Settling Velocity Analysis for Stormwater Solids in Urban Drainage BMPs

Settling velocity is a primary parameter when examining separation of solids by urban drainage unit operations (historically referred to as BMPs in North American practice). In all “volumetric” (for example settling tanks, basins, wetlands…) or “hydrodynamic” devices or systems (commonly as manufactured BMPs) the driving mechanism is gravitational settling. Unless the combination of residence time distribution, particle number density (PND) distribution, hydrodynamics and surface potential of the solids are favorable for particle-particle interaction, Type I discrete settling is the dominant settling mechanism as compared to Type II flocculant settling. Unmaintained unit operations (the common condition) can potentially generate Type II, III and IV settling mechanisms although the prevalence of scour under such conditions alters each of these mechanisms. The focus of this paper is Type I discrete solids (particulate matter, PM) settling in wet weather (stormwater). This study examines settling of largely inorganic PM with a hetero-disperse particle size distribution (PSD), typical of urban wet weather flow loadings. This paper illustrates the relative role of commonly measured or assumed parameters that are directly utilized (PM diameter, PM specific gravity) and indirectly utilized in Newton's Law (fluid temperature and salinity). For the common condition of a hetero-disperse PSD, (one reasonable gravimetric distribution is the New Jersey, NJDEP gradation, approximately 10 to 1000 μm), the relative role of PM diameter for a hetero-disperse PSD dominates all other parameters. The dominant role of PM diameter is followed by PM specific gravity, which can range from 1.1 to 2.8, although higher values can occur for example with mineral-based PM such as iron oxides. The relative role of temperature on settling velocity, within the typical range of urban drainage temperature, 0 to 30 °C primarily through altering fluid density and viscosity is minor in comparison to PM diameter and specific gravity. Finally, the relative role of salinity is insignificant for settling velocity as compared to PM diameter and specific gravity. It is noted that at salinity concentrations approaching that of seawater (30 ppt), salinity can have a potentially beneficial effect on PM surface potential although the Type II settling benefit is small compared to tri-valent species of iron and aluminum. Results illustrate that for the common condition of a hetero-disperse PSD such as NJDEP, Newton's Law can predict settling velocity (Vs) across a hetero-disperse PSD that ranges from clay-size to gravel-size PM. If these Type I settling velocity results are integrated across the entire hetero-disperse PSD the influence of urban drainage PM diameter on unit operation separation behavior dominates all other parameters and the role of temperature or salinity are very minor. Results have important implications with respect to pilot-scale controlled testing, and uncontrolled field monitoring and verification of unit operations for the separation of PM and PM-associated chemicals such as metals, organics and nutrients. Beyond measurements of hydrology and hydrodynamics for a given unit operation, the PND (concentration and PSD) and specific gravity of PM play the dominant roles in PM separation behavior as compared to temperature and salinity; in particular when results are integrated across the PM gradation.