GDM ist eine einfache, wartungsfreie Technologie für die Trinkwasserversorgung ohne Elektrizität und Chemie.
La gestion durable d'eaux potable et usées appliquée et mise en pratique dans le bâtiment modulaire NEST.
[[ element.title ]]
[[ element.title ]]
Biofilm models for the practitioner
Even though mathematical biofilm models are extensively used in biofilm research, there has been very little application of these models in the engineering practice so far. However, practitioners would be interested in models that can be used as tools to control plant operation under dynamic conditions or to help them handle complex interactions between particle removal, carbon oxidation, nitrification, denitrification and biological phosphorus removal. But even though there is a whole range of biofilm models available, it is difficult for the practitioner to select the appropriate modeling approach. Practitioners, experimenters and modelers should work together to identify the important processes that shoud be included in models. Guidance for model selection, calibration and application should be provided.
AQUASIM is a computer program for the identification and simulation of aquatic systems. The program includes a one-dimensional multisubstrate and multispecies biofilm model and represents a suitable tool for biofilm simulation. The program can be used to calculate substrate removal in biofilm reactors for any user specified microbial system. One-dimensional spatial profiles of substrates and microbial species in the biofilm can be predicted. The program also calculates the development of the biofilm thickness and of the substrates and microbial species in the biofilm and in the bulk fluid over time. Detachment and attachment of microbial cells at the biofilm surface and in the biofilm interior can be considered, and simulations of sloughing events can be performed. Furthermore, AQUASIM allows pseudo two-dimensional modeling of plug flow biofilm reactors by a series of biofilm reactor compartments. The most significant limitation of the model is that it only considers spatial gradients of substrates and microbial species in the biofilm in the direction perpendicular to the substratum.
Influence of shear on the production of extracellular polymeric substances in membrane bioreactors
Shear is used to control fouling in membrane bioreactor (MBR) systems. However, shear also influences the physicochemical and biological properties of MBR biomass. The current study examines the relationship between the level of shear and extracellular polymeric substance (EPS) production in MBRs. Two identical MBRs were operated in parallel where the biomass in one reactor was exposed to seven times greater shear forces. The concentrations of floc-associated and soluble EPS were monitored for the duration of the experiment. The stickiness of extracted floc-associated EPS from each reactor was also characterized using atomic force microscopy. A mathematical model of floc-associated and soluble EPS production was applied to quantitatively describe changes in EPS production with shear. Biomass grown in a high shear environment has lower floc-associated EPS production compared to biomass grown in a lower shear environment. This decrease in floc-associated EPS production also corresponds to a decrease in soluble EPS production, which can be explained by both the lower concentration of floc-associated EPS and the production of stickier floc-associated EPS that is more erosion resistant in the high shear reactor. This research suggests that mechanical stresses can have a significant impact on the production rates of floc-associated and soluble EPS—key parameters governing membrane fouling in MBRs.
Analyzing characteristic length scales in biofilm structures
The quantification of biofilm structure based on image analysis requires a statistical measure like representative elemental areas (REA) to determine the necessary size of biofilm area to be imaged. In this study, REAs for biofilm structure were calculated for the descriptors Gray level and Correlation (COR) derived from a spatial gray level dependence matrix analysis (SGLDM). An important difference between these two descriptors is their response to structural features at different spatial scales. Gray level is a scale-independent descriptor, whereas COR is scale-dependent. For scale-independent descriptors, the size of the individual images is not relevant when determining REAs. This is in contrast to scale-dependent descriptors for which REAs can only be determined when the area of each image covers the range of structural variability of the biofilm. We used COR to analyze scale dependence of structural heterogeneity at different length scales. A characteristic length of 400 µm in biofilm images provides structural information relevant for mass transport phenomena in biofilms. Overall REAs for gray level and COR were on average 3.4 mm2. The scale-dependent descriptor COR could not in all cases accurately be determined from combining individual image analysis results - even when the combined area resulted in the REA. Microscope and camera specifications define the upper and lower limit of detectable characteristic length that can be extracted from images and should therefore be considered in the experimental design when choosing an imaging technique.
Evaluating operating conditions for outcompeting nitrite oxidizers and maintaining partial nitrification in biofilm systems using biofilm modeling and Monte Carlo filtering
In practice, partial nitrification to nitrite in biofilms has been achieved with a range of different operating conditions, but mechanisms resulting in reliable partial nitrification in biofilms are not well understood. In this study, mathematical biofilm modeling combined with Monte Carlo filtering was used to evaluate operating conditions that (1) lead to outcompetition of nitrite oxidizers from the biofilm, and (2) allow to maintain partial nitrification during long-term operation. Competition for oxygen was found to be the main mechanism for displacing nitrite oxidizers from the biofilm, and preventing re-growth of nitrite oxidizers in the long-term. To maintain partial nitrification in the model, a larger oxygen affinity (i.e., smaller half saturation constant) for ammonium oxidizers compared to nitrite oxidizers was required, while the difference in maximum growth rate was not important for competition under steady state conditions. Thus, mechanisms for washout of nitrite oxidizing bacteria from biofilms are different from suspended cultures where the difference in maximum growth rate is a key mechanism. Inhibition of nitrite oxidizers by free ammonia was not required to outcompete nitrite oxidizers from the biofilm, and to maintain partial nitrification to nitrite. But inhibition by free ammonia resulted in faster washout of nitrite oxidizers.
Brockmann, D.; Morgenroth, E. (2010) Evaluating operating conditions for outcompeting nitrite oxidizers and maintaining partial nitrification in biofilm systems using biofilm modeling and Monte Carlo filtering, Water Research, 44(6), 1995-2009, doi:10.1016/j.watres.2009.12.010, Institutional Repository
The influence of aeration intensity on predation and EPS production in membrane bioreactors
Shear, in the form of vigorous aeration, is used to control fouling in membrane bioreactor (MBR) systems. However, shear also influences the physicochemical and biological properties of MBR biomass. The current study examines the relationship between the aeration intensity and extracellular polymeric substance (EPS) production in MBRs. Two identical submerged MBRs were operated in parallel but the aeration rate was three times greater in one of the MBRs. The concentrations of floc-associated and soluble EPS were monitored for the duration of the experiment. Microscopic images and floc-size measurements were also collected regularly. The membrane fouling potential of the biomass was quantified using the flux-step method. Increased aeration did not have a direct effect on soluble or floc-associated EPS production in the microfiltration MBRs. However, aeration intensity had a significant effect on predatory organisms. Large aquatic earthworms, Aeolosoma hemprichi, proliferated under lower shear conditions but were never observed in the high shear reactor. Predation by A. hemprichi resulted in increased floc-associated and soluble EPS production. Thus, the mixing conditions in the low shear MBR indirectly resulted in increased soluble EPS concentrations and higher fouling potential. This research suggests that predation can have a significant impact on the production rates of floc-associated and soluble EPS – key parameters driving membrane fouling in MBRs.
Predation influences the structure of biofilm developed on ultrafiltration membranes
This study investigates the impact of predation by eukaryotes on the development of specific biofilm structures in gravity-driven dead-end ultrafiltration systems. Filtration systems were operated under ultra-low pressure conditions (65 mbar) without the control of biofilm formation. Three different levels of predation were evaluated: (1) inhibition of eukaryotic organisms, (2) addition of cultured protozoa (Tetrahymena pyriformis), and (3) no modification of microbial community as a control. The system performance was evaluated based on permeate flux and structures of the biofilm. It was found that predation had a significant influence on both the total amount and also the structure of the biofilm. An open and heterogeneous structure developed in systems with predation whereas a flat, compact, and thick structure that homogeneously covered the membrane surface developed in absence of predation. Permeate flux was correlated with the structure of the biofilm with increased fluxes for smaller membrane coverage. Permeate fluxes in the presence or absence of the predators was 10 and 5 L m−2 h−1, respectively. It was concluded that eukaryotic predation is a key factor influencing the performance of gravity-driven ultrafiltration systems.
Activity of metazoa governs biofilm structure formation and enhances permeate flux during Gravity-Driven Membrane (GDM) filtration
The impact of different feed waters in terms of eukaryotic populations and organic carbon content on the biofilm structure formation and permeate flux during Gravity-Driven Membrane (GDM) filtration was investigated in this study. GDM filtration was performed at ultra-low pressure (65 mbar) in dead-end mode without control of the biofilm formation. Different feed waters were tested (River water, pre-treated river water, lake water, and tap water) and varied with regard to their organic substrate content and their predator community. River water was manipulated either by chemically inhibiting all eukaryotes or by filtering out macrozoobenthos (metazoan organisms). The structure of the biofilm was characterized at the meso- and micro-scale using Optical Coherence Tomography (OCT) and Confocal Laser Scanning Microscopy (CLSM), respectively. Based on Total Organic Carbon (TOC) measurements, the river waters provided the highest potential for bacterial growth whereas tap water had the lowest. An increasing content in soluble and particulate organic substrate resulted in increasing biofilm accumulation on membrane surface. However, enhanced biofilm accumulation did not result in lower flux values and permeate flux was mainly influenced by the structure of the biofilm. Metazoan organisms (in particular nematodes and oligochaetes) built-up protective habitats, which resulted in the formation of open and spatially heterogeneous biofilms composed of biomass patches. In the absence of predation by metazoan organisms, a flat and compact biofilm developed. It is concluded that the activity of metazoan organisms in natural river water and its impact on biofilm structure balances the detrimental effect of a high biofilm accumulation, thus allowing for a broader application of GDM filtration. Finally, our results suggest that for surface waters with high particulate organic carbon (POC) content, the use of worms is suitable to enhance POC removal before ultrafiltration units.
Derlon, N.; Koch, N.; Eugster, B.; Posch, T.; Pernthaler, J.; Pronk, W.; Morgenroth, E. (2013) Activity of metazoa governs biofilm structure formation and enhances permeate flux during Gravity-Driven Membrane (GDM) filtration, Water Research, 47(6), 2085-2095, doi:10.1016/j.watres.2013.01.033, Institutional Repository
Biofilm formation and permeate quality improvement in Gravity Driven Membrane ultrafiltration
The effects of biofilm development on ultrafiltration membranes with regards to permeate stability and permeation rates were investigated using Gravity-Driven Membrane (GDM) filtration. The first part of the study aimed at evaluating the influence of the biofilm on permeate flux quality and quantity with regards to Assimilable Organic Carbon (AOC) degradation. In addition, two types of biological pre-treatments were evaluated: slow sand filtration and packed bed bio-reactor, compared to a control (no treatment). Biofilm formation helped to decrease the AOC content of permeate water, compared to the influent. Both pre-treatments additionally reduced AOC level in permeate and thus increased its biological stability, however none of the systems were able to guarantee microbiologically stable water. Removal of AOC before the GDM filtration reduced the biofilm growth potential, which in turn influenced its physical structure and enhanced the permeation rates. Influence of inorganic particle removal by pre-sedimentation and its effect on the biofilm structure was also studied. Pre-sedimentation of particle populations selected fine and homogenous particle fractions, which lead to the formation of a homogenous biofilm structure characterized by an increased hydraulic resistance. This was clearly visible between horizontally and vertically installed membranes where the latter ones had a significantly reduced flux despite lower deposited particle mass. Presence of larger, heterogeneous particle fractions counterbalanced the negative effects of the fine particles, which overall resulted in enhanced permeation rates.
Chomiak, A.; Mimoso, J.; Koetzsch, S.; Sinnet, B.; Pronk, W.; Derlon, N.; Morgenroth, E. (2014) Biofilm formation and permeate quality improvement in Gravity Driven Membrane ultrafiltration, Water Science and Technology: Water Supply, 14(2), 274-282, doi:10.2166/ws.2013.197, Institutional Repository
Inorganic particles increase biofilm heterogeneity and enhance permeate flux
This study investigated the influence of inorganic particles on the hydraulic resistance of biofilm grown on membrane surface during low-pressure dead-end ultrafiltration. Gravity-driven ultrafiltration membrane systems were operated during several weeks without any flushing or cleaning. Smaller (kaolin d0.5 = 3.6 μm) or larger (kaolin with diatomaceous earth 50/50%, d0.5 = 18.1 μm) particles were added to pre-filtered creek water or to unfiltered creek water. It was demonstrated in both experiments that presence of finer particles in the feed water (kaolin) induced formation of compact and homogeneous biofilm structure. On the other hand presence of the larger particles (diatomite) helped to counterbalance the effect of fine particles due to the formation of more heterogeneous and permeable biofilm structure. The hydraulic resistance of biofilms formed with fine particles was significantly higher than the resistance of biofilm formed in (1) absence of any inorganic particles or (2) in presence of the mixed particle population. The membrane orientation (vertical or horizontal) determined which particles were accumulating at the membrane surface, with structural differences shown by Scanning Electron Microscopy (SEM). For vertical membranes, the larger particles were selectively removed due to sedimentation and did not contribute to the biofilm development. Thus the selection of smaller particles due to vertical membrane configuration negatively affected the biofilm structure and permeation rates, and such selective accumulation of fine particles should be avoided.