117,377 research outputs found
Surrogate based Global Sensitivity Analysis of ADM1-based Anaerobic Digestion Model
No full textIn order to calibrate the model parameters, Sensitivity Analysis routines are mandatory to rank the parameters by their relevance and fix to nominal values the least influential factors. Despite the high number of works based on ADM1, very few are related to sensitivity analysis. In this study Global Sensitivity Analysis (GSA) and Uncertainty Quantification (UQ) for an ADM1-based Anaerobic Digestion Model have been performed. The modified version of ADM-based model selected in this study was presented by Esposito and co-authors in 2013. Unlike the first version of ADM1, focused on sewage sludge degradation, the model of Esposito is focused on organic fraction of municipal solid waste digestion. It his recalled that in many applications the hydrolysis is considered the bottleneck of the overall anaerobic digestion process when the input substrate is constituted of complex organic matter. In Esposito's model a surfaced based kinetic approach for the disintegration of complex organic matter is introduced. This approach allows to better model the disintegration step taking into account the effect of particle size distribution on the digestion process. This model needs thus GSA and UQ to pave the way for further improvements and reach a deep understanding of the main processes and leading input factors. Due to the large number of parameters to be analyzed a first preliminary screening analysis, with the Morris' Method, has been conducted. Since two quantities of interest (QoI) have been considered, the initial screening has been performed twice, obtaining two set of parameters containing the most influential factors in determining the value of each QoI. A surrogate of ADM1 model has been defined making use of the two defined quantities of interest. The output results from the surrogate model have been analyzed with Sobol’ indices for the quantitative GSA. Finally, uncertainty quantification has been performed. By adopting kernel smoothing techniques, the Probability Density Functions of each quantity of interest have been defined
Anaerobic co-digestion of organic wastes
No full textOver the last years anaerobic digestion has been successfully established as technology to treat organic wastes. The perspective of turning, through a low-cost process, organic wastes into biogas, a source of renewable energy and profit, has certainly increased the interest around this technology and has required several studies aimed to develop methods that could improve the performance as well as the efficiency of this process. The present work reviews the most interesting results achieved through such studies, mainly focusing on the following three aspects: (1) the analysis of the organic substrates typically co-digested to exploit their complementary characteristics; (2) the need of pre-treating the substrates before their digestion in order to change their physical and/or chemical characteristics; (3) the usefulness of mathematical models simulating the anaerobic co-digestion process. In particular these studies have demonstrated that combining different organic wastes results in a substrate better balanced and assorted in terms of nutrients, pre-treatments make organic solids more accessible and degradable to microorganisms, whereas mathematical models are extremely useful to predict the co-digestion process performance and therefore can be successfully used to choose the best substrates to mix as well as the most suitable pre-treatments to be applied
Modelling the ecology of phototrophic-heterotrophic biofilms
No full textA mathematical model describing the microbial interactions in phototrophic-heterotrophic biofilms is presented. The main phenomena and factors involved in the model include: biomass growth and decay, substrates production, diffusion and consumption, biological invasion of planktonic species and detachment. In particular, non linear hyperbolic PDEs describe the growth of the microbial species while quasi-linear parabolic PDEs govern the dynamics of substrates and invading species. The whole system of PDEs is considered in a free boundary domain. The following syntrophic interactions are also modelled: the exchange of dissolved oxygen, organic carbon and inorganic carbon produced and released by phototrophs and heterotrophs, respectively. The positive effect of heterotrophic pioneers on the phototrophic growth is modelled by introducing a phototrophic colonization rate depending on the EPS fraction in the biofilm. Numerical simulations are performed to test model accuracy. Simulation results reproduce the main symbiotic mechanisms between phototrophs and heterotrophs reported in literature, such as the positive effect of heterotrophic pioneers and their EPS production on phototrophic growth and the effects of phototrophic organic carbon release on the invasion and growth of heterotrophic bacteria. Furthermore, model results highlight the role played by heterotrophic species under photoinhibition conditions, which provide a positive shading contribution to phototrophic growth. Light is confirmed as the most significant factor in the ecology of phototrophic-heterotrophic biofilms. Such results confirm the accuracy of the model that correctly predicts the evolution of a phototrophic-heterotrophic biofilm and the main phenomena involved, and can be seen as an auxiliary tool in different industrial applications, such as wastewater treatment and bioenergy production
Enhanced anaerobic digestion of food waste by thermal and ozonation pretreatment methods.
No full textTreatment of food waste by anaerobic digestion can lead to an energy production coupled to a reduction of the volume and greenhouse gas emissions from this waste type. According to EU Regulation EC1774/2002, food waste should be pasteurized/sterilized before or after anaerobic digestion. With respect to this regulation and also considering the slow kinetics of the anaerobic digestion process, thermal and chemical pretreatments of food waste prior to mesophilic anaerobic digestion were studied. A series of batch experiments to determine the biomethane potential of untreated as well as pretreated food waste was carried out. All tested conditions of both thermal and ozonation pretreatments resulted in an enhanced biomethane production. The kinetics of the anaerobic digestion process were, however, accelerated by thermal pretreatment at lower temperatures (<120 C) only. The best result of 647.5 ± 10.6 mlCH4/gVS, which is approximately 52% higher as compared to the specific biomethane production of untreated food waste, was obtained with thermal retreatment at 80 C for 1.5 h. On the basis of net energy calculations, the enhanced biomethane production could cover the energy requirement of the thermal pretreatment. In contrast, the enhanced biomethane production with ozonation pretreatment is insufficient to supply the required energy for the ozonator
Effect of Ammoniacal Nitrogen on One-Stage and Two-Stage Anaerobic Digestion of Food Waste.
No full textThis research compares the operation of one-stage and two-stage anaerobic continuously stirred tank reactor (CSTR) systems fed semi-continuously with food waste. The main purpose was to investigate the effects of ammoniacal nitrogen on the anaerobic digestion process. The two-stage system gave more reliable operation compared to one-stage due to: (i) a better pH self-adjusting capacity; (ii) a higher resistance to organic loading shocks; and (iii) a higher conversion rate of organic substrate to biomethane. Also a small amount of biohydrogen was detected from the first stage of the two-stage reactor making this system attractive for biohythane production. As the digestate contains ammoniacal nitrogen, re-circulating it provided the necessary alkalinity in the systems, thus preventing an eventual failure by volatile fatty acids (VFA) accumulation. However, re-circulation also resulted in an ammonium accumulation, yielding a lower biomethane production. Based on the batch experimental results the 50% inhibitory concentration of total ammoniacal nitrogen on the methanogenic activities was calculated as 3.8. g/L, corresponding to 146. mg/L free ammonia for the inoculum used for this research. The two-stage system was affected by the inhibition more than the one-stage system, as it requires less alkalinity and the physically separated methanogens are more sensitive to inhibitory factors, such as ammonium and propionic acid
Qualitative analysis and simulations of the biological fouling problem on filtration membranes
No full textThe mitigation of fouling formation and development in filtration systems represents the most critical aspect for water treatment, as it naturally affects both the operational costs for management procedures, and the duration of filtration devices. As it constitutes an artificial physical barrier for solid particles retention, membrane systems are perfect environments for the adhesion and development of biological fouling layers, especially in wastewater treatment reactors. Based on recent results, the present work focuses on the qualitative analysis of a mono dimensional continuous model for biofouling dynamics in microfiltration systems. The free boundary problem accounting for the evolution of the biofouling layer during the filtration regimen has been discussed in terms of existence and uniqueness of the solution. The achieved results represent a consistent base for numerical studies related to the correct prediction of transmembrane pressure in membrane systems. Numerical examples related to the heterotrophic-autotrophic interaction occurring in wastewater treatment plants have been presented to highlight the effect of crucial biological aspects, such as extracellular polymeric substances (EPS) accumulation, usually neglected in the classical membrane filtration modeling. The description of biofouling dynamics and membrane performance during the filtration regimen highlight key aspects for microfiltration system management in all industrial applications
Mathematical Modeling of Biofilms
No full textA continuum approach to mathematical modeling of multispecies biofilm formation and growth is presented. The equations governing the biological process are derived from mass balance principles in the general 3D situation. The 1D equations are inferred as special cases. A system of n nonlinear hyperbolic partial differential equations (PDEs) for the n bacterial species forming the biofilm, m semilinear parabolic (or elliptic) PDEs for the m substrates present in the biofilm, and an ordinary differential equation (ODE) for the motion of the biofilm boundary are obtained. In addition, a further system of parabolic PDEs is added in some situations, such as the invasion of new bacterial species and colonization into an already constituted biofilm. All equations mentioned are mutually connected and must be solved simultaneously in a domain that is a further unknown within the mathematical problem. Transforming the problem to characteristic coordinates yields positive answers about model consistency (uniqueness, existence, positiveness of solutions). The invasion problem is illustrated with some simulations based on the method of characteristics. The biofilm-reactor model is also discussed
Modelling the comparative influence of conjugation and transformation on plasmid spread in biofilms
No full textIn this work, we propose a multidimensional continuum model for plasmid dissemination in biofilms via horizontal gene transfer. The model is formulated as a system of nonlocal partial differential equations derived from mass conservation laws and reaction kinetics principles. Biofilm is modelled as a homogeneous, viscous, incompressible fluid with a velocity given by Darcy's law. The model considers plasmid-carrying cells as distinct volume fractions and their vertical and horizontal gene transfer via conjugation and natural transformation. The model encompasses local detoxification of biofilm due to plasmid-borne resistance gene and its effect at the community scale. The equations are solved numerically and simulations are performed to investigate how transformation and conjugation regulate the dynamics and the ecology of plasmid spread in both a multidimensional and one-dimensional biofilm system. Model results are able to predict relevant experimentally observed results in plasmid spread, such as the respective intensity of different horizontal gene transfer mechanisms and the importance of selective pressure. Moreover, model results predict coexistence of plasmid-carrying and plasmid-free bacteria even in conditions when one should out-compete the other, offering a simple modelling explanation on global plasmid persistence in bacterial communities
Modeling heavy metal sorption and interaction in a multispecies biofilm
No full textA mathematical model able to simulate the physical, chemical and biological interactions prevailing in multispecies biofilms in the presence of a toxic heavy metal is presented. The free boundary value problem related to biofilm growth and evolution is governed by a nonlinear ordinary differential equation. The problem requires the integration of a system of nonlinear hyperbolic partial differential equations describing the biofilm components evolution, and a systems of semilinear parabolic partial differential equations accounting for substrates diffusion and reaction within the biofilm. In addition, a semilinear parabolic partial differential equation is introduced to describe heavy metal diffusion and sorption. The biosoption process modeling is completed by the definition and integration of other two systems of nonlinear hyperbolic partial differential equations describing the free and occupied binding sites evolution, respectively. Numerical simulations of the heterotrophic-autotrophic interaction occurring in biofilm reactors devoted to wastewater treatment are presented. The high biosorption ability of bacteria living in a mature biofilm is highlighted, as well as the toxicity effect of heavy metals on autotrophic bacteria, whose growth directly affects the nitrification performance of bioreactors
Enhanced bio-methane production from co-digestion of different organic wastes
No full textThis paper deals with an experimental study aimed at assessing the effect of mixing different organic wastes on the anaerobic digestion process. Livestock manure and organic solid wastes have been taken into account as substrates to verify if their mixing gives rise to higher methane production rates and lower risk of process failure. Bio-methane potential (BMP) tests have been conducted using the following substrates: buffalo manure (BM), poultry manure (PM), organic fraction of the municipal solid waste (OFMSW), greengrocery waste (GW) and two different mixtures composed of BM and OFMSW. Mixing BM with OFMSW resulted in 12% and 30% higher methane volumes after 30 and 15 days from the test start, respectively. Experimental data have been also used to calibrate and validate a mathematical model previously proposed by the authors, showing its capability to reproduce the synergistic effect on methane production promoted by co-digesting BM and OFSMW. © 2012 Taylor and Francis
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