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

Abstract

The invention relates to a method for determining a start-up parameter of a digester suitable for producing a digestate and biogas in the presence of a microbial community. This parameter represents a supply of substrate for the digester from the start-up of the digester until nominal operating conditions are reached. 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 a useful microbial biomass inoculum capable of consuming the substrate, taking into account a parameter which represents the survival rate of the inoculum under the nominal operating conditions of the digester. The method then determines a mathematical function expressing said parameter as a function of time, this function being determined on the basis of the maximum growth rate, the concentration of useful microbial biomass in the inoculum, an initial inoculation volume, a parameter which represents the biodegradability of the substrate, and a rate of supplying the digester and withdrawing the digester.

Description

Description Title: Method for determining a start-up parameter of a digester and method for starting and ramping up a corresponding digester Field of invention [1] The invention relates to the field of digesters, also called biogas reactors or methanizers. [2] 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 [3] The objective of a digester is to produce biogas by anaerobic digestion. [4] Anaerobic digestion is a cascade of biochemical reactions that convert organic matter present in the digester into biogas, which is mainly a mixture of carbon dioxide and methane. The remaining material is called digestate. [5] This process is inherently complex because: [6] - each degradation step is carried out by distinct groups of microorganisms, each having their own kinetics, optimal growth conditions and specific inhibitory compounds; [7] - these groups of microorganisms interact with each other, which can result, for example, in phenomena of competition or syntrophy; [8] - the operating conditions of the process, such as the feeding method as well as the nature of the input or even the temperature, strongly influence the kinetics of these groups of microorganisms. [9] 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 in 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 m 3 , so that the digester can be fully inoculated, without logistical constraints. On an industrial scale, however, the volume of the reactors can vary from 2000 to 10000 m 3 so that inoculation generates significant constraints in terms of costs and logistics.

[0011] Scaling is usually implemented according to a strategy conventional method which consists of gradually increasing the organic matter supply at each feeding cycle while maintaining optimal methanogenic activity. The increase in load 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, an excessively rapid increase in load risks causing an accumulation of intermediate products (volatile fatty acids) which can acidify the reactor, potentially to the point of completely inhibiting the functioning of microorganisms which can consume these acids.This scenario is feared by methanizer operators, who have consequently historically implemented conservative but sub-optimal approaches to scaling up.

[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 escalation based on stability or performance measurements, consisting first of applying a low load, then gradually increasing it when stability or performance objectives have been achieved,

[0015] - progressive and automated ramp-up, assisted by modeling, based on stability or performance measurements,

[0016] - progressive increase in load predicted by an empirical model using an 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 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(0H)2, MgO, NILOH or NaHCCh. 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 ramp-up period can represent an additional cost with regard to digestate management (e.g., malfunctions, disposal, non-standard discharges).

[0018] The second approach avoids acid buildup 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 (TAS), and biogas production. Many criteria can thus be formulated, but all are based on quantities measured experimentally during monitoring of the methanizer. The initial load and 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 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., dehydration of digestate, recovery of biogas) also require start-up with their own deadlines. 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 duration 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 high-frequency feedback, 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 depends 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 based on 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 ones. (risk of acidification). While these approaches allow the kinetics of load increase to be predicted a priori, by explicitly linking the quantity of inoculum (volume or its quantity of DM or MV) with the duration of load increase, the rate of increase of the applied load is chosen arbitrarily or conservatively, and is therefore probably suboptimal. Indeed, there is no explicit method for determining 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 many microorganisms each having their own substrate, it is difficult to predict the growth yield and growth rate of this community in order to use it to control the start-up of a digester. There are models such as ADMl to represent the growth of these microbial communities ensuring methanization, however they depend on many 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 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 subject 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 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 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 in the nominal operating conditions of the digester to be started,

[0026] - 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.

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

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

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

[0030] - the specific operating features of the digester, in terms of feeding regime and withdrawal of incoming and outgoing flows,

[0031] which makes it possible to considerably reduce the digester's ramp-up time, particularly regardless of the biodegradability of the substrate.

[0032] Furthermore, the mathematical function does not take into account digester operating parameters such as temperature, pH, amount 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] Thus, the only variable in the mathematical function is time, the other parameters taken into account being constants, determined before the digester is started. The method according to the invention therefore does not require measurements to be taken in real time or at regular time intervals during start-up. It also allows a truly predictive approach to digester start-up, which can be implemented before the digester is started and thus allows for advance planning of the digester ramp-up.

[0034] The method according to the invention is advantageously capable of being implemented by computer.

[0035] 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 exhibiting the lowest growth rate among said microbial community.

[0036] 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.

[0037] The microbial biomass concentration of the inoculum can in particular be determined as a function of a parameter representative of a concentration of biodegradable matter of the substrate used in a digester dedicated to the production of inoculum, of a reduction rate of this parameter in the operating conditions of the digester dedicated to the production of inoculum and an overall microbial growth yield under the operating conditions of the digester dedicated to the production of inoculum.

[0038] 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.

[0039] 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 makes it possible to reduce the start-up cost of the digester while reducing the total start-up time of the digester.

[0040] 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:

[0041] - An inoculum production step in a dedicated digester different from the digester to be started,

[0042] - A step of introducing an initial volume of inoculum into the digester to be started,

[0043] - A step of introducing the substrate into the digester to start gradually over time.

[0044] Said method is characterized in that the step of introducing the substrate uses the start parameter determined by the determination method previously described.

[0045] Another subject 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 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:

[0046] - 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,

[0047] - 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,

[0048] - 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 digester feeding and withdrawal regime.

[0049] This device can advantageously be configured, in particular programmed, to implement the different stages of the determination method described above.

[0050] 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.

[0051] The device may also comprise a plurality of separate digital processors or computers, forming different means of the device, cooperating with each other.

[0052] 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.

[0053] 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

[0054] 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:

[0055] - [Fig. 1] is a flowchart of the determination method according to the invention;

[0056] - [Fig. 2] is a curve illustrating the costs of restarting a digester as a function of the initial volume of water and inoculum (digestate), used in example 1;

[0057] - [Fig. 3] represents the curves of accumulated volatile fatty acids, biogas flow rate and feed flow rate of example 1 as a function of time; the solid lines represent the optimal setpoints, the points represent the experimental measurements;

[0058] - [Fig. 4] is a curve representing the production of methane from propionate acid 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 y 71 Q_prop ionic _55 •

[0059] Definitions / Abbreviations

[0060] The chemical oxygen demand (COD or totCOD) of a sample is a measure of the total oxidizable substances, whether biodegradable or not, in that 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.

[0061] The biochemical oxygen demand (BOD) of a sample is a measure of the total amount of biodegradable oxidizable substances in the sample. It is expressed in kg / m3 of sample. BOD can be measured according to ISO 5815-1:2019.

[0062] 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 gravimetrically according to the standard NF T90-105-2: 1997.

[0063] Dry matter (DM) 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.

[0064] The reduction rate is defined as the percentage reduction in the values ​​of a parameter measured between the inlet and 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.

[0065] Method for determining a startup parameter

[0066] 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.

[0067] This start-up parameter is representative of a substrate feed to the digester from its start-up to nominal operating conditions. This parameter can typically be a flow rate of substrate to be introduced into the digester.

[0068] With reference to Figure 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.

[0069] By "microbial population" we mean one or more microorganisms, and in particular one or more species of microorganisms.

[0070] By "microbial community" we mean several microbial populations. The microbial community considered here is that present in the nominal operating conditions of the digester to be started.

[0071] 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 according to the load to be treated, the microbial community, and the volume of the digester. These nominal operating conditions thus include temperature, pH, hydraulic residence time, the concentration of total ammoniacal nitrogen produced from the input during digestion, and 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.

[0072] 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. These different species of microorganisms are likely to exhibit limiting kinetics depending on the operating conditions and the substrate.

[0073] The species of microorganisms growing in the digester do not have the same growth rate. Thus, some species will exhibit limiting kinetics because they will have a slower growth rate than certain other species, thus limiting the speed of the overall reaction of the digester.

[0074] 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 used to determine 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, thus ensuring growth of the entire microbial community with a controlled accumulation of acids.

[0075] A growth rate represents the change in cell number or mass per unit of time. It is usually expressed in hours. 1 (h' 1 ) or day 1 (I 1 ).

[0076] 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 experience, for example from laboratory tests reproducing the operation of the digester under nominal operating conditions.

[0077] The limiting microbial group can for example be identified from the literature (see e.g. Capson-Tojo et al., 2020, Unraveling the literature chaos around free ammonia inhibition in anaerobic digestion. Renewable and Sustainable Energy Reviews 117, 109487).

[0078] For example, the limiting kinetics associated with a conventional mesophilic wastewater sludge digester can be attributed to methanogenic archaea of ​​the family Methanosaetaceae. These organisms have a maximum growth rate of about 0.20 days. 1under 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).

[0079] An example of microorganisms exhibiting limiting kinetics is the archaeum Methanosarcina barkeri, which is cited in particular in the following article:

[0080] 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.

[0081] 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 gmax.

[0082] 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.

[0083] Then, the maximum growth rate of the limiting microorganism(s) can be determined from the literature, or based on parameters, including temperature, pH, and the concentration of inhibitory compound. These factors will in fact slow down or accelerate the reaction to a greater or lesser extent.

[0084] 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.

[0085] We can cite for example the following article: I. 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.

[0086] This paper presents a mathematical model of the growth rate of 4 groups of microorganisms (glucose-fermenting acidogens, propionate-degrading acetogens, butyrate-degrading acetogens, and acetic acid methanogens) as a function of pH, temperature, and ammonia inhibition.

[0087] The following article can also be cited as an example: Batstone, DJ, Keller, J., Angelidaki, 1, 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.

[0088] This article, for its part, introduces the ADM1 model, an international reference in the field, which is a dynamic model allowing the simulation of 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.

[0089] 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 inhibitory compound which will then be used to determine the maximum growth rate.

[0090] Inhibitory compounds are compounds that affect certain groups of microorganisms present within the digester. Therefore, if the concentration of these compounds increases, the productivity of the digester will greatly decrease.

[0091] The most common inhibitory compounds are hydrogen sulfide (LES) or hydrogen sulfide ion (HS'), but also ammonia (NEE) or ammonium ion (NH4 + ). Other inhibitory compounds may be present in the digester, such as heavy metals.

[0092] Examples include the following publications which present inhibitory compounds that may be present in the digester:

[0093] - Ying Jiang, et al. Ammonia inhibition and toxicity in anaerobic digestion: A critical review. Journal of Water Process Engineering, 2019.

[0094] - Qian Guoa, et al. Heavy metals interact with the microbial community and affect biogas production in anaerobic digestion: A review. Journal of Environmental Management,

[0095] - Hang P. Vu, et al. Hydrogen sulphide management in anaerobic digestion: A critical review on input control, process regulation, and post-treatment. Bioresource Technology, 2021.

[0096] For example, this concentration of inhibitor compound can be determined as described in document WO2023 / 203300.

[0097] We then proceed to a step 5 of estimating 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 therefore a concentration of “useful” microbial biomass initially present in the digester to be started.

[0098] This estimate is made 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.

[0099] The inoculum is produced in a dedicated inoculum digester, separate from the digester to be started. This inoculum digester is thus supplied with a substrate that may be different from the substrate used in the digester to be started. The substrate used for the inoculum digester may be wastewater treatment plant sludge, biowaste, or any other biodegradable organic matter.

[0100] 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.

[0101] The survival rate typically depends on digester operating parameters, including temperature, pH, salinity, and / or ammonia nitrogen concentration. Thus, the survival rate typically varies depending on differences in temperature, and / or pH, and / or salinity, and / or ammonia nitrogen between the digester dedicated to inoculum production and the digester to be started, especially when the latter is operating under nominal operating conditions.

[0102] 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.

[0103] 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:

[0104] - a concentration of the inoculum in a microbial population in the operating conditions of the digester dedicated to the production of inoculum and

[0105] - the concentration of the inoculum in the same microbial population under the nominal operating conditions of the digester to be started.

[0106] The microbial population considered has limiting kinetics under the nominal operating conditions of the digester to be started. It could therefore be the same microbial population as that for which the maximum growth rate was determined.

[0107] The survival rate can be expressed in particular as the ratio of the concentration of the inoculum into a microbial population under the nominal operating conditions of the digester to start on the concentration of the inoculum into the same microbial population under the operating conditions of the digester dedicated to the production of inoculum.

[0108] 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, a certain mortality of the microbial population is observed: 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.

[0109] 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.

[0110] The parameter representing 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.

[0111] Overall microbial growth efficiency 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.

[0112] Thus, as an example, the concentration of the inoculum in useful microbial biomass can be expressed by the following formula:

[0113] [Maths 1] ^inoc ^^feed ' ° ° slaughtered ' Y ' Ct (Eq 1)

[0114] with Xinoc: the concentration of useful microbial biomass present in the inoculum (gX.L -1 ), M Vaiimentation: the volatile matter concentration of the substrate used in the digester from which the inoculum comes (gMV.L -1), %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 1 ) in this same digester and a represents a survival rate of the microbial biomass whose value is included in the interval ]0; 1],

[0115] 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 inoculum production.

[0116] The useful microbial biomass concentration thus estimated corresponds to an initial biomass concentration which can then be used to explicitly calculate the quantity of substrate necessary to ensure its exponential growth according to the maximum growth rate (J-max previously determined.

[0117] Thus, during the determination step 6 of the start parameter, we determines a mathematical function expressing, as a function of time, the starting parameter representative of a substrate feed to be introduced into the digester to be started, from its start-up to nominal operating conditions.

[0118] This mathematical function is determined based on 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.

[0119] The initial inoculation volume, the parameter representing the biodegradability of the substrate and the feed and withdrawal regime of the digester are parameters whose values ​​are determined or fixed before the start-up of the digester and do not vary during start-up. The same applies to the previously determined maximum growth rate and the previously estimated useful microbial biomass concentration of the inoculum. It is therefore not necessary to plan measurements of these parameters during the start-up of the digester. The determination of the mathematical function is therefore simple and does not require additional data processing during the start-up of the reactor.

[0120] In one embodiment, the method according to the invention can thus comprise, prior to the step of determining the mathematical function, a step of determining and / or receiving the initial inoculation volume, the parameter representative of the biodegradability of the substrate and the feeding and withdrawal regime of the digester, for example implemented by determination and / or receiving means.

[0121] 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.

[0122] The parameter representing 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.

[0123] This parameter representative of substrate biodegradability can be measured experimentally (e.g. by methanogenic potential tests) or estimated using indirect methods (e.g. 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 (J-max.

[0124] This determination can typically be made by a material balance carried out on the process, depending on its feed and withdrawal regime.

[0125] 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:

[0126] [Maths 2] (Eq 2)

[0127] with Qaiimle substrate feed rate (m 3 .j _1 ), p ma x the maximum growth rate of the microbial group with limiting kinetics (f 1 ), Vo the initial volume of inoculum (m 3 ), X inoc the concentration of useful microbial biomass in the inoculum (gX.L -1 ), Yx is the overall growth yield of the microbial community on the substrate used during the ramp-up (gX.gMV 1 ) and Sbiodeg is the concentration of degradable matter in the substrate used during the ramp-up (gMV.L -1 ). Note that Vo, Xinoc, Yx, Sbiodeg are constants whose values ​​are known at the start of the digester and which are not modified during start-up. The only variable in equation 2 is therefore time.

[0128] An analogous 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:

[0129] [Maths 3]

[0130] with Qaiimle substrate feed rate (m 3 .j -1 ), p ma x the maximum growth rate of the microbial group with limiting kinetics (f 1 ), Vo the initial volume of inoculum (m 3 ), Vtot the working volume of the methanizer (m 3 ), Xinoc the concentration of useful microbial biomass in the inoculum (gX.L -1 ), Yx is the overall growth yield of the microbial community on the substrate used during the ramp-up (gX.gMV -1 ) and Sbiodeg is the concentration of degradable matter in the substrate used during the ramp-up (gMV.L -1). In this equation 3, the only variable is time.

[0131] 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,

[0132] Regardless of the embodiment, the mathematical time function is advantageously determined by material balance. For example, in the case of an infinitely mixed reactor, the mathematical time function is determined according to the following equation system, solved according to the feed and withdrawal mode of the digester to be started and the initial conditions of the ramp-up:

[0133] [Maths 4]

[0134] with Qaiim the substrate feed rate (m 3 .]' 1 ), Qsoutirage the digestate withdrawal flow rate (m 3 .]' 1 ), p ma x the maximum growth rate of the microbial group having limiting kinetics V the filling volume of the digester at time t (m 3 ), Xaiim the concentration of useful microbial biomass in the substrate used during the ramp-up (gX.L -1), X the concentration of useful microbial biomass present in the digester at time t (gX.L- x ), Yx is the overall growth yield of the microbial community on the substrate used during the ramp-up (gX.gMV 1 ), Sbiodeg is the concentration of matter degradable substrate used during the ramp-up (gMV.L -1 ) and S the concentration of degradable matter remaining in the digester at time t (gMV.L' 1 ).

[0135] The equation system detailed above can be used in particular to determine the mathematical function, regardless of the digester feed and withdrawal regime.

[0136] It is then possible to use the mathematical function which has just been determined in order to analytically identify the volume of inoculum allowing a techno-economic optimum to be reached depending on the constraints of the site.

[0137] 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.

[0138] Start-up and ramp-up process

[0139] 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.

[0140] For example, the substrate supply can be gradually increased, perhaps at set time intervals.

[0141] For example, we can introduce the initial volume of inoculum at t=0.

[0142] Then, at t= At, the substrate is introduced at a rate calculated using the mathematical function for time t= At. The substrate rate can be continued to be gradually increased at each time interval At, this rate being calculated using the mathematical function, and this can be continued until the substrate rate entering the digester is equal to the rate under the nominal operating conditions. For example, time intervals At of the order of a day can be used, in order to avoid changing the feed instructions too often. The invention is however not limited to a specific time interval which can be adapted according to the constraints of a site. Examples

[0143] Example 1

[0144] The method of the invention was applied to a conventional mesophilic digester for treating sewage treatment plant sludge.

[0145] For this type of digester, the Methanosaetaceae family is identified as the microbial group with the limiting kinetics, with a p ma x associated with 0.10 j 1 To inoculate this digester, digested sludge from another digester (digester dedicated to inoculum production) was used, which operates under conditions similar to those expected from the conventional mesophilic digester.

[0146] For the calculation of the concentration of useful microbial biomass, a value of 1 is thus retained for a.

[0147] Monitoring the operation of the digester producing the inoculum indicates that MVaimentation = 20 gMV.L-1 and %MVabatue = 0.39. Combined with a value of Y = 0.1 gX.gMV' 1 , it is possible to estimate by applying equation 1 (Eq 1) that Xinoc = 0.8 gX.L' 1 .

[0148] 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 1 .

[0149] The maximum mud flow rate available on site is 140 m 3 .j -1 . 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:

[0150] [Maths 5] Qalim (Eq5)

[0151] 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.m' 3 ), at the purchase price of biomethane (€113.MWh -1 ) and the cost of sludge disposal (244 E.tMS' 1), optimal volumes of 2200 m 3 of water and 1400 m 3 inoculum are identified (see Figure 2).

[0152] These initial conditions and the ramp-up profile were then applied to the reactor (see Figure 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).

[0153] The maximum load is reached after 21 days, slightly later than the optimal profile (17 days) due to operational constraints. No accumulation of volatile fatty acids above 0.3 gAcetate.L 1 could not be observed, indicating good stability of the methanizer during the ramp-up.

[0154] 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 ramp-up time of 100 days for a mesophilic digester fed with thermally treated sludge. The publication Hatzigeorgiou et al., Startup of Anaerobic Me sophilic Digesters, proc water environ fed 2006, 415-428 reports a startup time of 77 days for a conventional mesophilic digester treating wastewater treatment plant sludge.

[0155] Example 2: example of determining the parameter a for the case of a transition from mesophilic conditions to thermophilic conditions

[0156] The parameter a is a correction factor between 0 and 1 that 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.

[0157] In this example, the inoculum is taken from a methanizer treating sewage sludge under mesophilic conditions (37°C). The methanizer that we want to ramp up also treats sewage 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 with limiting kinetics during methanization. These substrates are provided at a concentration sufficient to make the measurement of biogas flow rates reliable, and low enough not to cause a toxic effect. For these tests, a concentration of 2 g COD / L was chosen. After bringing the inoculum into contact with the substrate, the reactors are inerted with nitrogen to ensure anaerobiosis, and equilibrated at the desired temperature. They are then operated in batch mode while measuring the methane production rate over time.

[0158] Once methane production is complete, the cumulative methane production curves versus time can be modeled to determine the parameters Xo i T (concentration of useful microbial biomass in the inoculum capable of consuming acid i at temperature T) and gmax 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 gmax i T. Once this growth rate is determined, a least-squares approach is implemented to estimate Xo i T, for example using the following model:

[0159] [Maths 6] PcH4 i T( 350 (Pff 6)

[0160] with V CH4 i T the cumulative methane volume (NmL), V reacteur the useful volume of the reactor (L), Yxs j _ T growth yield (gCOD / gCOD), . max _i_T I e maximum growth rate (j -1 ), t time (j) and X o i T the initial concentration of useful biomass (gCOD / L).

[0161] An application of this model to a methane production curve from propionic acid under thermophilic conditions is shown in Figure 4.

[0162] In the case studied, the population with the slowest growth in the conditions of the methanizer to be ramped up (i.e. the - max _i_55 I e lower) is the one consuming propionic acid. Indeed, the value of H max-P ropionique_55 is on average 0.4 days 1 while a value of 1.2 j' 1 is measured for H max _acetic_55- By least squares approach, the value of X o propionic acid _55 est about 10' 5gCOD / L. The same approach applied to reactors operated at 37°C indicates an X o propionic acid _37 of 0.2 gCOD / L. Thus, in this example, the parameter a can be calculated as the ratio of X Q propionic _37 on X Q propionic _55, that is to say 5. 10' 5 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 exhibiting 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 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, 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 a limiting kinetics under 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. A method of 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: 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.

8. 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 having 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 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, 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,of the useful microbial biomass concentration of the inoculum previously estimated, of an initial inoculation volume, of a parameter representative of the biodegradability of the substrate and of a feeding and withdrawal regime of the digester.,

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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