The workshop, Flocculation in Natural and Engineered Systems, held in September, 2003 at the Canada Centre for Inland Waters provided a unique perspective of floccu-lation processes through the integration of current knowledge obtained from natural and engineered systems. This multidisciplinary workshop incorporated scientists who work in freshwater and saltwater environments, and engineered systems. This allowed for a cross communication of ideas from disciplines that have largely remained isolated in their study of flocculation processes. This integration of ideas and methods provides researchers from different disciplines and work environments with different motivations in their effort to answer the very different questions dictated by the environment of investigation. The different approaches to the study of flocculation presented in this chapter allow individuals to look at it from an alternative perspective. With an integrated approach, new opportunities arise for flocculation research within all three environments.

As can be seen from the preceding chapters, flocculation plays an essential role in mediating the physical, chemical, and biological properties of not only the suspended flocs themselves, but also of the aquatic or engineered system as a whole. As such, the flocculation process has significant environmental and socioeconomic implications. For example, flocculation plays an important role in sediment associated contaminant fate and effect, reduces reservoir capacity and fisheries habitat due to increased sedimentation, is used as a remediation strategy for oil spills and toxic algal blooms, and dictates the efficiency of wastewater treatment systems. While untold billions of dollars are spent each year on issues for which flocculation is the key process, our understanding of the underlying mechanisms is still developing. The goal of this workshop was to improve our knowledge of flocculation and its role in what appear at first glance to be very different environments. The free exchange of methods, models, and ideas between researchers has identified areas of convergence and divergence within the study of flocculation. The workshop demonstrated that, in general, the important variables in the flocculation process are the same regardless of environmental constraints (freshwater, saltwater, or engineered systems). Apparent differences lie in the perceived relative importance of these variables and in the specific approach used for their assessment.

It is clear that there are three basic principles and one emerging issue which are supported in all three environments. The principles are (1) that flocculation is agreed, at its most simplistic level, to be the aggregating together of smaller particles to form larger particles; (2) that successful aggregation occurs through mechanisms described by the basic coagulation theory; and (3) that the substantive physical behavioral impact of flocculation is the modification (generally increasing) of the downward flux of sediments. The biology of flocculated sediment, particularly the microbial activity, is quickly emerging as an important issue; bacterial activities modify much of the physical, chemical, and biological behavior of the sediment particles and the system as a whole. The biological aspects of flocculation have, of course, always been an important issue in engineered systems (e.g., wastewater treatment) but are only in the last few decades being explored within the freshwater and saltwater environments as a significant mechanism influencing the flocculation process. Beyond these three principles and the emergence of biological activity as an important aspect of flocculation, all other aspects of this complex phenomenon were generally agreed to be ruled by site-specific parameters culminating in unique structures and chemical and biological behaviors within the medium of transport. This chapter summarizes the outcomes of the workshop and guides us toward a better understanding of the unifying principles of flocculation.

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