Stormwater will often contain polluting material that is best removed before it is allowed to reach water bodies in to which they are normally disposed of. Hydrodynamic separation technologies allow this removal of sediments.

Most hydrodynamic separators are devices that are designed in such a way that sediments and pollutants get settled and removed while the stormwater passes through them. The energy of the flowing water itself acts as a source of power in these separators, that allows the efficient separation of the sediments in the stormwater. This can be done either through indirect filtration or through a swirl action that the water is subjected to.

These separators are more effective when the sediments in the stormwater are heavy and can easily settle, or contain floatables that can be captured. They will not be so effective if the pollutants are dissolved or cannot settle easily. They can also be designed with baffles and other devices that reduce the velocity of the water flow, giving the solids more time to settle.

Hydrodynamic separation technologies are best used as part of an overall best practice for stormwater management. They are manufactured in various sizes and some are small enough to be fitted into standard manholes on a drainage system. This makes them ideal for use in areas where limited land is available. They can be placed in any location that is specified, but it is best to use them in areas that are hot spots for pollutants.

The inlets into these separators are positioned in such a way that they are eccentric to the chamber and thus create an automatic swirl in the water that helps in forcing the solid pollutants to the sidewalls, reducing their speed and forcing them to settle into the separator body and its solids collection hopper. The outlet for the stormwater is at a higher level and is only activated when the water in the separator starts overflowing. Some separators may contain additional filters or baffles that enable the separation process.

These hydrodynamic separators have no moving parts and are therefore not demanding of maintenance. They must be inspected periodically and are considered full and will become non-functional when the sediment level is within a foot of the top of the unit. They must then be cleared manually of the sediment or with vacuum trucks. Though hydrodynamic separators require very little maintenance, not paying attention to their sumps will reduce their efficiency.