Oil/Grit Separator¶
Source: NJ Stormwater BMP Manual, Chapter 11, Section 11.4 (2026)
Oil/grit separators (OGS) are below-grade vault structures installed in storm drain conveyance lines to capture coarse grit, sediment, and petroleum-based floatables before they reach downstream BMPs or water bodies. They function through sedimentation in a quiescent separation chamber and flotation of lighter-than-water petroleum products. Unlike hydrodynamic separators, conventional OGS units rely on gravity settling rather than vortex-induced separation.
Oil/grit separators are classified as Non-GI pretreatment devices in the 2026 NJ BMP Manual. They are widely used as the first element in a treatment train upstream of infiltration-based BMPs (infiltration trenches, bioretention) and manufactured treatment devices that would otherwise receive high coarse sediment and petroleum loading that accelerates clogging and degrades media performance.
Key Distinction from Hydrodynamic Separator: OGS units use passive gravity settling; hydrodynamic separators use engineered vortex flow. Both serve as pretreatment; both require NJDEP certification for formal water quality credit.
Primary stormwater functions:
- Coarse sediment removal — grit and coarse particles (≥ 75 μm) settle in separation chamber
- Oil/floatables removal — petroleum films and floatable debris trapped by baffle
- Pretreatment — protects downstream BMPs from petroleum and sediment loading
- Non-GI: no VRC credit; does not meet 80% TSS standard as standalone measure
Source: NJ Stormwater BMP Manual, Ch. 11, Section 11.4 (2026)
| Parameter | 2026 Requirement | 2023 Requirement | Notes |
|---|---|---|---|
| Unit sizing | Sized for WQV peak flow rate per manufacturer’s sizing table | Same | Certify flow rate compatibility with contributing drainage area |
| NJDEP certification | Must be certified per NJDEP MTD program | Same | Non-certified units not eligible for credit |
| TSS removal | 40–60% at design flow (coarse fraction removal) | Same | 80% TSS standard not met; downstream primary WQ BMP required |
| Sediment chamber | Minimum 1-ft sediment sump depth; sized per manufacturer for expected loading | Same | Cleanout feasibility drives sump design |
| Petroleum trap | Baffle configuration to retain floatables in separate chamber | Same | Ensure outlet configuration prevents floatable bypass at design flow |
| Access | 24-inch minimum access port for vacuum truck cleanout | Same | Confined space entry plan required if personnel entry is needed |
| Bypass | High-flow bypass weir required; unit not rated for large storm flows | Same | Size bypass path to convey 10-year storm |
Source: NJ Stormwater BMP Manual, Ch. 11, Section 11.4 (2026)
High-Petroleum Areas (Required Application)
- OGS are explicitly recommended upstream of infiltration-based BMPs receiving drainage from fueling stations, vehicle service areas, loading docks, and high-impervious parking lots
- Petroleum loading without pretreatment causes rapid biofilm clogging of infiltration media that is extremely difficult to remediate
Underground Utility Conflicts
- Vault installation requires full underground utility location; 811 required before siting
- Coordinate installation level with downstream conveyance structure inverts
Maintenance Access
- Unit must be accessible to vacuum truck within a clean paved or hard surface approach; vacuum truck requires 15–25 ft clear access path to the unit
Source: NJ Stormwater BMP Manual, Ch. 11, Section 11.4 (2026)
Cleanout Schedule — Annually or When 50% Full
- Inspect sediment sump depth through access port annually and after major storms; vacuum-clean sediment chamber when 50% of design sump volume is occupied
- Verify petroleum trap chamber does not show accumulated oil layer exceeding trap skimmer capacity
- Remove oil/debris from petroleum trap during same cleanout event
Waste Characterization
- Sediment from OGS units at fueling stations or vehicle service areas must be characterized as potentially petroleum-contaminated waste before disposal; consult waste disposal rules
Structural Inspection
- Inspect concrete vault for cracking, spalling, or joint sealant failure annually
- Confirm outlet baffle integrity; damaged baffles allow petroleum to bypass to conveyance
Source: NJ Stormwater BMP Manual, Ch. 8; Ch. 11, Section 11.4 (2026)
Design Errors
- Standalone water quality measure for WQV compliance — 40–60% TSS removal not sufficient; regulatory deficiency; downstream primary BMP required
- Unit not NJDEP-certified — manufacturer submits product that is not on current NJDEP approved list; water quality credit denied at permit review
- Insufficient access for petroleum area — OGS not provided upstream of infiltration trench or bioretention receiving parking lot drainage; downstream BMP fails early from petroleum clogging
Construction Issues
- Inverts not coordinated — OGS outlet set higher than downstream BMP inlet; backwater affects OGS treatment zone depth; below-design performance
Long-Term Performance Risks
- Maintenance neglect — after sediment fills chamber, device provides no treatment; coarse particles pass through to downstream BMP; accelerated clogging
- Petroleum saturation — oil trap fills between cleanout cycles; petroleum films discharge downstream; potential NJPDES permit violation if discharge point is a surface water body
Source: NJ Stormwater BMP Manual, Ch. 11, Section 11.4 (2026)
Governing Regulations
| Rule Section | Topic | Engineering Relevance |
|---|---|---|
| N.J.A.C. 7:8-5.3(d) | Water Quality Treatment | Pretreatment credit; does not meet 80% TSS standard alone |
| N.J.A.C. 7:8-5.3 | Green Infrastructure Requirement | Non-GI — no VRC credit |
| NJDEP MTD Certification | Manufactured Treatment Device Program | Unit must be certified for regulatory water quality credit |
BMP Manual Sources
- NJ Stormwater BMP Manual, Chapter 11, Section 11.4 (2026) — Oil/Grit Separators
- NJ Stormwater BMP Manual, Chapter 8 (2026) — Operation and Maintenance
Related OPAL Pages