L. F. Melo

Nitrifying and heterotrophic population dynamics in biofilm reactors: effects of hydraulic retention time and the presence of organic carbon.

Two biofilm reactors operated with hydraulic retention times of 0.8 and 5.0 h were used to study the links between population dynamics and reactor operation performance during a shift in process operation from pure nitrification to combined nitrification and organic carbon removal. The ammonium and the organic carbon loads were identical for both reactors. The composition and dynamics of the microbial consortia were quantified by fluorescence in situ hybridization (FISH) with rRNA-targeted oligonucleotide probes combined with confocal laser scanning microscopy, and digital image analysis. In contrast to past research, after addition of acetate as organic carbon nitrification performance decreased more drastically in the reactor with longer hydraulic retention time. FISH analysis showed that this effect was caused by the unexpected formation of a heterotrophic microorganism layer on top of the nitrifying biofilm that limited nitrifiers oxygen supply. Our results demonstrate that extension of the hydraulic retention time might be insufficient to improve combined nitrification and organic carbon removal in biofilm reactors.

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.

Biofilm formation: hydrodynamic effects on internal diffusion and structure

Diffusion in microbial films produced by Pseudomonas fluorescens under turbulent flow conditions was studied using an inert substance (LiCl). Mass transfer coefficients in the biofilm were measured during formation of the biological deposits and for biofilms developed under different fluid velocities. Mass transfer rates in the biofilm decreased with time, and more quickly in the case of biofilms subjected to high shear stresses. The latter show lower final thicknesses and lower internal diffusivities. The so‐called “active layer”;, if it exists, does not seem to have a fixed thickness (as proposed by some authors), since it will depend on the environmental conditions, particularly on fluid velocities.

Effect of flow regime on the architecture of a Pseudomonas fluorescens biofilm.

A comparison of the effects of laminar versus turbulent flow regime on the characteristics of a single-species biofilm is presented. The study was carried out by growing Pseudomonas fluorescens biofilms in a flow cell and studying the different layers of the biological matrix with a confocal laser-scanning microscope. The following conclusions were obtained: i) a higher concentration of cells was found in the upper layers of the microbial films than in their inner layers, regardless of the flow regime; ii) the fraction of cells in the overall biofilm mass decreased with time as the film grew; and iii) under laminar flow the total number of cells was higher than in biofilms formed under turbulent flow, but the latter had a higher number of cells per unit volume. Such conclusions, together with the fact that the biofilms were more dense and stable when formed in contact with turbulent flows, favor the design of more compact and efficient biofilm reactors operating in turbulent conditions.

Flow cell hydrodynamics and their effects on E. coli biofilm formation under different nutrient conditions and turbulent flow

Biofilm formation is a major factor in the growth and spread of both desirable and undesirable bacteria as well as in fouling and corrosion. In order to simulate biofilm formation in industrial settings a flow cell system coupled to a recirculating tank was used to study the effect of a high (550 mg glucose l−1) and a low (150 mg glucose l−1) nutrient concentration on the relative growth of planktonic and attached biofilm cells of Escherichia coli JM109(DE3). Biofilms were obtained under turbulent flow (a Reynolds number of 6000) and the hydrodynamic conditions of the flow cell were simulated by using computational fluid dynamics. Under these conditions, the flow cell was subjected to wall shear stresses of 0.6 Pa and an average flow velocity of 0.4 m s−1 was reached. The system was validated by studying flow development on the flow cell and the applicability of chemostat model assumptions. Full development of the flow was assessed by analysis of velocity profiles and by monitoring the maximum and average wall shear stresses. The validity of the chemostat model assumptions was performed through residence time analysis and identification of biofilm forming areas. These latter results were obtained through wall shear stress analysis of the system and also by assessment of the free energy of interaction between E. coli and the surfaces. The results show that when the system was fed with a high nutrient concentration, planktonic cell growth was favored. Additionally, the results confirm that biofilms adapt their architecture in order to cope with the hydrodynamic conditions and nutrient availability. These results suggest that until a certain thickness was reached nutrient availability dictated biofilm architecture but when that critical thickness was exceeded mechanical resistance to shear stress (ie biofilm cohesion) became more important.

Online Biofilm Monitoring

A review of online biofilm monitoring techniques is presented focusing on methods based on differential turbidimetry, light scattering, heat transfer, pressure drop, real-time measurement of metabolic products, image analysis, radiation signals (spectroscopy, fluorometry, photoacoustic spectroscopy, etc.), electric and mechanical (vibration) signals. The different methods are compared in terms of their applicability to practical situations and the know detection limits are reported.

Physiological changes induced by the quaternary ammonium compound benzyldimethyldodecylammonium chloride on Pseudomonas fluorescens

OBJECTIVES Antimicrobial resistance is a major public health concern, particularly in hospitals and other healthcare settings. For the rational design of disinfection strategies, it is of utmost importance to understand the mechanisms of action of antimicrobials. In this study, the mechanism of action of benzyldimethyldodecylammonium chloride (BDMDAC) was assessed against Pseudomonas fluorescens. METHODS The targets of antimicrobial action were studied using different bacterial physiological indices. The MIC, MBC, membrane permeabilization, intracellular potassium release, physico-chemical surface properties, surface charge, outer membrane protein (OMP) expression and morphological changes were assessed after BDMDAC exposure. RESULTS The MIC was found to be 20 mg/L and the MBC was 10 mg/L. BDMDAC led to a significant change in cell surface hydrophobicity and induced propidium iodide uptake. Such results suggest cytoplasmic membrane damage, corroborated by the release of intracellular potassium. The results obtained from the zeta potential measurement demonstrate a -31.2 mV value for untreated cells and -21.0 mV for cells at the MIC. Scanning electron microscopy revealed that cells treated with 20 mg/L were less bulky, and their membrane seemed to be rougher, wrinkled and deformed when compared with untreated cells. The overall bactericidal events occurred without detectable changes in OMP expression. CONCLUSIONS BDMDAC is an effective biocide against P. fluorescens. It binds by ionic and hydrophobic interactions to the cell membrane, causing changes in membrane properties and function, as manifested by phenomena such as cellular disruption and loss of membrane integrity with consequent leakage of essential intracellular constituents.

Polysaccharide production and biofilm formation by Pseudomonas fluorescen: effects of pH and surface material

Abstract Although the synthesis of extracellular polysaccharides was first recognized in certain bacterial cultures a long time ago, its role in bacterial adhesion is still subject to some debate. Several fermentation batch cultures were performed under different conditions of pH (pH 7, maintained with NaOH and HCl; pH 7 in phosphate buffer, and without pH control) in order to study the relation between the production of extracellular polysaccharides and biofilm formation on polymeric slides suspended in the culture medium. The polymers used were polystyrene, polypropylene, polyethylene and poly(vinyl chloride). The maximum amount of exopolysaccharides in the culture medium occurs at pH 7, although slightly thicker biofilms seem to be formed when there is no pH control. The biofilms were analysed by scanning electron microscopy and by wavelength dispersion spectroscopy. Biofilm morphology seems to be much more dependent on the type of substratium than on the pH of the medium; for different pH values, a polymeric network can be more clearly observed on biofilms formed on all surfaces except poly(vinyl chloride).

Feasibility of a pulsed Sequencing Batch Reactor with anaerobic aggregated biomass for the treatment of low strength wastewaters

This study concerns an assessment of a SBR operation that associates anaerobic aggregated biomass with a pulsed action during the reaction phase, a system named Pulsed Sequencing Batch Reactor (P-SBR). The system uses a diaphragm pump as a pulsator unit to increase the liquid-solid contact, in order to avoid dead zones and possible external mass transfer resistance. A preliminary study of the operation of the reactor was performed with a low strength synthetic wastewater with a COD near 1000 mg.1 −1 and a sub-optimal temperature of 22°C. A removal efficiency of 60-70% was attained after 5 and 6 hours of reaction time. The respective organic loads were 5 – 6 kg COD.m −3 . day −1 , thus supporting the feasibility of the P-SBR system for wastewater treatment in such conditions. The results also indicate that a ratio of 1.8%o between the swept volume delivered by the pump and the reactor volume was adequate to promote a flow turbulence in the sludge blanket and that a redox potential of near −400 mV was readily created by anaerobic bacteria after the reactor filling step.

Heat transfer and rheology of stirred yoghurt during cooling in plate heat exchangers

In the present work an experimental investigation was conducted to obtain a correlation for the determination of convective heat transfer coefficients of stirred yoghurt in a plate heat exchanger. A rheological study was carried out in order to characterise the stirred yoghurt flow behaviour, evaluating its dependency both on shear rate and temperature. A shift in the temperature dependency was evidenced at 25 °C. It is also shown that the material shows a complex flow behaviour, changing from a Bingham fluid to a power-law fluid at shear stresses in excess of approximately 6.7 Pa. As regards the heat transfer behaviour of the non-Newtonian stirred yoghurt a correlation for the convective heat transfer coefficient was obtained that reveals the large effects of the thermal entry length due to the high Prandtl numbers and to the short length of the plate heat exchanger.