Method for determining a start-up parameter of a digester and method for starting and ramping up a corresponding digester

Abstract

The invention relates to a method for determining a start-up parameter of a digester suitable for producing digestate and biogas in the presence of a microbial community. This parameter is representative of a substrate supply to the digester from its start-up up to nominal operating conditions. The method determines a maximum growth rate of a limiting microbial population under the nominal operating conditions of the digester and estimates the concentration of an inoculum in useful microbial biomass capable of consuming the substrate by taking into account a parameter representative of the survival rate of the inoculum under the nominal operating conditions of the digester.Then, the method determines a mathematical function expressing said parameter as a function of time, this function being determined as a function of the maximum growth rate, the concentration of useful microbial biomass in the inoculum, an initial inoculation volume, a parameter representative of the biodegradability of the substrate and a feeding and withdrawal regime of the digester. Abstract figure: Figure 1.

Description

Title of the invention: Method for determining a start-up parameter of a digester and method for starting and ramping up a corresponding digester Field of invention

[0001] The invention relates to the field of digesters, also called biogas reactors or methanizers.

[0002] More particularly, the invention relates to a method for determining a start-up parameter of a digester and its use in a method for starting and ramping up a digester. Description of the state of the art

[0003] The objective of a digester is to produce biogas by anaerobic digestion.

[0004] Anaerobic digestion corresponds to a cascade of biochemical reactions allowing the conversion of the organic matter present in the digester into biogas, mainly a mixture of carbon dioxide and methane. The remaining materials are called digestate.

[0005] This process is intrinsically complex because:

[0006] - each degradation step is carried out by groups of microorganisms distinct, each having their own kinetics, optimal growth conditions and specific inhibitory compounds;

[0007] - these groups of microorganisms interact with each other, which can result in example in phenomena of competition or syntrophy;

[0008] - the operating conditions of the process, such as the feeding mode as well as the nature of the input or even the temperature, strongly influence the kinetics of these groups of microorganisms.

[0009] Furthermore, the start-up and ramp-up, namely the start-up or restart, of a digester is an important step for the proper functioning of the process over the long term, and can represent a significant cost due to the typical durations of usual protocols (2 to 3 months, or even more).

[0010] From an operational point of view, start-up corresponds to the time required to bring the reactor to its nominal load (corresponding to the entire flow of organic matter to be treated) with a process efficiency in accordance with the desired specifications. The objective is to increase the applied volume load as quickly as possible, without inhibiting the ecosystem, with the introduction of an inoculum. At the laboratory scale, reactor sizes are small, generally less than 1 m3, so that the digester can be fully inoculated, without constraint logistics. On an industrial scale, however, the volume of reactors can vary from 2,000 to 10,000 m3, so inoculation creates significant constraints in terms of costs and logistics.

[0011] The ramp-up is generally implemented according to a conventional strategy which consists of gradually increasing the supply of organic matter at each feeding cycle while maintaining optimal methanogenic activity. The ramp-up is thus generally carried out in stages depending on the treatment capacity of the digester. It is then necessary to form in situ (i.e. inside the digester) the biomass necessary to ensure this treatment. This exercise is made difficult by the difference in kinetics of the different stages of methanization, the hydrolysis and fermentation stages (producing organic acids) being faster than the stages of consumption of acids for the formation of methane.Thus, too rapid a ramp-up risks leading to an accumulation of intermediate products (volatile fatty acids) that can acidify the reactor, potentially to the point of completely inhibiting the functioning of microorganisms that can consume these acids. This scenario is feared by methanizer operators, who have consequently historically implemented conservative but suboptimal ramp-up approaches.

[0012] These approaches are as follows:

[0013] - starting the digester with a constant applied load corresponding to the nominal load of the digester and applied after possible inoculation,

[0014] - gradual and manual load increase based on stability measurements or performance, consisting first of applying a low load, then gradually increasing it when stability or performance objectives have been achieved,

[0015] - progressive and automated load increase, assisted by modeling, on the basis stability or performance measures,

[0016] - progressive increase in load predicted by an empirical model using a indirect description of microbial growth.

[0017] The first approach leads to an initial accumulation of acids due to the difference in kinetics between the bacteria producing volatile fatty acids and the microorganisms capable of consuming them (syntrophic bacteria and methanogenic archaea). It is then necessary to add reagents in order to adjust the pH and maintain it in a zone favorable to microbial growth. For example, it has been proposed to take as a criterion the maintenance of a pH > 6.6 using the addition of Ca(OH)2, MgO, NH4OH or NaHCO3. The addition of these reagents represents a very significant cost. In addition, the products leaving the digester (digestates) are often post-treated and evacuated according to the qualities expected in a steady state. Thus, the period scaling up can represent an additional cost with regard to digestate management (e.g., malfunctions, elimination, non-standard discharges).

[0018] The second approach avoids the accumulation of acids and the costs associated with adding reagents. Usable stability or performance criteria include, for example, the reduction of volatile matter, the ratio of the quantity of volatile fatty acids produced to the total alkaline strength (TAC), and the production of biogas. Many criteria can thus be formulated, but all are based on quantities measured experimentally during monitoring of the methanizer. The initial load and the intermediate loads are therefore defined empirically. However, since methanization is a process with high inertia (slow kinetics), stabilizing the quantities measured and used to validate the ramp-up criteria requires time, which lengthens the ramp-up. In addition, it is not possible to predict the duration of the ramp-up, which imposes significant logistical constraints.For example, processes downstream of the methanization process (e.g., digestate dehydration, biogas recovery) also require start-up with their own timeframes. With this ramp-up strategy, the start-up schedule for these processes is uncertain. Finally, it is not possible to determine in advance an inoculation volume representing an economic optimum, because the relationship between inoculation volume and ramp-up time is not explicit.

[0019] The third approach is an automation of the previous approach to reduce reaction times and shorten the ramp-up. There are several methods based on the control of the digester control parameters during the ramp-up (feed rate, dilution of the methanization input) based on measurements from sensors (e.g. pH, flow rate and quality of biogas) or laboratory analyses (e.g. concentrations of volatile fatty acids, dry matter). This control can possibly be coupled with a methanization model (e.g. the AM2 model) in order to benefit from short-term predictive power. This third approach, however, requires feedback of information with a high frequency, associated with the acquisition of sensors or laboratory analytical costs. It is also necessary to install the actuators allowing the automation of the feed.Furthermore, control optimization approaches require the definition of mathematical constraints (e.g., min and max thresholds on monitoring parameters) and a cost function that depend on manually and empirically adjusted parameters (e.g., weights associated with different criteria). Finally, this approach does not allow for the determination in advance of an inoculation volume representing an economic optimum, because the relationship between inoculation volume and ramp-up time is not explicit.

[0020] The fourth approach takes advantage of an indirect description of microbial growth. Indeed, it is known that the growth of the microbial population follows an exponential law when it is not limited by its energy source (biodegradable organic substrate). To apply this approach, it is necessary to define an initial load, then an increase rate (i.e., exponential increase kinetics) of this load. The initial load can be determined arbitrarily on the basis of feedback. Two methods have thus been proposed. The first method is based on the concept of "active digester volume", initially corresponding to the volume of inoculum present in the digester and which will be the volume on which the increase rate will be applied to determine the volume fed. This first method does not take into account the quality of the inoculum and the substrate (only the volume), which can lead to malfunctions.This defect is partially corrected by the second method, which is based on a "substrate to inoculum" ratio and consists of providing a quantity of substrate corresponding to a ratio of dry matter -DM- (resp. volatile matter -VM-) of the substrate to that present in the digester of between 0.05 and 0.1 gDM / gDM (resp. 0.03 to 0.1 gVM / gVM). From this initial load, it is then possible to determine that of the following days by applying an increase rate. This second method, however, does not take into account the biodegradability of the substrate, nor the inert fraction of the inoculum, which can lead to very conservative scenarios (with an increase in the ramp-up time) or on the contrary too aggressive (risk of acidification).While these approaches allow predicting the kinetics of load ramping a priori, by explicitly linking the amount of inoculum (volume or its amount of MS or MV) with the duration of load ramping, the rate of increase of the applied load is chosen arbitrarily or conservatively, and is therefore probably suboptimal. Indeed, there is no explicit method to determine this rate of increase, the value of which is generally around 5%.

[0021] Furthermore, due to the complex microbial community present in the digester, this community comprising numerous microorganisms each having their own substrate, it is difficult to predict the growth yield and the growth rate of this community in order to use it to control the start-up of a digester. There are models such as ADM1 to represent the growth of these microbial communities ensuring methanization, however they depend on numerous kinetic and stoichiometry parameters and present high risks of overfitting, making their use difficult for predictive purposes. Summary of the invention

[0022] The invention aims to overcome all or part of the aforementioned defects by proposing a method for determining a start-up parameter of a digester, said parameter being representative of a substrate supply to the digester from its start-up up to nominal operating conditions. This determination is based on a prediction of the growth rate while taking into account the quality of the inoculum introduced and of the substrate. The parameter is thus established in a predictive manner, which makes it possible to supply the digester with substrate at its start-up in a safe and rapid manner.

[0023] To this end, a first object of the invention relates to a method for determining a start-up parameter of a digester, said parameter being representative of a substrate supply to the digester from its start-up to nominal operating conditions, the digester to be started being adapted to produce digestate and biogas in the presence of a microbial community, said method comprising the following steps:

[0024] - A step of determining a maximum growth rate of a population microbial population of said microbial community, this microbial population exhibiting limiting growth kinetics under the nominal operating conditions of the digester to be started,

[0025] - A step of estimating the concentration of an inoculum in microbial biomass useful capable of consuming said substrate as a function of a concentration in microbial biomass of the inoculum weighted by a parameter representative of the survival rate of the inoculum in the nominal operating conditions of the digester to be started,

[0026] - A step of determining said start-up parameter, during which one determines a mathematical function expressing said parameter as a function of time, said mathematical function being determined as a function of the previously determined maximum growth rate, the previously estimated useful microbial biomass concentration of the inoculum, an initial inoculation volume, a parameter representative of the biodegradability of the substrate and a feeding and withdrawal regime of the digester.

[0027] The mathematical function determined by the method according to the invention thus takes into account:

[0028] - the quality of the inoculum that will be used, in terms of actual biomass available,

[0029] - the quality of the substrate, in terms of biodegradability, namely the capacity to be degraded and therefore to promote growth, and

[0030] - the specificities of operation of the digester, in terms of feeding regime and extraction of incoming and outgoing flows,

[0031] which makes it possible to considerably reduce the digester's ramp-up time.

[0032] Furthermore, the mathematical function does not take into account operating parameters of the digester such as temperature, pH, quantity of volatile fatty acids formed, salinity, etc., so that it is not necessary to measure these parameters to carry out the ramp-up safely, thus reducing monitoring times and costs related to analyses.

[0033] The limiting microbial population of the microbial community may be known in advance. Otherwise, the method may comprise, upstream of the step of determining the maximum growth rate, a step of identifying a microbial population having the lowest growth rate among said microbial community.

[0034] Advantageously, the method according to the invention may comprise, upstream of the step of estimating the concentration of an inoculum in useful microbial biomass, a step of determining the parameter representative of the survival rate of the inoculum as a function of a concentration of the inoculum in a microbial population in the operating conditions of a digester dedicated to the production of inoculum and as a function of the concentration of the inoculum in the same microbial population in the nominal operating conditions of the digester to be started, the microbial population considered having limiting kinetics in the nominal operating conditions of the digester to be started. This limiting microbial population may in particular be the same as that for which the maximum growth rate is determined, and may possibly be previously identified by means of the aforementioned identification step.

[0035] The microbial biomass concentration of the inoculum can in particular be determined as a function of a parameter representative of a concentration of biodegradable material of the substrate used in a digester dedicated to the production of inoculum, of a reduction rate of this parameter under the operating conditions of the digester dedicated to the production of inoculum and of an overall microbial growth yield under the operating conditions of the digester dedicated to the production of inoculum.

[0036] Advantageously, said parameter representative of a concentration of biodegradable matter in the substrate used in the digester dedicated to the production of inoculum may be a concentration of the inoculum in a parameter chosen from COD, BOD, volatile matter and dry matter.

[0037] In one embodiment, the determination method may further comprise an optimization step during which the mathematical function is used to determine an optimal initial volume of inoculum as a function of one or more parameters chosen from the volume of inoculum, the cost of transporting the inoculum, the methanogenic potential of the substrate, the repurchase price of the biogas, the cost of disposing of the digestate, the cost of recovering the digestate, and revenues related to the treatment of the substrate. This helps reduce the start-up cost of the digester while reducing the total start-up time of the digester.

[0038] The invention also relates to a method for starting and ramping up a digester suitable for producing digestate and biogas in the presence of a microbial community, said method comprising the following steps:

[0039] - A step of inoculum production in a dedicated digester different from the digester to start,

[0040] - A step of introducing an initial volume of inoculum into the digester at to start up,

[0041] - A step of introducing the substrate into the digester to start progressively in the time.

[0042] Said method is characterized in that the step of introducing the substrate uses the start parameter determined by the determination method previously described.

[0043] Another object of the invention relates to a device for determining a start-up parameter of a digester, said parameter being representative of a substrate supply to the digester from its start-up up to nominal operating conditions, the digester to be started being adapted to produce a digestate and biogas in the presence of a microbial community, said device comprising:

[0044] - means for determining a maximum growth rate of a population microbial population of said microbial community, this microbial population exhibiting limiting growth kinetics under the nominal operating conditions of the digester to be started,

[0045] - means for estimating the concentration of an inoculum in biomass useful microbial capable of consuming said substrate as a function of a microbial biomass concentration of the inoculum weighted by a parameter representative of the survival rate of the inoculum under the nominal operating conditions of the digester to be started,

[0046] - means for determining said start-up parameter, during which one determines a mathematical function expressing said parameter as a function of time, said mathematical function being determined as a function of the previously determined maximum growth rate, the previously estimated useful microbial biomass concentration of the inoculum, an initial inoculation volume, a parameter representative of the biodegradability of the substrate and a feeding and withdrawal regime of the digester.

[0047] This device can advantageously be configured, in particular programmed, to implement the different steps of the determination method described above.

[0048] Typically, the device according to the invention here comprises a computer, or more generally at least one processor or any other type of digital calculator.

[0049] The device may also comprise a plurality of separate digital processors or computers, forming different means of the device, cooperating with each other.

[0050] The processor(s) may include storage means which may be random access memory (RAM), electrically erasable programmable read only memory (EEPROM), flash memory, external memory or the like. These storage means may, among other things, store received data, a control model and one or more computer programs.

[0051] The invention also relates to a digester suitable for producing digestate and biogas in the presence of a microbial community, comprising a device for determining a start-up parameter according to the invention. Detailed description of the invention

[0052] Other features and advantages of the invention will emerge from reading the description given below of a particular embodiment of the invention, given for information purposes, but not as a limitation, with reference to the appended drawings in which:

[0053] - [Fig.l] is a flowchart of the determination method according to the invention;

[0054] - [Fig.2] is a curve illustrating the costs of restarting a digester in function of the initial volume of water and inoculum (digestate), used in example 1;

[0055] - [Fig.3] represents the curves of accumulated volatile fatty acids, flow rate of biogas and feed flow rate of example 1 as a function of time; solid lines represent optimal setpoints, dots represent experimental measurements;

[0056] - [Fig.4] is a curve representing the production of methane from acid propionate at 55°C over time. The solid line represents the experimental measurement while the dotted line corresponds to equation 6 of example 2 after adjustment of the parameter

[0057] Definitions / Abbreviations

[0058] The chemical oxygen demand (denoted COD or DCOtot) of a sample is a measure of the totality of oxidizable substances, whether biodegradable or not, in this sample. It is expressed in kg / m3 of sample. COD can be measured according to the NFT 90-101-February 2001 or ISO 6060-1989 standard.

[0059] The biochemical oxygen demand (BOD) of a sample is a measure of the totality of biodegradable oxidizable substances in that sample. It is expressed in kg / m3 of sample. BOD can be measured according to ISO 5815-1:2019.

[0060] Volatile matter (VM), or suspended volatile matter, refers to the part of suspended matter that can be volatilized at 550°C. The determination of the volatile matter content of a sample is carried out by calcining at 550°C the suspended matter obtained after centrifugation and then drying at 105°C. The volatile matter content, expressed in kg / m3 of sample, can be determined by gravimetry according to standard NF T90-105-2: 1997.

[0061] Dry matter (denoted MS) includes both suspended matter and dissolved salts. It is expressed in g / L of sample and can be determined according to standard NF EN 12880- Nov 2000.

[0062] The reduction rate is defined as the percentage reduction in the values ​​of a parameter measured between the inlet and the outlet of a reactor. The reduction rate is calculated by dividing the difference between the value of the input parameter Pe and the output parameter Ps by the value of the parameter Pe at the inlet of the reactor according to the formula ((Pe-Ps) / Pe) xlOO.

[0063] Method for determining a start-up parameter

[0064] The determination method according to the invention aims to determine a start-up parameter of a digester, fed by an input such as sewage treatment plant sludge, biowaste or any other biodegradable organic matter, and producing at the output a digestate and biogas composed essentially of methane and carbon dioxide.

[0065] This start-up parameter is representative of a substrate feed to the digester from its start-up to nominal operating conditions. This parameter may typically be a flow rate of substrate to be introduced into the digester.

[0066] With reference to [Fig. 1], the determination method 10 comprises a step 3 of determining a maximum growth rate, during which a maximum growth rate of a microbial population exhibiting limiting growth kinetics under the nominal operating conditions of the digester is determined.

[0067] By “microbial population” is meant one or more microorganisms, and in particular one or more species of microorganisms.

[0068] By “microbial community” is meant several microbial populations. The microbial community considered here is that present in the nominal operating conditions of the digester to be started.

[0069] Nominal operating conditions are stationary operating conditions of the digester to be started, in which the entire load to be treated is introduced into the digester under conditions allowing the desired quantity of biogas to be produced. These nominal operating conditions therefore vary depending on the load to be treated, the microbial community, and the volume of the digester. These nominal operating conditions thus include the temperature, the pH, the residence time hydraulic, the concentration of total ammoniacal nitrogen produced from the input during digestion, salinity. For example, nominal operating conditions may be a temperature of 30 to 60 °C, a pH of 6.5 to 8.5, an average hydraulic residence time of 5 to 100 days, a concentration of total ammoniacal nitrogen produced from the input during digestion of 0.5 to 10 gN / L and a salinity of 0.01 to 1 mol / L.

[0070] In the digester to be started, several species of microorganisms may be present such as hydrogenotrophic methanogenic archaea, acetoclastic methanogenic archaea or even syntrophic bacteria oxidizing propionate or acetate.

[0071] The species of microorganisms developing in the digester do not have the same growth rate. Thus, certain species will exhibit limiting kinetics because they will have a lower growth rate than certain other species, thus limiting the speed of the overall reaction of the digester.

[0072] In this determination step 3, a maximum growth rate of the microbial population exhibiting limiting growth kinetics is determined, which will then be integrated into the mathematical function for determining the start-up parameter of the digester. In this way, the digester to be started will be fed in such a way as to support the growth rate of the kinetically limiting microbial population, which makes it possible to ensure growth of the entire microbial community with a controlled accumulation of acids.

[0073] The identification of the microorganism(s) exhibiting limiting kinetics can be carried out in several ways, either from databases created or found in the literature, or by experiment, for example from laboratory tests reproducing the operation of the digester under nominal operating conditions.

[0074] The limiting microbial group can for example be identified from the literature (see for example Capson-Tojo et al., 2020, Unraveling the literature chaos around free ammonia inhibition in anaerobic digestion. Renewable and Sustainable Energy Reviews 117, 109487).

[0075] For example, the limiting kinetics associated with a conventional mesophilic wastewater treatment plant sludge digester can be attributed to methanogenic archaea of ​​the Methanosaetaceae family. These organisms have a maximum growth rate of around 0.20 d1 under optimal conditions (De Vrieze et al., 2018, The active microbial community more accurately reflects the anaerobic digestion process: 16S rRNA ( gene ) sequencing as a predictive tool. Microbiome 6), which is reduced to 0.10 d-1 in the presence of 50 mgN.L-1 of NH3, as usually encountered in conventional sludge digesters (Moscoviz et al., 2023, Achieving stable anaerobic mono-digestion of concentrated waste activated sludge without any pretreatment. Bioresource Technology 380, 129114).

[0076] An example of microorganisms exhibiting limiting kinetics is the archaeum Methanosarcina barkeri which is cited in particular in the following article:

[0077] Gloria M. Maestrojuan and David R. Boone. Characterization of Methanosarcina barkeri MST and 227, Methanosarcina mazei S-6T, and Methanosarcina vacuolata 7-761. International Journal of Systematic Bacteriology, 1991.

[0078] Depending on the operating conditions (e.g. temperature, ammoniacal nitrogen concentration N-NH4, salinity), the person skilled in the art can also identify the limiting microbial population and deduce a maximum growth rate, noted hmax*

[0079] Thus, in one embodiment, the determination method 10 may comprise a step 1 of identifying a microbial population exhibiting the lowest growth rate among said microbial community, this identification step 1 being carried out upstream of the step 3 of determining the maximum growth rate.

[0080] Then, the maximum growth rate of the limiting microorganism(s) can be determined from the literature, or as a function of parameters, and in particular as a function of the temperature, the pH and the concentration of inhibitory compound. These factors will in fact more or less slow down or accelerate the reaction.

[0081] The dependence of the growth rate on these factors can be modeled by mathematical models. They are widely described in the literature for many known microorganisms present in digesters.

[0082] Examples include the following article: 1. Angelidaki, L. Ellegaard, and BK Ahring. A Mathematical Model for Dynamic Simulation of Anaerobic Digestion of Complex Substrates: Focusing on Ammonia Inhibition. Biotechnology and Bioengineering, 1993.

[0083] This article presents a mathematical model of the growth rate of 4 groups of microorganisms (glucose-fermenting acidogens, propionate-degrading acetogens, butyrate-degrading acetogens, and acetic methanogens) as a function of pH, temperature, and ammonia inhibition.

[0084] We can also cite as an example the following article: Batstone, DJ, Keller, J., Angelidaki, I., Kalyuzhnyi, SV, Pavlostathis, SG, Rozzi, A., Sanders, WTM, Siegrist, H., Vavilin, VA, 2002. The IWA Anaerobic Digestion Model No 1 (ADM1). Water Science and Technology, 2002.

[0085] This article, for its part, introduces the ADM1 model, an international reference in the profession, which is a dynamic model making it possible to simulate the phenomena governing the different stages of anaerobic digestion by integrating a large number of factors having an influence on the different kinetics, in particular the growth rate of microorganisms.

[0086] In certain embodiments, the method may thus comprise, upstream of the step of determining the maximum growth rate 3, a step of determining 2 the concentration of inhibitor compound which will then be used to determine the maximum growth rate.

[0087] Inhibitory compounds are compounds that affect certain groups of microorganisms present within the digester. Thus, if the concentration of these compounds increases, then the productivity of the digester will greatly decrease.

[0088] The most common inhibitory compounds are hydrogen sulfide (H2S) or hydrogen sulfide ion (HS), but also ammonia (NH3) or ammonium ion (NH4 +). Other inhibitory compounds may be present in the digester such as, for example, heavy metals.

[0089] Examples include the following publications which present inhibitory compounds that may be present in the digester:

[0090] - Ying Jiang, et al. Ammonia inhibition and toxicity in anaerobic digestion: A critical review. Journal of Water Process Engineering, 2019.

[0091] - Qian Guoa, et al. Heavy metals interact with the microbial community and affect biogas production in anaerobic digestion: A review. Journal of Environmental Management, 2019.

[0092] - Hang P. Vu, and al. Hydrogen sulphide management in anaerobic digestion: A critical review on input control, process regulation, and post-treatment. Bioresource Technology, 2021.

[0093] For example, this concentration of inhibitor compound can be determined as described in document WO2023 / 203300.

[0094] A step 5 is then carried out to estimate the concentration of the inoculum in useful microbial biomass capable of consuming the substrate to be treated by the digester to be started. This is thus a concentration of “useful” microbial biomass initially present in the digester to be started.

[0095] This estimate is carried out based on a concentration of microbial biomass in the inoculum weighted by a parameter representative of the survival rate of the inoculum under the nominal operating conditions of the digester to be started.

[0096] The inoculum is produced in a digester dedicated to the production of the inoculum, separate from the digester to be started. This digester dedicated to the production of the inoculum is thus supplied with a substrate which may be different from the substrate used in the digester to be started. The substrate used for the digester dedicated to the production of the inoculum can be sewage treatment plant sludge, biowaste or any other biodegradable organic matter.

[0097] The parameter representing the survival rate of the inoculum under the nominal operating conditions of the digester to be started is a parameter taking into account the survival rate of the microbial biomass present in the inoculum when it is injected into the digester to be started. The latter can in fact operate under conditions very different from those of the digester dedicated to the production of inoculum, which can then lead to a certain mortality of microbial biomass.

[0098] The survival rate typically depends on operating parameters of the digester and in particular the temperature, pH, salinity and / or the ammoniacal nitrogen concentration. Thus, the survival rate typically varies as a function of differences in temperature, and / or pH, and / or salinity and / or ammoniacal nitrogen between the digester dedicated to the production of the inoculum and the digester to be started, in particular when the latter operates under nominal operating conditions.

[0099] This parameter can be determined in different ways. For example, it can be determined from experimental tests or from literature data, as described for example by the publication of Moscoviz et al., 2023 mentioned above.

[0100] The method according to the invention can thus comprise a step 4 of determining the parameter representative of the survival rate of the inoculum as a function of:

[0101] - of a concentration of the inoculum in a microbial population in the operating conditions of the digester dedicated to the production of inoculum and

[0102] - of the concentration of the inoculum in the same microbial population in the nominal operating conditions of the digester to be started.

[0103] The microbial population considered has limiting kinetics under the nominal operating conditions of the digester to be started. It may therefore be the same microbial population as that for which the maximum growth rate has been determined.

[0104] The survival rate can in particular be expressed as the ratio of the concentration of the inoculum in a microbial population under the nominal operating conditions of the digester to be started to the concentration of the inoculum in the same microbial population under the operating conditions of the digester dedicated to the production of inoculum.

[0105] Most often, this parameter representing the survival rate can thus take a value included in the interval ]0; 1]. It is then equal to 1 when the survival rate is maximum (zero mortality rate), for example when the operating conditions of the digester to be started are similar to the operating conditions of the digester dedicated to the production of inoculum. Conversely, when the conditions are very different, we observes a certain mortality of the microbial population: the survival rate is then less than 1, more or less close to 0 depending on whether the nominal operating conditions are more or less favorable to the growth of this microbial population.

[0106] The microbial biomass concentration of the inoculum can be determined as a function of a parameter representative of a concentration of biodegradable matter of the substrate used in the digester dedicated to the production of inoculum, of a reduction rate of this parameter under the operating conditions of the digester dedicated to the production of inoculum and of an overall microbial growth yield under the operating conditions of the digester dedicated to the production of inoculum.

[0107] The parameter representative of a concentration of biodegradable matter in the substrate used in the digester dedicated to the production of inoculum can be chosen from a concentration of the inoculum in COD, BOD, volatile matter or dry matter.

[0108] The overall microbial growth yield can be determined from literature data. An example is the following publication: Moscoviz, R., Jimenez, J., 2021. Improving anaerobic digestion mass balance calculations through stoichiometry and usual substrate characterization. Bioresource Technology 337, 125402.

[0109] Thus, by way of example, the concentration of the inoculum in useful microbial biomass can be expressed by the following formula:

[0110] [Math.sl] ^inoc — MVaHmentation ' shot down ' @ (Eql)

[0111] with Xinoc: the concentration of useful microbial biomass present in the inoculum (gX.L1), MVaümentation: the concentration of volatile matter in the substrate used in the digester from which the inoculum comes (gMV.L1), %MVabattue is the fraction of MV slaughtered in this same digester, Y is the overall growth yield of the microbial community on this substrate (gX.gMV *) in this same digester and a represents a survival rate of the microbial biomass whose value is included in the interval ]0; 1].

[0112] This formula could also be implemented by replacing the volatile matter concentration with the BOD, COD or dry matter concentration of the substrate used in the digester dedicated to the production of inoculum.

[0113] The concentration of useful microbial biomass thus estimated corresponds to an initial concentration of biomass which can then be used to explicitly calculate the quantity of substrate necessary to ensure its exponential growth according to the maximum growth rate qmax previously determined.

[0114] Thus, during the step 6 of determining the start-up parameter, a mathematical function is determined expressing, as a function of time, the start-up parameter representative of a substrate feed to be introduced into the digester to be started, from its start-up up to nominal operating conditions.

[0115] This mathematical function is determined as a function of the previously determined maximum growth rate, the previously estimated useful microbial biomass concentration of the inoculum, an initial inoculation volume, a parameter representative of the biodegradability of the substrate and a feeding and withdrawal regime of the digester.

[0116] The substrate to be introduced into the digester to be started may contain a so-called inert fraction which will not be degraded in the digester. Only the biodegradable fraction present in the substrate will support microbial growth.

[0117] The parameter representative of the biodegradability of the substrate may for example be a concentration of biodegradable matter, for example a concentration of the substrate in COD, BOD, volatile matter or dry matter.

[0118] This parameter representative of the biodegradability of the substrate can be measured experimentally (for example by methanogenic potential tests) or estimated using indirect methods (eg prediction method from near-infrared spectra). By taking into account the biodegradability of the substrate, it is possible to explicitly determine the start-up parameter of the digester as a function of time, ensuring growth with a growth rate qmax.

[0119]

[0120]

[0121] This determination can typically be made by a material balance carried out on the process, depending on its feed and withdrawal regime. For example, in the case of feeding a fed-batch process (i.e. introducing the substrate into the digester continuously, without withdrawal), it can be shown that the mathematical function is written: [Math.s2] n (To — ..U t U If I — yv • C ahm\ / J x^biaieg (Eq2)

[0122] with Qaiim the substrate feed rate (m3.j '), qmax the maximum growth rate of the microbial group having limiting kinetics (j '), Vo the initial inoculum volume (m3), Xinoc the concentration of useful microbial biomass in the inoculum (gX.L1), Yx is the overall growth yield of the microbial community on the substrate used during the ramp-up (gX.gMV ') and SbiOdeg is the concentration of degradable matter in the substrate used during the ramp-up (gMV.L1).

[0123] A similar mathematical function can be established for a continuously operating process (i.e. continuous substrate feeding and continuous withdrawal), the mathematical function being able to be written as follows:

[0124] [Math.s3] (Eq 3)

[0125] with Qaiim the substrate feed rate (m3.j '), qmax the maximum growth rate of the microbial group having limiting kinetics (j '), Vo the initial inoculum volume (m3), Vtot the working volume of the methanizer (m3), Xinoc the concentration of useful microbial biomass in the inoculum (gX.L1), Yx is the overall growth yield of the microbial community on the substrate used during the ramp-up (gX.gMV1) and SbiOdeg is the concentration of degradable matter in the substrate used during the ramp-up (gMV.L1).

[0126] The invention is however not limited to the forms of the mathematical functions (Eq2, Eq3) previously described. The form of the mathematical function will in fact depend on the feeding and withdrawal regime of the digester to be started, a mathematical function specific to each feeding and withdrawal regime being able to be easily determined by a person skilled in the art by carrying out a material balance taking into account the supply of input (substrate), the possible withdrawal of digestate,

[0127] Whatever the embodiment, the time mathematical function is advantageously determined by material balance. For example, in the case of an infinitely mixed reactor, the time mathematical function is determined according to the following equation system, solved as a function of the feed and withdrawal mode of the digester to be started and the initial conditions of the ramp-up:

[0128] [Math.s4] dS _ y । o Qsoutirage^] dt — " FY ' + v( / ) ' ^biodeg " ^) ' «J dX _ Qsoutirag^} dt ~^max'A + v(t) ' y( / ) A (Eq4)

[0129] with Qaiim the substrate feed rate (m3.j '), Qsoutirage the digestate withdrawal rate (m3.j '), qmax the maximum growth rate of the microbial group having limiting kinetics (j '), V the filling volume of the digester at time t (m3), Xa,im the concentration of useful microbial biomass in the substrate used during the ramp-up (gX.L1), X the concentration of useful microbial biomass present in the digester at time t (gX.L '), Yx is the overall growth yield of the microbial community on the substrate used during the ramp-up (gX.gMV1), Sbiodeg is the concentration of degradable matter in the substrate used during the ramp-up (gMV.L4) and S the concentration of degradable matter remaining in the digester at time t (gMV.L1).

[0130] The equation system detailed above can in particular be used to determine the mathematical function, whatever the feed and withdrawal regime of the digester.

[0131] It is then possible to use the mathematical function which has just been determined in order to analytically identify the volume of inoculum making it possible to achieve a techno-economic optimum depending on the constraints of the site.

[0132] The determination method 10 can thus comprise an optimization step 7 during which the mathematical function is used to determine an optimal initial volume of inoculum as a function of one or more parameters chosen from the volume of inoculum, the cost of transporting the inoculum, the methanogenic potential of the substrate, the repurchase price of the biogas, the cost of disposing of the digestate, the cost of recovering the digestate, and revenues linked to the treatment of the substrate.

[0133] Start-up and ramp-up process

[0134] The start-up parameter determined by the method according to the invention can be used when starting up any digester suitable for producing digestate and biogas, in particular methane, in the presence of a microbial community.

[0135] For example, the substrate supply may be gradually increased, this increase being carried out, for example, at determined time intervals.

[0136] For example, the initial volume of inoculum can be introduced at t=0.

[0137] Then, at t= At, the substrate is introduced at a flow rate calculated using the mathematical function for the time t= At. It will be possible to continue to gradually increase the substrate flow rate at each time interval At, this flow rate being calculated using the mathematical function, and to continue in this way until the substrate flow rate entering the digester is equal to the flow rate under the nominal operating conditions. For example, time intervals At of the order of a day may be used, in order to avoid changing the feeding instructions too often. The invention is however not limited to a specific time interval which may be adapted according to the constraints of a site. Examples

[0138] Example 1

[0139] The method of the invention was applied to a conventional mesophilic digester for treating sewage treatment plant sludge.

[0140] For this type of digester, the Methanosaetaceae family is identified as the microbial group with the limiting kinetics, with an associated qmax of 0.10 d *. To inoculate this digester, digested sludge from another digester (digester dedicated to inoculum production) that operates under conditions similar to those expected from the conventional mesophilic digester was used.

[0141] For the calculation of the concentration of useful microbial biomass, a value of 1 is thus retained for a.

[0142] Monitoring the operation of the digester producing the inoculum indicates that MVaiimPnMinn = 20 gMV.L-1 and %MVabattue = 0.39. Combined with a value of Y = 0.1 gX.gMV ', it is possible to estimate by applying equation 1 (Eq 1) that Xinoc = 0.8 gX.L *.

[0143] Finally, the concentration of biodegradable material in the substrate used for the ramp-up was estimated by a methanogenic potential (BMP) test, with an average value of 30 gMV.L *.

[0144] The maximum sludge flow rate available on site is 140 m3.d *. The ramp-up is therefore considered complete when this feed rate is reached. All of these elements make it possible to generate a mathematical ramp-up function, here in fed-batch feed:

[0145] [Math.s5] ^alink / 75 (Eq 5)

[0146] Finally, taking into account this equation and the constraints of the methanizer (minimum volume for heating / agitation, maximum volume before overflow), the costs associated with the transport of the inoculum (19 €.m3), the repurchase price of the biomethane (113 €.MWh ') and the cost of sludge disposal (244 €.tMS '), optimal volumes of 2200 m3 of water and 1400 m3 of inoculum are identified (see [Fig.2]).

[0147] These initial conditions as well as the ramp-up profile were then applied to the reactor (see [Fig.3]). For this purpose, the substrate feed rate is modified every day: equation 5 is thus used to calculate the new flow rate each day (i.e. At = 1 day).

[0148] The maximum load is reached after 21 days, a little later than the optimal profile (17 days) due to operational constraints. No accumulation of volatile fatty acids above 0.3 gAcetate.L 1 could be observed, indicating the good stability of the methanizer during the load ramp-up.

[0149] The ramp-up time should be compared with typical times observed on an industrial scale. For example, the following publication: Batstone et al. (2010) Model assisted startup of anaerobic digesters fed with thermally hydrolysed activated sludge. Water Science and Technology 62, 1661-1666, reports a time of 100 days for ramping up a mesophilic digester fed with thermally treated sludge. The publication Hatzigeorgiou et al., Startup of Anaerobic Mesophilic Digesters. proc water environ fed 2006, 415-428 reports a time of 77 days to start up a conventional mesophilic digester treating wastewater treatment plant sludge.

[0150] Example 2: example of determination of the parameter a for the case of a transition from mesophilic conditions to thermophilic conditions

[0151] The parameter a is a corrective factor between 0 and 1 which takes into account the need or not to adapt the inoculum for the inoculated methanizer. Biologically, it represents a survival rate of the microbial population present in the inoculum and having limiting kinetics, when it is exposed to the environmental conditions of the new methanizer that we wish to increase in load. It is possible to experimentally measure this factor a using laboratory tests.

[0152] In this example, the inoculum is taken from a methanizer treating sewage treatment plant sludge under mesophilic conditions (37°C). The methanizer that we wish to ramp up also treats sewage treatment plant sludge, but at a higher temperature (55°C) corresponding to thermophilic conditions. The inoculum is taken and distributed in identical 0.5 L reactors, half of the reactors being operated at 37°C, and the other half at 55°C. For each temperature, the reactors can be separated into two groups of three experimental repetitions, each group treating a different methanization substrate. The two substrates considered here are acetic acid and propionic acid, because they represent the typical substrates of populations having limiting kinetics during methanization.These substrates are supplied at a concentration sufficient to make the measurement of biogas flow rates reliable, and low enough not to cause any toxic effects. For these tests, a concentration of 2 gCOD / L was chosen. After contacting the inoculum with the substrate, the reactors are inerted with nitrogen to ensure anaerobiosis, and equilibrated to the desired temperature. They are then operated in batch mode while measuring the methane production rate over time.

[0153] Once methane production is complete, the cumulative methane production curves as a function of time can be modeled to determine the parameters XOj_T (concentration of useful microbial biomass in the inoculum capable of consuming acid i at temperature T) and qmax _i T (maximum growth rate of the biomass consuming acid i at temperature T). This is made possible by noting that methane production is proportional to microbial growth. Thus, the growth rate observed on the cumulative methane production curve corresponds directly to qmax _iT. Once this growth rate is determined, a least squares approach is implemented to estimate XOj_T, for example using the following model:

[0154] [Math.s6] yyYS iT / \ i ~ 350 ' V reactor ' yYS t T ' ^0 i T ' (i T ) (Eq. 6)

[0155] with VCH4 i T 'c cumulative methane volume (NmL), V reactor the useful volume of the reactor (L), YXS i T the growth yield (gCOD / gCOD), ^maxj^r the maximum growth rate (j '), t the time (j) and XqjT the initial concentration of useful biomass (gCOD / L).

[0156] An application of this model to a methane production curve from propionic acid under thermophilic conditions is shown [Fig.4].

[0157] In the case studied, the population having the slowest growth in the conditions of the methanizer to be ramped up (i.e. the lowest j_55) is the one consuming propionic acid. Indeed, the value of f^max propionic 55 is on average 0.4 j1 while a value of 1.2 j1 is measured for ^max acetic 55- By least squares approach, the value of ^^ propionic 55 is approximately 105 g COD / L. The same approach applied to reactors operated at 37 °C indicates a Xq propionic 37 of 0.2 g COD / L. Thus, in this example, the parameter a can be calculated as the ratio of X(^propionic_v to ^0^^^55, i.e. 5. 105. This result indicates that during the transition from 37 to 55 °C, only 0.005% of the microbial population capable of consuming propionic acid and present in the inoculum was able to survive.

Claims

Claims

1. A computer-implemented method for determining a start-up parameter of a digester, said parameter being representative of a substrate feed to the digester from its start-up to nominal operating conditions, the digester to be started being adapted to produce digestate and biogas in the presence of a microbial community, said method comprising the following steps: - A step of determining a maximum growth rate of a microbial population of said microbial community, this microbial population having limiting growth kinetics under the nominal operating conditions of the digester to be started,- A step of estimating the concentration of an inoculum in useful microbial biomass capable of consuming said substrate as a function of a microbial biomass concentration of the inoculum weighted by a parameter representative of the survival rate of the inoculum under the nominal operating conditions of the digester to be started, - A step of determining said start-up parameter, during which a mathematical function is determined expressing said parameter as a function of time, said mathematical function being determined as a function of the previously determined maximum growth rate, the previously estimated useful microbial biomass concentration of the inoculum, an initial inoculation volume, a parameter representative of the biodegradability of the substrate and a feeding and withdrawal regime of the digester.,

2. Determination method according to claim 1, characterized in that it comprises, upstream of the step of determining a maximum growth rate: - a step of identifying a microbial population exhibiting the lowest growth rate among said microbial community.

3. Determination method according to claim 1 or 2, characterized in that it comprises, upstream of the step of estimating the concentration of an inoculum in useful microbial biomass: - a step of determining the parameter representative of the survival rate of the inoculum as a function of a concentration of the inoculum in a microbial population in the operating conditions of a digester dedicated to the production of inoculum and as a function of the concentration of the inoculum in the same microbial population in the nominal operating conditions of the digester to be started, the microbial population considered having limiting kinetics in the nominal operating conditions of the digester to be started.

4. Determination method according to any one of claims 1 to 3, characterized in that the microbial biomass concentration of the inoculum is determined as a function of a parameter representative of a concentration of biodegradable material of the substrate used in a digester dedicated to the production of inoculum, of a reduction rate of this parameter under the operating conditions of the digester dedicated to the production of inoculum and of an overall microbial growth yield under the operating conditions of the digester dedicated to the production of inoculum.

5. Determination method according to claim 4, characterized in that said parameter representative of a concentration of biodegradable matter of the substrate used in the digester dedicated to the production of inoculum is a concentration of this substrate in a parameter chosen from COD, BOD, volatile matter and dry matter.

6. Determination method according to any one of claims 1 to 5, characterized in that it further comprises: - an optimization step during which the mathematical function is used to determine an optimal initial volume of inoculum as a function of one or more parameters chosen from the volume of inoculum, the cost of transporting the inoculum, the methanogenic potential of the substrate, the repurchase price of the biogas, the cost of disposing of the digestate, the cost of recovering the digestate, and income linked to the treatment of the substrate.

7. Method for starting and ramping up a digester suitable for producing digestate and biogas in the presence of a

8. microbial community, said method comprising the following steps: - An inoculum production step in a dedicated digester different from the digester to be started, - A step of introducing an initial volume of inoculum into the digester to be started, - A step of introducing the substrate into the digester to start gradually over time, said method being characterized in that the step of introducing the substrate uses the start parameter determined by the method according to any one of claims 1 to 6. Device for determining a start-up parameter of a digester, said parameter being representative of a substrate supply to the digester from its start-up to nominal operating conditions, the digester to be started being adapted to produce digestate and biogas in the presence of a microbial community, said device comprising: - means for determining a maximum growth rate of a microbial population of said microbial community, this microbial population exhibiting limiting growth kinetics under the nominal operating conditions of the digester to be started, - means for estimating the concentration of an inoculum in useful microbial biomass capable of consuming said substrate as a function of a concentration in microbial biomass of the inoculum weighted by a parameter representative of the survival rate of the inoculum under the nominal operating conditions of the digester to be started, - means for determining said start-up parameter, during which a mathematical function is determined expressing said parameter as a function of time, said mathematical function being determined as a function of the previously determined maximum growth rate, the previously estimated useful microbial biomass concentration of the inoculum, an initial inoculation volume, a parameter representative of the biodegradability of the substrate and a feeding and withdrawal regime of the digester. 24

9. Digester adapted to produce digestate and biogas in the presence of a microbial community, comprising a device for determining a start-up parameter according to claim 8.

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