Hydrodynamic separators are primarily designed to treat stormwater runoff by removing pollutants such as sediment, debris, oil, and grease. They are typically installed to improve water quality before it enters receiving bodies such as rivers, lakes, oceans, or groundwater systems. Requirements are simple: the inlet and outlet pipes are sized in accordance with the flow, the hydrodynamic separator is sized in accordance with the pollutant removal, and water is allowed to flow freely from the inlet to the outlet of the system. 

But what happens when tailwater conditions obstruct the flow?

Tailwater refers to the water downstream of a hydraulic structure. Common causes include dams, spillways, bridges or culverts. Tidal influence and high groundwater can also cause tailwater conditions. In tailwater conditions, the flow of water is typically slower and above the outlet pipe invert.

When tailwater is present, it is essential to consider its effects and design accordingly. The flow may be slower and more sediment-laden compared to typical stormwater runoff. This could affect the performance of the hydrodynamic separator, as it may become overloaded with sediment or may not achieve the desired level of pollutant removal. Moving parts may become damaged if designed for flow in one direction only. Additional maintenance measures, such as plugging the outlet pipe, will also be necessary.

While hydrodynamic separators are not typically designed specifically for tailwater conditions, they can still be used in certain scenarios depending on the design and flow characteristics. For example, increasing the available driving head within the unit can ensure that the full design flow rate is still treated prior to any internal bypass. However, their effectiveness in tailwater conditions may vary depending on factors such as flow velocity, sediment load, and the specific design of the separator.

Before using a hydrodynamic separator in tailwater conditions, it's important to carefully consider the site-specific conditions and consult with engineers or specialists familiar with both hydrodynamic separators and tailwater environments. Additionally, modifications to the design or operation of the separator may be necessary to ensure optimal performance in tailwater conditions.