Sediment

=== Overview ===
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 and grit separators (OGS)
* Catch basins and pre-treatment sumps
* Vegetated swales
=== Types of Settling ===
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 ===
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.
Permanent pool storage, surface loading rates and detention times for sedimentation practices. Calculations are based on a one hectare drainage area with 85% imperviousness, peak flow rate for 4 hour Chicago distribution for 25 mm event and runoff coefficient of 0.81.References include: MOE, 2003; City of Toronto catch basin standard (T 705.010); OGS vendor technical manuals, TRCA OGS sizing review tool.
Practice 	Permanent Pool/m3/Ha 	Surface Area/m2/Ha 	Surface Loading Rate/L/min/m2 	Detention Time/hrs 	Assumptions
Enhanced Treatment Practices (80% TSS removal) 	Enhanced Treatment Practices (80% TSS removal) 	Enhanced Treatment Practices (80% TSS removal) 	Enhanced Treatment Practices (80% TSS removal) 	Enhanced Treatment Practices (80% TSS removal) 	Enhanced Treatment Practices (80% TSS removal)
Wet ponds/ Underground Tanks 	250 	167 	33 	>24 	Underground tanks sized like ponds, average permanent pool depth =1.5m
Stormwater Wetland 	140 	622 	8.8 	>24 	Average depth =0.225m
Pre-treatment Practices (approx. 30-50% TSS removal)	Pre-treatment Practices (approx. 30-50% TSS removal)	Pre-treatment Practices (approx. 30-50% TSS removal)	Pre-treatment Practices (approx. 30-50% TSS removal)	Pre-treatment Practices (approx. 30-50% TSS removal)	Pre-treatment Practices (approx. 30-50% TSS removal)
Oil Grit Separator 	12 	4.7 	1177 	Negligible 	Varies by manufacturer. 2.44m (8ft) diameter unit with permanent pool depth of 2.6m. 60% removal objective.
Catch Basin 	1.6 	1.8 	3054 	Negligible 	5 catch basins (0.6 x 0.6m) per Hectare. Permanent pool depth of 0.9m.
=== Particle Settling and Particle Size Distribution ===
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 ===
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.
Comparison of particle size distributions from previous studies in cold climates
Land use 	Number of observations	Location 	Samplinglocation 	Samplingmethod	Analyticalmethod 	d10/µm	d50/µm	d90/µm	Source
Mixedresidential 	66 	GTA 	Stormsewer 	Auto-sampler 	Coulter counterparticle analyzer 	0.9 	5.9 	44 	SWAMPhttps://sustainabletechnologies.ca/app/uploads/2013/01/Final_SWAMP_Synthesis.pdf
Parkinglot 	44 	Markham 	Stormsewer 	Auto-sampler 	Coulter counterparticle analyzer 	2.0 	9.0 	92 	SWAMP
Commercialparking lot	1 	Calgary 	Stormsewer 	NA 	Coulter counterparticle analyzer 	2.1 	8.5 	25.9 	
Public busyard 	11 	Toronto 	Stormsewer 	Auto-sampler 	Micro-imaging 	4.0 	19.2 	154.1 	
Commericalparking lot	45 	Madison,WI 	Stormsewer 	Auto-sampler 	Wet sieve (>32um);particle analyzer (<32um) 	<2 	55 	500 	
Mixed use 	20 	Madison, WI 	Stormsewer 	Auto-sampler 	Wet sieve (>32um);particle analyzer (<32um) 	<2 	42 	200 	
Parking lot 	94 	Madison, WI 	Stormsewer 	Auto-sampler 	Wet sieve (>32 um);particle analyzer (<32um)	2 	54 	350 	
Parking lot 	18 	NewHampshire 	Stormsewer 	Auto-sampler 	Particle analyzer 	- 	46 	- 	
Mixed use(roads) 	176 	Ohio 	Catchbasin 	Auto-sampler 	Coulter counterparticle analyzer 	11 	52.5 	145 	Winston, R. J., & Witter, J. D. (2019). Evaluating the particle size distribution and gross solids contribution of stormwater runoff from Ohio’s roads (Final Report, State Job Number 135258). Ohio Department of Transportation, Office of Statewide Planning & Research.
Mixed use 	7 	Toronto 	Stormoutfall 	Grab 	Coulter counterparticle analyzer 	4.0 	16.4 	55.1 	
Parking lot 	12 	Toronto,Vaughan, Brampton	Directrunoff, man hole	Grab 	Coulter counterparticle analyzer 	2.6 	12.4 	55.2 	
==References==