When Green Infrastructure is infeasible, Chapter 11 provides the framework. Six Non-GI BMP types — blue roofs, extended detention basins, sand filters, subsurface gravel wetlands, Non-GI wet ponds, and MTDs — analyzed under the 2023 and 2026 NJ BMP Manual editions.
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 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 constrained sites.
Manufactured Treatment Devices are proprietary stormwater treatment units — engineered and fabricated off-site — that provide water quality treatment within a compact, below-grade structure. They are designed for urban and suburban settings where land area for conventional surface-based stormwater practices is unavailable.
MTDs do not provide volumetric reduction. Their role is TSS removal toward the Water Quality Standard. For sites where the GI Requirement cannot be fully met with infiltration-based practices, MTDs may be used alongside GI BMPs to satisfy the TSS standard for the portion of runoff not captured by GI.
Stormwater enters a cylindrical chamber tangentially, inducing a rotational flow pattern. Centrifugal force drives denser suspended particles toward the outer walls and downward into a sump chamber, while treated effluent exits from a central overflow weir. Performance is velocity-dependent — most effective at moderate flow rates near the design treatment flow rate. Bypass piping diverts high-flow events around the treatment chamber to prevent scouring of captured sediment.
Stormwater passes through a media filter cartridge or screen-based separation element. Media types include engineered compost, activated carbon, zeolite, perlite, or proprietary blended materials. Can achieve higher pollutant removal than hydrodynamic separators for fine particles and dissolved pollutants, but require more frequent media replacement. Used where TSS removal alone is insufficient and nutrient or metal removal is also required.
Combine coarse trash and debris screening with hydrodynamic or filtration treatment in a single unit. Provide simultaneous gross pollutant capture and fine TSS reduction. Used at inlets receiving direct street runoff from paved catchments with significant trash loading.
Multi-state reciprocal protocol using controlled fill testing and field monitoring. Certifies the treatment flow rate at which the device achieves stated TSS removal efficiency. New Jersey accepts TARP Tier 1 and Tier 2 certifications. Required for all MTDs used in NJ major development stormwater permits.
NJDEP's own product certification 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 NJCAT certification are not eligible for use in NJ major development permits.
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.
Q_t = WQV ÷ Treatment Duration
Where treatment duration is typically 1–3 hours based on manufacturer's design guidance. The MTD must be sized so the design treatment flow rate does not exceed the device's rated treatment capacity. For flows exceeding Q_t (larger storm events), bypass piping is required.
Controlled slow-release rooftop detention systems. A drain control insert creates temporary ponding on a flat roof, releasing detained volume over 24–72 hours through a calibrated orifice. Provide peak rate attenuation only — no water quality treatment or volumetric reduction.
Dry retention basins with a low-flow orifice outlet that extends runoff discharge duration over a 24-hour or longer post-storm period. The extended detention time allows settling of fine particles. No permanent pool and no infiltration in standard Non-GI configuration.
Underground or surface vault systems in which stormwater passes through a sand filter media layer and drains through a perforated underdrain collection system. Installed over an impermeable base — no native soil infiltration. Provides high TSS removal and moderate nutrient removal.
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. Underlain by an impermeable liner — highest nitrogen removal of all Chapter 11 systems.
| BMP Type | TSS Removal | Mechanism | Peak Attenuation | Notes |
|---|---|---|---|---|
| Hydrodynamic separator MTD | 40–80% | Centrifugal separation | None | TARP/NJCAT cert. required |
| Filtration-based MTD | 75–90% | Media filtration | None | TARP/NJCAT cert. required |
| Sand filter with underdrain | ≥80% | Physical filtration through sand | Moderate | Meets 80% standard; liner required |
| Extended detention basin | 60–70% | Gravity settling | High | Does not meet 80% alone; needs supplemental |
| Subsurface gravel wetland | ≥80% TSS; 50–70% TN | Biological + physical | Moderate | Highest N removal of Ch. 11 systems |
| Blue roof | None | Detention only | High | Peak attenuation function only |
| Wet pond (Non-GI config.) | ≥80% at design HRT | Settling in permanent pool | High | Meets WQ + Qty standards; no VRC |
| Topic | 2023 | 2026 | Change Type |
|---|---|---|---|
| MTDs in Ch. 9 | Listed as Ch. 9 options | Removed to Ch. 11 | Reclassification |
| Sand filters (lined) in Ch. 9 | Listed as Ch. 9 options | Removed to Ch. 11 | Reclassification |
| Non-GI explicit labeling | Not present | Explicit Non-GI statement per sub-section | New |
| MTD bypass criteria | General description | Specific 150% trigger / downstream outlet requirements | Formalized |
| Subsurface gravel wetland TN removal | 40–50% cited | 50–70% (updated with field monitoring data) | Performance Updated |
| Blue roof release period | Not specified | ≥ 24 hours for storms ≤ 10-year — calibrated orifice required | New Requirement |
| GI retrofit cross-reference | Not present | Added to Ch. 11 intro — links to Ch. 8 retrofit pathway | New Cross-Reference |
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 reviewing engineers.