Biofiltració de contaminants gasosos en airecaracterització de paràmetres clau per l'estudi i modelització del creixement de biomassa

Resumen

Biofiltration has become an effective and economical alternative to traditional gas treatment systems. High costs of operation and energy consumption associated to conventional treatments have lead to increase the attention on biological processes. In general, a biofilter consists in a reactor packed with a carrier material (organic or inorganic) serving as a support for biofilm growth. The contaminated air stream to be treated is passed through the fixed-bed and the pollutant is transferred from the gas to the biofilm by absorption. In the biofilm, diffusion and biodegradation take place simultaneously. Thus, biofiltration employs the metabolic activity of microorganisms to degrade pollutants which are the energy source for microbial growth. In biofiltration, the mass transfer processes from the gas phase to the biofilm and the posterior biological degradation are the main phenomena involved in the abatement of volatile compounds in air. The correct description of biofilters, based on main phenomena, is related with physico-chemical properties of the packing materials used to immobilize the biomass and the operating conditions. Biofilters modeling has received less attention in comparison to experimental works, which is related with the complexity of the process, due to the physical, chemical and biological interactions. In this doctoral Thesis, firstly, research motivation, scope and outline of the work is presented (chapter 1). Secondly, in a general introduction (chapter 2), main physico-chemical treatments to abate volatile compounds in air are described in comparison to the different biological alternatives. In the same chapter, the key factors in the operation of biofilters are presented. Key factors lead to describe main mechanisms involved in the process of biofiltration, in order to incorporate the different phenomena in a mathematical model to describe the behavior of biofilters, underlining the biomass growth and their consequences in the operation. Objectives are in chapter 3 and Materials and Methods in chapter 4. Doctoral Thesis is presented as a compendium of 9 articles annexed in the Results’ chapter (chapter 5) to their latter general discussion (chapter 6). First 6 articles are related with experimental results of key factors and the other 3 articles are related with mathematical modeling of the processes which describe the behavior of biofilters. In the article “A comparative study based on physical characteristics of the most suitable packing materials for common situations in biofiltration” 10 packing materials commonly used as support media in biofiltration are analyzed and compared to evaluate their suitability according to physical characteristics. The materials studied were chosen according to previous works in the field of biofiltration including both organic and inorganic (or synthetic) materials. A set of nine different parameters were selected to cope with well-established factors such as material specific surface area, pressure drop, nutrients supply, water retentivity, sorption capacity and purchase cost. One ranking of packing materials was established per each parameter studied to define a relative suitability degree. Since biofiltration success generally depends on a combination of the ranked parameters, a procedure was defined to compare suitability of packing materials under common situations in biofiltration. Selected scenarios such as biofiltration of intermittent loads of pollutant and biofiltration of waste gases with low relative humidity were investigated. Among materials with high potential to be used as support media in biofilters, in the article “Evaluation of sludge-based carbon as packing material in biofiltration in comparison to classic materials”, a carbon obtained from sludge from wastewater treatment facilities is analyzed to use it as support material in a biofilter in comparison to classical materials used in this purpose. The study includes the performance of material in an operative, lab-scale biofilter. Apart from evaluating main properties of materials in their nature state, in the article “The role of water in performance of biofilters: parameterization of pressure drop and sorption capacities for common packing materials” is studied how the amount of water retained in biofilters affects physical properties of packing materials and packed beds. In this study, the influence of water on the pressure drop and sorption capacities of different packing materials were experimentally studied and compared. Pressure drop was characterized as a function of dynamic hold-up, porosity and gas flow rate. Sorption capacities for toluene were determined for both wet and dry materials to obtain information about the nature of interactions between the contaminant, the packing materials and the aqueous phase. The experimental sorption capacities of materials were fitted to different isotherm models for gas adsorption in porous materials. The corresponding confidence interval was determined by the Fisher information matrix. The results quantified the dynamic hold-up effect resulting from the significant increase in the pressure drop throughout the bed, i.e. the financial cost of driving air, and the negative effect of this air on the total amount of hydrophobic pollutant that can be adsorbed by the supports. Furthermore, the results provided equations for ascertaining water presence and sorption capacities that could be widely used in the mathematical modeling of biofilters. Later, in the article “Evaluation of mass transfer coefficients in biotrickling filters: experimental determination and comparison to correlations”, overall mass transfer coefficients were determined experimentally. A simple methodology based on overall mass balances and following a standard procedure allowed to calculate the mass transfer coefficients at different operating conditions corresponding to usual biofilter situations. Experimental results were fitted to existing and well-accepted correlations used in conventional biofilters or biotrickling filters modeling. Simple correlations for the experimental data obtained in this study were also suggested. Consequently, in the article “Development of a kinetic model for elemental sulfur and sulfate formation from the autotrophic sulfide oxidation using respirometric techniques” mass transfer is described in a respirometer in order to characterize a tool used usually in biofiltration to determine experimentally kinetic parameters. Finally, once physical phenomena have been characterized, biodegradation of a common pollutant (toluene) and the corresponding biomass growth from inoculation to biofilter clogging is studied towards future modeling of the system. In the article “Biomass accumulation in a biofilter treating toluene at high loads. Part 1: Experimental performance from inoculation to clogging” carbon dioxide production, oxygen consumption, toluene removal, pressure drop and biofilter weight were the parameters related with biomass growth which were monitored in 5 sampling ports along the height of the bioreactor during 120 days of operation. In the second step of the study, the main results in the modeling of biofilters are described according to the experimental data collected in the first part of the study. To achieve the final objective of modeling biomass growth inside biofilters, a base model is developed including the main phenomena involved in biofiltration. The validation of this base model, which predicts the outlet concentrations of pollutant according to system characteristics and operating conditions, has been used satisfactory to describe the evolution from bacterial to fungal population inside a biofilter, as it is detailed in “Modeling of a bacterial and fungal biofilter applied to toluene abatement: kinetic parameter estimation and model validation”. The mathematical model is based on detailed mass balances which include the main processes involved in the system: advection, absorption, diffusion and biodegradation. The model was calibrated and validated using experimental data obtained from two equal lab-scale biofilters. A novel procedure in gas biofilters modeling was considered for checking the model calibration, by the assessment of the parameters confidence interval based on the Fisher Information Matrix (FIM). Additionally, the incorporation of extra phenomena in the base model is studied and a sensitivity analysis is performed to distinguish the most significant parameters under a wide range of operation conditions. This part of the study is included in “Developing a general diffusion-reaction model applied to gas-phase VOCs removal by biofiltration: parameter analysis and phenomena study”. Experimental results in the study of the key factors and discussion of phenomena involved in the process, let to develop, calibrate and validate a dynamic model describing the behavior of biofilters for waste gas treatment considering biomass growth. The dynamic model predicts outlet concentrations along the height of the biofilter and the consequences of biomass growth in the progressive decrease in bed porosity and the increase in pressure drop, relating with the increase in the operating cost and the substitution of the packing material by bed clogging. The satisfactory predictions of the dynamic model to experimental data are presented in the article “Biomass accumulation in a biofilter treating toluene at high loads. Part 2: Model development, calibration and validation