Enhancing the environmental performance of biotrickling filters treating volatile organic compounds in air

Photocatalytic air purification is widely regarded as a promising technology, but it calls for more efficient photocatalytic materials and systems. Here we report a strategy to introduce an in-situ water (self-wetting) layer on WO3 by coating hygroscopic periodic acid (PA) to dramatically enhance the photocatalytic removal of hydrophilic volatile organic compounds (VOCs) in air. In ambient air, water vapor is condensed on WO3 to make a unique tri-phasic (air/water/WO3) system. The in-situ formed water layer selectively concentrates hydrophilic VOCs. PA plays the multiple roles as a water-layer inducer, a surface-complexing ligand enhancing visible light absorption, and a strong electron acceptor. Under visible light, the photogenerated electrons are rapidly scavenged by periodate to produce more •OH. PA/WO3 exhibits excellent photocatalytic activity for acetaldehyde degradation with an apparent quantum efficiency of 64.3% at 460 nm, which is the highest value ever reported. Other hydrophilic VOCs like formaldehyde that are readily dissolved into the in-situ water layer on WO3 are also rapidly degraded, whereas hydrophobic VOCs remain intact during photocatalysis due to the “water barrier effect”. PA/WO3 successfully demonstrated an excellent capacity for degrading hydrophilic VOCs selectively in wide-range concentrations (0.5−700 ppmv).

A pilot-scale biotrickling filter was installed at the Hyperion Treatment Plant in Los Angeles, California, to study hydrogen sulfide (odor) and volatile organic compound (VOC) removal from headworks waste air. The performance of the reactor was continuously monitored during a 10-month period. At an average empty bed gas residence time of 24 seconds, 10 to 50 ppm of hydrogen sulfide was consistently removed at greater than 98% efficiency, corresponding to an average volumetric elimination capacity of 5.2 g/m3 x h. Concentration profiles over the height of the reactor indicated nearly complete removal in the first section of the reactor, suggesting that elimination capacities up to 30 g/m3 x h could be obtained. The odor reduction (as dilution to threshold) was 98%, which correlated with the efficiency of removal of hydrogen sulfide as the primary pollutant. Volatile organic compounds were present at concentrations up to 225 ppb. Moderate but significant removal of toluene and benzene was observed when the biotrickling filter was operated with pH control to neutralize sulfuric acid production from hydrogen sulfide oxidation. Xylenes and chlorinated VOCs were not removed regardless of experimental conditions in the reactor. The results led to the conclusion that VOC removal is the limiting process in biotrickling filters for the simultaneous removal of hydrogen sulfide and VOCs at publicly owned treatment works.

Effect of inoculum type, packing material and operational conditions on the biofiltration of a mixture of hydrophobic volatile organic compounds in air

A method for the determination of 104 volatile organic compounds (VOCs) in ambient air based on double column multi dean switching gas chromatography-mass spectrometry/flame ionization detector (GC-MS/FID) coupled with sorbent assisted electronically controlled cryo-focusing unit was developed and evaluated. The sorbent assisted electronically controlled cryo-focusing unit was used for trapping, dehydration and focusing of VOCs sampled in summa canisters. The VOCs were split into two parts by the multi dean switching unit in GC-MS/FID. The C2-C3 components were determined in a PLOT capillary column with an FID detector, while the C4-C12 components were determined in an Intercap-624 capillary column with a MS detector. The C2-C3 components were qualitatively confirmed from the retention time and quantified by the calibration curves, while the C4-C12 components were qualitatively confirmed from the retention time and the relative abundance ratio of characteristic ions, and quantified by the internal standard calibration curves. The major factors influencing the cryo-focusing performance including the type of sorbent tube, the pressure employed in assisted pressure control unit (APC), and the split point in multi dean switching unit were investigated. The chromatographic and MS parameters were optimized. Under optimum conditions, a linear relationship was observed with the content of VOCs ranging from 0.0446 to 0.892 μmol/m3, and correlation coefficients (r) no less than 0.9984. The average spiked recoveries of the six VOCs at two levels of 0.0446 μmol/m3 and 0.223 μmol/m3 were 86.4%-116.1%, with relative standard deviations in the range of 0.9%-11.3% The method detection limits (MDLs) and the limits of quantification (LOQs) were 0.145-1.90 μg/m3 and 0.435-5.70 μg/m3, respectively. The method is accurate, sensitive and simple, and is suitable for the determination of the 104 VOCs in ambient air.

Determination of volatile organic compounds (VOCs) in various gases, including atmospheric and exhaled human air, is required to solve a wide range of environmental problems, control the composition of various gases and is increasingly used for diagnosis of various diseases. Lately, methods of soft ionization with minimal fragmentation of the components have been rapidly developed. In particular, our research group is developing an approach to direct analysis of mixtures of VOCs using time-of-flight pulsed glow discharge mass spectrometry. Previously, the effects of different gases and gas mixtures on ionization processes were not compared. Therefore, the ionization mechanisms of VOCs in argon, nitrogen, and air were investigated in the present work. Toluene, p-xylene, chlorobenzene and 1,2,4-trimethylbenzene were chosen as the model VOCs. Optimization of microsecond pulsed glow discharge parameters (period and duration of the discharge pulse, repelling pulse delay time and pressure in the discharge cell) for each compound and a gas mixture of several VOCs was carried out. The predominant ionization mechanisms are the formation of a VOC molecular ion by Penning ionization and the proton transfer reaction; their influence being different for various gases. It is shown that the use of argon even with a small addition of water leads to the predominance of the proton transfer reaction, whereas in nitrogen and air the Penning ionization predominates. The maximum VOC intensities were achieved in air, and the developed approach

Pilot-scale reactor for removing VOCs from a biowaste treatment plant: removal performance, degrading microorganisms, and their functional genes.

Due to increasingly stringent legal regulations as well as increasing social awareness, the removal of odorous volatile organic compounds (VOCs) from air is gaining importance. This paper presents the strategy to compare selected biological methods intended for the removal of different air pollutants, especially of odorous character. Biofiltration, biotrickling filtration and bioscrubbing technologies are evaluated in terms of their suitability for the effective removal of either hydrophilic or hydrophobic VOCs as well as typical inorganic odorous compounds. A pairwise comparison model was used to assess the performance of selected biological processes of air treatment. Process efficiency, economic, technical and environmental aspects of the treatment methods are taken into consideration. The results of the calculations reveal that biotrickling filtration is the most efficient method for the removal of hydrophilic VOCs while biofilters enable the most efficient removal of hydrophobic VOCs. Additionally, a simple approach for preliminary method selection based on a decision tree is proposed. The presented evaluation strategies may be especially helpful when considering the treatment strategy for air polluted with various types of odorous compounds.

Biochar for volatile organic compound (VOC) removal: Sorption performance and governing mechanisms

Comprehensive and time-dependent information (e.g., chemical composition, concentration) of volatile organic compounds (VOCs) in atmospheric, indoor, and breath air is essential to understand the fundamental science of the atmosphere, air quality, and diseases diagnostic. Here, we introduced a fully automated online dynamic in-tube extraction (ITEX)–gas chromatography/mass spectrometry (GC/MS) method for continuous and quantitative monitoring of VOCs in air. In this approach, modified Cycle Composer software and a PAL autosampler controlled and operated the ITEX preconditioning, internal standard (ISTD) addition, air sampling, and ITEX desorption sequentially to enable full automation. Air flow passed through the ITEX with the help of an external pump, instead of plunger up–down strokes, to allow larger sampling volumes, exhaustive extraction, and consequently lower detection limits. Further, in order to evaluate the ITEX system stability and to develop the corresponding quantitative ITEX method, two laboratory-made permeation systems (for standard VOCs and ISTD) were constructed. The stability and suitability of the developed system was validated with a consecutive 19 day atmospheric air campaign under automation. By using an electrospun polyacrylonitrile nanofibers packed ITEX, selective extraction of some VOCs and durability of over 1500 extraction and desorption cycles were achieved. Especially, the latter step is critically important for on-site long-term application at remote regions. This ITEX method provided 2–3 magnitudes lower quantitation limits than the headspace dynamic ITEX method and other needle trap methods. Our results proved the excellence of the fully automated online dynamic ITEX–GC/MS system for tracking VOCs in the atmospheric air.

Sericin-coated polyester based air-filter for removal of particulate matter and volatile organic compounds (BTEX) from indoor air

The purpose of this work was to evaluate the technical and economical feasibility of converting three chemical scrubbers in series to biotrickling filters (BTFs) for the simultaneous removal of H2S and volatile organic compounds (VOCs). The conversion of the full-scale scrubbers was based on previous conversion protocols. Conversion mainly required replacing the original carrier material and recycle pumps as well as modifying the controls and operation of the reactors. Complete removal of H2S and VOCs on a routine basis was reached at neutral pH in a longer period of time compared to previous conversions reported. Biotrickling filters operated at a gas contact time of about 1.4 s per reactor and at pH controlled between 6.5 and 6.8. Inlet average concentrations below 10 ppmv of H2S and below 5 ppmv for VOCs were often completely removed. The first and second bioreactors played a primary role in H2S removal. Year-round operation of the biotrickling filters proved the ability of the system to handle progressive load increases of H2S and VOCs. However, fast, sudden load changes often lead to reduced removal efficiencies. Odor analyses showed average removal efficiencies above 80%. Gas chromatography-mass spectrometry of selected samples showed that outlet odor concentration was due to limited removal of VOCs. The conversion showed was economically viable taking into account the theoretical consumption of chemicals needed for the absorption and oxidation of both H2S and VOCs.

Accumulations and spatial and dynamic variations of biofilms in the media of a biotrickling filter were simulated using mathematical models for Volatile Organic Compound (VOC) removal. Toluene was selected as the model VOC. Effects of toluene concentration and gas Empty Bed Contact Time (EBCT) on VOC removal were also investigated. Results showed that biofilm thickness increased with increased operation duration and the growth rate of biofilms increased with increased inlet toluene concentration and EBCT at a constant toluene loading. The profiles of the thickness and growth rate of biofilms along the medium depth dropped gradually at a certain time.

Removal of volatile organic compounds from the coal-fired flue gas by adsorption on activated carbon

The aim of the study was to monitor volatile organic compounds (VOCs) in outdoor air in nine cities/towns across the Republic of N. Macedonia and the Republic of Kosovo for nine consecutive months, using the Radiello® passive and diffusive samplers, and gas chromatography-mass spectrometry (GC-MS). In parallel, employing static headspace gas chromatography-mass spectrometry (SHS-GC-MS) technique, VOCs from different types of commercially available gasoline and diesel fuels in the monitored cities were analyzed in order to obtain the chemical profiles and to evaluate the presence of their components in ambient air. GC-MS analyses indicate that the gasoline fuel components (BTEX: benzene, toluene, ethylbenzene, and xylenes) and diesel fuel components (n-undecane, n-dodecane, n-tridecane, n-tetradecane, and other linear alkanes) account for approximately 60% of VOCs in the outdoor air in the sampling sites. The bulk of the VOCs in the all-sampling locations regardless of the season come from fossil fuels (automotive fuels) and only minor fraction comes from biogenic emission which is dominated by monoterpenes. Furthermore, the ratios of benzene to toluene (B/T) and xylenes to ethylbenzene (X/E) were used to assess the origins of VOCs emissions in the outdoor urban air at the monitored sites.

Current standard approaches for quantitation of volatile organic compounds (VOCs) in outdoor air are labor-intensive and/or require additional equipment. Solid-phase microextraction (SPME) is a simpler alternative; however, its application is often limited by complex calibration, the need for highly pure gases and the lack of automation. Earlier, we proposed the simple, automated and accurate method for quantitation of benzene, toluene, ethylbenzene and xylenes (BTEX) in air using 20 mL headspace vials and standard addition calibration. The aim of present study was to expand this method for quantitation of >20 VOCs in air. Twenty-five VOCs were chosen for the method development. Polydimethylsiloxane/divinylbenzene (PDMS/DVB) fiber provided better combination of detection limits and relative standard deviations of calibration slopes than other studied fibers. Optimal extraction time was 10 min. For quantification of all analytes except n-undecane, crimp top vials with samples should not stand on the autosampler tray for >8 h, while 22 most stable analytes can be quantified during 24 h. The developed method was successfully tested for automated quantification of VOCs in outdoor air samples collected in Almaty, Kazakhstan. Relative standard deviations (RSDs) of the responses of 23 VOCs were below 15.6%. Toluene-to-benzene concentration ratios were below 1.0 in colder days, indicating that most BTEX originated from non-transport-related sources.