Structural controls on floc strength and transport

An examination of how floc structure (internal and external) influences floc strength and transport is described. Samples were collected from both a natural river and a combined sewer outfall to provide significantly different structures for comparison. Regardless of floc type, the internal matrix was observed to be mediated by a dynamic microbial community. Microbial secretions of fibrilar extracellular polymeric substances (EPS) were found to be interwoven within the floc forming a framework and providing structural stability. This continually changing framework (due to floc physical, chemical, and biological activity) is shown to have extending influence over floc transport by modifying floc size, stability, density, porosity, and biochemical processes. This paper uses correlative microscopy to assess floc structure and behaviour in terms of the transport of sediments and associated contaminants. A conceptual model is provided which links floc strength to sediment transport (deposition and erosion) in the water column and at the sediment water interface.Key words: flocculation, bacteria, extracellular polymeric substances, stability, transport, structure.