Sedimentation facilities

Typical design

Although sedimentation facilities can take different forms, they all have in common that their primary aim is to use the process of particles settling to the bottom of a volume of water to remove pollutants.

Sedimentation requires a certain amount of time, depending on the size of the particle. Assuming a 2 m deep water body, large particles such as coarse sand (approximately2 mm) can settle in as little as 10 seconds, silt particles of approximately 62 µm can take 15 minutes, while even smaller particles (1 µm) this can take multiple weeks. A critical part of the design of any sedimentation facility is therefore to provide enough residence time for the water so that a substantial fraction of the particles can settle.

One common type of facility is a sedimentation pond or basin. These can be similar in design to retention ponds (wet ponds), since both are normally designed to have a permanent volume of water stored. However, for a sediment pond it is essential that the flow is evenly distributed over the whole pond surface so that the particles will have enough time to settle. Short-cuts or dead zones should be avoided. Therefore, it can be useful to use dams or baffles to increase the length of the flow path, or to choose a shape of pond where there is a larger distance between the inflow and outflow points. Vegetation in the pond can also help reduce flow velocities and improve sedimentation.

As with biofilters, a pre-sedimentation section can be included where the coarsest particles can settle. This volume of coarser, often less polluted, sediment can be emptied more easily.

Another type of sedimentation facilities are underground sedimentation chambers. In some cases, this can make it possible to provide some sedimentation when there is insufficient space on the surface. However, a downside is that it is more difficult to remove the accumulated sediment from the chamber.

Aims in stormwater management

Sedimentation facilities are primarily aimed at removing pollutants from stormwater. Therefore, they are designed for small rains with return intervals <1 year or with a certain rainfall depth (e.g. 10 or 20 mm). For larger rains the sedimentation facilities usually have an overflow or bypass to avoid damage to the facility or re-suspension of the sediment due to high flows, so there will be no effect on runoff rates or volumes.

Sedimentation works mainly for removing larger particles (for instance >125 – 250 µm) and pollutants associated with them from the water. If it is important to remove dissolved pollutants (or pollutants in very small particles) other facilities such as bioretention or constructed stormwater wetlands are better suited.

Implementation in catchments

As described above, sedimentation requires a certain amount of time and therefore the pond needs to be large enough to provide sufficient residence time. Common estimates are 1.5% of the impervious catchment area. With small ponds it can be difficult to reach sufficient residence time, storage depth and acceptable side slopes, so in general it is recommended that ponds should be at least 150-250 m2. This means sedimentation ponds are not very suitable to place throughout the catchment and are instead more suitable as an end-of-pipe solution.

Additional benefits

• Ponds can contribute to aesthetic values, biodiversity and recreational values.
• Underground facilities do in general not provide additional benefits.
• Floating baffles can be combined with a floating jetty to provide recreational values, e.g. for boats or walking.