Advantages of combined UV photodegradation and biofiltration processes to treat gaseous chlorobenzene

Reduction characteristic of chlorobenzene by a newly isolated Paenarthrobacter ureafaciens LY from a pharmaceutical wastewater treatment plant

A highly efficient chlorobenzene-degrading strain was isolated from the sludge of a sewage treatment plant associated with a pharmaceutical company. The strain exhibited a similarity of over 99.9% with multiple strains of Paenarthrobacter ureafaciens. Therefore, the strain was suggested to be P. ureafaciens LY. This novel strain exhibited a broad spectrum of pollutant degradation capabilities, effectively degrading chlorobenzene and other organic pollutants, such as 1, 2, 4-trichlorobenzene, phenol, and xylene. Moreover, P. ureafaciens LY co-metabolized mixtures of chlorobenzene with 1, 2, 4-trichlorobenzene or phenol. Evaluation of its degradation efficiency showed that it achieved an impressive degradation rate of 94.78% for chlorobenzene within 8 h. The Haldane-Andrews model was used to describe the growth of P. ureafaciens LY under specific pollutants and its concentrations, revealing a maximum specific growth rate (μmax ) of 0.33 h-1 . The isolation and characterization of P. ureafaciens LY, along with its ability to degrade chlorobenzene, provides valuable insights for the development of efficient and eco-friendly approaches to mitigate chlorobenzene contamination. Additionally, investigation of the degradation performance of the strain in the presence of other pollutants offers important information for understanding the complexities of co-metabolism in mixed-pollutant environments.

Enhanced chlorobenzene removal by internal magnetic field through initial cell adhesion and biofilm formation

Magnetic fields (MF) have been proven efficient in bioaugmentation, and the internal MFs have become competitive because they require no configuration, despite their application in waste gas treatment remaining largely unexplored. In this study, we firstly developed an intensity-regulable bioaugmentation with internal MF for gaseous chlorobenzene (CB) treatment with modified packing in batch bioreactors, and the elimination capacity increased by up to 26%, surpassing that of the external MF. Additionally, the microbial affinity to CB and the packing surface was enhanced, which was correlated with the ninefold increased secreted ratio of proteins/polysaccharides, 43% promoted cell surface hydrophobicity, and half reduced zeta potential. Furthermore, the dehydrogenase content was promoted over 3 times, and CB removal steadily increased with the rising intensity indicating enhanced biofilm activity and reduced CB bioimpedance; this was further supported by kinetic analysis, which resulted in improved cell adhesive ability and biological utilisation of CB. The results introduced a novel concept of adjustable magnetic bioaugmentation and provided technical support for industrial waste gas treatments. KEY POINTS: • Regulable magnetic bioaugmentation was developed to promote 26% chlorobenzene removal • Chlorobenzene mineralisation was enhanced under the magnetic field • Microbial adhesion was promoted through weakening repulsive forces.

Degradation of trichloroethylene by double dielectric barrier discharge (DDBD) plasma technology: Performance, product analysis and acute biotoxicity assessment

Trichloroethylene is carcinogenic and poorly degraded by microorganisms in the environment. Advanced Oxidation Technology is considered to be an effective treatment technology for TCE degradation. In this study, a double dielectric barrier discharge (DDBD) reactor was established to decompose TCE. The influence of different condition parameters on DDBD treatment of TCE was investigated to determine the appropriate working conditions. The chemical composition and biotoxicity of TCE degradation products were also investigated. Results showed that when SIE was 300 J L^-1, the removal efficiency could reach more than 90%. The energy yield could reach 72.99 g kWh^-1 at low SIE and gradually decreased with the increase of SIE. The k of the Non-thermal plasma (NTP) treatment of TCE was about 0.01 L J^-1. DDBD degradation products were mainly polychlorinated organic compounds and produced more than 373 mg m^-3 ozone. Moreover, a plausible TCE degradation mechanism in the DDBD reactors was proposed. Lastly, the ecological safety and biotoxicity were evaluated, indicating that the generation of chlorinated organic products was the main cause of elevated acute biotoxicity.

Effect of rhamnolipids on the fungal elimination of toluene vapor in a biotrickling filter under stressed operational conditions

The application of rhamnolipids in a fungal-cultured biotrickling filter (BTF) has a significant impact on toluene removal. Two BTFs were used; BTF-A, a control bed, and BTF-B fed with rhamnolipids. The effect of empty bed residence times (EBRTs) on toluene bioavailability was investigated. Removal of toluene was carried out at EBRTs of 30 and 60 s and inlet loading rates (LRs) of 23-184 g m^-3 h^-1. At 30 s EBRT, when inlet LR was increased from 23 to 184 g m^-3 h^-1, the removal efficiency (RE) decreased from 93% to 50% for the control bed, and from 94% to 87% for BTF-B. Increasing the EBRT simultaneously with inlet LRs, confirms that BTF-A was diffusion-limited by registering a RE of 62% for toluene inlet LR of 184 g m^-3 h^-1, whereas BTF-B, achieved RE > 96%, confirming a significant improvement in toluene biodegradability. Overall, the best performance was observed at 60 s EBRT and inlet LR of 184 g m^-3 h^-1, providing a maximum elimination capacity (EC) of 176.8 g m^-3 h^-1 under steady-state conditions. While a maximum EC of 114 g m^-3 h^-1 was observed under the same conditions in the absence of rhamnolipids (BTF-A). Measurements of critical micelle concentration showed that 150 mg L^-1 of rhamnolipids demonstrated the lowest aqueous surface tension and maximum formation of micelles, while 175 mg L^-1 was the optimum dose for fungal growth. Production rate of carbon dioxide, and dissolved oxygen contents highlighted the positive influence of rhamnolipids on adhesive forces, improved toluene mineralization, and promotion of microbial motility over mobility.

Using circular economy principles to recycle materials in guiding the design of a wet scrubber-reactor for indoor air disinfection from coronavirus and other pathogens

An arduous need exists to discover rapid solutions to avoid the accelerated spread of coronavirus especially through the indoor environments like offices, hospitals, and airports. One such measure could be to disinfect the air, especially in indoor environments. The goal of this work is to propose a novel design of a wet scrubber-reactor to deactivate airborne microbes using circular economy principles. Based on Fenton's reaction mechanism, the system proposed here will deactivate airborne microbes (bioaerosols) such as SARS-CoV-2. The proposed design relies on using a highly porous clay-glass open-cell structure as an easily reproducible and cheap material. The principle behind this technique is an in-situ decomposition of hydrogen peroxide into highly reactive oxygen species and free radicals. The high porosity of a tailored ceramic structure provides a high contact area between atomized oxygen, free radicals and supplied polluted air. The design is shown to comply with the needs of achieving sustainable development goals.

Carbon quantum dots-TiO2 nanocomposite as an efficient photocatalyst for the photodegradation of aromatic ring-containing mixed VOCs: An experimental and DFT studies of adsorption and electronic structure of the interface

In this work, we have developed and optimized TiO₂ nanoparticles decorated with carbon quantum dots to examine its potential use in the photocatalytic oxidation of aromatic ring containing gas-phase mixed volatile organic compounds, e.g., benzene, toluene, and p-xylene. Carbon quantum dots decorated TiO₂ demonstrated good photodegradation efficiency in contrast to pure TiO₂ under UV-vis light illumination. For example, with 0.5 wt% carbon quantum dots decorated on TiO₂, 64 % of the mixed volatile organic compounds were photodegraded, while pure TiO₂ only exhibited 44 % of the photodegradation efficiency. Also, the carbon quantum dots (0.5 wt%)/TiO₂ nanocomposite demonstrated considerable photocatalytic activity within the visible region. On the other hand, pure TiO₂ remained inactive within the visible region. The density functional theory study of the carbon quantum dots/TiO₂ interface revealed that C 2p states of carbon quantum dots incorporated new energy states around the Fermi level near the lowest conduction band. This might be accountable for the improved charge separation process and better conductivity of the photogenerated electrons. The improved photocatalytic performance of the carbon quantum dots/TiO₂ nanocomposites can be attributed to good light harvesting within the UV-vis region, charge separation, and adsorption capability.

Biological Waste Air and Waste Gas Treatment: Overview, Challenges, Operational Efficiency, and Current Trends

International contracts to restrict emissions of climate-relevant gases, and thus global warming, also require a critical reconsideration of technologies for treating municipal, commercial, industrial, and agricultural waste gas emissions. A change from energy- and resource-intensive technologies, such as thermal post-combustion and adsorption, as well to low-emission technologies with high energy and resource efficiency, becomes mandatory. Biological processes already meet these requirements, but show restrictions in case of treatment of complex volatile organic compound (VOC) mixtures and space demand. Innovative approaches combining advanced oxidation and biofiltration processes seem to be a solution. In this review, biological processes, both as stand-alone technology and in combination with advanced oxidation processes, were critically evaluated in regard to technical, economical, and climate policy aspects, as well as present limitations and corresponding solutions to overcome these restrictions.

Simultaneous Removal of Hexane and Ethanol from Air in a Biotrickling Filter—Process Performance and Monitoring Using Electronic Nose

Biofiltration is a well-accepted method for the removal of malodorous compounds from air streams. Interestingly, the mechanisms underlying this process are not fully understood. The aim of this paper was to investigate the simultaneous removal of hydrophobic hexane with hydrophilic ethanol, resulting in the enhanced removal of hexane in the presence of ethanol. Investigations were performed in a peat-perlite packed biotrickling filter and the process performance was monitored using both gas chromatography and electronic nose techniques. The results indicate that the length as well as the efficiency of biofiltration during the start-up period depend on the feed composition, with higher efficiency obtained when hexane and ethanol were fed together from the process initiation. The experiments in the steady-state period present the biofilter performance when different ratios of hydrophilic to hydrophobic compounds were fed to the biofilter. The obtained results show the synergistic effects of the addition of a hydrophilic compound on the removal efficiency of hydrophobic hexane. The influence of the ratio of hydrophilic to hydrophobic compounds is discussed in terms of enhancing the mass transfer phenomena for hydrophobic volatile organic compounds.

Improved degradation of n-hexane vapours using a hybrid system, a photoreactor packed with TiO2 coated-scoria granules and a multilayer biofilter

Biofiltration of hydrophobic and/or recalcitrant volatile organic compounds such as n-hexane is imperfect. In the present study, we applied a hybrid system consisting of a photoreactor packed with scoria granules coated with TiO₂ and a biofilter to improve the removal efficiency of n-hexane from the air stream. The experimental results showed that the hybrid system provided higher removal efficiencies than the single biofilter process with an inlet n-hexane concentration range of 0.11-1 g^-3 for empty bed residence times (EBRTs) of 30-120 s in the hybrid system. The removal efficiency of the single biofilter in EBRTs of 30, 60 and 120 s was 10.06%, 21.45%, and 45.98%, respectively. When the photoreactor was included as a pretreatment system (with residence time of 7-27 s) and the overall EBRTs of the system was adjusted to 30, 60 and 120 s, the removal efficiency of the hybrid system was increased to 39.79%, 63.08%, and 92.6%, respectively. The mass ratio of carbon dioxide produced as an indicator for n-hexane degradation in the hybrid system and the biofilter alone was 1.9 and 1.28, respectively. Bacterial community analysis with sequence analysis of 16S rDNA in the biofilter biomass revealed that Pseudomonas and Bacillus as predominant bacterial species were responsible for n-hexane biodegradation. Therefore, the application of the hybrid system is advantageous in enhanced n-hexane removal from the air stream.

Enhanced biodegradation of n-hexane by Pseudomonas sp. strain NEE2

Pseudomonas sp. strain NEE2 isolated from oil-polluted soils could biodegrade n-hexane effectively. In this study, the secretory product of n-hexane biodegradation by NEE2 was extracted, characterized, and investigated on the secretory product's enhanced effect on n-hexane removal. The effects of various biodegradation conditions on n-hexane removal by NEE2, including nitrogen source, pH value, and temperature were also investigated. Results showed that the secretory product lowered surface tension of water from 72 to 40 mN/m, with a critical micelle concentration of 340 mg/L, demonstrating that there existed biosurfactants in the secretory product. The secretory product at 50 mg/L enhanced n-hexane removal by 144.4% within 48 h than the control group. The optimum conditions for n-hexane removal by NEE2 were at temperature of 25-30 °C, pH value of 7-8, and (NH4)2SO4 as nitrogen source. Besides n-hexane, NEE2 could also utilize a variety of carbon sources. These results proved that NEE2 can consume hydrophobic volatile organic compounds (VOCs) to produce biosurfactants which can further enhance hydrophobic VOCs degradation.

Airborne disinfection using microwave-based technology: Energy efficient and distinct inactivation mechanism compared with waterborne disinfection

Microwave has been extensively applied to inactivate microorganisms in liquids, food, and surfaces. However, energy efficiency is a limiting factor for the environmental application. The utilization pathway and energy efficiency of the microwave in different media have not been investigated. In this study, the inactivation performance, energy utilization, and bactericidal mechanisms for microwave-irradiated airborne and waterborne Escherichia coli were compared. A Beer-Lambert law-based model was also developed and validated to compare the inactivation performance in different phases. Microwave had greater inactivation effect on airborne bacteria than waterborne bacteria. The inactivation rate constant for airborne E. coli (0.29 s^-1) was nearly 20 times higher than that of waterborne species (0.014 s^-1). Most of the absorbed microwave energy (92.3%) was converted to increase water temperature instead of inactivating the waterborne bacteria, because the microwave photons were easily absorbed by water molecules. By contrast, 45.4% of the absorbed energy could disinfect the airborne bacteria. Finally, the required energies for 1-log inactivation were calculated as 2.3 J and 116.9 J per log-inactivation for airborne and waterborne E. coli, respectively. The airborne and waterborne E. coli samples showed distinct microwave inactivation mechanisms. Waterborne E. coli disinfection was primarily due to thermal effect, while the non-thermal effect was the major mechanism for airborne E. coli inactivation.

Enhancing Chlorobenzene Biodegradation by Delftia tsuruhatensis Using a Water-Silicone Oil Biphasic System

In this study, a water-silicone oil biphasic system was developed to enhance the biodegradation of monochlorobenzene (CB) by Delftia tsuruhatensis LW26. Compared to the single phase, the biphasic system with a suitable silicone oil fraction (v/v) of 20% allowed a 2.5-fold increase in the maximum tolerated CB concentration. The CB inhibition on D. tsuruhatensis LW26 was reduced in the presence of silicone oil, and the electron transport system activity was maintained at high levels even under high CB stress. Adhesion of cells to the water-oil interface at the water side was observed using confocal laser scanning microscopy. Nearly 75% of cells accumulated on the interface, implying that another interfacial substrate uptake pathway prevailed besides that initiated by cells in the aqueous phase. The 8-fold increase in cell surface hydrophobicity upon the addition of 20% (v/v) silicone oil showed that silicone oil modified the surface characteristics of D. tsuruhatensis LW26. The protein/polysaccharide ratio of extracellular polymeric substances (EPS) from D. tsuruhatensis LW26 presented a 3-fold enhancement. These results suggested that silicone oil induced the increase in the protein content of EPS and rendered cells hydrophobic. The resulting hydrophobic cells could adhere on the water-oil interface, improving the mass transfer by direct CB uptake from silicone oil.

An innovative nutritional slow-release packing material with functional microorganisms for biofiltration: Characterization and performance evaluation

The type of packing material for biofiltration has a great impact on microbial growth and pollutant removal. This study evaluated the feasibility of a nutritional slow-release packing material with functional microorganisms (NSRP-FM) in a biofilter for the removal of gaseous n-butyl acetate. Through the emulsification-cross linked process and microbial immobilization, an innovative packing material was obtained, with a specific surface area of 2.45 m² g^-1 and a bulk density of 40.75 kg m^-3. The cumulative release rates of total phosphorus and total nitrogen were 90.6% and 75.6%, respectively, as measured while continuously spraying deionized water. To evaluate the performance of biofiltration, NSRP-FM was compared with the commercial polyurethane foam (PU-foam), in two identical biotrickling filters (BTFs). The BTF packed with the prepared NSRP-FM maintained a consistent removal efficiency (over 95%) without nutrients addition and pH adjustment. The other BTF had poor removal performance, and the removal efficiency declined to 65% when there was no pH adjustment. Energy dispersive X-ray spectroscopy (EDS) analysis of NSRP-FM showed that inorganic elements were released during the operation of BTF. The abundance of functional microorganisms suggested that the prepared NSRP-FM provided a better environment for microbial growth, despite changes in the operating conditions.

Significance of a Non-Thermal Plasma Treatment on LDPE Biodegradation with Pseudomonas Aeruginosa

The use of plastics has spanned across almost all aspects of day to day life. Although their uses are invaluable, they contribute to the generation of a lot of waste products that end up in the environment and end up polluting natural habitats such as forests and the ocean. By treating low-density polyethylene (LDPE) samples with non-thermal plasma in ambient air and with an addition of ≈4% CO₂, the biodegradation of the samples can be increased due to an increase in oxidative species causing better cell adhesion and acceptance on the polymer sample surface. It was, however, found that the use of this slight addition of CO₂ aided in the biodegradation of the LDPE samples more than with solely ambient air as the carbon bonds measured from Raman spectroscopy were seen to decrease even more with this change in gas composition and chemistry. The results show that the largest increase of polymer degradation occurs when a voltage of 32 kV is applied over 300 s and with a mixture of ambient air and CO₂ in the ratio 25:1.

Efficient degradation of chlorobenzene in a non-thermal plasma catalytic reactor supported on CeO2/HZSM-5 catalysts

Chlorobenzene removal was investigated in a non-thermal plasma reactor using CeO₂/HZSM-5 catalysts. The performance of catalysts was evaluated in terms of removal and energy efficiency. The decomposition products of chlorobenzene were analyzed. The results show that CeO₂/HZSM-5 exhibited a good catalytic activity, which resulted in enhancements of chlorobenzene removal, energy efficiency, and the formation of lower amounts of by-products. With regards to CO₂ selectivity, the presence of catalysts favors the oxidation of by-products, leading to a higher CO₂ selectivity. With respect to ozone, which is considered as an unavoidable by-product in air plasma reactors, a noticeable decrease in its concentration was observed in the presence of catalysts. Furthermore, the stability of the catalyst was investigated by analyzing the evolution of conversion in time. The experiment results indicated that CeO₂/HZSM-5 catalysts have excellent stability: chlorobenzene conversion only decreased from 78% to 60% after 75hr, which means that the CeO₂/HZSM-5 suffered a slight deactivation. Some organic compounds and chlorinated intermediates were adsorbed or deposited on the catalysts surface as shown by the results of Fourier Transform Infrared (FT-IR) spectroscopy, scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) analyses of the catalyst before and after the reaction, revealing the cause of catalyst deactivation.

Enhanced stable long-term operation of biotrickling filters treating VOCs by low-dose ozonation and its affecting mechanism on biofilm

For long-term operation of highly loaded biotrickling filters (BTFs), the prevention of excess biomass accumulation was essential for avoiding BTF failure. In this study, we proposed low-dose ozonation as a biomass control strategy to maintain high removal efficiencies of volatile organic compounds (VOCs) over extended operation of BTFs. To obtain an optimized biomass control strategy, the relative performance of five parallel BTFs receiving different ozone doses was determined, and the affecting mechanism of ozonation on biofilm was elucidated. Experimental results showed that the decline in ozone-free BTF performance began from day 150, which was correlated with excess biomass accumulation, abundant excretion of extracellular polymeric substances (EPS) and a decline in metabolic activity of biofilm over extended operation. Ozone of 5-10 mg m(-3) was effective in preventing excessive growth and uneven distribution of biomass, and eventually maintaining long-term stable operations. Ozone of over 20 mg m(-3) possibly inhibited microorganism growth severely, thereby deteriorating the elimination performance instead. Comparison of the biofilm EPS indicated that the presence of ozone reduce EPS contents to different extents, which was possibly beneficial for mass transfer and metabolic activity. Comparative community analysis showed that ozonation resulted in different microbial communities in the BTFs. Dyella was found to be the most abundant bacterial genera in all BTFs regardless of ozonation, indicating strong resistance to ozonation. Chryseobacterium and Burkholderia members were markedly enriched in the ozone-added biofilm, implying good adaptation to ozone presence. These findings provided an improved understanding of low-dose ozonation in maintaining a stable long-term operation of BTF.

Conversion characteristics and production evaluation of styrene/o-xylene mixtures removed by DBD pretreatment

The combination of chemical oxidation methods with biotechnology to removal recalcitrant VOCs is a promising technology. In this paper, the aim was to identify the role of key process parameters and biodegradability of the degradation products using a dielectric barrier discharge (DBD) reactor, which provided the fundamental data to evaluate the possibilities of the combined system. Effects of various technologic parameters like initial concentration of mixtures, residence time and relative humidity on the decomposition and the degradation products were examined and discussed. It was found that the removal efficiency of mixed VOCs decreased with increasing initial concentration. The removal efficiency reached the maximum value as relative humidity was approximately 40%-60%. Increasing the residence time resulted in increasing the removal efficiency and the order of destruction efficiency of VOCs followed the order styrene > o-xylene. Compared with the single compounds, the removal efficiency of styrene and o-xylene in the mixtures of VOCs decreased significantly and o-xylene decreased more rapidly. The degradation products were analyzed by gas chromatography and gas chromatography-mass spectrometry, and the main compounds detected were O3, COx and benzene ring derivatives. The biodegradability of mixed VOCs was improved and the products had positive effect on biomass during plasma application, and furthermore typical results indicated that the biodegradability and biotoxicity of gaseous pollutant were quite depending on the specific input energy (SIE).

Dichloromethane removal and microbial variations in a combination of UV pretreatment and biotrickling filtration

Biofiltration of hydrophobic and/or recalcitrant volatile organic compounds in industry is currently limited. A laboratory-scale system integrating ultraviolet (UV) photodegradation and a biotrickling filter (BTF) was developed to treat dichloromethane (DCM), and this was compared to BTF alone. A combined UV-BTF approach permitted faster biofilm formation and greater removal than BTF. DCM distribution and its photodegradation intermediates revealed that the lower filter of the UV-BTF contributed more to CO2 production; the upper filter assisted more with DCM removal. The UV-BTF kept secretion of extracellular polymeric substances at a normal level with an evenly distributed biomass. Pyrosequencing analysis showed that the dominant population in the combined biofilter was more diverse than that in BTF alone. Our data provide a foundation for understanding the effect of UV pretreatment on BTF performance and the microbial community.

Effects of surfactant and Zn (II) at various concentrations on microbial activity and ethylbenzene removal in biotricking filter

The effects of Tween-20, a non-ionic surfactant, and Zn (II) on microbial activity and removal performance for ethylbenzene in a biotrickling filter (BTF) were evaluated. Batch experiments were conducted to evaluate the surfactant and Zn (II) at various concentrations for their toxicity to microorganisms, and results indicated that Tween-20 was beneficial to microbial activity at all the tested concentration, while Zn (II) affected adversely when the concentration overpassed 5.0mgL(-1). Then effects of the two additives on removal efficiency of ethylbenzene were evaluated in a BTF at an empty-bed retention time of 30s and an ethylbenzene concentration of 1100mgm(-3). Results showed that the optimal concentrations of Tween-20 and Zn (II) were about 12 and 1.0mgL(-1), respectively. Compared to the results when neither of the two additives was added, Tween-20 improved ethylbenzene removal efficiency from 67% to 86% at the optimal condition, while on that basis, Zn (II) just increased the removal efficiency from 86% to 90%. The promoting effects of the two additives on recovering microbial activity and removing excessive biomass were also observed in this article.

Waste gas biofiltration: advances and limitations of current approaches in microbiology

As confidence in gas biofiltration efficacy grows, ever more complex malodorant and toxic molecules are ameliorated. In parallel, for many countries, emission control legislation becomes increasingly stringent to accommodate both public health and climate change imperatives. Effective gas biofiltration in biofilters and biotrickling filters depends on three key bioreactor variables: the support medium; gas molecule solubilization; and the catabolic population. Organic and inorganic support media, singly or in combination, have been employed and their key criteria are considered by critical appraisal of one, char. Catabolic species have included fungal and bacterial monocultures and, to a lesser extent, microbial communities. In the absence of organic support medium (soil, compost, sewage sludge, etc.) inoculum provision, a targeted enrichment and isolation program must be undertaken followed, possibly, by culture efficacy improvement. Microbial community process enhancement can then be gained by comprehensive characterization of the culturable and total populations. For all species, support medium attachment is critical and this is considered prior to filtration optimization by water content, pH, temperature, loadings, and nutrients manipulation. Finally, to negate discharge of fungal spores, and/or archaeal and/or bacterial cells, capture/destruction technologies are required to enable exploitation of the mineralization product CO(2).

Conversion characteristics and mechanism analysis of gaseous dichloromethane degraded by a VUV light in different reaction media

The photodegradation of gaseous dichloromethane (DCM) by a vacuum ultraviolet (VUV) light in a spiral reactor was investigated with different reaction media and initial concentrations. Through the combination of direct photolysis, O3 oxidation and HO* oxidation, DCM was ultimately mineralized into inorganic compounds (such as HCl, CO2, H2O, etc.) in the air with relative humidity (RH) of 75%-85%. During the photodegradation process, some small organic acids (including formic acid, acetic acid) were also detected and the intermediates were more soluble than DCM, providing a possibility for its combination with subsequent biodegradation. Based on the detected intermediates and the confirmed radicals, a photodegradation pathway of DCM by VUV was proposed. With RH 75%-80% air as the reaction medium, the DCM removal followed the second-order kinetic model at inlet concentration of 100-1000 mg/m3. Kinetic analysis showed that the reaction media affected the kinetic constants of DCM conversion by a large extent, and RH 80% air could cause a much lower half-life for its conversion. Such results supported the possibility that VUV photodegradation could be used not only for the mineralization of DCM but also as a pretreatment before biodegradation.

Combined biological and physicochemical waste-gas cleaning techniques

This review presents a general overview of physical, chemical and biological waste-gas treatment techniques such as adsorption, absorption, oxidation and biodegradation, focusing more extensively on combined processes. It is widely recognized that biological waste-gas treatment devices such as biofilters and biotrickling filters can show high performance, often reaching removal efficiencies above 90 % for pollutant concentrations below 5 g/m(3). However, for concentrations exceeding this limit and under transient shock-load conditions that are frequently encountered in industrial situations, a physicochemical gas cleaning process can sometimes be advantageously combined with a biological one. Besides improving the overall treatment efficiency, the non-biological, first-stage process could also serve as a load equalization system by reducing the pollutant load during periodic shock-loads, to levels that can easily be handled in the second-stage bioreactor. This article reviews the operational advantages of integrating different non-biological and biological processes, i.e., adsorption pre-treatment+bioreactor, bioreactor+adsorption post-treatment, absorption pre-treatment+bioreactor, UV pre-treatment+bioreactor, and bioreactor/bioreactor combinations, for waste-gas treatment, where different gas-phase pollutants have been tested.

Treatment of gaseous alpha-pinene by a combined system containing photo oxidation and aerobic biotrickling filtration

Biofiltration of hydrophobic and/or recalcitrant volatile pollutants is intrinsically limited. In the present study, a combined ultraviolet-biotrickling filter (UV-BTF) was developed to improve the removal of these compounds, and a single BTF as the control was operated under the same conditions. The experimental results showed that the UV-BTF provided higher removal efficiencies than the single BTF at an inlet concentration range of 600-1500 mg m(-3) under shorter residence times. The maximum elimination capacities (ECs) obtained were 94.2 mg m(-3)h(-1) and 44 mg m(-3)h(-1) in the combined UV-BTF and single BTF, respectively. The mass ratio of carbon dioxide produced to α-pinene removed in the UV-BTF was approximately 2.74, which was much higher than that of the single BTF (1.99). Polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) analysis indicated that there was more complicated microbial community in the UV-BTF than that in the single BTF. In addition, we investigated the effect of starvation or stagnation on re-acclimation and removal performance from an engineering standpoint. The results showed that the combined UV-BTF could deal with fluctuating conditions or periods without any flow (air or liquid) supply much better than the single BTF.

Modeling of a combined ultraviolet-biofilter system to treat gaseous chlorobenzene I: model development and parametric sensitivity

A new type of a combined ultraviolet (UV)-biofilter system for air pollution control is developed. In this paper, two conceptual mathematical submodels of the UV reactor and standalone biofilter are developed. All model parameters have been determined by independent experiments or have been taken from literature. Results from UV and the standalone biofilter submodels are in a good agreement with experimental data. However, the performance of the combined system has significantly deviated from those of the UV or standalone submodels because of the stimulating effects of UV irradiation products on the subsequent biofilter performance. A modified model that considers the stimulating effects has agreed well with experimental data over a wide range of operating conditions. Further analysis of the primary parametric sensitivity of the model has shown that inlet chlorobenzene concentrations, gas empty-bed residence time in the UV reactor, and light intensity are important operating conditions.

Stimulative effects of ozone on a biofilter treating gaseous chlorobenzene

Recalcitrant volatile organic compounds with low biodegradabilities pose challenges for biofiltration technologies. In this study, the effects and mechanism of adding ozone on the performance of a biofilter were investigated. A biofilter treating chlorobenzene was set up and operated continuously for 265 days under different inlet ozone concentrations. Results showed that ozone below 120 mg m(-3) could notably enhance the biofilter performance. The average chlorobenzene removal efficiency increased from 40 to 70% and then to 90% while the inlet ozone concentration rose from 0 to 40 mg m(-3) and 120 mg m(-3). Reducing ozone concentration resulted in a decrease in removal efficiency from 90 to 40%. Further analysis indicated that the thickness and extra-cellular polymer substance content of the biofilm were remarkably reduced while inlet ozone concentration was gradually increased. Meanwhile, the specific surface areas of the filter bed were found to increase from 784 to 820 and 880 m(2) m(-3). A respiratory quinone profile showed that the dominant quinone shifted from ubiquinone-8 to menaquinone-9(H(2)) after ozone was added. This indicated that some Gram-positive bacteria with thick cell wall became the dominant species under ozone compression.