Method for treating sludge from purification plants

Abstract

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

Description

Process for treating sludge from wastewater treatment plants

[0001] The present invention relates to the field of treatment of sludge from wastewater treatment plants. More particularly, the invention relates to a method for treating sludge from wastewater treatment plants, known as urban sludge, and in particular a method of thermal treatment for decomposing 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 water treatment process, can significantly reduce the volume of this type of waste. In fact, incineration, by converting the organic matter into the gas phase, reduces the volume of sludge by an average factor of 10. The residue obtained following incineration is composed exclusively of mineral matter.

[0003] When treating sludge by incineration, the sludge is generally dewatered before entering the incineration unit. This sludge can be of different types: primary, secondary, tertiary, mixed, digested or not, etc. Its dryness 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, press filters, etc.). The dryness of dewatered sludge usually 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 is made up of water and dry matter. Dryness is the mass percentage of dry matter. Thus, a sludge with a dryness of 10% has a humidity of 90%. Dryness is evaluated by the quantity of solid remaining after heating at 110 °C for two hours. It is generally expressed as a weight percentage. Conversely, we will talk about humidity level. Dryness is a mandatory data to know for all kinds of sludge handling during the wastewater treatment process, because the consistency of the sludge is a physical state dependent on its dryness. Dryness must therefore be high enough for the dried sludge to be stabilized and possibly sanitized.However, this dryness must not be too high in order to avoid the creation of dust (with a risk of significant flammability) and excessive energy consumption.

[0005] Sludge sent to an incineration unit preferably has dryness contents of less than 40% and greater than 20%. Indeed, between 40% and 60% the sludge acquires a particular "plastic" rheology. It then becomes complicated to pump or convey this type of sludge. In addition, above 40%, the fraction of organic matter becomes too high. The incineration process therefore becomes very exothermic (more material to burn and less water to evaporate). This leads to exceeding the usual incineration temperature thresholds (850-900 °C) beyond which the phenomena of ash fusion and NO formation xthermal are favored. These temperature ranges require more expensive furnace and exchanger materials and more advanced flue gas treatments.

[0006] There are also known so-called pyrolysis / incineration processes that can handle higher sludge drynesses (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. a single furnace or treatment reactor. However, they are not very well developed for sludge.

[0007] When the sludge arrives at the incineration unit inlet, it may have a so-called "non-autothermal" character. By non-autothermal, we mean sludge that has a Lower Calorific Value (LCV) insufficient to maintain the operating temperature of the incineration furnace. It should be noted that the operating temperature is the temperature that ensures the conversion of all the organic matter (into CO2 and H2O), as well as a moderate production of pollutants (NOX, SOX, dioxins, etc.).

[0008] To overcome the energy deficit of non-autothermal sludge, it is possible to provide energy through fuel. Burners for fuels such as natural gas, and generally biogas, are therefore systematically provided in sludge incineration furnaces. Despite the widespread use of burners in incinerators, this solution is considered expensive given the price of fuels and, moreover, not environmentally friendly, given the significant carbon footprint during fuel combustion.

[0009] Another possibility to promote the self-thermicity of the sludge is to couple the incineration furnace with a thermal pre-dryer. In this case, an incineration fumes / oil or fumes / steam heat exchanger is provided, which is installed to recycle the heat released by the process to the pre-dryer, in order to improve the energy performance of the installation. The sludge admitted to the incineration furnace is therefore pre-dried sludge with dry contents that can vary between 25 and 38% (depending on the PCI of the sludge treated). A plant comprising an incineration unit and a thermal pre-drying unit is interesting from the point of view of energy recovery. Thermal pre-dryers of the thin-film or "disc" type are thus integrated into the incineration unit to ensure the self-thermicity of the sludge.

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

[0011] Furthermore, installing a thermal pre-dryer represents a significant investment to consider. Finally, this solution requires the recycling of very high-quality thermal energy within the process, which is then no longer available for other uses such as exporting thermal energy, generating electricity, etc.

[0012] Another drying method associated with an incineration furnace can also be used and consists of implementing solar drying of the dewatered sludge, in a step prior to incineration, in order to prepare sludge whose dryness guarantees the self-thermicity of said sludge. This solar drying allows the elimination of free and bound water which cannot be eliminated by a mechanical process, during mechanical dehydration processes. Drying is therefore carried out 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 at the core diffuses towards the surface layer, called the boundary layer. The energy to be used is the sum of the latent heat and the binding energy of this water (low compared to the first).

[0013] Solar drying, which uses renewable and free solar energy to evaporate the water contained in the sludge, is an extensive process. Solar drying can also be implemented without the use of a greenhouse in the form of open drying without a greenhouse. However, solar drying has a major drawback because it generates odors, mainly linked to fermentation occurring due to the low dryness of the sludge introduced into a greenhouse. The implementation of solar pre-drying is therefore of limited interest when greenhouses are installed near urban areas because it then becomes mandatory to resolve this odor problem with a deodorization installation, which is expensive equipment.

[0014] Document EP 1 378 494 describes a sludge treatment process which comprises a thermal treatment of the incineration type preceded by a low-temperature drying step, in particular in a solar dryer. This process therefore proposes carrying out the drying in a solar dryer in a low temperature range up to a maximum of 60 to 70°C. It also comprises a storage step between the drying and the thermal treatment carried out in two stages. Dry and preheated air can be introduced into the solar drying device and comes from energy recovery from the incineration device. The dry and heated air introduced into the solar dryer is discharged directly into the environment in the form of a humid air flow.

[0015] According to the process described, low temperature or cold drying avoids any release of highly volatile compounds and only water is removed from the system, which prevents the formation of odors. However, it turns out that depending on the types of sludge treated, such a process does not 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 possible nuisances linked to odors, there is a loss of calories from the humid air evacuated from the greenhouse.

[0016] Furthermore, since this water is extracted from the greenhouse in the form of steam and the greenhouse air temperature is not very high (especially in winter), the steam concentration in the greenhouse atmosphere remains limited. As a result, it is necessary to extract a large flow of air from the greenhouse to remove the evaporated water. This high flow rate results in a high expenditure of electrical ventilation energy. In addition, the extracted volumes can still carry odorous compounds, which then requires expensive 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 aim, the invention relates to a method for treating sewage sludge, comprising at least the following steps:- solar drying in a solar sewage sludge drying unit, the solar drying unit comprising air regulated at a predetermined sludge drying temperature;- a heat treatment for decomposition of the organic matter of said dried sewage sludge in a heat treatment unit for 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 heat treatment unit for decomposition of the organic matter while the other fraction of this extracted air is subjected to a step of reducing the humidity level of said fraction in a range of 10 to 50%, preferably to a level of less than 30%, said air fraction then being subjected to a heating step at a predetermined temperature for drying the sludge and reintroduced into the solar drying unit.;

[0019] Thus, advantageously, the method according to the invention makes it possible to guarantee that the self-thermal capacity of the heat treatment reactor such as an incineration furnace is achieved 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 therein at least in part and by controlling bad odors by sending to the heat treatment unit a fraction of the air extracted from this solar drying unit loaded 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 characteristics, taken alone or in combination: - the thermal treatment for decomposition of the organic matter is an incineration; - the thermal treatment for decomposition of the organic matter is a pyrolysis such as pyrocarbonization, pyrogasification, pyroliquefaction; - the method comprises a step of heating the fraction of air extracted from the solar drying unit before it enters the thermal treatment unit for decomposition of the organic matter; - the method comprises a step of recovering the heat produced by the thermal treatment for decomposition of the organic matter from the sewage sludge, by means of recovering the heat at the outlet of the thermal treatment unit for decomposition of the organic matter;- the method comprises a step of generating energy such as electricity and / or heat from the recovered heat;- the step of reducing the humidity level of the extracted air fraction is a cooling step, followed by a step of heating 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. steam carrier,- the step of heating 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 step of heating 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 decomposing the organic matter; - the energy generation step is implemented by means of an organic Rankine cycle machine called an ORC machine (according to the English acronym for Organic Rankine Cycle), comprising a pump, a steam generator, an expansion machine and a condenser; - the step of heating the cooled air fraction is implemented in the organic Rankine cycle machine, the cold air circulating in the condenser as a refrigerating medium, the air thus heated being reinjected into the solar drying unit as a drying medium, i.e. a steam carrier;- the method comprises a step of treating fumes from the organic decomposition thermal treatment unit.;

[0021] The thermal treatment for the decomposition of organic matter can thus be chosen from so-called dry thermochemical processes such as pyrocarbonization, pyrogasification and pyroliquefaction. This type of process requires the introduction into the conversion reactor of dried sludge with a dryness greater than 80%. 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 in the thermal treatment (conversion) stage.

[0022] The solar drying unit advantageously consists of a closed greenhouse. Sludge drying is therefore carried out in a closed greenhouse, thanks to solar radiation which heats the air inside the greenhouse and therefore the surface of the sludge bed. The ventilation of the greenhouse allows the evaporation of the water contained in the sludge. The evaporated water is thus evacuated by natural convection thanks to a ventilation system, the drying air extracted from the greenhouse having a humidity level generally higher than 60%. Evaporation is optimized by air sweeping in the greenhouse and, possibly, by a sludge turning / spreading machine.

[0023] With the method according to the invention, it is thus proposed to recirculate the sweeping air extracted from the greenhouse with the orientation of a fraction of this air towards the heat treatment unit and a reintroduction into the greenhouse of the other fraction of extracted air in order to prevent the accumulation of micropollutants (e.g.: NH3 and H2S). The method according to the invention therefore allows operation in a closed system from the point of view of the sweeping air of the greenhouse, the fraction of this air sent to the heat treatment unit which can be previously equipped with a micropollutant treatment system, makes it possible to avoid the accumulation of odors in the greenhouse.

[0024] Furthermore, heating the greenhouse by injecting hot air that circulates between the greenhouse, cooling means to reduce the humidity level of the air and heating means, allows to reduce the costs related to 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] As a fraction of the air leaving the greenhouse is directed towards the reactor of the organic decomposition thermal treatment unit, to be destroyed there, this prevents the accumulation of micropollutants (NH3, H2S, etc.) in the greenhouse, which would occur if all the extracted air were reintroduced into the greenhouse.

[0026] Preferably, this air fraction is heated, thus allowing it to be dried before entering the heat treatment reactor such as an incineration furnace, which makes it possible to limit the impact on the energy balance by reintroducing humidity 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 oven, the dissipation into the outside environment of air that may be laden with olfactory pollutants is avoided while maintaining an acceptable level of odors within the greenhouse.

[0028] Advantageously, the process therefore comprises several stages, the first being the pre-drying of the sludge in a closed solar greenhouse. This pre-drying can be implemented by continuously feeding dehydrated sludge from purification into a solar greenhouse. This sludge is spread 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] Alternatively, a batch solar drying unit can be provided where a larger volume of sludge is spread at once in a greenhouse and dries on site without progressing within the greenhouse.

[0030] Following solar drying, the sludge pre-dried to the desired dryness is recovered at the greenhouse outlet 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 greater than 60% in the drying means, instead of pre-drying all of the sludge to less than 40% dryness, the objective being to provide enough energy to the thermal treatment means 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 then possible to plan to mix dried sludge with dewatered sludge from purification and not dried and to inject the mixture into the reactor. It is also possible to plan to inject the dried sludge and the dewatered sludge into the reactor at different injection points.

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

[0033] Advantageously, the method according to the invention further comprises a step of generating electricity and / or heat, for example by means of an organic Rankine cycle machine, called an ORC machine. This ORC machine is a thermodynamic machine producing electricity from heat, using a Rankine thermodynamic cycle and comprises a pump, a steam generator, an expansion machine and a condenser in which a thermal fluid circulates.

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

[0035] As already mentioned, the method according to the invention comprises a circulation of greenhouse sweep air in a loop. Thus, the fraction of the extracted air whose humidity level has been reduced (dehumidified) by cooling is reheated before its reintroduction into the drying unit. Advantageously, this reheating takes place in the ORC machine, the cold air circulating in the condenser as a refrigerating medium. The air will thus acquire calories and therefore increase its temperature and further lower its relative humidity. This heated air is then reinjected into the solar greenhouse, in the form of relatively hot and dry air, in order to act as a drying medium, i.e. a vapor carrier.

[0036] Thus, the sweep air 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 reduces the operating costs of the drying unit, 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 be implemented by other heating means as well.

[0038] Alternatively, the cooled and dehumidified air can be injected directly into the solar drying greenhouse. In fact, the cold water could then be used as a cooling medium.

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

[0040] The process also includes 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 decomposing the organic matter comprising a reactor into which the dried sludge is introduced; - as well as means for circulating air in the solar drying unit to create an air sweep in said solar drying unit, which comprise means for extracting the air laden with moisture from the solar drying unit, means for separating the air extracted from the solar drying unit into two fractions, means for reducing the humidity level n of a fraction of this extracted air 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 for supplying and introducing the other fraction of the extracted air to the thermal treatment unit for decomposing the organic matter.;

[0042] The system may further comprise one or more of the following features, taken alone or in combination: - means for reducing the humidity level of a fraction of the extracted air (for dehumidification) such as means for cooling the air fraction, means for supplying this cooled air to heating means to be heated there and means for supplying this heated air to the means for reintroduction into the solar drying unit; - means for recovering the heat produced by the thermal treatment unit for decomposing the organic matter such as the type comprising a system of exchangers and a thermal loop;- the means for heating the cooled fraction of extracted air using at least part of the heat recovered by the means for recovering the heat produced by the thermal treatment unit for decomposing organic matter, - means for generating energy in electrical and / or thermal form from the heat recovered by the means for recovering the heat produced by the thermal treatment unit for decomposing organic matter; - means for heating the cooled fraction of air by transferring energy from the energy produced by the means for generating energy such as electricity and / or heat from the heat recovered at the outlet of the thermal treatment unit for decomposing organic matter; - the solar drying unit consists of a closed solar greenhouse;- the thermal treatment unit for decomposing organic matter is an incineration unit comprising a reactor such as an incineration furnace, preferably of the fluidized bed furnace type, a pyrolysis furnace; - the means for generating energy in electrical and / or thermal form comprise a Rankine cycle machine called an ORC machine (according to the English acronym for Organic Rankine Cycle); - the means for generating energy in electrical and / or thermal form comprise a steam turbine, a boiler; - means for treating fumes chosen alone or in combination, from among means for treating dust by filtration such as a cyclone, a bag filter, an electrostatic filter, means for treating sulfur oxides (SOx), means for treating nitrogen oxides (NOx), means for capturing heavy metals, means for treating dioxins and other usual means for treating fumes;- means for admitting the sludge to be treated into the solar drying unit and means for 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 exchanges reach very high transfer coefficients. The dehydrated and pre-dried sludge fed into this bed is very quickly disintegrated by the turbulence of the sand, evaporation takes place instantly as well as the combustion of the organic matter with the fluidizing air as the oxidant.

[0044] A dense fluidized bed furnace is generally used without bed overflow and since there is no bed overflow, the height of the expanded bed remains stable (except for sand losses due to attrition). In fact, the mineral matter in the sludge has a very small particle size compared to the sand grains in the bed, so that all the mineral ash is pneumatically entrained into the upper zone of the furnace and then into the flue gas exhaust (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 refrigerating 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] a block diagram of the method according to the invention; and

[0049] a system diagram for the implementation of the process according to the installation.

[0050] As can be seen in the, the system according to the invention comprises: - a solar drying unit such as a closed greenhouse 1 for the solar drying 10 of dehydrated sludge B1 from a wastewater treatment plant; - a thermal treatment unit for decomposing the organic matter in the example an incineration unit comprising a furnace 2 into which the pre-dried sludge B2 is introduced; - means for recovering the incineration heat 5; - means for treating the fumes 6; - means for circulating the air in the greenhouse 1 to create an air sweep in the greenhouse 1 which comprise in particular means for extracting the air laden with moisture, this air generally having a humidity level greater than 60%, from the greenhouse 1 and means for 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 drying the sludge;- means of generating energy in electrical and / or thermal form 4.;

[0051] The means for circulating the extracted air laden with humidity A2 comprise means for dividing this flow of extracted air into two fractions, one of which is sent to the incineration furnace 2 and the other to means for treating this air with a view to its reintroduction into the greenhouse 1.

[0052] Preferably, the incineration furnace 2 is of the fluidized bed furnace type operating on the principle of suspending pre-sized 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 the heat exchanges reach very high transfer coefficients.

[0053] During the drying 10 of the sludge B1 from purification, in the greenhouse 1, the air in the greenhouse 1 is charged with evaporated water and to create the air sweep within the 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 above 60%.

[0054] This extracted air loaded with humidity A2 is subjected to a separation step 80 to then be divided into two fractions by division means such as distribution valves. A first fraction A3 is sent to means allowing a step of reducing the humidity level 30, these means comprising means of cooling 3 the air such as water washing means, making it possible to recover the water W which can be evacuated either directly to the outside or to be reused in the purification station for example.

[0055] The air thus dried and also cooled A5 which leaves 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 drying the sludge to then be reintroduced into the greenhouse 1.

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

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

[0058] The condenser then preferably constitutes the heating means allowing the implementation of the heating step 90 of the air coming from the cooling means 3 allowing the reduction of the humidity level. The dried and cooled air A5 then constitutes the refrigerating medium of the condenser of the means for generating energy in electrical and / or thermal form 4.

[0059] The air A5 will therefore acquire calories (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 in the form of relatively hot and dry air A6 in order to play the role of 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 A1.

[0060] Following the separation step 70, a fraction A4 of the air leaving the greenhouse 1 is directed towards the incineration unit 2. This step 110 of introducing 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 step 100 of heating the air A4 thus makes it possible to avoid a disturbance of the temperature prevailing within the incineration unit 2.

[0061] By introducing this fraction of extracted air A4 into the incineration unit 2, the destruction of the micropollutants (NH3, H2S, etc.) that it contains is enabled at the level of the incineration unit 2 and the fraction of air 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 makes it possible to recover the heat produced during the heat treatment step 20 in the furnace 2. The incineration heat recovery means 5 comprise a system of exchangers and thermal loops and make it possible to recover and send the heat C to the energy generation means in electrical and / or thermal form 4 where, thanks to the ORC machine type system, this heat C is transformed into electrical energy E E and / or thermal energy E T . Electricity or electrical energy E T produced in this way can be used directly on the installation, which reduces operating energy costs, or injected into the network.

[0063] Thermal energy E T produced is used and recovered mainly at the level of drying the sludge, in particular during the heating stage 90 of the air to be reintroduced into greenhouse 1.

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

[0065] ParameterUnitValueSludge flow rateKg / h317t / year10000Installation dimensionsPopulation equivalent200,000Incineration temperature (freebord)°C870Dryness before solar pre-drying% MS25Dryness after solar pre-drying% MS38Incineration furnace windbox temperature (fluidization air)°C300Sludge type-Digested sludgeLow Heating Value of VMMJ / kg VM23,3Ash content in sludge (before incineration)(% DM)40Available power at the outlet of the heat recovery unitkW531Electricity production efficiency (%)17Electricity production by the ORC unitkW90 (vs 28 for a conventional incineration furnace without pre-drying)Gain compared to a conventional incineration furnace (100 € / MWh)€ / year55 kHeat transfer between the ORC unit and the solar drying unitkW425Dryness of sludge entering the solar drying unit% DM25Dryness of sludge leaving after passing through the greenhouse or solar drying unit% DM38Total evaporated water flow rate in the greenhouseKg / h390Evaporation capacity (standard greenhouse system)Kg EW / m, 2 / an1600Greenhouse dimensionsm²2138 (1 to 2 greenhouses)Heating (in case of heated floor)W / m 2 200Heat transfer from ORC machine to solar drying unitkW425Heating (energy available from ORC machine)W / m 2199Heated airflow from the ORC machine (from 5 to 35 °C and relative humidity drop from 90 to 14%)Nm 3 / h40605Available energy of the ORC machine to evaporate waterMWh / t H2O ev1.09

[0066] The method also comprises a step of treating the fumes 60 and, for this purpose, the system comprises fume treatment means 6 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 usual means of treating fumes making it possible to separate the ash Ce and the fumes F.

[0067] Thus, the method according to the invention makes it possible, by using solar energy, to propose a method for achieving self-thermalization of sludge without consuming fuel and capable of promoting the production of thermal and / or electrical energy, while eliminating odor problems and maintaining favorable temperature conditions without using an expensive solution 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] In addition, the costs of installation (construction) and operation (operation, availability) of a solar drying unit as proposed in the installation according to the invention, i.e. without underfloor heating or odor treatment system, can be economically advantageous, particularly compared to a conventional thermal pre-dryer.

Claims

Method for treating sewage sludge, comprising at least the following steps:- solar drying (10) in a solar drying unit (1) of the sewage sludge (B1), 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 (90) 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.; Method according to claim 1, characterized in that the thermal treatment for decomposition of the organic matter (20) is incineration. 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. Method according to one of claims 1 to 3, characterized in that it comprises a step of heating (100) 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). 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). 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. 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. 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. 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. 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). 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. 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 decomposition of 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 heat 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 sludge drying temperature as well as means for reintroducing this fraction of air (A7) into the solar drying unit (1).;

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