Simulation of oxygen transfer in microbial slimes

A model based on diffusion mechanisms accounts for oxygen transfer from flowing nutrient medium into a film of organisms in slime. Profiles of dissolved oxygen concentration are generated at distances downstream from the start of the slime. In these studies, the nutrient medium was assumed rich, thus the rate of oxygen transport limited microbial respiration. Parameter sensitivity tests were performed for flow rate, oxygen concentration in the medium, and mass transfer coefficients.     

Biodegradation of toluene in a trickling filter

A trickling filter packed with PVC 16?mm Raschig rings was used to study the degradation of toluene in a polluted air stream, by means of a bacterial biofilm of Pseudomonas putida ATCC 17484. A polluted stream was simulated by blending air with a controlled amount of toluene. The mixing was accomplished in a special mixing chamber designed for that purpose. Induction of the enzymes of the toluene degradative pathway and adaptation of the inoculum were done in batch cultures with minimum mineral media and phenol. The continuous experiments were monitored by mass spectrometry for the quantification of the various gases and of toluene removal. A 94% toluene removal was achieved with contacting times above one minute and toluene concentrations up to 400?ppm.     

Microbial film development in a trickling filter

The transmission and scanning electron microscopes were employed to visualize the sequence of the biofilm development in the trickling wastewater filter. After the deposit of a small amount of debris upon a hard surface, the bacterial cells attach and develop the matrix on which the biofilm is formed. Protozoa invade the basic layer where they feed on the bacteria. The algae are seeded upon the bacterial matrix and grow so profusely that the bacteria must develop aerial colonies in the competition for food and oxygen. Destruction of the bacteria in the matrix and the weight and hydraulic pressure cause detachment of the biofilm and a new matrix must be developed.     

Microelectrode measurement of oxygen tensions in collagen-hepatocyte gels

Summary Microelectrodes have been used for the measurement of oxygen tension in various biological systems.(Silver, 1987) Although not previously reported, microelectrodes allow the direct measurement of oxygen tension profiles within collagen gels containing entrapped hepatocytes (collagcn-hepatocyte gels). These oxygen tension profiles, along with hepatocyte oxygen consumption data, allowed the estimation of a diffusion coefficient for oxygen in collagen-hepatocyte gels, D _g = 2.99 × 10^–5 cm²/s.     

Removal of toluene in waste gases using a biological trickling filter

The removal of toluene from waste gas was studied in a trickling biofilter. A high level of water recirculation (4.7 m h^–1) was maintained in order to keep the liquid phase concentration constant and to achieve a high degree of wetting. For loads in the range from 6 to 150 g m^–3 h^–1 the maximum volumetric removal rate (elimination capacity) was 35±10 g m^–3 h^–1, corresponding to a zero order removal rate of 0.11±0.03 g m^–2 h^–1 per unit of nominal surface area. The surface removal was zero order above the liquid phase concentrations of approximately 1.0 g m^–3, corresponding to inlet gas concentrations above 0.7–0.8 g m^–3. Below this concentration the surface removal was roughly of first order. The magnitude of the first order surface removal rate constant, k_1A , was estimated to be 0.08–0.27 m h^–1 (k_1A a=24–86 h^–1). Near-equilibrium conditions existed in the gas effluent, so mass transfer from gas to liquid was obviously relatively fast compared to the biological degradation. An analytical model based on a constant liquid phase concentration through the trickling filter column predicts the effluent gas concentration and the liquid phase concentration for a first and a zero order surface removal. The experimental results were in reasonable agreement with a very simple model valid for conditions with an overall removal governed by the biological degradation and independent of the gas/liquid mass transfer. The overall liquid mass transfer coefficient, K_La, was found to be a factor 6 higher in the system with biofilm compared to the system without. The difference may be explained by: 1. Difference in the wetting of the packing material, 2. Mass transfer occurring directly from the gas phase to the biofilm, and 3. Enlarged contact area between the gas phase and the biofilm due to a rough biofilm surface.     

Verification studies of a simplified model for the removal of dichloromethane from waste gases using a biological trickling filter

The removal of dichloromethane from waste gases in a biological trickling filter was studied experimentally as well as theoretically within the concentration range of 0–10,000 ppm. A stable dichloromethane elimination performance was achieved during two years of operation, while the start-up of the system only amounted to several weeks at constant inlet concentrations. The trickling filter system was operated co-currently as well as counter-currently.However, experimental and theoretical results revealed that the relative flow direction of the mobile phases did not significantly affect the elimination performance. Moreover, it was found that the gas-liquid mass-transfer resistance in the trickling filter bed applied was negligible, which leaves the biological process inside the biofilm to be the rate limiting step.A simplified model was developed, the Uniform-Concentration-Model, which showed to predict the filter performance close to the numerical solutions of the model equations. This model gives an analytical expression for the degree of conversion and can thus be easily applied in practice.The dichloromethane eliminating performance of the trickling filter described in this paper, is reflected by a maximum dichloromethane elimination capacity EC _max=157 g/(m³ · h) and a critical liquid concentration C _lcr=45 g/m³ at a superficial liquid velocity of 3.6 m/h, inpendent of the gas velocity and temperature.List of Symbols a _s m²/m³ specific area - a _w m²/m³ specific wetted area - A m² cross-sectional area - C _g g/m³ gas phase concentration - C _go g/m³ inlet gas phase concentration - C _gocr g/m³ critical gas phase concentration - C _g ^* C_g/C_go dimensionless gas concentration - C _l g/m³ liquid concentration - C _lcr g/m³ critical liquid concentration - C _lcr ^* mC_lcr/C_go dimensionless critical concentration - c _li g/m³ substrate concentration at liquid-biofilm interface - C _l ^* mC_l/C_go dimensionless liquid concentration - C _o g/m³ oxygen concentration inside the biofilm - C _oi g/m³ oxygen concentration at liquid-biofilm interface - C_s g/m³ substrate concentration inside the biofilm - C _si g/m³ substrate concentration at liquid-biofilm interface - D _eff m²/h effective diffusion coefficient in the biofilm - D _o m²/h effective diffusion coefficient for oxygen in the biolayer - E mu_g/u_l extraction factor - E _act kJ/mol activation energy for the biological reaction - EC g/(m³· h) K _o a _w : elimination capacity, or the amount of substrate degraded per unit of reactor volume and time - EC _max g/(m³ · h) K _o a_w: maximum elimination capacity - f degree of conversion - h m coordinate in height - H m height of the packed bed - K ₀ g/(m³ · h) _maxX_b/Y zeroth order reaction defined per unit of biofilm volume - k _og m/h overall gas phase mass transfer coefficient - K ^* dimensionless constant given by Eq. (A.5) - K _l ^* dimensionless constant given by Eq. (A.6) - K ₂ ^* dimensionless constant given by Eq. (A.6) - m C _g /C_l gas liquid distribution coefficient - N g/(m² · h) liquid-biofilm interfacial flux of substrate - N _og k_oga_wH/u_g number of gas phase transfer units - N _r k_o a_w H/u_g C_go number of reaction units - OL g/(m³· h) u _g C _go /H organic load - r _s g/(m³ ·h) zeroth order substrate degradation rate given by Eq. (1) - R _s g/(g TSS ·h) specific activity - T K absolute temperature - u _g m/h superficial gas velocity - u _t m/h superficial liquid velocity - X _b g TSS/m₃ biomass concentration inside biofilm - X _s g TSS/m₃ liquid suspended biomass concentration - x m coordinate inside the biofilm - Y g TSS/(g_DCM) yield coefficient Greek Symbols dimensionless parameter given by Eq. (2) - m averaged biofilm thickness - biofilm effectiveness factor given by Eqs. (7a)–(7c) - m penetration depth of substrate into the biofilm - _max d^–1 microbiological maximum growth rate - v _o stoichiometric utilization coefficient for oxygen - v _s stoichiometric utilization coefficient for substrate - dimensionless height in the filter bed - h H/u _g superficial gas phase contact time - _o (K ₀ /DC _ii )^1/2 - _o C _o /C _oi dimensionless oxygen concentration inside the biofilm - _s C _s /C _si dimensionless substrate concentration inside the biofilm Experimental results, verifying the model presented will be discussed Part II (to be published in Vol. 6, No. 4)     

The existence of a biological equilibrium in a trickling filter for waste gas purification

Clogging is well-known phenomenon in the application of a biological tricking filter for both waste gas and wastewater treatment. Nevertheless, no such observations or even significant changes in pressure drop have ever been recorded during the long-term processing of a waste gas containing dichloromethane (DCM) as a sole carbon source. To obtain more information about this phenomenon, a detailed investigation into the carbon balance of this system has been performed. During a period of operation of about 200 days the rate of DCM elimination and the overall rate of CO(2) production in a continuously operating filter were therefore recorded daily, thus allowing an evaluation of the overall conversion process. Furthermore pseudo-steady-state measurements were carried out on a regular basis. These experiments reveal more detailed information on the actual DCM conversion by Hyphomicrobium GJ21 within the biofilm. The combined results of the experiments described in this article show that on an overall basis a so-called biological equilibrium, i.e., a situation of no net biomass accumulation, is obtained in the course of time. It appeared that the overall rate of CO(2) production slowly increased until, after some 200 days, it finally counter-balanced the conversion rate of DCM on a molar-basis. As opposed to this result, all pseudo-steady-state experiments indicated that about 60% of the eliminated primary carbon source is converted into biomass. This is in good agreements with results from microkinetic experiments. Based on these results and evaluation of the experimental data, it is concluded that interactions between several microbial populations are involved in this biological equilibrium. These interactions include both biomass growth and biomass degradation. (c) 1994 John Wiley & Sons, Inc.     

Comparison of the trickling filter models for the treatment of synthetic dairy wastewater

This paper describes the design implications of four existing trickling filter models. Experimental data from the treatment of synthetic dairy wastewater was used to evaluate the kinetic parameters. The four trickling filter models were examined for their ability to model the present data. Among the four models studied, Kincannon and Stover model based on the independent variable of surface organic loading rate gave superior results compared to other models.     

Vertical distribution of nitrifying populations in bacterial biofilms from a full-scale nitrifying trickling filter

Cryosectioned biofilm from three depths (0.5, 3.0 and 6.0 m) in a full-scale nitrifying trickling filter (NTF) were studied using fluorescence in situ hybridization (FISH). A large number of sections were used to determine how the biofilm thickness, structure and community composition varied with depth along the ammonium concentration gradient in the NTF, and how the ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) were distributed vertically within the biofilm. Both the biofilm thickness and relative biomass content of the biofilm decreased with depth, along with structural differences such as void size and surface roughness. Four AOB populations were found, with two Nitrosomonas oligotropha populations dominating at all depths. A smaller population of Nitrosomonas europaea was present only at 0.5 m, while a population of Nitrosomonas communis increased with depth. The two N. oligotropha populations showed different vertical distribution patterns within the biofilm, indicating different ecophysiologies even though they belong to the same AOB lineage. All NOB were identified as Nitrospira sp., and were generally more associated with the biofilm base than the surface-associated dominating AOB population. Additionally, a small population of anaerobic ammonia-oxidizers was found at 6.0 m, even though the biofilm was well aerated.     

Some properties of a neutral phosphatase of a bacterium isolated from a trickling filter effluent

Summary The partial purification and some of the properties of a neutral phosphatase from a Gram-negative oxidative bacterium isolated from a sewage works effluent are described. The enzyme is active against a variety of organic and inorganic phosphates and, of a number of ions tested, cadmium, copper, mercury and zinc and tungstate and molybdate were the most powerful inhibitors of enzyme action. Inhibition by phosphate is competitive. The possible significance of bacterial neutral phosphatases in purification processes is discussed.     

Intermittent trickling bed filter for the removal of methyl ethyl ketone and methyl isobutyl ketone

Biodegradations of methyl ethyl ketone and methyl isobutyl ketone were performed in intermittent biotrickling filter beds (ITBF) operated at two different trickling periods: 12 h/day (ITBF-12) and 30 min/day (ITBF-0.5). Ralstonia sp. MG1 was able to degrade both ketones as evidenced by growth kinetic experiments. Results show that trickling period is an important parameter to achieve high removal performance and to maintain the robustness of Ralstonia sp. MG1. Overall, ITBF-12 outperformed ITBF-0.5 regardless of the target compound. ITBF-12 had high performance recovery at various inlet gas concentrations. The higher carbon dioxide production rates in ITBF-12 suggest higher microbial activity than in ITBF-0.5. Additionally, lower concentrations of absorbed volatile organic compound (VOC) in trickling solutions of ITBF-12 systems also indicate VOC removal through biodegradation. Pressure drop levels in ITBF-12 were relatively higher than in ITBF-0.5 systems, which can be attributed to the decrease in packed bed porosity as Ralstonia sp. MG1 grew well in ITBF-12. Nonetheless, the obtained pressure drop levels did not have any adverse effect on the performance of ITBF-12. Biokinetic constants were also obtained which indicated that ITBF-12 performed better than ITBF-0.5 and other conventional biotrickling filter systems.     

Docking and electron transfer studies between rubredoxin and rubredoxin:oxygen oxidoreductase

The interaction and electron transfer (ET) between rubredoxin (Rd) and rubredoxin:oxygen oxidoreductase (ROO) from Desulfovibrio gigas is studied by molecular modelling techniques. Experimental kinetic assays using recombinant proteins show that the Rd reoxidation by ROO displays a bell-shaped dependence on ionic strength, suggesting a non-trivial electrostatic dependence of the interaction between these two proteins. Rigid docking studies reveal a prevalence for Rd to interact, in a very specific way, with the surface of the ROO dimer near its FMN cofactors. The optimization of the lowest energy complexes, using molecular dynamics simulation, shows a very tight interaction between the surface of the two proteins, with a high probability for Rd residues (but not the iron centre directly) to be in direct contact with the FMN cofactors of ROO. Both electrostatics and van der Waals interactions contribute to the final energy of the complex. In these complexes, the major contributions for complex formation are polar interactions between acidic residues of Rd and basic residues of ROO, plus substantial non-polar interactions between different groups. Important residues for this process are identified. ET estimates (using the Pathways model), in the optimized lowest energy complexes, suggest that these configurations are efficient for transferring electrons. The experimental bell-shaped dependence of kinetics on ionic strength is analysed in view of the molecular modelling results, and hypotheses for the molecular basis of this phenomenon are discussed.     

The balance between charge transfer and non-charge transfer pathways in the sensitization of singlet oxygen by pi pi* triplet states.

A charge transfer (CT) channel and a non-CT deactivation channel, both leading to formation of O(2)((1)Sigma (g)(+)), O(2)((1) Delta(g)) and O(2)((3)Sigma(g)(-)), compete in the quenching of triplet states by O(2). Recent studies by our group demonstrated that these channels are described by rather simple and general quantitative relations. In the present paper we use the detailed kinetic data on the quenching by O(2) of pi pi* triplet sensitizers of three homologous aromatic series in CCl(4) to derive a parameter, which describes the balance between CT and non-CT deactivation. This quantity, p(CT), is the relative contribution of CT mediated deactivation and is easily calculated for a sensitizer of known triplet energy from its quenching rate constant. The parameter p(CT) quantitatively describes the balance between both deactivation channels without requiring any knowledge of oxidation potentials. It is shown how the variation of p(CT) influences the efficiencies and the rate constants of O(2)((1)Sigma(g)(+)), O(2)((1)Delta(g)) and O(2)((3)Sigma(g)(-)) formation in the quenching process.