Hydrodynamic separators are being studied for treating stormwater runoff in Utah Department of Transportation basins. The researchers identified pollutants like sediment, metals, and oil in stormwater, and developed methods to estimate pollutant loads. They found that hydrodynamic separators can remove sediment from stormwater, but not oil and grease. These separators are designed to treat small storm events and the first flush of larger storms. The researchers recommend using a design water quality storm event of 0.5 inches of rainfall to size the separators.