Volumetric Conversion Rate Research Articles

Improving carbon and nitrogen removal efficiency in high-strength nitrogen wastewater via two-stage nitritation-anammox process

Centralized systems employ biological treatments to remove nutrients, whereas decentralized systems, especially in rural areas, face challenges in implementing them. These systems are low flow but high carbon and nitrogen concentrations, and low C/N ratios. Source-separated wastewater (urine + flush) is an extreme example, like confined animal-feeding wastewater. High nutrient concentrations necessitate innovative technologies typically not employed for decentralized systems. This research demonstrates a simple two-stage nitritation-anammox system to treat high N concentration wastewater (pure urine + flush water). The systems consisted of a Membrane Aerated Biological Reactor (MABR) which performed carbon oxidation and nitritation and novel anammox reactors, (Pancopia AnammoX (PAXs)). The MABRs achieved OC removal over 108–198 g-C/m3-day with ∼ 97% removal, TAN oxidation rates up to 156 g-N/m3-day and nitrite/AN ratios were near 1 with no external control (e.g. pH, DO, cell wasting) other than urine loading rate. The PAXs achieved maximum AN removal efficiencies of 85–97%. The main limitation to complete TN removal in the PAXs was insufficient NO2- in the influent (nonideal influent NO2-/AN ratio). This system was not optimized for energy efficiency and had low volumetric conversion rates compared with other systems. However, it demonstrates that source-separated wastewater can be effectively treated with near complete N removal using a system with minimal process control requirements, lack of solid production, and elimination of diffuse aeration preventing odor generation. These attributes are attractive for applications where conditions do not allow for more complex high-rate systems such as developing societies and rural areas.

High-rate biological selenate reduction in a sequencing batch reactor for recovery of hexagonal selenium

Recovery of selenium (Se) from wastewater provides a solution for both securing Se supply and preventing Se pollution. Here, we developed a high-rate process for biological selenate reduction to elemental selenium. Distinctive from other studies, we aimed for a process with selenate as the main biological electron sink, with minimal formation of methane or sulfide. A sequencing batch reactor, fed with an influent containing 120 mgSe L−1 selenate and ethanol as electron donor and carbon source, was operated for 495 days. The high rates (419 ± 17 mgSe L−1 day−1) were recorded between day 446 and day 495 for a hydraulic retention time of 6 h. The maximum conversion efficiency of selenate amounted to 96% with a volumetric conversion rate of 444 mgSe L−1 day−1, which is 6 times higher than the rates reported in the literature thus far. At the end of the experiment, a highly enriched selenate reducing biomass had developed, with a specific activity of 856 ± 26 mgSe−1day−1gbiomass−1, which was nearly 1000-fold higher than that of the inoculum. No evidence was found for the formation of methane, sulfide, or volatile reduced selenium compounds like dimethyl-selenide or H2Se, revealing a high selectivity. Ethanol was incompletely oxidized to acetate. The produced elemental selenium partially accumulated in the reactor as pure (≥80% Se of the total mixture of biomass sludge flocs and flaky aggregates, and ~100% of the specific flaky aggregates) selenium black hexagonal needles, with cluster sizes between 20 and 200 µm. The new process may serve as the basis for a high-rate technology to remove and recover pure selenium from wastewater or process streams with high selectivity.

ЭКСПЕРИМЕНТАЛЬНОЕ ИССЛЕДОВАНИЕ ЭФФЕКТИВНОСТИ ПРИМЕНЕНИЯ ВИХРЕВЫХ АППАРАТОВ ДЛЯ РЕГЕНЕРАЦИОННОГО ИЗВЛЕЧЕНИЯ ЦИАНИСТОГО НАТРИЯ ИЗ ОТРАБОТАННЫХ РАСТВОРОВ

The article presents the results of the study on the effectiveness of the use of vortex apparatuses for the regenerative extraction of sodium cyanide from spent solutions. The authors emphasize the main advantages and factors explaining the higher efficiency of vortex devices compared with traditional technologies for the regeneration of NaCN from spent solutions. The study also highlights the results of experiments according to which the average volumetric conversion rate of the starting compounds is 40-60 times higher than in conventional bubble columns, which clearly shows the advantage of vortex devices.

The role of the external mass transfer resistance in nitrite oxidizing bacteria repression in biofilm-based partial nitritation/anammox reactors

A model-based study was developed to analyse the behaviour of Moving Bed Biofilm Reactor (MBBR) and Integrated Fixed-Film Activated Sludge (IFAS) reactor configurations for the removal of nitrogen in the main water line of municipal wastewater treatment plants via partial nitritation/anammox (PN/AMX). The basic principles and underlying mechanisms linking operating conditions to process performance were investigated, with particular focus on nitrite oxidizing bacteria (NOB) repression and resulting volumetric conversion rates. The external mass transfer resistance is a major factor differentiating granular sludge PN/AMX processes from MBBR or IFAS systems. The external mass transfer resistance was found to promote the metabolic coupling between anammox (AMX) and ammonia oxidizing bacteria (AOB), crucial for NOB repression in the biofilm. Operation at low bulk DO prevents NOB proliferation in the flocs of IFAS systems as AMX activity limits nitrite availability (the so-called AMX nitrite sink). Importantly, the effectiveness of the AMX nitrite sink strongly depends on the AMX sensitivity to oxygen. Also, over a broad range of operational conditions, the seeding of AOB from the biofilm played a crucial role in maintaining their activity in the flocs. From a practical perspective, while low DO promotes NOB repression, lower nitrogen loads have to be applied to maintain the same effluent quality. Thus, a trade-off between NOB repression and volumetric conversion capacity needs to be defined. To this end, IFAS allow for higher volumetric rates, but the window of operating conditions with effective NOB repression is smaller than that for MBBR. Ultimately, this study identified the principles controlling NOB in MBBR and IFAS systems and the key differences with granular reactors, allowing for the interpretation of (seemingly contradictory) published experimental results.

Assessment of Membrane-Aerated Biological Reactors (MABRs) for Integration into Space-Based Water Recycling System Architectures

Abstract This work investigates the suitability of membrane aerated biological reactors (MABRs) for biological treatment of a space-based waste stream consisting of urine, hygiene/grey water, and humidity condensate within an overall water recycling system. Water represents a critical limiting factor for human habitation and travel within space; thus, water recycling systems are essential. Biological treatment of wastewater provides a more efficient sustainable means of stabilizing the waste stream within water recycling system architectures in comparison to current chemical stabilization processes that utilize harsh chemicals, which represent both a hazardous and an unsustainable approach. To assess the capabilities of MABRs for providing microgravity compatible biological treatment and verify long duration operation and integration with desalination processes, two full-scale MABR systems were challenged with various loading rates and operational scenarios during sustained operation for over 1 year. The MABRs were able to maintain 196 g-C/m3-d and 194 g-N/m3-d volumetric conversion rates. Additionally the systems were able to handle intermittent loading and recover rapidly from system hibernation periods of up to 27 days. Overall, the use of MABRs within a wastewater treatment system architecture provides several potential benefits including minimizing the use of toxic chemical pretreatment solutions and providing an effluent solution that is easier to desalinate and dewater.

Intensifying the O2-dependent heterogeneous biocatalysis: Superoxygenation of solid support from H2O2 by a catalase tailor-made for effective immobilization

Besides merely destroying H2O2, an important use of the catalase reaction, H2O2→1/2 O2+H2O, is to supply O2 to oxygenation reactions. Due to convenient spatiotemporal control over O2 release, oxygenation from H2O2 is useful in particular for enzymatic reactions confined to solid supports. Because commercial catalases are difficult to immobilize, we have developed a one-step procedure of purification and immobilization of Bordetella pertussis catalase, recombinantly produced in Escherichia coli. Fusion of the catalase to a positively charged binding module enabled effective immobilization of the chimeric enzyme on anionic support (Relisorb SP 400), giving a controllable activity loading of between 5000 and 100,000 units/g support. Use of the immobilized catalase in combination with H2O2 feeding provided O2 to the reaction of glucose oxidase in solution for a range of volumetric conversion rates (0.2–1.5mM/min). Using optical sensing to measure the O2 concentration in the liquid but also in the solid phase, we showed that internal superoxygenation of the support was made possible under these conditions, resulting in an inverted (that is, negative) O2 concentration gradient between the bulk and the particle and allowing the internal O2 concentration to exceed by up to 4-fold the limit of atmospheric-pressure air saturation in solution. By tailored immobilization of B. pertussis catalase, therefore, an efficient biocatalytic system for hydrogen peroxide conversion in porous solid support was developed. This could find application for bubble-free oxygenation of O2-dependent enzymes co-immobilized with the catalase whereby enhanced internal availability of O2 would contribute to biocatalytic reaction intensification.

Highly efficient bioconversion of methane to methanol using a novel type I Methylomonas sp. DH‐1 newly isolated from brewery waste sludge

AbstractBackgroundMethane is the major component of natural and shale gas. Methane can be converted into methanol via a bioprocess using methanotrophs, and methanol is a valuable chemical feedstock for the production of value‐added chemicals. This work demonstrates highly effective bioconversion of methane to methanol using a newly isolated novel methanotroph, Methylomonas sp. DH‐1.ResultsA novel methanotroph strain was isolated from activated sludge from a brewery plant and characterized using phylogenetic analysis, electron microscopy and chemotaxonomic analysis. This aerobic, Gram‐negative, non‐motile rod‐shaped type I methanotroph was designated as Methylomonas sp. DH‐1. The growth condition of Methylomonas sp. DH‐1 and batch methane‐to‐methanol bioconversion conditions such as methane concentration, pH, biocatalyst loading, concentration of formate and MDH inhibitor were analyzed and optimized. Methanol was produced from methane with a 1.340 g L−1 titer, a 0.332 g L−1 h−1 volumetric conversion rate and a 0.0752 g g−1 cell h−1 specific methanol conversion rate.ConclusionIt was demonstrated that isolation and application of a new methanotroph strain is a practical way of improving bioconversion efficiency in the conversion of methane to methanol. Moreover, one promising feature of Methylomonas sp. DH‐1 for methanol production was its extremely high tolerance to methanol up to 7%(v/v), which is advantageous for high‐titer methanol production. © 2016 Society of Chemical Industry

High-rate volatile fatty acid (VFA) production by a granular sludge process at low pH.

Volatile fatty acids (VFA) are proposed platform molecules for the production of basic chemicals and polymers from organic waste streams. In this study we developed a granular sludge process to produce VFA at high rate, yield and purity while minimizing potential operational costs. A lab-scale anaerobic sequencing batch reactor (ASBR) was fed with 10 g l(-1) glucose as model substrate. Inclusion of a short (2 min) settling phase before effluent discharge enabled effective granulation and very high volumetric conversion rates of 150-300 gCOD l(-1) d(-1) were observed during glucose conversion. The product spectrum remained similar at the tested pH range with acetate and butyrate as the main products, and a total VFA yield of 60-70% on chemical oxygen demand (COD) basis. The requirement for base addition for pH regulation could be reduced from 1.1 to 0.6 mol OH(-) (mol glucose)(-1) by lowering the pH from 5.5 to 4.5. Solids concentrations in the effluent were 0.6 ± 0.3 g l(-1) but could be reduced to 0.02 ± 0.01 g l(-1) by introduction of an additional settling period of 5 min. The efficient production of VFA at low pH with a virtually solid-free effluent increases the economic feasibility of waste-based chemicals and polymer production. Biotechnol.

Full scale performance of the aerobic granular sludge process for sewage treatment

Recently, aerobic granular sludge technology has been scaled-up and implemented for industrial and municipal wastewater treatment under the trade name Nereda®. With full-scale references for industrial treatment application since 2006 and domestic sewage since 2009 only limited operating data have been presented in scientific literature so far. In this study performance, granulation and design considerations of an aerobic granular sludge plant on domestic wastewater at the WWTP Garmerwolde, the Netherlands were analysed. After a start-up period of approximately 5 months, a robust and stable granule bed (>8 g L−1) was formed and could be maintained thereafter, with a sludge volume index after 5 min settling of 45 mL g−1. The granular sludge consisted for more than 80% of granules larger than 0.2 mm and more than 60% larger than 1 mm. Effluent requirements (7 mg N L−1 and 1 mg P L−1) were easily met during summer and winter. Maximum volumetric conversion rates for nitrogen and phosphorus were respectively 0.17 and 0.24 kg (m3 d)−1. The energy usage was 13.9 kWh (PE150·year)−1 which is 58–63 % lower than the average conventional activated sludge treatment plant in the Netherlands. Finally, this study demonstrated that aerobic granular sludge technology can effectively be implemented for the treatment of domestic wastewater.

Efficient preparation of enantiopure D-phenylalanine through asymmetric resolution using immobilized phenylalanine ammonia-lyase from Rhodotorula glutinis JN-1 in a recirculating packed-bed reactor.

An efficient enzymatic process was developed to produce optically pure D-phenylalanine through asymmetric resolution of the racemic DL-phenylalanine using immobilized phenylalanine ammonia-lyase (RgPAL) from Rhodotorula glutinis JN-1. RgPAL was immobilized on a modified mesoporous silica support (MCM-41-NH-GA). The resulting MCM-41-NH-GA-RgPAL showed high activity and stability. The resolution efficiency using MCM-41-NH-GA-RgPAL in a recirculating packed-bed reactor (RPBR) was higher than that in a stirred-tank reactor. Under optimal operational conditions, the volumetric conversion rate of L-phenylalanine and the productivity of D-phenylalanine reached 96.7 mM h−1 and 0.32 g L−1 h−1, respectively. The optical purity (ee D) of D-phenylalanine exceeded 99%. The RPBR ran continuously for 16 batches, the conversion ratio did not decrease. The reactor was scaled up 25-fold, and the productivity of D-phenylalanine (ee D>99%) in the scaled-up reactor reached 7.2 g L−1 h−1. These results suggest that the resolution process is an alternative method to produce highly pure D-phenylalanine.

Mathematical modeling of high-rate Anammox UASB reactor based on granular packing patterns

A novel mathematical model was developed to estimate the volumetric nitrogen conversion rates of a high-rate Anammox UASB reactor based on the packing patterns of granular sludge. A series of relationships among granular packing density, sludge concentration, hydraulic retention time and volumetric conversion rate were constructed to correlate Anammox reactor performance with granular packing patterns. It was suggested that the Anammox granules packed as the equivalent simple cubic pattern in high-rate UASB reactor with packing density of 50–55%, which not only accommodated a high concentration of sludge inside the reactor, but also provided large pore volume, thus prolonging the actual substrate conversion time. Results also indicated that it was necessary to improve Anammox reactor performance by enhancing substrate loading when sludge concentration was higher than 37.8gVSS/L. The established model was carefully calibrated and verified, and it well simulated the performance of granule-based high-rate Anammox UASB reactor.

Enabling Unbalanced Fermentations by Using Engineered Electrode-Interfaced Bacteria

Cellular metabolism is a series of tightly linked oxidations and reductions that must be balanced. Recycling of intracellular electron carriers during fermentation often requires substrate conversion to undesired products, while respiration demands constant addition of electron acceptors. The use of electrode-based electron acceptors to balance biotransformations may overcome these constraints. To test this hypothesis, the metal-reducing bacterium Shewanella oneidensis was engineered to stoichiometrically convert glycerol into ethanol, a biotransformation that will not occur unless two electrons are removed via an external reaction, such as electrode reduction. Multiple modules were combined into a single plasmid to alter S. oneidensis metabolism: a glycerol module, consisting of glpF, glpK, glpD, and tpiA from Escherichia coli, and an ethanol module containing pdc and adh from Zymomonas mobilis. A further increase in product yields was accomplished through knockout of pta, encoding phosphate acetyltransferase, shifting flux toward ethanol and away from acetate production. In this first-generation demonstration, conversion of glycerol to ethanol required the presence of an electrode to balance the reaction, and electrode-linked rates were on par with volumetric conversion rates observed in engineered E. coli. Linking microbial biocatalysis to current production can eliminate redox constraints by shifting other unbalanced reactions to yield pure products and serve as a new platform for next-generation bioproduction strategies.

Enrichment of high activity nitrifers to enhance partial nitrification process

Partial nitrification (PN) process is an essential pre-treatment to Anammox process for nitrogen removal from ammonium-rich wastewaters. High activity nitrifiers (HANs) are considered as the key to high-rate PN process. The HANs could be successfully enriched at free ammonia (FA) concentrations of 10–20 mg NH 3–N L −1, which differed significantly from those enriched at FA concentrations of 1–3 mg NH 3–N L −1. The maximum specific ammonium conversion rate, half saturation constant, half inhibition constant, cell yield and decay rate were 2.87 g N g −1 VSS d −1, 5.82 mg N L −1, 8218.8 mg N L −1, 0.15 g VSS g −1 NH 4 + –N and 0.09 d −1 for HANs at 30 °C, respectively, which endowed the PN process a volumetric conversion rate (VCR) as high as 2.22 ± 0.33 kg N m −3 d −1. The cultivation mode of HANs was workable and the HANs were promising as inoculum for the PN process.

Nitrification‐Denitrification Biological Treatment of a High‐Nitrogen Waste Stream for Water‐Reuse Applications

This research was conducted to evaluate the use of biological nitrification-denitrification systems as pre-processors for recycling wastewater to potable water in support of space exploration. A packed-bed bioreactor and membrane-aerated nitrification reactor were operated in series with a 10:1 recycle ratio over varying loading rates. The dissolved organic carbon (DOC) removal exceeded 80% for all loading rates (theta = 1 to 6.8 days), while total nitrogen removal generally increased with decreasing retention time, with a maximum removal of 55%. The degree of nitrification generally declined with decreasing retention time from a high of 80% to a low of 60%. Maximum DOC and total nitrogen volumetric removal rates exceeded 1000 and 800 g/m3 x d, respectively, and maximum nitrification volumetric conversion rates exceeded 300 g/m3 x d. At low hydraulic loading rates, the system was stoichiometrically limited, while kinetic limitations dominated at high hydraulic loading rates. Incomplete nitrification occurred at high loading rates, likely as a result of the high pH and large concentrations of ammonia.

Optimizing sequencing batch reactor (SBR) reactor operation for treatment of dairy wastewater with aerobic granular sludge

The biological wastewater treatment using aerobic granular sludge is a new and very promising method, which is predominantly used in SBR reactors which have higher volumetric conversion rates than methods with flocculent sludge. With suitable reactor operation, flocculent biomass will accumulate into globular aggregates, due to the creation of increased substrate gradients and high shearing power degrees. In the research project described in this paper dairy wastewater with a high particle load was treated with aerobic granular sludge in an SBR reactor. A dynamic mathematical model was developed describing COD and nitrogen removal as well as typical biofilm processes such as diffusion or substrate limitation in greater detail. The calibrated model was excellently able to reproduce the measuring data despite of strongly varying wastewater composition. In this paper scenario calculations with a calibrated biokinetic model were executed to evaluate the effect of different operation strategies for the granular SBR. Modeling results showed that the granules with an average diameter of 2.5 mm had an aerobic layer in between 65-95 microm. Density of the granules was 40 kgVSS/m3. Results revealed amongst others optimal operation conditions for nitrogen removal with oxygen concentrations below 5 gO2/m3. Lower oxygen concentrations led to thinner aerobic but thicker anoxic granular layers with higher nitrate removal efficiencies. Total SBR-cycle times should be in between 360-480 minutes. Reduction of the cycle time from 480 to 360 minutes with a 50% higher throughput resulted in an increase of peak nitrogen effluent concentrations by 40%. Considering biochemical processes the volumetric loading rate for dairy wastewater should be higher than 4.5 kgCOD/(m3*d). Higher COD input load with a COD-based volumetric loading rate of 9.0 kgCOD/(m3*d) nearly led to complete nitrogen removal. Under different operational conditions average nitrification rates up to 5 gNH/(m3*h) and denitrification rates up to 3.7 gNO/(m3*h) were achieved.

Rapid conversion of toluene by an acinetobacter sp. Tol 5 mutant showing monolayer adsorption to water-oil interface

Two-liquid-phase partitioning bioreactors (TPPBs) consist of an aqueous phase containing microorganism cells and a water-immiscible, non-bioavailable organic phase, and have been investigated for use in bioconversion of toxic, hydrophobic chemicals. We achieved a toluene conversion rate 10-fold faster than any experimental data previously demonstrated, using a novel bacterium in a TPPB employing silicone oil. The bacterium was a bald mutant of toluene-degrading Acinetobacter sp. Tol 5, which has numerous filamentous cell appendages, hydrophobic cell surfaces, adheres to solid surfaces, and self-agglutinates. The bald mutant cells lack the appendages, have decreased adhesiveness to solid surfaces, retain the hydrophobic cell surface, and reversibly adsorb to the surface of oil droplets dispersed in an aqueous phase. The effects of toluene concentration, aqueous and oil phase volume ratios, and cell concentration on the initial conversion rate of toluene were investigated. Volume ratios did not significantly affect conversion rate, and the rate greater than 1 g/(l.h) was attained even in W/O emulsion containing 80% oil. As cell concentration increased, the volumetric conversion rate increased, but the specific conversion rate decreased. The highest volumetric conversion rate, 3.1 g/(l.h), was achieved using 8.20 g/l cells in a TPPB containing 20% oil.

Nitrate removal by an ultra-compact biofilm reactor

This study consisted of two stages. The first stage was to cultivate denitrifying biofilm with a simplified UCBR while the second stage was to investigate the performance of a standard UCBR for nitrate removal. The volumetric nitrate-N conversion rate increased steadily from less than 0.5 kg/m3.d to about 2.5 kg/m3.d towards the end of Stage 1. Reactor instability and poor biofilm morphology had led to a relatively low nitrate-N conversion rate during stage 1 of operation. In Stage 2, the conversion rate increased rapidly to 6.5 kg/m3.d within 4 days and increased steadily to 10.5 kg/m3.d, thereafter. The biofilm formation in Stage 2 was better than that attained in Stage 1. Biofilm density and biomass concentration in the reactor were in the range of 20.5 to 29.0 g/L and 1 to 3 g/L, respectively. The specific growth rate of the denitrifiers (μ) and the observed biomass growth yield coefficient (Yobs) in the reactor varied within the ranges of 1.5-4.2 d-1 and 0.20-0.65 g biomass/g nitrate-N, respectively. The specific nitrate-N conversion rate (γ) was within the range of 1.8-3.8 g nitrate-N/g biomass.d.

Degradation of phenol and benzoic acid in a three-phase fluidized-bed reactor

Degradation of phenol and benzoic acid was studied in a fluidized-bed reactor (liquid volume 2.17 L) under nonsterile conditions with special emphasis on maximizing the flow through the reactor and investigating reactor performance at fluctuating feeds. Reactor response to substrate pulses was investigated by applying substrate square-wave inputs at a liquid flow of 1.00 L h(-1). A twofold increase of the phenol and benzoic acid feed concentrations for 2.5 h did not lead to accumulation and breakthrough. The cells were able to survive four to fivefold increases of the feed concentration for 1 h without loss of viability, although the phenol pulse lead to phenol accumulation in the reactor. Reactor performance at constantly fluctuating loads was investigated by varying the feed concentrations using sine wave functions. No accumulation of phenol or benzoic acid was observed. Influence of induction was studied using shift experiments. After 35 days of operation (369 hydrodynamic residence times) with phenol as sole substrate (carbon source) the reactor was able to mineralize benzoic acid without any adaptation or lag phase. The capability of phenol degradation, on the other hand, was lost by most cells after only 3 days operation with benzoic acid as the sole substrate. The experiments underline the importance of induction. In order to maximize the flow through the reactor, the liquid flow was increased stepwise while the feed concentrations were reduced correspondingly, keeping the volumetric conversion rates of phenol (0.24 g L(-1) h(-1)) and benzoic acid (0.17 g L(-1) h(-1)) constant. By this means, liquid flow could be increased up to 13.32 L h(-1), which was more than 20-fold higher than the maximum liquid flow achievable in a chemostat using the same conditions.

The Performance of Structured Packings in Trickle-Bed Reactors

An experimental study was carried out to investigate whether the use of structured packings might improve the mass transfer characteristics and the catalyst effectiveness of a trickle-bed reactor. Therefore, the performances of a structured packing, consisting of KATAPAK elements, and a dumped packing, consisting of small diameter spherical particles, have been compared for both a chemisorption process and a process where a heterogeneously catalysed chemical reaction is carried out. The chemisorption of CO2 in aqueous amine solutions and the hydrogenation of ?-methylstyrene catalysed by palladium on ?-alumina were chosen as model reactions, respectively. The performance of the trickle-bed reactor was quantified by measuring the specific gas-liquid contact area and the volumetric liquid-side mass transfer coefficient in the case of the chemisorption process and the conversion rate in the case of the heterogeneously catalysed chemical reaction. These parameters were measured for both packing types as a function of a number of process parameters. In this paper, the experimental results are presented and a comparison is made of the performances of the two packing types for both types of processes. Both packing types showed similar mass transfer characteristics as well as volumetric conversion rates. However, the structured packing showed a much higher contact efficiency as well as a much higher catalyst effectiveness. A significant improvement is therefore expected when a structured packing is used with a higher specific geometrical area than that applied in this study. Furthermore, the structured packing is favoured in the case of fast exothermic liquid-phase reactions

Measurements of the photochemical production of aerosols in ambient air near a freeway for a range of SO 2 concentrations

The formation and growth of photochemically produced aerosol particles were studied for initially particle-free ambient air collected downwind from a freeway in two 14 m 3 transparent Teflon bags exposed to natural sunlight for SO 2 concentrations ranging from 0.006 to 0.067 ppm, by volume. Using a Thermo Systems Electrical Aerosol Analyzer, the concentration and size distribution of aerosol particles from 0.0056 to 0.36 μm dia. was measured as a function of time. The volumetric conversion rate of material from the vapor to the condensed phase was found to be a linear function of the initial SO 2 concentration and directly proportional to the initial non-methane hydrocarbon concentration in the bags. Filter samples of the photochemically produced aerosol particles were analyzed for sulfate, nitrate and nitrite. The sulfate and nitrate formation rates ranged from 0.72 to 2.6 μg m −3 h −1 and 0.76 to 3.2 μg m −3 h −1, respectively, while the nitrite formation rate ranged from 0.008 to 0.038 μg m −3 h −1. The overall rate of SO 2 to sulfate conversion was estimated to range from 0.55 to 1.3% h −1. X-ray photoelectron spectroscopy was used to determine the chemical states of sulfur, nitrogen and carbon on the surface of the filter samples. Sulfur was found in the form of SO 2− 4, as well as chemisorbed SO 2 and SO 3. Nitrogen was present as NO − 3 and as two reduced forms identified as amines or amides, and carbon was present as either elemental carbon or as paraffinic hydrocarbon.