Preventing biofouling in heat exchangers: an experimental assessment of the effects of water velocity and inorganic particles on deposit detachment

Biofouling is a costly problem in heat exchangers. Biocides can be used to minimize the formation of biofilms, but they are not always effective and, moreover, they are generally deleterious to the environment. The use of proper liquid velocities or of water jets in the exchanger tubes is also a means to prevent the build up of fouling deposits or to clean the surface once they are formed. Often, biofilms incorporate inorganic particles which modify the physical properties of the deposit and, thus, affect the effectiveness of anti-fouling measures. This paper presents experimental data that show the effects of the water velocity and of the presence of clay particles on the accumulation of biofilms and on their mechanical resistance to detachment caused by hydrodynamic forces. The results indicate that the fraction of dry biomass (micro-organisms plus extracellular biopolymers) in biofilms increases with the liquid velocity and that the deposits formed under higher hydrodynamic forces are more resistant to detachment. The resistance to detachment is even greater when the biofilms incorporate small (20 micrometer) clay particles

Intrinsic kinetics of biofilms formed under turbulent flow and low substrate concentrations

Reactor operating conditions strongly affect the behaviour of biofilm systems, namely their stability and the substrate removal. In this paper, the penetration of substrate and the activity of biofilms formed by Pseudomonas fluorescens under turbulent flow and low substrate concentrations, are studied. A first order diffusion-reaction model was applied to results of biofilm accumulation in steady and non-steady-state. The substrate consumption rate of the biofilm was calculated based on the on-line determination of the biofilm accumulated on the surface. This approach is important when the residence time or the substrate concentration on the reactor is very low. Also, the mass transfer of substrate inside the biofilm was measured for every case under study and introduced in the model. The fraction of biofilm penetrated by the substrate depends on the velocity of the fluid that contacts the biological matrix: contrary to biofilms formed at higher velocities, lower velocities give raise to non completely penetrated biofilms. This fact seems to be associated to the biofilm internal structure in terms of biomass density and compactness of the matrix. They remove more substrate per reactor volume, but are less resistant from an hydrodynamic point of view. In conclusion, biofilms formed at higher velocities in turbulent flow allow a more stable reactor operation.Fundação para a Ciência e a Tecnologia (FCT) - PRAXIS XXI, 2.1/BIO/37/94. União Europeia - Fundo de Desenvolvimento Regional Europeu (FDRE) - INTERREG, 01/REG II/6/96

Physical stability and biological activity of biofilms under turbulent flow and low substrate concentration

The paper focuses on biofilms subject to turbulent flow and high liquid velocity (of the order of 1 m s ˉ¹) which can be found in heat exchangers, water distribution systems and in some wastewater reactors. An overall model describing biofilm development is presented, which includes the effects of biomass detachment due to the hydrodynamic forces. A methodology for estimating substrate consumption from data obtained through continuous monitoring of biofilm growth is presented. Results show that the physical stability of the biofilm increases with the liquid velocity, while the rate of substrate consumption decreases.Fundação para a Ciência e a Tecnologia - PRAXIS XXI, 2.1/BIO/37/94 . Uniãoo Europeia - Fundo de Desenvolvimento Regional Europeu - Programme INTERREG - 01/REG II/6/9

Comparison of the Spherical Averaged Pseudopotential Model with the Stabilized Jellium Model

We compare Kohn-Sham results (density, cohesive energy, size and effect of charging) of the Spherical Averaged Pseudopotential Model with the Stabilized Jellium Model for clusters of sodium and aluminum with less than 20 atoms. We find that the Stabilized Jellium Model, although conceptually and practically more simple, gives better results for the cohesive energy and the elastic stiffness. We use the Local Density Approximation as well as the Generalized Gradient Approximation to the exchange and correlation energies.Comment: 13 pages, latex, 8 figures, compressed postscript version available at http://www.fis.uc.pt/~vieir

Chemical composition and activity of a biofilm during the start-up of an airlift reactor

For the successful operation of a biofilm reactor, it is important to characterise the fixed biomass, its activity and composition. The purpose of this research was to monitor the biofilm characteristics of an airlift reactor, namely its composition in terms of exopolymers (polysaccharides and proteins), attached biomass, thickness, total proteins, as well as its activity in terms of substrate consumption rates. During the early phases of biofilm development, a high exopolymer production was observed, which helped in promoting the initial cell adhesion. The high turbulence with strong abrasive conditions might also trigger the excretion of those polymers. Low levels of extracellular proteins were detected, showing that the proteins in the biofilm were mainly intracellular. The biofilm accumulation on the carrier was accompanied by an increased biological activity; however, at the end of the experiment, the specific substrate consumption rate decreased, probably because of a reduction of the cell density within the biofilm.Fundação para a Ciência e a Tecnologia (FCT) – PRAXIS XXI/BM/6699/95

Biological kinetics parameters in a pure culture biofilm developed in an airlift reactor

A diffusion-reaction model was fitted to data obtained with Pseudomonas fluorescens biofilms developed in an airlift reactor under different limiting substrate conditions, in order to determine the biofilm kinetic constants and the substrate concentration profiles within the biological films. Model predicted concentration profiles within the biofilms demonstrate that all films were completely penetrated by the substrate and that the reaction rate inside the biofilms was of zero order. The estimated kinetic constants (μmax = 0.24 h-1; Ks = 0.73x10-3 kg/m3) differ from those obtained in a suspended culture (μmax= 0.31 h-1; Ks = 6.21 kg/m3), as a result of the different metabolic state of microorganisms within biofilms

Biocorrosion of mild steel in drinking water conditions and disinfection

[Excerpt] Introduction: Corrosion in drinking water distribution system is a costly phenomenon, mainly due to the replacement of altered pipes. Bio..corrosion is also a problem in term of public health because of the suspected protection brought by the corroded surface to potentially harmful microorganisms, especially bacteria. The protection effect of corrosion is particularly relevant in the presence of disinfectant. In drinking water are present the conditions leading to microbialy induced corrosion: bacteria and metal-containing substrata joined closely together as biofilm attached to distribution system pipe walls. [...]info:eu-repo/semantics/publishedVersio