Biofilm reactors

After presenting the concept of biofilms, reference is made to their importance in industry and health. Although biofilms are also well known for their deleterious effects (biofouling), emphasis is here given to the beneficial use of biofilms in wastewater treatment. The main types of biofilm reactors are briefly described and the role of support material in the adhesion and stability of biofilms is explained, taking into account the mechanisms involved in biofilm attachment. Practical procedures for the start-up of biofilm reactors are also mentioned. Biofilm growth processes are described together with their properties, structure and Performance. The advantages and disadvantages of biofilm reactors versus suspended biomass systems are discussed. The main equations of the diffusion-reaction model are developed from engineering science principles. Equations derived from the diffusion-reaction model to calculate the reactor volume are presented, together with experimental values of the kinetic parameters. Practical empirical expressions or rules-of-thumb used in the design of fixed biomass reactors are also given. An overall model to predict the growth rate of biofilms and their final thickness or mass is established. The main problems concerning biofilm reactor modelling are discussed and the "missing links" for an optimised design are identified

Ripples in Tapped or Blown Powder

We observe ripples forming on the surface of a granular powder in a container submitted from below to a series of brief and distinct shocks. After a few taps, the pattern turns out to be stable against any further shock of the same amplitude. We find experimentally that the characteristic wavelength of the pattern is proportional to the amplitude of the shocks. Starting from consideration involving Darcy's law for air flow through the porous granulate and avalanche properties, we build up a semi-quantitative model which fits satisfactorily the set of experimental observations as well as a couple of additional experiments.Comment: 7 pages, four postscript figures, submitted PRL 11/19/9

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

Biofouling in water systems

The paper describes the mechanisms in the development of biofouling layers (initial surface conditioning, microbial transport and attachment, mass transfer of nutrients to the biofilm surface and through the microbial layer, cell metabolism, and detachment of cells and of larger parts of the biofilm) and summarizes the effects of several factors on the buildup and stability of biofilms (nutrient availability, fluid velocity and turbulence, temperature, surface condition, and nonliving particles). Mass transfer within biofilms is treated in more detail. A biofouling model applied to the development of biofilms in heat exchangers is presented. Finally, references are made to biofouling control methods (biocide and the proper design and operation of heat exchangers) and to future research needs in this area

Environmental technology education in Portugal : analysis and perspectives

The scope of the present communication is the current status of environmental engineering in Portugal. The different approaches concerning the environmental engineering courses available at undergraduate levels as well as some post-graduation studies are described. Furthermore, an analysis based on strengths, weakness, opportunities and trend issues regarding the environmental engineering profession in Portugal is presented