Environ. Sci. Technol. 1996, 30, 680-686
Floc Architecture in Wastewater and Natural Riverine Systems S T E V E N N . L I S S , * ,† I A N G . D R O P P O , ‡ DERRICK T. FLANNIGAN,§ AND G A R Y G . L E P P A R D ‡,§ Department of Applied Chemical & Biological Sciences, Ryerson Polytechnic University, Toronto, Ontario M5B 2K3, Canada, National Water Research Institute, Environment Canada, Burlington, Ontario L7R 4A6, Canada, and Department of Biology, McMaster University, Hamilton, Ontario L8S 4K1, Canada
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.
Introduction Flocculated particles are central to a number of significant environmental processes within engineered and natural systems. They can be defined as sedimenting units which can be isolated by cascade filtration and ultracentrifugation (1-3) and are composed of two or more primary particles (e.g., a bacterial cell and an inorganic particle) (4). Microbial flocs have typically been seen as structural entities serving only to transport/settle sediment and contaminants in * Author to whom all correspondence should be addressed; fax: (416) 979-5044; e-mail address:
[email protected]. † Ryerson Polytechnic University. ‡ National Water Research Institute. § McMaster University.
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natural or wastewater treatment systems. This approach has been restricted to the gross scale (>1 µm) and has focused primarily on size, shape, density, porosity, and settling velocity for the purpose of modeling and manipulating sediment and contaminant fate and effect. At the fine scale (submicron), the importance of the microorganisms and bioorganic material to floc development and properties is well recognized (5). Visual observations of natural flocs with high-resolution electron-optical microscopy techniques have revealed cross-linkages of floc primary particles by bridges of fibrillar extracellular polymeric substances (fibrils), 2-10 nm in diameter (6). Fibrils can be detected in pelagic associations between microbes and abiotic particles (7) and are common components of freshwater ecosystems (4, 7-9). These fibrillar extracellular polymeric substances are an important class of microbiallyderived colloids (particles
