Sediment
Overview[edit]
Sedimentation is a fundamental process used in stormwater treatment to remove pollutants from runoff. It relies on gravity to deposit suspended sediment and debris into a designated settling area for later removal. Many pollutants that adhere to solids, such as metals, phosphorus, and hydrocarbons, are also removed through this process. Stormwater treatment practices that primarily use sedimentation include:
- Stormwater wet ponds and dry ponds
- Underground storage tanks and chambers
- Stormwater wetlands
- Oil grit separators (OGS)
- Catch basins and pre-treatment sumps
- Vegetated swales
Types of Settling[edit]
Sedimentation occurs in two main forms:
Dynamic Settling – Takes place during active storm flow conditions when runoff is moving through the system.
Quiescent Settling – Occurs between storm events when water is retained in a storage system, allowing suspended particles to settle under still conditions.
Practices such as ponds and wetlands rely on quiescent settling, whereas systems like oil grit separators and catch basins rely more on dynamic settling. The effectiveness of sedimentation systems is largely governed by the surface loading rate (SLR), which is the rate at which water is introduced to the settling area. Higher SLRs generally result in lower pollutant removal efficiency.
Key Design Parameters[edit]
The performance of sedimentation systems depends on several factors:
Permanent Pool Volume – The volume of water permanently stored in a system, aiding quiescent settling.
Surface Loading Rate – The volume of water per unit surface area per unit time.
Detention Time – The length of time water remains in the system before discharge.
Particle Size Distribution – Determines the settling velocity of solids in runoff.
Inlet and Outlet Design – Properly designed structures reduce resuspension and short-circuiting of flow.
table[edit]
| Practice | Permanent Pool Volume (m³/ha) | Surface Area (m²/ha) | Surface Loading Rate (L/min/m²) | Detention Time (hrs)
Wet Ponds/Tanks
250
167
33
>24 - Stormwater Wetlands
140
622
8.8
>24 - Oil Grit Separator
12
4.7
1177
Negligible - Catch Basin
1.6
1.8
3054
Negligible } Particle Settling and Particle Size Distribution[edit]Settling velocity is influenced by particle size, shape, specific gravity, and water temperature. Coarser particles settle faster, whereas fine silts and organic matter take longer. Temperature affects viscosity, meaning lower temperatures reduce settling efficiency. Understanding particle size distributions at a site is critical for designing effective sedimentation systems. Research has shown wide variations in particle sizes in urban runoff, influenced by factors such as native soil texture, tree canopy cover, traffic levels, and road maintenance practices. While laboratory analyses help determine particle size, field studies provide more practical insights into sedimentation system performance. Best Practices for Sedimentation-Based Stormwater Treatment[edit]Design for Adequate Surface Area – Systems should have sufficient area to slow down flow and enhance settling. Maintain Low Surface Loading Rates – Lower SLRs improve treatment performance. Use Pre-Treatment Systems – Oil grit separators and catch basins can extend the life of downstream sedimentation systems. Incorporate Periodic Maintenance – Regular sediment removal prevents resuspension and system clogging. Account for Seasonal Variability – Adjust design parameters based on temperature effects on settling velocity. By understanding sedimentation principles and incorporating best practices into stormwater management design, communities can improve water quality and reduce the impact of urban runoff on aquatic ecosystems. |
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