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.

Description:
PRIORITY 
     Priority is claimed as a national stage application, under 35 U.S.C. §371, to international application No. PCT/IB2013/055033, filed Jun. 19, 2013, which claims priority to French application FR1255764, filed Jun. 20, 2012. The disclosures of the aforementioned priority applications are incorporated herein by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a process for treating organic waste, in particular sludge from wastewater treatment plants, in order to produce energy and/or render organic matter hygienic, comprising a first step of mesophilic or thermophilic digestion of at least one fraction of a stream of organic waste. 
     The field of the invention is that of the digestion of organic waste, in particular that which is produced during water treatment. 
     Description of the Related Art 
     The term “biogas” will subsequently denote the gas resulting from the digestion process. This biogas comprises a mixture essentially consisting of methane, carbon dioxide and water. 
     The term “difficult sludge” will denote any organic matter of which the mesophilic digestion at 35-40° C. for 20 days does not make it possible to convert into biogas, consisting of methane, more than 40% of the organic matter initially present. 
     The term “easy sludge” will denote any organic matter of which the mesophilic digestion at 35-40° C. for 20 days makes it possible to reduce the organic matter present by more than 40%. 
     The digestion of organic matter is a very efficient way of producing energy. This is why it is undergoing considerable development. However, it has a certain number of drawbacks: 
     1. Large size of the facility so as to have a substantial residence which enables “difficult” organic matter to be converted into methane. The organic molecule hydrolysis step is the limiting step. 
     2. Only a fraction of the organic matter can be converted into methane. In particular, the cell walls which are in the organic matter are difficult to digest. The overall yields of facilities for converting organic matter to sludge are rarely greater than 50%.
 
3. It is necessary to heat the digesters in order to keep them up to temperature, which consumes energy.
 
     Thermal hydrolysis processes are set up in order to counteract drawback No. 2. By virtue of thermal hydrolysis, a larger fraction of the organic matter can be digested in a shorter time, since the hydrolysis is no longer a limiting step, and the organic matter, in particular the cell walls, are decomposed and become digestible. 
     The hydrolysis also makes it possible to reduce the viscosity of the sludge, thereby making it possible to digest high concentrations of sludge in the digester while at the same time having uniform stirring. 
     However, thermal hydrolysis processes also have drawbacks:
         very expensive facility, said expense depending on the flow rate which passes through these facilities,   very expensive exploitation since it is necessary to heat the sludge with energy, sometimes noble energy.       

     The aqueous ammonia produced proportionally to the amount of organic matter digested is a poison for the digestion at high concentrations. This phenomenon therefore prevents the use of high concentrations of organic matter in the digester, thereby reducing the advantage of concentrating the dry matter so as to reduce the size of the thermal hydrolysis. The digestion yields, even amplified (“boosted”), rarely exceed 50%. 
     Wet oxidation (WO) processes are also known. Generally, the overall yield of biogas production does not exceed 30% of the initial organic matter. Furthermore, these WO processes have drawbacks:
         very expensive facility, said expense depending on the flow rate which passes through these facilities,   problems with the heat exchangers requiring the temperature to be raised sufficiently high for the combustion reaction to generate itself.       

     US 2005/0194311 discloses a process for treating organic waste comprising a two-phase digester: a first reactor performs the acidification hydrolysis function, and a second reactor performs the methanogenesis. It is the combination of the two reactors which forms a digester and performs digestion, i.e. methane production from organic matter. The fact that there is only one “digestion system”, which digests both the hydrolyzed sludge and the nonhydrolyzed sludge, limits the destruction yield. The system according to US 2005/0194311 does not provide fundamentally more than a hydrolysis upstream of the digestion system. 
     SUMMARY OF THE INVENTION 
     The aim of the invention is especially to provide a process of the type previously defined which makes it possible to improve the production of energy and of matter rendered hygienic, without requiring excessive facility and exploitation costs. 
     According to the invention, the process for treating organic waste, in particular the process for treating sludge from wastewater treatment plants, in order to produce energy and/or organic matter which has been rendered hygienic, comprises a first step of mesophilic or thermophilic digestion of at least one fraction of a stream of organic waste, and is characterized in that it comprises the following steps:
         dehydration of all of the digested and nondigested waste,   aerated thermal hydrolysis of the dehydrated waste, with the injection of an oxidizing agent in an amount lower than the stoichiometric amount for oxidizing the organic matter, and setting to the required temperature via a heating means,   and a second mesophilic or thermophilic digestion of the stream of hydrolyzed waste, separate from the first digestion step.       

     The invention defined comprises a system made up of the following successive steps:
         a first digestion,   a dehydration,   a hydrolysis,   a second digestion.       

     Preferably, the amount of oxidizing agent injected is between 10% and 50%, in particular between 20% and 30%, of the stoichiometric amount. 
     The means of heating during the thermal hydrolysis is advantageously made up of at least one injection of steam. As a variant, this heating means could be made up of a circulation of hot fluid around the external shell of a hydrolysis reactor, or of at least one electrical resistance in this reactor. 
     Advantageously, 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 the heating of the digesters. 
     A part, preferably less than 50%, of the hydrolyzed sludge is recycled, at the hydrolysis output, to the first digestion. 
     During the 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. Preferably, during the aerated thermal hydrolysis, 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. 
     The injection of oxidizing agent can be controlled by measuring carbon dioxide CO 2  in the gaseous headspace of a hydrolysis reactor and staging of the injection is carried out so as to provide a homogeneous hydrolysis reaction. 
     A dilution of the waste stream can be carried out by injecting water into the stream, upstream of the inlet of the second digester so as to control the operation of the second digestion. 
     It is possible to provide for a regulation of the heating means, in particular of the injection of steam, by controlling the temperature in the gaseous headspace of the hydrolysis reactor. 
     The duration of the first digestion, and likewise that of the second digestion, are advantageously between 7 and 19 days, preferentially 14 or 15 days under mesophilic conditions. 
     A regulation of the dilution can be carried out according to the aqueous ammonia content and the pH of the waste exiting the second digestion. 
     Preferably, the stream of organic waste exiting the thermal hydrolysis undergoes a heat exchange so as to provide heat to a loop of water, in particular while regulating the temperature of the organic waste exiting the thermal hydrolysis and the dilution. 
     Regulation of the temperature of the waste exiting the thermal hydrolysis and the dilution is advantageously envisioned for preheating the sludge at the entry of the first digestion. 
     The regulation of the temperature of the sludge, or of the waste, at the entry of the first digestion can be provided by heating with the loop of water, and/or by exchange with the hydrolyzed sludge. 
     The biogas produced by digestion is advantageously used as fuel in cogeneration for producing energy and heat, and the heat from the flue gases from combustion 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. 
     It is also possible to envision using the heat at the outlet of one or more cogeneration units through an exchanger in order to provide the energy required for heating the digester. 
     It is advantageous to envision the addition of a pretreatment, before the aerated thermal hydrolysis, in order to remove the tow and the grit according to the quality of the sludge to be treated. 
     During the aerated thermal hydrolysis, stirring is envisioned, enabling the homogenization of the sludge and of the injected oxidizing agent. 
     During the hydrolysis, an injection of acid or of base may be carried out such that an acidic pH of less than 4, preferably less than 2, is established by injecting acid, while a basic pH of greater than 10, preferably less than 12, is established by injecting a base. 
     The invention also relates to a facility for the implementation of a process as previously defined, this facility comprising a first mesophilic or thermophilic digester of at least one fraction of a stream of organic waste, and being characterized in that it comprises:
         a dehydrator of all of the digested and nondigested waste,   a reactor for aerated thermal hydrolysis of the dehydrated waste, with means for injection of an oxidizing agent and a heating means for setting to the required temperature,   and a second mesophilic or thermophilic digester of the stream of hydrolyzed waste, separate from the first digester.       

     According to the invention, the two distinct and separate digesters constitute two chambers each producing methane. 
     The means for heating during the thermal hydrolysis is advantageously made up of at least one steam injection. 
     As a variant, this heating means could be made up of a circulation of hot fluid around the external shell of the hydrolysis reactor, or of at least one electrical resistance in this reactor. 
     Advantageously, the facility comprises a heat exchanger through which the stream exiting the aerated thermal hydrolysis reactor passes and which makes it possible to reuse the heat for heating the sludge entering the digesters. 
     A means for injecting water into the stream, upstream of the inlet of the second digester, is envisioned in order to dilute the stream, and to control the operation of the second digester. 
     Preferably, the facility comprises a loop of water heated through a heat exchanger by the stream of organic waste exiting the thermal hydrolysis. 
     The facility preferably comprises at least one cogeneration unit for the combustion of the biogas produced by digestion and for producing energy and heat. An exchanger is envisioned at the outlet of one or more cogeneration units, so as to have biogas combustion flue gases pass through it and to provide the energy required for the aerated thermal hydrolysis and/or for heating the digester. 
     The aerated thermal hydrolysis reactor comprises a vertical shell, with entry of the waste in the lower part, means for heating the content of the shell, and means for injecting an oxidizing agent at least into the lower part, and preferably at a higher level. 
     The heating means may comprise means for injecting steam at least into the lower part, and preferably about halfway up. As a variant, the heating means may comprise a circulation of hot fluid around the external shell of the hydrolysis reactor, or at least one electrical resistance in this reactor. 
     The hydrolysis reactor also comprises a mechanical or physical stirring device (baffle, recirculation pumping, stirring, etc.) enabling the homogenization of the sludge and of the injected oxidizing agent. 
     The facility may comprise a means for regulating the pH in the hydrolysis reactor by injecting an acid or a base in order to improve the hydrolysis kinetics. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       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: 
         FIG. 1  is a diagrammatic representation in block form of a facility for implementing a process according to the invention. 
         FIG. 2  is a diagram of an aerated hydrolysis reactor according to the invention, and 
         FIG. 3  is a partial diagram of an exchanger implementation variant. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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. 
     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. 
     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 . 
     It should be noted that the difficult sludge  12  can also pass through this exchanger  11  if it is digested in the digester  13 . 
     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. 
     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. 
     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). 
     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. 
     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. 
     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  16   a  in order to heat to the required temperature. The aerated thermal hydrolysis can be carried out batchwise or continuously. 
     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 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 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 2 . 
     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. 
     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. 
     The injection of steam  16   a  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. 
     The injection of oxidizing agent is controlled by measuring carbon dioxide CO 2  in the gaseous headspace. A staging of the injection of oxidizing agent is envisioned in order to ensure a homogeneous hydrolysis reaction. 
     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. 
     The facility comprises a circuit or loop  20  of thermal fluid, preferentially water. 
     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. 
     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. 
     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  18   a  of hot sludge, originating from the hydrolysis reactor  16 , transmitting the heat to the sludge to be treated passing through a section  11   a  thermally coupled to the circuit  18   a.    
     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 . 
     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 . 
     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. 
     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 . 
     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. 
     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. 
     A control  41  of the pH and of the ammonia NH 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 3  in the sludge being too high, the dilution  40  is opened, i.e. increased, so as to reestablish the equilibrium. 
     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. 
     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 . 
     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. 
     An appropriate gas treatment  60  is envisioned for the gases (CO 2 , volatile organic compounds VOCs) exiting the thermal hydrolysis and which potentially cannot be directed to the digestion. 
       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. 
     The steam  16   a  is introduced via injection means  36   a , in particular injection nozzles, at least in the lower part of the reactor, and preferably via other injection means  36   b  approximately halfway up the reactor. The steam can be at a temperature of approximately 200° C. under a pressure of 12 bar. 
     The oxidizing agent  50 , in particular oxygen, is preferably injected at several points, in particular a bottom point with injection means, or nozzle,  50   a  and a point approximately halfway up with injection means  50   b.    
     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. 
     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  38   a  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  38   a  and the mass of hydrolyzed matter. 
     The invention provides many advantages. 
     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. 
     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 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.       

     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.       

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