Modelització matemàtica de la transferència de massa i energia en el procés de compostatge

El procés de compostatge és complex degut a que hi intervenen simultàniamentfenòmens de naturalesa molt diversa que interactuen entre ells. La modelitzaciómatemàtica és una eina que facilita l’anàlisi d’aquests sistemes complexos, però elnivell de desenvolupament i aplicació ha estat força limitat fins a l’actualitat. En elpresent treball es desenvolupa un model matemàtic determinista del procés decompostatge basat en les lleis bàsiques dels fenòmens físics i biològics que permetexplicar i quantificar els principals fenòmens observats durant el procés.El model desenvolupat estableix una metodologia de presentació dels fenòmensbiològics que permet acoblar al model físic proposat models desenvolupats per altresautors. Aquest considera el substrat com una matriu porosa constituïda per les tres fases,sòlida, líquida i gasosa, entre les que s’estableixen fluxos de massa i energia governatsper les lleis bàsiques de transferència. La fase gasosa, una mescla d’oxigen, CO2,amoníac, vapor d’aigua i nitrogen, és considerada com de mescla completa en el modelbase. La seva extensió a l’espai 1D incorpora el moviment de la fase gasosa degut agradients de pressió, el transport conductiu d’escalfor al sí de la matriu porosa,l’arrossegament convectiu d’energia, tant en forma de calor sensible com latent, i eltransport dels components de la fase gasosa.S’han dissenyat i construït tres reactors verticals estàtics d’aireig forçat a escala delaboratori i s’han operat amb diferents mescles de residus, a fi d’obtenir informació percontrastar la bondat del model desenvolupat. Durant els experiments s’ha mesurat ienregistrat l’evolució de diferents variables del procés: temperatura a diferents puntsdels reactors, concentracions d’oxigen i CO2 als gasos de sortida, massa total en elsreactors, cabal d’aire insuflat, pressió de l’aire a l’entrada dels reactors, així coml’assentament a diferents nivells del material en compostatge. A principi i final deprocés s’ha mesurat el contingut d’humitat a diferents nivells del material.Les simulacions numèriques, realitzades amb un únic conjunt de paràmetres físics,estequiomètrics i cinètics per als diferents experiments amb substrats diferents,reprodueixen satisfactòriament les tendències observades en les dades experimentals, toti que aquestes mostren que la hipòtesi de simetria radial en els reactors no sempre esverifica. L’anàlisi de sensibilitat realitzada ha permès identificar els paràmetres físicsque més influeixen en l’evolució del procés. Es proposa una equació de 3 paràmetresper quantificar el camp d’assentaments dins el material en compostatge en funció deltemps. A més, s’ha establert una equació que relaciona l’assentament del material ambel consum acumulat d’oxigen.En síntesi, el model desenvolupat simula satisfactòriament les tendències en l’evolucióde les variables de procés observades, integra els diferents fenòmens presents en elprocés de compostatge i permet quantificar la seva importància relativa. L’estructuraoberta del model facilita la incorporació de nous fenòmens.El proceso de compostaje es complejo debido a que en él intervienen simultáneamentefenómenos de naturaleza muy distinta interaccionando entre ellos. La modelizaciónmatemática es una herramienta que facilita el análisis de estos sistemas complejos, peroel nivel de desarrollo y aplicación al proceso de compostaje ha sido bastante limitadohasta la actualidad. En el presente trabajo se desarrolla un modelo matemáticodeterminista del proceso de compostaje basado en les leyes básicas de los fenómenosfísicos y biológicos que permite explicar y cuantificar los principales fenómenosobservados durante el proceso.El modelo desarrollado establece una metodología de presentación de los fenómenosbiológicos que permite acoplar al modelo físico propuesto modelos desarrollados porotros autores. Este considera el sustrato como una matriz porosa constituida por las tresfases, sólida, líquida i gaseosa, entre las que se establecen flujos de masa y energíagobernados por las leyes básicas de transferencia. La fase gaseosa, una mezcla deoxígeno, CO2, amoníaco, vapor de agua y nitrógeno, es considerada como de mezclacompleta en el modelo base. Su extensión al espacio 1D incorpora el movimiento de lafase gaseosa debido a gradientes de presión, el transporte conductivo de calor dentro dela matriz porosa, el arrastre convectivo de energía, tanto en forma de calor sensiblecomo latente, y el transporte de los componentes de la fase gaseosa.Se han diseñado y construido tres reactores verticales estáticos de aireación forzada aescala de laboratorio y se han operado con distintas mezclas de residuos, a fin deobtener información para contrastar la bondad del modelo desarrollado. Durante losexperimentos se ha medido y registrado la evolución de diferentes variables del proceso:temperatura a distintos puntos de los reactores, concentraciones de oxígeno y CO2 enlos gases de salida, masa total en los reactores, caudal de aire insuflado, presión del airea la entrada de los reactores, así como el asentamiento a diferentes niveles del materialen compostaje. A principio y final de proceso se ha medido el contenido de humedad adiferentes niveles del material.Las simulaciones numéricas, realizadas con un único conjunto de parámetros físicos,estequiométricos y cinéticos para los diferentes experimentos con sustratos distintos,reproducen satisfactoriamente las tendencias observadas en los datos experimentales, apesar de que éstos muestran que la hipótesis de simetría radial en los reactores nosiempre se verifica. El análisis de sensibilidad realizado ha permitido identificaraquellos parámetros físicos con más influencia en la evolución del proceso. Se proponeuna ecuación de 3 parámetros para cuantificar el campo de asentamiento en el materialen compostaje en función del tiempo. Además, se ha establecido una ecuación querelaciona el asentamiento del material con el consumo acumulado de oxígeno.En síntesis, el modelo desarrollado simula satisfactoriamente las tendencias en laevolución de las variables de proceso observadas, integra los distintos fenómenospresentes en el proceso de compostaje y permite cuantificar su importancia relativa. Laestructura abierta del modelo facilita la incorporación de nuevos fenómenosThe interaction of phenomena of different nature in the composting process makes it complex. Mathematical modelling is ahelpful tool to analyse such complex systems, but its development and application to the composting process has beenrather scarce up to date. The present study presents a deterministic mathematical model of the composting process whichallows the explanation and quantification of the main phenomena observed during the process and is based on the basiclaws of physical and biological elementary processes.The developed model sets up a methodology for the presentation of the biological phenomena involved, making possible tocouple models developed by different authors to the proposed physical model. This one considers the substrate as a porousmedia in which the three phases, solid, liquid and gaseous, are present. Mass and energy flows between them are governedby basic transfer laws. The gaseous phase is considered, in the base model, as a completely stirred mixture of oxygen, CO2,ammonia, water vapour and nitrogen, both considered as ideal gases. Its extension to the 1D space incorporates themovement of the gaseous phase due to pressure gradient, the conductive heat transfer in the solid matrix, the convectiveenergy transport, both as sensible and latent heat, and the convective mass transport of gaseous components.Three vertical static reactors at lab scale with forced aeration have been designed, built up and operated with different solidwaste mixtures in order to pick up data for checking the model predictions. Several process variables have been measuredand registered during the experiments: temperature at different points inside the reactors, oxygen and CO2 concentration inthe exhaust gases, total mass in the reactors, airflow rate, air pressure at the reactors inlet, and the settlement at differentlevels of the composting mass. The moisture content at different positions of the material has also been measured at thestart and at the end of the process.The simulations, run with a unique set of physical, stoichiometric and kinetic parameters for the experiments with differentsubstrates, reproduced satisfactorily the main trends observed by the experimental data, although these show that thehypothesis of radial symmetry in the reactors does not always holds on. A sensitivity analysis allowed the identification of thephysical parameters with greater influence on the process. A three parameter equation is proposed to describe thesettlement field inside the composting mass as a function of time. An equation linking settlement with the cumulative oxygenconsumption has also been proposed.In summary, the developed model simulates satisfactorily the main trends observed during the evolution of the observedprocess variables, integrates the different phenomena present in the composting process, and allows the quantification of itsrelative importance. The open structure of the model simplifies the incorporation of new phenomena

A new structured mathematical model of the composting process

Postprint (published version

1D-Space dynamic mathematical model of the composting process

Mathematical modelling is a helpful tool to analyse complex systems, but its development and application to the composting process has been rather scarce up to date. The use of mathematical models is common in complex biological processes such as the activated sludge wastewater treatment and the anaerobic digestion process, both systems operating on a single isothermal phase.Postprint (published version

Exergetic optimisation of a solar energy installation

This paper deals with the Exergetic optimisation stutdy of the PCM’s slabs- Solar Energy Storage Tank (SEST). Both melting temperature and the maximum power output taking into account the phase change process are determined.Peer ReviewedPostprint (published version

Efficient dynamic simulation of pH in processes associated to biofiltration of volatile inorganic pollutants

This work proposes a generic methodology to include the pH as a state variable in mathematical models of bioreactors. An ordinary differential equation for pH is stated and introduced into the general model structure of a biotrickling filter. All chemical equilibriums were considered and included into the model framework. A preliminary evaluation was performed by comparing results predicted by the model with experimental data obtained from the oxidation of thiosulfate by sulfide-oxidizing bacteria under alkaline conditions. The model was able to describe adequately the evolution of the main state variables including the pH for the initial complete oxidation of thiosulfate. The methodology presented here can be easily adapted to other mathematical models dealing with biological waste treatment processes in which pH appears as a key factor.Postprint (published version

Combined Acidification and Solar Drying of Pig Slurries for Nutrient Recovery and Controlled Atmospheric Emissions

The solar drying of pig slurries was tested in a pilot-scale greenhouse (10 m2 footprint), operated with forced ventilation under low and high solar irradiation in Mediterranean conditions. Gaseous emissions were prevented through slurry acidification and by the biofiltration of exhaust gases. Air relative humidity and temperature in and out the greenhouse, as well as the weight of a slurry sample, were monitored online to command the ventilation regime. Daily average drying rate values ranged from 0.3 to 2.8 kg m−2 d −1 and displayed a direct dependency with solar radiation until the pig slurry lost a 60% of its initial weight, with a solar energy efficiency of about 26%. Upon further drying, the water content from pig slurries stabilized at around 10%. Mass balances between the initial slurry and dried product were closed for total solids and organic matter, but the recovery of nutrients ranged from 69% to 81%, apparently because of precipitation and incrustation phenomena. The NPK composition of the final product was 4.3–2.5–3.8 and fulfilled current regulations for solid organic fertilizers. Operational costs of the drying process and fertilizing quality parameters were also discussed.This research was supported by the CERCA Programme/Generalitat de Catalunya. The assistance of the Catalan Ministry of Agriculture, Livestock, Fisheries and Food is also acknowledged. The authors from IRTA and the Universitat de Lleida belong to the Consolidated Research Group TERRA (ref. 2017 SGR 1290)

Effect of Harvesting Age and Size Reduction in the Performance of Anaerobic Digestion of Pennisetum Grass

In the rural zones of Latin American and Caribbean developing countries, the poorest households rely on traditional fuels such as firewood to meet their daily cooking needs. Many of those countries are located near the equator, where they have a tropical climate and grass is one of the most common biomass crops. The aim of this study was to evaluate the effect of harvesting age (30, 44, and 57 days) in the performance of anaerobic digestion of King Grass (Pennisetum purpureum cv. King Grass) grown under tropical climate conditions. Three reduction methods of crop size were also compared. Results showed that 44-day harvesting age presented the greater specific methane yield (347.8 mLCH4 g −1VS) and area-specific methane yield (9773 m3CH4 ha−1 y −1 ). The machine chopped method (1–3 cm for stems and 1–10 cm for leaves) was the reduction method that maximized the methane production. From those results, the calculated area required for grass cultivation to provide the cooking energy to a typical family in the Colombian rural zones is 154 m2

Estudio de procesos biológicos de tratamiento de residuos basado en la modelización

Los modelos matemáticos se han convertido en una herramienta básica para aumentar la comprensión de los procesos biológicos relacionados con el tratamiento de residuos orgánicos. En este sentido, la modelación permite crear un lenguaje de comunicación común, orientar el diseño experimental, evaluar resultados, contrastar hipótesis, revelar relaciones entre variables, prever la evolución de sistemas y, en definitiva, diseñar estrategias de gestión y tratamiento optimizados. En el presente artículo se presenta esta línea de trabajo y resultados concretos en dos ámbitos de aplicación: compostaje de mezclas de residuos y eliminación biológica de nitrógeno en purines de cerdo.Peer Reviewe

Microbial community dynamics in two-chambered microbial fuel cells : effect of different ion exchange membranes

BACKGROUND: The utilization of different kinds of ion exchange membrane is a common practice in bioelectrochemical systems such as two-chambered microbial fuel cells (MFCs). However, little is known about the effect of membrane materials on the anodic microbial community diversity.; RESULTS: The effect of two cationic and one anionic exchange membranes (Nafion N-117, Ultrex CMI-7000, and Ultrex AMI-7000) on the microbial community dynamics of Eubacteria and Archaea has been assessed in two-chambered MFCs. The experimental results indicated that the eubacterial community in the anodic chamber was not affected by the membrane materials, being predominantly populations of Bacteroidetes (Porphyromonadaceae) and -proteobacteria (Alcaligenaceae and Comamonadaceae). On the other hand, the archaeal counterpart appears to be highly dependent on the type of membrane used, as was evidenced by the selective enrichment of Methanosarcina sp. in the MFC equipped with the membrane Nafion N-117 which was the MFC that showed the highest current production.; CONCLUSIONS: The results obtained in the present study suggest that membrane materials affect archaeal diversity whereas both anodofilic eubacteria and methanogenic archaea populations could play an important role in the overall MFC process performance.Peer ReviewedPostprint (author’s final draft

1D-Space dynamic mathematical model of the composting process

Mathematical modelling is a helpful tool to analyse complex systems, but its development and application to the composting process has been rather scarce up to date. The use of mathematical models is common in complex biological processes such as the activated sludge wastewater treatment and the anaerobic digestion process, both systems operating on a single isothermal phase