Volatile fatty acids (VFAs) and methane from food waste and cow slurry: Comparison of biogas and VFA fermentation processes

The potential of various biomasses for the production of green chemicals is currently one of the key topics in the field of the circular economy. Volatile fatty acids (VFAs) are intermediates in the methane formation pathway of anaerobic digestion and they can be produced in similar reactors as biogas to increase the productivity of a digestion plant, as VFAs have more varying end uses compared to biogas and methane. In this study, the aim was to assess the biogas and VFA production of food waste (FW) and cow slurry (CS) using the anaerobic biogas plant inoculum treating the corresponding substrates. The biogas and VFA production of both biomasses were studied in identical batch scale laboratory conditions while the process performance was assessed with chemical and microbial analyses. As a result, FW and CS were shown to have different chemical performances and microbial dynamics in both VFA and biogas processes. FW as a substrate showed higher yields in both processes (435 ml CH₄/g VS_fed and 434 mg VFA/g VS_fed) due to its characteristics (pH, organic composition, microbial communities), and thus, the vast volume of CS makes it also a relevant substrate for VFA and biogas production. In this study, VFA profiles were highly dependent on the substrate and inoculum characteristics, while orders Clostridiales and Lactobacillales were connected with high VFA and butyric acid production with FW as a substrate. In conclusion, anaerobic digestion supports the implementation of the waste management hierarchy as it enables the production of renewable green chemicals from both urban and rural waste materials.     

A review of anaerobic digestion bio-kinetics

Anaerobic digestion is the key to sustainable wastewater management and bioenergy production. Kinetics plays an important role in the design of bioreactors, processes, and process scale-up in anaerobic digestion. This article focuses on a state-of-the-art literature review on the experimental kinetic studies of conventional anaerobic bioreactors and anaerobic membrane bioreactors. Various kinetic models that were used to fit the experimental data and derive the kinetic parameters are summarized and discussed in the literature. The values of the maximum specific growth rate µ_max, half saturation constant K_s, decay co-efficient k_d, sludge yield Y, and methane yield Y_CH4 from experimental studies are summarized for each model. This paper can serve as an updated comprehensive source of anaerobic bio-kinetic studies and digester design.     

Distributed model of solid waste anaerobic digestion: effects of leachate recirculation and pH adjustment

A distributed model of solid waste digestion in a 1-D bioreactor with leachate recirculation and pH adjustment was developed to analyze the balance between the rates of polymer hydrolysis/acidogenesis and methanogenesis during the anaerobic digestion of municipal solid waste (MSW). The model was calibrated on previously published experimental data generated in 2-L reactors filled with shredded refuse and operated with leachate recirculation and neutralization. Based on model simulations, both waste degradation and methane production were stimulated when inhibition was prevented rapidly from the start, throughout the reactor volume, by leachate recirculation and neutralization. An optimal strategy to reduce the time needed for solid waste digestion is discussed.     

A practical kinetic model that considers endproduct inhibition in anaerobic digestion processes by including the equilibrium constant

The classical Michaelis-Menten model is widely used as the basis for modeling of a number of biological systems. As the model does not consider the inhibitory effect of endproducts that accumulate in virtually all bioprocesses, it is often modified to prevent the overestimation of reaction rates when products have accumulated. Traditional approaches of model modification use the inclusion of irreversible, competitive, and noncompetitive inhibition factors. This article demonstrates that these inhibition factors are insufficient to predict product inhibition of reactions that are close the dynamic equilibrium. All models investigated were found to violate thermodynamic laws as they predicted positive reaction rates for reactions that were endergonic due to high endproduct concentrations. For modeling of biological processes that operate close to the dynamic equilibrium (e.g., anaerobic processes), it is critical to prevent the prediction of positive reaction rates when the reaction has already reached the dynamic equilibrium. This can be achieved by using a reversible kinetic model. However, the major drawback of the reversible kinetic model is the large number of empirical parameters it requires. These parameters are difficult to determine and prone to experimental error. For this reason, the reversible model is not practical in the modeling of biological processes.This article uses the fundamentals of steady-state kinetics and thermodynamics to establish an equation for the reversible kinetic model that is of practical use in bio-process modeling. The behavior of this equilibrium-based model is compared with Michaelis-Menten-based models that use traditional inhibition factors. The equilibrium-based model did not require any empirical inhibition factor to correctly predict when reaction rates must be zero due to the free energy change being zero. For highly exergonic reactions, the equilibrium-based model did not deviate significantly from the Michaelis-Menten model, whereas, for reactions close to equilibrium, the reaction rate was mainly controlled by the quotient of mass action ratio (concentration of all products over concentration of all substrates) over the equilibrium constant K. This quotient is a measure of the displacement of the reaction from its equilibrium. As the new equation takes into account all of the substrates and products, it was able to predict the inhibitor effect of multiple endproducts. The model described is designed to be a useful basis for a number of different model applications where reaction conditions are close to equilibrium.     

A fast kinetic model discrimination applied to the continuous two-phase anaerobic digestion of fruit and vegetable wastes

Summary A simple method to fit the constants of first order, Monod and Chen and Hashimoto models using only one single linear regression, is presented. The method can be applied to continuous operated systems. As an example, the kinetic behaviour of the anaerobic digestion of a mixture of fruit and vegetable wastes is studied. Chen and Hashimoto model yields the best fit.     

Three-reaction model for the anaerobic digestion of microalgae

Coupling an anaerobic digester to a microalgal culture has received increasing attention as an alternative process for combined bioenergy production and depollution. In this article, a dynamic model for anaerobic digestion of microalgae is developed with the aim of improving the management of such a coupled system. This model describes the dynamics of inorganic nitrogen and volatile fatty acids since both can lead to inhibition and therefore process instability. Three reactions are considered: Two hydrolysis–acidogenesis steps in parallel for sugars/lipids and for proteins, followed by a methanogenesis step. The proposed model accurately reproduces experimental data for anaerobic digestion of the freshwater microalgae Chlorella vulgaris with an organic loading rate of 1 gCOD L^?1 d^?1. In particular, the three‐reaction pathway allows to adequately represent the observed decoupling between biogas production and nitrogen release. The reduced complexity of this model makes it suitable for developing advanced, model‐based control and monitoring strategies. Biotechnol. Bioeng. 2012; 109:415–425. © 2011 Wiley Periodicals, Inc.     

Improvement of the anaerobic biodegradation of olive mill wastewaters by prior ozonation pretreatment

The purification of olive mill wastewaters (OMW) is investigated by a single anaerobic digestion in a batch reactor containing immobilized microorganisms, and by the combination of an ozonation pretreatment followed by an anaerobic digestion. In the single anaerobic digestion the removal of the COD is determined and the methane yield coefficient, which is the best measure of the extent of transformation of the biodegradable substrate, is also obtained, its value being 194?ml CH₄/g COD. A kinetic study is performed by using the Monod model combined with the Levenspiel model, due to the presence of inhibition effects. Both models lead to the determination of the kinetic parameters of this anaerobic treatment: kinetic constants, critical substrate concentration of inhibition and inhibitory parameter. In the combined process, the ozonation pretreatment of OMW achieves a great reduction in the phenolic compounds, leading to a significant increase in the methane yield coefficient in the following anaerobic digestion, its value being 266?ml CH₄/g COD.     

A dynamic mathematical model for sequential leach bed anaerobic digestion of organic fraction of municipal solid waste

A mathematical model that describes the operation of a sequential leach bed process for anaerobic digestion of organic fraction of municipal solid waste (MSW) is developed and validated. This model assumes that ultimate mineralisation of the organic component of the waste occurs in three steps, namely solubilisation of particulate matter, fermentation to volatile organic acids (modelled as acetic acid) along with liberation of carbon dioxide and hydrogen, and methanogenesis from acetate and hydrogen. The model incorporates the ionic equilibrium equations arising due to dissolution of carbon dioxide, generation of alkalinity from breakdown of solids and dissociation of acetic acid. Rather than a charge balance, a mass balance on the hydronium and hydroxide ions is used to calculate pH. The flow of liquid through the bed is modelled as occurring through two zones—a permeable zone with high flushing rates and the other more stagnant. Some of the kinetic parameters for the biological processes were obtained from batch MSW digestion experiments. The parameters for flow model were obtained from residence time distribution studies conducted using tritium as a tracer. The model was validated using data from leach bed digestion experiments in which a leachate volume equal to 10% of the fresh waste bed volume was sequenced. The model was then tested, without altering any kinetic or flow parameters, by varying volume of leachate that is sequenced between the beds. Simulations for sequencing/recirculating 5 and 30% of the bed volume are presented and compared with experimental results.     

Robustness of archaeal populations in anaerobic co-digestion of dairy and poultry wastes

The objective of this study is to investigate the responses of methanogen populations to poultry waste addition by comparing the archaeal microbial populations in continuous anaerobic digesters with or without the addition of poultry waste as a co-substrate. Poultry waste was characterized as an organic/nitrogen-rich substrate for anaerobic digestion. Supplementing dilute dairy waste with poultry waste for anaerobic co-digestion to increase organic loading rate by 50% resulted in improved biogas production. Elevated ammonia derived from poultry waste did not lead to process inhibition at the organic loadings tested, demonstrating the feasibility of the anaerobic co-digestion of dairy and poultry wastes for improved treatment efficiency. The stability of the anaerobic co-digestion process was linked to the robust archaeal microbial community, which remained mostly unchanged in community structure following increases in organic loading and ammonia levels. Surprisingly, Crenarchaeota archaeal populations, instead of the Euryarchaeota methanogens, dominated the archaeal communities in the anaerobic digesters. The ecological functions of these abundant non-methanogen archaeal populations in anaerobic digestion remain to be identified.     

Identification in an anaerobic batch system: global sensitivity analysis, multi-start strategy and optimization criterion selection

Several mathematical models have been developed in anaerobic digestion systems and a variety of methods have been used for parameter estimation and model validation. However, structural and parametric identifiability questions are relatively seldom addressed in the reported AD modeling studies. This paper presents a 3-step procedure for the reliable estimation of a set of kinetic and stoichiometric parameters in a simplified model of the anaerobic digestion process. This procedure includes the application of global sensitivity analysis, which allows to evaluate the interaction among the identified parameters, multi-start strategy that gives a picture of the possible local minima and the selection of optimization criteria or cost functions. This procedure is applied to the experimental data collected from a lab-scale sequencing batch reactor. Two kinetic parameters and two stoichiometric coefficients are estimated and their accuracy was also determined. The classical least-squares cost function appears to be the best choice in this case study.     

Influence of different natural zeolite concentrations on the anaerobic digestion of piggery waste

The effect of different natural zeolite concentrations on the anaerobic digestion of piggery waste was studied. Natural zeolite doses in the range 0.2-10 g/l of wastewater were used in batch experiments, which were carried out at temperatures between 27 degrees C and 30 degrees C. Total chemical oxygen demand (COD), total and volatile solids, ammonia and organic nitrogen, pH, total volatile fatty acids (TVFA), alkalinity (Alk) and accumulative methane production were determined during 30 days of digestion. The anaerobic digestion process was favored by the addition of natural zeolite at doses between 2 and 4 g/l and increasingly inhibited at doses beyond 6 g/l. A first-order kinetic model of COD removal was used to determine the apparent kinetic constants of the process. The kinetic constant values increased with the zeolite amount up to a concentration of 4 g/l. The values of the maximum accumulative methane production (Gm) increased until zeolite concentrations of 2-4 g/l. The addition of zeolite reduced the values of the TVFA/ Alk ratio while increasing the pH values, and these facts could contribute to the process failure at zeolite doses of 10 g/l.     

Analysis of the performance of an anaerobic digestion system at the Regina Wastewater Treatment Plant

From the performance analysis of the anaerobic digestion system at the Regina Wastewater Treatment Plant, it was found that the anaerobic digestion system at the Regina plant was generally operated in a stable condition as indicated by pH, volatile acids and alkalinity levels. The operation of the anaerobic digestion system was not optimal because of the low volatile solids concentration and low volatile solids loading rate, especially because of high HRT. Two options, thickening the primary sludge and increasing the volatile solids loading rate, were recommended for the optimal operation of the digestion system. After examining a number of kinetic models, it was found that the Chen-Hashimoto model could be used to predict the volumetric methane production rate and the first-order model could be used to predict the efficiency of volatile solids reduction. The study showed that utilization of digester gas for power production was the best alternative for the excess digester gas. 13.3% of the electrical demand and 35.5% of the plant's total energy could be met based on digester gas wasted, assuming 25% as the conversion efficiency.     

Sensitivity analysis and parameter estimation in a model of anaerobic waste water treatment processes with substrate inhibition

Anaerobic waste water treatment processes are commonly presented by the fifth order Hill and Barth non-linear model, describing three main stages of anaerobic digestion. The model investigated in the present work is a modified version of the Hill and Barth model, which includes substrate inhibition of growth of methanogenic bacteria. Parameter estimation of this model is a difficult problem because of the high number of parameters to be estimated and the rather restricted information concerning the variables. The aim of the present work is to use sensitivity theory to find a method for selecting the most significant parameters. In particular, we develop a sensitivity model for anaerobic digestion using relative sensitivity functions. Simulation results from the sensitivity model show that the number of parameters to be estimated can be reduced from 12 to 4. We suggest a 2-step procedure for parameter estimation, which is also based on sensitivity analysis. This procedure gives results, which allow for off-line determination of parameter values if the experimental data for biogas production rate are known.     

Kinetics of the hydrolysis of cellulose by beta-1,4-glucan cellobiohydrolase of Trichoderma viride

The cellulolytic enzyme beta-1,4-glucan cellobiohydrolase (CBH) has been isolated from the crude mixture of cellulase enzymes of Trichoderma viride by gel filtration and ion-exchange methods, and some aspects of its kinetic behaviour have been examined. Studies of the initial rates of the CBH-catalyzed production of cellobiose from fibrous alpha-cellulose show that (i) the dissociation constant for cellobiose competitive product inhibition of the reaction is Ki = (1.13 +/- 0.37) X 10(-3) M, (ii) the adsorption of CBH on fibrous alpha-cellulose and its subsequent reaction conform to kinetic equations developed in conjunction with the Langmuir adsorption isotherm, (iii) the rate-pH curve has a maximum at pH 5.2 and decreases at higher and lower pH values, exhibiting enzyme pK values of 3.8 and 6.5, and (iv) the energy of activation of the overall reaction between 5 and 60 degrees C is 5.3 +/- 0.3 kcal mol-1 at pH 5.2. Studies of the time course of the reaction over extended periods of time up to 40% hydrolysis of the cellulose show that (v) the data fit better to a competitive product inhibition model than to models of anticompetitive product inhibition or noncompetitive product inhibition.     

Mathematical modelling and kinetic study for CD production catalysed by Toruzyme® and CGTase from Bacillus firmus strain 37

A new mathematical model was developed for the kinetics of α-, β- and γ-cyclodextrin production, expanding an existing model that only included the production of β- and γ-cyclodextrins, because a detailed kinetic modelling of the reactions involved allows the manipulation of the process yields. The kinetic behaviour of the commercial enzyme Toruzyme^® was studied with maltodextrin as substrate at different concentrations and for CGTase from Bacillus firmus strain 37 at a concentration of 100 g L⁻¹. The mathematical model showed a proper fit to the experimental data, within the 24-h period studied, confirming that the considered hypotheses represent the kinetic behaviour of the enzymes in the reaction medium. The kinetic parameters generated by the model allowed reproducing previous observed qualitative tendencies as it can be seen that changing experimental conditions in the reaction process such as enzyme and substrate concentrations results in large changes in the enzyme kinetics and using high substrate concentrations does not guarantee the highest conversion rates due to enzyme inhibition and reverse reactions. In addition, this new mathematical model complements previous qualitative observations enabling the manipulation of the direct and reverse reactions catalysed by the enzyme by adjusting the reaction conditions, to target quantitative results of increased productivity and better efficiency in the production of a desired cyclodextrin.

     

Inhibition of anaerobic digestion process: a review

Anaerobic digestion is an attractive waste treatment practice in which both pollution control and energy recovery can be achieved. Many agricultural and industrial wastes are ideal candidates for anaerobic digestion because they contain high levels of easily biodegradable materials. Problems such as low methane yield and process instability are often encountered in anaerobic digestion, preventing this technique from being widely applied. A wide variety of inhibitory substances are the primary cause of anaerobic digester upset or failure since they are present in substantial concentrations in wastes. Considerable research efforts have been made to identify the mechanism and the controlling factors of inhibition. This review provides a detailed summary of the research conducted on the inhibition of anaerobic processes. The inhibitors commonly present in anaerobic digesters include ammonia, sulfide, light metal ions, heavy metals, and organics. Due to the difference in anaerobic inocula, waste composition, and experimental methods and conditions, literature results on inhibition caused by specific toxicants vary widely. Co-digestion with other waste, adaptation of microorganisms to inhibitory substances, and incorporation of methods to remove or counteract toxicants before anaerobic digestion can significantly improve the waste treatment efficiency.     

Study on the kinetics of enzymatic interesterification of triglycerides for biodiesel production with methyl acetate as the acyl acceptor

A new route for biodiesel production using methyl acetate instead of methanol as the acyl acceptor was proposed in our previous research, and it has been found that this novel route could enhance the stability of the immobilized lipase greatly. In this paper, the kinetics of lipase-catalyzed interesterification of triglycerides for biodiesel production with methyl acetate as the acyl acceptor was further studied. First, a simplified model based on Ping Pong Bi Bi with substrate competitive inhibition mechanism was proposed to describe the reaction kinetics of the interesterification. During our further study, it was observed that three consecutive and reversible reactions occurred in the interesterification of triglycerides and methyl acetate. So, a kinetic model based on mass balance of three second-order reversible reactions was developed and the reaction rate constant, k, was determined by solving the differential rate equations of the reaction system. The results showed that k_DG–MG (0.1124) and k_MG–TA (0.1129) were much higher than k_TG–DG (0.0311), which indicated that the first step reaction was the limit step for the overall interesterification.