ANAEROBIC BIOFILM REACTOR MODELING FOCUSED ON HYDRODYNAMICS

This work deals with an experimental and theoretical investigation of anaerobic biofilm reactors for treating wastewaters. Bioreactors are modeled as dynamic (gas-solid-liquid) three-phase systems. The anaerobic digestion model proposed by Angelidaki et al. (1999) was selected to describe the substrate degradation scheme and was applied to a biofilm system. The experimental setup consists of two mesophilic (36°±1°C) lab-scale anaerobic fluidized bed reactors (AFBRs) with sand as inert support for biofilm development. The experimental protocol is based on step-type disturbances applied on the inlet substrate concentration (glucose and acetate-based feeding) and on the feed flow rate considering the criterion of maximum efficiency. The predicted and measured responses of biological and hydrodynamic variables are investigated. Experimental data were used to estimate empirical values of biofilm detachment coefficients. Under the evaluated operating conditions, the proposed model for biofilm detachment rate, assumed as a first-order function of the energy dissipation parameter, is appropriate to represent the interaction between biofilm systems and fluidization characteristics in non-highly disturbed flow conditions. Model validation was carried out using the experimental data reported by Mussati et al. (2006). The results do not differ from those above. This seems to indicate that the proposed AFBR model is able to reproduce the main biological and hydrodynamic successes in the bioreactor.     

Combined application of 13C NMR spectroscopy and confocal laser scanning microscopy—Investigation on biofilm structure and physico-chemical properties

Long-term biofilm processes are influenced by the interplay of biofilm accumulation and detachment, which in turn depend partially on the biofilm structure and composition. In this study a combination of confocal laser scanning microscopy (CLSM) and nuclear magnetic resonance (NMR) spectroscopy was applied to analyze biofilm structure, composition and molecular mobility. Whereas CLSM delivers information about the structure of biofilms the NMR measurement provides detailed but not locally resolved information about the chemical composition of biofilm constituents. Heterotrophic mixed-species biofilms were cultivated in rotating annular reactors exposed to different flow conditions and glucose concentrations in order to obtain biofilms with diverse architectural structures. The growth state of the biofilms appeared to influence the composition of biofilm and detached biomass. The difference in the ¹³C NMR spectra between the differently structured biofilms or between biofilm and detached biomass was small, except for the still exponential growing biofilm supplied with the highest glucose concentration. More information was gained from the mobility of specific molecular groups within the biofilm biomass. Molecules within the biofilm biomass of the non-filamentous biofilms were more strongly bound than the molecules within the respective detached biomass. Glucose starvation resulted in a reduction in the biofilm molecular mobility. The opposite was observed in the filamentous biofilm. In this case, the molecular mobility in the biofilm increased after starvation and the molecules in the detached biomass were bound more strongly than in the respective biofilm biomass. It could be shown that the combination of CLSM and ¹³C NMR spectroscopy is a promising approach to analyze the interactions between biofilm architecture, composition or growth state and biofilm detachment.     

Open-cell polyvinylidene fluoride foams as carriers to promote biofilm growth for biological wastewater treatment

This article reports the design and fabrication of open-cell polyvinylidene fluoride (PVDF) foams as carriers that can promote biofilm growth and organic removal efficiency for biological wastewater treatment in attached growth bioreactors. Open-cell PVDF foams were fabricated by a manufacturing approach that integrated compression molding and particulate leaching. PVDF carriers were structured with two governing factors of leaching agent types (e.g., sodium chloride [NaCl] and sodium acetate [NaOAc]) and contents (e.g., 80 and 90 wt%). Open-cell PVDF foams possessed high porosity and high protected surface area (i.e., more than ×10 to ×20 of the areas of commercialized carriers), which promoted biofilm growth in these carriers. As a successful advantage, PVDF carriers used in the moving bed biofilm reactors (MBBR) were entirely covered by biofilm in both interior and exterior parts without clogging. This provides strong evidence of the bacterial compatibility of the fabricated open-cell PVDF foam carriers. Moreover, the specific morphology of the PVDF carriers in this article provided superior biofilm protection from the detachment in MBBR. Experimental results revealed that PVDF open-cell foams fabricated by 80 wt% of NaCl demonstrated higher mechanical strength with an organic removal efficiency of 77% ± 7% in the corresponding bioreactor containing them.     

Biofilms’ Role in Planktonic Cell Proliferation

The detachment of single cells from biofilms is an intrinsic part of this surface-associated mode of bacterial existence. Pseudomonas sp. strain CT07gfp biofilms, cultivated in microfluidic channels under continuous flow conditions, were subjected to a range of liquid shear stresses (9.42 mPa to 320 mPa). The number of detached planktonic cells was quantified from the effluent at 24-h intervals, while average biofilm thickness and biofilm surface area were determined by confocal laser scanning microscopy and image analysis. Biofilm accumulation proceeded at the highest applied shear stress, while similar rates of planktonic cell detachment was maintained for biofilms of the same age subjected to the range of average shear rates. The conventional view of liquid-mediated shear leading to the passive erosion of single cells from the biofilm surface, disregards the active contribution of attached cell metabolism and growth to the observed detachment rates. As a complement to the conventional conceptual biofilm models, the existence of a biofilm surface-associated zone of planktonic cell proliferation is proposed to highlight the need to expand the traditional perception of biofilms as promoting microbial survival, to include the potential of biofilms to contribute to microbial proliferation.     

Micro-scale modelling of substrate removal kinetics in multicomponent fixed-film systems

Modelling of fixed-film reactors, as for suspended growth systems has to account for all the major components and biochemical processes responsible for the removal of substrate. Traditionally fixed-film reactors are viewed as two-component (substrate/biomass) systems interacting with the governing effect of molecular diffusion. Recent developments however have shown the importance of viability and product formation in the modelling of biological systems. This paper emphasizes these two new parameters and evaluates the kinetic relationships between the bulk substrate concentration and biofilm composition as it affects substrate removal and residual product formation.     

Modeling coupled temperature and transport effects on biofilm growth using thermal lattice Boltzmann model

This paper aims to develop an integrated thermal lattice Boltzmann model and cellular automata to investigate the effects of different temperatures and velocities on biofilm growth in a microbioreactor. Compared with previous studies this model accounted for direct effects of transient temperature on biofilm growth and indirect effects caused by changes of fluid properties. In addition, the algorithms have been improved on variations in solid boundary conditions, detachment and extra mass transport. Results showed that temperature affected both maximum biofilm concentration and growth rate. An increase of 10–75% in biofilm concentration was observed roughly due to increases in temperature. The time required to reach maximum concentration decreased from 30 days at a low temperature to 5 days at a high temperature. This demonstrates the capability of the present model to simulate biofilm behavior in the microbioreactor and its potential industrial and clinical applications.     

Vertical upward flow of gas-solid two-phase mixtures through monolith channels

This paper reports some recent experimental observations of both gas and gas-solid two-phase flows through small monolith channels. For gas flows, the laminar-to-turbulent transition in monolith channels was observed to occur at a Reynolds number of ∼620, much lower than the conventional transition criterion of 2200 for large pipes. Surface roughness of and non-uniform distribution of gas in monolith channels were proposed to be possible reasons. For gas-solid two-phase flows, both pressure drop and solids hold-up were measured. It was found that the pressure drop of gas-solid two-phase flows through monolith channels was significantly lower than that through packed particle beds with even lower surface area per unit bed volume. Reprocessing of the pressure drop data in terms of the dimensionless groups showed that the Euler number depended approximately linearly on the solids-to-gas mass flux ratio for a given superficial gas velocity, and suspended particle size imposed little effect under the conditions of this study. Measurements of the solids hold-up showed that the hold-up in monolith channels increased with a decrease in both the gas velocity and the suspended particle size. The pressure drop results were also compared with semi-theories developed for pneumatic conveying. An overprediction was observed, an indication of the need for more controlled experiments for fundamental understanding of the hydrodynamics in monolith channels.The work reported here on gas-solid two-phase flows through monolith channels represents the first attempt in this area as no previously studies have been found in the literature.     

Sloughing and limited substrate conditions trigger filamentous growth in heterotrophic biofilms—Measurements in flow-through tube reactor

The aim of this study was to investigate the interaction between biofilm structure and sloughing in a flow-through tube reactor exposed to constant, limiting and non-limiting substrate conditions. Biofilm development and detachment were analysed by means of gravimetrical methods and confocal laser scanning microscopy (CLSM). This study revealed the impact of sloughing on biofilm structure. After six weeks of cultivation all biofilms were dominated by filamentous growth. In three out of four cultivations fungal networks developed after the first or second major sloughing event. In one biofilm, experiencing the highest substrate limitations, filamentous bacteria dominated the biofilm community prior to the first sloughing. Despite structural changes the overall biofilm substrate conversion rates remained rather constant. Several factors were identified, which possibly led to the first major sloughing event. For example, all biofilms had a density less than 40 kg m⁻³, a biofilm thickness above 80 μm, an increased surface roughness and presence of protozoa prior to sloughing. The observed fungal development may have several reasons: (1) small colonies dormant in the base biofilm adapted rapidly towards new conditions after sloughing, (2) spores attached after sloughing within the remaining base biofilm and (3) the absence of bacterial reseeding as a result of no recirculation of the bulk-fluid containing planktonic bacteria. Filamentous bacterial growth was due to the combination of limited substrate availability and high flow rates. These results can be significant for industrial systems where biofilm stability and sloughing as well as community composition are critical factors for process stability.     

Investigation and Modeling of Growth,Structure and Oxygen Penetration in Particle Supported Biofilms

Particle supported biofilms have been investigated with respect to biofilm formation, substrate transport and utilization. The investigated autotrophic and heterotrophic biofilms were cultivated in airlift suspension reactors. CLSM was used to describe the biofilm structure by recording volumes of bacteria and EPS glycoconjugates. Additionally, the microelectrode technique was used to measure transport and substrate utilization in the biofilm system. The experimental results on the microscopic scale were used to improve a mathematical model for biofilm growth. The oxygen profiles measured in the particle supported biofilms and the data from CLSM were used to optimize the model parameters.     

Parallel synthesis and fast screening of heterogeneous catalysts

We are presenting an effective method to prepare and test heterogeneous catalysts much faster than by the conventional way. A catalyst array was prepared via an incipient wetness method by combination of different amounts of Pt, Zr, and V on Al₂O₃ by means of an automatic liquid handler. For catalytic testing for methane oxidation a ceramic monolith reactor module, the channels of which contain the different catalyst compositions, was developed in which up to 250 catalyst compositions can be prepared and tested in parallel. Gas samples from each channel of the monolith were analysed sequentially by a mass spectrometer by moving the QMS inlet capillary into the channels using a three-dimensional positioning system which works at high temperatures. By comparison of the testing results with experiments carried out in flow reactors it is shown that the monolithic reactor is an efficient tool for fast screening of heterogeneous catalysts. This revised version was published online in June 2006 with corrections to the Cover Date.     

Transport and reaction in catalytic wall-flow filters

Diesel particulate filters composed of so-called wall-flow monoliths are well established devices for diesel particulate abatement. Recent improvements in production technology allow implementation of full-featured catalyst functionality in the filter walls.

From a reactor engineering point of view such wall-flow reactors with wall-integrated catalyst show fundamental differences compared to conventional flow-through monoliths. The complex interactions of convection, diffusion and reaction in the wall-flow monolith are studied by means of numerical simulation. A two-dimensional model for the flow in one pair of inlet/outlet channels with a generic first order reaction in the catalytic filter wall is developed. Concentration profiles in the reactor and a conventional flow-through catalyst are compared.

It is found that in the range of moderate reactor conversion concentration gradients along the inlet channel of the filter are small. Thus the reactor can be described by an approximate one-dimensional model, taking into account only the radial flux through the filter wall and assuming a constant inlet concentration in axial direction along the inlet channel.

Light-off curves are computed for the wall-flow and for the conventional flow-through monolith. Significantly better conversion is found for the wall-flow configuration. This can be explained by mass transfer limitation in the conventional flow-through monolith.     

Modeling of monolithic and trickle-bed reactors for the hydrogenation of styrene

A kinetic study into the styrene hydrogenation over a palladium on alumina catalyst has been made. Styrene was used as a model component for pyrolysis gasoline. A kinetic rate expression has been derived and the inhibiting effect of sulfur components has been included. Using this kinetics and mass-transfer models compiled from literature, the performance of two types of reactors for the styrene (pyrolysis gasoline) hydrogenation has been evaluated. A structured reactor such as a monolith has large advantages over a conventional trickle-bed reactor. For the monolithic reactor a more than 3 times higher volumetric productivity is obtained with much less catalyst. The modeling results indicate that deactivation by gum formation should be significantly less due to much better hydrogen mass transfer in the reactor.     

A comparative study of residence time distribution and selectivity in a monolith CDC reactor and a trickle bed reactor

A comparative study of the performance of a trickle bed reactor (TBR) and a monolith cocurrent downflow contactor (CDC) reactor in terms of selectivity and residence time distribution was conducted for the hydrogenation of 2-butyne-1,4-diol (B). Selectivity (S) towards 2-butene-1,4-diol was investigated with the solvent 2-propanol and a 30% (v/v) 2-propanol/water mixture (M) in batch recycle mode. Liquid residence time distribution (RTD) curves were obtained for both reactors. Although both reactors presented almost identical hydrodynamic behaviour, i.e. RTD, significant differences regarding selectivity towards the alkene were observed in both solvents. The use of 2-propanol gave lower selectivities in both reactors, but even then the monolith reactor was superior. In the monolith CDC, the liquid RTD curve was also obtained at different radial positions. RTD profiles across the monolith showed that from the centre to the column wall there is possibly an increased retention of material and despite this, overall selectivity does not appear to be considerably depressed by the backmixing that the above result implies in 2-propanol/water where the selectivity was found to be 100% towards the intermediate (C).

Modelling of the monolith CDC reactor was also conducted to predict RTD. The models tested were tanks-in-series, piston exchange and piston dispersion exchange; from which, piston exchange model was found to best predict and fit the experimental data.     

The effect of headspace pressure on the performance of a fluidised‐bed bioreactor

This study compares the biological performance of three fluidised‐bed biological reactors under conditions of different headspace pressures. The application of pressure can have a profound effect on the initial rate of bed growth. However, once the fluidised‐bed reaches full expansion, the biological performance at higher pressures is greater than those at lower pressures. There appears to be an almost linear relationship between the application of pressure and the performance of the fluidised‐bed biological reactors in removing soluble BOD₅. This can be attributed to the increase in the oxygen concentration in the bulk liquid and a greater oxygen penetration depth within the biofilm. © 2002 Society of Chemical Industry     

Influence of non-uniform distribution of shear stress on aerobic biofilms

This work deals with the detachment of biofilm subjected to a shear stress. Biofilms are developed on plates, under very low shear stress for one month and then subjected to an erosion test for 2 h in a Couette-Taylor reactor (CTR). During the erosion test, the plate was fixed on the external cylinder of the CTR. The presence of the plate modifies the velocity field in the CTR. A first zone close to the facing step region is characterized by the detachment of the stream lines. A second zone, downstream, is characterized by a pure shear flow: the distribution of the shear stress is uniform; the residual biofilm mass was measured and the detachment can be classically related to the magnitude of shear stress. In the first zone, the recirculating flow induces a strong non-uniform distribution of shear stress. The residual biofilm mass was also measured and found to be much lower than in the uniform shear stress zone, whereas the magnitude of shear stress is of the same order or even smaller. The assumption of elastic rheology for the biofilm enables the strong detachment observed in the region subjected to non-uniform shear stress to be explained.     

Design of monolithic catalysts for multiphase reactions

Monolith reactors are emerging as an attractive alternative for gas-liquid-solid reactor applications. The use of monolithic catalysts in new reactors as well as in retrofit designs should be based on an optimal choice of monolith geometry and operating conditions.In this contribution, we illustrate through fundamental modeling of the transport-kinetic interactions in a monolith catalyst how such an optimal design may be evolved. We also highlight the potential benefits a monolith catalyst has as compared to a pellet-based trickle bed reactor.     

Transient biofilter aerodynamics and clogging for VOC degradation

Removal of volatile organic compounds like toluene from waste gases with a biofilter can result in clogging of the reactor due to the formation of an excessive amount of biomass. Excessive biomass formation changes the bed's pore structure and leads to the progressive obstruction of the bed that is accompanied with a build-up in pressure drop and flow channeling. While the existing biofilter models appear to capture adequately the transport and reaction phenomena at the biofilm scale, they poorly address, or provide little insight about the connection between the aerodynamics, biological filtration (or clogging) and biokinetics at the bioreactor length scale. An attempt has been made with this contribution to fill in this gap by developing a unidirectional dynamic flow model based on the volume-average mass, momentum and species balance equations coupled with conventional diffusion/reaction equations describing apparent kinetics in the biofilm. Toluene biodegradation by biodegrading microbes immobilized on pelletized diatomaceous earth biological support media was chosen as a case study to illustrate the consequences of formation of excessive amounts of biomass. The simulation results were rationalized in terms of biofilm thickness, bed local porosity, gas-phase substrate residual concentration, and pressure drop rise in biological fixed-bed filters.     

Solids behaviour in a dilute gas-solid two-phase mixture flowing through monolith channels

This paper reports, for the first time, the solids behaviour in a dilute gas-solid two-phase mixture flowing through monolith channels. The non-intrusive positron emission particle tracking (PEPT) technique was used in the work, which allowed investigation of three-dimensional solids motion at the single suspended particle level. Processing of the PEPT data gave solids velocity and occupancy in the monolith channels. The results showed a non-uniform radial distribution of both the solids velocity and concentration. The highest axial solids velocity occurred in monolith channels located in the central part of the column, whereas the highest solids concentration took place at a position approximately 0.7 times the column radius. The axial distribution of the axial solids velocity showed an entrance region with a length of approximately 33 times the hydrodynamic diameter of a monolith channel under the conditions of this work. Analysis of the PEPT data also gave distributions of particle residence time and tortuosity in terms of solids motion. The distributions were approximately Gaussian-type with the tortuosity distribution more skewed toward the right hand side. The peak residence time and tortuosity decreased with increasing superficial gas velocity and the distributions were broadened at lower superficial gas velocities. The results of this work also provided a possible explanation to our previously observed early laminar-to-turbulent flow transition in monolith channels.     

Water treatment using sequenced ozonation and SBR biofilm reactors

The degradation of 3‐methylpyridine (3MEP), a model heterocyclic industrial molecule, was performed in a sequential batch ozonation–biofilm process. Four process steps (bubble‐column ozonation, heterotrophic biofilm degradation, biofilm nitrification, and biofilm denitrification) were combined in different sequences. Three packed‐bed biofilm reactors were started up so as to have separate, specific activities (heterotrophic, nitrification and denitrification). Batch experiments with acetate, ammonia, and nitrate proved that all reactors displayed degradation activity for all substances. Different batch sequences of these reactors were tested with the products of batch ozonation of 3MEP as the first step. The best results were obtained using a two‐step process, in which the ozonation was followed by a single fluidized‐bed, heterotrophic biofilm reactor. The high C/N ratio of 3MEP and the appreciable non‐specific activity of this reactor made it possible to achieve all the biodegradation in the one reactor. Establishing the optimal batch ozonation time (80 min) was determined by an ozone electrode and by stopping the process when the dissolved ozone concentration rose above an initial low level. The identifiable products of 3MEP ozonation were nitrate, acetate, formate, pyruvate, oxalate and ammonium. A C‐balance, compared with TOC measurement, indicated that about 50% of the carbon was in unidentified, but biogradable, ozonation products. Copyright © 2003 Society of Chemical Industry