High-solids anaerobic digestion requires a trade-off between total solids, inoculum-to-substrate ratio and ammonia inhibition

Increasing total solids in anaerobic digestion can reduce the methane yield by highly complex bio-physical–chemical mechanisms. Therefore, understanding those mechanisms and their main drivers becomes crucial to optimize this waste treatment biotechnology. In this study, seven batch experiments were conducted to investigate the effects of increasing the initial total solids in high-solids anaerobic digestion of the organic fraction of municipal solid waste. With inoculum-to-substrate ratio = 1.5 g VS/g VS and maximum total solids ≤ 19.6%, mono-digestion of the organic fraction of municipal solid waste showed a methane yield = 174–236 NmL CH4/ g VS. With inoculum-to-substrate ratio ≤ 1.0 g VS/g VS and maximum total solids ≥ 24.0%, mono-digestion experiments acidified. Co-digestion of the organic fraction of municipal solid waste and beech sawdust permitted to reduce the inoculum-to-substrate ratio to 0.16 g VS/g VS while increasing total solids up to 30.2%, though achieving a lower methane yield (117–156 NmL CH4/ g VS). At each inoculum-to-substrate ratio, higher total solids corresponded to higher ammonia and volatile fatty acid accumulation. Thus, a 40% lower methane yield for mono-digestion was observed at a NH3 concentration ≥ 2.3 g N–NH3/kg reactor content and total solids = 15.0%. Meanwhile, co-digestion lowered the nitrogen content, being the risk of acidification exacerbated only at total solids ≥ 20.0%. Therefore, the biodegradability of the substrate, as well as the operational total solids and inoculum-to-substrate ratio, are closely interrelated parameters determining the success of methanogenesis, but also the risk of ammonia inhibition in high-solids anaerobic digestion

Semi-continuous mono-digestion of OFMSW and Co-digestion of OFMSW with beech sawdust: Assessment of the maximum operational total solid content

In this study, mono-digestion of the organic fraction of municipal solid waste (OFMSW) and co-digestion of OFMSW with beech sawdust, simulating green waste, were used to investigate the maximum operational total solid (TS) content in semi-continuous high-solids anaerobic digestion (HS-AD). To alleviate substrate overloading in HS-AD, the effluent mass was relatively reduced compared to the influent mass, extending the mass retention time. To this aim, the reactor mass was daily evaluated, permitting to assess the reactor content removal by biogas production. During mono-digestion of OFMSW, the NH3 inhibition and the rapid TS removal prevented to maintain HS-AD conditions (i.e. TS ≥ 10%), without exacerbating the risk of reactor acidification. In contrast, the inclusion of sawdust in OFMSW permitted to operate HS-AD up to 30% TS, before acidification occurred. Therefore, including a lignocellulosic substrate in OFMSW can prevent acidification and stabilize HS-AD at very high TS contents (i.e. 20-30%)

Modelling non-ideal bio-physical-chemical effects on high-solids anaerobic digestion of the organic fraction of municipal solid waste

This study evaluates the main effects of including ‘non-ideal’ bio-physical-chemical corrections in high-solids anaerobic digestion (HS-AD) of the organic fraction of municipal solid waste (OFMSW), at total solid (TS) between 10 and 40%. As a novel approach, a simple ‘non-ideal’ module, accounting for the effects of ionic strength (I) on the main acid-base equilibriums, was coupled to a HS-AD model, to jointly evaluate the effects of ‘non-ideality’ and the TS content dynamics on the HS-AD bio-physical-chemistry. ‘Non-ideality’ influenced the pH, concentration of inhibitors (i.e. NH3), and liquid-gas transfer (i.e. CO2), particularly at higher TS (i.e. ≥ 20%). Meanwhile, fitting the experimental data for batch assays at 15% TS showed that HS-AD of OFMSW might be operated at I ≥ 0.5 M. Therefore, all HS-AD simulations should account for ‘non-ideal’ corrections, when assessing the main inhibitory mechanisms (i.e. NH3 buildup and acidification) potentially occurring in HS-AD of OFMSW

High-solids anaerobic digestion model for homogenized reactors

During high-solids anaerobic digestion (HS-AD) of the organic fraction of municipal solid waste (OFMSW), an important total solid (TS) removal occurs, leading to the modification of the reactor content mass/volume, in contrast to ‘wet’ anaerobic digestion (AD). Therefore, HS-AD mathematical simulations need to be approached differently than ‘wet’ AD simulations. This study aimed to develop a modelling tool based on the anaerobic digestion model 1 (ADM1) capable of simulating the TS and the reactor mass/volume dynamics in the HS-AD of OFMSW. Four hypotheses were used, including the effects of apparent concentrations at high TS. The model simulated adequately HS-AD of OFMSW in batch and continuous mode, particularly the evolution of TS, reactor mass, ammonia and volatile fatty acids. By adequately simulating the reactor content mass/volume and the TS, this model might bring further insight about potentially inhibitory mechanisms (i.e. NH3 buildup and/or acidification) occurring in HS-AD of OFMSW

Assessing practical identifiability during calibration and cross-validation of a structured model for high-solids anaerobic digestion

High-solids anaerobic digestion (HS-AD) of the organic fraction of municipal solid waste (OFMSW) is operated at a total solid (TS) content ≥ 10% to enhance the waste treatment economy, though it might be associated to free ammonia (NH3) inhibition. This study aimed to calibrate and cross-validate a HS-AD model for homogenized reactors in order to assess the effects of high NH3 levels in HS-AD of OFMSW, but also to evaluate the suitability of the reversible non-competitive inhibition function to reproduce the effect of NH3 on the main acetogenic and methanogenic populations. The practical identifiability of structural/biochemical parameters (i.e. 35) and initial conditions (i.e. 32) was evaluated using batch experiments at different TS and/or inoculum-to-substrate ratios. Variance-based global sensitivity analysis and approximate Bayesian computation were used for parameter optimization. The experimental data in this study permitted to estimate up to 8 biochemical parameters, whereas the rest of parameters and biomass contents were poorly identifiable. The study also showed the relatively high levels of NH3 (i.e. up to 2.3 g N/L) and ionic strength (i.e. up to 0.9 M) when increasing TS in HS-AD of OFMSW. However, the NH3 non-competitive function was unable to capture the acetogenic/methanogenic inhibition. Therefore, the calibration emphasized the need for target-oriented experimental data to enhance the practical identifiability and the predictive capabilities of structured HS-AD models, but also the need for further testing the NH3 inhibition function used in these simulations

Modelling the dynamics of valerate acetogenesis by modified ADM1 with variable stoichiometry and t hermodynamic control

The thermodynamics analysis of hevalerate and propionateacetogenesi sindicaes thatthisprocessi snotfavouredathighac etateconcen tr atio n aPostprint (published version

Evaluation expérimentale et modélisation des principaux mécanismes et cinétiques bio-physico-chimiques de la digestion anaérobie à haute teneur en solides de déchets organiques

The organic fraction of municipal solid waste (OFMSW) includes readily biodegradable wastes such as food waste, and slowly biodegradable wastes such as lignocellulosic materials. Anaerobic digestion (AD) is a mature treatment biotechnology in which OFMSW is decomposed to a mixture of methane (CH4) and carbon dioxide (CO2), known as biogas. Due to the elevated CH4 content (50-70%), biogas can be used as a source of renewable energy. Moreover, AD yields a partially stabilized digestate, allowing the recycle of nutrients to agriculture. High-solids anaerobic digestion (HS-AD) is a well-suited strategy to enhance the overall AD efficiency for OFMSW treatment. HS-AD is operated at a total solid (TS) content ≥ 10%, permitting to reduce the reactor size and overall operational costs. Nonetheless, the TS increase can result into biochemical instability, and even reactor failure by acidification. Both the high organic load and the buildup of inhibitors can be responsible for the HS-AD instability. The most notable inhibitor in HS-AD of OFMSW is NH3. Therefore, a balance is often required between enhancing the HS-AD economy and the ‘undesired’ instability for OFMSW treatment. This PhD research investigated the main bio-physical-chemical mechanisms and kinetics in HS-AD of OFMSW, with the aim to optimize the industrial application and maximize the kinetic rates. Laboratory-scale batch and semi-continuous experiments highlighted the main strengths and weaknesses of HS-AD. Simultaneously, the development of a HS-AD model permitted to condense the experimental knowledge about the bio-physical-chemical effects occurring when increasing the TS content in HS-AD.HS-AD batch experiments required a tradeoff between the initial TS, the inoculum-to-substrate ratio (ISR), the alkalinity and the nitrogen content, to assess the effects of increasing the initial TS content upon the methane yield, TS removal and chemical oxygen demand conversion. Particularly, a low ISR led to acidification, whereas the NH3 buildup led to volatile fatty acid (VFA) accumulation, reducing the methane yield, whether or not co-digestion of OFMSW with beech sawdust was used.In semi-continuous experiments, HS-AD of OFMSW required a reduced effluent compared to the influent to counterbalance the organic mass removal associated to the biogas production. Nonetheless, mono-digestion of readily-biodegradable OFMSW could not sustain a TS ≥ 10% without exacerbating the risk of substrate overload. Overloading was associated to the high biodegradability and the NH3 buildup. Thus, adding sawdust to OFMSW permitted to operate the reactors up to 30% TS, due to the lower biodegradability and nitrogen content of lignocellulosic substrates. As the main novelty of this PhD research, a HS-AD model based on the Anaerobic Digestion Model No.1 (ADM1) was developed. This model simulates the reactor mass and TS in HS-AD, in contrast of models focusing on ‘wet’ AD simulations (TS < 10%). Moreover, the HS-AD model considers also the TS concentration effect on soluble species. A ‘non-ideal’ bio-physical-chemical module, modifying predominantly the acid-base equilibriums, was subsequently coupled to the HS-AD model. Noteworthy, HS-AD is often characterized by a high ionic strength (I ≥ 0.2 M), affecting the pH, NH3 concentration and CO2 liquid-gas transfer, as the most important triggers for HS-AD inhibition. The HS-AD model calibration required multiple experimental datasets to circumvent parameter non-identifiability. The model calibration showed that HS-AD of OFMSW might be operated at I up to 0.9 M and NH3 up to 2.3 g N/L, particularly at higher TS (25-30%). Moreover, the model calibration suggested that the non-competitive NH3 inhibition should be further tested. Further HS-AD model developments (e.g. precipitation) were also recommended. All these results might aid in the optimization of HS-AD for organic waste treatment, renewable energy and nutrient recoveryLa fraction fermentescible des ordures ménagères (FFOM) comprend des déchets facilement biodégradables (alimentaires), et des lentement biodégradables (lignocellulosiques). La digestion anaérobie (DA) est une biotechnologie dans laquelle la FFOM est décomposé dans biogaz (CH4 + CO2). En raison de la teneur élevée en CH4 (50-70%), le biogaz pouvant être utilisé comme source d'énergie. En outre, DA produit un digestat partiellement stabilisé, riche d'éléments nutritifs. La DA à haute teneur en solides est une stratégie pour l'amélioration de l'efficacité. Elle correspond à une opération avec une teneur en matières sèches (MS) ≥ 10%, qui permet de réduire la taille du réacteur et les coûts de fonctionnement. Toutefois, l'augmentation de la MS peut entraîner une instabilité biochimique, et même une défaillance par acidification, à cause de la forte charge organique et l'accumulation d'inhibiteurs. L'inhibiteur le plus notable est NH3. Par conséquent, un équilibre entre l'amélioration de l'économie et l'instabilité est requis pour le traitement de la FFOM par DA à haute teneur en solides. Cette thèse de doctorat porte sur les principaux mécanismes cinétiques bio-physiques-chimiques mis en jeu lors de la DA à haute teneur en solides, dans le but d’optimiser son application. Des expériences de laboratoire ont mis en œuvre pour élucider les principales forces et faiblesses de ce procédé. Simultanément, le développement d'un modèle spécifique à la DA à haute teneur en solides a permis de condenser les connaissances expérimentales sur les effets qui se produisent lors de l'augmentation de la teneur de la MS. Les expériences en réacteur batch ont nécessité un compromis entre la teneur initiale en MS, le rapport entre l'inoculum et le substrat (X/S), l'alcalinité et la teneur en azote, afin d'évaluer les effets de l'augmentation de la teneur initiale en MS sur le rendement en CH4, l’élimination de la MS et la conversion de la demande chimique en oxygène. En particulier, des ratios X/S bas ont conduit à l'acidification, tandis que l'accumulation de NH3 a conduit à une accumulation d’acides gras volatils (AGV). Dans des expériences en semi-continue, la DA à haute teneur en solides nécessitait de diminuer le débit de l’effluent pour contrer l'élimination de la masse. Cependant, la mono-digestion de la FFOM facilement biodégradable ne peut pas supporter MS ≥ 10% sans augmenter le risque de surcharge. La surcharge était associée à la forte biodégradabilité et à l'accumulation de NH3. Par conséquent, l'ajout de sciure de bois à FFOM a permis à des réacteurs semi-continus de fonctionner jusqu'à 30% de MS, en raison de la biodégradabilité et de la teneur d'azote plus faibles ce substrat. La principale nouveauté de cette thèse est le développement d'un modèle pour la DA à haute teneur en solides. Ce modèle permet de simuler la dynamique masse et de MS dans des digesteurs, contrairement aux modèles sur des simulations de MS < 10%. Ce modèle prend également en compte l’effet de la concentration en MS sur les espèces solubles. Un module bio-physico-chimique « non idéal », modifiant les constantes d’équilibre acide-base, a été couplé ensuite au modèle. Il est à noter que la DA à haute teneur en solides est souvent caractérisée par une force ionique élevée (I ≥ 0,2 M), affectant le pH, la concentration en NH3 et le transfert de CO2 liquide-gaz. L'étalonnage du modèle a montré que la DA à haute teneur en solides requis plusieurs jeux de données expérimentaux pour contourner la « non-identifiabilité » des paramètres. La DA à haute teneur en solides pouvait fonctionner à une I allant jusqu'à 0,9 M et NH3 allant jusqu'à 2,3 g N/L, à des teneurs en MS élevées (25-30%). En outre, l'étalonnage a suggéré que l'utilisation d'une inhibition non-compétitive de NH3 devrait être testée plus avant. Il a également été recommandé de mettre au point d'autres développements du modèle. Ces résultats pourraient aider à l'optimisation de la DA à haute teneur en solides

Evaluation expérimentale et modélisation des principaux mécanismes et cinétiques bio-physico-chimiques de la digestion anaérobie à haute teneur en solides de déchets organiques

La fraction fermentescible des ordures ménagères (FFOM) comprend des déchets facilement biodégradables (alimentaires), et des lentement biodégradables (lignocellulosiques). La digestion anaérobie (DA) est une biotechnologie dans laquelle la FFOM est décomposé dans biogaz (CH4 + CO2). En raison de la teneur élevée en CH4 (50-70%), le biogaz pouvant être utilisé comme source d'énergie. En outre, DA produit un digestat partiellement stabilisé, riche d'éléments nutritifs. La DA à haute teneur en solides est une stratégie pour l'amélioration de l'efficacité. Elle correspond à une opération avec une teneur en matières sèches (MS) ≥ 10%, qui permet de réduire la taille du réacteur et les coûts de fonctionnement. Toutefois, l'augmentation de la MS peut entraîner une instabilité biochimique, et même une défaillance par acidification, à cause de la forte charge organique et l'accumulation d'inhibiteurs. L'inhibiteur le plus notable est NH3. Par conséquent, un équilibre entre l'amélioration de l'économie et l'instabilité est requis pour le traitement de la FFOM par DA à haute teneur en solides. Cette thèse de doctorat porte sur les principaux mécanismes cinétiques bio-physiques-chimiques mis en jeu lors de la DA à haute teneur en solides, dans le but d’optimiser son application. Des expériences de laboratoire ont mis en œuvre pour élucider les principales forces et faiblesses de ce procédé. Simultanément, le développement d'un modèle spécifique à la DA à haute teneur en solides a permis de condenser les connaissances expérimentales sur les effets qui se produisent lors de l'augmentation de la teneur de la MS. Les expériences en réacteur batch ont nécessité un compromis entre la teneur initiale en MS, le rapport entre l'inoculum et le substrat (X/S), l'alcalinité et la teneur en azote, afin d'évaluer les effets de l'augmentation de la teneur initiale en MS sur le rendement en CH4, l’élimination de la MS et la conversion de la demande chimique en oxygène. En particulier, des ratios X/S bas ont conduit à l'acidification, tandis que l'accumulation de NH3 a conduit à une accumulation d’acides gras volatils (AGV). Dans des expériences en semi-continue, la DA à haute teneur en solides nécessitait de diminuer le débit de l’effluent pour contrer l'élimination de la masse. Cependant, la mono-digestion de la FFOM facilement biodégradable ne peut pas supporter MS ≥ 10% sans augmenter le risque de surcharge. La surcharge était associée à la forte biodégradabilité et à l'accumulation de NH3. Par conséquent, l'ajout de sciure de bois à FFOM a permis à des réacteurs semi-continus de fonctionner jusqu'à 30% de MS, en raison de la biodégradabilité et de la teneur d'azote plus faibles ce substrat. La principale nouveauté de cette thèse est le développement d'un modèle pour la DA à haute teneur en solides. Ce modèle permet de simuler la dynamique masse et de MS dans des digesteurs, contrairement aux modèles sur des simulations de MS < 10%. Ce modèle prend également en compte l’effet de la concentration en MS sur les espèces solubles. Un module bio-physico-chimique « non idéal », modifiant les constantes d’équilibre acide-base, a été couplé ensuite au modèle. Il est à noter que la DA à haute teneur en solides est souvent caractérisée par une force ionique élevée (I ≥ 0,2 M), affectant le pH, la concentration en NH3 et le transfert de CO2 liquide-gaz. L'étalonnage du modèle a montré que la DA à haute teneur en solides requis plusieurs jeux de données expérimentaux pour contourner la « non-identifiabilité » des paramètres. La DA à haute teneur en solides pouvait fonctionner à une I allant jusqu'à 0,9 M et NH3 allant jusqu'à 2,3 g N/L, à des teneurs en MS élevées (25-30%). En outre, l'étalonnage a suggéré que l'utilisation d'une inhibition non-compétitive de NH3 devrait être testée plus avant. Il a également été recommandé de mettre au point d'autres développements du modèle. Ces résultats pourraient aider à l'optimisation de la DA à haute teneur en solides.The organic fraction of municipal solid waste (OFMSW) includes readily biodegradable wastes such as food waste, and slowly biodegradable wastes such as lignocellulosic materials. Anaerobic digestion (AD) is a mature treatment biotechnology in which OFMSW is decomposed to a mixture of methane (CH4) and carbon dioxide (CO2), known as biogas. Due to the elevated CH4 content (50-70%), biogas can be used as a source of renewable energy. Moreover, AD yields a partially stabilized digestate, allowing the recycle of nutrients to agriculture. High-solids anaerobic digestion (HS-AD) is a well-suited strategy to enhance the overall AD efficiency for OFMSW treatment. HS-AD is operated at a total solid (TS) content ≥ 10%, permitting to reduce the reactor size and overall operational costs. Nonetheless, the TS increase can result into biochemical instability, and even reactor failure by acidification. Both the high organic load and the buildup of inhibitors can be responsible for the HS-AD instability. The most notable inhibitor in HS-AD of OFMSW is NH3. Therefore, a balance is often required between enhancing the HS-AD economy and the ‘undesired’ instability for OFMSW treatment. This PhD research investigated the main bio-physical-chemical mechanisms and kinetics in HS-AD of OFMSW, with the aim to optimize the industrial application and maximize the kinetic rates. Laboratory-scale batch and semi-continuous experiments highlighted the main strengths and weaknesses of HS-AD. Simultaneously, the development of a HS-AD model permitted to condense the experimental knowledge about the bio-physical-chemical effects occurring when increasing the TS content in HS-AD.HS-AD batch experiments required a tradeoff between the initial TS, the inoculum-to-substrate ratio (ISR), the alkalinity and the nitrogen content, to assess the effects of increasing the initial TS content upon the methane yield, TS removal and chemical oxygen demand conversion. Particularly, a low ISR led to acidification, whereas the NH3 buildup led to volatile fatty acid (VFA) accumulation, reducing the methane yield, whether or not co-digestion of OFMSW with beech sawdust was used.In semi-continuous experiments, HS-AD of OFMSW required a reduced effluent compared to the influent to counterbalance the organic mass removal associated to the biogas production. Nonetheless, mono-digestion of readily-biodegradable OFMSW could not sustain a TS ≥ 10% without exacerbating the risk of substrate overload. Overloading was associated to the high biodegradability and the NH3 buildup. Thus, adding sawdust to OFMSW permitted to operate the reactors up to 30% TS, due to the lower biodegradability and nitrogen content of lignocellulosic substrates. As the main novelty of this PhD research, a HS-AD model based on the Anaerobic Digestion Model No.1 (ADM1) was developed. This model simulates the reactor mass and TS in HS-AD, in contrast of models focusing on ‘wet’ AD simulations (TS < 10%). Moreover, the HS-AD model considers also the TS concentration effect on soluble species. A ‘non-ideal’ bio-physical-chemical module, modifying predominantly the acid-base equilibriums, was subsequently coupled to the HS-AD model. Noteworthy, HS-AD is often characterized by a high ionic strength (I ≥ 0.2 M), affecting the pH, NH3 concentration and CO2 liquid-gas transfer, as the most important triggers for HS-AD inhibition. The HS-AD model calibration required multiple experimental datasets to circumvent parameter non-identifiability. The model calibration showed that HS-AD of OFMSW might be operated at I up to 0.9 M and NH3 up to 2.3 g N/L, particularly at higher TS (25-30%). Moreover, the model calibration suggested that the non-competitive NH3 inhibition should be further tested. Further HS-AD model developments (e.g. precipitation) were also recommended. All these results might aid in the optimization of HS-AD for organic waste treatment, renewable energy and nutrient recover

Modelling the dynamics of valerate acetogenesis by modified ADM1 with variable stoichiometry and t hermodynamic control

The thermodynamics analysis of hevalerate and propionateacetogenesi sindicaes thatthisprocessi snotfavouredathighac etateconcen tr atio n