Phase 2D - Non-GI Stormwater Practices and Manufactured Treatment Devices (MTDs)¶
Source Documents: - 2023: BMP 11.1, BMP 11.2, BMP 11.3, BMP 11.4, BMP 11.5, BMP 11.6, BMP 9.5 - 2026: BMP 11.1, BMP 11.2, BMP 11.3, BMP 11.4, BMP 11.5, BMP 11.6, BMP 9.5
Overview¶
Chapter 11 in both the 2023 and 2026 NJ Stormwater BMP Manuals remains the non-GI chapter family used when a project needs detention- or treatment-based BMPs that do not meet the definition of green infrastructure. The six BMPs reviewed here are blue roofs, extended detention basins, non-GI MTDs, sand filters with underdrains, subsurface gravel wetlands, and wet ponds. Read alongside Chapter 9.5, the manuals also keep a stable MTD boundary: GI-qualified MTDs stay in Chapter 9.5, while Chapter 11.3 addresses the MTDs that require a waiver or variance from N.J.A.C. 7:8-5.3. (2023 BMP 11.1-11.6; 2026 BMP 11.1-11.6; 2023 BMP 9.5; 2026 BMP 9.5)
These BMPs still matter wherever the project cannot rely on GI alone, but the comparison across the two editions is more continuity-heavy than many early summaries suggested. The chapter tables, waiver-or-variance framing, and core design criteria for the Chapter 11 family are materially consistent between 2023 and 2026, with the clearest classification distinction continuing to be the split between Chapter 9.5 GI MTDs and Chapter 11.3 non-GI MTDs. (2023 BMP 11.1-11.6; 2026 BMP 11.1-11.6; 2023 BMP 9.5; 2026 BMP 9.5)
Section 1: Overview of Non-GI Stormwater Treatment Approaches¶
1.1 Role of Non-GI Practices in Stormwater Management¶
The Chapter 11 BMPs do not all serve the same compliance function, and the manuals are careful about that distinction. Blue roofs are quantity BMPs. Non-GI MTDs and subsurface gravel wetlands are quality BMPs. Extended detention basins, sand filters with underdrains, and wet ponds can be used for quantity and water-quality purposes only within the limits stated in their chapter tables and only with the required waiver or variance from the GI requirements. Across both editions, the more reliable reading is chapter-by-chapter rather than a single shorthand statement about what all non-GI systems do. (2023 BMP 11.1-11.6; 2026 BMP 11.1-11.6)
That chapter-specific reading also matters because the performance range is uneven across the family. Blue roofs are controlled-release detention systems with no chapter-stated TSS credit. Extended detention basins are settling BMPs credited at 40-60% TSS depending on detention time. Wet ponds range from 50-90% TSS depending on permanent-pool ratio and extended detention. Sand filters with underdrains are 80% TSS BMPs, while subsurface gravel wetlands are described as the Chapter 11 system built around denitrification in addition to solids removal. Non-GI MTDs are different again because their TSS value is device-specific and comes from Department certification. (2023 BMP 11.2-11.6; 2026 BMP 11.2-11.6)
1.2 Situations Where Non-GI Systems Are Used¶
The chapter family itself gives a practical picture of where these systems fit. Blue roofs are described as most effective where roofs make up the majority of site impervious cover or where there is little space available at grade for other BMPs. Non-GI MTDs are aimed at sites where the physical size limits of the installation area matter. Sand filters with underdrains are presented as especially well-suited to regulated motor vehicle surfaces, urban areas, and paved catchments with higher solids, metals, or hydrocarbon loadings. Extended detention basins are described as traditional tools at sites expecting significant runoff increases from development. Wet ponds and subsurface gravel wetlands remain chapter-specific treatment systems rather than generic fallback devices. (2023 BMP 11.1-11.6; 2026 BMP 11.1-11.6)
Section 2: Manufactured Treatment Devices (MTDs)¶
2.1 Purpose and Function¶
The manuals draw a clean line between the two MTD chapters. Chapter 11.3 is limited to non-GI MTDs that do not meet the definition of green infrastructure and therefore require a waiver or variance from N.J.A.C. 7:8-5.3 if they are to be used for stormwater runoff quality. Chapter 9.5 addresses the separate subset of GI MTDs that do meet the GI definition. That boundary is not a new 2026 creation; it is already present in both editions. (2023 BMP 11.3; 2026 BMP 11.3; 2023 BMP 9.5; 2026 BMP 9.5)
Within Chapter 11.3, the role of a non-GI MTD is narrow but important. It is a proprietary device used to treat stormwater runoff quality, not quantity or recharge. The manuals require a Department-issued certification letter and sizing in accordance with the published verification report, and they state that the approved TSS removal rate is either 50% or 80% depending on the posted certification for the specific device. (2023 BMP 11.3; 2026 BMP 11.3)
2.2 Treatment Mechanisms¶
The source text identifies two certified non-GI MTD categories. Hydrodynamic sedimentation devices remove pollutants by settling through a swirling vortex, baffle system, laminar plates, or a combination of those mechanisms. Filtration devices remove pollutants by passing runoff through filter media. That is the chapter-backed technology split in both editions, and it is also the right level of generality for comparison purposes. (2023 BMP 11.3; 2026 BMP 11.3)
Chapter 9.5 uses a related but distinct GI MTD framework. GI MTDs are tied to the Department's GI certification list, the 80% TSS rate associated with those GI devices, and the same certification-letter and verification-report logic used to establish acceptable sizing. Read together, the two chapters show that the manuals are not treating all proprietary treatment devices as one undifferentiated category. They separate GI-qualified devices from non-GI devices and then apply different compliance consequences to each. (2023 BMP 9.5; 2026 BMP 9.5; 2023 BMP 11.3; 2026 BMP 11.3)
Section 3: Design and Implementation Considerations¶
3.1 MTD Sizing and Siting¶
The MTD chapters are much more specific about sizing than the current draft suggested. Both editions say the designer must know the peak flow rate of the Water Quality Design Storm, the contributory drainage area, and the physical size limits of the installation area before selecting a device. The device must then be accepted through a Department-issued certification letter and sized in accordance with its published verification report. For non-GI MTDs, the chapter also requires use of the NRCS standard unit hydrograph with a peak rate factor of 484, prohibits use of the DelMarVa unit hydrograph, and ties the sheet-flow travel time to the projected 2-year storm depth under N.J.A.C. 7:8-5.7(d). (2023 BMP 11.3; 2026 BMP 11.3)
The chapter examples also show why a single universal sizing shortcut is not the right summary. For GI MTDs in Chapter 9.5 and filtration-type non-GI MTDs in Chapter 11.3, the manuals direct the designer to compare the maximum allowable drainage area evaluation and the maximum treatment flow rate evaluation in the certification letter and then select the higher minimum configuration. For HDS non-GI MTDs, the example turns on the maximum treatment flow rate evaluation alone. That is a certification-driven sizing method rooted in the verification documents. (2023 BMP 11.3; 2026 BMP 11.3; 2023 BMP 9.5; 2026 BMP 9.5)
The design criteria also stay concrete across both editions. Future connections are prohibited if they would exceed the existing MTD's maximum stormwater quality treatment flow rate. The device must be installed in the same configuration used during NJCAT verification. HDS devices may not use direct grate inlets unless specifically tested that way, and their inlet and outlet pipe angles must stay consistent with the tested configuration unless another geometry was specifically verified. Blind downstream connections are prohibited, and access points such as inspection ports or manholes are required where the MTD connects to down-gradient facilities. (2023 BMP 11.3; 2026 BMP 11.3)
3.2 Non-MTD Chapter 11 Systems¶
The rest of the Chapter 11 family reads just as specifically when each BMP is allowed to speak in its own terms:
- Blue roofs (Chapter 11.1) remain controlled rooftop detention systems used for stormwater runoff quantity only. The manuals state that no standing water may remain
72 hoursafter a rain event and that all blue roof systems must drain within72 hours. They are most effective where roofs dominate site impervious cover or where little at-grade space is available for other BMPs. (2023 BMP 11.1; 2026 BMP 11.1) - Extended detention basins (Chapter 11.2) remain non-GI settling BMPs that can address quantity and, with waiver or variance, water quality. Their approved TSS range is
40-60%, based on detention time. The chapters keep the same core design logic in both editions: on-line or off-line configuration, one-foot SHWT separation,72-hourfull drain requirement, and no exfiltration credit in routing. (2023 BMP 11.2; 2026 BMP 11.2) - Sand filters with underdrains (Chapter 11.4) remain the non-GI sand-filter chapter in both editions. The approved performance is
80%TSS with waiver or variance. The chapters keep the same underdrained design posture: no exfiltration may be counted in quantity routing, the design drain time is36 hours, the maximum allowable drain time is72 hours, and the underdrained chapter is explicitly separated from the infiltrating sand filters discussed in Chapters 9.9 and 10.3. (2023 BMP 11.4; 2026 BMP 11.4) - Subsurface gravel wetlands (Chapter 11.5) remain quality-only non-GI BMPs. Both editions describe them as surface-marsh plus saturated-gravel systems that rely on settling, vegetation uptake and filtration, and denitrification in the submerged gravel cells. The introductory chapter text in both editions states
90%TSS and90%nitrogen removal, while the 2026 applicability table states90%TSS and keeps denitrification as a core chapter mechanism. (2023 BMP 11.5; 2026 BMP 11.5) - Wet ponds (Chapter 11.6) are already titled
Wet Ponds (Non-GI)in both editions. The approved TSS framework remains50%at a1:1permanent-pool ratio, increasing up to90%when extended detention is also provided. Quantity use remains limited to systems designed as on-line facilities and only with waiver or variance. (2023 BMP 11.6; 2026 BMP 11.6)
Section 4: Pollutant Removal and Performance¶
4.1 Chapter-Stated Performance Roles¶
| BMP Type | Chapter-stated performance | Dominant mechanism | Regulatory role in Chapter 11 |
|---|---|---|---|
| Blue roof | No TSS removal rate stated | Controlled rooftop detention | Quantity only, with waiver or variance |
| Extended detention basin | 40-60% TSS depending on detention time |
Settling | Quantity and quality, with waiver or variance |
| Non-GI MTD | 50% or 80% TSS depending on certification |
HDS settling or filtration media | Quality only, with waiver or variance |
| Sand filter with underdrain | 80% TSS |
Settling, filtration, adsorption | Quantity and quality, with waiver or variance |
| Subsurface gravel wetland | 90% TSS; chapter text also describes 90% nitrogen removal |
Settling, vegetation uptake, denitrification | Quality only, with waiver or variance |
| Wet pond (Non-GI) | 50-90% TSS depending on pool ratio and extended detention |
Permanent-pool settling and detention | Quantity when on-line, and quality, with waiver or variance |
The useful comparison point is that each BMP chapter states its own treatment logic instead of collapsing the whole family into one performance shorthand. Extended detention basins are detention-time BMPs. Wet ponds are permanent-pool-ratio and detention BMPs. Sand filters with underdrains are chapter-stated 80% TSS filters. Non-GI MTDs depend on product certification. Subsurface gravel wetlands are the denitrification chapter. Blue roofs remain detention-only. (2023 BMP 11.1-11.6; 2026 BMP 11.1-11.6)
4.2 Conceptual Performance Descriptions¶
That chapter-specific performance logic also keeps the narrative cleaner. Blue roofs reduce peaks by controlled release. Extended detention basins rely on settling during the detention period and are capped at 60% TSS credit. MTDs are compact proprietary treatment devices whose approved removal depends on the device certification rather than a universal chapter formula. Sand filters with underdrains rely on pretreatment, settling, filtration, and adsorption through the sand bed. Subsurface gravel wetlands add biological uptake and denitrification in saturated gravel cells. Wet ponds combine permanent storage and, where used, extended detention to move from lower to higher TSS credit. (2023 BMP 11.2-11.6; 2026 BMP 11.2-11.6)
Section 5: Key Updates Between the 2023 and 2026 Manuals¶
5.1 Continuity Is the Main Story¶
The strongest comparison point is continuity, not reorganization. The reviewed source text already shows the Chapter 11 family in 2023 and again in 2026, and the titles for the principal non-GI chapters already carry the same structure: Manufactured Treatment Devices (Non-GI), Sand Filters with Underdrains (Non-GI), and Wet Ponds (Non-GI). The chapter tables and core design criteria for Chapters 11.2 through 11.6 are materially parallel across the two editions. (2023 BMP 11.2-11.6; 2026 BMP 11.2-11.6)
5.2 The Chapter 9.5 / 11.3 MTD Boundary Is Stable¶
The clearest durable distinction is the one between GI and non-GI MTDs. In both editions, Chapter 9.5 addresses GI MTDs and Chapter 11.3 addresses the non-GI MTDs that require waiver or variance. That means the 2023 to 2026 comparison is not a story about MTDs newly leaving Chapter 9 in 2026; it is a story about a classification split that already existed and remains legible in both manuals. (2023 BMP 9.5; 2026 BMP 9.5; 2023 BMP 11.3; 2026 BMP 11.3)
5.3 Blue Roofs, Wet Ponds, and Sand Filters Read as Continuity¶
The same continuity-heavy reading applies to the other chapters that produced the most overstatement in the earlier draft. Blue roofs carry the same 72-hour drainage rule in both editions. Wet ponds are already titled Wet Ponds (Non-GI) in both editions, so 2026 is not the first time that BMP appears in Chapter 11 as a non-GI chapter. Sand filters with underdrains also remain in Chapter 11.4 in both editions, with materially parallel quantity, quality, and underdrain criteria. (2023 BMP 11.1; 2026 BMP 11.1; 2023 BMP 11.4; 2026 BMP 11.4; 2023 BMP 11.6; 2026 BMP 11.6)
5.4 Summary of the Comparison¶
| Topic | 2023 | 2026 | Practical reading |
|---|---|---|---|
| Chapter 11 family | Full non-GI family already present | Full non-GI family still present | Continuity outweighs novelty |
| GI vs non-GI MTD split | Chapter 9.5 GI / Chapter 11.3 non-GI | Same split remains | Stable classification boundary |
| Blue roof drain rule | 72-hour drainage rule stated |
Same 72-hour drainage rule stated |
No new minimum-release narrative supported |
| Sand filter with underdrain | Chapter 11.4 non-GI underdrained filter | Same chapter role and core criteria | Compare as parallel chapters, not missing content |
| Subsurface gravel wetland | 90% TSS and chapter-stated nitrogen-removal language |
90% TSS table and continued denitrification / nitrogen-removal language in chapter text |
No support for an invented 2023-to-2026 percentage jump |
| Wet pond placement | Wet Ponds (Non-GI) already in Chapter 11.6 |
Same title and same chapter role | Better read as continuity than relocation |
Synthesis¶
Chapter 11 remains the part of the BMP Manual that explains what happens when a project must use non-GI detention or treatment measures. The six BMP chapters do not form a single interchangeable fallback category; instead, they divide into quantity-only, quality-only, and mixed quantity/quality tools, each with its own waiver-or-variance posture and its own chapter-backed performance logic. Chapter 9.5 sharpens that reading by preserving a separate GI lane for the proprietary devices that do meet the GI definition. (2023 BMP 11.1-11.6; 2026 BMP 11.1-11.6; 2023 BMP 9.5; 2026 BMP 9.5)
For designers and reviewers, the more useful takeaway is precision rather than broad narrative. Blue roofs should be read as rooftop detention with a 72-hour drain requirement. MTDs should be sized from certification letters and published verification reports. Extended detention, sand filters with underdrains, gravel wetlands, and wet ponds should be read through their own chapter tables instead of through a generic non-GI shorthand. In that sense, the 2023 to 2026 comparison is less about a newly formalized non-GI regime and more about continued chapter-specific guidance for BMPs that remain outside the GI definition. (2023 BMP 11.1-11.6; 2026 BMP 11.1-11.6)