Floc Architecture in Wastewater and Natural Riverine Systems

Abstract

The study of flocs has largely been devoted to the gross (>1 µm) scale so that the behavior of flocs (i.e., transport and settling) can be observed and modeled. With the assistance of a newly developed field kit and correlative microscopy [which includes transmission electron microscopy (TEM), scanning confocal laser microscopy (SCLM), and conventional optical microscopy (COM)], this paper begins to bridge the resolution gap between the gross and fine (submicron) scales in order to better understand the role of floc ultrastructure in outward floc behavior for both natural and engineered systems. Results from both systems have demonstrated that pores which appeared to be devoid of physical structures under the optical microscopic techniques (SCLM and COM) were observed to be composed of complex matrices of polymeric fibrils (4-6 nm diameter) when viewed by high-resolution TEM. These fibrils were found to represent the dominant physical bridging mechanism between organic and inorganic components of the flocs and contributed to the extensive surface area per unit volume of the flocs. In this way, the microbial floc resembles a biofilm and will likely support similar processes with respect to contaminants and the physical-chemical environment.

The Prevalence of Freshwater Flocculation in Cold Regions: A Case Study from the Mackenzie River Delta, Northwest Territories, Canada

“…Bacteria have been shown to be an integral part of most natural floc structures (Logan and Hunt, 1987;Droppo and Ongley, 1989, 1992Muschenheim et al, 1989;Liss et al, 1996) and were observed to be an important constituent of MRD flocs for the September samples. While often the majority of bacteria are free-floating in freshwater environments (Geesey and Costerton, 1979;Kirchman, 1983), there are reports of environments and conditions where the attached bacteria are dominant (Goulder, 1976;Bell and Albright, 1981;Lind and Lind, 1991).…”

“…While attached bacteria may not always be dominant, the significance of this sediment-bacteria relationship lies in the fact that bacteria associated with particulate surfaces demonstrate greater metabolic activity than free-floating bacteria (Logan and Hunt, 1987;Kasimir, 1992). Liss et al (1996) have demonstrated that it is the bacteria's metabolic production of EPS which is the dominant mechanism controlling floc development, structure, stability, and behaviour.…”

“…Flocculation also alters the chemical and biological behaviour of sediment in terms of how it interacts with contaminants and the biological community and how it alters or degrades the contaminants or nutrients assimilated within or around the floc. In effect, a floc can be considered a micro-ecosystem capable of modifying not only itself but the aquatic environment as a whole (Liss et al, 1996;Droppo et al, 1997).…”

Optimal preparation of water samples for the examination of colloidal material by transmission electron microscopy

“…This is a result of their sorptive capacities, great surface area relative to mass, and tendency to aggregate into particles subject to sedimentation (O'Melia 1985, More1 & Gschwend 1987, Leppard 1993. Fibrillar EPS colloids (fibrils), as defined morphologically by TEM, have been recognized as abundant colloids with multiple roles (Leppard 1992a(Leppard , 1993, including a role in bridging other colloids (Filella et al 1993, Perret et al 1994, Liss et al 1996. Analysis of the native associations established by bridging, and the evolution of bridged colloids into flocs, is a subject (like marine TEP formation) which requires an optimal methodology for TEM preparation of water samples.…”

The effect of pulp mill effluent on fine-grained sediment morphology and storage in the Fraser River at Prince George, B. C.

“…There are two processes by which bacteria can increase the mutual attraction between particles in suspension: exuding extracelluar polymeric fibre (Bar-Or and Shilo, 1987;Droppo and Ongley, 199Gb;Droppo et al, 1996;Liss et al, 1996;Droppo et al, 1997), and hydrophobic surface adhesion (van Loosdrecht et al, 1987a,b). The polymeric fibre is a macromolecule with a high charge density, which short-circuits the electrostatic double-layer.…”