Abrasion of suspended biofilm pellets in airlift reactors: effect of particle size

The detachment of biomass from suspended biofilm pellets in three-phase internal loop airlift reactors was investigated under non-growth conditions, and in the presence of bare carrier particles. In the experiments the size of biofilm pellets and bare carrier particles was varied. Results show that an increase in particle size drastically increases the abrasion rate caused by particle collisions. This increase is larger than predicted by conventional collision theory, which accounts for changes in collision frequency and collision impact. However, collision theory was formulated for neutrally buoyant particles which follow the liquid flow. This condition does not hold for biofilm pellets and carrier particles. The difference might therefore be caused by differences in particle responses to flow fluctuations. An empirical relationship, including this flow response, was formulated. The collision impact is also strongly affected by the roughness of a bare carrier particle: sharp and edgy particles cause much more damage than smoother ones. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 55: 206-215, 1997.     

Dynamics of biofilm detachment in biofilm airlift suspension reactors

The dynamic change in the overall detachment rate of spherical biofilms in a biofilm airlift suspension reactor was measured after a downshift of the substrate loading rate to zero while all other conditions remained constant. In contrast to the expectations, the overall detachment rate decreased rapidly to a nearly stable level. Correlations available from literature were not able to describe this phenomenon. Concepts were formulated which can describe the observations from this study. Research under dynamic conditions and careful monitoring of the biofilm surface area and biofilm morphology are necessary to elucidate and discriminate biofilm detachment mechanisms. (c) 1995 John Wiley & Sons, Inc.     

Abrasion of suspended biofilm pellets in airlift reactors: importance of shape, structure, and particle concentrations

The detachment of biomass from suspended biofilm pellets in three-phase internal loop airlift reactors was investigated under nongrowth conditions and in the presence of bare carrier particles. In different sets of experiments, the concentrations of biofilm pellets and bare carrier particles were varied independently. Gas hold-up, bubble size, and general flow pattern were strongly influenced by changes in volume fractions of biofilm pellets and bare carrier particles. In spite of this, the rate of biomass detachment was found to be linear with both the concentration of biofilm pellets and the bare carrier concentration up to a solids hold-up of 30%. This implies that the detachment rate was dominated by collisions between biofilm pellets and bare carrier particles. These collisions caused an on-going abrasion of the biofilm pellets, leading to a reduction in pellet volume. Breakage of the biofilm pellets was negligible. The biofilm pellets were essentially ellipsoidal, which made three-dimensional size determination necessary. Calculating particle volumes from two-dimensional image analysis measurements and assuming a spherical shape led to serious errors. The abrasion rate was not equal on all sides of the biofilm pellets, resulting in an increasing flattening of the pellets. This flattening was oriented with the basalt carrier inside the biofilm and independent of the absolute abrasion rate. These observations suggest that the collisions causing abrasion are somehow oriented. The internal structure of the biofilms showed two layers, a cell-dense outer layer and an interior with a low biomass density. Taking this density gradient into account, the washout of detached biomass matched observed changes in volume of the biofilm pellets. No gradient in biofilm strength with biofilm depth was indicated. (c) 1997 John Wiley & Sons, Inc.     

Formation of nitrifying biofilms on small suspended particles in airlift reactors

For a stable and reliable operation of a BAS-reactor a high, active biomass concentration is required with mainly biofilm-covered carriers. The effect of reactor conditions on the formation of nitrifying biofilms in BAS-reactors was investigated in this article. A start-up strategy to obtain predominantly biofilm-covered carriers, based on the balancing of detachment and a biomass production per carrier surface area, proved tp be very successful. The amount of biomass and the fraction of covered carrier were high and development of nitrification activity was fast, leading to a volumetric conversion of 5 kg(N) . m(-3) . d(-1) at a hydraulic retention time of 1h. A 1-week, continuous inoculation with suspended purely nitrifying microorganisms resulted in a swift start-up compared with batch addition of a small number of biofilms with some nitrification activity. The development of nitrifying biofilms was very similar to the formation of heterotrophic biofilms. In contrast to heterotrophic bio-films, the diameter of nitrifying biofilms increased during start-up. The detachment rate from nitrifying biofilms decreased with lower concentrations of bare carrier, in a fashion comparable with heterotrophic biofilms, but the nitrifying biofilms were much more robust and resistant. Standard diffusion theory combined with reaction kinetics are capable of predicting the activity and conversion of biofilms on small suspended particles. (c) 1995 John Wiley & Sons Inc.     

Macromixing hydrodynamic study in draft-tube airlift reactors using electrical resistance tomography

The present study summarizes results of mixing characteristics in a draft tube airlift bioreactor using ERT. This technique offers the possibility for noninvasive and nonintrusive visualization of flow fields in the bioreactor and has rarely been utilized previously to analyze operating parameters and mixing characteristics in this type of bioreactors. Several operating parameters and geometric characteristics were examined. In general, results showed that the increase in superficial gas velocity corresponds to an increase in energy applied and thus, to a decrease in mixing time. This generally corresponded to an increase in liquid circulation velocity and shear rate values. Bottom clearances and draft tube diameters affected flow resistance and frictional losses. The influence of sparger configurations on mixing time and liquid circulation velocity was significant due to their effect on gas distribution. However, the effect of sparger configuration on shear rate was not significant, with 20% reduction in shear rates using the cross-shaped sparger. Fluid viscosity showed a marked influence on both mixing times and circulation velocity especially in the coalescing media of sugar and xanthan gum (XG) solutions. Results from this work will help to develop a clear pattern for operation and mixing that can help to improve several industrial processes, especially the ones related to emerging fields of technology such as the biotechnology industry.     

Detachment of biomass from suspended nongrowing spherical biofilms in airlift reactors

In three-phase internal loop airlift reactors, the detachment of biomass from suspended biofilm pellets in the presence of bare carrier particles was investigated under nongrowth conditions. The detachment rate was dominated by collisions between bare carrier particles and biofilm pellets. The concentration of bare carrier particles and the carrier roughness strongly influenced the detachment rate. A change in flow regime from bubbling to slug flow considerably increased the detachment rate. Otherwise, the superficial gas velocity did not directly affect the detachment rate. The influence of particle size was not clear. The bottom clearance did not affect the detachment rate within the tested range. Other aspects of reactor geometry might be important. The main detachment processes were abrasion and breakage of biofilm pellets. During the detachment process, two phases could be distinguished. In the first phase the detachment was relatively high, and both breakage and abrasion of biofilm pellets occurred. During the second phase, breakage dominated and the detachment rate was lower. The two-phase behavior is explained by differences in strength between the inner and outer biofilm layers, possibly caused by variations in local growth rates during biofilm formation. Differences in growth history might also explain the various detachment rates observed with different biofilm batches. (c) 1995 John Wiley & Sons, Inc.     

Formation and growth of heterotrophic aerobic biofilms on small suspended particles in airlift reactors

In this article, the conditions for aerobic biofilm formation on suspended particles, the dynamics of biofilm formation, and the biomass production during the start-up of a Biofilm Airlift Suspension reactor (BAS reactor) have been studied. The dynamics of biofilm formation during start up in the biofilm airlift suspension reactor follows three consecutive stages: bare carrier, microcolonies or patchy biofilms on the carrier, and biofilms completely covering the carrier. The effect of hydraulic retention time and of substrate loading rate on the formation of biofilms were investigated. To obtain in a BAS reactor a high biomass concentration and predominantly continuous biofilms, which completely surround the carrier, the hydraulic retention time must be shorter than the inverse of the maximum growth rate of the suspended bacteria. At longer hydraulic retention times, a low amount of attached biomass can be present on the carrier material as patchy biofilms. During the start-up at short hydraulic retention times the bare carrier concentration decreases, the amount of biomass per biofilm particle remains constant, and biomass increase in the reactor is due to increasing numbers of biofilm particles. The substrate surface loading rate has effect only on the amount of biomass on the biofilm particle. A higher surface load leads to a thicker biofilm.A strong nonlinear increase of the concentration of attached biomass in time was observed. This can be explained by a decreased abrasion of the biofilm particles due to the decreasing concentration of bare carriers. The detachment rate per biofilm area during the start-up is independent of the substrate loading rate, but depends strongly upon the bare carrier concentration.The Pirt-maintenance concept is applicable to BAS reactors. Surplus biomass production is diminished at high biomass concentrations. The average maximal yield of biomass on substrate during the experiments presented in this article was 0.44 +/- 0.08 C-mol/C-mol, the maintenance value 0.019 +/- 0.012 C-mol/(C-mol h). The lowest actual biomass yield measured in this study was 0.15 C-mol/C-mol. (c) 1994 John Wiley & Sons, Inc.     

Application of two component biodegradable carriers in a particle-fixed biofilm airlift suspension reactor: development and structure of biofilms

Two component biodegradable carriers for biofilm airlift suspension (BAS) reactors were investigated with respect to development of biofilm structure and oxygen transport inside the biofilm. The carriers were composed of PHB (polyhydroxybutyrate), which is easily degradable and PCL (caprolactone), which is less easily degradable by heterotrophic microorganisms. Cryosectioning combined with classical light microscopy and CLSM was used to identify the surface structure of the carrier material over a period of 250 days of biofilm cultivation in an airlift reactor. Pores of 50 to several hundred micrometers depth are formed due to the preferred degradation of PHB. Furthermore, microelectrode studies show the transport mechanism for different types of biofilm structures, which were generated under different substrate conditions. At high loading rates, the growth of a rather loosely structured biofilm with high penetration depths of oxygen was found. Strong changes of substrate concentration during fed-batch mode operation of the reactor enhance the growth of filamentous biofilms on the carriers. Mass transport in the outer regions of such biofilms was mainly driven by advection.     

Shortcut biological nitrogen removal in hybrid biofilm/suspended growth reactors

A shortcut biological nitrogen removal (SBNR) process converts ammonium directly through nitrite to nitrogen gas, thus requiring less aeration and carbon. We evaluated a hybrid SBNR (HSBNR) reactor containing an anoxic tank followed by an aerobic tank and a settling tank. The aerobic tank was filled with polyvinyl alcohol sponge media (20%, v/v) to attach and retain ammonium oxidizers. Two configurations of the HSBNR reactor were tested for treating a wastewater with high strength ammonium and organic electron donor. The HSNBR reactors accumulated nitrite stably for 1.5 years and maintained a high free ammonia (FA) concentration (20–25 mg/L) and a low dissolved oxygen (DO) concentration (<1 mg/L) in the aerobic tank. Apparently, the biofilm carriers increased the solids retention time (SRT) for ammonium oxidizers, while high FA and low DO selected against nitrite oxidizers and promoted direct denitrification of nitrite in the aerobic tank. The significant amount of chemical oxygen demand (COD) was removed by shortcut denitrification of nitrite in the anoxic tank.     

Anammox enrichment from reject water on blank biofilm carriers and carriers containing nitrifying biomass: operation of two moving bed biofilm reactors (MBBR)

The anammox bacteria were enriched from reject water of anaerobic digestion of municipal wastewater sludge onto moving bed biofilm reactor (MBBR) system carriers-the ones initially containing no biomass (MBBR1) as well as the ones containing nitrifying biomass (MBBR2). Duration of start-up periods of the both reactors was similar (about 100?days), but stable total nitrogen (TN) removal efficiency occurred earlier in the system containing nitrifying biomass. Anammox TN removal efficiency of 70% was achieved by 180?days in both 20?l volume reactors at moderate temperature of 26.0°C. During the steady state phase of operation of MBBRs the average TN removal efficiencies and maximum TN removal rates in MBBR1 were 80% (1,000?g-N/m(3)/day, achieved by 308?days) and in MBBR2 85% (1,100?g-N/m(3)/day, achieved by 266?days). In both reactors mixed bacterial cultures were detected. Uncultured Planctomycetales bacterium clone P4, Candidatus Nitrospira defluvii and uncultured Nitrospira sp. clone 53 were identified by PCR-DGGE from the system initially containing blank biofilm carriers as well as from the nitrifying biofilm system; from the latter in addition to these also uncultured ammonium oxidizing bacterium clone W1 and Nitrospira sp. clone S1-62 were detected. FISH analysis revealed that anammox microorganisms were located in clusters in the biofilm. Using previously grown nitrifying biofilm matrix for anammox enrichment has some benefits over starting up the process from zero, such as less time for enrichment and protection against severe inhibitions in case of high substrate loading rates.     

Hydrodynamic characteristics and gas-liquid mass transfer in a biofilm airlift suspension reactor

The hydrodynamics and mass transfer, specifically the effects of gas velocity and the presence and type of solids on the gas hold-up and volumetric mass transfer coefficient, were studied on a lab-scale airlift reactor with internal draft tube. Basalt particles and biofilm-coated particles were used as solid phase. Three distinct flow regimes were observed with increasing gas flow rate. The influence of the solid phase on the hydrodynamics was a peculiar characteristic of the regimes. The volumetric mass transfer coefficient was found to decrease with increasing solid loading and particle size. This could be predominantly related to the influence that the solid has on gas hold-up. The ratio between gas hold-up and volumetric mass transfer coefficient was found to be independent of solid loading, size, or density, and it was proven that the presence of solids in airlift reactors lowers the number of gas bubbles without changing their size. To evaluate scale effects, experimental results were compared with theoretical and empirical models proposed for similar systems.     

A comparison between the reaction rates in biofilm reactors and free suspended cells bioreactors

A theoretical equation for comparison of the bioreaction rates in a biofilm reactor and in a bioreactor with free suspended cells at different input substrate concentrations and substrate conversions is proposed. The main assumption is that there is no kinetic change after fixation of the cells. Some previously published experimental data are used for verification of this equation. It is shown that difference between the experimental and calculated data is within 19%.     

Early stages in biofilm development in methanogenic fluidized-bed reactors

Summary Biofilm development in methanogenic fluidized-bed reactors with sand as the carrier was studied on a laboratory scale. The microorganisms present in consecutive layers of the biofilm of mature sludge granules were preliminarily characterized on the basis of their morphology, element composition and adhesion capacity and were compared to bacteria which take part in the initial colonization of sand. The early phase of biofilm development was monitored with reactors receiving waste-waters containing different mixtures of volatile fatty acids and inoculated with fluidized-bed reactor effluent for different lengths of time. The results obtained indicate that facultative anaerobic bacteria abundantly present in the outermost biofilm layers of mature sludge granules are probably the main primary colonizers of the sand. Methanothrix spp. or other methanogens were rarely observed among the primary colonizers. The course of biofilm formation was comparable under the various start-up conditions employed including variations in waste-water composition, inoculation and anaerobicity. However, omission of waste-water and thus of substrate resulted in rapid wash-out of the attached biomass. Offprint requests to: W. Heinen     

Biohydrogen production from glucose in upflow biofilm reactors with plastic carriers under extreme thermophilic conditions (70 degrees C)

Biohydrogen could efficiently be produced in glucose-fed biofilm reactors filled with plastic carriers and operated at 70 degrees C. Batch experiments were, in addition, conducted to enrich and cultivate glucose-fed extreme-thermophilic hydrogen producing microorganisms from a biohydrogen CSTR reactor fed with household solid waste. Kinetic analysis of the biohydrogen enrichment cultures show that substrate (glucose) likely inhibited hydrogen production when its concentration was higher than 1 g/L. Different start up strategies were applied for biohydrogen production in biofilm reactors operated at 70 degrees C, and fed with synthetic medium with glucose as the only carbon and energy source. A biofilm reactor, started up with plastic carriers, that were previously inoculated with the enrichment cultures, resulted in higher hydrogen yield (2.21 mol H(2)/mol glucose consumed) but required longer start up time (1 month), while a biofilm reactor directly inoculated with the enrichment cultures reached stable state much faster (8 days) but with very low hydrogen yield (0.69 mol H(2)/mol glucose consumed). These results indicate that hydraulic pressure is necessary for successful immobilization of bacteria on carriers, while there is the risk of washing out specific high yielding bacteria.     

Simulation of growth and detachment in biofilm systems under defined hydrodynamic conditions

Detachment from biofilms was evaluated using a mixed culture biofilm grown on primary wastewater in a tube reactor. The growth of biofilms and the detachment of biomass from biofilms are strongly influenced by hydrodynamic conditions. In a long-term study, three biofilms were cultivated in a biofilm tube reactor. The conducted experiments of biofilm growth and detachment can be divided into three phases: 1) an exponential phase with a rapid increase of the biofilm thickness, 2) a quasi-steady-state with spontaneous fluctuation of the biofilm thickness between 500 and 1,200 microm in the investigated biofilm systems, and 3) a washout experiment with increased shear stress in three to four steps after several weeks of quasi-steady-state. Whereas the biofilm thickness during the homogeneous growth phase can be regarded constant throughout the reactor, it was found to be very heterogeneous during the quasi-steady-state and the washout experiments. Growth and detachment during all three phases was simulated with the same one-dimensional biofilm model. For each of the three phases, a different detachment rate model was used. During the homogeneous growth phase, detachment was modeled proportional to the biofilm growth rate. During the quasi-steady-state phase, detachment was described by random detachment events assuming a base biofilm thickness. Finally, the washout experiment was simulated with detachment being a function of the biofilm thickness before the increase of the shear stress.     

The influence of vessel height and top-section size on the hydrodynamic characteristics of airlift fermentors

Fermentations of the yeast Saccharomyces cerevisiae were carried out in a 90 to 250-L working volume concentric tube airlift fermentor. Measurements of liquid circulation velocity, gas hold-up, and liquid mixing were made under varying conditions of gas flowrate, vessel height, and top-section size. Both liquid circulation velocity and mixing time increased with vessel height. Liquid velocity varied approximately in proportion to the square root of column height, supporting a theoretically based relationship. The effect of vessel height on gas hold-up was negligible. The height of the top-section had a significant effect on liquid mixing. Mixing time decreased with increasing size of the top-section up to a critical height. As the top-section was expanded beyond this height, little improvement in mixing was seen. This indicated the presence of a two-zone flow pattern in the top-section. Liquid velocity and gas hold-up were essentially independent of top-section height. (c) 1994 John Wiley & Sons, Inc.     

Effects of geometrical design on hydrodynamic and mass transfer characteristics of a rectangular-column airlift bioreactor

Gas holdup and gas–liquid mass transfer coefficients were measured in a 21-L rectangular-column airlift bioreactor with aspect ratio of 10 and working volumes ranging from 10 to 16 L. The effect of the bottom and top clearances was investigated using water and mineralized CMC solutions and covering a range of effective viscosity from 0.02 to 0.5 Pa s and surface tension from 0.065 to 0.085 N m⁻¹. The gas holdup and mass transfer results were successfully correlated using expressions derived via dimensional analysis. The separator gas holdup was found to be similar to the total gas holdup in the airlift bioreactor. The downcomer gas holdup (ɛ_d) increased two-fold when the bottom clearance (h_b) was increased from 0.014 to 0.094 m while the top clearance (h_t) had no effect. Increasing h_b decreased the mass transfer by 50% compared to 31% when the top clearance (h_t/D_hr) was increased. It was found that the gas–liquid separator diameter ratio (D_hs/D_hc) exerted the maximal influence of over 65% on mass transfer as compared to both clearances.     

Influence of surface topography on biofilm development: Experiment and modeling

The effect of surface topography on the long-term development (≈10 weeks) of biofilms has been investigated using a monitoring technique based on images produced by a flat-bed scanner and initially developed for flat surfaces. The biofilm response to rotation speed changes in lab-scale rotating biological contactors (RBCs) has been studied. Two RBCs, each containing five discs (two with flat surfaces and three with rough surfaces) were run initially at two different rotation speeds: 4 rpm for reactor I and 40 rpm for reactor II. After 47 days, the rotation speed was increased in reactor I to 40 rpm and decreased in reactor II to 4 rpm. Prior to the rotation speed change, the biofilm on the flat discs underwent large detachments in both reactors, but the biofilm on rough discs was less extensively damaged. The increase in rotation speed induced large detachments of the biofilm in reactor I on all discs, but the biofilm on the rough discs recovered more effectively with faster regrowth. In reactor II, the decrease in rotation speed favored the development of the biofilm. Wall stress distributions obtained from CFD simulations on flat and rough discs at different rotation speeds were well correlated with experimental observations.     

Interaction between biofilm development, structure and detachment in rotating annular reactors

In biotechnology, composition of biofilms and suspended bioaggregates can be crucial for system performance or product quality. Consequently, understanding biofilm dynamics is important for any process optimisation. The aim of this study was to investigate biofilm development and detachment under different hydrodynamic conditions and varying glucose load. Confocal laser scanning microscopy proved to be a fast method providing information about structure, distribution and volume ratio of bacteria and extra cellular polymers (EPS) within biofilms and detached biomass. As a result, it could be shown that biofilm structure, in terms of density and EPS volume, was largely influenced by hydrodynamic conditions. Furthermore, it was demonstrated that the EPS:bacteria ratio and distribution was largely influenced by substrate load. Finally, the characteristics in biofilm structure and development were reflected in the composition and quantity of the detached biomass.