Modeling biodegradation of toluene in rotating drum biofilter.

Rotating drum biofilters (RDBs) are cost-effective for control of emissions of volatile organic compounds (VOCs) from waste gas streams. In this paper, a dynamic mathematical model is presented which simulates and predicts the variation in performance of a multi-layer RDB with time on the basis of the two-film theory. The model takes into account factors including biofilm growth and biomass loss, and a changing biofilm surface area and thickness assuming quasi-steady-state conditions in the two-phase system and uniform bacterial population. Toluene was assumed to be the only rate-limiting substrate. The model equations for the gas-phase mass balance and biofilm growth were solved using MATLAB based on the fourth-fifth-order Runge-Kutta technique, and the concentration profiles in the biofilms were obtained using the method of orthogonal collocation. Simulation results showed that the toluene removal efficiency decreased with increased toluene loading or increased duration of operation of the biofilter. Calculation results were compared to the experimental results, which demonstrated that the dynamic model provided a good simulation of the performance of the biofilter.

Effect of H2O2 dose on the thermo-oxidative co-treatment with anaerobic digestion of excess municipal sludge.

Thermo-oxidative treatment, applied at moderate temperatures below the boiling point of water and using hydrogen peroxide as oxidant, has been shown to enhance anaerobic digestion of excess municipal sludge. Due to the high economic impact of chemical addition on the process, a balance between process performance and economic cost has to be established. The effect of three low hydrogen peroxide dosages (0.1, 0.25 and 0.5 gH2O2/gVSSinfluent) applied at 90 degrees C was evaluated. Increases in solids destruction of 13.9, 18.9 and 25.6% with respect to a control were observed when the thermo-oxidative co-treatment was operated at 0.1, 0.25 and 0.5 gH2O2/gVSSinfluent, respectively.

Oxidative and thermo-oxidative co-treatment with anaerobic digestion of excess municipal sludge.

The effect of oxidative and thermo-oxidative co-treatment of excess municipal sludge was investigated. A mixture of primary and waste activated sludge was anaerobically treated using two different configurations: i) two stages and ii) a single stage with recycling. Oxidative or thermo-oxidative co-treatment placed in between the reactor or in the recycle line was studied. A two-stage configuration with no co-treatment served as a control and resulted in 50.1% overall solids removal. Compared to the control, an increase in solids removal of 10.8 and 2.7% was observed when oxidative co-treatment was placed between reactors and in the recycle line respectively. When thermo-oxidative co-treatment was placed between the two stages or in the recycle line an increase in solids removal of 25.1 and 26.9% respectively was observed. The performances of the different configurations were also evaluated with parameters such as COD, TKN, ammonia, and fecal coliform concentration.

Challenges in biodegradation of trace organic contaminants--gasoline oxygenates and sex hormones.

Advances in analytical methods have led to the identification of several classes of organic chemicals that are associated with adverse environmental effects. Two such classes of organic chemicals, gasoline oxygenates and sex hormones, are used to illustrate challenges associated with the biodegradation of trace organic contaminants. Gasoline oxygenates can be present in groundwater, alone, or commingled with xylene, at appreciable concentrations. However, target-treated water standards dictate that gasoline oxygenates be reduced to the microgram-per-liter concentration range before consumption. Sex hormones, on the other hand, are present in wastewater matrixes in the part-per-trillion concentration range, and the biggest challenge that must be met, before optimizing their removal, is facilitating their detection.

Biodegradation of sediment-bound PAHs in field-contaminated sediment.

The biodegradation of polycyclic aromatic hydrocarbons (PAHs) has been reported to occur under aerobic, sulfate reducing, and denitrifying conditions. PAHs present in contaminated sites, however, are known for their persistence. Most published studies were conducted in systems where PAHs were freshly spiked, and biodegradation was often tested using pure cultures or enrichments. This paper investigated the degradation potentials of PAHs that were present in aged contaminated sediment by indigenous bacteria, where the limited bioavailability of PAHs due to aging played an important role. The sediment and the overlaying water were collected from a contaminated site to prepare sediment-water slurries, and the sediment served as both the media containing PAH substrates and the habitat for the indigenous microorganisms. Reduced sulfur compounds present in the sediment caused rapid oxygen depletion due to extensive activities of sulfur-oxidizing bacteria and could result in a dramatic pH drop. Once oxygen depletion and acidification problems were avoided, substantial removals of two-, three-, four-, and five-ring PAHs were achieved aerobically, though the extent of degradations was smaller than what was reported for freshly spiked PAHs. The amendment of inorganic N and P, co-substrates, or surfactant Triton X-100 did not enhance the level of degradations appreciably. Under denitrifying conditions, no distinct PAH degradation was observed, while the complete denitrification of nitrate to nitrogen occurred stoichiometrically with a concomitant increase in sulfate concentration, indicating the dominance of autotrophic denitrifiers. The addition of ethanol or acetic acid did not stimulate PAH degradation. Substantial PAH degradation attributed to sulfate reduction was only observed for phenanthrene, the low-ring PAH existing in a highest initial concentration. Addition of ethanol or acetic acid did not change this finding. This is the first study to our knowledge that revealed the importance of indigenous bacteria involved in natural sulfur cycling in determining degradation behavior of PAHs.

Biomass accumulation patterns for removing volatile organic compounds in rotating drum biofilters.

A rotating drum biofilter (RDB) with multi-layered foam media was developed for the improvement of current biofiltration technology. The biofilter was used to investigate the effects of organic loadings and influent volatile organic compound (VOC) concentrations on VOC removal efficiency and biomass accumulation. These effects were evaluated using diethyl ether and toluene separately as model VOCs at an empty bed contact time (EBCT) of 30 s. When the toluene loading increased from 2.0 to 4.0 and 8.0 kgCOD m(-3) day(-1), toluene removal efficiency of the biofilter decreased from over 99% to 78% and 74%, respectively. The biomass distribution was found to be more even within the medium when removing toluene than when removing diethyl ether. Higher organic loading also resulted in the more even distribution of the biomass. The ratios of biomass accumulation rates in the medium of the outermost, middle and innermost layers ranged from 1:0.11:0.02 when removing diethyl ether at 2.0 kgCOD m(-3) day(-1) to 1:0.69:0.51 when removing toluene at 8.0 kgCOD m(-3) day(-1). Review of these ratios revealed three biomass accumulation patterns: surface pattern, in-depth pattern and shallow pattern. Different patterns represent different removal mechanisms in the biofiltration process. Improved biofilter design and operation should be based on the biomass accumulation pattern.

Biodegradation of the energetic compound TNT through a multiple-stage treatment approach.

Biodegradation of the energetic compound 2,4,6-trinitrotoluene (TNT) and its intermediate 2,4,6-triaminotoluene (TAT) was investigated in this study. From previous investigations, a relationship between the biological utilization of ethanol as co-substrate for the reduction of TNT under anaerobic conditions was proposed using an anaerobic fluidized-bed reactor (AFBR). In this study, the theoretical co-substrate requirement for reduction of TNT to TAT was further investigated through the systematic lowering of the ethanol loading to the reactor. Near complete reduction to TAT was observed up to a critical ethanol loading point, as well as the production of methane from the limited excess available ethanol. Once ethanol deficient loading conditions were established, the increased presence of incompletely reduced degradation intermediates, such as 2,4-diamino-6-nitrotoluene, and even TNT, was observed. The cessation of methanogenesis confirmed that no excess ethanol was available. Degradation of the TAT intermediate in the reactor effluent was investigated using two second-stage reactors under oxidizing conditions. The first was an aerobic activated sludge reactor, and the second was a denitrifying fluidized-bed reactor (DenFBR). The aerobic reactor was successful in lowering the chemical oxygen demand (COD), but complete removal of TAT was not accomplished. Because of TAT polymerization and auto-oxidation under aerobic conditions, it was difficult to confirm to what extent of TAT removal was biological. In the DenFBR, incompletely reduced TNT intermediates were not successfully degraded, but strong evidence existed for the degradation of TAT. This is the first known report of second stage degradation of TAT under denitrifying conditions.

Impact of soluble organic compounds on permeate flux in an aerobic membrane bioreactor.

The relative impact of mixed liquor suspended solids and soluble organic compounds on the permeate flux in an aerobic membrane bioreactor (MBR) were investigated during long-term operation. A statistical correlation analysis performed on data obtained over an approximately 700 day operational period revealed that permeate flux was strongly correlated to soluble organic compounds such as soluble sugars and proteins and was not correlated to total mixed liquor COD. Organic compounds with sizes less than 0.10 microm exhibited the strongest correlation to permeate flux. Specific filtration tests conducted on the MBR showed that the effect of soluble COD was most pronounced in the range of 110-210 mg 1(-1). A critical level of soluble COD was established at 500 mg 1(-1) after which point no correlation was present. The effluent quality remained high throughout the study at below 5 mg 1(-1) total COD, indicating that the membrane was able to retain most organic compounds regardless of mixed liquor soluble COD content. It was concluded that MBR permeate fluxes are enhanced when operating at conditions where bio-degradation is improved and soluble organic compounds are reduced.

Biodegradation of methyl tert-butyl ether under various substrate conditions.

Five aerobic enrichments efficient at degrading methyl tert-butyl ether (MTBE) under different substrate conditions were developed in well-mixed reactors containing a polyethlene porous pot for biomass retention. The five substrate conditions were as follows: MTBE alone; MTBE and diethyl ether (DEE); MTBE and diisopropyl ether (DIPE); MTBE and ethanol (EtOH); and MTBE with benzene, toluene, ethylbenzene, and xylene (BTEX). All five cultures demonstrated greater than 99.9% removal of MTBE. Addition of alternative substrate was found to have no effect on the performance of the reactors. The bacterial communities of the reactors were monitored periodically by denaturing gradient gel electrophoresis (DGGE) to determine when homeostasis was achieved. Phylogenetic analysis of the excised DGGE bands was done in order to compare the bacterial community compositions of the reactors. All cultures were found to be mixed cultures, and each enrichment was shown to have a unique composition. A majority of the bands in all reactors represented a group of organisms belonging to the Cytophaga-Flexibacter-Bacterioides (C-F-B) Phylum of bacteria. This was also the only group found in all of the reactors. This study demonstrates that MTBE can be degraded effectively in bioreactors under several substrate conditions and gives insight into the microorganisms potentially involved in the process.

Effect of solids retention time on the performance and biological characteristics of a membrane bioreactor.

The purpose of this study was to compare the performance of a pilot-scale membrane bioreactor (MBR) in the treatment of municipal strength wastewater at solid retention times (SRTs) ranging from 30 days to two days. Cumulative nitrogen and phosphorus mass balances resulted in closures exceeding 90% at each steady state period. Biomass production rate and biomass viability generally increased with decreasing SRT, whereas overall enzymatic activity did not change significantly at most SRTs, but was highest at the two day SRT. Nitrification decreased at two day SRT but did not fail completely. At higher SRTs, nitrification was not noticeably affected by the sludge age. Phospholipid fatty acid (PLFA) analysis showed substantially diverse biomass in the sludge at different SRTs. Different ratios of gram positive bacteria, eukaryotic organisms, and yeast cells were observed in the mixed liquor at varying SRTs. On the other hand, BIOLOG analysis indicated that the overall capacity of the biomass to degrade different carbon substrates did not change significantly at different SRTs. The concentration of metals in the MBR mixed liquor declined steadily with decreasing SRT. The MBR effluent contained negligible amounts of Fe, Zn, Mn, and Co at each condition, indicating the retention of these metals regardless of the SRT.

Effectiveness of an anaerobic granular activated carbon fluidized-bed bioreactor to treat soil wash fluids: a proposed strategy for remediating PCP/PAH contaminated soils.

An integrated system has been developed to remediate soils contaminated with pentachlorophenol (PCP) and polycyclic aromatic hydrocarbons (PAHs). This system involves the coupling of two treatment technologies, soil-solvent washing and anaerobic biotreatment of the extract. Specifically, this study evaluated the effectiveness of a granular activated carbon (GAC) fluidized-bed reactor to treat a synthetic-waste stream of PCP and four PAHs (naphthalene, acenaphthene, pyrene, and benzo(b)fluoranthene) under anaerobic conditions. This waste stream was intended to simulate the wash fluids from a soil washing process treating soils from a wood-preserving site. The reactor achieved a removal efficiency of greater than 99.8% for PCP with conversion to its dechlorination intermediates averaging 46.5%. Effluent, carbon extraction, and isotherm data also indicate that naphthalene and acenaphthene were removed from the liquid phase with efficiencies of 86 and 93%, respectively. Effluent levels of pyrene and benzo(b)fluoranthene were extremely low due to the high-adsorptive capacity of GAC for these compounds. Experimental evidence does not suggest that the latter two compounds were biochemically transformed within the reactor.

Biofilm structure and mass transfer in a gas phase trickle-bed biofilter.

Mass transport phenomena occurring in the biofilms of gas phase trickle-bed biofilters are investigated in this study. The effect of biofilm structure on mass transfer mechanisms is examined using experimental observation from the operating of biofilters, microelectrode techniques and microscopic examination. Since the biofilms of biofilters used for waste gas treatment are not completely saturated with water, there is not a distinguishable liquid layer outside the biofilm. Results suggest that due to this characteristic, gas phase substrates (such as oxygen or volatile organic compounds) may not be limited by the aqueous phase because transport of the compound into the biofilm can occur directly through non-wetted areas. On the other hand, for substrates that are present only in the liquid phase, such as nitrate, the mass transfer limitation is more serious because of the limited liquid supply. Microscopic observations show that a layered structure with void spaces exists within the biofilm. Oxygen concentration distributions along the depth of the biofilms are examined using an oxygen microelectrode. Results indicate that there are some high dissolved oxygen zones inside the biofilm, which suggests the existence of passages for oxygen transfer into the deeper sections of the biofilm in a gas phase trickle-bed biofilter. Both the low gas-liquid mass transfer resistance and the resulting internal structure contribute to the high oxygen penetration within the biofilms in gas phase trickle-bed biofilters.

Anaerobic degradation of 2,4,6-trinitrotoluene in granular activated carbon fluidized bed and batch reactors.

In this study, an anaerobic fluidized bed reactor (AFBR) was used to treat a synthetically produced pink water waste stream containing trinitrotoluene (TNT). The synthesized waste consisted of 95 mg/l-TNT, the main contaminant in pink water, which was to be co-metabolized with 560-mg/l ethanol. Granular activated carbon was used as the attachment medium for biological growth. TNT was reduced to a variety of compounds, mainly 2,4,6-triaminotoluene (2,4,6-TAT), 2,4-diamino-6-nitrotoluene (2,4-DA-6-NT), 2,6-diamino-4-nitrotoluene (2,6-DA-4-NT), 2-amino-4,6-dinitrotoluene (2-A-4,6-DNT), and 4-amino-2,6-dinitrotoluene (4-A-2,6-DNT). These conversions resulted through the oxidation of ethanol to carbon dioxide under anoxic conditions, or reduction to methane under methanogenic conditions. The anaerobic reactor was charged with 1.0 kg of 16 x 20 U.S. Mesh Granular Activated Carbon (GAC) and was pre-loaded with 200 g of TNT prior to the addition of the mixed seed culture. During the first three weeks of operation, ethanol was completely degraded and no methane was produced. Effluent inorganic carbon revealed stoichiometric conversion of the feed ethanol to dissolved inorganic carbon with accumulation of carbon dioxide in the headspace of the reactor. GAC extraction showed incremental reduction of the nitro groups to amino groups, with 2,4,6-TAT as the final product. After three weeks, the oxygen from the nitro groups was depleted and methane production commenced. The reproducibility of this phenomenon was confirmed by repeating the experiment in the same manner using an identical AFBR. Furthermore, serum bottle tests were conducted using TNT loading ratios of 0.2, 0.4, 0.8, 1.0 g-TNT/g-GAC as well as experiments in the absence of GAC. Similar behavior to that of the columns was observed, with degradation rates varying according to the particular condition. GAC greatly enhanced the degradation rates and the higher TNT loading resulted in slower degradation rates of ethanol.

Complete remediation of PCE contaminated unsaturated soils by sequential anaerobic-aerobic bioventing.

Bioventing principles have been applied to completely dechlorinate tetrachloroethylene vapors in the unsaturated zone in a sequential anaerobic-aerobic pattern. The aerobic step yields trans-DCE and VC as PCE reductive dechlorination byproducts, while TCE and cis-DCE are observed as intermediates. The aerobic step results in rapid oxidation of the VC and trans-DCE to carbon dioxide. Hydrogen was delivered in the gas phase as a reducing agent for the anaerobic step at levels of 1%, and oxygen at 4.2% was used as an electron acceptor in the aerobic step. PCE and VC half lives in the anaerobic and aerobic steps respectively, where less than 10 min.

Removal of ammonia from contaminated air by trickle bed air biofilters.

A trickle bed air biofilter (TBAB) was evaluated for the oxidation of NH3 from an airstream. Six-millimeter Celite pellets (R-635) were used for the biological attachment medium. The efficiency of the biofilter in oxidizing NH3 was evaluated using NH3 loading rates as high as 48 mol NH3/m3 hr and empty-bed residence times (EBRTs) as low as 1 min. Excess biomass was controlled through periodic backwashing of the biofilter with water at a rate sufficient to fluidize the medium. The main goal was to demonstrate that high removal efficiencies could be sustained over long periods of operation. Ammonia oxidation efficiencies in excess of 99% were consistently achieved when the pH of the liquid nutrient feed was maintained at 8.5. Quick recovery of the biofilter after backwashing was observed after only 20 min. Evaluation of biofilter performance with depth revealed that NH3 did not persist in the gas phase beyond 0.3 m into the depth of the medium (26% of total medium depth).

Aerobic biodegradation of gasoline oxygenates MTBE and TBA.

MTBE degradation was investigated using a continuously stirred tank reactor (CSTR) with biomass retention (porous pot reactor) operated under aerobic conditions. MTBE was fed to the reactor at an influent concentration of 150 mg/l (1.70 mmol/l). A second identical reactor was operated as a control under the same conditions with the addition of 2.66 g/l of sodium azide, to kill any biological activity. Results from these experiments suggest that biomass retention is critical to the degradation of MTBE. The rate of MTBE removal was shown to be related to the VSS concentration. MTBE removal exceeded 99.99% when the VSS concentration in the reactor was over 600 mg/l. Results obtained from batch experiments conducted on mixed liquor samples from the porous pot reactor indicate that the individual rates of biodegradation of MTBE and TBA were higher for initial concentrations of 15 mg/l than for concentrations of 5 mg/l. The presence of TBA at lower concentrations did not effect the rate of MTBE degradation, however higher concentrations of TBA did reduce the rate of biodegradation of MTBE. Denaturing Gradient Gel Electrophoresis (DGGE) analysis reveals that the culture consisted of a community of bacterial organisms of about 6 species.

Anaerobic bioventing of unsaturated zone contaminated with DDT and DNT.

Initial degradation of highly chlorinated compounds and nitroaromatic compounds found in munition waste streams is accelerated under anaerobic conditions followed by aerobic treatment of the degradation products. The establishment of anaerobic environment in a vadose zone can be accomplished by feeding appropriate anaerobic gas mixture, i.e., "anaerobic bioventing". The gas mixture contains an electron donor for the reduction of these compounds. Lab scale study was conducted to evaluate potential of anaerobic bioventing for the treatment of an unsaturated zone contaminated with 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) and 2,4-dinitrotoluene (DNT). Hydrogen was used as the electron donor. Using the soil columns innoculate with anaerobic microorganisms, it was observed that by feeding a gas mixture of 1% hydrogen, 1% carbon dioxide and nitrogen, methanogenic conditions were established and DDT was reductively dechlorinated. 1,1-dichloro-2,2-bis(p-chlorophenyl)ethane (DDD) accumulated as the intermediate product. The half life of DDT was calculated to be 8.5 months. DNT completely disappeared after six months of operation and no intermediates could be detected.

Removal of PAHs from highly contaminated soils found at prior manufactured gas operations.

Removal of PAHs from highly contaminated soil found at a manufactured gas site was evaluated using solvent washing with mixed solvents. The following solvents were considered as water miscible co-solvents in mixed solvents: ethanol, 2-propanol, acetone, and 1-pentanol. In batch solvent extraction of soil, ethanol and 2-propanol were selected as primary components of mixed solvents in addition to 1-pentanol. Using ternary solutions containing either ethanol or 2-propanol with a volume fraction of 1-pentanol ranging from 5 to 25% and a water volume fraction ranging from 5 to 30%, ethanol was more effective than 2-propanol in extracting PAHs from soil. A solvent mixture of 5% 1-pentanol, 10% water and 85% ethanol was selected as the extraction solvent. Using a 1g:4ml soil:solvent extraction ratio, extraction kinetics showed that from 65 to 90% of the extractable PAHs were removed within an hour of contact between soil and solvent. Using this 1g:4ml extraction ratio, PAHs were removed in a three-stage cross-current solvent washing process where the same batch of soil was extracted with clean solvent for 1h in each stage. PAH removals in three-stage cross-current solvent washing were comparable to PAH removals obtained with Soxhlet extraction.

Evaluation of trickle bed air biofilter performance as a function of inlet VOC concentration and loading, and biomass control.

The 1990 Amendments to the Clear Air Act have stimulated strong interest in the use of biofiltration for the economical, engineered control of volatile organic compounds (VOCs) in effluent air streams. Trickle bed air biofilters (TBABs) are especially applicable for treating VOCs at high loadings. For long-term stable operation of highly loaded TBABs, removal of excess accumulated biomass is essential. Our previous research demonstrated that suitable biomass control for TBABs was achievable by periodic backwashing of the biofilter medium. Backwashing was performed by fluidizing the pelletized biological attachment medium with warm water to about a 40% bed expansion. This paper presents an evaluation of the impact of backwashing on the performance of four such TBABs highly loaded with toluene. The inlet VOC concentrations studied were 250 and 500 ppmv toluene, and the loadings were 4.1 and 6.2 kg COD/m3 day (55 and 83 g toluene/m3 hr). Loading is defined as kg of chemical oxygen demand per cubic meter of medium per day. Performance deterioration at the higher loading was apparently due to a reduction of the specific surface of the attached biofilm resulting from the accumulation of excess biomass. For a toluene loading of 4.1 kg COD/m3 day, it was demonstrated that the long-term performance of biofilters with either inlet concentration could be maintained at over 99.9% VOC removal by employing a backwashing strategy consisting of a frequency of every other day and a duration of 1 hr.

Gas treatment in trickle-bed biofilters: biomass, how much is enough?

The objective of this article is to define and validate a mathematical model that desribes the physical and biological processes occurring in a trickle-bed air biofilter for waste gas treatment. This model considers a two-phase system, quasi-steady-state processes, uniform bacterial population, and one limiting substrate. The variation of the specific surface area with bacterial growth is included in the model, and its effect on the biofilter performance is analyzed. This analysis leads to the conclusion that excessive accumulation of biomass in the reactor has a negative effect on contaminant removal efficiency. To solve this problem, excess biomass is removed via full media fluidization and backwashing of the biofilter. The backwashing technique is also incorporated in the model as a process variable. Experimental data from the biodegradation of toluene in a pilot system with four packed-bed reactors are used to validate the model. Once the model is calibrated with the estimation of the unknown parameters of the system, it is used to simulate the biofilter performance for different operating conditions. Model predictions are found to be in agreement with experimental data. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 54: 583-594, 1997.