Biofilm growth

Fig. 5.1.8 MRM maps of velocity (bottom row) resolution is 54.7 im per pixel (128 x 128 -and T2 magnetic relaxation (top row) as a pixels) in plane, so the data reflect pore scale function of biofilm growth time (left to right), spatial distributions of velocity over a 1000-pm Day 1 shows the clean porous media. Spatial slice and biomass over a 200-gm slice.

This equation is plotted in Fig. 11, showing that for relatively dense support particles, biofilm growth can reduce the settling velocity if the biofilm density is less than that of the biofilm-free particle. As such bioparticles gain biomass, they will rise to the top of the bed and may even elutriate from the reactor (Sreekrishnan et al., 1991 Myska and Svec, 1994), reducing achievable conversion rates. This situation could be resolved by using lower density particles, such as expanded polystyrene or... 

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

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 choice of solid carriers spans a wide spectrum (Table 1) from materials most suitable for research purposes (sintered glass beads, laterite stone deposited on a gramophone disk) to industrial materials (pumice, activated carbon, etc.). Key properties that affect the performance of the carrier are porosity (from impervious to controlled-size pores), composition (from ceramics to activated carbon), and hydrophilic behavior. It is difficult to perform a direct comparison of different carriers. Colonization and biofilm growth depend strongly on the nature of bacteria and on their intrinsic propensity to adhere on hydrophilic vs. hydrophobic surfaces. 

Fluidized beds, both in the conventional and in the airlift configurations, require more careful operation. Proper selection of the operating conditions makes it possible to control biofilm-growth while preventing reactor clogging. Typically, the reactor is operated as a CSTR by establishing large recycle of the liquid stream [36, 37],... 

The continuous exchange of cells between the biofilm (X) and the liquid phase (Xl) was described by means of a combined attachment/detachment mechanism. The net rate of detachment balances biofilm growth under steady state conditions. 

Nielsen, P.H., K. Raunkjaer, N.H. Norsker, N.Aa. Jensen, and T. Hvitved-Jacobsen (1992), Transformation of wastewater in sewer systems —Areview, Water Sci. Tech., 25(6), 17-31. Norsker, N.-H., P.H. Nielsen, and T. Hvitved-Jacobsen (1995), Influence of oxygen on biofilm growth and potential sulfate reduction in gravity sewer biofilm, Water Sci. Tech., 31(7),... 

Details concerning biofilm growth and activity in sewers are at present not available to the same extent as is the case for the suspended biomass. Therefore, a simple expression for biofilm growth and respiration compared to other well-known deterministic biofilm models, e.g., as described by Gujer and Wanner (1990), is selected. 

Aesoey, A., M. Storfjell, L. Mellgren, H. Helness, G. Thorvaldsen, H. Oedegaard, and G. Bentzen (1997), A comparison of biofilm growth and water quality changes in sewers with anoxic and anaerobic (septic) conditions, Water Sci. Tech., 36(1), 303—310. 

Norsker, N.-H., P.H. Nielsen, and T. Hvitved-Jacobsen (1995), Influence of oxygen on biofilm growth and potential sulfate reduction in gravity sewer biofilm, Water Sci. Tech., 31(7), 159-167. 

Measurement of biofilm activity can be performed based on laboratory reactor experiments or with a technique combining biofilm growth taking place in a sewer followed by measurements in laboratory scale (Raunkjaer et al., 1997 Bjerre et al., 1998). Huisman et al. (1999) developed a sewer in situ biofilm respiration chamber. It includes a DO sensor and a chamber that can be pressed onto the sewer wall. It is designed to achieve an even and unidirectional flow distribution over the entire measurement area. Pure oxygen is injected for oxygenation. 

Microbial fouling is best dealt with before biofilm becomes mature. Biofilm protects the microorganisms from the action of shear forces and biocidal chemicals used to attack them. Microbes can be destroyed using chlorine, ozone, ultraviolet radiation, or some non-oxidizing biocides (see Chapters 8.2.1,8.2.2, 8.1.8, and 8.2.5, respectively). An effective method to control bacteria and biofilm growth usually involves a combination of these measures. Specifically, chlorination or ozonation of the pretreatment system, followed by dechlorination to protect the membranes, or UV distraction followed by periodic sanitation with a non-oxidizing biocide used directly on the membranes. 

Many operating costs are the same as those for any conventional chemical production process, such as operating labor, raw materials, and equipment maintenance. However, some expenses, such as cleaning optical surfaces or preventing biofilm growth on the photobioreactor surfaces, are unique to photobiological processes. 

Electroless Ag-PTFE composite coatings were prepared on stainless steel substrates for the prevention of biofilm growth.87 The anticorrosion properties of Ag-PTFE composite coatings were investigated in 0.9% NaCl solution. The results showed that the corrosion resistance of the Ag-PTFE composite coating was superior to that of stainless steel. 

The leveling off of the biofilm growth is attributed to a balance between growth and removal, depending on nutrient availability and temperature on the one hand, and shear forces caused by water flow on the other. 

Fig. 3 Idealized concept of biofilm growth on a surface with time. (From Ref...

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