AU2020322315A1

AU2020322315A1 - Process for anaerobic digestion of carbonaceous material

Info

Publication number: AU2020322315A1
Application number: AU2020322315A
Authority: AU
Inventors: Valéry GEAUGEY; Mathieu Haddad; Pierre Emmanuel Pardo
Original assignee: Suez International SAS
Current assignee: Suez International SAS
Priority date: 2019-07-29
Filing date: 2020-07-24
Publication date: 2022-03-10
Also published as: EP4003921A1; MX2022001315A; WO2021018780A1; CO2022001321A2

Abstract

The invention concerns a process of treatment of carbonaceous material, such as wastewater sludge or organic waste, comprising the steps of : (1) performing a thermal treatment of said carbonaceous material, thereby providing thermally treated carbonaceous material, (2) cooling of said thermally treated carbonaceous material, thereby providing cooled carbonaceous material, and (3) performing a post-treatment of said cooled carbonaceous material, wherein the cooling of step 2) is performed using a vacuum cooling step.

Description

PROCESS FOR ANAEROBIC DIGESTION OF CARBONACEOUS MATERIAL

TECHNICAL FIELD

The present invention relates to carbonaceous materials treatment comprising a thermal pre treatment step. More specifically, it concerns a sludge or organic waste treatment including at least one biological treatment step such as anaerobic digestion.

TECHNOLOGICAL BACKGROUND

Anaerobic digestion is a key process to recover the energy initially present in carbonaceous material, such as wastewater and organic waste, in the form of biogas. The improvement in digestion efficiency leads to increased energy production on the road to the energetically self- sufficient WWTP (waste water treatment plant) and waste treatment facilities. However, anaerobic digestion shows certain limitations in the first hydrolytic step, leading to slow degradation of the organic matter and high retention times in the digester. To improve the kinetics of anaerobic biodegradation, many pre-treatment technologies have been developed with the aim of accelerating the hydrolysis limiting step and enhancing biogas productivity, as well as the characteristics of the digested carbonaceous material (or sludge).

A state-of-the-art improved digestion process, with different examples of hydrolysis pre treatment steps implemented upstream a mesophilic digester, is displayed in Figure 1 . The hydrolysis pre-treatment step (PT) may either be a thermal hydrolysis (TH) or a biological hydrolysis (BH), performed in a tank (H). The pre-treatment step may also comprise a pre dewatering step (performed in a pre-dewatering unit DU) and storing the carbonaceous material in a storage unit (SS), wherein the carbonaceous material is pre-treated before entering the thermal treatment reactor (H).

Thermal hydrolysis (TH or THP) is the most widespread pre-treatment technology used to enhance sludge anaerobic digestion (AD) in WWTP. Thermal hydrolysis has the objective of improving digestion performances and dewaterability on biological or mixed sludge by breaking down bacteria’s cellular walls; cell content consequently becoming easily degradable anaerobically.

Another method used to enhance the rate limiting hydrolysis step is based on biological hydrolysis (BH) also known as Temperature Phased Anaerobic Digestion (TPAD or 2PAD). TPAD typically combines a short (1 -3 days) thermophilic pre-treatment stage (typically 50 to 70Ό or 750) applied prior to a conventional mesop hilic anaerobic digestion (typically 35Ό to 38Ό, 10-20 days). The TPAD is usually fed with a c arbonaceous material exhibiting a dry solid (DS) content ranging from 3 to 8% DS. It combines a short thermophilic pre-treatment stage applied prior to a conventional mesophilic anaerobic digestion. Thermophilic-mesophilic TPAD or 2PAD has been shown to be an effective treatment for increasing methane production and volatile solids (VS) destruction, compared with a single-stage mesophilic digestion.

As compared with thermal hydrolysis, biological hydrolysis shows inferior performances in terms of sludge dewaterability and digestion performance (biogas production, the produced biogas being stored in a biogas storage BS). However, biological hydrolysis is an interesting alternative because it requires lower capital expenditures, and is, in particular, more relevant in emerging countries.

The TPAD allows for the production of US EPA class B biosolids whereas the 2PAD allows for the production of class A biosolids. The beneficial reuse of biosolids through land application is governed in the United States (US) by the Environmental Protection Agency (EPA), more specifically through the EPA’s CFR 40 Part 503 rule for biosolids. Within the part 503 rule, definition of quality of Biosolids is defined and have two commonly identified terminologies: Class A Biosolids; and Class B Biosolids. While the part 503 rule also defines, quality surrounding such things as heavy metals, the focus of AD processes and digestion enhancements are focused on meeting the requirements surrounding pathogen and vector attraction reductions.

In a TPAD/2PAD system, the sludge has to be cooled between the (thermophilic) biological hydrolysis reactor (ca. 550) H and the mesophilic digester D, usually at 370. Today, downstream the hydrolysis step of a TPAD, sludge cooling occurs via heat exchangers (recovery heat exchangers or water/sludge, further designated as HEx) as displayed in figure 2.

The water used in HEx is, typically, the process water of the waste water treatment plant.

The heat exchanger size can become significantly large as it depends on the temperature of the process water. Indeed, the sizing of the HEx depends on the delta temperature in the heat exchanger between the cold side (cooling water) and the hot side (hydrolysed sludge). The smaller the delta temperature, the larger the HEx.

Multiple limitations of the current configurations are found with the current HEx.

Viscosity varies significantly depending on the type of sludge, and from one site to another, which can heavily impact the size of the heat exchanger (heat exchange coefficient variability) and induce significant energy cost for pumping (important head losses). This can render the heat recovery HEx uneconomical or even unfeasible. This also limits the scalability of the process from one site to another.

Many emerging countries are found in tropical or warm climate regions where the process water temperature (used for cooling) can be too hot to efficiently cool down the sludge (temperature of process water often being superior to 20^tC). This significantly increases the HEx size, in which the generated head loss requires using large pumping capacity and size and consequently renders the solution energy intensive and/or uneconomical. This is not compatible with the requirement to reduce energy consumption in countries where energy costs are high.

This is the reason why very few actors on the market offer a TPAD/2PAD on large industrial installations, where the HEx size issues become unmanageable.

Lastly, deposits that occur in the heat exchanger can also diminish the quality of the exchange as they reduce the heat exchange coefficient.

In order to overcome the above-mentioned limitations, heat exchangers designs include numerous safety margins to ensure proper operations. Consequently, cooling heat exchangers can be very lengthy. The longer the heat exchanger, the bigger the head loss, and with it the electrical consumption of the pumps.

In addition, the heat exchanger must be maintained and kept clear of fouling (grease, solids and mineral deposits may occur). The latter continuously decrease the global heat exchange to a value below the one desired, in addition to increasing the head losses of the pump.

And as the TPAD may be operated in batch, in which case it is named 2PAD, the heat exchanger may be used only a fraction of the day, with no flow going through it. During these batch phases, significant deposits may occur and permanently stick to the HEx walls, which further accelerates future deposits. Another difficulty of TPAD/2PAD current processes is the preheating step of the sludge prior to be fed to the biological hydrolysis reactor.

In a TPAD/2PAD system, as the first stage (BH) is operated under thermophilic conditions, it needs to be pre-heated. To reduce the heat requirements of this first stage, it is possible to use the hot hydrolysed sludge to heat up the cold raw sludge feeding the system. This heat recovery is usually carried out in a double heat exchanger hot sludge/water /cold sludge. The water is the energy transfer media that carries the energy from the hot sludge to the cold sludge.

This leads to the need to build another heat exchanger to recover the energy. Another possibility is to have a hot sludge/ cold sludge heat exchanger. In this case, the heat exchange coefficient is very limiting, and in terms of overall mass, the mass of this sludge/sludge HEx is equivalent to the one of the sludge/water/sludge HEx and consequently their cost is also in the same range.

The same shortcomings are found in anaerobic digestion processes including a TH pre treatment step upstream of the anaerobic digestion, or when the carbonaceous material has to be pasteurized prior to being digested.

SUMMARY OF THE INVENTION

The present invention almost erases the above described intermediate HEx drawbacks. The pre-heating steps drawbacks and additional costs may also be significantly reduced.

The present invention is in particular directed towards a process of temperature-phased anaerobic digestion of wastewater sludge, organic waste, or any kind of carbonaceous material, either continuous (TPAD) or batch (2PAD), where the cooling of sludge downstream the biological hydrolysis step is performed using a vacuum cooling step.

In a first aspect, the present invention relates to a process for treating carbonaceous material, such as wastewater sludge or organic waste, comprising the steps of:

(1 ) performing a thermal treatment of said carbonaceous material, thereby providing thermally treated carbonaceous material,

(2) cooling of said thermally treated carbonaceous material, thereby providing cooled carbonaceous material and recovered steam, and

(3) performing a post-treatment of said cooled carbonaceous material,

wherein the cooling of step 2) is performed using a vacuum cooling step.

As used herein, a“carbonaceous material” is understood as a mixture of organic and inorganic materials, such as biomass. In the invention, it may also be referred to as“organic matter”. The carbonaceous material is typically wet. Its dry solid content is advantageously between 3 and 25%. Examples of carbonaceous material are organic waste and/or sludge, and more particularly sludge from organic waste or drinking water or wastewater treatment plants. Typically, in the present invention, the carbonaceous material is a sludge, such as a wastewater treatment sludge. Examples of sludge are municipal sludge, biological sludge, and fresh or raw sludge.

As used herein, a“vacuum cooling” is understood as an evaporation under vacuum, that is to say at a pressure below atmospheric pressure. In general, it refers to a rapid cooling technique for evaporating water from any suspension comprising organic matter, such as sludge or organic waste, said evaporation being performed under vacuum. In some instances, it is referred to as “flash cooling” or “vacuum flash cooling”, as the evaporation is almost “instantaneous”. Vacuum Cooling is typically operated at an absolute pressure ranging from 0.055 to 0.480 bar (i.e. 5 500 to 48 000 Pa), such as from 0.055 to 0.170 bar (i.e. 5 500 to 17 000 Pa), or from 0.15 to 0.48 bar (i.e. 15 000 to 48 000 Pa).

In said vacuum cooling step, part of the water content of the sludge is evaporated, producing steam at a temperature determined by the absolute pressure in the vacuum cooling vessel, herein referred to as“recovered steam”. In the following, it may also be referred to as“flash steam” or simply “steam”. Recovered steam is distinguished from “Off-gas” or “non condensable gas”, which are herein understood as gas produced downstream of the cooling unit, not condensed for instance in a subsequent heat recovery step (typically at a temperature of between 500 and 800, and at a pressure equal o r close to the pressure in the vacuum cooling unit/step, i.e. at an absolute pressure ranging from 0.055 to 0.48 bar (i.e. 5 500 to 48 000 Pa)). Off-gas generally comprise or consist essentially of N₂ (nitrogen), H₂S (hydrogen sulphide), C0₂ (Carbon dioxide), light hydrocarbons (saturated, linear or ramified C1 -C4 hydrocarbon chains, in particular methane), and/or NH₃ (ammonia).

Advantageously, the thermally-treated carbonaceous material (in particular sludge) is then cooled down (for instance to a temperature around 370) prior to be temporarily stored (typically held in an intermediate holding tank) and/or to be submitted to a post-treatment.

As used herein, the“thermal treatment” of step 1 ) is understood as comprising heating the carbonaceous material to 500 or more, typically be tween 500 and 900. Examples of thermal treatment comprise low temperature thermal hydrolysis (TH), biological hydrolysis (BH, corresponding to the first step of a temperature-phased anaerobic digestion), thermophilic anaerobic digestion, or pasteurization.

“Pasteurization” is well-known in the art. It is usually understood as a process in which a liquid product is treated with mild heat, usually at a temperature of less than 1000, advantageously between 70Ό and 75Ό, to eliminate pathogens.

Anaerobic digestion is a process involving microorganisms that break down carbonaceous material in the absence of oxygen. This process produces a digestate and a gaseous fraction comprising methane, and typically consisting essentially of methane and C0₂, also called biogas. Anaerobic digestion is usually performed at pH conditions between 7,0 and 7,5, preferably between 7,0 and 7,2.

“Thermophilic anaerobic digestion” is well-known in the art. It is an anaerobic digestion typically performed at a temperature of between 50Ό and QOΌ .

“Low temperature thermal hydrolysis”, abbreviated as“TH”, is well known in the art. As used herein, it is understood as a process aiming at improving digestion performances and dewaterability on carbonaceous material (typically biological or mixed sludge) by breaking down bacteria’s cellular walls; cell content consequently becoming easily degradable anaerobically.

A typical prior art TH process is depicted in Figure 1 . In this process, carbonaceous material - usually sludge, preferably with a dry solid (DS) content ranging from 12 to 22 % - is heated to a temperature of between 140 and 165^*0, typically f or 30 minutes. The hydrolysed sludge is then cooled down in a flash tank before being fed to the digester D wherein the anaerobic digestion is performed. Following hydrolysis, sludge is typically diluted to approximately 10% dry matter content before being injected into the digester: Mixing is effective despite this high concentration as thermal hydrolysis decreases sludge viscosity. The produced biogas is recovered in a tank T. The following publications disclose further TH processes:

• Barber, W. P. F. "Thermal hydrolysis for sewage treatment: a critical review." Water Research 104 (2016): 53-71 .

• Sandino, Julian, et al. "Thermal Hydrolysis and Incineration of Sludge: Evaluating Their Role in Optimizing Energy Profiles at Advanced BNR Facilities." Proceedings of the Water Environment Federation 2018.18 (2018): 372-386.

• Gonzalez, A., et al. "Pre-treatments to enhance the biodegradability of waste activated sludge: elucidating the rate limiting step." Biotechnology advances 36.5 (2018): 1434- 1469.

“Biological hydrolysis” (BH) is well known in the art. As used herein, it is the first step of Temperature Phased Anaerobic Digestion (TPAD or 2PAD). Typically, BH is a thermophilic digestion step, advantageously operated at a temperature of between 5013 et 75Ό (while the second step of a TPAD/2PAD is usually a mesophilic digestion operated at a temperature of between 30-40^tC, advantageously between 35Ό and 38 ^tC). Prior art TPAD/2PAD including a BH step are for instance described in ES2430739, DK3008193 and KR100588166B1 .

Descriptions of state of the art TPAD/2PAD processes can also be found in the following publications:

• https://www.epa.gov/biosolids/examples-equivalent-processes-pfrp-and-psrp (search 2PAD)

• Huyard, A., B. Ferran, and J-M. Audic. "The two phase anaerobic digestion process: sludge stabilization and pathogens reduction." Water Science and Technology 42.9 (2000): 41 -47.

• Willis, John, and Perry Schafer. "Advances in thermophilic anaerobic digestion." Proceedings of the Water Environment Federation 2006.7 (2006): 5378- 5392.

• Ge, Huoqing, Paul D. Jensen, and Damien J. Batstone. "Temperature phased anaerobic digestion increases apparent hydrolysis rate for waste activated sludge." Water Research45.4 (201 1 ): 1597-1606.

• Ge, Huoqing, Paul D. Jensen, and Damien J. Batstone. "Increased temperature in the thermophilic stage in temperature phased anaerobic digestion (TPAD) improves degradability of waste activated sludge." Journal of Hazardous Materials 187.1 -3 (201 1 ): 355-361 .

• Ge, Huoqing, Paul D. Jensen, and Damien J. Batstone. "Pre-treatment mechanisms during thermophilic-mesophilic temperature phased anaerobic digestion of primary sludge." Water research 44.1 (2010): 123-130.

• Watts, S., G. Hamilton, and J. Keller. "Two-stage thermophilic-mesophilic anaerobic digestion of waste activated sludge from a biological nutrient removal plant." Water science and technology 53.8 (2006): 149-157.

• Akgul, D., M. A. Celia, and C. Eskicioglu. "Temperature phased anaerobic digestion of municipal sewage sludge: a Bardenpho treatment plant study." Water Practice and Technology 1 1 .3 (2016): 569-573.

As used herein, the“post-treatment” of step 3) may comprise or be a mechanical and/or a biological treatment. An example of “mechanical treatment” is a dewatering step. As used herein, a“biological treatment” is understood as a thermophilic acidogenesis, an aerobic digestion, an anaerobic digestion or a fermentation.

“Fermentation” is a process well-known in the art and may be defined as a biological anaerobic process extracting energy from carbohydrates in the absence of oxygen, to produce small molecules (organic substrates), in particular RBCs, through the action of enzymes in particular. No <3H is produced, or only traces amounts. There are five main types of fermentation: • Alcoholic Fermentation, yielding mainly ethanol,

• Lactic Acid Fermentation, yielding lactate,

• Propionic Acid Fermentation, yielding propionate,

• Butyric Acid / Butanol Fermentation, yielding butyrate and butanol,

• Mixed Acid Fermentation, yielding VFAs (mainly acetate, but also propionate, lactate, butyrate).

The fermentation process may be controlled by the retention time of the sludge into the anaerobic tank, temperature and pH in the anaerobic tank, as well as by the specific microbial population involved in the fermentation process (i.e. by the choice of microbial strains in the anaerobic tank).

In a preferred embodiment, the process further comprises a step of pre-heating the carbonaceous material with the recovered steam of step 2).

In a preferred embodiment, pre-heating of the carbonaceous material (such as raw sludge) entering the thermal treatment step 1 ) is performed by direct contact of the recovered steam produced in the vacuum cooling step 2), with the carbonaceous material. Alternatively, it may be performed by direct injection of the recovered steam produced in the flash cooling step (2), into the carbonaceous material (such as raw sludge).

In another preferred embodiment, the recovered steam of step 2) is directly contacted with the carbonaceous material (raw sludge) upstream the thermal treatment step 1 ). Accordingly, the carbonaceous material (raw sludge) is directed first to a heat recovery vessel wherein said sludge is in contact with the recovered steam produced in the vacuum cooling step. Then the sludge enters a reactor of a first unit where it is submitted to a thermophilic biological treatment or to a mechanical treatment.

In a particular embodiment, the process according to the invention comprises the steps of

(1 ) Performing a first thermal treatment of wastewater sludge or organic matter or any carbonaceous material, at a temperature T 1 between 50 and QOΌ, preferably between 50Ό and 75Ό, thereby producing thermally treated carbonaceous material,

(2) Cooling said resulting thermally treated carbonaceous material downstream of step (1 ) to a temperature T2 (T2 lower than T1 ) of between 34-750 in a cooling unit (flash cooler) operating under vacuum (operating typically from 0.055 to 0.170 bar (absolute pressure, i.e. 5 500 to 17 000 Pa), thereby producing cooled carbonaceous material,

(3) Performing a post-treatment of said cooled carbonaceous material.

The thermal treatment of step (1 ) may be a pasteurization, a thermophilic biological treatment and/or a low temperature thermal hydrolysis.

The post-treatment of step (3) may be any suitable treatment. This post-treatment may be a mechanical treatment like a dewatering process, temporary storage. It may also be an anaerobic treatment, such as a fermentation or an anaerobic digestion. Said anaerobic digestion may be a two-stage digestion or a mesophilic digestion

In a particular embodiment, the thermal treatment of step (1 ) is a pasteurization or a low temperature thermal hydrolysis, preferably a pasteurization. In such case, the post treatment advantageously comprises or is an anaerobic treatment, such as a fermentation or an anaerobic digestion. Said anaerobic digestion may be a two-stage digestion or a mesophilic digestion.

Step (2) may be performed in one step or in several sub-steps. In other words, the vacuum cooling step may comprise several cooling stages. In this variant, the process of the invention typically comprises:

(1 ) Performing a first thermal treatment of carbonaceous material, at a temperature Ti of between 50 and 90 O, preferably between 50Ό and 7 5Ό, thereby providing thermally treated carbonaceous material,

(2) a) Cooling said resulting thermally treated carbonaceous material downstream of step (1 ) to an intermediate temperature T_2a (T_2a lower thanTi) of between 500 and 800 in a cooling unit (flash cooler) operating under vacuum (operating typically from 0.15 to 0.48 bar (absolute pressure, i.e. 15 000 to 48 000 Pa), thereby producing intermediate cooled carbonaceous material,

(2) b) Cooling said intermediate cooled carbonaceous material downstream step 2a) to a final temperature T_2b (T_2blower than T_2a) of between 340 and 47 O in a cooling unit (flash cooler) operating under vacuum (operating typically from 0.05 to 0.1 bar, (absolute pressure), i.e. 5 000 to 10 000 Pa), and preferably 0,055 to 0,17 bar (absolute pressure), i.e. 5 500 to 17 000 Pa), thereby producing cooled carbonaceous material,

(3) Performing a post-treatment of the cooled carbonaceous material.

The cooling temperature T_2a of step (2a) is preferably around 500 when step ( 1 ) is performed at 55-600, around 650 when step (1 ) is performed at 70 - 750 and around 800 when step (1 ) is performed at 850 - 900.

In this variant, step (1 ) is preferably a pasteurization step. In such case, the post treatment is preferably an anaerobic treatment, such as a fermentation or an anaerobic digestion. Said anaerobic digestion may be a two-stage digestion or a mesophilic digestion.

(1 ) Performing a first thermophilic biological treatment of wastewater sludge or organic matter or any carbonaceous material, at a temperature between 50 and 75 O,

(2) Cooling the resulting hydrolysed sludge or organic matter or carbonaceous material downstream of the first stage from 50-75 Ό to 35-4 2€ in a flash cooler operating under vacuum (operating typically from 0.05 to 0.1 bar (absolute pressure, i.e. 5.000 to 10.000 Pa), preferably 0,055 to 0,17 bar, thereby producing cooled sludge,

(3) Performing a post-treatment of the cooled sludge.

When the post-treatment of the resulting hydrolysed cooled carbonaceous material such as sludge (i.e. carbonaceous material produced in step (2) is a mesophilic digestion step, it is advantageously performed in a second reactor at 35-42^tC.

Recovered steam produced in step (2), at low pressure and low temperature, may be condensed in other unit operations.

In a second aspect, the present invention is directed towards an installation to implement said process and a flash cooling unit designed for implementation of said process.

The new installation is designed so that:

• The viscosity of the carbonaceous material (notably sludge) has no influence on the design of the system,

• Grit settling has no influence on the design of the system,

• The temperature of the raw carbonaceous material (notably sludge) has no influence on the design of the system,

• the head loss generated is insignificant compared to a heat exchanger,

• Maintenance is significantly reduced. The invention thus relates to an installation comprising:

o at least a thermal treatment unit (1 ) for thermally treating carbonaceous material at a temperature of between dO-QOΌ, preferably 50-75° C,

o at least a vacuum cooling unit (2) downstream the thermal treatment unit for cooling the carbonaceous material, and

o at least a post-treatment unit (3), downstream the vacuum cooling unit (2), for post-treating the cooled carbonaceous material.

In one embodiment, an installation according to the invention comprises:

• A first unit with one or multiple reactor(s) wherein a thermal treatment like thermophilic biological treatment of sludge/organic matter is performed between 50-90° C, preferably This unit can be segmented in m ultiple subunits in series or operated as one tank.

• A vacuum cooling unit downstream the first thermal treatment unit(s),

• A second or multiple unit(s) for post-treatment downstream the vacuum cooling unit.

The installation may thus comprise:

• a thermal treatment unit for thermally treating carbonaceous material at a temperature of between dO-QOΌ, preferably dO^d , having a fi rst inlet \^ and a first outlet O1, said thermal treatment unit being configured to be fed at the first inlet \^ with carbonaceous material, and to produce a thermally treated carbonaceous material , recovered at the first outlet O1,

• a vacuum cooling unit having a first inlet I2 and a first outlet O2, and optionally a second outlet O2’, said first inlet I2 being in fluid connection with the first outlet O1 of the thermal treatment unit, said vacuum cooling unit being configured to be fed at the first inlet I2 with said thermally treated carbonaceous material , and to produce cooled carbonaceous material recovered at the first outlet O2, and optionally recovered steam at the second outlet O2’,

• a post-treatment unit , having a first inlet I3 and a first outlet O3, said first inlet I3 being in fluid connection with the first outlet O2 of the vacuum cooling unit , said post-treatment unit being configured to be fed at the first inlet I3 with said cooled carbonaceous material , and to produce post-treated carbonaceous material recovered at the first outlet O3.

As used herein, a“thermal treatment unit” is understood as a unit suitable for performing a thermal treatment of carbonaceous material at a temperature of between dO-QOΌ, preferably 50-75TT The thermal treatment unit may be a TH uni t, a BH unit, a thermophilic anaerobic digester, or a pasteurization unit.

As used herein, a“vacuum cooling unit” is understood as a unit suitable for performing evaporation under vacuum, that is to say at a pressure below atmospheric pressure, typically operating at an absolute pressure ranging from 0.055 to 0.480 bar (i.e. 5 500 to 48 000 Pa), such as from 0.055 to 0.170 bar (i.e. 5 500 to 17 000 Pa), or from 0.15 to 0.48 bar (i.e. 15 000 to 48 000 Pa).

As used herein, a“unit for post-treatment” or a“post-treatment unit” is understood as a unit suitable for performing a further biological treatment or a mechanical treatment. In a preferred embodiment, the post-treatment step comprises a further biological treatment, in particular a mesophilic digestion step.

Advantageously, the installation further includes a heat recovery vessel downstream the vacuum cooling unit. In other words, the installation advantageously further comprises:

• a heat recovery vessel, having a first inlet U, a first outlet O₄, and a second outlet C ·, said first inlet U being in fluid connection with the second outlet O_2' of the vacuum cooling unit, said heat recovery vessel being configured to be fed at the first inlet U with said recovered steam , said recovered steam being then contacted with carbonaceous material (or fresh organic matter) into vessel , to produce pre-heated carbonaceous material recovered at the first outlet C , and further recovered steam recovered at the second outlet . Advantageously, said outlet O₄ of said heat recovery vessel is in fluid connection with inlet . Alternatively, said thermal treatment unit comprises a second inlet lr, and said outlet O₄ of said heat recovery vessel is in fluid connection with inlet lr, said thermal treatment unit being configured to be fed with said pre-heated carbonaceous material .

Advantageously, the installation further comprises a heat recovery vessel upstream of the thermal treatment unit for contacting the carbonaceous material with the recovered steam.

Advantageously, the installation further comprises a condenser having a first inlet I _Hex, a first outlet OH_QC, and a second outlet C (not shown), said first inlet IH_QC being in fluid connection with the second outlet O_4’ of the heat recovery vessel, said condenser being configured to be fed at the first inlet IH_QC with said recovered steam, and to produce off-gas recovered at the first outlet OH_QC, and condensed liquid (generally process water) recovered at the second outlet OH_QC'. Said condenser is preferably a direct or indirect heat exchanger.

In a variant, the installation further comprises a second heat recovery vessel downstream the first heat recovery vessel and upstream the thermal treatment unit.

In a particular embodiment, the first unit or thermal treatment unit is a reactor suitable for thermophilic digestion. In this variant, the second unit or post-treatment unit is preferably a reactor suitable for mesophilic digestion.

In a particular embodiment, the vacuum cooling unit comprises two vacuum cooling units installed in series. In other words, the vacuum cooling unit is a two-stage vacuum cooling unit. In this embodiment, the treated carbonaceous material is directed from the thermal treatment unit to a first cooling unit , then the cooled carbonaceous material from the first cooling unit is further cooled in a second cooling unit and directed towards the post-treatment unit .

In this embodiment, the installation thus comprises:

• a thermal treatment unit for thermally treating carbonaceous material at a temperature of between 50-900, preferably 50-750, having a fi rst inlet \^ and a first outlet O₁ , said thermal treatment unit being configured to be fed at the first inlet \^ with carbonaceous material, and to produce a thermally treated carbonaceous material , recovered at the first outlet O₁ ,

• a first vacuum cooling unit having a first inlet h_a and a first outlet 0_2a, and optionally a second outlet 0_2a·, said first inlet h_a being in fluid connection with the first outlet O₁ of the thermal treatment unit, said first vacuum cooling unit being configured to be fed at the first inlet h_a with said thermally treated carbonaceous material, and to produce intermediate cooled carbonaceous material recovered at the first outlet 0_2a, and optionally recovered steam at the second outlet 02_a·,

• a second vacuum cooling unit having a first inlet h_b and a first outlet 0_2b, and optionally a second outlet 0_2b’, said first inlet h_b being in fluid connection with the first outlet 0_2a of the first vacuum cooling unit, said second vacuum cooling unit being configured to be fed at the first inlet h_b with said intermediate cooled carbonaceous material and to produce cooled carbonaceous material recovered at the first outlet 0_2b, and optionally recovered steam at the second outlet 02_b’, and

• a post-treatment unit, having a first inlet h and a first outlet O₃, said first inlet l_pt being in fluid connection with the first outlet 0_2b of the second vacuum cooling unit, said post treatment unit being configured to be fed at the first inlet h with said cooled carbonaceous material, and to produce post-treated carbonaceous material recovered at the first outlet O₃. In this embodiment, the recovered steam produced in the first and second cooling units is condensed in one or several condensers. Said condenser may be a direct or indirect heat exchanger Hex (more specifically a water/steam heat exchanger Hex).

A“heat exchanger” is known in the art: it is a system used to transfer heat between two or more fluids (at least one cool fluid and one hot fluid), which may be used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing (indirect heat exchanger) or they may be in direct contact (direct heat exchanger). In the invention, heat exchangers are used to cool down the recovered steam. The cool fluid is typicaly process water, while the hot fluid is steam.

The steam produced in the second cooling unit may be recovered and directed to a second heat recovery vessel wherein the steam is contacted with carbonaceous material (preferably fresh organic matter), thereby producing a first pre-heated carbonaceous material, said first pre-heated carbonaceous material being then sent to the second heat recovery unit , wherein the first pre-heated carbonaceous material is contacted with the recovered steam from the first cooling unit.

Advantageously, the steam produced in the first cooling unit is recovered and sent to a heat recovery vessel wherein the steam is contacted with carbonaceous material (preferably fresh organic matter).

In a particular embodiment, an installation according to the invention comprises:

• A first unit with one or multiple sub-units (reactor) wherein a thermal treatment like thermophilic biological treatment of sludge/organic matter is performed between 50- 90Ό, preferably 50-75TT This unit can be segmente d in multiple subunits (reactors) in series or operated as one tank (reactor).

• A vacuum cooling unit downstream the first thermal unit,

• A second unit in one or multiple sub-units for post-treatment downstream the vacuum cooling unit.

An installation according to the invention may also include:

• A holding tank containing (fresh primary, mixed or biological sludge /organic matter or any carbonaceous matter to be treated) ahead of the first reactor. As an alternative, the readily degradable organic matter (carbonaceous material) can be sent directly to the post-treatment unit while the sludge is first processed in the first reactor.

• A heat exchanger on both the first unit and the second unit (or solely on the first thermal treatment unit) to maintain the carbonaceous material temperature at the required set point.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated more in details in the following figures wherein:

Figure 1 illustrates an hydrolysis pre-treatment implemented upstream a mesophilic digester of wastewater sludge of the state of the art;

Figure 2 illustrates state of the art temperature phased anaerobic digestion (TPAD) process of waste water sludge;

Figure 3 illustrates an installation to perform a temperature phased anaerobic digestion (TPAD) process of waste water sludge according to the invention; Figure 4 represents an installation for implementing a process of waste water sludge treatment according to the invention;

Figure 5 represents an installation for implementing a process of waste water sludge treatment according an embodiment of the invention;

Figure 6 represents an installation for implementing a process of waste water sludge treatment according an embodiment of the invention;

Figure 7 represents an installation for implementing a process of waste water sludge treatment according an embodiment of the invention.

DETAILED DESCRIPTION

According to the invention, the process is performed in an installation that comprises a first reactor 1 wherein a thermal treatment takes place to treat municipal sludge, organic matter or any carbonaceous material from a holding tank 1 a. The hot organic matter is then cooled in a vacuum cooling unit 2 downstream the thermal reactor 1 before entering into a second reactor 3 wherein the cooled organic matter can be treated by a post-treatment like anaerobic digestion.

In other words, the installation of figure 3 comprising:

More specifically, the installation of figure 3 comprises:

• a thermal treatment unit (1 ) for thermally treating carbonaceous material at a temperature of between dO-QOΌ, preferably 50-75^tC, having a first inlet \^ and a first outlet Oi , said thermal treatment unit being configured to be fed at the first inlet \^ with carbonaceous material, and to produce a thermally treated carbonaceous material (TL2), recovered at the first outlet Oi ,

• a vacuum cooling unit (2) having a first inlet h and a first outlet O2, and optionally a second outlet O2', said first inlet I2 being in fluid connection with the first outlet Oi of the thermal treatment unit, said vacuum cooling unit (2) being configured to be fed at the first inlet I2 with said thermally treated carbonaceous material (TL2), and to produce cooled carbonaceous material (TL3) recovered at the first outlet O2, and optionally recovered steam (SL1 ) at the second outlet O2',

• a post-treatment unit (3), having a first inlet I₃ and a first outlet O₃, said first inlet I₃ being in fluid connection with the first outlet O₂ of the vacuum cooling unit (2), said post-treatment unit (3) being configured to be fed at the first inlet I₃ with said cooled carbonaceous material (TL3), and to produce post-treated carbonaceous material recovered at the first outlet O₃.

In a preferred variant of the first embodiment, the produced steam in the cooling step is preferably put in contact by a stream line SL1 with the raw sludge in a reactor 4 placed upstream the reactor 1 of the first unit (see figure 3). The contact can be done using a counter current direct contact tower or any other means in a way to have a limited headloss of the steam while maintaining a maximum contact between the raw sludge and the steam to maximise condensation. As an alternative, the produced steam of vacuum cooling step (2) produced in the unit 2 may be sent back (SL2) to the reactor 1 to pre-heat the latter (direct injection in the reactor 1 ).

The biogas produced in reactors 1 , 3 of the first and second units may be sent respectively by a biogas line BL3, BL4 to a biogas recovery unit 5, typically a CHP (Combined Heat and Power) unit. Heat may be recovered from the biogas recovery unit 5 and sent to reactor 1 by a heat line HL.

As another alternative, a heat exchanger HEx can be used on the produced steam to heat up the raw (primary, mixed or biological) sludge (less preferred).

If it is uneconomical to inject the entire flowrate of steam into the reactor 1 of the first unit, or the holding tank 1 a, or the heat exchanger, part of the steam can be condensed in a condenser while the rest is sent to pre-heat the incoming sludge (see figure 4).

The installation of figure 4 further comprises:

• a heat recovery vessel (4), having a first inlet U, a first outlet O₄, and a second outlet O_4’, said first inlet U being in fluid connection with the second outlet O_2' of the vacuum cooling unit (2), said heat recovery vessel (4) being configured to be fed at the first inlet U with said recovered steam (SL1 ), said recovered steam (SL1 ) being then contacted with carbonaceous material (or fresh organic matter) (TL1 ) into vessel (4), to produce pre-heated carbonaceous material (TL1’) recovered at the first outlet O4, and further recovered steam SL3 recovered at the second outlet O_4'. Advantageously, said outlet O₄ of said heat recovery vessel (4) is in fluid connection with inlet \ ^ . Alternatively, said thermal treatment unit (1 ) comprises a second inlet lr, and said outlet O4 of said heat recovery vessel (4) is in fluid connection with inlet lr, said thermal treatment unit (1 ) being configured to be fed with said pre-heated carbonaceous material (TLT).

Advantageously, the installation of figure 4 further comprises a condenser Hex having a first inlet I H_ex, a first outlet CW, and a second outlet C (not shown), said first inlet W being in fluid connection with the second outlet O_4' of the heat recovery vessel (4), said condenser Hex being configured to be fed at the first inlet W with said recovered steam SL3, and to produce off-gas recovered at the first outlet CW, and condensed liquid (generally process water) recovered at the second outlet CW. Said condenser is preferably a direct or indirect heat exchanger.

This reinjection of low pressure and temperature recovered steam enables the carbonaceous material (depending on step 1 , it may be primary, mixed or biological sludge), to be heated up from a temperature of 10 to 20Ό to a temperature 0 f 20 to 45 Ό (typically between 30Ό and 40Ό, for instance between 30Ό and 35Ό), thus lea ding to global energy saving in the process.

The recovered steam from step (2) is at a pressure between 0.055 to 0.480 bar (absolute pressure), i.e. 5 500 to 48 000 Pa). Thanks to a vacuum pump (i.e. a diaphragm vacuum pump) placed downstream the cooling unit (flash cooler), said recovered flash steam may be put in contact with the carbonaceous material (which may be primary/mixed/biological sludge). Due to the low solid content of said sludge (between 3 and 25% mass%, in particular between 3 and 8 mass %), such contact is possible without any further intermediate device.

As illustrated in figure 4, in a variant of the embodiment, the process comprises the steps of : (1 a) contacting the wastewater sludge or organic matter or any carbonaceous material issued from the storage tank 1 a in a heat recovery vessel 4 with the recovered steam SL1 from a cooling step (2) downstream the reactor 1 of the first unit wherein is performed a thermal treatment either a thermophilic biological treatment of sludge/organic matter/ carbonaceous material is performed between 50 and 90 Ό, prefera bly between 50Ό and 75Ό or a mechanical treatment of sludge/organic matter (carbonaceous material);

(1 ) submitting the sludge/organic matter/carbonaceous material pre-heated in the previous step to a thermal treatment, e.g. a thermophilic biological treatment between 50-90Ό in the reactor 1 of the first unit;

(2) cooling the resulting sludge or organic matter or carbonaceous material downstream the first unit from 50-90 Ό to 35-42 Ό in a vacuum co oling unit 2 operating under vacuum (vacuum pump VP) (operating typically from 0.05 to 0.1 bar (absolute pressure), i.e. 5000 to 10 000 Pa), preferably 0,055 to 0,17 bar thereby producing cooled carbonaceous material;

(3) performing a post-treatment of the cooled sludge/organic matter/carbonaceous material in a reactor 3 of the second unit.

The recovered steam (SL1 ) from cooling unit 2 is sent to a heat recovery vessel 4. As a result, recovered steam (SL1 ) is at least partly condensed in vessel 4. Off-gas (SL2) is sent to a vacuum pump to be evacuated from the installation or further treated.

Said embodiment is illustrated in Figures 4 and 5.

If in heat recovery vessel 4, the contact between the carbonaceous material (more specifically fresh sludge) and the recovered steam is a direct one, then a foam abatement step, as for example a mechanical foam abatement like recirculation of the sludge in the vessel or a chemical foam abatement like anti-foaming product injection may be performed.

In another embodiment (not shown), a first part of the recovered steam SL1 is advantageously sent back to reactor 1 of the thermal treatment unit to pre-heat the latter (direct injection in reactor 1 ) and a second part (SL2) is sent to the heat recovery vessel 4 to heat the fresh sludge (see figure 4).

In figure 5, is illustrated a step of vacuum cooling with a minimum number of subunits/elements. This kind of installation can then present a minimal size with a minimal pre-heating step. The cooling step is made in one step so the steam temperature will be equal to the sludge temperature in the vacuum cooler, which is the lowest achievable temperature with this system.

The carbonaceous material is directed in the treatment line TL1 into the recovery heat vessel 4 to be contacted with steam. The pre-heated carbonaceous material may be optionally then hold in a holding tank 1 b. The carbonaceous material is directed towards the reactor 1 of the first unit through the treatment line TL1” to be thermally or mechanically treated. The non condensable products may be processed in a further treatment step(s) such as an odour treatment unit.

The hot carbonaceous material is directed from the reactor 1 of the first unit through TL2 to the vacuum cooling unit 2. Then the cooled matter is sent by TL3 into the reactor 3 of the second unit that may be an anaerobic digestion tank 3. The steam recovered from the cooling unit 2 is directed by SL1 into the heat recovery unit vessel 4 to perform the pre-heating of the raw carbonaceous material.

The steam in excess in the vessel 4 may be directed by SL3 to a condenser HEx and the non condensable products may be sent by a non condensable line to a biogas recovery or to a further treatment step. The condenser Hex can be a direct one or indirect condenser like a contactor. In figure 6, the illustrated installation proposes a two-substeps vacuum cooling units with a maximal heat recovery in the case of limited availability of cooling fluid (such as process water). Consequently, the installation provides two cooling units 2a, 2b installed in series.

The organic matter treated in the first reactor 1 is cooled in a first cooling unit 2a and then in a second cooling unit 2b. The steam produced in the second cooling unit 2b is recovered and sent (SL1 b) to a heat recovery vessel 4b wherein the steam is contacted with fresh organic matter. The pre-heated organic matter is then sent by TLT to a second heat recovery unit 4a wherein the first pre-heated organic matter is contacted with the recovered steam (SL1 a) from the first cooling unit 2a. Consequently, the organic matter is first pre-heated by the coldest recovered steam and then by the hottest recovered steam. These two steps of pre-heating the organic matter allows the improvement of the rheological properties of the organic matter as the organic matter is diluted and pre-heated before the second step of pre-heating.

That avoids the need of cooling water to condensate the produced steam in the second sub step of vacuum cooling, the steam being condensed in the organic matter. The condenser may be optional as the steam can all be consumed in the preheating step, avoiding the need for water and also for a condenser.

In that example of installation, the recovered steam (SL3a, SL3b) produced by the two vacuum cooling units may be condensed in condenser HEx. In the figures two condensers HEx are represented but the condensation may be performed in one condenser. The condenser Hex may be a direct or indirect condenser, such as a contactor. All the non-condensable matters may then be treated in subsequent treatment steps such odour treatment line, or evacuated in the atmosphere or joined to the produced biogas in the post-treatment.

The installation of figure 6 comprises:

• a thermal treatment unit for thermally treating carbonaceous material at a temperature of between dO-QOΌ, preferably dO-TdΌ, having a fi rst inlet \^ and a first outlet Oi , said thermal treatment unit being configured to be fed at the first inlet \^ with carbonaceous material, and to produce a thermally treated carbonaceous material (TL2), recovered at the first outlet Oi ,

• a first vacuum cooling unit (2a) having a first inlet 1_2a and a first outlet 0_2a, and optionally a second outlet 02_a·, said first inlet 1_2a being in fluid connection with the first outlet O_tt of the thermal treatment unit, said first vacuum cooling unit (2a) being configured to be fed at the first inlet 1_2a with said thermally treated carbonaceous material (TL2), and to produce intermediate cooled carbonaceous material (TL2’) recovered at the first outlet 0_2a, and optionally recovered steam (SL1 a) at the second outlet 0_2a·,

• a second vacuum cooling unit (2b) having a first inlet 1_2b and a first outlet 0_2b, and optionally a second outlet 02_b·, said first inlet h_b being in fluid connection with the first outlet 0_2a of the first vacuum cooling unit, said second vacuum cooling unit (2b) being configured to be fed at the first inlet h_b with said intermediate cooled carbonaceous material (TL2’), and to produce cooled carbonaceous material (TL3) recovered at the first outlet 0_2b, and optionally recovered steam (SL3b) at the second outlet 0_2b’, and

• a post-treatment unit (3), having a first inlet I3 and a first outlet O3, said first inlet I3 being in fluid connection with the first outlet 0_2b of the second vacuum cooling unit, said post treatment unit (3) being configured to be fed at the first inlet I3 with said cooled carbonaceous material, and to produce post-treated carbonaceous material recovered at the first outlet O3.

In this embodiment, the recovered steam produced in the first and second cooling units (SL3a and SL3b) is condensed in one or several condensers. Said condenser may be a direct or indirect heat exchanger Hex (more specifically a water/steam heat exchanger Hex).

The steam produced in the second cooling unit (2b) may be recovered and directed to the heat recovery vessel (4b) wherein the steam is contacted with carbonaceous material (preferably fresh organic matter), thereby producing a first pre-heated carbonaceous material, said first pre-heated carbonaceous material being then sent to the second heat recovery unit (4a), wherein the first pre-heated carbonaceous material is contacted with the recovered steam (SL1 a) from the first cooling unit (2a).

Advantageously, the steam produced in the first cooling unit (2a) is recovered and sent to a heat recovery vessel (4) wherein the steam is contacted with carbonaceous material (preferably fresh organic matter).

The process allows a maximal energy recovery with very viscous flow of dry organic matter, highly concentrated in dry matter and/or an installation wherein cooling water availability is limited.

In figure 7, the illustrated installation provides an equilibrium between optimising heat recovery and minimising the number of subunits/elements of said installation. This installation can be considered when the cooling water is available, and the organic matter flow is easy to pre heat.

Consequently, the installation provides also two vacuum cooling units 2a, 2b installed in series but the treated organic matter issued from the reactor 1 is cooled by the first cooling unit 2a and the recovered steam (SL1 a) from this unit 2a is used in a heat recovery vessel 4 to pre heat the organic matter before the reactor 1 .

The cooled organic matter is then cooled a second time in the cooling unit 2b before to be directed TL3 towards the second reactor 3 for post-treatment.

The excess steam produced in the first and second cooling units is condensed SL3a and SL3b in a condenser like a water/steam heat exchanger HEx, that can be direct or indirect heat exchanger. The condenser Hex may be a direct or indirect condenser, such as a contactor. This installation provides an optimal balance cost/ energy gain even if cooling water is needed.

ADVANTAGES OF THE INVENTION

Cooling using a vacuum cooling system compared to a conventional heat exchanger operating with process water has many beneficial effects:

• Cooling efficiency doesn’t depend on the process water temperature but on the vacuum pressure, which is much easier to control. Cooling is nearly instantaneous and doesn’t require a significant HRT (Hydraulic Retention Time) as in a HEx.

• Viscosity variation of the sludge is no longer an issue.

• In the absence of a HEx, there is no need for maintenance on sludge transfer pumps & pipes.

TPAD/2PAD processes according to the invention can be implemented on large industrial installations, where the HEx size issues would become unmanageable with state-of-the-art processes.

When operated in batch configuration (2PAD), the flash cooling allows for a significant reduction in the sludge withdrawal sequence from the thermophilic reactor. Consequently, this increases the batch time, which reduces the size of the thermophilic reactor heating HEx and/or the feeding pumps (feeding can occur for a longer period).

Raw sludge pre-heating based on direct steam injection of the off-gas produce by the flash under vacuum pressure, into the raw sludge has the following advantages over conventional heat-exchanger based pre-treatment in know TPAD/2PAD.

Heat recovery is independent from the sludge viscosity. The off-gas generated by a vacuum cooling system can be problematic (odour issue due to the presence of H₂S and NH₃). Usually this issue is dealt with by injecting this gas into the digester once it is condensed. Reusing this off gas without condensing it to preheat the raw sludge, allows to treat it according to standard processes, via the plant odour treatment unit 6. In the present application, the low temperature steam reduces the quantity of odours and contaminants (organic material vaporised) compared to conventional flash systems.

In the absence of a HEx, there is no need for maintenance on sludge transfer pumps and pipes.

In the field of anaerobic digestion of sludge, vacuum cooling is usually linked with cooling high temperature sludge (around 165Ό) that is treated w ith a THP or with other thermal treatments, down to a temperature of around l OOTT The pressure following such a flash (in the field of sludge treatment) remains higher than atmospheric pressure (delta P of the flash > atmospheric pressure).

Secondary vacuum cooling may be used to cool sludge from a temperature between 100 to 1 10 Ό down to a temperature around 60TT Said seco ndary flash might operate under vacuum.

The last cooling step, in order to cool the sludge down to a temperature of 37-38 Ό appropriate for Anaerobic digestion (mesophilic step), is performed by diluting the sludge with process water or mixing it with raw primary sludge produced by primary settling tanks

Using vacuum cooling to reduce the temperature of the sludge under the 60Ό threshold was never described in the field of municipal sludge treatment.

In state-of-the-art processes, the non-condensable gas generated during the flash cooling step(s) following steam cooling and condensation to S O-SeO, is fed into the mesophilic reactor.

Claims

1 . Process of treatment of carbonaceous material, such as wastewater sludge or organic waste, comprising the steps of :

(2) cooling of said thermally treated carbonaceous material, thereby providing cooled carbonaceous material, and

(3) performing a post-treatment of said cooled carbonaceous material,

wherein the cooling of step 2) is performed using a vacuum cooling step.

2. The process of claim 1 , comprising a step of pre-heating of the carbonaceous material 1 a, 4 before directing to the thermal treatment step 1 ) (TL1 ), performed by direct steam contact of the recovered steam (SL1 ) produced by the vacuum cooling step 2) with the carbonaceous material.

3. The process of claim 2, wherein the carbonaceous material is directed first to a heat recovery vessel wherein said carbonaceous material is in contact with the recovered steam produced by the vacuum cooling step.

4. The process of any one of claims 1 to 3, comprising the steps of

(1 ) Performing a first thermal treatment of carbonaceous material, at a temperature T1 between 50 and QOΌ, preferably between 50Ό and 75 O, thereby producing thermally treated carbonaceous material,

(2) Cooling said resulting thermally treated carbonaceous material downstream of step (1 ) to a temperature T2 lower than T1 of between 34-75°C, in a cooling unit operating under vacuum, thereby producing cooled carbonaceous material,

(3) Performing a post-treatment of said cooled carbonaceous material

5. The process of any one of claims 1 to 4, wherein step 2) is performed in one or several sub steps.

6. The process of the claim 5, wherein step 2 comprises:

(2a) cooling the resulting thermally treated carbonaceous material downstream of step (1 ) to an intermediate temperature T2a lower than T1 , of between 50Ό and 80Ό in a cooling unit operating under vacuum (typically from 0.15 to 0.48 bar (absolute pressure, i.e. 15.000 to 48.000 Pa), thereby producing intermediate cooled carbonaceous material,

(2b) cooling said intermediate cooled carbonaceous material downstream step 2a) to a final temperature T2b lower than T2a of between 34Ό and 47 Ό in a cooling unit operating under vacuum (typically from 0.05 to 0.1 bar, (absolute pressure, i.e. 5.000 to 10.000 Pa), preferably 0,055 to 0,17 bar (absolute pressure, i.e. 5.500 to 17.000 Pa)), thereby producing cooled carbonaceous material.

7. The process of any one of claims 1 to 6, wherein the thermal treatment of step 1 ) is a pasteurization, a thermophilic biological treatment or a low temperature thermal hydrolysis.

8. The process of anyone of claims 1 to 7, wherein the post-treatment step 3) is:

- a mechanical treatment step such as a dewatering step, or a temporary storage step, or

- an anaerobic treatment step, such as a fermentation, an anaerobic digestion like a two-stage digestion, a mesophilic digestion step.

9. The process of anyone of claims 1 to 8, wherein the carbonaceous material is organic waste and/or sludge, in particular municipal sludge.

10. Installation for implementing the process of claims 1 to 9, comprising:

at least a thermal treatment unit (1 ) for thermally treating carbonaceous material at a temperature of between dO-QOΌ, preferably

at least a vacuum cooling unit (2) downstream the thermal treatment unit for cooling the carbonaceous material, and

at least a post-treatment unit, downstream the vacuum cooling unit (2), for post-treating the cooled carbonaceous material.

1 1 . The installation of claim 10, comprising a heat recovery vessel (4,4b) upstream of the thermal treatment unit for contacting the carbonaceous material with the recovered steam.

12. The installation of claim 1 1 , comprising a second heat recovery vessel (4a) downstream the first heat recovery vessel (4b) and upstream the thermal treatment unit.

13. The installation of claim 1 1 or 12, wherein the vacuum cooling unit comprises two vacuum cooling units (2a, 2b) installed in series, the treated carbonaceous material from the first unit (1 ) by a treatment line (TL1 ) being cooled in a first cooling unit (2a), then the cooled carbonaceous material being cooled in a second time in the cooling unit (2b) and directed through a treatment line (TL3) towards the second unit (3) for post-treatment.

14. The installation of claim 12, wherein the recovered steam produced in the first and second cooling units (SL3a and SL3b) is condensed in one or several condensers.

15. The installation of claim 1 1 , wherein the recovered steam produced in the second cooling unit (2b) may be recovered and directed to the heat recovery vessel (4b) wherein the steam is contacted with carbonaceous material (preferably fresh organic matter), thereby producing a first pre-heated carbonaceous material, said first pre-heated carbonaceous material being then sent to the second heat recovery unit (4a), wherein the first pre-heated carbonaceous material is contacted with the recovered steam (SL1 a) from the first cooling unit (2a).

16. The installation of claim 12, wherein the recovered steam produced in the first cooling unit (2a) is recovered and sent to a heat recovery vessel (4) wherein the steam is contacted with carbonaceous material.