Editor’s note: This article from the Stormwater archive has not been updated. Some details may no longer be accurate.
This is the first article in a series on gravity separation, eventually leading to the best method for sizing basins, using field data as confirmation of the method. Gravity separation is a set of unit processes in which gravity removes settleable solids and associated pollutants, floatables, and dispersed petroleum products. Gravity separation is the primary mechanism of pollutant removal in stormwater treatment systems. Removal occurs downward for solids denser than water like sediment, and upward for solids lighter than water such as dispersed droplets of petroleum, oil, and paper. The former is sedimentation; the latter is flotation.
Editor’s note: This article from the Stormwater archive has not been updated. Some details may no longer be accurate. This is the first article in a series on gravity separation, eventually leading to the best method for sizing basins, using field data as confirmation of the method. Gravity separation is a set of unit processes in which gravity removes settleable solids and associated pollutants, floatables, and dispersed petroleum products. Gravity separation is the primary mechanism of pollutant removal in stormwater treatment systems. Removal occurs downward for solids denser than water like sediment, and upward for solids lighter than water such as dispersed droplets of petroleum, oil, and paper. The former is sedimentation; the latter is flotation. [text_ad] Figure 1 conceptually displays gravity separation. The fundamental engineering principle of gravity separation is the settling velocities of particles, and recognition of two types of settling in stormwater treatment: dynamic and quiescent. Settling velocity is affected by particle size, shape, and specific gravity and by water temperature. This article discusses sedimentation, the downward removal of stormwater particles. Settleable Solids There is a distinction between suspended and settleable solids. Total suspended solids (TSS) are those solids captured by a laboratory filter. Some are so small they will not settle. Settleable solids are defined as those solids that settle in an Imhoff cone. An Imhoff cone is made of clear plastic or glass. Imagine a traffic cone placed upside down: That is the vision of an Imhoff cone. Settleable solids are measured by placing one liter of a thoroughly mixed sample into the cone and measuring the number of milliliters of solids accumulated at the bottom after 60 minutes (milliliters per liter). However, this definition of settleable solids understates what actually settles, because the detention time, also referred to as the hydraulic residence time (HRT), in settling basins is typically greater than one hour. The true settable solids are, in the case of wet basins, those that settle over a period of several days. Solids are further categorized by type: organic, inorganic, biological, and chemical. The specific gravities and shape of each type and therefore settling velocities differ significantly. Organic solids are lighter than inorganic solids. Some are lighter than water, although most have specific gravities between 1.1 and 1.5. The specific gravity of most inorganic solids ranges from 1.5 to 2.65, with sand 2.45 to 2.65. Some clays have a much lower specific gravity because of entrained water, on the order of 1.5. Biological solids refer to the bacterial flocs created in wastewater treatment systems. Chemical floc is produced in coagulation processes. The engineer’s definition of clay is by size, rather than by mineral form as with soils. Hence, solids as defined in by engineers, rather than by soil scientists, likely contains organic matter. [caption id="attachment_43314" align="aligncenter" width="600"]
Figure 1. Elements of gravity separation[/caption] Types of Sedimentation There are four types of sedimentation as applied to stormwater, water, and wastewater treatment: discrete, flocculent, hindered, and compression. Only the first two are relevant to stormwater treatment. An exception may be treatment basins at construction sites in which the final two types might be relevant. For a solution where the suspension is initially relatively dilute, the suspension is considered discrete. As settling progresses, particles may flocculate as they contact each other. Near the bottom of the basin, the suspension density is sufficient to hinder settling, reducing the settling velocity. At the bottom the solids thicken by compression from the weight of the accumulation. Depending on the suspended solids, the suspension may not flocculate, moving directly from discrete to hindered settling. [text_ad use_post='27751'] The stormwater solids suspension is likely to be both discrete and flocculating. The suspension has three overlapping components: coarse materials that settle quickly as discrete particles (>50 microns), intermediate size silt that tends to naturally flocculate while settling (5 to 50 microns), and clay-size particles (<5 microns) that neither settle easily nor flocculate quickly because they are colloidal, negatively charged, and resist coagulation. Furthermore, the settling of clay-size particles is inhibited by Brownian motion. The classification system applies to vaults and basins, flotation systems, and swirl concentrators. Editor's note: This story is continued here: "The Principles of Gravity Separation, Part 2".
Figure 1 conceptually displays gravity separation. The fundamental engineering principle of gravity separation is the settling velocities of particles, and recognition of two types of settling in stormwater treatment: dynamic and quiescent. Settling velocity is affected by particle size, shape, and specific gravity and by water temperature. This article discusses sedimentation, the downward removal of stormwater particles.
Settleable Solids
There is a distinction between suspended and settleable solids. Total suspended solids (TSS) are those solids captured by a laboratory filter. Some are so small they will not settle. Settleable solids are defined as those solids that settle in an Imhoff cone. An Imhoff cone is made of clear plastic or glass. Imagine a traffic cone placed upside down: That is the vision of an Imhoff cone. Settleable solids are measured by placing one liter of a thoroughly mixed sample into the cone and measuring the number of milliliters of solids accumulated at the bottom after 60 minutes (milliliters per liter). However, this definition of settleable solids understates what actually settles, because the detention time, also referred to as the hydraulic residence time (HRT), in settling basins is typically greater than one hour. The true settable solids are, in the case of wet basins, those that settle over a period of several days.
Solids are further categorized by type: organic, inorganic, biological, and chemical. The specific gravities and shape of each type and therefore settling velocities differ significantly. Organic solids are lighter than inorganic solids. Some are lighter than water, although most have specific gravities between 1.1 and 1.5. The specific gravity of most inorganic solids ranges from 1.5 to 2.65, with sand 2.45 to 2.65. Some clays have a much lower specific gravity because of entrained water, on the order of 1.5. Biological solids refer to the bacterial flocs created in wastewater treatment systems. Chemical floc is produced in coagulation processes. The engineer’s definition of clay is by size, rather than by mineral form as with soils. Hence, solids as defined in by engineers, rather than by soil scientists, likely contains organic matter.
Types of Sedimentation
There are four types of sedimentation as applied to stormwater, water, and wastewater treatment: discrete, flocculent, hindered, and compression. Only the first two are relevant to stormwater treatment. An exception may be treatment basins at construction sites in which the final two types might be relevant. For a solution where the suspension is initially relatively dilute, the suspension is considered discrete. As settling progresses, particles may flocculate as they contact each other. Near the bottom of the basin, the suspension density is sufficient to hinder settling, reducing the settling velocity. At the bottom the solids thicken by compression from the weight of the accumulation. Depending on the suspended solids, the suspension may not flocculate, moving directly from discrete to hindered settling.
The stormwater solids suspension is likely to be both discrete and flocculating. The suspension has three overlapping components: coarse materials that settle quickly as discrete particles (>50 microns), intermediate size silt that tends to naturally flocculate while settling (5 to 50 microns), and clay-size particles (<5 microns) that neither settle easily nor flocculate quickly because they are colloidal, negatively charged, and resist coagulation. Furthermore, the settling of clay-size particles is inhibited by Brownian motion. The classification system applies to vaults and basins, flotation systems, and swirl concentrators.
Editor’s note: This story is continued here: “The Principles of Gravity Separation, Part 2“.
