Oil Grit Separators: Protecting Water Quality from First Flush Runoff

When it rains, the very first water that washes off roads, parking lots, and other paved surfaces carries a heavy load of pollutants. This “first flush runoff” often contains oil, grease, sediment, trash, and heavy metals that can quickly degrade water quality if left untreated. That’s where Oil Grit Separators (OGS) come in.

These stormwater management devices are designed to capture and treat the most polluted portion of runoff before it reaches storm drains, waterways, or underground detention systems. For property managers and municipalities, installing and maintaining oil grit separators is a proactive step in keeping facilities compliant with local stormwater regulations while protecting the environment.

What Is An Oil Grit Separator?

An oil grit separator (OGS), sometimes called a hydrodynamic separator, is a stormwater treatment structure that removes pollutants from runoff through a combination of:

• Gravity separation – allowing heavier sediments and grit to settle at the bottom.
• Oil and grease trapping – floating hydrocarbons are captured at the water’s surface.
• Trash screening – larger debris is held back from entering downstream systems.

They are commonly installed beneath parking lots, at roadway intersections, near fueling areas, and at other high-pollutant sites where runoff quality is a concern.

Common Oil Grit Separator Brands

While the term oil grit separator (OGS) is a general category, many systems are sold under proprietary brand names. Some of the most widely used include:

• Stormceptor® (Contech Stormwater Solutions)
• CDS® Unit (Continuous Deflective Separation, Contech)
• Downstream Defender® (Hydro International)
• BaySeparator® (BaySaver Technologies, ADS)
• Aqua-Swirl® (AquaShield, Inc.)
• Vortechs® System (Environment 21, Contech)

Each brand uses slightly different hydrodynamic separation technology, but they all serve the same purpose: capturing sediment, oil, and debris from stormwater runoff to protect downstream systems and water quality.

Why First Flush Runoff Matters

The first 0.5 to 1 inch of rainfall carries the majority of pollutants. According to the U.S. Environmental Protection Agency (EPA), this first flush effect can account for over 80% of the total pollutant load in a storm event.

Without treatment, this contaminated runoff can lead to:

• Clogged stormwater systems.
• Oil slicks and sediment buildup in local streams and ponds.
• Regulatory violations under the Clean Water Act and local stormwater ordinances.
• Increased costs for long-term maintenance and remediation.

Benefits of Oil Grit Separators

Installing an OGS system offers multiple benefits for both property managers and the environment:

• ✅ Improved water quality – captures oil, grit, and floatables before they reach waterways.
• ✅ Regulatory compliance – helps meet NPDES, MS4, and local stormwater permit requirements.
• ✅ System protection – prevents clogging and sediment overload in downstream detention ponds, infiltration systems, or filters.
• ✅ Low maintenance – requires routine inspection and cleaning, but no electricity or chemicals.
• ✅ Space efficiency – compact design works well in urban and commercial settings.

Maintenance Requirements for Oil Grit Separators

Like any stormwater BMP, oil grit separators only work if they’re maintained. Over time, sediment and oil build up inside the unit, reducing performance.

Best practices for OGS maintenance include:

• Routine inspections – typically quarterly or after major storm events.
• Cleaning frequency – usually 1–2 times per year, depending on pollutant load.
• Vacuum pumping – specialized equipment removes accumulated sediment, oil, and debris.
• Documentation – keeping inspection logs to remain in compliance with local and state regulations.

Failing to maintain an OGS can result in bypassing, where polluted runoff flows straight into storm drains, undoing the benefits of the system.

Where Oil Grit Separators Fit in a Stormwater Management Plan

An OGS is often used as a pretreatment measure rather than a standalone solution. They work best in combination with other BMPs, such as:

• Detention and retention ponds – for long-term storage and treatment.
• Bioretention systems – filtering water through engineered soils.
• Underground storage – for managing peak flows.
• Infiltration systems – to recharge groundwater.

By capturing the dirtiest portion of runoff first, oil grit separators extend the life and efficiency of downstream stormwater systems.

Key Takeaways

• Oil Grit Separators (OGS) are critical for removing oil, sediment, and debris from polluted first flush runoff.
• They help property managers, municipalities, and contractors stay compliant while protecting water quality.
• Regular maintenance – typically 1-2 cleanings per year – is essential for proper function.
• OGS units are most effective when integrated into a comprehensive stormwater management plan.

Partner with Stormwater Experts

If your property includes high-traffic areas, fueling stations, or large parking lots, an oil grit separator may be the best way to protect your stormwater system and stay compliant. At iSTORMWATER, we provide inspection, maintenance, and compliance services to keep your systems performing at their best.

📞 Call us at (410) 231-3455 or 💬 Contact us online to schedule an inspection or discuss stormwater solutions tailored to your site.