Process for treating sludge from wastewater treatment plants

Abstract

The invention relates to a process for treating sewage sludge, comprising at least the following steps: - solar drying (10) in a solar drying unit for sewage sludge, the solar drying unit comprising air regulated to a predetermined sludge drying temperature; - thermal treatment for decomposition of organic matter (20) of said dried sewage sludge in a thermal treatment unit for decomposition of organic matter, - extraction (70) of air from the solar drying unit following drying.The invention consists in the fact that the extracted air is divided (80) into two fractions, one fraction of this air being conveyed and introduced into the thermal treatment unit for the decomposition of organic matter while the other fraction of this extracted air is subjected to a step of reducing the moisture content (30) of said fraction in a range of 10 to 50%, preferably to a rate less than 30%, said fraction of air then being subjected to a heating step (80) to a predetermined temperature for the drying of the sludge and reintroduced into the solar drying unit as a drying medium, i.e. vapor carrier. Figure 1.

Description

Title of the invention: Process for treating sludge from wastewater treatment plants

[0001] The present invention relates to the field of sludge treatment from wastewater treatment plants. More particularly, the invention relates to a process for treating sludge from wastewater treatment plants, known as urban sludge, and in particular a thermal treatment process for the decomposition of the organic matter in said sludge.

[0002] A thermal treatment for the decomposition of organic matter, such as the incineration of sludge from the urban and / or industrial wastewater treatment process, significantly reduces the volume of this type of waste. Indeed, incineration, by converting the organic matter into a gaseous phase, reduces the volume of sludge by an average factor of 10. The residue obtained after incineration consists exclusively of mineral matter.

[0003] During sludge treatment by incineration, the sludge is generally dewatered before entering the incineration unit. This sludge can be of various types: primary, secondary, tertiary, mixed, digested or undigested, etc. Its dryness content at the outlet of the mechanical dewatering unit will depend on the nature of the sludge treated and the dewatering technology used (centrifuge, belt filters, filter presses, etc.). The dryness content of dewatered sludge typically varies between 18 and 30%.

[0004] As a reminder, the dryness of sludge is determined by an index used in the field of wastewater treatment. Sludge consists of water and dry matter. Dryness is the mass percentage of dry matter. Thus, sludge with a dryness of 10% has a moisture content of 90%. Dryness is assessed by the amount of solids remaining after heating at 110°C for two hours. It is generally expressed as a weight percentage. Conversely, we speak of moisture content. Dryness is essential information for all types of sludge handling during the wastewater treatment process, as the consistency of the sludge is a physical state dependent on its dryness. The dryness must therefore be sufficiently high for the dried sludge to be stabilized and potentially sanitized.However, this dryness must not be too high in order to avoid the creation of dust (with a significant risk of flammability) and excessive energy consumption.

[0005] Sludge sent to an incineration unit preferably has a dry solids content of less than 40% and greater than 20%. Indeed, between 40% and 60%, the sludge acquires a particular "plastic" rheology. It then becomes difficult to pump or convey this type of sludge. Moreover, above 40%, the fraction of the material The organic matter content becomes too high. The incineration process therefore becomes highly exothermic (more material to burn and less water to evaporate). This leads to exceeding the usual incineration temperature thresholds (850-900 °C), beyond which ash melting and the formation of thermal NOx are favored. These temperature ranges require more expensive furnace and heat exchanger materials and more advanced flue gas treatment.

[0006] Pyrolysis / incineration processes are also known, which can handle higher sludge dryness levels (greater than 60% – beyond the plastic range). These processes rely on two separate reactors and are therefore more complex to operate than a so-called "one-pot process" incineration solution, i.e., one with a single furnace or reactor. However, they are not widely used for sludge.

[0007] When the sludge arrives at the inlet of the incineration unit, it may be of a "non-autothermal" nature. Non-autothermal sludge is defined as sludge with a Lower Heating Value (LHV) insufficient to maintain the operating temperature of the incineration furnace. It should be noted that the operating temperature is the temperature at which all the organic matter is converted (into CO2 and H2O), and a moderate production of pollutants (NOx, SOx, dioxins, etc.) is achieved.

[0008] To address the energy deficit of non-autothermal sludge, it is possible to add energy using a fuel. Fuel burners, such as natural gas or biogas, are therefore systematically included in sludge incineration furnaces. Despite the widespread use of burners in incinerators, this solution is considered costly given the price of fuels and, moreover, environmentally unfriendly, as the carbon footprint is significant during fuel combustion.

[0009] Another way to promote the self-heating of sludge is to couple the incineration furnace with a thermal pre-dryer. In this case, a flue gas / oil or flue gas / steam heat exchanger is installed to recycle the heat released by the process to the pre-dryer, thereby improving the plant's energy efficiency. The sludge admitted to the incineration furnace is therefore pre-dried sludge with dry solids contents that can vary between 25 and 38% (depending on the lower heating value of the treated sludge). A plant comprising an incineration unit and a thermal pre-drying unit is advantageous from an energy recovery perspective. Thin-film or disc-type thermal pre-dryers are thus integrated into the incineration unit to ensure the self-heating of the sludge.

[0010] However, the operation of a thermal pre-dryer (although less restrictive) (that a thermal dryer) drastically increases the complexity of the process. Indeed, a thermal dryer is equipment that requires active and vigilant operation, and the problems known to affect it can reduce the availability of the incineration unit.

[0011] Furthermore, the installation of a thermal pre-dryer represents a significant investment that must be taken into account. Finally, this solution requires the recycling within the process of very high-quality thermal energy, which is then no longer available for other uses such as thermal energy export, electricity production, etc.

[0012] Another drying method associated with an incineration furnace is also usable and consists of implementing solar drying of the dewatered sludge as a step prior to incineration, in order to prepare sludge with sufficient dryness to guarantee its self-heating properties. This solar drying allows the elimination of free and bound water that cannot be removed by a mechanical process during mechanical dewatering. Drying therefore occurs by evaporation, which requires the creation of a temperature gradient between the outer surface of the grains or flocs constituting the sludge and their core. During this process, the water vapor formed in the core diffuses towards the surface layer, known as the boundary layer. The energy required is the sum of the latent heat and the binding energy of this water (which is small compared to the former).

[0013] Solar drying, which uses free and renewable solar energy to evaporate the water contained in sludge, is an extensive process. Solar drying can also be implemented without the use of a greenhouse, in the form of open-cell drying. However, solar drying has a major drawback: it generates odors, mainly due to fermentation resulting from the low dryness of the sludge introduced into a greenhouse. Therefore, implementing solar pre-drying is of limited benefit when greenhouses are located near urban areas, as it then becomes necessary to address this odor problem with a deodorization system, which is expensive equipment.

[0014] Document EP 1 378 494 describes a sludge treatment process comprising incineration-type heat treatment preceded by a low-temperature drying step, notably in a solar dryer. This process therefore proposes to carry out the drying in a solar dryer at a low temperature range up to a maximum of 60 to 70°C. It also includes a storage step between the drying and the two-stage heat treatment. Dry, preheated air can be introduced into the solar drying device and comes from energy recovery from the incineration device. The dry, heated air introduced into the Solar dryer exhaust is discharged directly into the environment in the form of a stream of humid air.

[0015] According to the described process, low-temperature or cold drying prevents the release of highly volatile compounds, and only water is removed from the system, thus preventing the formation of odors. However, it turns out that, depending on the type of sludge being treated, such a process cannot guarantee that there will be no release of volatile compounds, and in particular, compounds that generate unpleasant odors. This air is then released into the environment, and it can therefore be considered that, in addition to the potential nuisance related to odors, there is a loss of heat from the humid air exhausted from the greenhouse.

[0016] Furthermore, since this water is extracted from the greenhouse in the form of vapor and the greenhouse air temperature is not very high (especially in winter), the vapor concentration in the greenhouse atmosphere remains limited. Consequently, a large volume of air must be extracted from the greenhouse to remove the evaporated water. This high flow rate results in a significant expenditure of electrical energy for ventilation. In addition, the extracted volumes may still carry odorous compounds, which then necessitates costly deodorization treatment.

[0017] The present invention therefore aims to propose a process for treating sewage sludge in which the energy balance is optimized while controlling the risk of emission into the environment of odorous compounds linked to solar pre-drying.

[0018] In order to achieve this goal, the invention relates to a process for treating sewage sludge, comprising at least the following steps: - solar drying in a solar drying unit for sewage sludge, the solar drying unit comprising air regulated to a predetermined sludge drying temperature; - a thermal treatment for the decomposition of organic matter in said dried sewage sludge in a thermal treatment unit for the decomposition of organic matter, - an extraction of air from the solar drying unit following drying, characterized in that the extracted air is divided into two fractions, one fraction of this air being conducted and introduced into the thermal treatment unit for the decomposition of organic matter while the other fraction of this extracted air is subjected to a step of reducing the moisture content of said fraction in a range of 10 to 50%, preferably to a rate below 30%, said fraction of air then being subjected to a heating step to a predetermined temperature for the drying of the sludge and reintroduced into the solar drying unit.

[0019] Thus, advantageously, the method according to the invention makes it possible to guarantee achieving the self-thermality of the thermal treatment reactor such as an incineration furnace by using renewable energy such as solar energy to pre-dry the sludge while using the air extracted from the solar drying unit as a heat transfer medium to heat said solar drying unit by being reintroduced at least in part and controlling bad odors by sending to the thermal treatment unit a fraction of the air extracted from this solar drying unit laden with bad odors, thus avoiding the accumulation of these odors at the level of the solar drying unit.

[0020] The method according to the invention may further comprise one or more of the following features, taken alone or in combination: - the thermal treatment for the decomposition of organic matter is incineration; - the thermal treatment of decomposition of organic matter is a pyrolysis such as pyrocarbonation, pyrogasification, pyroliquefaction; - the process includes a step of heating the fraction of air extracted from the solar drying unit before it enters the thermal treatment unit for the decomposition of organic matter; - the process includes a step of recovering the heat produced by the thermal treatment of decomposition of organic matter from sewage sludge, by means of recovering heat at the outlet of the thermal treatment unit for decomposition of organic matter; - the process includes an energy generation step such as electricity and / or heat from recovered heat; - the step of reducing the humidity level of the extracted air fraction is a cooling step, followed by a heating step of the cooled air fraction to a predetermined sludge drying temperature, the air thus heated being reintroduced into the solar drying unit as a drying medium, i.e. vapor carrier, -the heating step of the cooled air fraction is implemented by at least part of the heat produced by the thermal treatment of decomposition of the organic matter of the sewage sludge and recovered; - the heating step of the cooled air fraction is implemented by transferring energy from the energy produced by the energy generation step such as electricity and / or heat from the heat recovered at the outlet of the thermal treatment unit for the decomposition of organic matter; - The energy generation stage is implemented using an Organic Rankine Cycle (ORC) machine, comprising a pump, a steam generator, and a... expansion and a condenser; -the heating stage of the cooled air fraction is implemented in the organic Rankine cycle machine, the cold air circulating in the condenser as a refrigerant medium, the air thus heated being reinjected into the solar drying unit as a drying medium, i.e. vapor carrier; - the process includes a step of treating fumes from the thermal treatment unit of organic decomposition.

[0021] The thermal treatment for the decomposition of organic matter can thus be chosen from among so-called dry thermochemical processes such as pyrocarbonation, pyrogasification, and pyroliquefaction. This type of process requires introducing dried sludge with a dryness content greater than 80% into the conversion reactor. A fraction of the products of these processes, such as pyrolysis gases, char, or oil, is burned, which provides heat that can be recovered for use in the drying stage and the thermal treatment (conversion) stage.

[0022] The solar drying unit advantageously consists of a closed greenhouse. The sludge is thus dried in a closed greenhouse, thanks to solar radiation which heats the air inside the greenhouse and, consequently, the surface of the sludge bed. Ventilation of the greenhouse allows the water contained in the sludge to evaporate. The evaporated water is then removed by natural convection via a ventilation system, the drying air extracted from the greenhouse generally having a humidity level above 60%. Evaporation is optimized by air sweeping within the greenhouse and, optionally, by a sludge turning / spreading machine.

[0023] The process according to the invention thus proposes to recirculate the exhaust air extracted from the greenhouse, directing a portion of this air towards the thermal treatment unit and reintroducing the other portion into the greenhouse to prevent the accumulation of micropollutants (e.g., NH3 and H2S). The process according to the invention therefore allows for closed-system operation with respect to the greenhouse exhaust air. The portion of this air sent to the thermal treatment unit, which can be pre-equipped with a micropollutant treatment system, prevents the accumulation of odors in the greenhouse.

[0024] In addition, heating the greenhouse by injecting hot air which circulates between the greenhouse, cooling means to reduce the humidity level of the air and heating means, makes it possible to reduce the costs associated with maintaining a sufficient temperature for drying in the greenhouse and without requiring equipment such as a heated floor which is expensive to purchase and from a functional point of view, generally used in this type of solar drying unit.

[0025] Since a fraction of the air exiting the greenhouse is directed towards the reactor of the organic decomposition thermal treatment unit, to be destroyed there, this avoids the accumulation of micropollutants (NH3, H2S ...) in the greenhouse which would occur if all of the extracted air were reintroduced into the greenhouse.

[0026] Preferably, this fraction of air is heated, thus allowing it to be dried before entering the thermal treatment reactor such as an incineration furnace, which limits the impact on the energy balance by reintroducing moisture at the level of said incineration furnace, for example.

[0027] By recovering the air extracted from the greenhouse and circulating it back to the greenhouse and to the combustion furnace, the dissipation into the outside environment of air that may be laden with olfactory pollutants is thus avoided while maintaining an acceptable level of odors within the greenhouse.

[0028] Advantageously, the process therefore comprises several steps, the first being the pre-drying of the sludge in a closed solar greenhouse. This pre-drying can be carried out by continuously feeding the dewatered sludge from wastewater treatment into a solar greenhouse. This sludge is spread out at the entrance of the greenhouse on the floor, for example, a concrete slab. The sludge is conveyed throughout the greenhouse by a turning system while drying.

[0029] A batch solar drying unit can also be provided where a larger volume of sludge is spread at once in a greenhouse and dries on the spot without progressing within the greenhouse.

[0030] Following solar drying, the pre-dried sludge at the desired dryness is collected at the outlet of the greenhouse and conveyed to a thermal treatment reactor, for example using automatic conveying means to be treated there, for example incinerated.

[0031] Alternatively, it is possible to dry a fraction of the sludge, which then has a dryness content greater than 60%, in the drying equipment, instead of pre-drying all the sludge to less than 40% dryness. The objective is to provide sufficient energy to the thermal treatment equipment, such as an incinerator. In the case of pre-drying only, the reactor of the thermal treatment unit has a single sludge inlet. In the case of drying a fraction of the sludge (dryness >60%), it is possible to mix dried sludge with dewatered, undried sludge from wastewater treatment and inject the mixture into the reactor. It is also possible to inject the dried and dewatered sludge into the reactor at different injection points.

[0032] Advantageously, the process according to the invention also includes a heat recovery step, the heat produced released by the thermal treatment of the sludge being recovered via heat recovery means such as a system of exchangers and a thermal loop, preferably placed at the outlet of the thermal treatment unit.

[0033] Advantageously, the process according to the invention further comprises a stage of generating electricity and / or heat, for example by means of an organic Rankine cycle machine, known as an ORC machine. This ORC machine is a thermodynamic machine that produces electricity from heat, using a thermodynamic Rankine cycle, and comprises a pump, a steam generator, an expansion machine, and a condenser through which a heat transfer fluid circulates.

[0034] The heat recovered from thermal treatment such as incineration is thus transformed: - in electricity, which can be consumed on site, or injected into the network; and - in heat, which can be used mainly in sludge drying.

[0035] As already mentioned, the process according to the invention comprises a closed-loop air circulation system for sweeping the greenhouse. Thus, the fraction of extracted air whose humidity level has been reduced (dehumidified) by cooling is reheated before being reintroduced into the drying unit. Advantageously, this reheating takes place in the ORC machine, with the cold air circulating in the condenser acting as a refrigerant medium. The air thus acquires heat, thereby increasing its temperature and further lowering its relative humidity. This heated air is then reinjected into the solar greenhouse as relatively warm and dry air, in order to act as a drying medium, i.e., a vapor carrier.

[0036] Thus, the air swept from the drying unit is used both as a heat transfer medium for drying in the greenhouse and as a refrigerant for the ORC. This allows the operating costs of the drying unit to be reduced, in particular by saving on electricity consumption for ventilation in the greenhouse.

[0037] The step of heating the air before its reintroduction into the solar drying unit can also be implemented by other means of heating.

[0038] In an alternative embodiment, the cooled and dehumidified air can be injected directly into the solar drying greenhouse. Indeed, the cold water could then be used as a refrigerant medium.

[0039] In this alternative embodiment, the preheated water exiting the ORC can then be sent into the greenhouse via underfloor heating or through pipes connected to air heaters. The heat contained in the preheated water is diffused into the sludge and the drying air. In this case, the greenhouse itself constitutes a heat exchanger, an alternative to the "air / cold fluid" exchanger described in the previous option.

[0040] The process further comprises a step of treating fumes from the heat treatment.

[0041] The present invention also relates to a system, in particular for implementing the method according to the invention, this system comprising: - a solar drying unit for sewage sludge; - a thermal treatment unit for the decomposition of organic matter comprising a reactor into which the dried sludge is introduced; - as well as means for circulating air within the solar drying unit to create airflow within said solar drying unit, which include means for extracting moisture-laden air from the solar drying unit, means for separating the air extracted from the solar drying unit into two fractions, means for reducing the moisture content (n) of a fraction of this extracted air and then means for heating this fraction of the extracted air to a predetermined sludge-drying temperature, as well as means for reintroducing this fraction of air into the solar drying unit; and means of supplying and introducing the other fraction of extracted air towards the thermal treatment unit for the decomposition of organic matter.

[0042] The system may further include one or more of the following features, taken alone or in combination: - means of reducing the humidity level of a fraction of the extracted air (for dehumidification) such as means of cooling the air fraction, means of bringing this cooled air to heating means to be heated there and means of bringing this heated air to means of reintroduction into the solar drying unit; - means of recovering heat produced by the thermal treatment unit for the decomposition of organic matter such as of the type including a system of exchangers and a thermal loop; - the means of heating the fraction of the extracted air that has been cooled, using at least part of the heat recovered by the means of recovering heat produced by the thermal treatment unit for the decomposition of organic matter, - means of generating energy in electrical and / or thermal form from the heat recovered by the means of recovering heat produced by the thermal treatment unit for the decomposition of organic matter; - means of heating the fraction of air cooled by energy transfer from the energy produced by the means of energy generation such as electricity and / or heat from the heat recovered at the outlet of the thermal treatment unit for the decomposition of organic matter; - the solar drying unit consists of a closed solar greenhouse; - the thermal treatment unit for the decomposition of organic matter is an incineration unit comprising a reactor such as an incineration furnace, preferably of the fluidized bed type, a pyrolysis furnace; - means of generating energy in electrical and / or thermal form use a Rankine cycle machine called an ORC machine (according to the English acronym for Organic Rankine Cycle); -the means of generating energy in electrical and / or thermal form include a steam turbine, a boiler; - means of treating fumes chosen alone or in combination, from among means of treating dust by filtration such as a cyclone, a bag filter, an electrostatic filter, means of treating sulfur oxides (SOx), means of treating nitrogen oxides (NOx), means of capturing heavy metals, means of treating dioxins and other means of usual fume treatment; - means allowing the admission of sludge to be treated into the solar drying unit and means of bringing the dried sludge to the thermal treatment unit such as automatic conveying means such as belt conveyors.

[0043] The incineration furnace is preferably of the fluidized bed type, operating on the principle of suspending pre-calibrated sand particles in a stream of preheated hot air. The fluidized sand bed is generally heated to the incineration temperature (700-800°C) and constitutes an extremely turbulent medium in which heat exchange reaches very high transfer coefficients. The dewatered and pre-dried sludge fed into this bed is very rapidly disintegrated by the turbulence of the sand; evaporation occurs instantaneously, as does the combustion of the organic matter with the fluidized air as the oxidant.

[0044] A dense fluidized bed furnace without bed overflow is generally used, and since there is no bed overflow, the height of the expanded bed remains stable (except for sand losses due to attrition). Indeed, the mineral matter of the sludge has a very small particle size compared to the sand grains of the bed, so that all the mineral ash is pneumatically carried to the upper zone of the furnace and then into the flue gas ducts (fly ash).

[0045] The incineration process implemented in a furnace of this type therefore makes it possible to destroy the organic matter in the sludge. It is the most widespread method for carrying out this destruction while complying with regulatory standards.

[0046] The air extracted from the solar drying unit, once dried and cooled, constitutes a refrigerant medium in the condenser of the ORC machine, which constitutes a means of heating this fraction of extracted air before reintroducing it into the solar drying unit.

[0047] The invention will now be described in more detail with reference to the drawing which represents:

[0048] [Fig. 1] a block diagram of the process according to the invention; and

[0049] [Fig.2] a diagram of the system for implementing the process according to the installation.

[0050] As can be seen in [Fig.2], the system according to the invention comprises: -a solar drying unit such as a closed greenhouse 1 for solar drying 10 of dewatered sludge Bi from a wastewater treatment plant; - a thermal treatment unit for the decomposition of organic matter, in the example an incineration unit comprising a furnace 2 into which pre-dried B2 sludge is introduced; - means of recovering heat from incineration 5; - means of treating fumes 6; - means of air circulation in greenhouse 1 to create an air sweep in greenhouse 1 which include in particular means of extracting the air charged with humidity, this air generally having a humidity level greater than 60%, out of greenhouse 1 and means of reintroducing the extracted air into the solar drying unit whose humidity level has been reduced and this fraction of air heated to a predetermined temperature for the drying of the sludge; - means of generating energy in electrical and / or thermal form 4.

[0051] The means for circulating the extracted air laden with moisture A2 include means for dividing this extracted airflow into two fractions, one of which is sent to the incineration furnace 2 and the other to means for treating this air for reintroduction into the greenhouse 1.

[0052] Preferably, the incineration furnace 2 is of the fluidized bed type, operating on the principle of suspending pre-calibrated sand particles in a stream of preheated hot air. The fluidized sand bed is generally heated to the incineration temperature (800-850°C) and constitutes an extremely turbulent medium in which heat exchange reaches very high transfer coefficients.

[0053] During the drying 10 of the Bi sludge from purification, in greenhouse 1, the air in greenhouse 1 becomes charged with evaporated water and to create the air sweep within greenhouse 1, an extraction step 70 is carried out, for example using fans, extractors, of the air charged with humidity A2>generally with a humidity level greater than 60%.

[0054] This extracted air, laden with moisture A2, is subjected to a separation step 80 and then divided into two fractions by means such as distribution valves. A first fraction A3 is sent to means enabling a moisture reduction step 30, these means including air cooling means 3 such as water scrubbing means, allowing the recovery of water W which can be discharged either directly to the outside or reused in the wastewater treatment plant, for example.

[0055] The air thus dried and also cooled A5 which exits the cooling means 3 is subjected to a heating step 90, and is for this purpose sent to heating means to heat this air to a predetermined temperature for the drying of the sludge to then be reintroduced into greenhouse 1.

[0056] The process also includes a heat recovery step 50, in which the heat produced released by the thermal treatment of the sludge is recovered via heat recovery means such as a system of heat exchangers and a thermal loop 5, preferably placed at the outlet of the thermal treatment unit 2.

[0057] The process then includes an energy generation step 40 implemented by the energy generation means 4 which recovers thermal energy from the energy recovery means 5. These energy generation means consist, in the example shown, of a system of the type organic Rankine cycle machine, also called ORC, comprising a pump, a steam generator, an expansion machine and a condenser.

[0058] The condenser then preferably constitutes the heating means enabling the implementation of the heating step 90 of the air coming from the cooling means 3, thus reducing the humidity level. The dried and cooled air A5 then constitutes the refrigerant medium of the condenser of the energy generation means in electrical and / or thermal form 4.

[0059] The air A5 will therefore acquire heat (increase in its temperature and decrease in its relative humidity) in the condenser, and at the outlet of the condenser, the air is returned and reinjected into the solar greenhouse 1 as relatively hot and dry air A6 in order to act as a drying medium capable of transporting water vapor resulting from the solar drying of the sludge. To this air A6 reinjected into the greenhouse 1 is added air introduced from outside Ab

[0060] Following the separation step 70, a fraction A4 of the air exiting the greenhouse 1 is directed to the incineration unit 2. This step of introducing 110 of the air into the incineration unit 2 can be carried out directly but preferably the air A4 is first heated, preferably in a heat exchanger at the level of the heat recovery means 5. This heating step 100 of the air A4 thus makes it possible to avoid a disturbance of the temperature prevailing within the incineration unit 2.

[0061] By introducing this extracted air fraction A4 into the incineration unit 2, the destruction of the micropollutants (NH3, H2S ...) it contains is allowed at the level of the incineration unit 2 and the air fraction A6 reintroduced into the greenhouse 1 also contains less of it, which makes it possible to avoid the accumulation of micropollutants in said greenhouse 1.

[0062] The heat recovery step 50 recovers the heat produced during the heat treatment step 20 in the furnace 2. The incineration heat recovery means 5 comprise a system of heat exchangers and a heat loop and allow the heat C to be recovered and sent to the energy generation means 4 in electrical and / or thermal form, where, thanks to the ORC machine-type system, this heat C is transformed into electrical energy Ee and / or energy Thermal ET. The electricity or electrical energy ET produced in this way can be used directly on the installation, which reduces operating energy costs, or injected into the grid.

[0063] The thermal energy ET produced is used and utilized mainly in the drying of the sludge, particularly during the heating step 90 of the air to be reintroduced into the greenhouse 1.

[0064] The following table shows the results of an example implementation of the method according to the invention in a system according to the given example:

[0065] [Tables 1] Parameter Unit Value Sludge flow rate Kg / h 317 t / year 10000 Installation dimensions Population equivalent 200,000 Incineration temperature (freeboard) °C 870 Dryness before solar pre-drying % DM 25 Dryness after solar pre-drying % DM 38 Incineration furnace airbox temperature (fluidization air) °C 300 Sludge type - Digested sludge Low Heating Value of VM MJ / kg VM 23,3 Ash content in sludge (before incineration) (%DM) 40 Available power at the outlet of the heat recovery unit kW 531 Electrical production efficiency (%) 17 Electrical power production by the ORC unit kW 90 (vs 28 for a conventional incineration furnace without pre-drying) Savings compared to a conventional incineration furnace (€100 / MWh) € / year 55 k Heat transfer between the ORC unit and the solar drying unit kW 425 Sludge dryness upon entry to the solar drying unit %DM 25 Sludge dryness upon exit after passing through the greenhouse or solar drying unit %DM 38 Total water evaporated in the greenhouse Kg / h 390 Evaporation capacity (standard greenhouse system) Kg EW / m2 / year 1600 , Greenhouse dimensions m2 2138 (1 to 2 greenhouses) Heating (if heated floor) W / m2 200 Heat transfer from the ORC machine to the solar drying unit kW 425 Heating (energy available from the ORC machine) W / m2 199 Heated airflow from the ORC machine (from 5 to 35 °C and relative humidity reduction from 90 to 14%) NmVh 40605 Energy available from the ORC machine to evaporate water MWh / t H2O eV 1.09

[0066] The process also includes a fume treatment step 60 and, for this purpose, the system includes fume treatment means 6 selected alone or in combination, from dust treatment means by filtration such as a cyclone, a bag filter, an electrostatic filter, sulfur oxide (SOx) treatment means, nitrogen oxide (NOx) treatment means, heavy metal capture means, dioxin treatment means and other conventional fume treatment means allowing the separation of ash Ce and fumes F.

[0067] Thus, the process according to the invention makes it possible, by using solar energy, to propose a process to achieve the self-thermality of sludge without fuel consumption and likely to promote the production of thermal and / or electrical energy, while eliminating the problems of odors and maintaining favorable temperature conditions without the use of costly solutions such as underfloor heating.

[0068] The process according to the invention makes it possible to reduce the carbon footprint while having an interesting energy yield.

[0069] Moreover, the installation (construction) and operating (operation, availability) costs of a solar drying unit as proposed in the installation according to the invention, i.e. without underfloor heating or odor treatment system, may be economically advantageous, in particular compared to a conventional thermal pre-dryer.

Claims

Claims

1. A method for treating sewage sludge, comprising at least the following steps: - solar drying (10) in a solar drying unit (1) of the sewage sludge (Bi), the solar drying unit (1) comprising air regulated at a predetermined sludge drying temperature;- a heat treatment for decomposition of the organic matter (20) of said dried sewage sludge (B2) in a heat treatment unit for decomposition of the organic matter (2), - an extraction (70) of the air from the solar drying unit (A2) following the drying, characterized in that the extracted air (A2) is divided (80) into two fractions, one fraction (A4) of this air being conducted and introduced into the heat treatment unit for decomposition of the organic matter (2) while the other fraction (A3) of this extracted air (A2) is subjected to a step of reducing the humidity level (30) of said fraction in a range of 10 to 50%, preferably to a level lower than 30%, said air fraction (A5) then being subjected to a heating step (80) to a predetermined temperature for drying the sludge and reintroduced (A6) into the solar drying unit (1) as a medium drying, that is, steam carrier.;

2. Method according to claim 1, characterized in that the thermal treatment for decomposition of the organic matter (20) is incineration.

3. Method according to claim 1, characterized in that the thermal treatment for decomposition of the organic matter (20) is a pyrolysis such as pyrocarbonization, pyrogasification, pyroliquefaction.

4. Method according to one of claims 1 to 3, characterized in that it comprises a step of heating (90) the air fraction (A4) extracted from the solar drying unit (1) before its entry into the thermal treatment unit for decomposition of the organic matter (2).

5. Method according to one of claims 1 to 4, characterized in that it comprises a step of recovering the heat (50) produced by the thermal treatment of decomposition of the organic matter (20) of the sewage sludge, by means of recovering the heat (5) at the outlet of the thermal treatment unit for decomposition of the organic matter (2).

6. Method according to claim 5, characterized in that the step of heating the cooled air fraction is carried out by at least part of the heat produced by the thermal treatment of decomposition of the organic matter of the sewage sludge and recovered.

7. Method according to claim 5 or 6, characterized in that it comprises a step of generating energy (40) such as electricity and / or heat from the recovered heat.

8. Method according to claim 7, characterized in that the step of heating the cooled air fraction is implemented by transferring energy from the energy produced by the step of generating energy such as electricity and / or heat from the heat recovered at the outlet of the thermal treatment unit for decomposing the organic matter.

9. Method according to claim 7, characterized in that the energy generation step (40) is implemented by means of an organic Rankine cycle machine called an ORC machine (4) comprising a pump, a steam generator, an expansion machine and a condenser.

10. Method according to one of claims 1 to 9, characterized in that the step of reducing the humidity level of the fraction of the extracted air is a cooling step (30).

11. A method according to claim 9, characterized in that the step of heating (80) the cooled air is carried out in the organic Rankine cycle machine (4), the cold air circulating in the condenser as a refrigerating medium.

12. System for implementing the method according to one of claims 1 to 11, comprising: - a solar drying unit (1) for solar drying of sewage sludge; - a thermal treatment unit for decomposing organic matter (2) comprising a reactor into which the pre-dried sludge is introduced; characterized in that it also comprises means for circulating air in the solar drying unit (1) to create an air sweep in said solar drying unit (1), which comprise means for extracting the air laden with moisture from the solar drying unit (1), means for separating the extracted air (A2) from the solar drying unit (1) into two fractions, means for supplying and introducing a fraction (A4) of the extracted air to the thermal treatment unit for decomposing the organic matter (2) and means for reducing the humidity level of the other fraction (A3) of this extracted air then means for heating (4) this fraction (A6) of the extracted air to a predetermined temperature for drying the sludge as well as means for reintroducing this fraction of air (A7) into the solar drying unit (1).

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