Manufactured Treatment Device

National baseline

Among Tier A manuals that mention manufactured treatment devices, numeric guidance for this practice is largely expressed through site-wide water-quality volume (WQv), drawdown, and seasonal high water table (SHWT) separation requirements that apply to the development or BMP portfolio—not as device-specific depth, detention, or setback numbers for the unit itself. Where WQv depth is stated (n=30), the mode and median are both 1 inch, with reported values from about 0.37 to 1.61 inches across jurisdictions such as Boston, Baltimore, Chesapeake, Detroit, and many state manuals. Maximum drawdown time appears in 36 manuals with a mode of 72 hours and median of 48 hours, spanning roughly 4 to 120 hours (e.g., Florida FSESCI Tier 1 at 4 hours, Mississippi at 120 hours, and many manuals at 48 or 72 hours). SHWT separation is documented in 36 manuals with a mode of 24 inches and median of 36 inches, over a range of 12 to 120 inches. Committee discussion should treat these figures as the temperate, Type II–ish jurisdictional sizing context in which MTDs are typically sized and credited, pending practice-specific baselines where manuals do not isolate MTD geometry or hydraulics.

Matrix summary

From Tier A manuals mentioning manufactured treatment (37/76), extracted site-wide numerics show WQv depth n=30 with mode and median 1 inch (range 0.37–1.61 in.), max drawdown n=36 with mode 72 hours and median 48 hours (range 4–120 hr), and SHWT separation n=36 with mode 24 in. and median 36 in. (range 12–120 in.); these counts reflect jurisdictional sizing context cited alongside MTD practice mentions, not a unified national standard for device geometry or performance.

Cross-state parameters

Regional modifiers

Open issues

Tier A coverage mentions manufactured treatment in 37 of 76 scanned manuals, but nearly half of the pool lacks extracted numeric values for WQv depth (45 missing), drawdown (38 missing), and SHWT separation (38 missing), so national numeric consensus for MTD-specific design—treatment flow path, internal storage, maintenance frequency, and independent verification—is not established from this matrix. Site-wide mode values (1 inch WQv, 72-hour drawdown mode with 48-hour median, 24-inch SHWT mode with 36-inch median) describe jurisdictional stormwater sizing context, not verified facility specifications for proprietary devices. Extreme drawdown outliers (4 hours in Florida FSESCI Tier 1, 120 hours in Mississippi) and SHWT span (12 to 120 inches, including San Diego 120 inches) indicate regional policy divergence that the synthesis does not resolve into a single MTD design envelope. Chapter proxies and draft manuals in the extract set add uncertainty for committees seeking adopted language.