In the history of environmental science, there has probably been no greater struggle than the attempt to control the impact of the sediment and solids generated by nature and human influence (including industrial processing) on the terrestrial and aquatic environment and on socioeconomics in general. Untold billions of dollars are spent each year on dredging to maintain navigation channels and harbors. Further costs are added by the need to treat these sediments prior to disposal because of high levels of contamination resulting from anthropogenic impacts on the environment. Significant financial burdens arise as a result of the need to remove solids during drinking water and wastewater treatment processes, a necessity for sustainable development, and the protection of human and aquatic health. It is now well established that the majority of particles within natural (freshwater and saltwater) systems are present in a flocculated form (i.e., flocs), and that the formation of flocs is essential for the effective performance of engineering processes such as biological wastewater treatment.

Flocculation is the process of aggregating smaller particles together to form larger composite particles via various physical, chemical, and biological interactions. These larger composite particles behave differently in terms of their physical (e.g., transport, settling), chemical (e.g., contaminant uptake and transformation), and biological (e.g., community structure activities and metabolism) behavior relative to their constituent individual particles due to differences in size, shape, porosity, density, and compositional characteristics. Given these significant behavioral differences between flocs per se and their individual component parts, flocculation influences a wide array of environmental phenomena related to sediment-water and sedimentsediment interactions. A few of these include sediment and contaminant transport in various aquatic ecosystems, remediation of contaminated bed sediments, contaminated bed sediment stability, and habitat destruction resulting from sedimentation (e.g., coral reef, salmon spawning grounds, mollusk habitat degradation). These concerns, coupled with the ubiquitous nature of flocs within natural and engineered systems and the potential to influence floc properties to control better the environmental and engineering processes, have generated an increased emphasis on floc research.

The traditional disciplines within saltwater, freshwater, and engineering research have, however, remained somewhat mutually exclusive in their approach to the study of flocculation processes. This reality is facilitated by differences in external variables (e.g., environmental conditions), focus driven research, and discipline bias. Regardless of differences in discipline or approach, there is great scope and utility for the sharing of information between scientists who work in these three floc environments. Often methods used in one environment can, and should, be used in another to further our understanding of flocculation processes. While new developments in genomics, nanotechnology, sampling, and modeling permit increasingly revealing investigations into floc structure, processes, and impact, there is still a fundamental lack of knowledge related to many aspects of the flocculation process.

In light of the importance of flocculation within natural and engineered systems, an international workshop was held on September 4 and 5, 2003, at the Canada Centre for Inland Waters, Burlington, Ontario, Canada. The workshop brought together academics and government scientists from around the globe to address the critical issue of sediment flocculation within freshwater, saltwater, and engineered systems. During the workshop, participants representing these three environments presented their research findings. Three focus areas were used to structure the workshop: (a) modeling, (b) physicochemical, and (c) biological aspects of floc-culation. Following individual presentations, the participants were divided up into three working groups to address assigned topics in the focus areas. Each focus group contained researchers from the freshwater, saltwater, and engineered systems to ensure a cross-communication of ideas between environments and to facilitate an understanding of the unifying principles of flocculation. Participants ranged from geographers/geomorphologists who investigate flocculation as it relates to sediment source, transport, and fate within river systems, sedimentologists interested in floc-culation's influence within depositional environments, biologists focusing on the biopolymeric matrices and microbial consortia of flocs, oceanographers investigating sediment transport and delivery within estuaries and open ocean environments, and wastewater engineers/biologists interested in floc behavior within engineered systems.

The peer-reviewed 20 chapters that comprise this text are organized by their environment of investigation. The final chapter identifies the unifying principles that were discussed within the workshop focus groups and from the preceding chapters. The text provides a unique perspective in that it integrates the natural sciences and engineering fields as they relate to the central phenomenon of flocculation. We hope that the array of information provided in this book will be valuable to all those interested in flocculation issues within any environment.

Ian G. Droppo Gary G. Leppard Steven N. Liss Timothy G. Milligan

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