Phase 2D — Non-GI Stormwater Practices and Manufactured Treatment Devices (MTDs)¶
Source Materials: NJ Stormwater BMP Manual, Chapter 11 (all sub-sections, 11.1–11.6) — 2023 and 2026 Editions Date: March 5, 2026 | OPAL Stormwater Engineering Knowledge System
Overview¶
Not every site can support Green Infrastructure. Shallow bedrock, high seasonal water tables, contaminated fill, subsurface utilities, and dense urban development patterns all constrain or eliminate the native soil infiltration capacity on which GI BMPs depend. Chapter 11 of the NJ Stormwater BMP Manual covers the Non-GI stormwater practices and Manufactured Treatment Devices (MTDs) available when GI is infeasible or insufficient. These systems cannot generate Volumetric Reduction Credit (VRC) toward the GI Requirement, but they remain essential tools for meeting the Water Quality Standard (≥80% TSS removal) and the Quantity Standard (peak rate attenuation) on sites where the infiltration-based GI framework cannot be fully applied.
The 2026 edition substantially reorganizes Chapter 11 to make the Non-GI classification explicit, to formalize the MTD performance verification protocol, and to address the newly defined GI Retrofit pathway (see Phase 2A) that allows some Non-GI systems to be converted to GI status.
Section 1: Overview of Non-GI Stormwater Treatment Approaches¶
1.1 Role of Non-GI Practices in Stormwater Management¶
Non-GI stormwater BMPs serve a defined and necessary role in the New Jersey regulatory framework even though they cannot satisfy the volumetric reduction component of the GI Requirement. Their functions include:
Water Quality Treatment (the TSS Standard): N.J.A.C. 7:8 requires that major development achieve ≥80% TSS removal by mass from the water quality storm (WQV event). Non-GI systems are capable of meeting this standard and may be used to satisfy TSS removal obligations where GI is infeasible.
Stormwater Quantity Control (the Peak Rate Standard): N.J.A.C. 7:8 requires that post-development peak discharge rates not exceed pre-development rates for specified storm frequencies (2-year and 100-year storms in most cases). Dry detention basins, extended detention basins, and blue roofs provide the storage volume required for peak attenuation independent of infiltration.
Partial GI Compliance: On sites where only a portion of the WQV can be addressed through GI BMPs (due to site constraints), Non-GI practices address the remaining water quality and quantity obligations for the portion of runoff that cannot be infiltrated, evapotranspirated, or harvested.
1.2 Situations Where Non-GI Systems Are Used¶
Typical site conditions that require Non-GI approaches: - Seasonal high water table within 2.0 feet of proposed BMP bottom — prohibits native infiltration - Native soil Ksat below qualifying threshold (HSG D soils; heavily compacted or contaminated fill) — prohibits infiltration-based GI - Dense urban sites with constrained footprints — insufficient land for infiltration-based BMP surface area - Contaminated sites (brownfields) where infiltration would recontaminate groundwater or mobilize pollutants - Ultra-urban settings where maintenance access for vegetated GI is not practical - Sites where structural constraints (underground parking, utilities, existing foundations) prevent excavation to natural soil
Section 2: Manufactured Treatment Devices (MTDs)¶
2.1 Purpose and Function¶
Manufactured Treatment Devices (MTDs) are proprietary stormwater treatment units — engineered and fabricated off-site — that provide stormwater quality treatment within a compact, below-grade structure. They are designed for installation in urban and suburban settings where land area for conventional surface-based stormwater practices is unavailable, and where underground installation in existing or new paved areas is the only practical treatment option.
MTDs do not provide volumetric reduction — they treat stormwater that passes through them but do not permanently reduce the volume of runoff leaving a site. Their role is TSS removal toward the Water Quality Standard. For sites where the GI Requirement cannot be fully met, MTDs may be used alongside GI BMPs to satisfy the TSS standard for the portion of runoff not captured by GI practices.
2.2 Treatment Mechanisms¶
The NJ BMP Manual identifies two primary MTD treatment technology categories:
Hydrodynamic Separator / Swirl Concentrator: Stormwater enters a cylindrical chamber tangentially, inducing a rotational flow pattern. Centrifugal force drives denser suspended particles (sediment, debris) toward the outer walls and downward into a sump chamber, while treated effluent exits from a central overflow weir or outlet. Performance is velocity-dependent — the system is most effective at moderate flow rates near the design treatment flow rate. At very low flows (insufficient turbulence) and at high flows exceeding the treatment flow rate, efficiency decreases. Bypass piping diverts high-flow storm events around the treatment chamber to prevent scouring of previously captured sediment.
Filtration-Based MTDs: Stormwater passes through a media filter cartridge or screen-based separation element. Media types include engineered compost media, activated carbon, zeolite, perlite, or proprietary blended materials targeting specific pollutants. Filtration MTDs can achieve higher pollutant removal efficiency than hydrodynamic separators for fine particles and dissolved pollutants but require more frequent media replacement to maintain performance. Used where TSS removal alone is insufficient and nutrient or metal removal is also required.
Screening/Trash Capture Hybrids: Some proprietary MTD systems combine coarse trash and debris screening with hydrodynamic or filtration treatment. These provide simultaneous gross pollutant capture and fine TSS reduction, and are used at inlets receiving direct street runoff from paved catchments.
Section 3: Design and Implementation Considerations¶
3.1 MTD Sizing and Siting¶
MTDs are sized based on the treatment flow rate — the design volumetric flow rate at which the device achieves its stated TSS removal efficiency under NJDEP's performance testing protocol. The treatment flow rate is calculated as:
Treatment Flow Rate (Q_t) = WQV / Treatment Duration
Where treatment duration is typically set at 1 to 3 hours based on the manufacturer's design guidance, generating a flow rate that the device must treat at rated efficiency. An MTD must be sized such that the design treatment flow rate does not exceed the device's rated treatment flow.
For flows exceeding the treatment flow rate (larger storm events), bypass piping is required to divert excess flow around the MTD without generating scour velocities within the treatment chamber that would re-suspend previously captured sediment.
Siting requirements: - Access manhole or cleanout at each inlet and outlet port for inspection and maintenance - Minimum 18 inches of headroom above the inlet pipe invert for inspection access in the treatment chamber - Installation must maintain manufacturer-specified inlet and outlet pipe slopes and diameters to preserve rated hydraulic performance - Do not install upstream of wet ponds or detention basins without confirming that backwater conditions will not influence MTD hydraulics
3.2 Non-MTD Chapter 11 Systems¶
Blue Roofs (Chapter 11.1): Controlled slow-release rooftop detention systems. A rooftop drain control insert creates temporary ponding on a flat roof surface, releasing detained volume over 24–72 hours through a calibrated orifice. Blue roofs provide peak rate attenuation only — no water quality treatment or volumetric reduction. Classified as Non-GI in both editions. Primarily used on dense urban buildings where ground-level BMPs are infeasible.
Extended Detention Basins (Chapter 11.2): Dry retention basins with a low-flow orifice outlet that extends the discharge duration of captured runoff over a 24-hour or longer post-storm period. The extended detention time allows settling of fine suspended particles. No permanent pool and no infiltration in standard Non-GI configuration. Provide TSS removal (typically 60–70% — below the 80% threshold) and peak attenuation. May require supplemental treatment upstream to reach the 80% TSS standard.
Sand Filters with Underdrain (Chapter 11.4): Underground or surface vault systems in which stormwater passes through a sand filter media layer and drains through a perforated underdrain collection system to a lower outlet elevation. The sand filter is installed over an impermeable base — no native soil infiltration. Provides high TSS removal (≥80%) and moderate nutrient removal. Commonly used in ultra-urban settings and parking garages. Non-GI in all configurations with impermeable liner.
Subsurface Gravel Wetlands (Chapter 11.5): Below-grade gravel media systems planted with emergent wetland vegetation at the surface. Stormwater flows horizontally through the gravel matrix, with biological treatment occurring in the root zone. The gravel bed is underlain by an impermeable liner — prohibiting native infiltration. Provides high TSS and nutrient removal (subsurface gravel wetlands are among the highest-performing systems for total nitrogen removal). Non-GI in standard configuration.
Wet Ponds — Non-GI (Chapter 11.6): For wet ponds designed without native soil infiltration (standard configuration with liner or low-permeability soils), the 2026 manual explicitly moves them to Chapter 11 to reflect their Non-GI status. Design guidance identical to Chapter 10.5 for the water quality and quantity sizing; the chapter placement change clarifies regulatory classification.
Section 4: Pollutant Removal and Performance¶
4.1 TSS Removal Performance for Chapter 11 Systems¶
| BMP Type | TSS Removal Estimate | Mechanism | Notes |
|---|---|---|---|
| Hydrodynamic separator MTD | 40–80% (device-specific) | Centrifugal separation | Performance certification required per NJCAT/TARP |
| Filtration MTD | 75–90% | Media filtration | Performance certification required |
| Sand filter with underdrain | ≥80% | Physical filtration through sand | Achieves 80% standard; liner required |
| Extended detention basin | 60–70% (typical) | Gravity settling | Typically does not meet 80% alone |
| Subsurface gravel wetland | ≥80% TSS; 50–70% TN | Biological + physical | Highest N removal of Ch. 11 systems |
| Blue roof | No TSS removal | Detention only | Peak attenuation function only |
| Wet pond (Non-GI config) | ≥80% at design HRT | Settling in permanent pool | Meets standard when HRT ≥ 21 days |
4.2 NJDEP Performance Verification Protocol for MTDs¶
A key distinction of MTDs compared to conventional BMP types is the requirement for independent performance verification. Both editions require that all MTDs installed under New Jersey stormwater permits have demonstrated their TSS removal performance through one of two protocols:
TARP (Technology Acceptance Reciprocity Partnership): A multi-state reciprocal protocol under which an MTD is evaluated through controlled fill testing and field monitoring. TARP verification documents the treatment flow rate at which the device achieves stated TSS removal efficiency. New Jersey accepts TARP Tier 1 and Tier 2 certifications for permitting purposes.
NJCATS (NJ Certification for Proprietary Treatment Systems): NJDEP's own product certification process, used for proprietary systems that have not undergone TARP evaluation. Requires applicant-submitted performance data reviewed and approved by NJDEP Division of Water Quality.
MTDs without current TARP or NJCATS certification are not eligible for use in New Jersey major development stormwater management permits under either edition.
Section 5: Key Updates Between the 2023 and 2026 Manuals¶
5.1 Chapter Reorganization — Non-GI Classification Made Explicit¶
The most structurally significant 2026 change to the Non-GI and MTD content is the chapter reorganization. In the 2023 manual, BMPs now classified as Non-GI (sand filters, MTDs, certain wet ponds) were distributed across Chapter 9 and Chapter 11 without clear classification labeling. The 2026 edition:
- Moves all Non-GI practices to Chapter 11 — making the chapter exclusively Non-GI BMPs
- Adds explicit classification statements to each Chapter 11 BMP sub-section: "This BMP type is classified as Non-GI and does not generate Volumetric Reduction Credit toward the GI Requirement under N.J.A.C. 7:8."
- Removes MTDs from Chapter 9 — MTDs are no longer listed as a Chapter 9 BMP option, eliminating the risk of a designer inadvertently classifying an MTD as a GI practice
5.2 MTD Bypass Design Criteria Formalized¶
The 2026 Chapter 11.3 adds explicit bypass design requirements for MTDs that were described only generally in the 2023 edition. The 2026 standard specifies: - Bypass must be designed so that flows exceeding 150% of the treatment flow rate are diverted before entering the MTD chamber - Bypass outlet must discharge to a receiving conveyance system at a point downstream of the MTD outlet — not upstream — to prevent re-entry of bypassed flow - Bypass piping must include a cleanout to confirm bypass is free of obstructions before each inspection period
5.3 Subsurface Gravel Wetland Nitrogen Performance Updated¶
The 2026 Chapter 11.5 updates the nitrogen removal performance data for subsurface gravel wetlands based on field monitoring data collected since the 2023 edition. The 2023 edition cited a 40–50% total nitrogen removal range; the 2026 edition revises this upward to 50–70% for properly designed and maintained systems operating under the amended media and vegetation management protocols. This update may affect pre-application discussions for projects seeking maximum nitrogen removal in impaired watershed areas.
5.4 Blue Roof Detention Period Requirement¶
The 2026 Chapter 11.1 adds a minimum release period requirement for blue roofs used to meet the Quantity Standard: stored runoff must be released over a minimum 24-hour period from any storm event up to the 10-year design storm. The 2023 edition described extended release without specifying a minimum duration. This change requires that blue roof drain control inserts be calibrated to a maximum orifice flow rate corresponding to the 24-hour release standard, not simply "restricted flow."
5.5 GI Retrofit Pathway Cross-Reference (New 2026)¶
The 2026 Chapter 11 introduction adds a cross-reference to the GI Retrofit pathway in Chapter 8 (see Phase 2A), indicating that certain Non-GI systems in Chapter 11 — specifically sand filters with impermeable liners and extended detention basins — may be eligible for conversion to GI-classified infiltrating systems through the defined retrofit protocol. This cross-reference formalizes the relationship between the Non-GI BMP inventory and the emerging retrofit-to-GI compliance pathway.
5.6 Summary of Chapter 11 Changes¶
| Topic | 2023 | 2026 | Impact |
|---|---|---|---|
| MTDs in Ch. 9 | Listed as Ch. 9 options | Removed to Ch. 11 | Eliminates GI misclassification risk |
| Sand filters (lined) in Ch. 9 | Listed as Ch. 9 options | Removed to Ch. 11 | Clearer Non-GI classification |
| Non-GI explicit labeling | Not present | Explicit Non-GI statement per sub-section | Reduces permit submission errors |
| MTD bypass criteria | General description | Specific 150% trigger / downstream outlet requirements | Standardizes bypass design |
| Subsurface gravel wetland TN removal | 40–50% cited | 50–70% (updated with field data) | Higher N removal credit available |
| Blue roof minimum release period | Not specified | ≥24 hours for storms ≤ 10-year | Calibrated orifice design now required |
| GI retrofit cross-reference | Not present | Added to Ch. 11 introduction | Formalizes rehabilitation pathway |
Synthesis¶
Chapter 11 Non-GI BMPs and MTDs occupy a critical but bounded role in the 2026 NJ stormwater framework. They serve sites where the GI Requirement cannot be satisfied through infiltration-based practices, but they cannot substitute for GI — they satisfy the TSS standard and the peak rate standard, while the volumetric reduction gap must be addressed through whatever GI capacity the site can support. The 2026 manual's explicit Non-GI classification, the MTD bypass formalization, and the GI retrofit cross-reference collectively make the relationship between Non-GI systems and the path toward GI compliance cleaner and more navigable for both designers and reviewers. For the OPAL training system, Chapter 11 establishes the boundary conditions of the NJ stormwater compliance framework — it answers the question: "What happens on sites that can't fully infiltrate?"