Hydrodynamic Separator¶
Source: NJ Stormwater BMP Manual, Chapter 11, Section 11.3 (2026)
Hydrodynamic separators (HDS) are manufactured pre-cast concrete units installed below grade at storm drain inlets or in manholes. They use centrifugal or swirling flow dynamics to separate suspended solids, coarse sediment, and floatables from stormwater runoff before discharge to the conveyance system, a downstream BMP, or a water body. Flow enters the unit tangentially or through a vortex chamber, generating rotational motion that drives sediment to the chamber floor and floatables to a center oil/debris trap while treated water exits through an outlet weir or riser.
Hydrodynamic separators are classified as Non-GI manufactured devices in the 2026 NJ BMP Manual. They do not provide groundwater recharge, do not generate VRC, and by themselves do not consistently achieve the 80% TSS removal standard required for primary water quality compliance under N.J.A.C. 7:8. Their regulatory role is typically as a pretreatment measure protecting a downstream primary BMP from coarse sediment and oil loading, or as supplemental treatment in constrained urban sites.
2026 Classification Update: The 2026 manual explicitly states that hydrodynamic separators as standalone water quality measures do not satisfy the 80% TSS requirement for the WQV standard. They must be combined with a primary water quality BMP.
Primary stormwater functions:
- Sediment and floatable removal — efficient at coarse particles (≥ 75 μm) and oils
- Pretreatment — protects downstream engineered BMPs from coarse sediment loading
- Compact urban footprint — fits within existing or new below-grade utility structure
Source: NJ Stormwater BMP Manual, Ch. 11, Section 11.3 (2026)
| Parameter | 2026 Requirement | 2023 Requirement | Notes |
|---|---|---|---|
| Unit sizing | Manufacturer’s sizing table; based on WQV flow rate (design storm peak flow) | Same | Manufacturer certification data required by NJDEP |
| NJDEP certification | Unit must be on NJDEP list of approved manufactured treatment devices | Same | Non-certified units not eligible for regulatory credit |
| TSS removal efficiency | 50–60% at design flow (coarse fraction only) | Same | Does not meet 80% TSS standard; must be paired with primary WQ BMP |
| Floatables removal | Effective for petroleum films, debris, floatables | Same | Oil/grease trap chamber must be sized per manufacturer |
| Bypass | High-flow bypass required; unit not designed for flows above design capacity | Same | Sizing must include bypass flow path design |
| Maintenance access | 24-inch minimum access opening; confined space protocol required for cleanout | Same | Confined space entry plan required in O&M Agreement |
| Cleanout frequency | Annually or when sediment chamber is 50% full, whichever comes first | Same | Determine by inspection record; adjust frequency based on actual loading |
Source: NJ Stormwater BMP Manual, Ch. 11, Section 11.3 (2026)
NJDEP Certified Devices Only
- Only units on the current NJDEP list of certified manufactured treatment devices are eligible for water quality credit in SWM Reports; verify manufacturer certification status at time of design, not at time of concept planning
Underground Installation
- Below-grade installation requires coordination with other utilities; minimize conflicts with water mains, gas lines, and existing storm sewer
- Unit must be accessible for vacuum truck access within the 24-inch access opening
Flow Rate Matching
- Unit must be sized for the design WQV peak flow rate; undersizing results in poor sediment removal efficiency and overflow through bypass at design event
Not Standalone for WQV
- Cannot be the sole water quality measure on a regulated project; must be part of a treatment train with a primary WQ BMP meeting 80% TSS standard
Source: NJ Stormwater BMP Manual, Ch. 11, Section 11.3 (2026)
Annual Inspection and Cleanout
- Inspect sediment chamber depth annually using a measuring rod through the access port; clean when sediment chamber is at 50% capacity or greater
- Remove sediment and floating debris with a vacuum truck; dispose of per waste characterization requirements (sediment may contain petroleum; confirm disposal classification)
- Inspect outlet weir or riser for debris, biofilm, and structural integrity
Confined Space Safety
- Entry into any hydrodynamic separator unit for maintenance requires a written Confined Space Entry Program per OSHA 29 CFR 1910.146; certify personnel before entry
- Most routine maintenance can be performed with a vacuum truck from the surface through the access port without personnel entry
High-Flow Bypass
- Confirm bypass structure is not obstructed after major storm events; sediment deposited in the bypass weir zone reduces hydraulic capacity
Source: NJ Stormwater BMP Manual, Ch. 8; Ch. 11, Section 11.3 (2026)
Design Errors
- Used as standalone WQV BMP — 50–60% TSS removal does not meet 80% standard; regulatory deficiency discovered at permit review or post-construction inspection
- Unit not on NJDEP certified device list — engineering report cites uncertified device; NJDEP reviewer requires revision; project permit delay
- Undersizing — unit flow capacity below design WQV peak flow; bypass activated during design storm; sediment removal efficiency at actual flow far below design
Construction Issues
- Outlet elevation not coordinated with downstream BMP inlet — head loss through unit backs up into catch basins upstream; stormwater system does not drain as designed
Long-Term Performance Risks
- Maintenance neglect — sediment completely fills separation chamber; device functions as a flow-through pipe with no treatment; oil and sediment pass to downstream BMP media; downstream BMP clogging rate increases significantly
- Oil saturation — without scheduled cleaning, oil/grease reservoir fills; no additional floating petroleum is captured; oil discharges to receiving water or BMP media
Source: NJ Stormwater BMP Manual, Ch. 11, Section 11.3 (2026)
Governing Regulations
| Rule Section | Topic | Engineering Relevance |
|---|---|---|
| N.J.A.C. 7:8-5.3(d) | Water Quality Treatment | Does not meet 80% TSS standard; pretreatment credit only; must pair with primary WQ BMP |
| N.J.A.C. 7:8-5.3 | Green Infrastructure Requirement | Non-GI — no VRC generated |
| NJDEP MTD Program | Manufactured Treatment Device Certification | Unit must be NJDEP-certified for water quality credit |
BMP Manual Sources
- NJ Stormwater BMP Manual, Chapter 11, Section 11.3 (2026) — Hydrodynamic Separators
- NJ Stormwater BMP Manual, Chapter 8 (2026) — Operation and Maintenance
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