Biofilm thickness

The biofilm thickness is small compared to the diameter of the substrate therefore, the biofilm can be regarded as a flat surface. 

Freitas dos Santos, L.M., Livingston, A.G., Membrane attached biofilms novel technique for measurement of biofilm thickness, density and diffusivity, Proc. lAWQ Conf. Workshop on Biofilm Structure, Growth and Dynamics - Need for New Concept , Noordwijkerjout, The Netherlands, August 1995. 

Freitas dos Santos, L.M., Pavasant, P., Pistikopoulos, E.N., Livingston, A.G., Growth of immobilised cells results and predictions for membrane-attached biofilms using a novel in situ biofilm thickness measurement technique. Immobilised Cells Basics and Applications, Ed. Wijfells, R.H., Buitelaar, R.M., Bucke, C., Tramper, J., Progess in Biotech. 11, pp.290-297, 1996. 

Detachment includes two processes erosion and sloughing. Sloughing is a process in which large pieces of biofilm are rapidly removed, frequently exposing the surface. The causes are not well understood. Biofilm erosion is defined as continuous removal of single cells or small groups of cells from the biofilm surface and is related to shear stress at the biofilm/fluid interface. An increase in shear stress increases the erosion rate and decreases the biofilm accumulation rate. Empirical observations indicate that the erosion rate is related to biofilm thickness and density. 

The basic biofilm model149,150 idealizes a biofilm as a homogeneous matrix of bacteria and the extracellular polymers that bind the bacteria together and to the surface. A Monod equation describes substrate use molecular diffusion within the biofilm is described by Fick s second law and mass transfer from the solution to the biofilm surface is modeled with a solute-diffusion layer. Six kinetic parameters (several of which can be estimated from theoretical considerations and others of which must be derived empirically) and the biofilm thickness must be known to calculate the movement of substrate into the biofilm. 

Particle sizes used for aerobic waste treatment generally range from 100-1000 pm in size, with most being less than 500 pm in diameter, and reported biofilm thicknesses range from 40-1200 pm, with 100-200 pm being typical (Fan, 1989). 

Modeling studies have shown that when the bed is not monosized and the difference in particle sizes is great enough, it is possible that the large particles will not reach a lower settling velocity than the small particles hence the large particles will remain in the bottom of the bed (Myska and Svec, 1994). If biofilm thickness control is desired, removal of particles at a location other than the free surface would have to be implemented in this case. 

Intraparticle Mass Transfer. One way biofilm growth alters bioreactor performance is by changing the effectiveness factor, defined as the actual substrate conversion divided by the maximum possible conversion in the volume occupied by the particle without mass transfer limitation. An optimal biofilm thickness exists for a given particle, above or below which the particle effectiveness factor and reactor productivity decrease. As the particle size increases, the maximum effectiveness factor possible decreases (Andrews and Przezdziecki, 1986). If sufficient kinetic and physical data are available, the optimal biofilm thickness for optimal effectiveness can be determined through various models for a given particle size (Andrews, 1988 Ruggeri et al., 1994), and biofilm erosion can be controlled to maintain this thickness. The determination of the effectiveness factor for various sized particles with changing biofilm thickness is well-described in the literature (Fan, 1989 Andrews, 1988)... 

The biofilm thickness (Lf) and density (X = 50 g/L) were assumed uniform and the biofilm treated as a continuum. A substrate diffusion-reaction model assuming spherical particle was used. Diffusion coefficient of phenol and oxygen in the biofilm were assessed according to Fan et al. [64] ... 

Figure 7 shows phenol, dye, oxygen and suspended biomass concentrations and biofilm thickness as a function of time. 

Aerobic phase Steady state values of phenol concentration (40 mg/L) and biofilm thickness (170 pm) were approached after a 5 h transient period, which reproduces fairly well the experimental dynamical patterns reported in Fig. 6. However, biomass was present also in the liquid phase as a consequence of biofilm detachment. 

In spite of all of this variety of approaches, covering a wide array of metabolism pathways, limitations also exist. Differences in the vulnerability of biofilms have been found to depend on the age, community composition and succession status of the community. In dense biofilms the transfer of contaminants may be limited, resulting in decreased bioavailable concentrations of nutrients or toxicants for the algae. Biofilms show an inverse relationship between metal toxicity and biomass accrual [26], and a similar relationship has been established with nutrients. Therefore, the colonisation time or biofilm thickness are relevant factors to be included in the procedure uses. 

Station (Distance in km from Treatment Plant) Typical COD Value (gCOD nr3) Average DO Removal Rate r = k-So (g02 nrr2 lr1) fl2 DO Concentration (g02 m-3) Biofilm Thickness (pm)... 

The potential production of sulfide depends on the biofilm thickness. If the flow velocity in a pressure main is over 0.8-1 ms-1, the corresponding biofilm is rather thin, typically 100-300 pm. However, high velocities also reduce the thickness of the diffusional boundary layer and the resistance against transport of substrates and products across the biofilm/water interphase. Totally, a high flow velocity will normally reduce the potential for sulfide formation. Furthermore, the flow conditions affect the air-water exchange processes, e.g., the emission of hydrogen sulfide (cf. Chapter 4). 

The thickness of the sewer biofilm affects sulfide formation. Reduction of the biofilm thickness by increasing the wastewater velocity may lead to reduced sulfide problems. At very low velocities in an arerobic gravity sewer, a biofilm thickness may be more than 50 mm however, it may be substantially reduced to typically 1-5 mm when the velocity is increased. The thickness of... 

The vendor cites the following advantages of the GAC-FBR system a large surface area for biomass attachment high biomass concentrations and the ability to control and optimize biofilm thickness minimal plugging or channeling no off-gas production low hydraulic retention times biomass carrier can be tailored to optimize system performance and skid-mounted units with small footprints are available for most applications. 

The pore wall consists of metal oxides. Its reactive part is consumed in proportion to depth because of metabolic activity within a biofilm. The biofilm separates the bulk solution from the pore wall. The horizontal biofilm concentration profiles on the left side of Figure 6 correspond to the center of each of the five boxes. (Concentration is on the vertical axis and biofilm thickness is on the horizontal axis.) Excess organic matter is assumed within the upper few centimeters. The arrows indicate net flux densities of various substances in and out of the biofilm. 

Biofilms that become too thick, however, may prevent full penetration of substrates the reaction rates become more dependent on the diffusion of substrates inside the film. Therefore, procedures for controlHng the biofilm thickness may be necessary in some reactor systems [53]. Oxygen may also become limited within such biofilms, reducing transformation rates of MTBE. It is possible to estimate whether oxygen or MTBE is Hmited inside a biofilm by the following expression [49] ... 

Equations 5.1 and 5.4 relate the physical properties of a fluid with the velocity in the system. In general terms as the bulk velocity is increased the velocity gradient increases since the velocity near the solid surface is extremely low or zero. Increased velocity gradient in turn increases the shear force near the deposit. It is this shear force that is often regarded as the force for the removal of solid material from the surface. Indeed if the velocity across a deposit of loosely bound particles is increased, the deposit thickness decreases [Hussain 1982]. Biofilm thickness decreases with velocities above about 1 i5- [Miller 1979] and deposits in combustion systems are dependent on velocity [Tsados 1986]. 

Data reported in the literature are often obtained from tubes with similar diameters operated at different velocities, i.e. the Reynolds numbers (and hence the turbulence level) were different for each tube. Work by Pujo and Bott [1991] sought to examine the effect of velocity at fixed Reynolds number. An example of their work is shown on Fig. 12.11. For a given Reynolds number of 12,200 it may be seen that velocity does still affect the extent of the biofilm. Biofilm thickness passes through a peak value with increasing velocity till at a velocity of 2 m/s the biofilm thickness is severely limited and may be attributed to the shear effects. 

FIGURE 12.9. The change in biofilm thickness with velocity for Pseudomonas fluorescens growing on aluminium tubes... 

FIGURE 12.10. The effect of velocity on plateau values of biofilm thickness... 

Fig. 12.15 shows a remarkable increase in biofilm thickness for only 5°C temperature change. These data were obtained for Escherichia coli in an experimental apparatus with flow Reynolds number of 6.3 x 10, i.e. turbulent [Bott and Pinheiro 1977]. The optimum temperature for maximum growth for this species is around 35 - 40°C. Work by Harty [1980] using the same species (Escherichia coli) demonstrated similar effects. Fig. 12.16 also shows that as velocity is increased (i.e. increased shear at the surface) in the particular system studied, the effect of temperature becomes less. It would be expected that the effects of shear are likely to be more pronounced than the effects of temperature on growth. 

FIGURE 12.16. Maximum biofilm thickness versus velocity for temperatures 37 and 42°C... 

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