Biofilms velocity effects

Among the physical factors, current velocity has a special significance for benthic biofilms because it can modulate the diffusion of metals through the biofilm and their effects [18, 40]. pH and organic complexation are particularly significant for metal bioavailability [42]. Therefore, metal toxicity will also depend on the influence that environmental variability has on its bioavailability. 

The effect of biofilm volume on the terminal settling velocity can be described by Eq. (9) (Andrews and Przezdziecki, 1986) ... 

Figure 11. Effect of biofilm growth on terminal settling velocity.

Detergency Biodispersants exert a detergency or cleaning effect by lowering the adhesion of biofilms and other slimes to the wood or metal substrate. The applied mechanical force (water velocity) aids in the removal of the matter. 

Beyenal, H., and Z. Lewandowski. 2000. Combined effect of substrate concentration and flow velocity on effective diffusivity in biofilms. Water Research 34 528-538. 

Gantzer, C. J., B. E. Rittmann, and E. E. Herricks. 1991. Effect of long-term water velocity changes on streambed biofilm activity. Water Research 25 15-20. 

Fig. 7 The effect of velocity on biofilm growth at constant concentration. (From Ref.. )...

Sly LI, Hodgkinson MC, Arunpairojana V. 1988. Effect of water velocity on the early development of manganese-depositing biofilm in a drinking water distribution system. FEMS Microbiol Ecol 53 175-186. 

Because of complex effect of pipe material, water quality and flow velocity on the biofilm in the studied water distribution system, the biofilm formation process was studied in a model system. It was determined the highest bacterial density of the biofilm developed on carbon steel and the lowest ones of the biofilms on plastic pipes (Fig. 2). The results showed strong influence of the pipe material on biofilm density during initial phases of the process compared with mature biofilm. 

It can be supposed that biofdms grew under higher flow velocities were more rigid and strongly attached because of shear stress and as a result bacteria detachment was reduced. Increasing impact of mature biofilms under higher flow velocities 0.7 m/s or 1 m/s found 175 day after the beginning of the process can be related to the critical value of biofdm thickness reached to and shear stress effect on the biofdms surface layer. 

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

The effect of velocity on the development of the biofilm affects its density [Bott and Pinheiro 1977, Vieira et al 1993], At higher velocities the biofilm density is increased and this will in turn, affect the mass transfer through the biofilm as the higher density biofilm will offer a greater resistance to molecular difiusion. 

The discussion so far has concentrated on the effects of velocity but another variable of considerable influence on the development of biofilms is temperature. In Section 12.2 it was stated that all micro-organisms have an optimum temperature for maximum growth. 

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. 

Pujo, M. and Bott, T.R., 1991, Effects of fluid velocities and Reynolds Numbers on biofilm development in water systems, in Keffer, J.F., Shah, R.K. and Ganic, E.N. eds. Experimental Heat Transfer, Fluid Mechanics and Thermodynamics. Elsevier, New York, 1358 - 1362. 

The effect of flow velocity on biofilm removal is illustrated on Fig. 14.14 [Nesaratnam and Bott 1984]. The curves show that as the velocity is increased the rate of removal of established biofilm increases, and the apparent final biofilm thickness is less. There are two effects related to velocity and include ... 

FIGURE 14.14. The effect of flow velocity on the removal of biofilms ot Pseudomonas fluorescens using sodium hypochlorite. 

Low levels of residual chlorine may be regarded as a biostat, i.e. preventing colonisation or restricting biofilm development. In a particular laboratory experiment [Kaur et al 1988] chlorine levels as low as 0.2 - 0.6 mg/l inhibited biofilm attachment and growth. The effect of flow velocity and chlorine concentration are shown on Fig. 14.14. 

The effect of velocity on biofilms subject to glutaraldehyde treatment, will be similar to its effect with other biocides described in this chapter, i.e. the higher the fluid stress the higher the effectiveness of a given concentration of glutaraldehyde. 

Nesaratnam, R.N. and Bott, T.R., 1984, Effects of velocity and sodium hypochlorite derived chlorine concentration on biofilm removal from aluminium tubes. Proc. Biochem. 19,14 -18. 

Higher cross-flow velocity provides higher shear forces, and thus, a thinner biofilm. However, the shear forces of the turbulent flow do not penetrate the viscous sublayer and affect the bacterial monolayer. Fluid velocities in SW modules have very Uttle effect on impeding the initial rate of microbial (or colloidal) foulant deposition, although they can reduce the thickness of the fouling layer. 

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