Continuous air purification by front flow photocatalytic reactor: Modelling of the influence of mass transfer step under simulated real conditions

In this work, a solution for the treatment of toxic gases based on a photocatalytic process using TiO₂ coated on a cellulosic support, has been investigated. Here, cyclohexane was chosen as the reference for testing its removal efficiency via a continuous front flow reactor as type A anti-gas filters. The photocatalytic support was firstly characterized by EDX, to confirm its elemental composition. Then, the experiments were carried out, starting with a batch reactor in order to evaluate the degradation efficiency of the photocatalytic media, as well as the monitoring of the photocatalytic process which allowed the establishing of a carbon mass balance corresponding to the stoichiometric number of our target pollutant. The transition to a continuous treatment with a front flow reactor aims to highlight the influence of the input concentration (0.29-1.78 mM m^-3) under different flow rates (12, 18 and 36 L min^-1). The relative humidity effect was also investigated (from 5 to 90% of humidity) where an optimum rate was obtained around 35-45%. In addition, the mineralization rate was monitored. The major rates obtained were for a cyclohexane input concentration of 0.29 mM m^-3 in wet condition (38%) at an air flow rate of 18 L min^-1, where the CO₂ selectivity reached 77% for an abatement of 62%. In order to understand the limiting steps of the photocatalytic process, a model considering the reactor geometry and the hydraulic flow was developed. The obtained results showed that the mass transfer must be considered in the photocatalytic process for a continuous treatment. The Langmuir-Hinshelwood bimolecular model was also developed to represent the influence of the humidity.

Adsorption onto ACFC of mixture of pharmaceutical residues in water - experimental studies and modelling

The presence of pharmaceutical residues in water resources is a critical issue for the production of drinking water, even though trace concentrations are mostly encountered. The adsorption of eight micropollutants, in mixture, onto a microporous activated carbon fibre cloth was investigated. For each compound, the kinetics and isotherms of adsorption were studied in batch reactors with ultrapure water, groundwater and half-diluted groundwater. Experimental data were generated and compared to values calculated by the association of Ideal Adsorbed Solution Theory (IAST) model and the Homogeneous Surface Diffusion Model (HSDM). The impact of the nature and the content of Natural Organic Matter (NOM) was modelled considering an Equivalent Background Compound (EBC). The presence of NOM in the groundwater is largely detrimental for the adsorption of trace micropollutants.

Body fluid analog chlorination: Application to the determination of disinfection byproduct formation kinetics in swimming pool water

Disinfection by-products (DBPs) are formed in swimming pools by the reactions of bather inputs with the disinfectant. Although a wide range of molecules has been identified within DBPs, only few kinetic rates have been reported. This study investigates the kinetics of chlorine consumption, chloroform formation and dichloroacetonitrile formation caused by human releases. Since the flux and main components of human inputs have been determined and formalized through Body Fluid Analogs (BFAs), it is possible to model the DBPs formation kinetics by studying a limited number of precursor molecules. For each parameter the individual contributions of BFA components have been quantified and kinetic rates have been determined, based on reaction mechanisms proposed in the literature. With a molar consumption of 4 mol Cl₂/mol, urea is confirmed as the major chlorine consumer in the BFA because of its high concentration in human releases. The higher reactivity of ammonia is however highlighted. Citric acid is responsible for most of the chloroform produced during BFA chlorination. Chloroform formation is relatively slow with a limiting rate constant determined at 5.50 × 10^-3 L/mol/sec. L-histidine is the only precursor for dichloroacetonitrile in the BFA. This DBP is rapidly formed and its degradation by hydrolysis and by reaction with hypochlorite shortens its lifetime in the basin. Reaction rates of dichloroacetonitrile formation by L-histidine chlorination have been established based on the latest chlorination mechanisms proposed. Moreover, this study shows that the reactivity toward chlorine differs whether L-histidine is isolated or mixed with BFA components.

Photocatalytic treatment of petroleum industry wastewater using recirculating annular reactor: comparison of experimental and modeling

In this study, the treatment of petroleum wastewater has been investigated by applying heterogeneous photocatalytic process using a recirculating annual reactor. An attempt has been made to study the effect of operating parameters such as TiO₂ load, initial concentration of the pollutant, emitted photonic flux, and pH of the solution. The degradation efficiency of toluene and benzene, as target molecules, was studied. In fact, result showed that the toluene is better degraded alone than when it is in a mixture. The rate of elimination of toluene separately was 89.5%, while it was 76.19 and 79.55% in the binary (toluene/benzene) and the ternary mixtures (toluene/benzene/xylene), respectively. Moreover, the mineralization of the solution decreased more rapidly when toluene was pure with a rate of 83.13% compared to binary and ternary mixtures. A mathematical model is proposed taking into account the parameters influencing the process performances. The mass transfer step, the degradation, and the mineralization kinetics of the pollutants were defined as model parameters. To build the model, mass balances are written in bulk region and catalyst phase (solid phase). The degradation mechanism on solid phase is divided in two stages. Firstly, the removal of toluene gives an equivalent intermediate (EI). Secondly, EI is oxidized into carbon dioxide (CO₂). This approach gives a good agreement between modeling and empirical data in terms of degradation and mineralization. It also allows for the simulation of toluene kinetics without knowing the plausible chemical pathway. A satisfactory fit with experimental data was obtained for the degradation and mineralization of toluene.

Comparative analysis of wood and solid recovered fuels gasification in a downdraft fixed bed reactor

This study aims to qualify air gasification performances of Solid Recovered Fuels (SRF) in a downdraft fixed bed gasifier. Five fuels have been studied: Wood, SRF wood, and three different mixtures of SRF wood and 20 %w of either SRF tire (mix A), SRF plastics (mix B) or SRF sewage sludge (mix C). The tests were carried out in a pilot scale reactor in a batch-fed mode using a fuel mass ranging from 5 to 8 kg, and an air inlet flow of 170-180 L/min, which led to an Equivalence Ratio (ER) ranging between 0.20 and 0.29. Considering Poplar wood as a reference, we observed a similar syngas quality for SRF wood, but SRF mixes led to a slight decrease of H₂ and CO contents along with an increase of CO₂, CH₄, C₂H₄ and C₂H₆ contents. However syngas Low Heating Value (LHV) remained close and ranged between 4.9 and 5.4 MJ/m³ which led to a Cold Gas Efficiency (CGE) ranging from 38 to 52%. Wood and SRF wood had a similar condensate content (159-202 g/m³), but adding 20 %w of non-woody fuel led to an increased condensate content up to 369 g/m³ for mix A and 411 g/m³ for mix C. Tar analyses confirmed the similarity of Wood, SRF wood and mix C in air gasification, but highlighted large increases in aromatics and Polycyclic Aromatic Hydrocarbon (PAH) contents when adding 20 %w of tire. This study confirms the ability of downdraft gasifier to handle non-woody fuels, with an upper limit of 20%w share in a wood-based fuel.

Upscaling fixed bed adsorption behaviors towards emerging micropollutants in treated natural waters with aging activated carbon: Model development and validation

A scale-up procedure was assessed in this study to predict the fixed bed adsorption behaviors with aging granular activated carbon (GAC) for various micropollutants (pesticides, pharmaceuticals). Two assumptions of this upscaling methodology (i.e., involving equal adsorption capacities and surface diffusivities between the batch test and the fixed bed) were studied for the first time to investigate the aging effect on the adsorption capacity and kinetics of carbon at full scale. This study was conducted in natural waters (the Seine River) treated by Veolia Eau d'Ile de France in Choisy-Le-Roi, a division of Syndicat des Eaux d'Ile de France, aiming to monitor real industrial conditions. The isotherms showed that the adsorption capacity for most compounds was significantly affected by aging. For the mass transfer coefficients (i.e., as determined by the homogeneous surface diffusion model (HSDM)), different patterns of adsorbate/adsorbent behaviors were observed, suggesting different competition mechanisms. The model predictions (i.e., HSDM) performed with all parameters obtained during the batch tests tended to overestimate the full-scale pilot adsorption performance. This overestimation could be compensated for by applying a scaling factor. Finally, an empirical pseudo-first order function was used to model the impact of the GAC service time on the characteristic adsorption parameters. Thus, our scale-up procedure may enable the prediction of long-term fixed bed adsorption behaviors and increase the model efficiency for practical implementation.

Pilot scale degradation of mono and multi volatile organic compounds by surface discharge plasma/TiO2 reactor: Investigation of competition and synergism

This paper mainly deals with the isovaleraldehyde degradation with the help of a nonthermal plasma surface discharge (NPSD) coupled with photocatalysis. The efficiency of NPSD reactor, for gas treatment, was studied for different binary mixtures: (1) mixture of aldehydes (Isovaleraldehyde and Butyraldehyde) and (2) mixture of aldehyde and amine (Isovaleraldehyde and Trimethylamine). A planar continuous reactor is used to investigate the effect of addition of another pollutant on the performance of oxidation process. A synergetic effect was observed by combining NPSD and photocatalysis for the degradation of mixture of pollutants. In addition, combined NPSD/photocatalysis has significantly enhanced the CO₂ selectivity, as compared to NPSD alone. This is attributed to the formation of more reactive species due to the presence of TiO₂ in the plasma discharge zone. Moreover, ozone and UV light on TiO₂, produced by plasma, have activated the surface leading to enhanced mineralization. In addition, the byproducts of each binary mixture were identified and evaluated.

Adsorption of pharmaceuticals onto activated carbon fiber cloths - Modeling and extrapolation of adsorption isotherms at very low concentrations

Activated carbon fiber cloths (ACFC) have shown promising results when applied to water treatment, especially for removing organic micropollutants such as pharmaceutical compounds. Nevertheless, further investigations are required, especially considering trace concentrations, which are found in current water treatment. Until now, most studies have been carried out at relatively high concentrations (mg L(-1)), since the experimental and analytical methodologies are more difficult and more expensive when dealing with lower concentrations (ng L(-1)). Therefore, the objective of this study was to validate an extrapolation procedure from high to low concentrations, for four compounds (Carbamazepine, Diclofenac, Caffeine and Acetaminophen). For this purpose, the reliability of the usual adsorption isotherm models, when extrapolated from high (mg L(-1)) to low concentrations (ng L(-1)), was assessed as well as the influence of numerous error functions. Some isotherm models (Freundlich, Toth) and error functions (RSS, ARE) show weaknesses to be used as an adsorption isotherms at low concentrations. However, from these results, the pairing of the Langmuir-Freundlich isotherm model with Marquardt's percent standard of deviation was evidenced as the best combination model, enabling the extrapolation of adsorption capacities by orders of magnitude.

Isovaleraldehyde elimination by UV/TiO2 photocatalysis: comparative study of the process at different reactors configurations and scales

A proposal for scaling-up the photocatalytic reactors is described and applied to the coated catalytic walls with a thin layer of titanium dioxide under the near ultraviolet (UV) irradiation. In this context, the photocatalytic degradation of isovaleraldehyde in gas phase is studied. In fact, the removal capacity is compared at different continuous reactors: a photocatalytic cylindrical reactor, planar reactor, and pilot unit. Results show that laboratory results can be useful for reactor design and scale-up. The flowrate increases lead to the removal capacity increases also. For example, with pilot unit, when flowrate extends four times, the degradation rate varies from 0.14 to 0.38 g h(-1) mcat (-2). The influence of UV intensity is also studied. When this parameter increases, both degradation rate and overall mineralization are enhanced. Moreover, the effects of inlet concentration, flowrate, geometries, and size of reactors on the removal capacity are also studied.

Odorous gaseous emissions as influence by process condition for the forced aeration composting of pig slaughterhouse sludge

Compost sustainability requires a better control of its gaseous emissions responsible for several impacts including odours. Indeed, composting odours have stopped the operation of many platforms and prevented the installation of others. Accordingly, present technologies collecting and treating gases emitted from composting are not satisfactory and alternative solutions must be found. Thus, the aim of this paper was to study the influence of composting process conditions on gaseous emissions. Pig slaughterhouse sludge mixed with wood chips was composted under forced aerationin 300 L laboratory reactors. The process conditions studied were: aeration rate of 1.68, 4.03, 6.22, 9.80 and 13.44 L/h/kg of wet sludge; incorporation ratio of 0.55, 0.83 and 1.1 (kg of wet wood chips/kg of wet sludge), and; bulking agent particles size of <10, 10<20 and 20<30 mm. Out-going gases were sampled every 2 days and their composition was analysed using gas chromatography coupled with mass spectrometry (GC-MS). Fifty-nine compounds were identified and quantified. Dividing the cumulated mass production over 30 days of composting, by odour threshold, 9 compounds were identified as main potential odour contributors: hydrogen sulphide, trimethylamine, ammonia, 2-pentanone, 1-propanol-2-methyl, dimethyl sulphide, dimethyl disulphide, dimethyl trisulphide and acetophenone. Five gaseous compounds were correlated with both aeration rate and bulking agent to waste ratio: hydrogen sulphide, trimethylamine, ammonia, 2-pentanone and 1-propanol-2-methyl. However, dropping the aeration rate and increasing the bulking agent to waste ratio reduced gaseous odour emissions by a factor of 5-10, when the required threshold dilution factor ranged from 10(5) to 10(6), to avoid nuisance at peak emission rates. Process influence on emissions of dimethyl sulphide, dimethyl disulphide, dimethyl trisulphide were poorly correlated with both aeration rate and bulking agent to waste ratio as a reaction with hydrogen sulphide was suspected. Acetophenone emissions originated from the wood chips. Olfactory measurements need to be correlated to gaseous emissions for a more accurate odour emission evaluation.

Ozonation effect on natural organic matter adsorption and biodegradation--application to a membrane bioreactor containing activated carbon for drinking water production

More stringent legislation on dissolved organic matter (DOM) urges the drinking water industry to improve in DOM removal, especially when applied to water with high dissolved organic carbon (DOC) contents and low turbidity. To improve conventional processes currently used in drinking water treatment plants (DWTPs), the performances of a hybrid membrane bioreactor containing fluidized activated carbon were investigated at the DWTP of Rennes. Preliminary results showed that the residual DOC was the major part of the non-biodegradable fraction. In order to increase the global efficiency, an upstream oxidation step was added to the process. Ozone was chosen to break large molecules and increase their biodegradability. The first step consisted of carrying out lab-scale experiments in order to optimise the necessary ozone dose by measuring the process yield, in terms of biodegradable dissolved organic carbon (BDOC). Secondly, activated carbon adsorption of the DOC present in ozonated water was quantified. The whole process was tested in a pilot unit under field conditions at the DWTP of Rennes (France). Lab-scale experiments confirmed that ozonation increases the BDOC fraction, reduces the aromaticity of the DOC and produces small size organic compounds. Adsorption tests led to the conclusion that activated carbon unexpectedly removes BDOC first. Finally, the pilot unit results revealed an additional BDOC removal (from 0.10 to 0.15 mg L(-1)) of dissolved organic carbon from the raw water considered.

Photocatalytic degradation of indole in a circulating upflow reactor by UV/TiO2 process--influence of some operating parameters

The present work involves the photocatalytic degradation of indole on a recirculating reactor. The effects of various factors as initial concentration of indole, catalyst-loading, pH, agitation and flow rate of the solution on the photodegradation were examined. The experimental results indicate that the optimal pH for indole elimination is about 6-7; the effect of catalyst loading shows an optimal value (1g/L) which is necessary to degrade indole; the increase of recirculating rate leads to a decrease of degradation rate due to the reduction of the residence time; the agitation speed has a slight influence on the indole degradation by improving the mass transfer step. Finally, L-H model was used to fit experimental results concerning the influence of experimental data. L-H model constants' were determined also.

Revisiting the determination of langmuir parameters--application to tetrahydrothiophene adsorption onto activated carbon

The selection of a proper sorbent for a given application is a complex problem. The design and efficiency of adsorption processes require an equilibrium adsorption model. Linear transformation is one of the methods available to estimate the adjustable parameters of isotherm models but possesses limitations compared to nonlinear regressions. A different approach to calculate predicted equilibrium isotherm values leading to an alternative nonlinear regression is presented in this paper and compared with usual regression methods. Adsorption isotherm data of gaseous THT onto three activated carbon materials constitute an experimental basis for the discussion. Assessment of the goodness-of-fit of the Langmuir model is supported by different selected test functions. The new nonlinear approach did not obtain the best results for each test function, but raises questions about the inherent combined error in regression procedures.

Photocatalytic degradation of pesticides in pure water and a commercial agricultural solution on TiO2 coated media

Heterogeneous photocatalysis of pesticides is an effective process for removing pesticides from pure water. With a view to treating real agricultural effluents, this paper deals with the degradation of the chlortoluron and cyproconazole pesticides in pure water and the treatment of commercial solutions by photocatalysis on TiO2 coated media. The process was effective in degrading and mineralizing the pesticides. The changes of the fate of heteroatoms showed that during irradiation of the chlortoluron and cyproconazole, NH4+ and NO3(-) ions were produced. A release of chloride ions was observed from the beginning of the irradiation and stoichiometry was achieved. The photodegradation of chlortoluton and cyproconazole in commercial solutions was studied. For the degradation of chlortoluton in a commercial solution, the mineralization was completely achieved whereas in the case of the commercial cyproconazole solution, the degradation kinetic was lower. These results highlight the fact that the chemical nature of the additives in the commercial pesticide solutions does significantly affect the degradation yield of the target compound by photocatalysis.

Particulate products and new polymers for a more efficient removal of dissolved organic matter in drinking water resources

More restricting legislation on dissolved organic carbon (DOC), especially when applied to waters with high DOC contents and low turbidity, urges the drinking water industry to improve the removal of dissolved organic matter (DOM). Jar tests were carried out on sand-filtered water (SFW) and raw water (RW), with respectively 50 different particulate compounds and different cationic polymers. Analytical measurements of DOC and UV absorbance at 254 nm, as well as a characterisation by size exclusion chromatography (SEC), were performed in order to determine the efficiency of the process. Experiments on SFW showed that activated carbon was the only compound able to remove efficiently the residual DOC remaining after conventional treatments (up to 40% of reduction). Other trials on RW using mixes of ferric chloride (FeCl3) and cationic polymers (polyamines and polyDADMAC) pointed out that the molecular mass and the reticulation ratios have a significant influence on the efficiency of the treatment. Finally, the addition of activated carbons to the previous reagents significantly improved the observed removal efficiencies by similar amounts.

Photocatalytic degradation of two volatile fatty acids in an annular plug-flow reactor; kinetic modeling and contribution of mass transfer rate

This study investigates the influence of inlet concentration and of flow rate on the degradation rate of two Volatile Fatty Acids (butyric and propionic acids). TiO2-coated nonwoven fiber textile was used as the photocatalyst in an annular plug-flow reactor at laminar flow regime. The kinetic follows a Langmuir-Hinshelwood form. The oxidation rate increased with the flow rate, which emphasizes the influence of the mass transfer. A first design equation is proposed considering that the mass transfer could be neglected. Despite a good accuracy of the model, the determined kinetic constants are dependent on the flow rate which highlights the contribution of the mass transfer rate on the global degradation rate. Thus, a new design equation which includes the mass transfer rate was developed. Using this model, the degradation rate can be determined for any given flow rate. Moreover, it allows the estimation of the contribution of mass transfer and chemical reaction steps at given experimental conditions; and thus providing an interesting tool for reactor optimization or design.

Chlorination kinetics of glyphosate and its by-products: modeling approach

Chlorination reactions of glyphosate, glycine, and sodium cyanate were conducted in well-agitated reactors to generate experimental kinetic measurements for the simulation of chlorination kinetics under the conditions of industrial water purification plants. The contribution of different by-products to the overall degradation of glyphosate during chlorination has been identified. The kinetic rate constants for the chlorination of glyphosate and its main degradation products were either obtained by calculation according to experimental data or taken from published literature. The fit of the kinetic constants with experimental data allowed us to predict consistently the concentration of the majority of the transitory and terminal chlorination products identified in the course of the glyphosate chlorination process. The simulation results conducted at varying aqueous chlorine/glyphosate molar ratios have shown that glyphosate is expected to degrade in fraction of a second under industrial aqueous chlorination conditions. Glyphosate chlorination products are not stable under the conditions of drinking water chlorination and are degraded to small molecules common to the degradation of amino acids and other naturally occurring substances in raw water. The kinetic studies of the chlorination reaction of glyphosate, together with calculations based on kinetic modeling in conditions close to those at real water treatment plants, confirm the reaction mechanism that we have previously suggested for glyphosate chlorination.

Importance of surface diffusivities in pesticide adsorption kinetics onto granular versus powdered activated carbon: experimental determination and modeling

Three pesticides (atrazine, bromoxynil and diuron) and two granular activated carbons are involved in equilibrium and kinetic adsorption experiments. Equilibrium is represented by Freundlich isotherm law and kinetic is described by the Homogeneous Surface Diffusion Model, based on external mass transfer and intraparticle surface diffusion. Equilibrium and long-term experiments are conducted to compare Powdered Activated Carbon and Granular Activated Carbon. These first investigations show that crushing GAC into PAC improves the accessibility of the adsorption sites without increasing the number of these sites. In a second part, kinetics experiments are carried out using a Differential Column Batch Reactor. Thanks to this experimental device, the external mass transfer coefficient k(f) is calculated from empirical correlation and the effect of external mass transfer on adsorption is likely to be minimized. In order to obtain the intraparticle surface diffusion coefficient D. for these pesticides, comparisons between experimental kinetic data and simulations are conducted and the best agreement leads to the Ds coefficient. This procedure appears to be an efficient way to acquire surface diffusion coefficients for the adsorption of pesticides onto GAC. Finally it points out the role of surface diffusivity in the adsorption rate. As a matter of fact, even if the amount of the target-compound that could be potentially adsorbed is really important, its surface diffusion coefficient may be small, so that its adsorption may not have enough contact time to be totally achieved.