Biofilms surfaces

The particle size and porosity of the filter media, since operating efficiency is directly related to the available biofilm surface area. 

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. 

The simple kinetics for uptake of soluble substrate of the bacteria in a biofilm is traditionally described by a combination of mass transport across the water/biofilm interface, transport in the biofilm itself and the corresponding relevant biotransformations. Transport through the stagnant water layer at the biofilm surface is described by Fick s first law of diffusion. Fick s second law of diffusion and Michaelis-Menten (Monod) kinetics are used for describing the combined transport and transformations in the biofilm itself (Williamson... 

The biological processes in biofilms are either described by 1-order or 0-order kinetics. However, the 0-order reaction is of specific importance for sewer biofilms as is also the case for treatment processes of wastewater in biofilters. The saturation constant, Ks, is normally insignificant compared with the substrate concentration, and the biofilm kinetics [cf. Equation (2.20)], is therefore 0-order. As shown in Figure 2.8, two different conditions exist the biofilm is either fully penetrated or partly penetrated, corresponding to either a fully effective or a partly effective biofilm. The distinction between these two situations can be expressed by means of a dimensionless constant, P, called the penetration ratio (Harremoes, 1978). For each of these two situations, the flux of substrate across the biofilm surface can neglect the stagnant liquid film being calculated [Equations (2.23) and (2.25)] ... 

Sw = substrate concentration in the bulk water phase (g m-3) ra = biofilm surface flux (g nr2 s-1, g nr2 h 1 or g nr2 d-1) ku2 = 1/2-order rate constant per unit biofilm surface area... 

Particles may be trapped on the biofilm surface or in voids of the biofilm where any organics may be hydrolyzed and further take part in the transformation processes. A number of factors influence adsorption and desorption of particles, such as particle size, surface charge, pH, etc., as well as biofilm surface properties and bulk water flow pattern. Studies of model biofilms have shown that water flows into the biofilm in small channels, making the prediction of transport of particles as well as soluble compounds complex (Norsker et al., 1995). 

Recently [ 152], a novel strategy has been described for controlling biofilms through generation of a biocide (hydrogen peroxide) at the biofilm-surface interface rather than simply applied extrinsically. In this procedure, the colonized surface incorporated a catalyst that generated active biocide from a treatment agent. 

As the biofilm develops, the nutrient availability to the bulk biofilm may become affected. The biofilm, despite its voids and channels, offers a further resistance to mass transfer. The cells within the biofilm consume nutrients that diffuse through the biofilm in response to the difference in concentration between nutrients at the biofilm surface and the cells attached to the conditioning layer. As a consequence, it is entirely possible that cells in the region of the solid surface are likely to become starved of nutrients. The properties of the biofilm may be different, therefore, in the layers where nutrient is available compared with the regions where there is little or no nutrient. For instance, the lack of oxygen may encourage anaerobic species to develop (some bacteria can exist as aerobes or anaerobes), with attendant changes to the quality of the biofilm. 

Here [g m" -h ] is the maximum mass flux through biofilm surface. The physical model used by these authors was the same as that proposed by Atkinson, extended to double S limitation (glucose and O2). Their authors case A corresponds to no S or O2 limitation, case B involves an inactive aerobic film (S limitation), and case C involves O2 limitation (partly anaerobic film). With a modified Thiele modulus and a pseudo-first-order approximation for the enzyme-catalyzed rate under the given surface conditions, Fig. 4.40 shows the result of the calculation of versus (/>j as a function of Sj /K under various conditions. For biofilm reactor operation, is more convenient in the form... 

Ammar Y, Swailes D, Bridgens B, Chen J. Influence of surface roughness on initial formation of biofilm. Surface Coating Technology 2015 284 410-6. 

The biofilm subsists on the oxidation of an organic substrate, S (mM), which is delivered to the biofilm matrix via diffusive mass transport. The substrate is supplied at a constant concentration to a large, well-mixed anodic chamber. This allowed us to assume that the bulk concentration of substrate is constant because of the size of the chamber and the relatively slow consumption rate of substrate by the biofilm. The concentration at the biofilm surface is equal to the bulk concentration because of mixing in the anodic chamber and because of simplification of the model. The substrate utilization rate is controlled by both the substrate (electron donor) concentration and the eleetron acceptor concentration, through multiplicative Monod substrate utilization equations [37, 38]. Equations 9.1 and 9.2 simply state that the biofilm can only metabolize in the presence of both an electron donor and an electron acceptor. The lack of either one prevents biofilm metabolic activity. In our model, we assume that there are two possible electron transfer pathways thus, there are two substrate utilization equations. For diffusion-based EET, substrate utilization is given by ... 

A commonly applied alternative to fighting pathogenic microbes with soluble biocides is to prevent their proliferation, which majorly occurs on surfaces composed of virtually any material in the form of biofilms. Surface modifications that prevent the formation of biofilms are not only hindering the spread of infections, but also protect the materials from deterioration. While nature has applied the concept of non-fouling surfaces, known from the excellent non-fouling activity of albumin, microbe-killing surfaces seem to be a concept beyond nature s imagination . 

Wood, P., Jones, M., Bhakoo, M. and Gilbert, P., 1996. A novel strategy for control of microbial biofilms through generation of biocide at the biofilm-surface interface. Applied and Environmental Microbiology, 62, 2598-2602. 

Let us take the simplifying approach that there is no external mass transfer limitation, as this approach will yield an upper limit on the chemical flux into a biofilm that is not limited by mass transfer outside the biofilm. To do that, we assume that the concentration at the biofilm surface is the bulk substrate concentration. 

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