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Institut für Wasser- und Umweltsystemmodellierung - IWS



"The Characterization of Biostabilized Flocs Applying Floc Settling Column and Image Analysis"

Sediment transport models in consideration of effects brought about by the biological component on the sediment stabilization, have increasingly become a focal point towards unravelling the actual forces behind sediment dynamics. Sediment erosion and transport as a whole is highly dependent on the extracellular polymeric substances (EPS) embedded on them before and after erosion. This is bound to its distribution on the surface of the sediment and the degree of cementing. Sediment biostabilization therefore is vital in enhancing the physical architecture of sediments which affects erosion, transportation and deposition. Influence of microbial biofilms and their polymeric matrix (EPS) on features decisive on sediment transport and deposition takes center stage of this research. EPS carbohydrates and proteins have been analyzed to give insight on their level of stabilization to the artificial glass beads. Incubation of the sediment with freshwater obtained from River Enz, Germany is conducted in the six laboratory flumes for a period of 61 days and the shear stress and the settling velocity of the eroded flocs investigated using Gust chamber and the settling column respectively. Floc images obtained by the use CCD camera are subjected to an image analysis program where information concerning floc geometrical features after undergoing biostabilization process is extracted for further probe. Particle bulk density, loss on ignition (LOI) and biological analysis has also been conducted. There is increased erosion threshold which correlates with the increasing EPS carbohydrates and proteins. A single eroded chunk is characterized by few to hundreds of glass beads that have been glued together by the biofilm, with the ratio of organic matter content increasing with time. This is a clear indication that biofilm makes non-cohesive sediments to become cohesive hence enhanced stability. The general hydrodynamics of the sediment are transformed. Flocs equivalent diameter and loss on ignition increases as the bulk density decreases. This is an indication that the organic matter content (LOI) from the grown biofilm on the artificial sediment have increased. The geometric parameters of pure glass beads (with diameters between 100μm-200μm) such as equivalent diameter, form factor, roundness, etc, changes leading to an increase in floc settling velocity, Vs. The average settling velocity recorded few days after setting up of experiment is 6.3mm/s. This changes from the average settling velocity of the glass beads used in this research as artificial sediment that ranged from 2.5 to 4 mm/s. It is concluded that the extracellular polymeric substances enhances the structural formation and the behaviour of the sediment. Biofilm also changes topography of the sediment surface from smooth to rugged surface. There has been also intensive invasion of the biofilm spreading in 3D (in the x, y and z directions) hence creating visible vertical profiles that increases in depth with time and environmental conditions administered to the flumes during incubation period. Image analysis method applied in this study has been helpful towards extracting individual floc geometries, to determine their frequency distribution and calculate floc settling velocity, Vs which enables prediction of sediment transport paths in aquatic systems.