Contribución al estudio de los fotorreactores de mezcla perfecta

[spa] El objeto del presente trabajo es el desarrollo, resolución y comprobación experimental de un modelo matemático para el cálculo de fotorreactores anulares de mezcla perfecta operando en continuo. Este tema incluido en una de las líneas de investigación de este Departamento, complementa un trabajo anterior sobre fotorreactores tubulares realizado por el Dr. Francisco Vall Garriga para la obtención de su grado de doctor. La Introducción se halla dividida en tres Secciones. En la primera se tratan los fundamentos de ingeniería de las reacciones fotoquímicas, es decir las fuentes de radiación ultravioleta existentes en la actualidad, los procesos fotoquímicos industriales y las principales diseños de fotorreactores, finalizando con un estudio sobre el estado de conocimientos sobre el fotorreactor objeto de este trabajo en el inicio de la investigación. Los antecedentes de la parte experimental se dsscriben en la segunda Sección,y los objetivos del presente trabajo se mencionan en la tercera Sección. El Capítulo 3 contiene los métodos de análisis matemática y cálculo del fotorreactor anular continuo de mezcla perfecta, centrándose principalmente en el cálculo del perfil de intensidades, que es la diferencia esencial con las reactores de mezcla perfecta convencionales. En el Capítulo 4 se describe la instalación experimental y en el capítulo 5 la metodología a seguir para la realización de los experimentos. Los resultados experimentales se muestran en el Capítulo 6 que se ha dividido en dos Secciones. La Sección 6.1 comprende los resultedo·s obtenidos mediante la simulación de un proceso usando un ordenador; En la Sección 6.2 se exponen los resultados obtenidos en la instalación experimental: actinometría mediante le descomposición fotoquímica del ácido oxálico en presencia de sales de uranilo, el efecto de diversas soluci'noes filtrantes sobre la reacción anterior y la fotodescomposición de soluciones acuosas de ácido fórmico. Las conclusiones a que se ha llegado, así como varies recomendaciones para trabajos posteriors, se exponen en el Capítulo 7. Los listados de los programas de cálculo matemático utilizados en este trabejo se muestran en el Apéndice junto con unas notas explicativas sobre su funcionamiento. Finaliza esta Memoria con la exposición de la nomenclatura utilizada y la Bibliografía consultada

Abatement of ozone-recalcitrant micropollutants during municipal wastewater ozonation: kinetic modelling and surrogate-based control strategies

Although ozonation is nowadays recognized as one of the most efficient technologies for micropollutants abatement in municipal wastewater effluents, several of the compounds potentially present in those waters exhibit a strong resistance to direct ozone oxidation. In addition, the real-time control of the removal process is still challenging. In this work, the abatement of ozone-recalcitrant micropollutants during wastewater ozonation of six different wastewater effluents was explored using the pesticide acetamiprid as hydroxyl radical (¿OH) probe. By means of this data, the oxidation efficiency (i.e., hydroxyl radical exposure per consumed ozone) could be described by means of a two-stage model based on the ROHO3 concept. This was possible using a semi-continuous bubbling ozone contactor in all experiments, which permitted the inclusion of the ozone mass balance in the model. ROHO3 values of (1.53-7.60)·10-7 s for initial ozonation stage and (0.61-2.95)·10-6 s for the secondary stage were obtained allowing the characterization and comparison of the process performance in a wide range of effluent qualities, including water matrices with a high content of dissolved and particulate organic matter (total organic carbon (TOC), dissolved organic carbon (DOC) and turbidity tested ranges: 6.7-50 mg C L-1, 6.6-27.6 mg C L-1 and 0.3-28.6 NTU, respectively). Finally, a surrogate strategy involving ¿OH exposure estimation by means of ultraviolet absorbance at 254 nm (UVA254) measurements was proposed based on the ROHO3 concept, and by means of its application the removal of atrazine and ibuprofen in six different wastewaters could be rightly predicted (R2 > 0.98)

Characterization and fate of EfOM during ozonation applied for effective abatement of recalcitrant micropollutants

Alterations occurring in the effluent organic matter (EfOM) during ozonation could be detrimental depending on the final application of the treated effluent. In this work, the fate of EfOM in different ozonized wastewaters was assessed through the monitoring of general water quality parameters and organic fractions determined through size-exclusion chromatography combined with organic carbon detection (SEC-OCD) analysis. These different components of EfOM were distinguished based on relative molecular weights and assigned to fractions named as biopolymers, humic substances, building blocks and low molecular weight neutrals and acids. The significant abatement (60-90%) of an ozone-refractory micropollutant (MP) was employed as reference to simulate potential scenarios in which also the presence of these species is wanted to be attenuated. Ultraviolet absorbance at 254 nm (UVA254) and chemical oxygen demand (COD) reductions ranged from 40 to 80% and from 10 to 45%, respectively, for ozone doses between 0.6 and 1.0 mM, depending on the organic matter content (both dissolved and suspended) and alkalinity of the effluents. Dissolved organic carbon (DOC) analysis showed 21-27% reductions in Membrane bioreactor (MBR) effluents, whereas for conventional activated sludge (CAS) samples this value increased (6-35%) during the oxidative treatment. This was attributed to the continuous solubilization of humic substances, according to SEC-OCD results. Moreover, accumulation of lower molecular weight fractions such as building blocks or acids was observed in all the tested effluents, and attributed to the breakdown of largest EfOM fractions, mainly humic substances. Relationships proposed in this work between humic substances evolution, water quality (UVA254) and process parameters (immediate ozone demand (IOD), IOD-normalized hydroxyl radical exposure (∫[¿OH]dt/IOD) and transferred ozone dose (TOD)) might be useful for EfOM variations estimations along ozonation

Exploring ozonation as treatment alternative for methiocarb and formed transformation products abatement

Despite the high toxicity and resistance to conventional water treatments exhibited by methiocarb (MC), there are no reports regarding the degradation of this priority pesticide by means of alternative purification technologies. In this work, the removal of MC by means of ozonation was studied for the first time, employing a multi-reactor methodology and neutral pH conditions. The second-order rate constants of MC reaction with molecular ozone (O3) and formed hydroxyl radicals (OH·) were determined to be 1.7·106 and 8.2·109 M−1 s−1, respectively. During degradation experiments, direct ozone reaction was observed to effectively remove MC, but not its formed intermediates, whereas OH· could oxidize all species. The major identified TPs were methiocarb sulfoxide (MCX), methiocarb sulfoxide phenol (MCXP) and methiocarb sulfone phenol (MCNP), all of them formed through MC oxidation by O3 or OH· in combination with hydrolysis. A toxicity assessment evidenced a strong dependence on MCX concentration, even at very low values. Despite the OH· capability to degrade MC and its main metabolites, the relative resistance of TPs towards ozone attack enlarged the oxidant dosage (2.5 mg O3/mg DOC) necessary to achieve a relatively low toxicity of the medium. Even though ozonation could be a suitable technique for MC removal from water compartments, strategies aimed to further promote the indirect contribution of hydroxyl radicals during this process should be explored

Priority pesticides abatement by advanced water technologies: the case of acetamiprid removal by ozonation

With the aim of exploring treatment alternatives for priority insecticide acetamiprid (ACMP) abatement, the removal of this compound from water by ozonation was studied for the first time, paying special attention to the kinetic, mechanistic and toxicological aspects of the process. The second order rate constants of reactions between ACMP and both molecular ozone (O3) and hydroxyl radicals (OH·) were determined to be 0.25 M-1s-1 and 2.1·109 M-1s-1, respectively. On the basis of kinetic results, the degradation of ACMP during ozonation could be well-explained by the reactivity of this pesticide with OH·. HPLC/MS analysis of the ozonated ACMP showed ACMP-N-desmethyl, 6-chloronicotinic acid, N'cyano-N-methyl acetamidine and N'-cyano acetamidine as the major transformation products (TPs), all of them formed through amine α carbon oxidation in combination with hydrolysis. Microtox bioassays revealed an increase in the toxicity of the medium during ACMP ozonation process, followed by a decrease to relatively low values. These changes could be attributed to the synergistic effects between TPs as well as to the presence of toxic intermediate aldehydes. Even though adopting strategies to further promote ozone decomposition to hydroxyl radicals appears to be essential, ozonation can be an effective treatment process for complete ACMP removal and associated toxicity abatement

Priority pesticide dichlorvos removal from water by ozonation process: reactivity, transformation products and associated toxicity

The treatability of waters contaminated with priority pesticide dichlorvos (DDVP) by means of ozonation has been assessed for the first time. In order to do so, reaction kinetics, transformation mechanisms and associated toxicity have been inspected in detail. Second-order rate constants of DDVP reactions with O3 and OH· were determined to be 590 and 2.2·109 M-1s-1, respectively. These values partly explained the degradation profiles obtained during experiments with and without the presence of an OH· scavenger, in which the significant contribution of the indirect degradative route in the removal of DDVP was revealed. LC-MS analyses for ozonated samples allowed the elucidation of desmethyl dichlorvos (d-DDVP), dichloroacetic acid (DCA) and dimethyl phosphate (DMP) as main transformation products (TPs), being the latter a common intermediate of both O3 and OH·-mediated oxidations. Three possible degradation routes were proposed for OH· degradation, whereas the direct oxidation by O3 was only well-explained by the addition of this oxidant to the double bond of DDVP dichlorovinyl moiety. Microtox® bioassays revealed the inability of molecular ozone to reduce the toxicity of the medium and pointed out the importance of OH· contribution in the degradation process. In general, ozonation could be a suitable treatment alternative for DDVP, formed TPs and associated toxicity abatement

Continuous versus single H2O2 addition in peroxone process: performance improvement and modelling in wastewater effluents

The extension of ozonation up to ozone (O3) doses beyond immediate ozone demand (IOD) completion combined with continuous addition of hydrogen peroxide (H2O2) was studied as potential strategy of treatment aimed to the effective abatement of ozone-resistant micropollutants (MPs) from wastewater effluents. Through experiments involving the continuous addition of H2O2 in a semi-continuous ozone contactor, it was demonstrated that this new approach could lead to a 36% reduction of the overall O3 needs for a constant H2O2/O3 molar ratio of 0.25 compared with single ozonation, representing a 28% reduction in the energy consumption. This improvement, however, was mainly attributed to H2O2 addition during the secondary ozonation stage, where the direct ozone demand becomes less important. The ¿OH-exposure per consumed ozone (ROHO3) calculation demonstrated that larger H2O2/O3 ratios (0.5-1) lead to a little improvement on oxidation performance during the IOD stage, whereas relationships of 0.25 work markedly better during the secondary stage of the process. Moreover, continuous versus total initial addition of H2O2 were compared and the first one showed better performance, with differences in estimated energy costs up to 21%. Finally, and since monitoring the fate of O3-recalcitrant MPs during the process is essential, two different strategies for the real-time control of the O3-recalcitrant MPs fate during the process were tested, one based on the ROHO3 concept and the other on continuous measurements of ultraviolet absorbance at 254 nm (UVA254). They both showed accurate predictions (R2 > 0.96) for different compounds, effluents and processes

Characterization and Control Strategies of an Integrated Chemical-Biological System for the Remediation of Toxic Pollutants in Wastewater: A case of study

In a previous work, a hybrid system consisting of an advanced oxidation process (AOP) named Photo-Fenton (Ph-F) and a fixed bed biological treatment operating as a sequencing batch biofilm reactor (SBBR) was started-up and optimized to treat 200 mg·L-1 of 4-chlorophenol (4-CP) as a model compound. In this work, studies of reactor stability and control as well as microbial population determination by molecular biology techniques were carried out to further characterize and control the biological reactor. Results revealed that the integrated system was flexible and even able to overcome toxic shock loads. Oxygen uptake rate (OUR) in situ was shown to be a valid tool to control the SBBR operation, to detect toxic conditions to the biomass, and to assess the recovery of performance. A microbial characterization by 16S rDNA sequence analysis reveals that the biological population was varied, although about 30% of the bacteria belonged to the Wautersia genus

Mixtures of chelating agents to enhance photo-Fenton process at natural pH: Influence of wastewater matrix on micropollutant removal and bacterial inactivation

Three organic fertilizers (EDTA (Ethylenedinitrilotetraacetic acid), EDDS (Ethylenediamine-N, N′-disuccinic acid) and DTPA (Diethylene triamine pentaacetic acid)) were tested as Fe-complexes in photo-Fenton process at natural pH for micropollutants (MPs) abatement and simultaneous E.coli inactivation. Less stable Fe-complexes show high iron precipitation, stopping MPs degradation. On the contrary, stable Fe-complexes imply low kinetic rates for MPs removal. To solve these inconveniences, three mixtures of organic fertilizers were also tested, trying to improve the kinetic rates of micropollutants oxidation and overcome iron precipitation. Three different pollutants (propranolol (PROP), acetamiprid (ACMP) and sulfamethoxazole (SMX)) were used as the target compounds. As the iron release is, in part, linked to the hardness of water, two water matrices from two different secondary wastewaters (Membrane Bioreactor (MBR) and Conventional Activated Sludge (CAS)) were tested. The best performance in micropollutant degradation and E.coli inactivation was achieved with the combination of EDDS + EDTA, accomplishing a good equilibrium between iron precipitation and rate of MPs removal. For instance, total removal of propranolol was achieved at 45 min in MBR, while it was only 85.7% in CAS, being an improvement of the process comparing with that obtained using single organic fertilizers. At the end of the treatment, 2.1 log-inactivation for E.coli was reached in CAS. The differences observed between both wastewaters were related to CAS' higher DOC, turbidity, and hardness. Finally, from the physicochemical characterization conducted, including Biochemical Oxygen Demand at 5 days and phytotoxicity, it is possible to highlight the suitability of these treated effluents for its reuse in irrigation, as long as in CAS matrix the final values of E. coli are within the legal limit

Performance and kinetic modelling of photolytic and photocatalytic ozonation for enhanced micropollutants removal in municipal wastewaters

In this work, the performances of ozonation, photolytic ozonation (UV-C/O3), and photocatalytic ozonation (UV-A/TiO2/O3) in degrading ozone recalcitrant micropollutants in four different real domestic wastewaters were evaluated in semi-continuous operation, together with the influence of water matrices in the ozone mass transfer and pollutant degradation rates. The ¿OH exposure per consumed ozone ratio, defined as ROH,O3, was applied for single ozonation and modified for light-assisted ozonation processes to evaluate and compare the contribution of radical pathway on micropollutants abatement for the different wastewaters studied. ROH,O3 plots presented good fitting (R2 > 0.95) in two stages, corresponding to different ozone mass transfer regimes, for all cases. Light-assisted ozonation attained higher pollutant degradation for all water matrices compared to single ozonation, although the performance of UV-assisted processes was more sensitive to matrix factors like composition and turbidity. Moreover, the improvement brought by both light-based processes on ROHO3 values mainly took place during the second stage. Thus, photocatalytic ozonation reached ROHO3 values higher than double for all wastewaters, compared with single ozonation (between 105% and 127% increase). These values represent a saving of almost half of the overall ozone needs (42%) for the same ozone recalcitrant micropollutant depletion, although it would require the adoption of higher ozone doses than the currently employed for ozonation in wastewater treatment plants