Compare oil/water separators, hydrodynamic separators, catch basin inserts and booms for capturing debris and oil. Learn what each device removes, where it stops, and how to size and verify one.
A debris and oil stormwater treatment system can be an oil/water separator, a hydrodynamic separator, a catch basin insert or a boom. Each captures trash, sediment and free or particulate-bound oil. Dissolved hydrocarbons need sorptive media. Size the device to treat the full design water quality event without bypass.
Which stormwater systems capture both debris and oil?
Four device families combine physical debris capture with an oil stage: oil/water separators, hydrodynamic separators, catch basin inserts and booms. The table summarizes what each captures and where it stops.
| Device | Debris and sediment | Free oil | Dissolved hydrocarbons |
|---|---|---|---|
| Oil/water separator | Screens, grit settling | Gravity or coalescing plates | Not targeted (free oil only) |
| Hydrodynamic separator | Swirl and screens, litter sump | Floatables and surface oil trapped | Poor fit for dissolved pollutants |
| Catch basin insert | Trash and sediment held in the insert | Media removes petroleum products | Needs a sorptive media chosen for the purpose |
| Boom | Floatable trash | Can be designed to absorb oils | Not targeted |
How do oil/water separators work?
An oil/water separator, also called a water quality inlet or oil/grit separator, uses a series of chambers that promote sedimentation of coarse materials and separation of free oil, as opposed to emulsified or dissolved oil. A typical unit has a sedimentation chamber, an oil separation chamber and a discharge chamber. Most also have screens for larger or floating debris, and newer designs add a coalescing unit.
Separators use two main methods of intercepting oil: a conventional gravity separator and a coalescing plate interceptor. They fit sites with automotive contamination or other high hydrocarbon loads. If your problem is a visible floating layer, look at this family first.
How do hydrodynamic separators differ on free oil and dissolved hydrocarbons?
A hydrodynamic separator is a modified traditional oil-grit separator. A cylindrical chamber creates a swirling motion that separates sediment. Extra chambers may trap oil and other floatables. It takes up a smaller working area than a conventional gravity separator. It removes debris, sediment and surface grease and oil.
The limit is in the fraction. Free oil and gross litter are a good match. Poorly settleable solids and dissolved hydrocarbons pass through.
What fraction of oil is actually free, particulate or dissolved?
Oil and grease arrives in several forms, so the fraction decides which stage you add. Free oil is what water quality inlets are designed to separate. Emulsified and dissolved oil need something else.
A working rule follows from that:
- Free oil goes to a separator or hydrodynamic unit.
- Particulate-bound oil goes to sediment and trash capture.
- Emulsified and dissolved hydrocarbons go to a sorptive stage, tested on your own water.
How do catch basin inserts and hydrophobic media capture oil, sediment and litter?
An insert holds trash and sediment in its body while the media treats oil. EPA describes a box that fits directly into the catch basin, removes petroleum products as stormwater passes through, and captures trash and sediment. Some insert products offer different filter elements, with hydrocarbons and heavy metals handled depending on the element chosen.
Hydrophobic, oil-attracting, polymeric-based media absorbs and retains hydrocarbons. Activated-carbon-type media targets dissolved hydrocarbons and organic compounds. The limits are practical. Loading, maintenance and the character of your water decide how long an insert works, and a full insert can stop capturing. For help choosing one, see this guide to catch basin retrofit stormwater treatment.
What about booms and skimmers at outfalls?
Booms add an end-of-pipe stage for floating material. They use floatation structures with suspended curtains to capture buoyant materials and can be designed to absorb oils and grease. Crews anchor them to the shoreline and bottom downstream of one or more outfalls. Booms suit floating sheen and litter. They do not replace upstream treatment for oil that is emulsified or dissolved.
What third-party verification shows a device works?
Ask for independent, verified test data that matches your conditions. Manufactured treatment device removal efficiencies are variable and highly dependent on storm size, influent pollutant concentrations, rainfall intensity and other factors, so a single headline percentage tells you little. Check that the test covered the fraction you care about: free, particulate or dissolved. For dissolved hydrocarbons, laboratory column testing on a sample of your own water is the most direct check before a full-scale purchase.
What trash and oil and grease requirements apply to permits?
Requirements vary by permit, so confirm them in your own documents. For an MS4, read the trash provisions that apply to your jurisdiction. For industrial sites, find the oil and grease benchmark or action level in your MSGP or state general permit, and note the sampling method it specifies. The benchmark tells you which fraction to treat. A sheen problem points to separation. A repeated exceedance on a clean-looking sample may point to dissolved or emulsified oil.
How should you size and site the system?
Size the device to treat the whole design water quality event with no bypass, and place it where the pollutants originate. One county manual says to use devices designed to treat the entire water quality event with no bypass. Where it applies, it also says to model the 1-year 24-hour storm with 1.8 inches of rainfall. That is one county's value, so confirm the figure your own jurisdiction uses.
EPA notes that water quality inlets, because they remove hydrocarbons, are typically located at sites with automotive-related contamination or other sites with high hydrocarbon concentrations. Work from the design flow and drainage area to the available footprint, then check maintenance access. Those inputs also decide whether a passive format works or a pumped system is needed. You can compare treatment stages in this guide to what makes up a stormwater management system, and find related topics on the Abtech blog.
If you want help matching media and format to your site, a site assessment is the usual first step. Contact Abtech Industries to get started.