Types Of Freshwater Flocs

Flocs in freshwater ecosystems are fundamentally no different from those in saltwater (Section II) or engineered (Section III) ecosystems, although saltwater flocs are sometimes exceptionally large.22,31 At first this similarity may seem nonsensical, given the extreme differences in overlying conditions and industrial manipulations. However, if one examines flocs from these environments they are all composed of inorganic particles, organic (living and nonliving) particles (Figure 2.2), and water. The difference lies in the relative proportions and specific composition of individual entities comprising these general base components. In addition, it is evident that the factors influencing flocculation will remain the same regardless of environment, only the relative importance of each will vary as defined by site specific conditions. It is these relative compositional and mechanistic differences which will give the floc population its site specific distinctive characteristics. As such, while extreme examples within this generalized view of flocs have been defined in the literature (biota-rich flocs,10,16,17 mineral-rich flocs,32 and aggregated humic substances33-35) their common link is that they all have an inorganic and organic (living and nonliving) component and water as constituents, although in some instances a single component may be dominant.

Within freshwater systems, flocs can be classified into four categories based on their location of origin: (a) formed within the water column, (b) eroded from the bed, (c) derived from the terrestrial environment and washed into the system by overland or subsurface flow (and usually referred to as "aggregates"), and (d) decaying organic matter (e.g., from plants). This chapter focuses primarily on the first classification of flocs. While these categories of flocs are known because of our understanding of soils, microbiology, hydraulics, and flocculation theory, they, at this point in time, cannot be differentiated within a single sample.36 The lack of differentiation reflects a lack of existing methods to discriminate these forms, as the majority of sediment analysis instruments are indirect and nondiscriminative (e.g., laser particle sizers).

Flocs formed in the water column via various physical, chemical, and biological means, as discussed in this chapter, will generally appear as open matrix, low density, high water content particles which may be more fragile than those derived from the other three categories.36 Flocs derived from bed sediment erosion are generally more compact but with a larger organic fraction due to biofilm growth providing for a low density. The compaction is a result of self weighted consolidation processes and biostabilization.37 The significant biological component provides the floc with strength due to the sticky nature of the material. These particles will often have denser areas within them that may represent water stable soil "aggregates" that have settled quickly to the bed. Such particles can be referred to as hybrid particles (i.e., a particle composed of both floc and aggregate components). Flocs derived from soil surfaces are typically not truly flocs but rather aggregates formed through soil processes. Nonetheless, these particles are similar in structure, containing similar constituent particles, and once within the water, are quickly colonized by aquatic bacteria. These particles are compact and dense with settling velocities one to two orders of magnitude higher than flocs formed directly in a water body.36

Freshwater flocs derived from the microbial decomposition of suspended plant, algal, and zooplankton debris are receiving renewed attention as a result of an accelerating interest in aquatic microbial ecology.22,38 The focus of many recent studies has been on bacterial colonization, bacterial/algal interactions, decomposition phenomena, the cycling of nutrients and elements of biogeochemical interest, and the flux of energy in aquatic ecosystems. Some of this research reveals the fact that a small chunk of decomposing debris takes on the aspect of a microbiota-rich floc, as the debris per se becomes increasingly consumed during its conversion to microbial biomass and associated EPS. In fact, Grossart and Simon38 point out similarities between such biota-rich flocs and activated sludge flocs in sewage treatment plants. The association of microbiota and suspended debris during the decomposition process is sometimes called a macroscopic organic "aggregate," not to be confused with the soil "aggregate" (described earlier in the chapter) or the submicrometer-scale "aggregate" of nanoscale colloids to be described in the following sections.

In the authors' examination of thousands of floc images from multiple freshwater environments (rivers, lakes, storm waters, and combined sewer systems)5,13,16,17,39,40 and also of those in the literature,6,11,41-43 very rarely are flocs seen in excess of 500 ¡m, with the majority of flocs below 100 ¡m. As with all environments, the size of freshwater flocs will be dictated by local shear conditions and developmental factors described in Figure 2.3 to Figure 2.7 below. On average, Droppo44 demonstrated that the general size of flocs relative to environment is as follows: combined sewers > lakes > rivers. This relative difference is related to organic concentrations being highest in the sewer systems and shear being the strongest in river systems.

Density relationships for flocs in freshwater are no different than those in engineered or marine systems. In all cases, as floc size increases the density decreases, approaching that of water. This relationship is related to larger flocs becoming more porous (approaching 100%) due to an increase in contact points and therefore retaining more bound water. As described below, pores in freshwater flocs are generally small,

FIGURE 2.3 The characteristics of inorganic particles that will influence the internal and external behavior of flocs. (Adapted from Droppo (2001) and reproduced with permission.)
FIGURE 2.4 The characteristics of the microbial community/organic particles that will influence the internal and external behavior of flocs. (Adapted from Droppo (2001) and reproduced with permission.)
FIGURE 2.5 The characteristics of microbial extracellular polymeric fibrils and their influence on the internal and external behavior of flocs. (Adapted from Droppo (2001) and reproduced with permission.)
FIGURE 2.6 The characteristics of water within a floc and its influence on the internal and external behavior of flocs. (Adapted from Droppo (2001) and reproduced with permission.)

particularly due to the prominent functional existence of EPS fibrils, and thus they "trap" water rather than allowing convective flow through flocs. This has concomitant effects on diffusion gradients within the floc. A comparison of densities from multiple environments can be found in Droppo44 and Leppard and Droppo.40 While data on

FIGURE 2.7 The characteristics of floc pores and their influence on the internal and external behavior of flocs. (Adapted from Droppo (2001) and reproduced with permission.)

freshwater floc density is limited, the authors have demonstrated that generally the typical range is from 1.01 to 1.3 mgcm-3.

Generally within the literature, researchers have illustrated flocs to comprise three gross scale morphological and compositional forms; they show flocs which are highly enriched in either microbiota or mineral colloids or humic substances. While these characterizations were based on the dominant primary particle within the floc, such characterization is misleading as the freshwater floc is highly heterogeneous in structure and composition (although extreme exceptions do occur such as mineral flocs collected at the snout of glaciers — Woodward et al.32). From our experience, freshwater floc types tend to differ in terms of the relative amounts of common colloidal subcomponents; different floc types grade into each other. While the relative importance of each of these components will vary greatly with differing flocs, the processes operating within the floc are the same. Commonly found compounds, materials and life forms within flocs are revealed in Figure 2.1, while specific colloids found within flocs as abundant subcomponents are shown in Figure 2.2. The associations between various colloids typically appear almost random, with little repetitiveness of specific arrangements. Nevertheless, there is order and this order is related to microbial function and to microbial modifications to improve habitat, a fact being increasingly well demonstrated in related researches with biofilms45-48 and with experimental bacterial populations.49 As is the case for biofilms, among the spectrum of floc types there may be environmentally significant "machines" for adjusting water quality and modulating biogeochemical processes.

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