Consideration of CSO Floc Characteristics for Modeling the Removal of Solids in a Detention Tank
Within the past decade, there has been a renewed interest in urban runoff quality control strategies and technologies. These technologies include wet and dry stormwater detention ponds, constructed wetlands, holding (detention) tanks, vortex/swirl separators, filter basins and filter inlets, grass buffer strips and swales, ultraviolet radiation, porous pavements, and improved street sweeping methods (Tracy and Craig, 1993; Urbonas, 1993; Driscoll and Strecker, 1993; Cheung et al., 1995; Stirrup, 1996; Field et al., 1997; Rochefort et al., 1997; Papa et al., 1997; Sutherland and Jelen, 1997; Kresin et al., 1997; Horsley, 1997; Anderson et al., 1997). The success of many of these technologies will be affected by the characteristics of the sediment transported in the runoff (e.g. particle size, shape, density, composition). In a combined sewer system, the sediment characteristics may vary, depending on factors such as the relative contribution of sanitary flow and stormwater runoff, but generally this sediment moves as flocs (i.e. two or more discrete particles that are joined and transported as a single, larger particle).
Lawler (1993) noted that mathematical approaches to describe the kinetics of flocculation were developed as early as 1917. However, Lawler (1993) also indicated that this older work still serves as the foundation for understanding changes in particle size distribution due to flocculation, in part because the complexity of flocculation dynamics has been difficult to capture mathematically and because "the ability to measure the size distribution in practice is also difficult...". Some recent modeling efforts have attempted to consider flocculation effects, although most open water and sewer models assume that particles behave as discrete particles (Gailani et al., 1991;Ongleyet al., 1992; Bertrand- Krajewski et al., 1993;Coghlin et al., 1993; Wen et al., 1994). The ability to develop satisfactory models of the flocculation process and floc transport dynamics is being facilitated by the increasing sophistication of non-destructive analytical techniques to determine sediment size and settling characteristics (e.g. Lau and Krishnappan, 1992; Kranck et al., 1992; Pisano and Brombach, 1996; Droppo et al., 1997; Marsalek et al., 1998). This type of non-destructive, empirically determined size and settling data also can be valuable in the design and evaluation of stormwater quality control technologies.
The objectives of this study were twofold. First, size (floc and discrete particle) distribution and settling velocity data were summarized for suspended solids collected from a combined sewer system that discharges to Hamilton Harbour, Hamilton, Ontario. Second, these sediment data were used as input to the STORAGE block of PCSWMM4 to assess the sensitivity of the model to variations in sediment characteristics. A standard, hypothetical detention tank was designed and used as input to the STORAGE block for the model evaluations.
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