Method for treating organic waste, in particular sludge from wastewater treatment plants, and facility for implementing said method

Abstract

The invention relates to a method for treating organic waste, in particular to a method for treating sludge from wastewater treatment plants, in order to produce power and/or hygienized organic matter, including a first step of mesophilic or thermophilic digestion (13) of at least one fraction of a stream of organic waste, and comprising the following steps: dehydrating (15) all of the digested and non-digested waste; aerated thermal hydrolysis (16) of the dehydrated waste, including an injection of an oxidizing agent in a quantity lower than the stoichiometric quantity for oxidizing organic matter, and setting to the required temperature by a heating means; and a second mesophilic or thermophilic digestion (17) of the stream of hydrolyzed waste.

Claims

1. A process for treating organic waste or for treating sludge from wastewater treatment plants, in order to produce energy and/or organic matter which has been rendered hygienic, comprising: a first step of mesophilic or thermophilic digesting of at least one fraction of a stream of organic waste in a first mesophilic or thermophilic digester; dehydrating all of the digested and nondigested waste, the dehydration being carried out to 10-20% of dry matter; aerated thermal hydrolyzing of the dehydrated waste, with the injection of an oxidizing agent in an amount between 10% and 50% of the stoichiometric amount for the oxidation of the organic matter, and setting to the required temperature via a heater, the oxidizing agent being at least one selected from the group consisting of oxygen, ozone, hydrogen peroxide and air, the injection of oxidizing agent being in stages controlled by measuring CO.sub.2 in a headspace of a hydrolysis reactor, the oxidizing agent being injected at several points including a bottom of the hydrolysis reactor and half way up the hydrolysis reactor; and a second mesophilic or thermophilic digesting of the stream of only hydrolyzed waste in a second mesophilic or thermophilic digester, separate from the first digestion step.

2. The process as claimed in claim 1, wherein the heater comprises at least one steam injector.

3. The process as claimed in claim 1, wherein, after the aerated thermal hydrolysis, and before the second digestion, the stream of waste is subjected to cooling by heat exchange making it possible to reuse the heat exiting the aerated thermal hydrolysis for heating the first and the second digesters.

4. The process as claimed in claim 1, wherein a part of the hydrolyzed sludge is recycled, at the output of the aerated thermal hydrolysis, to the first digestion.

5. The process as claimed in claim 1, wherein, during the aerated thermal hydrolysis, the organic waste is heated at a temperature between 90 and 240° C., under a pressure of 1 to 45 bar, for 5 to 90 minutes.

6. The process as claimed in claim 5, wherein, during the aerated thermal hydrolysis, the temperature is between 120° C. and 200° C., while the pressure is between 4 and 12 bar.

7. The process as claimed in claim 1, wherein staging of the injection of the oxidizing agent is carried out in order to provide a homogeneous reaction.

8. The process as claimed in claim 1, wherein an injection of water into the stream of waste is carried out upstream of an inlet of the second digester in order to provide a dilution of the stream and to control the operation of the second digestion.

9. The process as claimed in claim 2, further comprising a regulation of the injection of steam by controlling the temperature in a gaseous headspace of a hydrolysis reactor.

10. The process as claimed in claim 1, wherein the duration of the first digestion, like that of the second digestion, is between 7 and 19 days, under mesophilic conditions.

11. The process as claimed in claim 8, wherein a regulation of the dilution is carried out according to an ammonia content and a pH on exiting the second digestion.

12. The process as claimed in claim 8, wherein the stream of organic waste exiting the thermal hydrolysis undergoes a heat exchange in order to supply heat to a loop of water, with regulation of the temperature of the organic waste exiting the aerated thermal hydrolysis and the dilution.

13. The process as claimed in claim 12, wherein a regulation of the temperature of the sludge is provided at an entry into the first digestion, by heating with a loop of water, and/or by exchange with the hydrolyzed sludge.

14. The process as claimed in claim 1, wherein biogas produced by digestion is used as fuel in cogeneration for producing energy and heat, and in that the heat from the flue gases of the biogas at the outlet of one or more cogeneration units is used through an exchanger in order to provide the energy required for the aerated thermal hydrolysis.

15. The process as claimed in claim 14, wherein a use of the heat at the outlet of one or more cogeneration units, through an exchanger, in order to provide the energy required for heating the first and second digesters.

16. The process as claimed in claim 1, further comprising pretreating aimed at removing the tow and grit according to the quality of the sludge to be treated.

17. The process as claimed in claim 1, wherein, during the aerated thermal hydrolysis, an injection of acid or a base is carried out such that an acidic pH of less than 4, is attained by injecting acid, while a basic pH greater than 10, is attained by injecting a base.

18. The process as claimed in claim 1, wherein the amount of oxidizing agent injected is between 20% and 30 of the stoichiometric amount.

19. The process as claimed in claim 5, wherein, during the aerated thermal hydrolysis, the temperature is equal to 160° C., while the pressure is 8 bar.

20. The process as claimed in claim 1, wherein, during the hydrolysis, an injection of acid or a base is carried out such that an acidic pH of less than 2, is attained by injecting acid, while a basic pH less than 12, is attained by injecting a base.

21. The process as claimed in claim 1, wherein the dehydration is carried out to 17% of dry matter.

22. A process for treating organic waste or for treating sludge from wastewater treatment plants, in order to produce energy and/or organic matter which has been rendered hygienic, consisting of: a first step of mesophilic or thermophilic digesting of at least one fraction of a stream of organic waste in a first mesophilic or thermophilic digester; dehydrating all of the digested and nondigested waste, the dehydration being carried out to 10-20% of dry matter; aerated thermal hydrolyzing of the dehydrated waste, with the injection of an oxidizing agent in an amount between 10% and 50% of the stoichiometric amount for the oxidation of the organic matter, and setting to the required temperature via a heater, the oxidizing agent consisting of at least one selected from the group consisting of oxygen, ozone, hydrogen peroxide and air, the injection of oxidizing agent being in stages controlled by measuring CO.sub.2 in a headspace of a hydrolysis reactor, the oxidizing agent being injected at several points including a bottom of the hydrolysis reactor and half way up the hydrolysis reactor; and a second mesophilic or thermophilic digesting of the stream of only hydrolyzed waste in a second mesophilic or thermophilic digester, separate from the first digestion step.

Description

BRIEF DESCRIPTION OF THE DRAWING FIGURES

(1) The invention consists, apart from the arrangements set out above, of a certain number of other arrangements to which reference will be more explicitly made hereinafter with respect to an implementation example described with reference to the attached drawings, but which is in no way limiting. On these drawings:

(2) FIG. 1 is a diagrammatic representation in block form of a facility for implementing a process according to the invention.

(3) FIG. 2 is a diagram of an aerated hydrolysis reactor according to the invention, and

(4) FIG. 3 is a partial diagram of an exchanger implementation variant.

DETAILED DESCRIPTION OF THE INVENTION

(5) The process of the invention aims to produce energy and/or organic matter which has been rendered hygienic. The description which follows is given using, in the interest of simplification, the term “sludge” which should be understood in a very general sense which is not limited to sludge from wastewater treatment plants, but which applies to any stream of organic waste.

(6) Referring to the drawings, in particular to FIG. 1, it can be seen that the sludge to be treated is categorized into two categories: easy sludge 10, and difficult sludge 12, as initially described.

(7) A fraction of the stream of sludge, corresponding at least to the easy sludge 10, undergoes heating in a heat exchanger 11. The exchanger 11 is of the water-sludge type, and acts on the stream of nonhydrolyzed sludge, entering a digestion step in a digester 13.

(8) It should be noted that the difficult sludge 12 can also pass through this exchanger 11 if it is digested in the digester 13.

(9) The exchanger 11 makes it possible, using the heat recovered from the sludge exiting 21 the aerated thermal hydrolysis, to preheat the sludge entering the digester 13 so as not to have to heat the latter continuously.

(10) The first digestion step in the digester 13 may be mesophilic, corresponding to an operating temperature between 30° C. and 45° C., preferably between 35° C. and 40° C., or thermophilic, corresponding to an operating temperature between 50° C. and 60° C.

(11) The digester 13 treats at least the easy sludge and a fraction of the cooled hydrolyzed sludge, forming a stream 24. Depending on the economic balance of the assembly, the digester 13 can also treat a part or all of the difficult sludge 12. The digester 13 is dimensioned for a residence time of between 9 and 19 days, preferentially 14 days under mesophilic conditions. The digester has its own backup heating circuit (not represented).

(12) The digested sludge exiting the digester 13 is mixed with the difficult sludge 12, for the case where said difficult sludge was not introduced into the digester 13. This mixing can be carried out in an apparatus for pretreating 14 all of the digested sludge and difficult sludge in order to remove the tow and the grit which may disrupt the aerated thermal hydrolysis. The apparatus 14 may comprise a disintegrator and also a grit remover, depending on the quality of the sludge, in order to protect the downstream equipment.

(13) All of the digested and nondigested sludge is then subjected to a dehydration in a dehydrator 15. This dehydration makes it possible to reduce the volume of sludge to be treated by the aerated thermal hydrolysis and therefor the energy consumptions. The dehydration is carried out to 10-20% of dry matter (ratio of the weight of the dry matter to the total dry matter+liquid weight), preferentially to 17% of dry matter in order to optimize the size of the aerated thermal hydrolysis according to the technology used and in order to optimize the size of a second digester 17. This value of 17% makes it possible to reconcile a sludge viscosity which is not too high and an acceptable economic cost of the facility.

(14) The dehydrated sludge is then subjected to an aerated thermal hydrolysis in a hydrolysis reactor 16, with injection of an oxidizing agent 50 (oxygen, and/or ozone, and/or hydrogen peroxide, or air by default) and the injection of steam 16a in order to heat to the required temperature. The aerated thermal hydrolysis can be carried out batchwise or continuously.

(15) During this hydrolysis, the sludge is heated at a temperature between 90 and 240° C., under a pressure of 1 to 45 bar, for 5 to 90 minutes in the presence of a small amount of oxidizing agent (air, oxygen, ozone) in order to hydrolyze and oxidize the organic molecules and to break the cell membranes. The amount of oxidizing agent injected is less than the stoichiometric amount for the total oxidation of the waste, i.e. ensuring the conversion of the carbon of the organic matter into carbon dioxide CO.sub.2. Preferably, the amount of oxidizing agent injected is between 10% and 50%, in particular between 20% and 30%, of the stoichiometric amount. The oxygen O.sub.2 added via the oxidizing agent is partly consumed for breaking the molecules and producing COH and COOH bonds, and partly for producing carbon dioxide CO.sub.2.

(16) The reduced amount of oxidizing agent injected, while improving the hydrolysis and the breaking of the molecules, makes it possible to preserve organic matter suitable for undergoing a digestion and for producing biogas, while avoiding complete conversion of the carbon of the molecules into carbon dioxide.

(17) The heating at the desired temperature during the aerated thermal hydrolysis is advantageously provided by direct injection of steam into the matter undergoing hydrolysis. This injection of steam makes it possible to dispense with heating of the matter undergoing hydrolysis by passing it through a heat exchanger. Preferably, the temperature is between 120° C. and 200° C., in particular equal to 160° C., while the pressure is between 4 and 12 bar, in particular equal to 8 bar.

(18) The injection of steam 16a in order to heat at the desired temperature is controlled by measuring the temperature in the gaseous headspace of the hydrolysis reactor. The injection of steam is reduced or eliminated if the temperature exceeds a predetermined limit, maximum of 240° C., whereas the injection is increased if the opposite is true.

(19) The injection of oxidizing agent is controlled by measuring carbon dioxide CO.sub.2 in the gaseous headspace. A staging of the injection of oxidizing agent is envisioned in order to ensure a homogeneous hydrolysis reaction.

(20) The sludge in the hydrolysis reactor is in the liquid state. A device is envisioned for stirring the sludge in the reactor. The stirring device may be mechanical or physical: the stirring may be provided by recirculation pumping, or statically by means of baffles. Mechanical stirring, in particular by means of a propeller as seen in FIG. 2, is also possible. The stirring enables the homogenization of the sludge and the injected oxidizing agent.

(21) The facility comprises a circuit or loop 20 of thermal fluid, preferentially water.

(22) The loop 20 of thermal fluid comprises the heat exchanger 18 in which the hydrolyzed sludge is cooled so as to reach the temperature (35° C. to 65° C.) required for the operation of a second digester 17, according to a dilution 40 applied to the sludge by injecting water into the sludge.

(23) The loop 20 can be split into two different loops, one comprising the exchanger 11 and the other comprising the exchanger 18, in such a way that the heating and cooling functions are always provided.

(24) The regulation of the temperature at the entry of the digester 17 can be carried out by means of a circuit (not represented) for bypassing the exchanger 18 via the loop of water 20. Depending on the dimensioning of the two digesters 13, 17, the exchangers 11 and 18 may become common, in the form of a sludge/sludge exchanger Eb, according to FIG. 3, with section 18a of hot sludge, originating from the hydrolysis reactor 16, transmitting the heat to the sludge to be treated passing through a section 11a thermally coupled to the circuit 18a.

(25) The loop 20 also comprises the heat exchanger 11. The heat supplied to the loop 20 of thermal fluid makes it possible to heat the sludge at the inlet of the digester 13 in a controlled manner, through this exchanger 11.

(26) The loop 20 also comprises a heat exchanger 32 between the water of the loop and steam obtained from the heat produced by a cogeneration unit 30. In the event of a need for heat in the sludge at the outlet 23 of the exchanger 11, the steam produced is used to heat the loop of hot water 20.

(27) The loop 20 also comprises, for the case where there is a surplus of heat, a cooler 33 of the air condenser type, or water/water exchanger or any other type of cooler, which makes it possible to cool the loop of water such that the exchanger 18 always performs its role of cooling the sludge.

(28) A circulation 24 of cooled hydrolyzed sludge, preferably less than 50%, is advantageously envisioned in the digester 13 in order to increase the overall biogas conversion yield and to reduce the dilution 40 imposed by the aqueous ammonia concentrations in the digester 17.

(29) The dilution 40 is carried out by injecting water, downstream of the exchanger 18 and of the bypass pipe 24 to the digester 13, and upstream of the digester 17. This dilution, in particular, is carried out according to the continuously measured concentration of the amount of aqueous ammonia at the outlet of the digester 17. The dilution is increased when the amount of aqueous ammonia at the outlet increases, and conversely in the event of a decrease. This regulation makes it possible to optimize the operation of the digester 17 by guaranteeing operating conditions which are always optimal.

(30) The hydrolyzed sludge, after dilution 40, is subjected to a second digestion in the mesophilic or thermophilic second digester 17. The digester 17 recovers a part of the hydrolyzed sludge, since another part is recycled to the first digester 13. The digester 17 is dimensioned for a residence time of between 9 and 19 days, preferentially 15 days under mesophilic conditions. The digester 17 has its own backup heating circuit, not represented.

(31) A control 41 of the pH and of the ammonia NH.sub.3 content of the sludge is carried out at the outlet of the digester 17, or in a recirculation loop of the digester. This control makes it possible to verify that the digester 17 is always under optimal operating conditions. In the event of the value of the free ammonia NH.sub.3 in the sludge being too high, the dilution 40 is opened, i.e. increased, so as to reestablish the equilibrium.

(32) The biogas, essentially methane, produced in the digesters is sent to an assembly 30 made up of one or more cogeneration units which make it possible to use, optionally after treatment, the biogas which serves as a fuel for energy production purposes. The cogeneration unit may be a simple boiler or an electricity-heat cogeneration unit.

(33) The facility advantageously comprises a steam production unit 31 on the outlet of one or more cogeneration units. The unit 31 may be made up of an exchanger of heat between flue gases from the cogeneration unit and steam. This steam is used to feed the thermal hydrolysis with thermal energy, but also to supplement the heat required for the digester 13, either by heating the heat loop 20 through an exchanger 32, or by heating the internal heating loop (not represented) of the digester 13.

(34) It is also possible to envision a means for regulating the pH in the hydrolysis reactor by injecting an acid or a base in order to improve the hydrolysis kinetics. An acidic pH of less than 4, preferably less than 2, is established by injecting acid, while a basic pH greater than 10, preferably less than 12, is established by injecting a base. When such an injection of acid or base is carried out, the heating of the hydrolysis reactor can be reduced, without the hydrolysis yield being lowered.

(35) An appropriate gas treatment 60 is envisioned for the gases (CO.sub.2, volatile organic compounds VOCs) exiting the thermal hydrolysis and which potentially cannot be directed to the digestion.

(36) FIG. 2 is a diagrammatic vertical section of an aerated thermal hydrolysis reactor 16 according to the invention. The reactor comprises a vertical cylindrical shell 34 designed to withstand the pressure prevailing during the hydrolysis. The dehydrated sludge is introduced in the lower part of the reactor via a tube 35. The sludge temperature at the entry of the reactor 16 can be approximately 200° C.

(37) The steam 16a is introduced via injection means 36a, in particular injection nozzles, at least in the lower part of the reactor, and preferably via other injection means 36b approximately halfway up the reactor. The steam can be at a temperature of approximately 200° C. under a pressure of 12 bar.

(38) The oxidizing agent 50, in particular oxygen, is preferably injected at several points, in particular a bottom point with injection means, or nozzle, 50a and a point approximately halfway up with injection means 50b.

(39) The device for stirring the matter contained in the reactor 16 may be mechanical and may comprise for example a propeller 37 carried by a rotating vertical shaft which passes through the upper wall of the reactor in a leaktight manner. As a variant, the stirring device may consist of a recirculation pump on the reactor; injection of oxidizing agent is then advantageously carried out in the recirculation loop. According to another variant, the stirring device may comprise baffles.

(40) A pipe 38, equipped with an automatic valve, starting from the upper part of the reactor, makes it possible to discharge, to the treatment 60, gases originating from the gaseous headspace 38a of the reactor. The temperature of these gases can be approximately 200° C. A safety valve 39 is installed on the upper wall of the reactor. The pipe 21 for discharging the hydrolyzed sludge opens into the reactor 16 below the level of separation between the gaseous headspace 38a and the mass of hydrolyzed matter.

(41) The invention provides many advantages.

(42) During the aerated hydrolysis, the oxygen or any other oxidizing agent present makes it possible to reduce the parasitic reactions resulting in refractory products, such as Maillard reactions.

(43) Compared with a simple digestion or with a thermal hydrolysis-assisted digestion, the invention makes it possible to obtain: an increase in the amount of biogas produced, it being possible for the yield of conversion of the organic matter into biogas to go above 70%; this is due to the reduced amount of oxidizing agent injected which makes it possible, while breaking the molecules, to avoid converting all the organic carbon into CO.sub.2 and to preserve organic matter suitable for undergoing digestion and for producing biogas, a decrease in the amount of final residue, which can go above an 80% reduction in the organic matter, a sludge which is very easy to dehydrate and has been completely rendered hygienic, a decrease in digester size, therefore a decrease in costs, a cost virtually identical to that of a thermal hydrolysis-assisted digestion since the reactor sizes are identical and only the means for injecting an oxidizing agent: air, oxygen, ozone, are added.

(44) Finally, the invention also provides as advantages: a decrease in the amount of biogas used for energy purposes (heating of the aerated thermal hydrolysis, heating of the digester) by virtue of the reductions defined above and effective looping of energy, optimization of the operation of the digesters by regulation of the dilution at the entry of the final digester, reliability of the operation process, a decrease in the size of the thermal hydrolysis, a decrease in energy consumption both for the thermal hydrolysis and for the digestion.

(45) According to the invention, the first digester 13 is dimensioned in such a way that the digestion yield is optimized. Depending on the easy sludge used, this dimensioning will be for from 7 to 19 days, preferentially 14, under mesophilic conditions, instead of the 20 days normally used. The fraction of hydrolyzed sludge recycled will be taken into account in the residence time.

(46) Likewise, the second digester 17 is dimensioned with an optimized digestion time according to the digestion, namely between 7 and 19 days, preferentially 15 days, under mesophilic conditions, instead of the 20 days normally used.

(47) These times are a compromise between the equilibrium of the digestion (equilibrium of the bacteria populations) and the yield achievable by the digestion.

(48) Thus, compared with a conventional digestion of 20 days, for a sludge consisting of 50% by volume of easy sludge and 50% by volume of difficult sludge, at a concentration of 5% DM, the digestion volume is decreased by more than 30%. The amount of biogas produced is increased by more than 50%.