Silt traps specifically work with suspended sediment as a result of construction activity or water runoff. In addition they are used for the treatment of waste water from commercial discharge.
The design is to slow the influent to allow contaminated water full of silt and other contaminates to settle before discharging into the sewer system.
While the water is in the trap, the sediment will settle to the bottom of the trap until it can be removed. This also helps ensure the stormwater system works effectively and efficiently.