Valorization of commercial food waste via anaerobic processes

La production croissante de déchets alimentaires dans le monde et des nouvelles réglementations internationales exigent le développement de nouveaux procédés pour le traitement de ce type de déchets. Parmi toutes les possibilités existantes, les procédés anaérobies représentent une approche durable qui permet le traitement et la valorisation de ces déchets. Ce doctorat vise à comprendre les processus biochimiques régissant la digestion anaérobie des déchets alimentaires, en fournissant des éléments pour le développement de procédés applicables à l'échelle industrielle.Dans un premier temps, un screening a été effectué pour élucider les paramètres principaux affectant la digestion anaérobie des déchets alimentaires, en évaluant différentes charges de substrat, teneurs en matière sèche, proportions de co-digestion et des inocula microbiens de différentes origines. Après avoir conclu l'importance cruciale de l'inoculum utilisé et de la charge du substrat, différentes stratégies de stabilisation des procédés de méthanisation ont été testées à l'aide de réacteurs discontinus consécutifs. Ce travail a permis de confirmer l'effet positif de la supplémentation des oligoéléments et à identifier le principal verrou: l'accumulation d'acide propionique. Dans le but de trouver une solution, deux expériences ont été axées sur l'évaluation de la capacité des matériaux conducteurs à base de carbone à résoudre ce problème. Le dosage de ces matériaux favorisait la cinétique de la digestion, améliorant significativement les productions volumétriques du méthane.Cette thèse fournit des connaissances nouvelles, à la fois sur les principaux mécanismes régissant la digestion anaérobie des déchets alimentaires et sur les implications qu'elles présentent pour la valorisation de ces déchets. En outre, des solutions possibles pour lever les verrous opérationnels ont été développés, permettant de fournir des recommandations pour l’implantation d’un procédé de digestion à l’échelle industrielle.The increasing production of food waste worldwide and new international regulations call for the development of novel processes for the treatment of this waste. Among all the existing possibilities, anaerobic processes represent a sustainable-modern approach that allows waste treatment and valorization. This PhD thesis aims at understanding the biochemical processes governing anaerobic digestion of food waste, eventually providing a stable process applicable at industrial scale.As a first step, a screening was performed to elucidate the main parameter affecting anaerobic digestion of food waste, evaluating different substrate loads, solid contents, co-digestion proportions and microbial inocula from different origins. After concluding the critical importance of the inoculum used and the substrate load, different strategies for process stabilization for methane production were tested using consecutive batch reactors. This served for confirming the positive effect of supplementation of trace elements and to identify the main issue that was found: accumulation of propionic acid. Aiming at finding a solution, the final experiments were focused on assessing the capability of carbon-based conductive materials to solve this problem. The dosing of these materials favored the digestion kinetics, improving greatly the methane volumetric productivities.This thesis provides novel insights, both on the main mechanisms governing food waste anaerobic digestion and on the implications that they present for the valorization of this waste. In addition, potential solutions for the complications found are given, aiding to the development of a feasible industrial digestion process

Valorisation des biodéchets alimentaires commerciaux par des procédés anaérobies

The increasing production of food waste worldwide and new international regulations call for the development of novel processes for the treatment of this waste. Among all the existing possibilities, anaerobic processes represent a sustainable-modern approach that allows waste treatment and valorization. This PhD thesis aims at understanding the biochemical processes governing anaerobic digestion of food waste, eventually providing a stable process applicable at industrial scale. As a first step, a screening was performed to elucidate the main parameter affecting anaerobic digestion of food waste, evaluating different substrate loads, solid contents, co-digestion proportions and microbial inocula from different origins. After concluding the critical importance of the inoculum used and the substrate load, different strategies for process stabilization for methane production were tested using consecutive batch reactors. This served for confirming the positive effect of supplementation of trace elements and to identify the main issue that was found: accumulation of propionic acid. Aiming at finding a solution, the final experiments were focused on assessing the capability of carbon-based conductive materials to solve this problem. The dosing of these materials favored the digestion kinetics, improving greatly the methane volumetric productivities. This thesis provides novel insights, both on the main mechanisms governing food waste anaerobic digestion and on the implications that they present for the valorization of this waste. In addition, potential solutions for the complications found are given, aiding to the development of a feasible industrial digestion process.La production croissante de déchets alimentaires dans le monde et des nouvelles réglementations internationales exigent le développement de nouveaux procédés pour le traitement de ce type de déchets. Parmi toutes les possibilités existantes, les procédés anaérobies représentent une approche durable qui permet le traitement et la valorisation de ces déchets. Ce doctorat vise à comprendre les processus biochimiques régissant la digestion anaérobie des déchets alimentaires, en fournissant des éléments pour le développement de procédés applicables à l'échelle industrielle. Dans un premier temps, un screening a été effectué pour élucider les paramètres principaux affectant la digestion anaérobie des déchets alimentaires, en évaluant différentes charges de substrat, teneurs en matière sèche, proportions de co-digestion et des inocula microbiens de différentes origines. Après avoir conclu à l'importance cruciale de l'inoculum utilisé et de la charge du substrat, différentes stratégies de stabilisation des procédés de méthanisation ont été testées à l'aide de réacteurs discontinus consécutifs. Ce travail a permis de confirmer l'effet positif de la supplémentation du milieu réactionnel en oligoéléments sur les performances de production de biogaz et à identifier le principal verrou: l'accumulation d'acide propionique. Dans le but de trouver une solution, deux expériences ont été axées sur l'évaluation de la capacité des matériaux conducteurs à base de carbone à résoudre ce problème. Le dosage de ces matériaux favorise la cinétique de la digestion, améliorant significativement les productions volumétriques du méthane. Cette thèse fournit des connaissances nouvelles, à la fois sur les principaux mécanismes régissant la digestion anaérobie des déchets alimentaires et sur les implications qu'elles présentent pour la valorisation de ces déchets. En outre, des solutions possibles pour lever les verrous opérationnels ont été développés, permettant de fournir des recommandations pour l’implantation d’un procédé de digestion à l’échelle industrielle

Considering syntrophic acetate oxidation and ionic strength improves the performance of models for food waste anaerobic digestion

Current mechanistic anaerobic digestion (AD) models cannot accurately represent the underlying processes occurring during food waste (FW) AD. This work presents an update of the Anaerobic Digestion Model no. 1 (ADM1) to provide accurate estimations of free ammonia concentrations and related inhibition thresholds, and model syntrophic acetate oxidation as acetate-consuming pathway. A modified Davies equation predicted NH3 concentrations and pH more accurately, and better estimated associated inhibitory limits. Sensitivity analysis results showed the importance of accurate disintegration kinetics and volumetric mass transfer coefficients, as well as volatile fatty acids (VFAs) and hydrogen uptake rates. In contrast to the default ADM1, the modified ADM1 could represent methane production and VFA profiles simultaneously (particularly relevant for propionate uptake). The modified ADM1 was also able to predict the predominant acetate-consuming and methane-producing microbial clades. Modelling results using data from reactors dosed with granular activated carbon showed that this additive improves hydrogen uptakeGabriel Capson-Tojo is grateful to the Xunta de Galicia for his postdoctoral fellowship (ED481B-2018/017). Sergi Astals is thankful to the Spanish Ministry of Science, Innovation and Universities for his Ramon y Cajal fellowship (RYC-2017-22372). Ángel Robles is grateful for the support from the Spanish Ministry of Science and Innovation and the Partnership for Research and Innovation in the Mediterranean Area (Grant PCI2020-112218)S

Modelling anaerobic digestion of food waste: the importance of syntrophic acetate oxidation and correct free ammonia estimation

International audienceThe aim of this study was to assess the importance of two major modifications of the IWA ADM1 on its ability to model food waste anaerobic digestion: (i) estimating free ammonia nitrogen using a modified Davies equation and (ii) including syntrophic acetate oxidation as main acetate-consuming pathway. The obtained results show that, in agreement with the literature, including the Davies equation for free ammonia estimation avoided overestimations of up to 30% when compared to the ideal equation (standard in the ADM1). Further including syntrophic acetate oxidation allowed to properly represent the dynamics of methane production and the volatile fatty acid profiles (e.g. propionate accumulation). This also allowed to predict more accurately the structure of the microbial communities (e.g. with hydrogenotrophs as dominant methanogens). Combined, these modifications allowed to obtain more precise values of the inhibition constants for different microbial clades. In addition, a threshold inhibition function was used, which allowed a more accurate inhibition representation. Finally, the kLa values were drastically reduced to account for the decreased mass transfer rates. The presented work shows that these modifications must be considered to achieve an accurate representation of food waste anaerobic digestion using mechanistic models. Although further calibration and validations must be performed, the results are promising when considering the high ammonia nitrogen and transient volatile fatty acid concentrations in the reactors and the performance of the unmodified ADM1

Modelling anaerobic digestion of food waste: the importance of syntrophic acetate oxidation and correct free ammonia estimation

International audienceThe aim of this study was to assess the importance of two major modifications of the IWA ADM1 on its ability to model food waste anaerobic digestion: (i) estimating free ammonia nitrogen using a modified Davies equation and (ii) including syntrophic acetate oxidation as main acetate-consuming pathway. The obtained results show that, in agreement with the literature, including the Davies equation for free ammonia estimation avoided overestimations of up to 30% when compared to the ideal equation (standard in the ADM1). Further including syntrophic acetate oxidation allowed to properly represent the dynamics of methane production and the volatile fatty acid profiles (e.g. propionate accumulation). This also allowed to predict more accurately the structure of the microbial communities (e.g. with hydrogenotrophs as dominant methanogens). Combined, these modifications allowed to obtain more precise values of the inhibition constants for different microbial clades. In addition, a threshold inhibition function was used, which allowed a more accurate inhibition representation. Finally, the kLa values were drastically reduced to account for the decreased mass transfer rates. The presented work shows that these modifications must be considered to achieve an accurate representation of food waste anaerobic digestion using mechanistic models. Although further calibration and validations must be performed, the results are promising when considering the high ammonia nitrogen and transient volatile fatty acid concentrations in the reactors and the performance of the unmodified ADM1

Moving towards a realistic application of purple phototrophic bacteria for resource recovery

International audiencePurple phototrophic bacteria are receiving increasing attention due to their unique capability of growing photoheterotrophically, using energy from light to simultaneously recover carbon and nutrients in the form of a different value-added products. In this work, PPB-based applications and potential products are reviewed to identify major challenges and opportunities. A comprehensive analysis of data has shown that, despite the potential of this technology, most of the research on PPB applications has been carried out using pure cultures, axenic conditions and artificial illumination. If a real application of this technology is to be developed, research on PPB should be performed using enriched non-axenic cultures and natural light, aiming at producing results that can be extrapolated to economically-feasible, full-scale systems. Amongst the products obtained from PPB, using the biomass as fish feed represents the most profitable approach, with a potential revenue of 1.14 $•kgbiomass-1

Oxygen in anaerobic purple phototrophic bacteria systems for resource recovery: friend or foe?

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Implications of light attenuation for the upscaling of mixed purple phototrophic bacteria processes

International audienceLight attenuation in purple phototrophic bacteria (PPB) has been studied. The results show that increased biomass concentrations lead to higher light attenuation, and that PPB absorb both visible and near-infrared wavelengths. Both spectrum fractions were equally absorbed at PPB concentrations above 1 g COD•L-1. A flat plate configuration showed less attenuation than top-illuminated cylindrical reactors, representative of open ponds. Neither a complex wastewater matrix nor the presence of polyhydroxyalkanoates affected light attenuation. The concentration of pigments (e.g. carotenoids and bacteriochlorophyll) had a strong effect, with much higher attenuation in the presence of pigments compared to non-pigmented biomass. In dense outdoor PPB cultures (≥1 g COD•L-1), effective light penetration is only 5 cm. This, together with the increased oxygen diffusion in large open ponds, biases design away from horizontal lagoons, and towards multi-panel systems such as flat plate reactors

Light attenuation in enriched purple phototrophic bacteria cultures: Implications for modelling and reactor design

International audienceLight attenuation in enriched purple phototrophic bacteria (PPB) cultures has not been studied, and its understanding is critical for proper process modelling and reactor design, especially for scaled systems. This work evaluated the effect of different biomass concentrations, reactor configurations, wastewater matrices, and growth conditions, on the attenuation extent of near infra-red (NIR) and ultraviolet-visible (UV-VIS) light spectra. The results show that increased biomass concentrations lead to higher light attenuation, and that PPB absorb both VIS and NIR wavelengths, with both fractions of the spectrum being equally absorbed at biomass concentrations above 1,000 g COD⋅m − 3. A flat plate configuration showed less attenuation compared with cylindrical reactors illuminated from the top, representative for open ponds. Neither a complex wastewater matrix nor the presence of polyhydroxyalkanoates (under nutrient limited conditions) affected light attenuation significantly. The pigment concentration (both bacteriochlorophyll and carotenoids) however, had a strong effect, with significant attenuation in the presence of pigments. Attenuation predictions using the Lambert-Beer law (excluding scattering) and the Schuster model (including scattering) indicated that light scattering had a minimal effect. A proposed mathematical model, based on the Lambert-Beer law and a Monod function for light requirements, allowed effective prediction of the kinetics of photoheterotrophic growth. This resulted in a half saturation coefficient of 4.6 W⋅m − 2. Finally, the results showed that in dense outdoor PPB cultures (≥1,000 g COD⋅m − 3), effective light penetration is only 5 cm, which biases design away from horizontal lagoons, and towards nonincident multi-panel systems such as flat plate reactors

Photo anaerobic model 2: a mechanistic model for resource recovery using enriched purple phototrophic bacteria grown outdoors

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