Hydrodynamic separators are stormwater management systems that employ the use of cyclonic and other separation methods to manage water pollution. These systems are designed as flow-through structures that have a separation or settling unit to remove pollutants and sediment. They are considered structural BMPs (best management practices) and are generally used to pre-treat and treat stormwater runoffs.
The term “hydrodynamic separators” itself refers to a set of stormwater management technologies which primarily treat stormwater by using phase separation to remove floating materials (like grease and free oils) and gravity to eliminate settle-able sediment from the water matrix. It is worth noting, however, that these systems aren’t that effective when it comes to the removal of dissolved pollutants or super-fine solids. The technology doesn’t attenuate flows since the units have small detention storage. Due to this, HDS systems are generally used as standalone structures in areas where water quantity control isn’t required or in combination with quantity control technology in areas where stormwater runoff is an issue.
HDS systems are normally installed underground as flow-through structures and serve as part of storm drainage systems. It is also worth noting that these systems are commonly referred to as OGS or Oil and Grit Separators. In addition to phase separation and gravity, OGS systems often include other functions that offer treatment. Some systems are even designed with a single separation technique while others employ the use of various separation techniques, including coalescence action, screening action, swirl action, and by-pass.
By-pass separators: These separators only allows low flows to be treated while higher flows are kept out of the treatment chamber. This keeps particles in the water matrix from re-suspending, something that often happens due to the turbulence synonymous with high inflows.
Screening action separators: These employ custom-made screens to remove solids from water. Flow direction in swirl action separators is tangential to the screen’s surface, which allows stormwater to pass while retaining solids on the inside. Additional settling of solids is attained as flow velocities reduce as water continues to pass through the screen.
Swirl action separators: Separators that use swirl action allow stormwater to enter the system on a tangent to the chamber that stimulates a swirling motion. Gravity removes sediments and deposits them at the bottom of the separator’s chamber.
Coalescence action separators: These separators consist of a series of parallel plates that are normally positioned at an angle of the flow of direction. Small droplets of oil that are suspended in the water matrix adhere to the plate’s surface; as the oil accumulates, larger drops eventually break off from the surface of the plate and float to the surface of the water where they’re trapped.
HDS systems are an excellent option for treating more significant flows of water in single utility hole chambers. Typical types of HDS are made up of modified concrete structures, but lighter-weight, high-performance plastic structure are available too. It is worth noting that hydrodynamic separators need frequent maintenance to operate effectively and can only function as they should if used in conjunction with an enforceable and effective maintenance plan.