JiHyeon Song

Effect of vapor-phase bioreactor operation on biomass accumulation, distribution, and activity: Linking biofilm properties to bioreactor performance

Excess biomass accumulation and activity loss in vapor‐phase bioreactors (VPBs) can lead to unreliable long‐term operation. In this study, temporal and spatial variations in biomass accumulation, distribution and activity in VPBs treating toluene‐contaminated air were monitored over a 96‐day period. Two laboratory‐scale bioreactors were subjected to a toluene loading rate of 45.8 g/m3‐h with one VPB operating in a unidirectional (UD) mode and a second identical VPB operating in a directionally switching (DS) mode. In the UD bioreactor, the contaminated air stream was continuously fed to the bottom of the reactor, while, in the DS bioreactor, the direction of the contaminated gas flow was reversed every three days. Overall, the DS system performed better with respect to biomass distribution and microbial activity across the bioreactor, resulting in more stable bioreactor performance. In contrast, most of the biomass accumulation and activity was confined to the front half of the UD bioreactor column which caused high pressure drops, rapid activity loss and eventually toluene breakthrough. A carbon balance reveals that excess biomass accumulated continuously in both bioreactors, and biomass yield coefficients were very similar (0.59 g dry biomass/g toluene for the UD and 0.63 g dry biomass/g toluene for the DS). The viable biomass population remained relatively constant in both bioreactors over the operational period, while the inactive biomass fraction steadily increased over the same time frame. Biodegradation activity determined by the dehydrogenase enzyme activity assay was found to be a function of biomass accumulation and reflected pollutant removal profiles along the columns. In addition, biomass activity correlated well with the toluene‐degrading fraction of the total bacterial population.

Effects of acid pre-treatment on bio-hydrogen production and microbial communities during dark fermentation.

Optimal conditions for acid pre-treatment were investigated for the enrichment of hydrogen-producing bacteria (HPB) in a mixed culture using three strong acids: HCl, HNO(3), and H2SO4 x HCl was selected as a suitable acid for the enrichment of HPB in the fermentation process. The volume of bio-hydrogen produced when the mixed culture was pre-treated using HCl at pH 2 was 3.2 times higher than that obtained without acid pre-treatment. Changes in the microbial community during acid pre-treatment were monitored using images obtained by the fluorescent in situ hybridization (FISH) method and the Live/Dead cell viability test. The tests clearly indicated that the Clostridium species of cluster I were the predominant strains involved in bio-H(2) fermentation, and could be selectively enriched by HCl pre-treatment.

Factors affecting nitrous oxide production: a comparison of biological nitrogen removal processes with partial and complete nitrification.

Nitrous oxide (N2O) emission from biological nitrogen removal (BNR) processes has recently received more research attention. In this study, two lab-scale BNR systems were used to investigate the effects of various operating parameters including the carbon to nitrogen (C/N) ratio, ammonia loading, and the hydraulic retention time on N2O production. The first system was operated in a conventional BNR mode known as the Ludzack–Ettinger (LE) process, consisting of complete denitrification and nitrification reactors, while the second one was operated in a shortcut BNR (SBNR) mode employing partial nitrification and shortcut denitrification, which requires less oxygen and carbon sources. As the C/N ratio was decreased, a significant increase in N2O production was observed only in the anoxic reactor of the LE process, indicating that N2O was released as an intermediate of the denitrification reaction under the carbon-limited condition. However, the SBNR process did not produce significant N2O even at the lowest C/N ratio of 0.5. When the SBNR process was subjected to increasing concentrations of ammonia, N2O production from the aerobic reactor was rapidly increased. Furthermore, the increasing production of N2O was observed mostly in the aerobic reactor of the SBNR process with a decline in hydraulic retention time. These experimental findings indicated that the increase in N2O production was closely related to the accumulation of free ammonia, which was caused by an abrupt increase of the ammonium loading. Consequently, the partial nitrification was more susceptible to shock loading conditions, resulting in a high production of N2O, although the SBNR process was more efficient with respect to nitrogen removals as well as carbon and oxygen requirements.

Effects of adsorptive properties of biofilter packing materials on toluene removal

Various adsorptive materials, including granular activated carbon (GAC) and ground tire rubber (GTR), were mixed with compost in biofilters used for treating gaseous toluene, and the effects of the mixtures on the stability of biofilter performance were investigated. A transient loading test demonstrated that a sudden increase in inlet toluene loading was effectively attenuated in the compost/GAC biofilter, which was the most significant advantage of adding adsorptive materials to the biofilter packing media. Under steady conditions with inlet toluene loading rates of 18.8 and 37.5 g/m(3)/h, both the compost and the compost/GAC biofilters achieved overall toluene removal efficiencies greater than 99%. In the compost/GAC mixture, however, biodegradation activity declined as the GAC mass fraction increased. Because of the low water-holding capacity of GTR, the compost/ground tire mixture did not show a significant improvement in toluene removal efficiency throughout the entire operational period. Furthermore, nitrogen limitations affected system performance in all the biofilters, but an external nitrogen supply resulted in the recovery of the toluene removal efficiency only in the compost biofilter during the test periods. Consequently, the introduction of excessive adsorptive materials was unfavorable for long-term performance, suggesting that the mass ratio of the adsorptive materials in such mixtures should be carefully selected to achieve high and steady biofilter performance.

Nitrogen utilization in a vapor-phase biofilter.

The effect of media nitrogen levels on biofilter performance was investigated in a lab-scale biofilter treating toluene and p-xylene. Nitrogen utilization rates and the quantity of nitrogen recycled to meet microbial demand in the biofilm were estimated using a nitrogen balance approach. Experimental data imply that overall biofilter performance was a strong function of normalized nitrogen levels in the synthetic media. The biodegradation of p-xylene was found to be more sensitive to media nitrogen levels than was the degradation of toluene. However, increasing the nitrogen supply improved both toluene (>99%) and p-xylene removal efficiencies (>90%). Nitrogen balance calculations indicate that substantial recycling of nitrogen occurred in the biofilm even under nitrogen-rich conditions. The fraction of nitrogen demand met by recycling nitrogen increased when the external supply of nitrogen was terminated, and the biofilm became nitrogen limited. However, to avoid severe nitrogen limitation conditions, an external nitrogen source must be provided to sustain high pollutant removals in the biofilter.

Modeling and simulations of the removal of formaldehyde using silver nano-particles attached to granular activated carbon

A combined reaction, consisting of granular activated carbon (GAC) adsorption and catalytic oxidation, has been proposed to improve the removal efficiencies of formaldehyde, one of the major indoor air pollutants. In this study, silver nano-particles attached onto the surface of GAC (Ag-GAC) using the sputtering method were evaluated for the simultaneous catalytic oxidation and adsorption of formaldehyde. The evolution of CO(2) from the silver nano-particles indicated that formaldehyde was catalytically oxidized to its final product, with the oxidation kinetics expressed as pseudo-first order. In addition, a packed column test showed that the mass of formaldehyde removed by the Ag-GAC was 2.4 times higher than that by the virgin GAC at a gas retention time of 0.5s. However, a BET analysis showed that the available surface area and micro-pore volume of the Ag-GAC were substantially decreased due to the deposition of the silver nano-particles. To simulate the performance of the Ag-GAC, the homogeneous surface diffusion model (HSDM), developed for the prediction of the GAC column adsorption, was modified to incorporate the catalytic oxidation taking place on the Ag-GAC surface. The modified HSDM demonstrated that numerical simulations were consistent with the experimental data collected from the Ag-GAC column tests. The model predictions implied that the silver nano-particles deposited on the GAC reduced the adsorptive capacity due to decreasing the available surface for the diffusion of formaldehyde into the GAC, but the overall mass of formaldehyde removed by the Ag-GAC was increased due to catalytic oxidation as a function of the ratio of the surface coverage by the nano-particles.

Enhanced toluene removal using granular activated carbon and a yeast strain Candida tropicalis in bubble-column bioreactors.

The yeast strain Candida tropicalis was used for the biodegradation of gaseous toluene. Toluene was effectively treated by a liquid culture of C. tropicalis in a bubble-column bioreactor, and the toluene removal efficiency increased with decreasing gas flow rate. However, toluene mass transfer from the gas-to-liquid phase was a major limitation for the uptake of toluene by C. tropicalis. The toluene removal efficiency was enhanced when granular activated carbon (GAC) was added as a fluidized material. The GAC fluidized bioreactor demonstrated toluene removal efficiencies ranging from 50 to 82% when the inlet toluene loading was varied between 13.1 and 26.9 g/m(3)/h. The yield value of C. tropicalis ranged from 0.11 to 0.21 g-biomass/g-toluene, which was substantially lower than yield values for bacteria reported in the literature. The maximum elimination capacity determined in the GAC fluidized bioreactor was 172 g/m(3)/h at a toluene loading of 291 g/m(3)/h. Transient loading experiments revealed that approximately 50% of the toluene introduced was initially adsorbed onto the GAC during an increased loading period, and then slowly desorbed and became available to the yeast culture. Hence, the fluidized GAC mediated in improving the gas-to-liquid mass transfer of toluene, resulting in a high toluene removal capacity. Consequently, the GAC bubble-column bioreactor using the culture of C. tropicalis can be successfully applied for the removal of gaseous toluene.

Temperature effects on nitrification in polishing biological aerated filters (BAFs)

The effects of temperature on nitrification in a polishing biological aerated filter (BAF) were investigated using a 75‐mm diameter pilot‐scale BAF with a gravel media size of 5 mm and a depth of 1.7 m. Influent soluble chemical oxygen demand (sCOD) and ammonia‐nitrogen (NH3‐N) concentrations were approximately 50 mg/L and 25 mg/L simulating the effluent from an aerated lagoon system. For an influent wastewater temperature of 6.5 °C, approximately 95% of NH3‐N was nitrified at a hydraulic retention time (HRT) of 2 hours. By recirculating 200% of the effluent back into the BAF for a HRT of 1 hour and at 6.5 °C, NH3‐N percentage removal improved from 54% to 92%. For NH3‐N loading larger than 0.9 kg NH3‐N/m3‐day at 24 °C, the mass of NH3‐N removed in kg NH3‐N/m3‐day reached an asymptotic value of 0.63 kg NH3‐N/m3‐day. The NH3‐N concentrations within the column at different temperatures were modelled using zero‐order biotransformation rate kinetics. The results showed that gravel BAF operating at an HRT of 1 hour with 100% or 200% recirculation can be used as an add‐on technology for nitrification for cold weather conditions.

Sewage sludge reduction and system optimization in a catalytic ozonation process.

The main objective of this study was to suggest a feasible, effective process for the reduction of sewage sludge using ozone oxidation catalysed by metal ion. A series of lab‐scale experiments was conducted to select a suitable catalyst and its proper dose to achieve optimum sludge reduction. Using a central composite design under response surface methodology (RSM), system optimization with respect to sludge reduction and cost‐effectiveness was performed by varying the independent parameters: dosages of ozone and ions. Five metal ions, Mn2+, Fe2+, Zn2+, Cu2+, and Al3+, were tested, and the manganese ion showed the highest sludge reduction, as measured by a decrease in total suspended solids. The ozone/Mn combination achieved approximately twice as much sludge reduction as the ozonation alone. Furthermore, the Mn dose of 10 mg/g‐TS (total solids) resulted in the highest sludge reduction efficiency among the different doses, which ranged from 0 to 20 mg‐Mn/g‐TS. The predicted efficiency of sewage sludge reduction using the RSM was found to agree well with the experimental results, and the statistical analyses predicted optimum ranges for the doses of ozone and Mn ions, taking into account the overall cost for sewage sludge treatment.

Biodegradation of toluene using Candida tropicalis immobilized on polymer matrices in fluidized bed bioreactors

A yeast strain, Candida tropicalis, was whole-cell-immobilized on polymer matrices of polyethylene glycol (PEG) and polyethylene glycol/activated carbon/alginate (PACA). The polymer matrices were used as fluidized materials in bubble-column bioreactors for the biodegradation of toluene. Simultaneously, another bubble-column bioreactor using granular activated carbon (GAC) and a conventional compost biofilter were operated for comparison. In the compost biofilter, the toluene removal efficiency gradually deteriorated due to the limitation of microbial activity. The toluene removal in the GAC bioreactor was relatively high because of an increase of toluene mass transfer. However, low toluene removal efficiencies were observed in the PEG bioreactor, presumably because the synthetic polymer alone was not suitable for yeast cell immobilization. In the PACA bioreactor, toluene removal was found to be greater than 95% overall. The CO(2) yield coefficient calculated at the highest toluene loading condition for the PACA bioreactor was found to be higher than those observed in the other bioreactors. Furthermore, almost complete elimination capacities were observed in the PACA bioreactor at short-term toluene loading up to 180 g/m(3)/h. In conclusion, the immobilization of C. tropicalis in the PACA matrix resulted in enhanced toluene biodegradation because of the increases of both mass transfer and microbial activity.