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Biological processes have been proven to be economical and effective for control of volatile organic compounds (VOCs) with concentration of <1,000 mg C/m3 and VOCs such as toluene, xylenes, and dichloromethane(DCM) have been shown to be biodegradable. An activated sludge tank can be used as a bioscrubber for the purpose. Into the tank, an air stream is introduced through the spargers originally for aeration. Water-soluble contaminants in the air stream are absorbed into the mixed liquid and, subsequently, degraded by the microorganisms in the aeration tank. In this study, a pilot-scale activated-sludge tank (0.4 m¡Ñ0.4 m in cross-sectional area and 3 m in water depth) was used as a bioscrubber to study the treatability of gas-borne toluene, xylenes, and DCM. Synthetic gases with the three VOCs with concentrations in the rang of 250-1000 ppm (measured as methane) were used as the test gases. Effects of volumetric aeration rate (Q/V), influent VOC concentration, and submerged liquid depth on the removal efficiency of the influent VOCs were tested. Results show that with Q/V = 3.75-11.25 m3/m2.h, pH = 6-8, MLSS (mixed liquor suspended soilds concentration) = 1600-2800 mg/L, and submerged liquid depth D = 1.1 m, removal efficiencies were 93.1, 93.9, and 98.8%, respectively, for toluene, xylenes, and DCM. The removal efficiencies were nearly independent of the operating conditions.

The Henry's law constant is important in the gas-liquid mass transfer process. This study investigates the apparent dimensionless Henry's law constant, also known as the gas-liquid partition coefficient (K'H), of both hydrophilic (methanol, isopropanol and acetone) and hydrophobic (toluene and p-xylene) organic compounds. The K'H in deionized (DI) water, wastewater with a maximum total dissolved organic carbon (DOC) content of 700 mg/L, and DI water mixed with a maximum activated sludge suspended solid (SS) concentration of 40,000 mg/L, are measured by the single equilibrium technique (SET) at 288 K, 293 K, 298 K and 303 K. Experimental results indicate that the K'H of the three tested volatile organic compounds (VOCs) varies according to three situations. First, the K'H of the hydrophilic compounds in mixed liquor with the maximum suspended solid concentration is higher than that in DI water. The maximum rates of increase of K'H from KH in the SS range between 288 K and 303 K are 27.2% for methanol, 23.5% for isopropanol and 16.1% for acetone. Second, the K'H values for toluene and p-xylene are lower than those in DI water. Between 288 K and 303 K, the maximum reduce rate of K'H from KH in the SS range are 87.3% for toluene and 93.0% for p-xylene. Third, the K'H values of all of the test compounds in the wastewater are lower than those in DI water. The maximum rates of decrease of K'H from KH in the DOC concentration range between 288 K and 303 K are 1.2% for methanol, 1.1% for isopropanol, 1.7% for acetone, 14.3% for toluene and 20.1% for p-xylene. A model is presented to related K'H to wastewater DOC and concentration of SS in the activated sludge, using an organic carbon-water partition coefficient (KOC) and activated sludge-water partition coefficient (KSS) as model parameters. The model is verified, and model parameters for test compounds are estimated. Temperature variations of K'H can be regressed using van't Hoff's equation. The gas-water phase change enthalpy £GH'gw rises approximately linearly with increasing SS for hydrophobic toluene and p-xylene. K'H variation can be controlled by SS concentration and temperature in activated sludge system. The temperature effect of KOC and KSS are also observed. KOC and KSS decrease with as the temperature rises from 288 K to 303 K. The VOCs phase change enthalpy and entropy can be calculated using thermodynamic analysis. The partition of VOCs into the organic carbon phase releases energy, and thus prefers low temperatures. Lower temperatures and higher SS concentrations lead to lower K'H for hydrophobic VOCs, while higher SS concentrations result in higher K'H for hydrophilic VOCs. Organic-rich wastewater or low-biomass mixed liquor can successfully remove hydrophilic VOCs from contaminated air streams by bioscrubber. However, bioreactors scrubbed with high biomass-containing mixed liquor perform better than regular activated sludge concentration in removing hydrophobic VOCs. The experimental results thus support the design and operation of bioscrubber and suspended operable bioreactors.

Bioprocesses for air pollution control can generally be categorized as bioscrubber, biofilter, and biotrickling filter systems. These processes have been proven to be economical and effective for control of volatile organic compounds (VOCs) with concentrations of <1,000 mg C m-3 in gas streams. First, an activated sludge aeration tank (W x L x H = 40 x 40 x 300 cm) with a set of 2 mm orifice air spargers was utilized to treat gas-borne VOCs (toluene, p-xylene, and dichloromethane) in air streams. The effects of liquid depth (Z), aeration intensity (G/A), the overall mass transfer rate of oxygen in clean water (KLaO2), the Henry¡¦s law constant of the tested VOC (H), and the influent gaseous VOC concentration (C0) on the efficiency of removal of VOCs were examined and compared with a literature-cited model. Results show that the measured VOC removal efficiencies and those predicted by the model were comparable at a G/A of 3.75 ¡V 11.25 m3 m-2 hr-1 and C0 of around 1,000 ¡V 6,000 mg m-3. Experimental data also indicate that the designed gas treatment reactor with KLaO2 = 5 ¡V 15 hr-1, could achieve > 85% removal of VOCs with H = 0.24 ¡V 0.25 at an aerated liquid depth of 1 m, and > 95% removal of dichloromethane with H = 0.13 at a 1 m liquid depth. The model predicts that, for gas treatment in common activated sludge tanks, with KLaO2 = 5 ¡V 10 hr-1, depth = 3 ¡V 4.5 m, G/A = 9 ¡V 18 m3 m-2 hr-1, > 92% VOC removal can be achieved with operating parameters of Z of 3.0 m and KLaVOC/(G/A) of about 0.28 m-1, for VOCs with H < 0.3, such as most oxygen-containing hydrocarbons with low molecular weights, and benzene, toluene, ethylbenzene, and dichloromethane. Second, an activated sludge aeration tank and a sieve-plate column with six sieve plates were utilized to treat gas-borne VOCs in air streams. The tank was used for the biodegradation of the absorbed VOCs from the column which utilized the activated mixed liquor drawn from the tank as a scrubbing liquor. This research proposed a model for VOC absorption to a down-flow activated sludge liquor in a sieve-plate column. The experimental setup consisted of a pilot-scale activated-sludge tank and a sieve-plate tower, as demonstrated. The sieve-plate tower was constructed from a 25 x 25 x 162 cm (W x L x H) acrylic column with six custom-made sieve plates. Each plate has 382 holes which are 3 mm in diameter arranged on a square pitch. The holes give an open area of 3.82% of the whole plate area for gas flow. Two 25 mm-i.d. down-comer pipes were also equipped to allow for the downflow of the activated sludge liquor. Ports were provided at the column inlet, outlet, and each plate for gas and liquid sampling. Experiments were conducted and the model verified based on the results of tests on the removal efficiencies of isopropyl alcohol (IPA), toluene and p-xylene in the system operated at a range of influent VOC concentrations, air application rates, and liquid/gas flow ratios (L/G). The model developed by a material balance for the gaseous- and liquid-VOC over each plate of the column was developed and experimentally verified in this study. Superficial gas velocity over the column plate (U), number of plates (N), volumetric liquid-phase VOC-transfer coefficient (KLaVOC), aerated liquid depth over the plate (Z), volumetric liquid/gas flow-rate ratio (L/G), dimensionless Henry¡¦s law coefficient of the VOC to be absorbed (H), VOC content of the influent scrubbing liquor (xN+1), and the biodegradation rate constant of the VOC in the activated sludge mixed liquor (k) are among the affecting parameters to the effectiveness of the VOC removal. Model application by the model for effects of affecting parameters on the VOC removal effectiveness indicates that L/G, plate number N, biodegradation rate constant k, Henry¡¦s law constant of VOC H are among the important ones. A L/GH of greater than 2 and N of around 6 are enough for the effective (>90%) removal of the influent VOCs with H < 0.01 if no biodegradation occurred in the column. However, a N of over 16 is required for the influent VOCs with H of around 0.2. Biodegradation with a rate constant of around 100 hr-1 in the column greatly improves the column performance.

This thesis examined the biological treatment of gas streams containing benzene, toluene, ethylbenzene and o-xylene (BTEX) using solid-liquid two-phase partitioning bioscrubbers (SL-TPPBs). SL-TPPBs consist of a cell containing aqueous phase and a polymeric solid phase that sequesters poorly water soluble and/or toxic substrates, mitigating substrate toxicity in the aqueous phase and improving the gas mass transfer during treatment of VOC contaminated gases. An initial investigation of oxygen transport determined that the polymers in a stirred-tank SL-TPPB enhance gas-liquid mass transfer. In addition, a study on biodegradation kinetics of BTEX by a bacterial consortium identified and quantified substrate interactions such as inhibition, enhancement and cometabolism. The stirred-tank SL-TPPB was then experimentally investigated for treatment of BTEX gas streams during steady-state and dynamic step-change operation to determine performance of the system relative to other biotreatment methods. A mathematical model was developed to predict system performance, which included the microbial kinetic model structure and parameters estimated during kinetic and oxygen mass transfer studies. As a less energy intensive alternative, an airlift SL-TPPB was operated and characterized. The airlift SL-TPPB was compared to an airlift liquid-liquid TPPB (silicone oil as sequestering phase) and a single phase airlift over dynamic step-change loadings, which showed that the airlift SL-TPPB outperformed the single phase airlift by >30% and had similar performance to the liquid-liquid airlift. However, the airlift SL-TPPB performance was lower relative to the stirred-tank SL-TPPB by >15%. Steady-state operation of the airlift SL-TPPB identified a range of operating conditions that provided maximum performance and conditions that were not oxygen limited. This prompted a study of oxygen mass transfer and hydrodynamics in the airlift system, which identified that the addition of polymers to an airlift does not cause physical enhancement of the gas-liquid mass transfer coefficient, but improves aqueous phase mixing and enhances overall oxygen transfer rate. A tanks-in-series mathematical model was formulated to predict performance of the airlift SL-TPPB, wherein the number of tanks-in-series to describe mixing in the airlift was obtained from a residence time distribution analysis of the airlift system completed during the hydrodynamic investigation. This thesis contributes a low-energy solution for the effective treatment of gases contaminated with BTEX. / Thesis (Ph.D, Chemical Engineering) -- Queen's University, 2009-08-18 16:16:22.598