Introduction

Globally, freshwater represents only 2.5% of the world's water resources.1 Water, particularly freshwater, is the most essential and significant component for sustaining human life and many other aspects of global survival. Globally, the integrity of freshwater is jeopardized by contaminant and particulate inputs from soil erosion, atmospheric deposition, and anthropogenic point and nonpoint sources of pollution. With clean drinking water one of the most significant issues impacting mankind,1 a better understanding of its particulate component, the component carrying the majority of contaminants, is critical for freshwater sustainable development.

Flocculation is a universal process occurring within aquatic ecosystems that incorporate both inorganic and organic cohesive particles. Certainly the freshwater systems, consisting primarily of rivers and lakes (although other systems such as urban sewer systems and stormwater detention ponds also have been studied2), are dominated by cohesive sediments from a variety of sources and with a variety of compositions.

©2005 by CRC Press 25

While some river loads such as that for the Mississippi will be dominated by sand transport, flocculation of the cohesive fraction will play an equally important role in moderating contaminant transport.3 Within the majority of cohesive sediment transport rivers, flocculated particles are consistently shown to represent greater than 80% of the total volume of sediment in transport.4,5 This fact has been dismissed within many engineering and scientific applications of the past. In fact, coastal and estu-arine models and researchers often treated the river inputs to the marine system as unflocculated, and only when mixed with saltwater was flocculation believed to be significant (due to electrochemical effects). Over the last few decades though, there has been enlightenment as to the importance of flocculation in the freshwater system. For example, freshwater flocs are shown to be an integral component of interstitial pores within gravel bed rivers, with concomitant effects on salmonid egg survival.6,7 Urban engineering projects such as storm, sanitary and combined sewer systems, stormwater detention ponds, inline detention basins, artificial marsh lands, and other products of best management practices are taking into account the influence that flocculation has on the controls of sediment and contaminant transport.2 Models of urban environments, however, lag behind those of the natural water systems, owing largely to the purely engineering approach to system design. Flocculated particles have also been given greater consideration as to their impact on the transport of contaminants.8-14 A tangible impact of flocculation is its effect on reservoir infilling by significantly increasing the deposition rate of sediments. Flocculation's impact on reservoir infilling, fisheries, habitat destruction, and contaminant transport have resulted in significant financial burdens for remediation and restoration projects. All of the above examples are related to the important relationship of floc structure to floc behavior. Specifically, floc form or structure will impact floc physical (transport), chemical (uptake/transformation), and biological (biocommunity dynamics) behavior within the floc itself or within a given system as a whole.9,14-22

This chapter provides an overview of freshwater flocculation, and the nature of the resultant flocs, with subsequent chapters addressing studies which have investigated many of the above issues as they relate to flocculation. While our focus is on freshwater, other studies/methods from the engineering and marine fields are discussed in this chapter when they are applicable to freshwater.

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