °C.

[0020] The temperature of the cooled wet-oxidized fraction supplied to the mixing step is typically 35-100 °C and preferably 50-99 °C, such as 60-99 °C, such as 70-99 °C, such as 80-99 °C.

[0021] The volumetric ratio of sludge water to cooled wet-oxidized fraction supplied to the mixing step is typically between 2.0:1 and 20:1 and preferably between 2.5:1 and 10:1, such as 3:1 and 8:1.

[0022] The bacterial nitrogen removal treatment preferably comprises bacterial nitrification, which in turn may comprise oxidation of ammonium to nitrite by 10 ammonia-oxidizing bacteria (AOB). In one embodiment, the bacterial nitrification further comprises oxidation of nitrite to nitrate by nitrite-oxidizing bacteria (NOB).

[0023] In one embodiment, bacterial nitrogen removal treatment comprises bacterial denitrification, e.g. by heterotrophic bacteria. These heterotrophic bacteria may be so called “denitrifiers”, which are heterotrophic bacteria (most of them facultative anaerobic) that couple the oxidation of organic substrates to the reduction of N0₃ to either N₂0 or N₂.

[0024] Nitrogen removal based on bacterial nitrification and denitrification is known to the skilled person. One such treatment is based on a Sequencing Batch Reactor (SBR), which is also known to the skilled person.

[0025] In one embodiment, the bacterial nitrogen removal treatment comprises treatment with anammox bacteria. Such treatment is further described in Kuenen (Nature Reviews Microbiology volume 6, pages 320-326 (2008)).

[0026] The inventors have found that the parameter that best predicts the pumpability of the dewatered sludge is the total organic content. Accordingly, the inventors have identified 8.6% as a suitable upper limit for the total organic content of the sludge that is supplied to the heat-treating step, i.e. the dewatered and optionally diluted sludge. Preferably, the total organic content is 8.0% or lower, such as 7.4% or lower. A typical lower limit maybe 5.0% or 6.0%.

[0027] The dry matter content of the dewatered and optionally diluted sludge that is supplied to the heat-treating step is typically below 10%.

[0028] The dry matter content is preferably determined by evaporation at 105 °C until a steady mass is obtained. This is a standard procedure known to the skilled person. The total organic content may then be determined by combustion of the dry matter (“total organic content” = “dry matter content” - “ash content”). In the incoming sludge, the total organic content is typically 45%-8o% of the dry matter content (see e.g. table 1 below). In one embodiment, the total organic content is 52%- 80% of the dry matter content.

[0029] When the dewatered sludge is diluted, it is preferred to use process water separated from the heat-treated slurry in the dilution. In an embodiment, at least 90% of the process water separated from the heat-treated slurry is used to dilute the dewatered sludge. 11

[0030] The heat-treated slurry is preferably subjected to flashing in at least one step to obtain at least one steam fraction that is/are added in the heat-treating step. However, all the steam obtained in the flashing(s) is not necessarily recirculated to the heat-treating step. “Excess steam” may also be produced. Such excess steam may for example be used in a separate method (e.g. in the wastewater treatment plant) or to heat the sludge water or the reject water mixture. When the method comprises separation of the process water fraction, the flashing is preferably performed prior to such separation.

[0031] Similarly, at least one steam fraction obtained in said flashing of the wet- oxidized fraction is/are typically added in the heat-treating step. Again, all the steam obtained in the flashing(s) is not necessarily recirculated to the heat-treating step. “Excess steam” may also be produced. Utilization of excess steam is discussed above.

[0032] In one embodiment, the temperature of the process water used to dilute the dewatered sludge is at least 45 °C, such as at least 50 °C, preferably at least 60 °C. Further, this temperature is typically below 100 °C since the heat-treated slurry typically has been subjected to flashing prior to the separating step.

[0033] The sludge obtained in the first step typically has a dry matter content below 10%, such as below 8%. Normally, it is 3 6-5%. Further, sludge obtained in the first step may have a total organic content below 8%, such as below 6%, such as 2-4%.

[0034] The dewatered sludge, i.e. the sludge that have undergone the dewatering step, but not any dilution step, preferably has a dry matter content above 15% and/or a total organic content above 10%. Preferably, it has a dry matter content above 20%, such as at least 25% and/or a total organic content above 13%, such as above 15%.

[0035] The dewatering step is preferably carried out by means of a screw press. Suitable screw presses are commercially available, i.a. from Huber (Germany). A benefit of using the screw press is that it can remove high amounts of water from sludge at a relatively modest energy consumption.

[0036] It follows that at least 50% of the water of the sludge obtained in the first step is normally removed as sludge water in the dewatering step. In one embodiment, at least 75% of the water of the sludge obtained in the first step is removed as sludge water in the dewatering step. 12

[0037] In a preferred embodiment, the sludge is obtained from a wastewater treatment plant and the treated reject water is returned to a wastewater treatment plant. The wastewater treatment plant from which the sludge is obtained may be, but is not necessarily, the same wastewater treatment plant that also receives the treated reject water.

[0038] In one embodiment, the above method is “centralized” and receives sludge from several wastewater treatment plants. In this embodiment, the treated reject water may be received by one of the wastewater treatment plants or distributed among several of them (such as all of them).

[0039] By means of the dewatering step and the (optional) subsequent dilution step of the above method, the water content of the processed sludge can be controlled such that energy is not wasted on heating water, while the flowability of the sludge is sufficient for efficient mixing with steam and pumping.

[0040] When the dilution step is included, “cold” water in the incoming sludge may be replaced with “warm” process water, which improves heat-efficiency. Also, unreacted/non-degraded components (dissolved and undissolved) in the process water are recycled for another round of processing, which reduces the chemical oxygen demand (COD) of the water expelled from the process.

[0041] Possibly the most significant benefit of the combination of a dewatering step and a dilution step is that the degree of dewatering can be adapted to “make room” for substantially all available process water, which reduces the risk that insufficiently processed water must be expelled and treated further elsewhere, typically in the wastewater treatment plant form which the sludge was obtained.

[0042] When the process water is used to dilute the dewatered sludge, its temperature is higher than the temperature of the incoming sludge, i.e. the sludge obtained in the first step of the method. During the colder seasons when the temperature of the incoming sludge is particularly low (e.g. below 15 °C), the dilution step is particularly beneficial.

[0043] The heat-treating step comprises separation into a first fraction, which is the heat-treated slurry, and a second fraction, which has lower total suspended solids (TSS) than the heat-treated slurry. The separation may be carried out in a reactor used for the heat-treating step (as shown in WO 2017/003358) or in a separate 13 separation device as shown in figure 2 and discussed in Example 2 below. The first and the second fraction typically have a temperature in the range of 180-250 °C, preferably 180-230 °C and more preferably 190-225 °C.

[0044] As discussed above, the second fraction is subjected to wet oxidation to obtain a wet-oxidized fraction. The wet oxidation reactions are exothermic and reduces the COD. The temperature of the wet-oxidized fraction is typically 220-275 °C, preferably 230-250 °C, more preferably 235-245 °C.

[0045] As also discussed above, the wet-oxidized fraction is subjected to flashing in at least one step. In an embodiment, the temperature of the steam fraction obtained in the first step of flashing the wet-oxidized fraction is 9-40 °C higher than the temperature of the second fraction before wet oxidation. This embodiment is particularly beneficial the steam fraction obtained in the first step of flashing the wet- oxidized fraction is the last steam fraction to be added in the heat-treating step.

[0046] In a second aspect of the present disclosure, the heat-treating step comprises no separation into fractions of different TSS. However, a water fraction that has undergone wet oxidation is still generated and then mixed with sludge water from a dewatering of incoming sludge.

[0047] The sludge treatment method of the second aspect comprises the steps of:

- obtaining a sludge, preferably from a wastewater treatment plant;

- dewatering the sludge to obtain dewatered sludge and sludge water;

- mixing sludge water from the dewatering step with at least part of the process water fraction to obtain a reject water mixture; and

- subjecting the reject water mixture to a bacterial nitrogen removal treatment to obtain treated reject water. 14

[0048] As understood by the skilled person, the temperature of the reject water mixture is typically higher than the temperature of the sludge water supplied to said mixing.

[0049] In an embodiment of the second aspect, the dewatered sludge is diluted before the heat-treating step with a part of the process water fraction. Accordingly, one part of the process water fraction is mixed with the sludge water and another part of the process water fraction dilutes the dewatered sludge.

[0050] The temperature of the process water supplied to the mixing step may be 45-99 °C, such as 55-99 °C.

[0051] The volumetric ratio of sludge water to process water supplied to the mixing step is typically between 1.5:1 and 20:1, preferably between 2:1 and 10:1.

[0052] At least one steam fraction obtained in the flashing step is/are preferably added in the heat-treating step.

[0053] The wet oxidation of the heat-treated slurry typically results in a temperature-increase of at least 10 °C, such as an increase to a temperature of in the range of 220-260 °C, preferably 230-250 °C, more preferably 235-245 °C.

[0054] The wet-oxidized slurry is subjected to flashing in at least one step. Thereby at least one steam fraction that can be added to the diluted sludge in the heat-treating step is/are obtained. The temperature of the steam fraction obtained in the first flashing step may be 9-40 °C higher than the temperature of the heat-treated slurry before wet oxidation. Such a steam fraction is typically the last steam fraction to be added in the heat-treating step.

[0055] In an embodiment, the separating step comprises filtering at a temperature of 60 °C or lower. In such case, the cooled slurry from the flashing step typically needs further cooling, advantageously by heat exchange with the sludge water or the reject water mixture.

[0056] Otherwise, the embodiments of the first aspect apply to the second aspect mutatis mutandis.

[0057] As a third aspect of the present disclosure there is provided a system for sludge treatment, in particular a sludge treatment according to the first aspect. The system of the third aspect comprises: 15

- an arrangement for bacterial nitrogen removal treatment arranged to treat a mixture of sludge water from the press and the cooled wet-oxidized fraction from the flashing arrangement.

[0058] Examples of bacteria that may contained in the arrangement for bacterial nitrogen removal treatment are discussed above in connection with the first aspect.

[0059] The mixture of sludge water from the press and the cooled wet-oxidized fraction from the flashing arrangement may be formed in the arrangement for bacterial nitrogen removal treatment or in a mixing device arranged upstream the arrangement for bacterial nitrogen removal treatment. Accordingly, the system may comprise a sludge water line for routing sludge water from the press to the arrangement for bacterial nitrogen removal treatment or the mixing device. Further, the system may comprise an oxidized water line for routing the cooled wet-oxidized fraction to the arrangement for bacterial nitrogen removal treatment or the mixing device.

[0060] The heating and separation arrangement may comprise a reactor comprising at least one first outlet for the heat-treated slurry and at least one second outlet for the second fraction, wherein the at least one first outlet is/are arranged above the at least one second outlet. Thereby the separation is facilitated. This is further described in WO 2017/003358.

[0061] In one embodiment, the system comprises a steam routing arrangement arranged to route at least one steam fraction from the flashing arrangement to the 16 heating and separation arrangement. Preferably, the heating and separation arrangement comprises steam mixers to which the steam routing arrangement is connected. Such steam mixers are preferably arranged upstream any reactor or equipment for separating the second fraction.

[0062] In an embodiment of the third aspect, the system comprises a slurry flashing arrangement arranged to subject the heat-treated slurry to flashing in at least one step and thereby produce at least one steam fraction and a cooled heat-treated slurry. Accordingly, the steam routing arrangement (discussed above) may be arranged to also route at least one steam fraction from the slurry flashing arrangement to the heating and separation arrangement.

[0063] The system of the third aspect may further comprise a separation device for separating the cooled heat-treated slurry into a process water fraction and a solids fraction. Also, the system may comprise a recycling line for routing process water from the separation device to the mixing device, thereby enabling dilution of the dewatered sludge with process water.

[0064] Otherwise, the embodiments of the first aspect apply to the third aspect mutatis mutandis.

[0065] Various embodiments of the third aspect are also described in examples 1 and 2 below.

[0066] As a fourth aspect of the present disclosure there is provided a system for sludge treatment, in particular a sludge treatment according to the second aspect.

The system of the fourth aspect comprises:

- a heating arrangement arranged to heat dewatered and optionally diluted sludge from the press or the mixing device to obtain a heat-treated slurry;

- a flashing arrangement arranged to subject the wet-oxidized slurry to flashing in at least one step and thereby produce a cooled wet-oxidized slurry; 17

[0067] The mixture of sludge water from the press and the at least part of the process water fraction from the separation device may be formed in the arrangement for bacterial nitrogen removal treatment or in a mixing device arranged upstream the arrangement for bacterial nitrogen removal treatment. Accordingly, the system may comprise a sludge water line for routing sludge water from the press to the arrangement for bacterial nitrogen removal treatment of the mixing device. Further, the system may comprise a process water line for routing process water from the separation device to the arrangement for bacterial nitrogen removal treatment or the mixing device. The process water line may comprise a recycling line for routing process water from the separation device to the mixing device, thereby enabling dilution of the dewatered sludge with process water.

[0068] Otherwise, the embodiments of the first, second and third aspect apply to the fourth aspect mutatis mutandis.

[0069] Various embodiments of the fourth aspect are also described in examples 3 and 4 below.

[0070] The sludge of the present disclosure may be digested or undigested municipal or industrial sludge, typically from biological waste water treatment. Municipal sludge is preferred. The sludge may also be digested or undigested food waste or manure.

EXAMPLES Example 1

[0071] A system 100 according to an embodiment of the third aspect of the present disclosure for carrying out a first exemplary embodiment of the method of the first aspect of the present disclosure is illustrated in figure 1.

[0072] A sludge is received by an inlet of a screw press 101, typically from a wastewater treatment plant 123. The incoming sludge typically has a dry matter content of 3-5%. The temperature of the incoming sludge is typically in the range of 18 io°C to 20°C depending on the season. The screw press 101 produces a dewatered sludge and a sludge water fraction. The dewatered sludge typically has a dry matter content of at least 20%. The dewatered sludge is mixed with a process water fraction in a mixer 102 to produce a diluted sludge, which typically has a dry matter content of 10% or lower and a total organic content of 7.4% or lower.

[0073] The diluted sludge is heated by stepwise additions of steam, e.g. in a first 103, a second 104, a third 105 and a fourth 106 steam mixer arranged in series. A pump i03p, i04p, 105P, io6p is preferably arranged downstream each steam mixer 103, 104, 105, 106. The heated sludge from the last steam mixer 106 is routed HTC reactor 107. The HTC reactor 107 not only subjects the heated sludge to HTC, it also separates it into a heat-treated slurry and a second fraction, wherein the second fraction has lower total suspended solids (TSS) than the heat-treated slurry. An example of an HTC reactor designed for such a separation is shown in WO 2017/003358. The temperature of the heat-treated slurry provided by the HTC reactor 107 is typically 190-215 °C.

[0074] Collectively, the steam mixers 103, 104, 105, 106 and the HTC reactor 107 are referred to a “heating and separation arrangement” 108.

[0075] Oxygen in the form of air, oxygen-enriched air or even almost pure oxygen is added 109 to the second fraction, which is then allowed to undergo wet oxidation in a wet oxidation reactor no. Collectively, the point of oxygen addition 109 and the wet oxidation reactor 110 are referred to a “wet oxidation device” 111. In one embodiment, the wet oxidation device 111 comprises two wet oxidation reactors arranged in series. The wet oxidation reduces the COD of the second fraction and increases its temperature, e.g. to a temperature of 220-250 °C.

[0076] The wet-oxidized fraction is then subjected to flashing in at least one step, such as in a first 112, a second 113 and a third 114 flashing vessel arranged in series. The flashing produces a cooled wet-oxidized fraction, which is mixed with the sludge water fraction from the screw press 101 to provide a reject water mixture. In an arrangement for bacterial nitrogen removal treatment 119, which may comprise an SBR, the reject water mixture undergoes a bacterial nitrogen removal treatment. Thereby, treated reject water that can be returned to the wastewater treatment plant 123 is obtained. 19

[0077] The reject water mixture may be formed in the arrangement for bacterial nitrogen removal treatment 119 or in a mixing device arranged upstream the arrangement for bacterial nitrogen removal treatment 119. Accordingly, the system

100 comprises a sludge water line 122 for routing sludge water from the screw press

101 to the arrangement for bacterial nitrogen removal treatment 119 or the mixing device. Further, the system 100 comprises an oxidized water line 121 for routing the cooled wet-oxidized fraction to the arrangement for bacterial nitrogen removal treatment 119 or the mixing device.

[0078] Steam from the flashing vessels 112, 113, 114 is typically added in the steam mixers 106, 105, 104. The steam from the first flashing vessel 112 after the wet oxidation is typically added in the last steam mixer 106. However, in case of extensive wet oxidation, excess steam from any of the flashing vessels 112, 113, 114 may be used in a separate process or to increase the temperature of the reject water mixture. The latter, which is particularly suitable during colder seasons, may be accomplished by heating the sludge water or the reject water mixture with the excess steam.

[0079] The heat-treated slurry is also subjected to flashing in at least one step, such as in a first 115, a second 116 and a third 117 flashing vessel arranged in series. The flashing produces a cooled slurry that is separated by a separation device 118, thereby providing the process water fraction used in the mixer 102 and a solids fraction. Accordingly, a recycling line 120 is provided for routing the process water fraction from the separation device 118 to the mixing device 102. The process water fraction provided by the separation device typically has a temperature of 55-99 °C, i.e. a higher temperature than that of the incoming sludge.

[0080] Steam from the flashing vessels 115, 116, 117 is typically added in the steam mixers 105, 104, 103. The steam from the last flashing vessel 117 is typically added in the first steam mixer 103.

[0081] In case of extensive wet oxidation, excess steam from any of the flashing vessels 115, 116, 117 may be used in a separate process or to increase the temperature of the reject water mixture as described above.

[0082] Another way of increasing the temperature of the reject water mixture, in particular in the colder seasons, is to exchange heat between the cooled slurry and the sludge water or the reject water mixture. This maybe beneficial since some filter 20 presses require further cooling (e.g. down to 50-60 °C) of the cooled slurry before the separation device 118.

[0083] As an illustrative example, there may be an inflow of 100001/h of sludge having a dry matter content of 4% and a temperature of 14 °C. This sludge is dewatered to a dry matter content of 20% in the screw press 101, which means that the flow of dewatered sludge and sludge water is 20001/h and 80001/h, respectively. The dewatered sludge is diluted to a dry matter content of 9% (corresponding to a flow of diluted sludge of 44401/h) in the mixer 102, which requires 24401/h of process water from the separation device 118. After processing, the flow and temperature of the cooled wet-oxidized fraction maybe 16601/h and 96 °C, while only 3401/h of water leaves the process with the solids fraction (which typically has a dry matter content of about 60%). The ratio volumetric ratio of sludge water to cooled wet-oxidized fraction is thus 8000:1660 (i.e. 4.8:1). When the cooled wet-oxidized fraction has been mixed with the sludge water, a flow of 96601/h of reject water mixture having a temperature of 28.1 °C has been provided (given that the sludge water has the same temperature as the incoming sludge, i.e. 14 °C). This temperature is close to ideal for the bacterial treatment. If the temperature of the process water from the separation device is 55 °C, the temperature of the diluted sludge will be about 37 °C. This is significantly higher than the temperature of the incoming sludge.

[0084] Should the temperature of the incoming sludge be lower than 14 °C, a reject water mixture of 28 °C can be reached by adapting the process, e.g. by heating with excess steam or heat exchanging, as discussed above.

Example 2

[0085] A system 200 according to another embodiment of the third aspect of the present disclosure for carrying out a second exemplary embodiment of the method of the first aspect of the present disclosure is illustrated in figure 2.

[0086] The system 200 of figure 2 is identical to that of figure 1 except from that the HTC reactor 207r does not have a built-in separation capacity. Instead, an external, downstream separator 207s provides the heat-treated slurry and the second fraction.

[0087] Collectively, the steam mixers 103, 104, 105, 106, the HTC reactor 207r and the separator 207s are referred to a “heating and separation arrangement” 208. 21

Example ¾

[0088] A system 300 according to an embodiment of the fourth aspect of the present disclosure for carrying out a first exemplary embodiment of the method of the second aspect of the present disclosure is illustrated in figure 3.

[0089] Just as in Example 1, sludge is received by an inlet of a screw press 301, typically from a wastewater treatment plant 323. The incoming sludge typically has a dry matter content of 3-5%. The temperature of the incoming sludge is typically in the range of io°C to 20°C depending on the season. The screw press 301 produces a dewatered sludge and a sludge water fraction. The dewatered sludge typically has a dry matter content of at least 20%. The dewatered sludge is mixed with process water in a mixer 302 to produce a diluted sludge, which typically has a dry matter content of 10% or lower and a total organic content of 7.4% or lower.

[0090] The diluted sludge is heated by stepwise additions of steam, e.g. in a first 303, a second 304 and a third 305 steam mixer arranged in series. A pump 303P, 304P, 305P is preferably arranged downstream each steam mixer 303, 304, 305. The heated sludge from the last steam mixer 305 is routed to a reactor 307, which produces a heat-treated slurry that typically has a temperature of 190-225 °C.

[0091] The sludge preferably undergoes HTC in the reactor.

[0092] Collectively, the steam mixers 303, 304, 305 and the reactor 307 are referred to a “heating arrangement” 308.

[0093] Oxygen in the form of air, oxygen-enriched air or even almost pure oxygen is added 309 to the heat-treated slurry, which is then allowed to undergo wet oxidation in at least one wet oxidation reactor 310. Collectively, the point of oxygen addition 309 and the wet oxidation reactor 310 are referred to a “wet oxidation device”. In one embodiment, the wet oxidation device comprises two wet oxidation reactors arranged in series. The wet oxidation reduces the COD of the heat-treated slurry and increases its temperature, e.g. to a temperature in the range of 230-275 °C, such as 230-260 °C.

[0094] The wet-oxidized slurry is subjected to flashing in at least one step, such as in a first 315, a second 316 and a third 317 flashing vessel arranged in series. The flashing produces a cooled wet-oxidized slurry that is separated by a separation device 318, thereby providing a process water fraction and a solids fraction. The 22 process water provided by the separation device typically has a temperature of 55-99 °C, i.e. a higher temperature than that of the incoming sludge. A recycling line 320 is provided for routing a part of the process water fraction to the mixing device 302.

[0095] Another part of the process water fraction is mixed with the sludge water fraction from the screw press 301 to obtain a reject water mixture. In an arrangement for bacterial nitrogen removal treatment 319, which may comprise an SBR, the reject water mixture undergoes a bacterial nitrogen removal treatment. Thereby, treated reject water that can be returned to the wastewater treatment plant 323 is obtained.

[0096] The reject water mixture may be formed in the arrangement for bacterial nitrogen removal treatment 319 or in a mixing device arranged upstream the arrangement for bacterial nitrogen removal treatment 319. Accordingly, the system

300 comprises a sludge water line 322 for routing sludge water from the screw press

301 to the arrangement for bacterial nitrogen removal treatment 319 or the mixing device. Further, the system 300 comprises a process water line 321 for routing process water from the separation device 318 to the arrangement for bacterial nitrogen removal treatment 319 or the mixing device.

[0097] Steam from the flashing vessels 315, 316, 317 is typically added in the steam mixers 305, 304, 303. The steam from the first flashing vessel 317 is typically added in the last steam mixer 305.

[0098] In case of extensive wet oxidation, excess steam from any of the flashing vessels 315, 316, 317 may be used in a separate process or to increase the temperature of the reject water mixture. The latter, which is particularly suitable during colder seasons, may be accomplished by heating the sludge water or the reject water mixture with the excess steam.

[0099] Another way of increasing the temperature of the reject water mixture, in particular in the colder seasons, is to exchange heat between the cooled wet-oxidized slurry and the sludge water or the reject water mixture. This may be beneficial since some filter presses require further cooling (e.g. down to 50-60 °C) of the cooled wet- oxidized slurry before the separation device 318. 23

Example 4.

[ooioo] A system 400 according to another embodiment of the fourth aspect of the present disclosure for carrying out a second exemplary embodiment of the method of the second aspect of the present disclosure is illustrated in figure 4.

[00101] The system 400 of figure 4 is identical to that of figure 3 except from that the reactor 307 has been omitted. Instead, this embodiment relies on the heat treatment the sludge undergoes in the piping leading to the wet oxidation reactor 310 and in the wet oxidation reactor 310.

Example 5

[00102] The flow characteristics of sludge from eight different sources were studied. In more detail, it was determined at which dry matter content and total organic content each sludge started to flow. The results are presented in table 1 below.

[00103] Table 1.

[00104] From table 1 it can be concluded that total organic content better predicts the flow point than dry matter content. It also shows that for processability purposes, the total organic content of the sludge is preferably reduced to 8.6% or lower, more preferably 8.0% or lower, most preferably 7.4% or lower.

Claims

24 CLAIMS

1. A method of sludge treatment, comprising the steps of:

- obtaining a sludge, preferably from a wastewater treatment plant;

- dewatering the sludge to obtain dewatered sludge and sludge water;

2. The method of claim 1, wherein the reject water mixture obtained in the mixing step has a temperature of 20-40 °C, such as 24-35 °C, such as 26-33 °C, such as 27-32 °C.

3. The method of claim 1 or 2, wherein the temperature of the sludge water supplied to the mixing step is below 25 °C, such as below 25 °C, such as below 15 °C, such as below 12 °C.

4. The method of any one of the preceding claims, wherein the temperature of the cooled wet-oxidized fraction supplied to the mixing step is 35-100 °C, such as 50-99 °C, such as 60-99 °C, such as 70-99 °C.

5. The method of any one of the preceding claims, wherein the volumetric ratio of sludge water to cooled wet-oxidized fraction supplied to the mixing step is between 2.0:1 and 20:1, such as between 2.5:1 and 10:1.

6. The method of any one of the preceding claims, wherein the bacterial nitrogen removal treatment comprises bacterial nitrification. 25

7. The method of any one of the preceding claims, wherein the heat-treating step is carried out after dilution of the dewatered sludge to a total organic content of 8.6% or lower, such as 8.0% or lower, such as 7.4% or lower.

8. The method of claim 7, wherein the heat-treated slurry is subjected to flashing in at least one step to produce at least one steam fraction and a cooled slurry, the cooled slurry is separated into a process water fraction and a solids fraction and at least part of the process water fraction is used in the dilution of the dewatered sludge.

9. The method of any one of the preceding claims, wherein the sludge obtained in the first step has a dry matter content below 10%, such as below 8%, such as 3%-5%.

10. The method of any one of the preceding claims, wherein the sludge obtained in the first step has a temperature below 25 °C, such as below 20 °C, such as below 15 °C.

11. The method of any one of the preceding claims, wherein the dewatered sludge has a dry matter content above 10%, such as above 15%, such as above 20%, such as at least 25%.

12. A method of sludge treatment, comprising the steps of:

- obtaining a sludge, preferably from a wastewater treatment plant;

- dewatering the sludge to obtain dewatered sludge and sludge water;

- mixing sludge water from the dewatering step with at least part of the process water fraction to obtain a reject water mixture wherein the temperature of the reject water mixture is higher than the temperature of the sludge water supplied to said mixing; and

- subjecting the reject water mixture to a bacterial nitrogen removal treatment to obtain treated reject water. 26

13. The method of claim 12, wherein the dewatered sludge is diluted before the heat-treating step with a part of process water fraction.

14. A system (100, 200) for sludge treatment, comprising:

- a dewatering device, such as a screw press (101), for dewatering sludge such that dewatered sludge and sludge water are produced;

- optionally a mixing device (102) arranged to dilute dewatered sludge from the press (101) with process water;

- a heating and separation arrangement (108, 208) arranged to heat dewatered and optionally diluted sludge from the press (101) or the mixing device (102) and process it into a heat-treated slurry and a second fraction, which has lower total suspended solids (TSS) than the heat-treated slurry;

- a wet oxidation device (111) arranged to subject the second fraction to wet oxidation and thereby produce a wet-oxidized fraction;

- an arrangement for bacterial nitrogen removal treatment (119) arranged to treat a mixture of sludge water from the press (101) and the cooled wet-oxidized fraction from the flashing arrangement.

15. A system (300, 400) for sludge treatment, comprising:

- a dewatering device, such as a screw press (301), for dewatering sludge such that dewatered sludge and sludge water are produced;

- optionally a mixing device (302) arranged to dilute dewatered sludge from the press (301) with process water;

- a heating arrangement (308, 408) arranged to heat dewatered and optionally diluted sludge from the press or the mixing device (302) to obtain a heat-treated slurry;

- a wet oxidation device arranged to subject heat-treated slurry from the heating arrangement to wet oxidation and thereby produce a wet-oxidized slurry;

- a flashing arrangement arranged to subject wet-oxidized slurry from the wet oxidation device to flashing in at least one step and thereby produce a cooled wet- oxidized slurry;

- a separation device (318) arranged to separate cooled wet-oxidized slurry from the flashing arrangement into a process water fraction and a solids fraction; and 27

- an arrangement for bacterial nitrogen removal treatment (319) arranged to treat a mixture of sludge water from the press (301) and at least part of the process water fraction from the separation device (318).