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SE546126C2 - Sludge treatment - Google Patents

Sludge treatment

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Publication number
SE546126C2
SE546126C2 SE2150614A SE2150614A SE546126C2 SE 546126 C2 SE546126 C2 SE 546126C2 SE 2150614 A SE2150614 A SE 2150614A SE 2150614 A SE2150614 A SE 2150614A SE 546126 C2 SE546126 C2 SE 546126C2
Authority
SE
Sweden
Prior art keywords
sludge
fraction
heat
wet
steam
Prior art date
Application number
SE2150614A
Other languages
Swedish (sv)
Other versions
SE2150614A1 (en
Inventor
Fredrik Björnerbäck
Peter Axegård
Original Assignee
C Green Ab
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by C Green Ab filed Critical C Green Ab
Priority to SE2150614A priority Critical patent/SE546126C2/en
Priority to PCT/SE2022/050469 priority patent/WO2022240350A1/en
Priority to EP22807946.3A priority patent/EP4337617A4/en
Publication of SE2150614A1 publication Critical patent/SE2150614A1/en
Publication of SE546126C2 publication Critical patent/SE546126C2/en

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Classifications

    • 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
    • C02F11/12—Treatment of sludge; Devices therefor by de-watering, drying or thickening
    • C02F11/121—Treatment of sludge; Devices therefor by de-watering, drying or thickening by mechanical de-watering
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F9/00—Multistage treatment of water, waste water or sewage
    • 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
    • C02F11/10—Treatment of sludge; Devices therefor by pyrolysis
    • 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 OR C10K; LIQUIFIED PETROLEUM GAS; USE OF ADDITIVES TO FUELS OR FIRES; FIRE-LIGHTERS
    • C10L9/00—Treating solid fuels to improve their combustion
    • C10L9/08—Treating solid fuels to improve their combustion by heat treatments, e.g. calcining
    • C10L9/086—Hydrothermal carbonization
    • 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
    • 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
    • C02F11/12—Treatment of sludge; Devices therefor by de-watering, drying or thickening
    • C02F11/121—Treatment of sludge; Devices therefor by de-watering, drying or thickening by mechanical de-watering
    • C02F11/125—Treatment of sludge; Devices therefor by de-watering, drying or thickening by mechanical de-watering using screw filters
    • 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
    • C02F11/18—Treatment of sludge; Devices therefor by thermal conditioning

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Organic Chemistry (AREA)
  • Environmental & Geological Engineering (AREA)
  • Water Supply & Treatment (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Hydrology & Water Resources (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Mechanical Engineering (AREA)
  • Combustion & Propulsion (AREA)
  • Oil, Petroleum & Natural Gas (AREA)
  • Treatment Of Sludge (AREA)

Abstract

There is provided 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;- diluting the dewatered sludge to obtain diluted sludge;- heat-treating the diluted sludge to obtain a heat-treated slurry;- separating the heat-treated slurry into a process water fraction and a solids fraction; and- using at least part of the process water fraction in the step of diluting the dewatered sludge.

Description

TECHNICAL FIELD id="p-1"
[0001] The present disclosure relates to the field of treatment of sludge, preferably sludge from a wastewater treatment plant.
BACKGROUND id="p-2"
[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. id="p-3"
[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. id="p-4"
[0004] The reactor disclosed in WO 2017/ 003358 may be used for the HTC and separating steps of the method in WO 2017/ id="p-5"
[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 id="p-6"
[0006] Objectives of the present disclosure are to improve the efficiency of sludge treatment and to facilitate the sludge treatment in 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 plantsrgï* - diluting the dewatered sludge to obtain diluted sludge; - heat-treating the diluted sludge to obtain a heat-treated slurry; - separating the heat-treated slurry into a process water fraction and a solids fraction; and - using at least part of the process water fraction in the step of diluting the dewatered sludge. 2. The method of item 1, wherein the heat-treated slurry is subjected to flashing in at least one step prior to the separating step to obtain at least one steam fraction that is / are added to the diluted sludge in the heat-treating step. 3. The method of item 1 or 2, wherein the temperature of the process water used to dilute the dewatered sludge is at least 45 °C, preferably at least 50 °C, preferably at least 60 °C. 4. The method of any one of the preceding items, wherein the sludge has a dry matter content below 10%, such as below 8%, such as 3-5%.
. The method of any one of the preceding items, wherein the sludge has a total organic content below 8%, such as below 6%, such as 2-4%. 6. The method of any one of the preceding items, wherein the sludge has a \' Ä *l :ti °C, such as below 15 °C. temperature below 7. The method of any one of the preceding items, wherein the dewatered sludge \' 't -\ ~ >* \' has a dry matter content g 1.: 3:; 1:; :;:::_. g *f ;:;:::.I: 11:: :.:;;.::::f" .. ::.:.;:;Åï:: 11:: c, _ _ ____ __- at least 25%.8. The method of any one of the preceding items, wherein the dewatered sludge has a total organic content above 10%, such as above 13%, such as above 15%. 9. The method of any one of the preceding items, wherein the diluted sludge has a dry matter content of 10% or lower.
. The method of any one of the preceding items, wherein the diluted sludge has a total organic content of 8.6% or lower, such as below 8.0% or lower, such as 7.4% or lower. 11. The method of any one of the preceding items, wherein 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. 12. The method of item 11, wherein the second fraction is subjected to wet oxidation to obtain a wet-oxidized fraction. 13. The method of item 12, wherein the wet-oxidized fraction is subjected to flashing in at least one step to obtain at least one steam fraction that is / are added to the diluted sludge in the heat-treating step and a cooled wet-oxidized fraction. 14. The method of item 13, wherein the sludge is obtained from a wastewater treatment plant and the cooled wet-oxidized fraction is returned to the wastewater treatment plant, optionally after further cooling and/ or purification.
. The method of item 14, wherein the cooled wet-oxidized fraction is merged with sludge water obtained in the dewatering step and then subjected to purification before being returned to the wastewater treatment plant. 16. The method of any one of items 11-15, wherein at least 90% of the process water fraction is used to dilute the dewatered sludge. 17. The method of any one of items 1-10, wherein the heat-treated slurry is subjected to wet oxidation prior to the separating step and prior to any flashing. 18. The method of item 17, wherein the temperature of the heat-treated slurry that has been subjected to wet oxidation is 220-260 °C, preferably 230-250 °C and more preferably 235-245 °C.19. The method of item 17 or 18, wherein part of the process water fraction is merged with sludge water obtained in the dewatering step and then subjected to purification before being returned to the wastewater treatment plant.
. 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Q. ,-... _.. ...fw "s . .\ . ...s . ,- .Ä .,~..-\,-\ ss.. » .s t sms.. ...ssëë ...s u... .s\.~.~,.,,.s ..-s.\.._. ,,.:_\.-s.~ sussu. ...s .s ._s- , y.. ._.~\s...\.-\.- is \.-\.~..-.\....,. vvs... .,.~.:...\ss.«_....... m. u.s.s.s\.~. s. __.__By means of the dewatering step and the 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 (if the water content is too low, pumpability is impaired. Further, "cold" water in the incoming sludge is 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.ï¿_ï§:'j_.f_~;3:§ïlš_____Possibly the most significant benefit of the above method 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. ____ __In one embodiment, the dewatering step of the above method 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. ______ __The sludge water (also referred to as reject water) from the dewatering step of the above method is typically returned to the wastewater treatment plant, which typically is already adapted to process it (since it used to carry out the dewatering step itself). The need for such further processing may however be reduced by further treatment before return. This is further discussed below.
BRIEF DESCRIPTION OF THE DRAWINGS id="p-1"
[0001] Figure 1 illustrates an embodiment of the system of the present disclosure for carrying out an exemplary embodiment of the method of the present disclosure. id="p-2"
[0002] Figure 2 illustrates another embodiment of the system of the present disclosure for carrying out another exemplary embodiment of the method of the present disclosure. id="p-3"
[0003] Figures 3 and 4 illustrate systems for carrying out yet other exemplary embodiments of the method of the present disclosure.
DETAILED DESCRIPTION id="p-4"
[0004] As a first aspect of the present disclosure, 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; - diluting the dewatered sludge to obtain diluted sludge; - heat-treating the diluted sludge to obtain a heat-treated slurry; - separating the heat-treated slurry into a process water fraction and a solids fraction; - using at least part of the process water fraction in the step of diluting the dewatered sludge. [0005] The heat-treating step may comprise hydrothermal carbonization (HTC). id="p-6"
[0006] 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 has typically been subjected to flashing prior to the separating step as further discussed below. id="p-7"
[0007] 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. id="p-8"
[0008] In any case, the temperature of the process water used to dilute the dewatered sludge is higher than the temperature of the incoming sludge, i.e. the sludge obtained in the first step of the method. The temperature of the incoming sludge is typically below 30 °C and normally below 25 °C. Often it is below 20 °C and sometimes it is below 15 °C, especially during the colder seasons of the year. At such lower temperatures, the method of the first aspect is particularly beneficial. id="p-9"
[0009] The sludge, i.e. the sludge obtained in the first step of the method, typically has a dry matter content below 10%, such as below 8%, such as 3-5%. Further, it may have a total organic content below 8%, such as below 6%, such as 2-4%. id="p-10"
[0010] 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%-80% 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. id="p-11"
[0011] The dewatered sludge, i.e. the sludge that have undergone the dewatering step, but not the 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%.[0012] 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. id="p-13"
[0013] The inventors have found that 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 diluted sludge obtained from the dilution step. Preferably, the total organic content of the diluted sludge obtained from the dilution step is 8.0% or lower, such as 7.4% or lower. A typical lower limit may be 5.0% or 6.0%. id="p-14"
[0014] Further, the dry matter content of the diluted sludge obtained from the dilution step is preferably 10% or lower. id="p-15"
[0015] In an embodiment of the first aspect, the heat-treated slurry is subjected to flashing in at least one step prior to the separating step. Thereby at least one steam fraction that can be added to the diluted sludge in the heat-treating step is obtained.
Such internal heat recovery is heat efficient. id="p-16"
[0016] In an embodiment, the diluted sludge subjected to heat-treating is separated 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. In other words, the heat-treating step of the embodiment comprises separation into the first fraction and the second fraction. 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 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. id="p-17"
[0017] The second fraction is preferably 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. id="p-18"
[0018] The wet-oxidized fraction is preferably subjected to flashing in at least one step. Thereby at least one steam fraction that can be added to the diluted sludge inthe heat-treating step is/ are obtained. The flashing further provides a cooled wet- oxidized fraction. id="p-19"
[0019] The temperature of the steam fraction obtained in the first flashing step may be 9-40 °C higher than the temperature of the second fraction before wet oxidation. id="p-20"
[0020] The cooled wet-oxidized fraction may be further treated by a wastewater treatment plant, such as the Wastewater treatment plant from which the (incoming) sludge of the first step was obtained. However, the cooled wet-oxidized fraction can be subjected to further cooling and/ or purification before reaching the wastewater treatment plant. In one embodiment, the cooled wet-oxidized fraction is merged with sludge water from the dewatering step and then subjected to purification before being returned to the wastewater treatment plant. id="p-21"
[0021] In an embodiment, at least 80%, such as at least 90%, of the process water fraction is used to dilute the dewatered sludge. This embodiment is particularly relevant when a second fraction is separated as described above. id="p-22"
[0022] In an embodiment, the heat-treated slurry is subjected to wet oxidation prior to the separating step and prior to any flashing. This embodiment is particularly relevant when a second fraction is not separated (see e.g. figures 3 and 4 and examples 3 and 4 below). Accordingly, part of the process water fraction may be diverted and not recycled to the dilution step in this embodiment. Instead, this part of the process water fraction is typically processed further in a wastewater treatment, such as the wastewater treatment plant from which the (incoming) sludge of the first step was obtained. As an example, it may be merged with sludge water from the dewatering step and then subjected to purification before being returned to the wastewater treatment plant. The purification may comprise removal of nitrogen compounds. id="p-23"
[0023] 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. id="p-24"
[0024] Whether wet-oxidized or not, the heat-treated slurry is preferably 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. When theheat-treated slurry is wet-oxidized, 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. id="p-25"
[0025] As a second aspect of the present disclosure, there is provided a system for sludge treatment, e.g. according to the embodiment of the first aspect comprising separation and wet oxidation of the second fraction. id="p-26"
[0026] The system of the second aspect comprises: - a screw press for dewatering sludge; - a mixing device arranged to dilute dewatered sludge from the screw press with a process water fraction; - a heating and separation arrangement arranged to heat diluted sludge from 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 first flashing arrangement arranged to subject the heat-treated slurry to flashing in at least one step and thereby produce a cooled slurry; - a separation device for separating the cooled slurry from the first flashing arrangement into the process water fraction and a solids fraction; - a recycling line for routing the process water fraction from the separation device to the mixing device; - a wet oxidation device arranged to subject the second fraction to wet oxidation and thereby produce a wet-oxidized fraction; and - a second flashing arrangement arranged to subject the wet-oxidized fraction to flashing in at least one step and thereby produce a cooled wet-oxidized fraction. id="p-27"
[0027] The heating and separation arrangement may comprise a reactor 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/ id="p-28"
[0028] In one embodiment, the system comprises a steam routing arrangement arranged to route at least one steam fraction from the first flashing arrangement and/ or at least one steam fraction from the second flashing arrangement to the heating and separation arrangement. Preferably, the heating and separation arrangement comprises steam mixers to which the steam routing arrangement is 1O connected. Such steam mixers are preferably arranged upstream any reactor or equipment for separating the second fraction. id="p-29"
[0029] Various embodiments of the second aspect are described in examples 1 and 2 below. id="p-30"
[0030] 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 WEISÉC OI' IIIEIIIUTC.
EXAMPLES Example[0031] A system 100 according to an embodiment of the second 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 id="p-32"
[0032] A sludge is received by an inlet of a dewatering screw 101, typically from a wastewater treatment plant. The incoming sludge typically has a dry matter content of 3-5% The temperature of the incoming sludge is typically in the range of 10°C to 20°C depending on the season. The dewatering screw 101 produces a dewatered sludge and a sludge water fraction. The dewatered sludge typically has a dry matter content of at least 25%. 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 % or lower and a total organic content of 7.4% or lower. id="p-33"
[0033] 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 103p, 104p, 105p, 106p is preferably arranged downstream each steam mixer 103, 104, 105, 106. The heated sludge from the last steam mixer 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 is typically 190-215 °C.[0034] Collectively, the steam mixers 103, 104, 105, 106 and the HTC reactoris referred to a "heating and separation arrangement" id="p-35"
[0035] 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 110. Collectively, the point of oxygen addition and the wet oxidation reactor is referred to a "wet oxidation device" 111. In one embodiment, the wet oxidation device 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. id="p-36"
[0036] 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 typically routed back to the wastewater treatment plant, optionally after mixing with the sludge water fraction from the screw press and optionally after further treatment id="p-37"
[0037] 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 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 Sepafate PYOCCSS. id="p-38"
[0038] 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. id="p-39"
[0039] 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. However, in case of extensive wet oxidation, excess steam from any of the flashing vessels 115, 116, 117 may be used in a separate process.Example[0040] A system 200 according to another embodiment of the second 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 id="p-41"
[0041] 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. id="p-42"
[0042] Collectively, the steam mixers 103, 104, 105, 106, the HTC reactor 207r and the separator 207s are referred to a "heating and separation arrangement" Example[0043] A system 300 for carrying out a third exemplary embodiment of the method of the first aspect of the present disclosure is illustrated in figure id="p-44"
[0044] Just as in Example 1, sludge is received by an inlet of a dewatering screw 301, typically from a Wastewater treatment plant. The incoming sludge typically has a dry matter content of 3-5%. The temperature of the incoming sludge is typically in the range of 10°C to 20°C depending on the season. The dewatering screw 101 produces a dewatered sludge and a sludge water fraction. The dewatered sludge typically has a dry matter content of at least 25%. 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. id="p-45"
[0045] 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 is routed to a reactor 307, which produces a heat-treated slurry that typically has a temperature of 190-225 °C. id="p-46"
[0046] 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. 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 230-260 °C.[0047] 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 slurry that is separated by a separation device 318, thereby providing the process water used in the mixer 302 and a solids fraction. Accordingly, a recycling line 320 is provided for routing process water fraction the separation device 318 to the mixing device 302. The 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. id="p-48"
[0048] For water balancing purposes and/ or to avoid accumulation, part of the process water provided by the separation device 318 may be diverted and typically routed back to the wastewater treatment plant, optionally after mixing with the sludge water fraction from the screw press 301 and optionally after further treatment 319- id="p-49"
[0049] 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 17 is typically added in the last steam mixer 305. However, in case of extensive wet oxidation, excess steam from any of the flashing vessels 315, 316, 317 may be used in a separate process.
Example[0050] A system 400 for carrying out a fourth exemplary embodiment of the method of the first aspect of the present disclosure is illustrated in figure id="p-51"
[0051] 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 reactorand in the wet oxidation reactor 310. Example[0052] 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 tablebelow. id="p-53"
[0053] TableSludge # 1 2 3 4 5 6 7 8 Ash content in dry matter (%) 35 33.7 37 47 55 21 41 21 Organic content in dry matter (%) 65 66 63 53 45 79 59 79 Dry matter content in sludge @ flow point (%) 11.6 12 12.4 13.9 18 10.9 11.5 1o Total organic content in sludge @ flow point (%) 7.5 8.0 7.8 7.4 8.o 8.6 6.8 7.[0054] 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.o% or lower, most preferably 7.4% or lower.

Claims (14)

1. A method of sludge treatment, comprising the steps of: - obtaining a sludge, preferably from a wastewater treatment plant, sludge has a temperature below 25 °C; - dewatering the sludge having a temperature below 25 °C to obtain dewatered sludge having a dry matter content above 20% and sludge water; - diluting the dewatered sludge to obtain diluted sludge; - heat-treating the diluted sludge to obtain a heat-treated slurry; - separating the heat-treated slurry into a process water fraction and a solids fraction; and - using at least part of the process water fraction in the step of diluting the dewatered sludge.
2. The method of claim 1, wherein the heat-treated slurry is subjected to flashing in at least one step prior to the separating step to obtain at least one steam fraction that is / are added to the diluted sludge in the heat-treating step.
3. The method of claim 1 or 2, 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.
4. The method of any one of the preceding claims, wherein the sludge has a dry matter content below 10%, such as below 8%, such as 3%-5%.
5. The method of any one of the preceding claims, wherein the sludge has a temperature below 20 °C, such as below 15 °C.
6. The method of any one of the preceding claims, wherein the dewatered sludge has a dry matter content of at least 25%.
7. The method of any one of the preceding claims, wherein the diluted sludge has a dry matter content of 10% or lower.
8. The method of any one of the preceding claims, 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.
9. The method of any one of the preceding claims, wherein 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.
10. The method of claim 9, wherein the second fraction is subjected to wet oxidation to obtain a wet-oxidized fraction.
11. The method of claim 10, wherein the wet-oxidized fraction is subjected to flashing in at least one step to obtain at least one steam fraction that is/ are added to the diluted sludge in the heat-treating step and a cooled wet-oxidized fraction.
12. The method of claim 11, wherein the sludge is obtained from a wastewater treatment plant and the cooled wet-oxidized fraction is returned to the Wastewater treatment plant, optionally after further cooling and/ or purification.
13. The method of claim 12, wherein the cooled wet-oxidized fraction is merged with sludge water obtained in the dewatering step and then subjected to purification before being returned to the wastewater treatment plant.
14. The method of any one of claims 9-13, wherein at least 90% of the process water fraction is used to dilute the dewatered sludge.
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