Tratamiento de emisiones a la atmósfera de acetato de etilo y tolueno mediante biofiltros de turba.

En las últimas décadas, la calidad del aire y el control de la contaminación atmosférica vienen siendo líneas principales de actuación en política medioambiental a fin de evitar daños sobre la salud humana y el medio ambiente. Los compuestos orgánicos volátiles, entre los que se encuentran el acetato de etilo y el tolueno, son uno de los principales contaminantes atmosféricos de origen industrial. Las nuevas reglamentaciones asociadas a la emisión de compuestos orgánicos volátiles procedentes del uso de disolventes en determinadas instalaciones industriales hacen necesaria la búsqueda de técnicas o procedimientos para reducir, minimizar o eliminar este tipo de emisiones. En este sentido, los sistemas biológicos de tratamiento como la biofiltración se presentan como una alternativa económica y respetuosa con el medio ambiente. En este trabajo de tesis doctoral se presenta un estudio del proceso de biofiltración en el que se han abordado dos líneas de trabajo principales:· Estudio experimental en bioreactores para la depuración de aire contaminado con acetato de etilo y/o tolueno, utilizando turba fibrosa como material de relleno, bajo condiciones de estado estacionario y bajo condiciones de alimentación intermitente.· Desarrollo de un modelo matemático para su uso como herramienta de predicción y de diseño.En los ensayos llevados a cabo bajo condiciones de estado estacionario se ha realizado un seguimiento de la operación de los biofiltros durante 2 años de experimentación. En primer lugar se ha evaluado la influencia que tiene el uso de inóculos previamente adaptados a los compuestos a degradar sobre el tiempo de puesta en marcha del biofiltro. A continuación se ha evaluado la influencia de la carga volumétrica de entrada de contaminante en la capacidad de eliminación trabajando con un tiempo de residencia constante de 90 s, para posteriormente analizar la influencia del tiempo de residencia en la eficacia del proceso, ampliando el estudio a tiempos de residencia comprendidos entre 22 y 60 s. En muchas ocasiones, las emisiones industriales suelen presentar un carácter intermitente y oscilante asociado al proceso productivo del que provienen. A fin de evaluar estas variaciones sobre la respuesta transitoria del sistema, los tres biofiltros se han sometido a condiciones de alimentación intermitente con interrupciones nocturnas y de fin de semana del aporte de contaminante. A su vez, la influencia que las perturbaciones en la alimentación tienen sobre el rendimiento del sistema se ha estudiado mediante la aplicación de variaciones instantáneas de la concentración de entrada del compuesto y del caudal de aire contaminado. Finalmente, se ha evaluado el tiempo necesario para recuperar la eficacia de eliminación en cada uno de los tres biofiltros tras una interrupción prolongada de 15 días en el aporte de contaminante, situación que representa una posible parada vacacional en los procesos industriales. De forma rutinaria se ha realizado un seguimiento periódico de la variación de la concentración de biomasa a lo largo del lecho, implementando para ello, una técnica de recuento por tinción mediante fluorocromos a este sistema. Finalmente, se presenta el desarrollo de un modelo matemático para la predicción del funcionamiento de los tres biofiltros en condiciones de estado estacionario. El modelo incluye como hipótesis principal la existencia de una densidad celular no uniforme a lo largo del lecho, asumiendo que se establece un perfil decreciente en la concentración de biomasa activa a lo largo del lecho. Además, se ha empleado un procedimiento basado en la Matriz de Información de Fisher para obtener los intervalos de confianza del 95% de los parámetros estimados. La verificación del modelo planteado se ha realizado utilizando los datos experimentales procedentes del tratamiento de la mezcla equimásica de ambos contaminantes.The volatile organic compound (VOC), like ethyl acetate and toluene, are one of the main air pollutants from industrial sources. The biofiltration is an economic and environment-friendly technology to removal these compounds. In this doctoral thesis has been carried out an experimental study in three bioreactors for the treatment of air contaminated with ethyl acetate and/or toluene, using fibrous peat as packing material under steady state and intermittent loading conditions. Besides, it has been developed a mathematical model which can be used for prediction and design purposes. In the experiments under steady state conditions (2 years of experimentation), initially, the biofilter start-up time has been evaluated using an inoculum previously adapted to the compounds to degrade. Subsequently, the elimination capacity has been determined working with empty bed residence times between 22 and 90 s. In the experiments under intermittent loading conditions, the three biofilters have been operated with overnight and weekend shutdowns. Concurrently, instantaneous inlet concentration and flow rate changes have been carried out. Finally, it has been evaluated the time needed to recover the removal efficiency in each of the biofilters after of a 15 days starvation period. The living and the dead cells concentrations along the bed biofilters have been monitored by the fluorescence microscopy using LIVE/DEAD® BacLightTM. Finally, a mathematical model has been developed. The model includes as main hypothesis the existence of a non-uniform cell density throughout the bed. It has also used the Fisher Information Matrix for the confidence intervals of 95% of the estimated parameters. The model validation has been carried out using experimental data from the treatment of the 1:1 (w:w) mixture

Modelling mass transfer properties in a biotrickling filter for the removal of isopropanol

A study was carried out to model mass transfer properties in biotrickling filters, treating isopropanol as the target pollutant. This study was extended to the mass transfer of oxygen related to the fact that the treatment of hydrophilic compounds by biotrickling filtration is often limited by oxygen. A simple method for each compound was developed based on their physical properties. The influence of temperature on the Henry"s law constant of isopropanol was determined. An increase of 1.8 per 10ºC for the dimensionless Henry"s law constant was obtained. The determination of the overall mass transfer coefficients of isopropanol (KGa) was carried out, obtaining values between 500 and 1800 h-1 for gas velocities of 100 and 300 m h-1. No significant influences were observed for either the liquid velocity or packing material. Also, the determination of overall mass transfer coefficients of oxygen (KLa) were carried out, obtaining values between 20 and 200 h-1 depending on the packing material for liquid velocities between 2 and 33 m h-1. Structured packing materials exhibited greater mass transfer coefficients, while for random packing materials, the mass transfer coefficients clearly benefited from the high specific surface area. Mathematical correlations found in the literature were compared with the empirical data, showing that neither was capable of reproducing the mass transfer coefficients obtained empirically. Thus, empirical relationships between the mass transfer coefficients and the gas and liquid velocities are proposed to characterise the syste

Performance of peat biofilters treating ethyl acetate and toluene mixtures under non-steady-state conditions

[Abstract] This paper presents the response of peat biofilters to loading changes corresponding to industrial practices such as overnight and weekend shutdowns, intermittent emission or inlet concentration peaks. Three laboratory-scale reactors fed with air contaminated with ethyl acetate, toluene or a 1:1 mixture of ethyl acetate and toluene were operated under 65 g m-3 h-1 inlet load and 60 s EBRT during 16 h/day, 5 days/week. Dynamic behavior after feed resumption after night and weekend closures showed a 1-2 h period of transient response to recover stable CO2 production values. No increase in VOC emission was observed, except for biofilters treating toluene for which a transient peak in VOC emission during 4-8 h after weekend closures was detected. More stressful conditions such as intermittent emissions (2 h-on/ 2 h-off, 16 h/day, 5 days/week), or inlet concentration peaks (40-min, 50% increase) were successfully handled in the biofilter treating only ethyl acetate; but deterioration in the operation was observed in presence of toluene. The system performance after 15-days starvation period was fully recovered in less than 8 h of re-acclimation period. Living and dead cells monitoring results are also presented

Comparison of simultaneous saccharification and fermentation and separate hydrolysis and fermentation processes for butanol production from rice straw

Rice straw (RS) is one of the lignocellulosic wastes with the highest global production. The main objective of this study was to maximise the butanol production by Clostridium beijerinckii DSM 6422 from RS pretreated by microwave-assisted hydrothermolysis. Two different fermentation strategies were compared: separate hydrolysis and fermentation (SHF, two-step process) and simultaneous saccharification and fermentation (SSF, one-step process). In parallel, the variables that significantly affected the butanol production were screened by using fractional factorial designs. Butanol concentration and productivity at 48 h were, respectively, 8% and 173% higher in SSF than in SHF. A one-step process was more efficient than a two-step process, especially considering the time savings derived from much higher productivity. From these results, SSF was further optimised by response surface methodology with central composite design over the key factors on the butanol production at 48 h: initial pH, enzyme loading and yeast extract concentration. The optimum point yielded a butanol productivity of 0.114 g L-1h−1, with a butanol-biomass ratio of 51 g kg−1 of raw RS (ABE-biomass ratio of 77.0 g kg−1 of raw RS). The parameter with the greatest effect was enzyme loading, with an optimal value of 13.5 FPU g-dw-1. This study showed that microwave-processed RS has great potential as a substrate for the butanol production from ABE fermentation when combining process stages by SSF

Intermittent operation of UASB reactors treating wastewater polluted with organic solvents: process performance and microbial community evaluation

The effect of intermittent feeding on the treatment of wastewater polluted with ethanol, ethyl acetate and 1-ethoxy-2-propanol in anaerobic upflow sludge blanket reactors was investigated. Three laboratory-scale reactors, one periodically supplemented with chitosan, were operated in an intermittent pattern (16 hours per day; 5 days per week) during 5 months. Removal efficiencies higher than 94% were obtained at organic loading rates up to 50 kg COD m−3 d−1. The addition of chitosan positively affected the specific methanogenic activity of the granular sludge. Although partial deterioration of the granules was observed, it was not correlated with variations in the production of extracellular polymeric substances, the percentage of granules remained between 57 and 84%. Microbial community analysis showed the prevalence of bacteria of the genus Geobacter and archaea of the Methanocorpusculum genus were the most abundant methanogens, suggesting that hydrogenotrophic methanogenesis, with the syntrophic oxidation of the substrate, was an important pathway for solvent degradation

Evolution of bacterial community in isopropanol-degrading biotrickling filters by fluorescence in situ hybridization (FISH)

In this study, the bacterial population of two biotrickling filters (BFTs) treating isopropanol by using fluorescence in situ hybridization (FISH) is analyzed. The experimental system consists in two identical laboratoryscale BFTs named as BFT1 and BFT2. The two bioreactors were operated in parallel during an experimental period of one year working under intermittent feeding conditions Operating conditions and maintenance were identical in both BFT

Performance and feasibility of biotrickling filtration in the control of styrene industrial air emissions

The performance and feasibility of a pilot unit of biotrickling filter (BTF) for the treatment of industrial emissions polluted by styrene was investigated for one year at a fiber reinforced plastic industrial site. The pilot unit was packed with a structured material with a volume of 0.6 m3. Monitoring results have shown successful treatment of the industrial styrene emissions working at empty bed residence times (EBRT) between 31 and 66 s. The best performance was obtained after 300 days when a more stable biofilm had been developed, obtaining the highest elimination capacity of 18.8 g m−3 h−1 (removal efficiency of 75.6%) working at 31 s of EBRT. In addition, a photocatalytic reactor was evaluated as pretreatment of the biological process, but results have shown very low capacity for improving the BTF performance due to catalyst deactivation. The economic feasibility of the BTF was evaluated. The total direct cost, excluding capital recovery, of the biotrickling filter technology was estimated in 0.71 year−1 per Nm3 h−1 of treated air whereas 2.27 year−1 per Nm3 h−1 was obtained for the regenerative catalytic oxidizer equipped with a zeolite pre-concentrator. Results show that this technology is economically and environmentally competitive in comparison with thermal treatment

Control of VOCs from printing press air emissions by anaerobic bioscrubber: Performance and microbial community of an on-site pilot unit

A novel process consisted of an anaerobic bioscrubber was studied at the field scale for the removal of volatile organic compounds (VOCs) emitted from a printing press facility. The pilot unit worked under high fluctuating waste gas emissions containing ethanol, ethyl acetate, and 1-ethoxy-2-propanol as main pollutants, with airflows ranging between 184 and 1253 m3 h−1 and an average concentration of 1126 ± 470 mg-C Nm−3. Three scrubber configurations (cross-flow and vertical-flow packings and spray tower) were tested, and cross-flow packing was found to be the best one. For this packing, daily average values of VOC removal efficiency ranged between 83% and 93% for liquid to air volume ratios between 3.5·10−3 and 9.1·10−3. Biomass growth was prevented by periodical chemical cleaning; the average pressure drop was 165 Pa m−1. Rapid initiation of anaerobic degradation was achieved by using granular sludge from a brewery wastewater treatment plant. Despite the intermittent and fluctuating organic load, the expanded granular sludge bed reactor showed an excellent level of performance, reaching removal efficiencies of 93 ± 5% at 25.1 ± 3.2 °C, with biogas methane content of 94 ± 3% in volume. Volatile fatty acid concentration was as low as 200 mg acetic acid L−1 by treating daily average organic loads up to 3.0 kg COD h−1, equivalent to 24 kg COD m−3 bed d−1. The denaturing gradient gel electrophoresis (DGGE) results revealed the initial shift of the domains Archaea and Bacteria associated with the limitation of the carbon source to a few organic solvents. The Archaea domain was more sensitive, resulting in a drop of the Shannon index from 1.07 to 0.41 in the first 123 days. Among Archaea, the predominance of Methanosaeta persisted throughout the experimental period. The increase in the proportion of Methanospirillum and Methanobacterium sp. was linked to the spontaneous variations of operating temperature and load, respectively. Among Bacteria, high levels of ethanol degraders (Geobacter and Pelobacter sp.) were observed during the trial